European Journal of Neuroscience, Vol. 20, pp. 3424–3432, 2004
ª Federation of European Neuroscience Societies
Functional dissociation between serotonergic pathways in dorsal and ventral hippocampus in psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition in rats Snezana Kusljic1,2 and Maarten van den Buuse1,2 1 Behavioural Neuroscience Laboratory, Mental Health Research Institute of Victoria, 155 Oak Street, Parkville, Victoria 3052, Australia 2 Department of Pharmacology, The University of Melbourne, Melbourne, Australia
Keywords: amphetamine, dopamine, hippocampus, phencyclidine, schizophrenia, serotonin
Abstract Altered hippocampal function and brain serotonin activity are implicated in the development and symptoms of schizophrenia. We have previously shown that lesions of the median raphe nucleus, but not the dorsal raphe nucleus, produced a marked enhancement of locomotor hyperactivity induced by phencyclidine and disruption of prepulse inhibition. The dorsal and ventral hippocampus receive serotonin projections predominantly from the median raphe nucleus and dorsal raphe nucleus, respectively. Therefore, we investigated the effect of local lesions of serotonin projections into the dorsal and ventral hippocampus on psychotomimetic druginduced locomotor hyperactivity and prepulse inhibition. Male Sprague–Dawley rats were anaesthetized with pentobarbitone and stereotaxically microinjected with 5 lg of the serotonergic neurotoxin 5,7-dihydroxytryptamine into either the dorsal or the ventral hippocampus. Two weeks after surgery, dorsal hippocampus-lesioned rats showed a 100% enhancement of the locomotor hyperactivity caused by phencyclidine treatment and a slight but significant reduction of the effect of amphetamine. Prepulse inhibition was significantly disrupted in lesioned rats and serotonin levels in the dorsal hippocampus were reduced by 80%. Rats with lesions of the ventral hippocampus showed 85% depletion of serotonin and partial disruption of prepulse inhibition, but no significant changes in the effect of phencyclidine or amphetamine. These results suggest that serotonin projections from the median raphe nucleus to the dorsal hippocampus play an important role in locomotor hyperactivity and prepulse inhibition in rats, animal models of aspects of schizophrenia. This suggests that these serotonin projections may be involved in the pathophysiology of schizophrenia symptomology.
Introduction Dysfunction of the hippocampal formation has been implicated in neuropsychiatric disorders, including schizophrenia (Dierks et al., 1999; Weinberger, 1999). This has been supported by preclinical results (Swerdlow et al., 2000; Finamore et al., 2001), and neuroanatomical (Harrison, 1999) and clinical findings (Sachdev, 1998). Magnetic resonance imaging studies have reported that, in patients with chronic schizophrenia and first-episode psychosis, there are structural changes in the hippocampus present at the onset of illness (Velakoulis et al., 1999). However, this hippocampal volume reduction could not be explained simply by a loss of hippocampal neurons, nor by other gross morphological changes (Weinberger, 1999). Thus, despite good evidence for hippocampal dysfunction in schizophrenia (Silbersweig et al., 1995; Csernansky & Bardgett, 1998; Heckers et al., 1998), the exact nature of this abnormality remains unclear. Psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition of the startle reflex are two of the most widely used animal models of aspects of schizophrenia. In order to characterize the role of hippocampal functioning in schizophrenia, animal behavioural
Correspondence: Dr M. van den Buuse, 1Behavioural Neuroscience Laboratory, as above. E-mail:
[email protected] Received 26 August 2004, revised 6 October 2004, accepted 7 October 2004
doi:10.1111/j.1460-9568.2004.03804.x
models may be used. Psychotomimetic drugs, such as amphetamine and phencyclidine, can induce abnormal behaviours in animals and mimic certain aspects of psychosis in humans (Geyer & Markou, 1995). Psychotomimetic drugs activate similar brain regions in rats as those implicated in psychosis in humans (Pradhan, 1984; Laruelle et al., 1996; Drevets et al., 2001; Duncan et al., 2001). Prepulse inhibition of the startle reflex is an animal model of sensorimotor gating and sensory information processing (Geyer et al., 1990; Swerdlow & Geyer, 1998). In schizophrenia and other mental illnesses there is a deficiency in prepulse inhibition, which may lead to sensory flooding and cognitive fragmentation (Geyer et al., 1990; Swerdlow et al., 1994). Prepulse inhibition is defined as a reduction in the reflex response when a startle-producing stimulus is preceded by a weak prepulse (Wiley, 1994; Geyer & Swerdlow, 1998). Studies using pharmacological manipulations of hippocampal activity in experimental animals, for example hippocampal lesions and microinfusions, revealed that this brain region is involved in the modulation of locomotor hyperactivity and prepulse inhibition (Geyer & Braff, 1987; Swerdlow et al., 2000; Bast & Feldon, 2003). The dopamine hypothesis of schizophrenia (Carlsson & Lindqvist, 1963; Harrison, 1999) has dominated neuropsychopharmacological schizophrenia research for several decades. However, there are several lines of evidence suggesting an important role of other
Serotonin, hippocampus and behaviour 3425 neurotransmitter systems, particularly serotonin, as well in the development and symptoms of this debilitating disorder. For example, post-mortem studies have shown significant alterations in serotonin systems in schizophrenia (Laruelle et al., 1993; Dean et al., 1999). Furthermore, we have shown that the brain serotonin system is differentially involved in the regulation of behaviour in rat models of schizophrenia (Kusljic et al., 2003). In our previous study we assessed the effect of 5,7-dihydroxytryptamine (5,7-DHT) lesions of either the dorsal raphe nucleus or the median raphe nucleus in animal models of aspects of schizophrenia. Rats with median raphe nucleus lesions showed significant enhancement of phencyclidine-induced but not amphetamine-induced locomotor hyperactivity and a marked disruption of prepulse inhibition. In contrast, rats with dorsal raphe nucleus lesions did not show changes in psychotomimetic drug-induced locomotor hyperactivity but displayed partial disruption of prepulse inhibition. (Kusljic et al., 2003). Because the dorsal hippocampus and ventral hippocampus receive serotonin projections predominantly from the median raphe nucleus and dorsal raphe nucleus, respectively, we postulated that the effect of median raphe nucleus lesions was mediated by depletion of serotonin in the dorsal hippocampus. To test this hypothesis and because of the importance of the hippocampus in schizophrenia, the present study aimed to examine the effect of local lesions of serotonin projections into the dorsal or ventral hippocampus on psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition.
exposed and a small hole was drilled. A 25-gauge stainless steel cannulae was lowered into the hippocampus. Each rat received bilateral injection of 1 lL of 5 lg ⁄ mL 5,7-DHT into either the dorsal or the ventral hippocampus. In the dorsal hippocampus 5,7-DHT was delivered via two injections of 0.5 lL each per side, because preliminary experiments with one single injection per side of the dorsal hippocampus resulted in only 40% serotonin depletion. With bregma as zero and the stereotaxic arm at 0, the coordinates were as follows (Paxinos & Watson, 1998): for the dorsal hippocampus (n ¼ 13 for behavioural experiments, n ¼ 1 for histology): )3.6 mm posterior, ±1.5 mm lateral and )3.8 mm ventral of bregma and )3.6 mm posterior, ±3.5 mm lateral and )3.8 mm ventral of bregma; for the ventral hippocampus (n ¼ 10 for behavioural experiments, n ¼ 1 for histology): )5.6 mm posterior, ±4.8 mm lateral and )8 mm ventral of bregma. 5,7-DHT was microinjected by hand, using a micrometer, over a period of 2 min. Sham-operated controls underwent the same surgical procedure and received an equal volume of vehicle solution containing ascorbic acid into either the dorsal hippocampus (n ¼ 10) or the ventral hippocampus (n ¼ 9). After injection, the cannula was left in place for a further 2 min to avoid backflow of the solution. The animals were administered 5 mg ⁄ kg carprofen (s.c.) (Heriot AgVet, Rowville, Vic., Australia), a nonsteroidal, anti-inflammatory analgesic, to reduce post-operative inflammation and discomfort. Rats were placed on a veterinary heat pad until they had recovered from the anaesthesia. After surgery, rats were allowed to recover for 2 weeks, during which they were handled and health checks were made 2–3 times a week.
Materials and methods Animals Experiments were carried out on 44 male Sprague–Dawley rats (Department of Pathology, University of Melbourne), weighing 250– 300 g at the time of the surgery. The rats were housed under standard conditions in groups of 2–3, with free access to food and water. They were maintained on a 12-h light : 12-h dark cycle (lights on at 07:00 h) at a constant temperature of 21 C. The experimental protocol and surgical procedures were approved by the Animal Experimentation Ethics Committee of the University of Melbourne, Australia.
Drugs and solutions d-Amphetamine sulphate (Sigma Chemical Co., St. Louis, MO, USA) and phencyclidine HCl (PCP, Sigma) were dissolved in 0.9% saline solution and injected subcutaneously (s.c.) in the nape of the neck, directly after the 30 min of baseline locomotor activity measurements. Desipramine HCl (Sigma) was dissolved in distilled water and injected intraperitoneally (i.p.) 30 min prior to 5,7-DHT. All treatments were administered in an injection volume of 1 mL ⁄ kg body weight. 5,7DHT (Sigma) was dissolved in 0.1% ascorbic acid (BDH Chemicals, Kilsyth, Vic., Australia) in saline to prevent oxidation of the neurotoxin.
Surgery Rats were pretreated with 20 mg ⁄ kg desipramine to prevent destruction of noradrenergic neurons by 5,7-DHT (Jonsson, 1980), and anaesthetized with sodium pentobarbitone (60 mg ⁄ kg i.p., Rhone Merieux, Qld, Australia). The rat was mounted in a Kopf stereotaxic frame (David Kopf Instruments, Tujunga, CA, USA) with the incisor bar set at )3.3 mm (Paxinos & Watson, 1998). The skull surface was
Locomotor activity and prepulse inhibition Locomotor activity was monitored using eight automated photocell cages (31 · 43 · 43 cm, h · w · l, ENV-520, MED Associates, St. Albans, VT, USA). The position of the rat at any time was detected with 16 infrared sources and sensors on each of the four sides of the monitor. Locomotor activity was expressed as cumulative 30-min distance moved. Behavioural tests were performed starting 2 weeks after the surgery, each session including random numbers of dorsal hippocampus (DH)-lesioned and DH sham-operated rats or ventral hippocampus (VH)-lesioned and VH sham-operated rats. Three locomotor activity tests were performed with 3–4-day intervals to prevent habituation due to repeated testing and to allow clearance of the drugs. Rats were treated with saline, 0.5 mg ⁄ kg amphetamine or 2.5 mg ⁄ kg phencyclidine in random sequence (Kusljic et al., 2003). In the experiments, the rats were placed in the locomotor photocell cages for 30 min to establish baseline locomotor activity and to allow habituation to the test environment, after which they were injected and locomotor activity recorded over a further 90 min. Prepulse inhibition testing was done using four automated startle chambers (SR-LAB, San Diego Instruments, San Diego, CA, USA) consisting of clear Plexiglas cylinders, 9 cm in diameter, resting on a platform inside a ventilated, sound-attenuated and illuminated chamber. Whole-body startle responses of the animal in response to acoustic stimuli caused vibrations of the Plexiglas cylinder, which were then converted into quantitative responses by a piezoelectric accelerometer unit attached beneath the platform. Percentage prepulse inhibition was calculated as 100 · [response to pulse-alone trials ) (response to prepulse + pulse trials)] ⁄ (response to pulse-alone trials) (Geyer & Swerdlow, 1998). Prepulse inhibition measurements were carried out in the same group of control and lesioned rats 1 week after the last locomotor activity tests. A single prepulse inhibition session lasted for about 45 min and consisted of high- and low-intensity stimulus
ª 2004 Federation of European Neuroscience Societies, European Journal of Neuroscience, 20, 3424–3432
3426 S. Kusljic and M. van den Buuse combinations with continuous background noise of 70 dB. The session started and ended with a block of ten pulse-alone trails. These blocks, together with 20 randomly presented pulse-alone trials during the prepulse inhibition protocol, were used to calculate basal startle reactivity and startle habituation. Prepulse inhibition was assessed by random presentation of 115-dB pulses and ten of each of prepulse-2, -4, -8, -12 and -16, and ten of ‘no-stim’. For example, a prepulse-8 (PP8) is a 20-ms prepulse of 8 dB above the background noise, i.e. 78 dB, followed 100 ms later by a 40-ms 115-dB pulse (Van den Buuse & Eikelis, 2001).
Histology Animals were killed by decapitation and brains were removed from the skull. For histological assessment of the location of the injection sites, 20-lm sections of the region of the hippocampus of one DHlesioned and one VH-lesioned rat were cut on a cryostat and mounted on gelatin-coated glass slides. Sections were then stained with cresyl violet (ProSciTech, Thuringowa, Qld, Australia), dehydrated, cleared with xylene, coverslipped and examined microscopically to verify the location of the tips of the infusion cannula.
HPLC measurement of tissue levels of serotonin Hippocampi were dissected bilaterally on a cold plate into the dorsal and ventral components. Dissected structures were weighed and stored in Eppendorf tubes at )80 C until biochemical assays. The tissue samples were homogenized in 500 lL of 0.1 m perchloric acid by ultrasonication and centrifuged at 15 500 g for 5 min. A 50-lL aliquot of the supernatant was injected into the high-pressure liquid chromatography (HPLC) system to determine the content of serotonin (ng ⁄ mg tissue). The HPLC system consisted of a Waters Model 510 Solvent delivery system, a Waters U6K injector, an Alphabond C18 125A 10 U 150 · 3.9 mm column, and a Column & Spectra-Physics 970 D-A1 fluorescence spectrometer. The output signal from the fluorescence detector was analysed with the chromatography software package 810 Baseline, version 3.31. The mobile phase used consisted of 9.8 g ⁄ L KH2PO4 (Ajax Chemicals, Sydney, NSW, Australia), 1.0 g ⁄ L Na2EDTA (Mallinckrodt, KT, USA), 5% acetonitrile and 1 mL ⁄ L triethylamine. Once dissolved, the pH of the mobile phase solution was adjusted to 3.0 with 1 m HCl. The solution was then filtered and degassed and it was delivered to the HPLC at a flow rate of 1 mL ⁄ min. Prior to sample testing, standards for serotonin were run through the system: 12.5, 25, 50, 100 and 200 ng ⁄ mL. Calibration curves were constructed and the level of serotonin in tissue samples was calculated according to standards. Each run was 8 min and the retention time for serotonin was 2.6 min.
prepulse inhibition experiments, factors were Group and Habituation (four blocks of ten startle responses) or Group and Prepulse (five different prepulse intensities), where Habituation and Prepulse were repeated-measures factors. For HPLC measurements, one-way anova was used. A P-value of P < 0.05 was considered to be statistically significant.
Results Histology: injection sites Inspection of cresyl violet-stained brain sections revealed that the tip of the infusion cannula was situated within the boundaries of the dorsal or ventral hippocampi (Fig. 1), as delineated by the Paxinos and Watson rat brain atlas (Paxinos & Watson, 1998).
HPLC: 5-HT depletions Behavioural data of two DH-lesioned rats with partial depletions, as measured by HPLC, were excluded from the study. In the remaining animals (n ¼ 40), local injection of 5,7-DHT caused a marked reduction in serotonin concentration in the dorsal hippocampus or ventral hippocampus. After microinjection of 5,7-DHT into the dorsal hippocampus the content of serotonin was reduced by 80% in the dorsal but not the ventral hippocampus (Fig. 2). anova revealed significant reduction of serotonin levels in the dorsal hippo-
Statistical analysis Data were expressed as the mean ± standard error of the mean (SEM). A statistical analysis of all data was performed using the statistical software package systat 9.0 (SPSS Inc., Chicago, IL, USA). All data were analysed with analysis of variance (anova) with repeated measures where appropriate. In the locomotor activity experiments, distance moved data were summed in 30-min blocks and these blocks were used to assess main effects of lesion type (Group), treatment with amphetamine or phencyclidine (Time) and interactions between these factors. In this analysis, Time was a repeated-measures factor. In the
Fig. 1. Representative photomicrographs of the injection sites in the dorsal hippocampus (top panel) and ventral hippocampus (bottom panel) in cresyl violet-stained brain sections. Panels on the right side illustrate diagrams of the rat brain showing the location of dorsal and ventral hippocampus (Paxinos & Watson, 1998).
ª 2004 Federation of European Neuroscience Societies, European Journal of Neuroscience, 20, 3424–3432
Serotonin, hippocampus and behaviour 3427
Fig. 2. Serotonin content in the dorsal and ventral hippocampus after sham surgery or 5,7-DHT microinjection into the dorsal or ventral hippocampus. Data are expressed as average serotonin concentration (ng ⁄ mg of tissue wet weight) ± SEM. Top panels show serotonin levels in dorsal hippocampus in the DH-lesioned group (left) and VH-lesioned group (right). Bottom panels show serotonin levels in ventral hippocampus in the DH-lesioned group (left) and VH-lesioned group (right). ***P < 0.001 for the difference between serotonin levels in lesioned rats and their respective sham-operated control rats as indicated by anova.
campus compared with controls (F1,17 ¼ 139.3, P < 0.001). After microinjection of 5,7-DHT into the ventral hippocampus, serotonin content of the dorsal hippocampus was at the level of controls, whereas there was approximately 85% reduction in serotonin level in the ventral hippocampus. anova revealed significant reduction of serotonin levels in the ventral hippocampus compared with controls (F1,17 ¼ 147.7, P < 0.001).
Effects of microinjection of 5,7-DHT on baseline and saline locomotor activity Spontaneous baseline locomotor activity was assessed over a period of 30 min in the photocell cage prior to drug administration. When expressed as total distance moved, neither DH-lesioned nor VHlesioned rats showed differences in spontaneous baseline activity levels compared with the sham-operated controls nor were there differences between the three sessions (Figs 3 and 4). Furthermore, after saline injection, locomotor activity levels were very low and there was no significant difference between the lesioned groups and controls (Figs 3 and 4).
Effects of microinjection of 5,7-DHT on amphetamine- and phencyclidine-induced locomotor hyperactivity Compared with sham-operated controls, amphetamine-induced locomotor hyperactivity was slightly but significantly reduced in the DH-lesioned group. When comparing the time-course of locomotor activity after amphetamine injection (Fig. 3), there was a main effect of Group (F1,19 ¼ 6.1, P ¼ 0.024). Although there was the expected main effect of Time (F2,38 ¼ 32.1, P < 0.001), reflecting the increase in activity caused by amphetamine, there was a lack of interaction of Time by Group, suggesting that the time-course of the amphetamine
Fig. 3. Time course of the effects of subcutaneous injection of saline, 0.5 mg ⁄ kg amphetamine or 2.5 mg ⁄ kg phencyclidine on locomotor activity in rats with 5,7-DHT-induced dorsal hippocampal lesions. Locomotor hyperactivity is expressed as average distance moved (cm) ± SEM, for sham-operated (n ¼ 10, d) and DH-lesioned rats (n ¼ 11, s). ***P < 0.001 and *P < 0.05, for the difference between responses in DH-lesioned rats and control rats as indicated by anova.
effect was not altered after DH lesions. Cumulative distance moved in the 90 min after amphetamine injection was reduced by 24% in DH-lesioned rats compared with sham-operated controls. In contrast to amphetamine treatment, the locomotor activity levels induced by phencyclidine injection were markedly increased in DH-lesioned rats. When comparing locomotor activity after phencyclidine injection between sham-operated and DH-lesioned rats (Fig. 3), there was a main effect of Group (F1,19 ¼ 18.6, P < 0.001). Again there was an expected main effect of Time (F2,38 ¼ 12.7, P < 0.001) but there was no overall change in the time-course of the phencyclidine effect. Cumulative distance moved in the 90 min after phencyclidine
ª 2004 Federation of European Neuroscience Societies, European Journal of Neuroscience, 20, 3424–3432
3428 S. Kusljic and M. van den Buuse
Fig. 5. Effect of 5,7-DHT-induced lesions of the dorsal hippocampus (n ¼ 11) or sham surgery (n ¼ 10) on startle amplitude, startle habituation and prepulse inhibition of the acoustic startle. Top panel illustrates basal startle reactivity and startle habituation. Data are expressed as mean startle amplitudes ± SEM for each of the four blocks of ten 115-dB pulses. Bottom panel illustrates prepulse inhibition of the acoustic startle. Prepulse inhibition is expressed as percentage inhibition ± SEM at different prepulse intensities. anova indicated a main effect of lesion, P ¼ 0.020, when comparing sham and DH-lesioned rats. Prepulse inhibition was significantly reduced at PP2, PP4 and PP8.
Fig. 4. Time course of the effects of subcutaneous injection of saline, 0.5 mg ⁄ kg amphetamine or 2.5 mg ⁄ kg phencyclidine on locomotor activity in rats with 5,7-DHT-induced ventral hippocampal (VH) lesions. Locomotor hyperactivity is expressed as average distance moved (cm) ± SEM, for shamoperated (n ¼ 9, d) and VH-lesioned rats (n ¼ 10, s).
injection was increased by 100% in DH-lesioned rats compared with sham-operated controls. There was no significant effect of VH lesions on either the amphetamine or the phencyclidine response, only a main effect of Time (F2,34 ¼ 31.3, P < 0.001 and F2,34 ¼ 7.6, P ¼ 0.002, respectively), reflecting the hyperactivity induced by these treatments (Fig. 4). The lack of a main effect of Group or a Time–Group interaction suggested that this hyperactivity did not differ between sham-operated rats and rats with VH lesions.
Effect of microinjection of 5,7-DHT on startle, habituation and prepulse inhibition Startle amplitude in pulse-alone trials was not different between the DH-lesioned rats and their sham-operated controls (Fig. 5). There was
a trend towards an increase in startle amplitude in VH-lesioned rats compared with sham-operated controls, but this was not statistically significant (Fig. 6). Inspection of the data revealed that VH-lesioned rats showed a tendency towards increased startle amplitudes, particularly in the first and second startle pulse blocks (Fig. 6). However, a rapid decline of startle amplitudes occurred after the second block and lesioned rats habituated to the levels of controls (Fig. 6). When comparing habituation data from lesioned rats and sham-operated controls, there was no significant main effect of Group or Habituation– Group interaction (Figs 5 and 6). An increase in prepulse intensity led to a proportional reduction of the startle response, and consequently to a proportional degree of percentage inhibition. Thus, there were main effects of Prepulse for all groups (data not shown). There was a marked reduction in prepulse inhibition in the DH-lesioned group (Fig. 5). anova revealed a significant main effect of Group (F1,19 ¼ 6.4, P ¼ 0.02), but no Group–Prepulse interaction. Analysis of separate prepulse intensities revealed that prepulse inhibition was significantly decreased at PP2, PP4 and PP8 in DH-lesioned rats compared with sham-operated controls. In contrast to DH lesions, rats with VH lesions displayed disruption of prepulse inhibition but this effect appeared to be dependent on the prepulse intensity (Fig. 6). anova revealed a significant Group–Prepulse intensity interaction (F4,68 ¼ 2.9,
ª 2004 Federation of European Neuroscience Societies, European Journal of Neuroscience, 20, 3424–3432
Serotonin, hippocampus and behaviour 3429
Fig. 6. Effect of 5,7-DHT-induced lesions of the ventral hippocampus (n ¼ 10) or sham surgery (n ¼ 9) on startle amplitude, startle habituation and prepulse inhibition of the acoustic startle. Top panel illustrates basal startle reactivity and startle habituation. Data are expressed as mean startle amplitudes ± SEM for each of the four blocks of ten 115-dB pulses. Bottom panel illustrates prepulse inhibition of the acoustic startle. Prepulse inhibition is expressed as percentage inhibition ± SEM at different prepulse intensities. anova indicated a prepulse–lesion interaction, P ¼ 0.027, when comparing sham and VH-lesioned rats. Prepulse inhibition was significantly reduced at PP2.
P ¼ 0.027), but no significant main effect of Group. Analysis of separate prepulse intensities revealed that prepulse inhibition was significantly decreased at PP2 (Fig. 6).
DH-lesioned rats compared with sham-operated controls; (iii) prepulse inhibition was disrupted in both DH- and VH-lesioned groups but the effect in VH-lesioned rats was smaller and dependent on prepulse intensity. The serotonergic innervation of the hippocampus is differentially distributed. The dorsal hippocampus and ventral hippocampus receive serotonin projections predominantly from the median raphe nucleus and dorsal raphe nucleus, respectively (Azmitia & Whitaker-Azmitia, 1995; McQuade & Sharp, 1997; Mokler et al., 1998). Therefore, our present findings clearly support the idea that serotonin projections from the median raphe nucleus to the dorsal hippocampus are involved in the psychomotor-activating effects of psychotomimetic drugs, such as phencyclidine and amphetamine, and normal regulation of prepulse inhibition. A limitation of the present study could be that we did not verify possible effects of 5,7-DHT injections on hippocampal levels of noradrenaline or dopamine. However, a large number of studies support our conclusion. The neurotoxins 5,6- and 5,7-DHT are accepted tools for ‘chemical degeneration’ of serotonergic axons in the CNS (Baumgarten et al., 1982). Pre-treatment with desipramine has been shown to prevent damage to catecholaminergic pathways (Jonsson, 1980). Administration of 25 mg ⁄ kg desipramine, before intraventricular injection of 5,7-DHT, protected noradrenergic neurons (Gately et al., 1986). In this study, injection of 5,7-DHT produced a significant reduction in 5-HT level in the hippocampus (80–95%), but no significant change in noradrenaline content (Gately et al., 1986). Similar injection of 5,7-DHT, combined with desipramine treatment, induced an almost complete depletion of 5-HT in the medial prefrontal cortex, nucleus accumbens, medial corpus striatum and hippocampus, with no changes in noradrenaline or dopamine concentrations (Lipska et al., 1992). Even a low dose of desipramine (5 mg ⁄ kg) protected noradrenaline neurons from the neurotoxic effects of 5,7-DHT injection into the raphe nuclei (Adell & Myers, 1995). Furthermore, in rats pretreated with 25 mg ⁄ kg desipramine, 5,7-DHT injections into the fimbria-fornix and cingular bundle did not significantly affect noradrenaline and dopamine levels in either dorsal or ventral hippocampus (Lehmann et al., 2000). Finally, Fletcher and colleagues assayed for 5-HT and noradrenaline content following 5,7-DHT lesions of the raphe nuclei. In their study, rats were pretreated with 10 mg ⁄ kg of desipramine. Intra-raphe administration of 5,7-DHT resulted in 90% reduction in 5-HT levels in the hippocampus, whereas the levels of noradrenaline were not significantly affected (Fletcher et al., 2001). Therefore, we believe that the depletion procedure used here would not significantly affect noradrenaline and dopamine levels and we conclude that serotonin depletion mediates the effects observed.
Discussion We previously reported that the brain serotonin system is differentially involved in the regulation of behaviour in rat models of aspects of schizophrenia (Kusljic et al., 2003). We hypothesized that the effects of raphe lesions were mediated by differential serotonin depletion in the hippocampal formation, producing impaired subcortical dopaminergic and glutamatergic activity and inappropriate initiation of behavioural responses to external stimuli. This hypothesis was tested here by studying the effects of serotonin depletion in either dorsal or ventral hippocampus on psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition. The most important findings of the present study were: (i) phencyclidine-induced locomotor hyperactivity was markedly increased in DH-lesioned rats compared with sham-operated controls; (ii) amphetamine-induced locomotor hyperactivity was reduced in
Serotonin in hippocampus and phencyclidine effects The enhancement of the effect of phencyclidine treatment on locomotor activity in rats with dorsal hippocampus lesions is consistent with the effect of median raphe nucleus lesions (Kusljic et al., 2003). Brain structures involved in the expression of phencyclidine-induced hyperlocomotion are the dorsal hippocampus, prefrontal cortex and nucleus accumbens. It has been shown that acute administration of phencyclidine activates dopamine release in the prefrontal cortex and nucleus accumbens (Jentsch et al., 1998) and also stimulates serotonin release in the dorsal hippocampus and frontal cortex (Martin et al., 1998a, b). By contrast, stimulation of locomotor activity by phencyclidine appears to be independent of dopamine release in the nucleus accumbens (Carlsson et al., 2001). Our results
ª 2004 Federation of European Neuroscience Societies, European Journal of Neuroscience, 20, 3424–3432
3430 S. Kusljic and M. van den Buuse suggest that serotonin release from the dorsal hippocampal terminals exerts an inhibitory influence on the expression of phencyclidineinduced hyperactivity. Extrinsic excitatory projections to the nucleus accumbens synapse primarily onto spines of medium-sized, densely spiny neurons, which form at least 90% of accumbens neurons (French & Totterdell, 2002). Non-dopaminergic inputs to the nucleus accumbens from the hippocampus are known to contribute to the behavioural effects of psychostimulants (Burns et al., 1993; Hitchcott & Phillips, 1997; Vorel et al., 2001). The core part of the nucleus accumbens receives glutamatergic projections predominantly from the dorsal hippocampus (Amaral & Witter, 1989; Verwer et al., 1997; Pitkanen et al., 2000) and it is associated with the regulation of locomotor activity and prepulse inhibition (Jongen-Relo et al., 2002). The shell part of the nucleus accumbens, due to its limbic nature, is considered to be more associated with motivated behaviours and less with motor functions (Jongen-Relo et al., 2002). Glutamatergic neurons comprise 90% of the neurons in the hippocampus, and the remaining 10% of cells are GABAergic interneurons (Freund & Buzsaki, 1996). It is likely that serotonin release from terminals in the dorsal hippocampus has an inhibitory effect on glutamatergic projections to the core of the nucleus accumbens. Disruption of this inhibition results in increased glutamatergic transmission in the nucleus accumbens and therefore enhancement of phencyclidine-induced hyperlocomotion.
Serotonin in hippocampus and amphetamine effects The reduction of amphetamine-induced locomotor hyperactivity in DH-lesioned rats is similar to the findings of previous studies showing that serotonergic denervation of the hippocampus by injections of 5,7DHT into the fimbria-fornix ⁄ cingular bundle inhibited amphetamineinduced hyperlocomotion (Lehmann et al., 2000). By contrast, serotonergic lesions of the median raphe nucleus, causing hippocampal serotonin depletion, had no effect on the action of amphetamine (Lehmann et al., 2000; Kusljic et al., 2003). This finding raises two questions: (1) how does the mechanism of the effect of DH lesions compare between the behavioural actions of phencyclidine and amphetamine; and (2) how does the effect of DH lesions on the action of amphetamine compare with that of median raphe nucleus lesions? The nucleus accumbens is located at the interface of limbic glutamatergic projections from the hippocampus, amygdala and prefrontal cortex and of ascending dopamine fibres arising from the midbrain (David et al., 2004). This anatomical organization plays a major role in motor-activating properties of psychostimulant drugs, such as amphetamine (David et al., 2004). In fact, strong evidence suggests a complex interaction of dopamine and glutamate neurotransmission in the nucleus accumbens to modulate the stimulant effect of amphetamine (David & Abraini, 2003). NMDA receptors appear to have an agonist action on dopamine D-1 receptor function and an antagonist action on dopamine D-2 receptor function, whereas for non-NMDA receptors this is the opposite (David et al., 2004). Although our results with phencyclidine suggest that serotonin depletion in the dorsal hippocampus causes enhanced glutamatergic transmission to the nucleus accumbens, the effect of this enhancement on the amphetamine response will depend on the balance of NMDA and non-NMDA receptor activation interacting with dopamine D-1 and D-2 receptors. Further experiments will be needed to identify the involvement of these receptors in the effect of serotonin depletion. Moreover, activation of serotonin release in the hippocampus could be predicted to enhance amphetamine-induced locomotor hyperactivity, a
finding that would be in line with predicted enhanced serotonergic activity in the hippocampus in patients with schizophrenia (Scarr et al., 2001). DH lesions in the present study and median raphe nucleus lesions in our previous study would be expected to have the same effect on the amphetamine response, similar to their effect on serotonin levels in the dorsal hippocampus and on the phencyclidine response. However, as outlined above, median raphe nucleus lesions did not appear to influence amphetamine-induced locomotor hyperactivity (Kusljic et al., 2003). It is possible that, given the complex interaction of glutamatergic and dopaminergic receptors (David et al., 2004), the net effect of median raphe nucleus lesions appeared to be nil. Alternatively, we believe that the contradiction can be explained by the role of serotonergic projections to the nucleus accumbens itself. Serotonergic projections to the nucleus accumbens arise from both the median raphe nucleus and the dorsal raphe nucleus (Kapur & Remington, 1996; Abi-Dargham et al., 1997). The difference between serotonin depletion caused by median raphe nucleus in our previous study and serotonin depletion in the dorsal hippocampus is that the median raphe nucleus lesions would cause serotonin depletion in the nucleus accumbens whereas the DH lesions do not. Thus, whereas hippocampal serotonin depletion caused by 5,7-DHT injections into either the median raphe nucleus or the dorsal hippocampus could be expected to induce the same reduction of amphetamine-induced locomotor hyperactivity, in the case of median raphe nucleus lesions, additional serotonin depletion in the nucleus accumbens itself may off-set or compensate for the serotonin depletion in the dorsal hippocampus, leading to an apparent lack of effect of median raphe nucleus lesions on amphetamine-induced hyperactivity. This model would need to be tested by studying the effect of local depletion of serotonin in the nucleus accumbens, which would be predicted to enhance amphetamine-induced locomotor hyperactivity.
Serotonin in hippocampus and prepulse inhibition Swerdlow et al. (1995) showed that lesions of the ventral hippocampus increased sensitivity of adult rats to the sensorimotor gatingdisruptive effects of apomorphine. Moreover, decreased prepulse inhibition was observed in adult rats that had been subjected to VH lesions on postnatal day 7 (Lipska et al., 1995). In addition, rats with excitotoxic lesions of either the dorsal or the ventral subiculum displayed differential effects on startle reactivity and prepulse inhibition (Caine et al., 2001). These findings suggest that the hippocampal formation modulates sensorimotor gating as measured by prepulse inhibition, but the role of serotonergic projections to the dorsal and ventral hippocampus in this modulation has not been studied before. DH-lesioned rats displayed overall marked disruption of prepulse inhibition, and disruption of prepulse inhibition in the VH-lesioned group was seen particularly at low prepulse intensity. Prepulse inhibition is modulated by a neuronal circuit consisting of cortico-limbic brain structures in which the nucleus accumbens plays an important role (Geyer et al., 1990; Koch & Schnitzler, 1997; Swerdlow & Geyer, 1998). This functional dorsal–ventral hippocampal difference may be related to the different connections of the dorsal and ventral hippocampus with the nucleus accumbens, the amygdala and the prefrontal cortex, which have been implicated in the regulation of prepulse inhibition (Zhang et al., 2002). The present study suggests that the dorsal hippocampus is likely to be involved in responses to a broader range of prepulse intensities, whereas the ventral hippocampus has less involvement or may modulate low prepulse intensities. Further studies are needed to investigate if disruption of prepulse
ª 2004 Federation of European Neuroscience Societies, European Journal of Neuroscience, 20, 3424–3432
Serotonin, hippocampus and behaviour 3431 inhibition by drugs, such as apomorphine or phencyclidine, is also modulated by serotonin depletion in hippocampal areas.
Conclusion Our results highlight a functional dissociation between serotonergic pathways in the dorsal and ventral hippocampus in psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition, animal models of aspects of schizophrenia. Furthermore, these results suggest that serotonin projections from the median raphe nucleus to the dorsal hippocampus may be involved in the pathophysiology of schizophrenia symptomology and may help to explain the importance of serotonin receptor modulation in the action of atypical anti-psychotic drugs (Meltzer et al., 2003).
Acknowledgements This work was supported by the National Health and Medical Research Council of Australia. M.v.d.B. was supported by a Griffith Senior Research Fellowship of the University of Melbourne. The Mental Health Research Institute is a Stanley Research Centre supported by the Stanley Medical Research Institute, Bethesda, MD, USA.
Abbreviations DH, dorsal hippocampus lesioned rats; VH, ventral hippocampus lesioned rats; 5,7-DHT, 5,7-dihydroxytryptamine.
References Abi-Dargham, A., Laruelle, M., Aghajanian, G.K., Charney, D. & Krystal, J. (1997) The role of serotonin in the pathophysiology and treatment of schizophrenia. J. Neuropsych. Clin. Neurosci., 9, 1–17. Adell, A. & Myers, R.D. (1995) Selective destruction of midbrain raphe nuclei by 5,7-DHT: is brain 5-HT involved in alcohol drinking in Sprague–Dawley rats? Brain Res., 693, 70–79. Amaral, D.G. & Witter, M.P. (1989) The three-dimensional organization of the hippocampal formation: a review of anatomical data. Neuroscience, 31, 571–591. Azmitia, E.C. & Whitaker-Azmitia, P.M. (1995) Anatomy, cell biology, and plasticity of the serotonergic system. Neuropsychopharmacological implications for the actions of psychotropic drugs. In Bloom, F.E. & Kupfer, D.J. (Eds), Psychopharmacology: the Fourth Generation of Progress. Raven Press, New York, pp. 443–449. Bast, T. & Feldon, J. (2003) Hippocampal modulation of sensorimotor processes. Prog. Neurobiol., 70, 319–345. Baumgarten, H.G., Klemm, H.P., Sievers, J. & Schlossberger, H.G. (1982) Dihydroxytryptamines as tools to study the neurobiology of serotonin. Brain Res. Bull., 9, 131–150. Burns, L.H., Robbins, T.W. & Everitt, B.J. (1993) Differential effects of excitotoxic lesions of the basolateral amygdala, ventral subiculum and medial prefrontal cortex on responding with conditioned reinforcement and locomotor activity potentiated by intra-accumbens infusions of d-amphetamine. Behav. Brain Res., 55, 167–183. Caine, S.B., Humby, T., Robbins, T.W. & Everitt, B.J. (2001) Behavioral effects of psychomotor stimulants in rats with dorsal or ventral subiculum lesions: locomotion, cocaine self-administration, and prepulse inhibition of startle. Behav. Neurosci., 115, 880–894. Carlsson, A. & Lindqvist, M. (1963) Effect of chlorpromazine or haloperidol on formation of 3-methoxytyramine and normetanephrine in mouse brain. Acta. Pharmacol. Toxicol., 20, 140–144. Carlsson, A., Waters, N., Holm-Waters, S., Tedroff, J., Nilsson, M. & Carlsson, M.L. (2001) Interactions between monoamines, glutamate, and GABA in schizophrenia: new evidence. Annu. Rev. Pharmacol. Toxicol., 41, 237–260. Csernansky, J.G. & Bardgett, M.E. (1998) Limbic-cortical neuronal damage and the pathophysiology of schizophrenia. Schizophrenia Bull., 24, 231–248. David, H.N. & Abraini, J.H. (2003) Blockade of the locomotor stimulant effects of amphetamine by group I, group II, and group III metabotropic glutamate receptor ligands in the rat nucleus accumbens: possible
interactions with dopamine receptors. Neuropharmacology, 44, 717– 727. David, H.N., Sissaoui, K. & Abraini, J.H. (2004) Modulation of the locomotor responses induced by D1-like and D2-like dopamine receptor agonists and damphetamine by NMDA and non-NMDA glutamate receptor agonists and antagonists in the core of the rat nucleus accumbens. Neuropharmacology, 46, 179–191. Dean, B., Hussain, T., Hayes, W., Scarr, E., Kitsoulis, S., Hill, C., Opeskin, K. & Copolov, D.L. (1999) Changes in serotonin2A and GABAA receptors in schizophrenia: studies on the human dorsolateral prefrontal cortex. J. Neurochem., 72, 1593–1599. Dierks, T., Linden, D.E., Jandl, M., Formisano, E., Goebel, R., Lanfermann, H. & Singer, W. (1999) Activation of Heschl’s gyrus during auditory hallucinations. Neuron, 22, 615–621. Drevets, W.C., Gautier, C., Price, J.C., Kupfer, D.J., Kinahan, P.E., Grace, A.A., Price, J.L. & Mathis, C.A. (2001) Amphetamine-induced dopamine release in human ventral striatum correlates with euphoria. Biol. Psychiatry, 49, 81–96. Duncan, E.J., Madonick, S.H., Parwani, A., Angrist, B., Rajan, R., Chakravorty, S., Efferen, T.R., Szilagyi, S., Stephanides, M., Chappell, P.B., Gonzenbach, S., Ko, G.N. & Rotrosen, J.P. (2001) Clinical and sensorimotor gating effects of ketamine in normals. Neuropsychopharmacology, 25, 72–83. Finamore, T.L., Seybold, K.S., Noble, M. & Port, R.L. (2001) Contributions of hippocampal cellular damage and NMDA receptor dysfunction to behavioral markers of schizophrenia. Int. J. Neurosci., 109, 61–70. Fletcher, P.J., Selhi, Z.F., Azampanah, A. & Sills, T.L. (2001) Reduced brain serotonin activity disrupts prepulse inhibition of the acoustic startle reflex. Effects of 5,7-dihydroxytryptamine and p-chlorophenylalanine. Neuropsychopharmacology, 24, 399–409. French, S.J. & Totterdell, S. (2002) Hippocampal and prefrontal cortical inputs monosynaptically converge with individual projection neurons of the nucleus accumbens. J. Comp. Neurol., 446, 151–165. Freund, T.F. & Buzsaki, G. (1996) Interneurons of the hippocampus. Hippocampus, 6, 347–470. Gately, P.F., Segal, D.S. & Geyer, M.A. (1986) The behavioral effects of depletions of brain serotonin induced by 5,7-dihydroxytryptamine vary with time after administration. Behav. Neural Biol., 45, 31–42. Geyer, M.A. & Braff, D.L. (1987) Startle habituation and sensorimotor gating in schizophrenia and related animal models. Schizophrenia Bull., 13, 643– 668. Geyer, M.A. & Markou, A. (1995) Animal models of psychiatric disorders. In Bloom, F. & Kupfer, D. (Eds) Psychopharmacology: the Fourth Generation of Progress. Raven Press, New York, pp. 787–798. Geyer, M.A. & Swerdlow, N.R. (1998) Measurement of startle response, prepulse inhibition, and habituation. Current Protocols Neurosci., Unit 8.7, 8.7.1–8.7.15. Geyer, M.A., Swerdlow, N.R., Mansbach, R.S. & Braff, D.L. (1990) Startle response models of sensorimotor gating and habituation deficits in schizophrenia. Brain Res. Bull., 25, 485–498. Harrison, P.J. (1999) The neuropathology of schizophrenia. Brain, 122, 593– 624. Heckers, S., Rauch, S.L., Goff, D., Savage, C.R., Schacter, D.L., Fischman, A.J. & Alpert, N.M. (1998) Impaired recruitment of the hippocampus during conscious recollection in schizophrenia. Nat. Neurosci., 1, 318–323. Hitchcott, P.K. & Phillips, G.D. (1997) Amygdala and hippocampus control dissociable aspects of drug-associated conditioned rewards. Psychopharmacology (Berl.), 131, 187–195. Jentsch, J.D., Taylor, J.R. & Roth, R.H. (1998) Subchronic phencyclidine administration increases mesolimbic dopaminergic system responsivity and augments stress- and psychostimulant-induced hyperlocomotion. Neuropsychopharmacology, 19, 105–113. Jongen-Relo, A.L., Kaufmann, S. & Feldon, J. (2002) A differential involvement of the shell and core subterritories of the nucleus accumbens of rats in attentional processes. Neuroscience, 111, 95–109. Jonsson, G. (1980) Chemical neurotoxins as denervation tools in neurobiology. Ann. Rev. Neurosci., 3, 169–187. Kapur, S. & Remington, G. (1996) Serotonin–dopamine interaction and its relevance to schizophrenia. Am. J. Psychiatry, 153, 466–476. Koch, M. & Schnitzler, H.U. (1997) The acoustic startle response in rats – circuits mediating evocation, inhibition and potentiation. Behav. Brain Res., 89, 35–49. Kusljic, S., Copolov, D.L. & van den Buuse, M. (2003) Differential role of serotonergic projections arising from the dorsal and median raphe nuclei in locomotor hyperactivity and prepulse inhibition. Neuropsychopharmacology, 28, 2138–2147.
ª 2004 Federation of European Neuroscience Societies, European Journal of Neuroscience, 20, 3424–3432
3432 S. Kusljic and M. van den Buuse Laruelle, M., Abi-Dargham, A., Casanova, M.F., Toti, R., Weinberger, D.R. & Kleinman, J.E. (1993) Selective abnormalities of prefrontal serotonergic receptors in schizophrenia. A postmortem study. Arch. Gen. Psychiatry, 50, 810–818. Laruelle, M., Abi-Dargham, A., van Dyck, C.H., Gil, R., D’Souza, C.D., Erdos, J., McCance, E., Rosenblatt, W., Fingado, C., Zoghbi, S.S., Baldwin, R.M., Seibyl, J.P., Krystal, J.H., Charney, D.S. & Innis, R.B. (1996) Single photon emission computerized tomography imaging of amphetamine-induced dopamine release in drug-free schizophrenic subjects. Proc. Natl Acad. Sci. USA, 93, 9235–9240. Lehmann, O., Jeltsch, H., Bertrand, F., Lazarus, C., Will, B. & Cassel, J. (2000) When injected into the fimbria-fornix ⁄ cingular bundle, not in the raphe, 5,7dihydroxytryptamine prevents amphetamine-induced hyperlocomotion. Behav. Brain Res., 114, 213–217. Lipska, B.K., Jaskiw, G.E., Arya, A. & Weinberger, D.R. (1992) Serotonin depletion causes long-term reduction of exploration in the rat. Pharmacol. Biochem. Behav., 43, 1247–1252. Lipska, B.K., Swerdlow, N.R., Geyer, M.A., Jaskiw, G.E., Braff, D.L. & Weinberger, D.R. (1995) Neonatal excitotoxic hippocampal damage in rats causes post-pubertal changes in prepulse inhibition of startle and its disruption by apomorphine. Psychopharmacology (Berl.), 122, 35–43. Martin, P., Carlsson, M.L. & Hjorth, S. (1998a) Systemic PCP treatment elevates brain extracellular 5-HT: a microdialysis study in awake rats. Neuroreport, 9, 2985–2988. Martin, P., Waters, N., Schmidt, C.J., Carlsson, A. & Carlsson, M.L. (1998b) Rodent data and general hypothesis: antipsychotic action exerted through 5-HT2A receptor antagonism is dependent on increased serotonergic tone. J. Neural Transm., 105, 365–396. McQuade, R. & Sharp, T. (1997) Functional mapping of dorsal and median raphe 5-hydroxytryptamine pathways in forebrain of the rat using microdialysis. J. Neurochem., 69, 791–796. Meltzer, H.Y., Li, Z., Kaneda, Y. & Ichikawa, J. (2003) Serotonin receptors: their key role in drugs to treat schizophrenia. Progr. Neuro-Psychopharmacol. Biol. Psych., 27, 1159–1172. Mokler, D.J., Lariviere, D., Johnson, D.W., Theriault, N.L., Bronzino, J.D., Dixon, M. & Morgane, P.J. (1998) Serotonin neuronal release from dorsal hippocampus following electrical stimulation of the dorsal and median raphe nuclei in conscious rats. Hippocampus, 8, 262–273. Paxinos, G. & Watson, C. (1998) The Rat Brain in Stereotaxic Co-Ordinates, 4th edn. Academic Press, New York. Pitkanen, A., Pikkarainen, M., Nurminen, N. & Ylinen, A. (2000) Reciprocal connections between the amygdala and the hippocampal formation, perirhinal cortex, and postrhinal cortex in rat. A review. Ann. NY Acad. Sci., 911, 369–391.
Pradhan, S.N. (1984) Phencyclidine (PCP): some human studies. Neurosci. Biobehav. Rev., 8, 493–501. Sachdev, P. (1998) Schizophrenia-like psychosis and epilepsy: the status of the association. Am. J. Psychiatry, 155, 325–336. Scarr, E., Copolov, D.L. & Dean, B. (2001) A proposed pathological model in the hippocampus of subjects with schizophrenia. Clin. Exp. Pharmacol. Physiol., 28, 70–73. Silbersweig, D.A., Stern, E., Frith, C., Cahill, C., Holmes, A., Grootoonk, S., Seaward, J., McKenna, P., Chua, S.E., Schnorr, L.& et al. (1995) A functional neuroanatomy of hallucinations in schizophrenia. Nature, 378, 176–179. Swerdlow, N.R., Braff, D.L., Taaid, N. & Geyer, M.A. (1994) Assessing the validity of an animal model of deficient sensorimotor gating in schizophrenic patients. Arch. Gen. Psychiatry, 51, 139–154. Swerdlow, N.R. & Geyer, M.A. (1998) Using an animal model of deficient sensorimotor gating to study the pathophysiology and new treatments of schizophrenia. Schizophrenia Bull., 24, 285–301. Swerdlow, N.R., Lipska, B.K., Weinberger, D.R., Braff, D.L., Jaskiw, G.E. & Geyer, M.A. (1995) Increased sensitivity to the sensorimotor gating-disruptive effects of apomorphine after lesions of medial prefrontal cortex or ventral hippocampus in adult rats. Psychopharmacology (Berl.), 122, 27–34. Swerdlow, N.R., Taaid, N., Halim, N., Randolph, E., Kim, Y.K. & Auerbach, P. (2000) Hippocampal lesions enhance startle gating-disruptive effects of apomorphine in rats: a parametric assessment. Neuroscience, 96, 523–536. Van den Buuse, M. & Eikelis, N. (2001) Estrogen increases prepulse inhibition of acoustic startle in rats. Eur. J. Pharmacol., 425, 33–41. Velakoulis, D., Pantelis, C., McGorry, P.D., Dudgeon, P., Brewer, W., Cook, M., Desmond, P., Bridle, N., Tierney, P., Murrie, V., Singh, B. & Copolov, D. (1999) Hippocampal volume in first-episode psychoses and chronic schizophrenia: a high-resolution magnetic resonance imaging study. Arch. Gen. Psychiatry, 56, 133–141. Verwer, R.W., Meijer, R.J., Van Uum, H.F. & Witter, M.P. (1997) Collateral projections from the rat hippocampal formation to the lateral and medial prefrontal cortex. Hippocampus, 7, 397–402. Vorel, S.R., Liu, X., Hayes, R.J., Spector, J.A. & Gardner, E.L. (2001) Relapse to cocaine-seeking after hippocampal theta burst stimulation. Science, 292, 1175–1178. Weinberger, D.R. (1999) Cell biology of the hippocampal formation in schizophrenia. Biol. Psychiatry, 45, 395–402. Wiley, J.L. (1994) Clozapine’s effects on phencyclidine-induced disruption of prepulse inhibition of the acoustic startle response. Pharmacol. Biochem. Behav., 49, 1025–1028. Zhang, W.N., Bast, T. & Feldon, J. (2002) Prepulse inhibition in rats with temporary inhibition ⁄ inactivation of ventral or dorsal hippocampus. Pharmacol. Biochem. Behav., 73, 929–940.
ª 2004 Federation of European Neuroscience Societies, European Journal of Neuroscience, 20, 3424–3432
