Potential antidepressant properties of 8-hydroxy-2-(di-n-propylamino) tetralin, a selective serotonin1A receptor agonist

European Journal of Pharmacology, 144 (1987) 223-229

223

Elsevier EJP 50046

Potential antidepressant properties of 8-hydroxy-2-(di-n-propylamino) tetralin, a selective serotonin IA receptor agonist L. C e r v o * a n d R. Samanin Istituto di Ricerehe Farmaeologiche "Mario Negri" Via Eritrea 62, 20157 Milan, Italy

Received 27 May 1987, revised MS received 18 September 1987, accepted 22 September 1987

8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective serotoninlA receptor agonist, was studied for its anti-immobility activity in the forced swimming test after different schedules of treatment. Single doses of 0.250 and 0.500 mg/kg 8-OH-DPAT s.c. reduced the immobility time of rats with no effect on open-field activity. Similar results were obtained with a three-injection course of 8-OH-DPAT in 24 h (doses from 0.125 to 0.500 mg/kg s.c.) or with a once daily injection of 0.250 mg/kg s.c. 8-OH-DPAT for 7 or 21 days. Methiothepin 0.2 mg/kg s.c. and 1-propranolol 20 mg/kg s.c. significantly antagonized the anti-immobility effect of three injections of 0.25 mg/kg s.c. 8-OH-DPAT but 2 mg/kg i.p. metergoline had no such effect. The effect of 8-OH-DPAT was also antagonized both by 0.5 mg/kg i.p. haloperidol and 100 mg/kg i.p. sulpiride, and in animals given an intracerebroventricular injection of 150 #g 5,7-dihydroxytryptamine to deplete brain serotonin levels. The results show that 8-OH-DPAT, by acting on serotonin neurons in the brain, produces disinhibitory effects in a rat model predictive of antidepressant activity and suggest that serotoninlA agonists such as 8-OH-DPAT could constitute a novel class of rapid-acting antidepressant agents. Serotonin; SerotoninlA receptor agonists; Forced swimming test; Antidepressant activity; (Rat) 1. Introduction The role of serotonin (5-HT) in the etiology of depression is controversial The fact that various drugs which specifically block 5-HT uptake show antidepressant activity (Heel et al., 1982; Pedersen et al., 1982) and that various tricyclic antidepressants enhance the electrophysiological responses to 5-HT in the brain after chronic treatment (De Montigny and Aghajanian, 1978) favours the hypothesis of a deficit in 5-HT neurotransmission in depression. This is apparently supported by the finding of reduced central 5-HT turnover and metabolism in one population of depressed patients (Murphy et al., 1978). In contrast, the ability of various tricyclic antidepressants to act as 5-HT antagonists (Fuxe et * To whom all correspondence should be addressed.

al., 1978; Maj et al., 1978; Ogren et al., 1982) and the reduction in the number of 5-HT 2 receptors found after long-term treatment with various antidepressants (Peroutka and Snyder, 1980; Blackshear and Sanders-Bush, 1982) have led to the suggestion that an excess of 5-HT at some receptors, probably the 5-HT2 type, could be involved in depression. This suggestion is apparently supported by a recent report on antidepressant properties of ritanserin, a selective 5-HT2 receptor antagonist (Reyntjens, Waelkens, Gelders, Ceulemans and Janssen, 1984, Proceedings 14th C.I.N.P. Congress, Florence, p. 566). 8 - H y d r o x y - 2 - ( d i - n - p r o p y l a m i n o ) t e t r a l i n (8OH-DPAT), a selective agonist at 5-HTIA receptors (Middlemiss and Fozard, 1983), has been found to reduce selectively brain 5-HT synthesis (Hjorth et al., 1982), cause behavioural effects compatible with reduced central 5-HT function

0014-2999/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)

224 such as increases in eating and sexual behaviour, and have anxiolytic activity (Dourish et al., 1985; Ahelnius et al., 1981; Engel et al., 1984). Evidence has been provided that activation of autoreceptors located on serotonergic cell bodies in midbrain raphe nuclei is involved in the effects on feeding (Bendotti and Samanin, 1986) but 5HT1A sites are also found postsynaptically to serotonergic neurons in various brain regions, particularly the cerebral cortex and some limbic areas (Pazos and Palacios, 1985; Marcinkiewicz et al., 1984; Verge et al., 1985). Thus an action of 8O H - D P A T at postsynaptic 5-HT1A receptors might contribute to its effects. In view of the selective action of 8-OH-DPAT on 5-HT receptors (Middlemiss and Fozard, 1983; Hjorth et al., 1982) it was of interest to study the effects of different doses and treatment schedules of 8-OH-DPAT on the immobility of rats in the forced swimming test, which has been shown to be sensitive to a wide variety of antidepressants including tricyclics, MAO inhibitors and atypical antidepressants such as iprindole, mianserin and viloxazine (Porsolt et al., 1978; Borsini et al., 1985). Another experiment served to study various drugs known to antagonize the action of 5-HT on 5-HT 1 receptors for their ability to prevent the action of 8-OH-DPAT. In order to establish whether pre- or post-synaptic 5-HT receptors were involved, the effect of 8-OH-DPAT was also studied in animals in which 5-HT-containing neurons had been destroyed by intracerebroventricularly (i.c.v.) injected 5,7-dihydroxytryptamine (5,7-DHT). Since previous studies have shown that dopamine (DA) receptor blocking agents block the anti-immobility effect of various tricyclic antidepressants (Borsini et al., 1984; 1985; Pulvirenti and Samanin, 1986), the ability of haloperidol and sulpiride to counteract the effect of 8-OH-DPAT was studied as well.

2. Materials and methods

2.1. Animals Male CD-COBS rats (Charles River, Italy), weighing 220-250 g at the beginning of the experi-

ment, were housed four to a cage at constant room temperature ( 2 1 + 1 ° C ) and relative humidity (60%), with water and food ad libitum. Each exoeriment group consisted of at least eight animals.

2.2. Measurement of immobility Rats were placed individually in Plexiglas cylinders (height 40 cm, diameter 18 cm) containing 17 cm of water at 25 ° C, and 15 min later they were removed to a 3 0 ° C drying room for 30 min (pre-test). For drug testing, the animals were replaced in the cylinders and the total period of immobility was recorded for 5 min by an observer who did not know which treatment the rats had received. A rat was judged to be immobile when it remained floating in the water, in an upright position, making only small movements to keep its head above water.

2.3. Drug treatments All the experiments were performed 1 h after the last 8-OH-DPAT injection since previous experiments had shown that behavioural effects such as intermittent locomotion alternated with flat body posture and movements of the head started rapidly and disappeared within 30-50 min (Bendotti and Samanin, 1986). Four groups of animals underwent different treatment schedules with 8OH-DPAT. One group received a single subcutaneous (s.c.) injection of 8-OH-DPAT (0.125-0.500 m g / k g ) 23 h after the pre-test, 1 h before they were replaced in the cylinders. A second group received various doses of 8-OH-DPAT (0.125-0.500 m g / k g s.c.) as three injections 24, 5 and 1 h before the test. The remaining two groups were treated once daily for 7 or 21 days with 0.250 m g / k g s.c. 8-OH-DPAT. The first dose was injected immediately after the 30 min drying period, the last dose 1 h before the 5 min test. The doses, routes and pretreatment time (before 8-OH-DPAT) for the other drugs used were those reported to have significant effects on 5-HT or dopamine (DA) mechanisms (the appropriate references for each compound are given in parentheses): methiotepin maleate 0.2 m g / k g s.c., 30 min (Tricklebank et al., 1984); 1-propranolol hydrochloride 20 m g / k g s.c.,

225 30 min (Middlemiss et al., 1977; Nahorski and Willcocks, 1983); metergoline maleate 2 m g / k g i.p., 180 rain (Samanin et al., 1977; 1979); sulpiride-Dobren® 100 m g / k g i.p., 30 min (Ljungberg and Ungerstedt, 1978); haloperidol 0.5 m g / k g i.p., 30 rain (Ljungberg and Ungerstedt, 1978).

2.4. Measurement of open-field activity In order to see whether changes in immobility were associated with changes in motor activity, naive animals treated with 8-OH-DPAT were tested for activity in an open-field. The apparatus was a square box (100 × 100 cm) with 40 cm high plastic walls. The floor consisted of a sheet of white plastic painted with a black grid dividing the field into 25 (5 x 5) equal squares. The illumination was identical to that used for the immobility test and the animals underwent the same treatment schedules. One hour after 8-OH-DPAT the animals were placed in a comer of the apparatus and the number of times an animal entered a square with all four paws in 5 min was recorded by an observer unaware of the treatments.

transmission, functional impairment is most evident during the first week after 5,7-DHT injection (Berge et al., 1983; Romandini et al., 1986; Bendotti and Samanin, 1986). After testing, the vehicle- and 5,7-DHT-treated animals were killed by decapitation for biochemical assay of 5-HT. The brains were rapidly removed, frozen on dry ice and stored at - 2 0 ° C until assay. 5-HT was determined by high performance liquid chrom a t o g r a p h y with electrochemical detection according to Ponzio and Jonsson (1979).

2.6. Drugs 8-Hydroxy-2-(di-n-propylamino) tetralin HBR (R.B.I., Wayland, MA, U.S.A.), methiothepin maleate (Hoffman-La Roche, Basel, Switzerland) and l-propranolol (Icpharma, Milan, Italy) were dissolved in distilled water. Metergoline maleate (Farmitalia-Carlo Erba, Milan, Italy) was dissolved in a 1% ascorbic acid solution. Sulpiride used as Dobren ® (Ravizza, Milan, Italy) and haloperidol (Lusofarmaco, Milan, Italy) were dissolved in distilled water with a few drops of 1 N HC1.

2.5. 5, 7-Dihydroxytryptamine (5, 7-DHT)injection

2. 7. Statistics

The rats, anesthetized with equ~tensin, were immobilized in a Kopf stereotaxic instrument. 5,7D H T creatinine sulphate (Serva, Feinbiochemical, Heidelberg) was dissolved in a solution of ascorbic acid (1 mg/ml). 5,7-DHT, 150 #g, (calculated as free base) in a volume of 20 /xl was infused into the right lateral ventricle. The control animals received only the ascorbic acid solution. To protect noradrenaline-containing neurons from the action of 5,7-DHT (Baumgarten et al., 1973), the rats received desipramine 25 m g / k g i.p., an inhibitor of noradrenaline uptake into the nerve endings (Samanin et al., 1975) 30 rain before 5,7-DHT. Since marked aggressive behaviour was observed in grouped rats treated with 5,7-DHT the rats were kept in single cages. The same condition was applied to control animals. Testing started three days after the neurotoxin injection. This time was selected on the basis of previous evidence that, because of dynamic adaptation of 5-HT neuro-

The immobility test results were analyzed by Dunnett's test (two-tailed) for multiple comparison or by factorial analysis of variance (ANOVA 2 × 2) followed by Tukey's test. Statistical analysis of the data from open-field experiments was done by non-parametric analysis of variance (KruskallWallis test). Post-hoc comparisons with the control group (saline-treated) were made using Ryan's procedure. Biochemical data were analyzed by Student's t-test.

3. Results

Table 1 shows the effects of different treatment schedules with 8-OH-DPAT on the immobility time and open-field activity of rats. Single doses of 0.250 and 0.500 m g / k g 8-OH-DPAT reduced the immobility time but 0.125 m g / k g had no effect. None of the doses significantly modified

226 TABLE 1

TABLE 2

Effects of various 8-OH-DPAT treatment schedules on the total duration of immobility in the forced swimming test and on open-field activity. Values are the means 5: S.E. for at least 8 rats. The single and the last doses of 8-OH-DPAT were administered s.c. 1 h before the 5 rain test. The immobility test results were analyzed by analysis of variance and post-hoc comparison with the control group using Dunnett's test. Data from open-field experiments were analyzed by non-parametric analysis of variance (Kruskall-Wallis test). Post-hoc comparisons with control (saline) condition were made according to Ryan's procedure.

Effects of 5-HT or DA receptor antagonists on the reduction of immobility induced by 8-OH-DPAT. Values are means 5: S.E. for at least 8 animals. Three doses of 0.250 m g / k g 8-OH-DPAT were administered 24, 5 and 1 h before the test. Injection times before the test were 240 min for metergoline, and 90 min for haloperidol, sulpiride, methiothepin and 1-propranolol. Data were analyzed by factorial analysis of variance (ANOVA 2 X 2) followed by Tukey's test.

Treatment (mg/kg)

Vehicle Methiothepine l-Propranolol Metergoline

Immobility time (s)

Open-field (total counts)

231.5 + 6.7 231.6 5:11.5 181.65:19.0 a 174.95:13.3 a

86.6 + 10.6 88.2 5:14.4 75.5 5:15.5 110.45:7.1

Single doses Saline 0.125 0.250 0.500

Three doses in 24 h Saline 0.125 0.250 0.500

267.75:6.3 225.45:11.4 b 197.75:12.0 b 192.6+ 5.8 b

90.0+ 11.5 88.05:9.3 69.5 5:13.5 100.7+10.1

Treatment

Dose (mg/kg) 0.2 20.0 2.0

Vehicle Haloperidol

0.5

Vehicle Sulpiride

100

Immobility time (s) Saline

8-OH-DPAT

254.4 5:7.1 255.0+2.6 243.7 5:8.7 251.95:4.4

194.2 5:13.0 249.7_+ 9.8 249.1 5 : 7 . 9 220.45:5.9

249.45:6.3 272.65:3.9 a

192.05:6.0 b 252.6-t- 7.1 c

250.25:4.6 261.75:5.1

202.65:8.1 b 249.75:5.8 c

b c c ns

a p < 0.05 vs. respective vehicle; b p < 0.01 vs. respective saline; c p < 0.01 (ANOVA 2 x 2); ns not significant (ANOVA 2 × 2).

One dose daily for 7 days Saline 0.250

265.45:7.0 215.45:4.7 b

103.7 5:11.4 107.0+11.3

One dose daily for 21 days Saline 0.250

262.5+ 2.8 227.25:6.1 b

118.65:7.8 119.15:17.6

P < 0.05, b p < 0.01, Dunnett's test.

the activity of rats in the open-field, although a tendency to an increase was observed with 0.500 mg/kg. All doses of 8-OH-DPAT administered on a three-injection course in 24 h significantly reduced the immobility time of rats without affecting their activity in the open-field. Similar results were obtained with 0.250 m g / k g 8-OH-DPAT given once daily for 7 and 21 days. Immobility was reduced but there were no changes in openfield activity. In no instance were behavioural effects observed which could have interfered with the measurement of immobility. Intermittent locomotion alternated with flat body posture and movements of the head were noted for about 30 rain after the highest doses of 8-OH-DPAT (0.250 and 0.500 m g / k g ) but they were no longer visible 1 h after injection. The same applies to changes in body temperature: 0.250 and 0.500 m g / k g 8-OHDPAT, as a three-injection course in 24 h, signifi-

cantly reduced the body temperature of the rats 30 but not 60 min after the last dose. Since the three-injection course of 8-OH-DPAT caused the most significant reduction of immobility time, this schedule was selected for studying the effect of various pretreatments on the reduction of immobility caused by 8-OH-DPAT. As shown in table 2, methiothepin 0.2 m g / k g and 1-propranolol 20 m g / k g significantly antagonized the effect of 8-OH-DPAT (F(1,28)= 9.3, P < 0.01 and F(1,28) = 12.0, P < 0.01, respectively) while metergoline 2 m g / k g had no effect TABLE 3 Effect of 5,7-DHT i.c.v, on the reduction of immobility induced by 8-OH-DPAT. Values are means + S.E. of at least 7 animals. 5,7-DHT 150 # g / 2 0 #1 was injected i.c.v, three days before testing. Three doses of 0.250 m g / k g 8-OH-DPAT were administered 24, 5 and 1 h before the test. Data were analyzed by factorial analysis of variance (ANOVA 2 × 2) followed by Tukey's test. Treatment

Vehicle 5,7-DHT

Dose

Immobility time (s)

(#g/rat)

Saline

8-OH-DPAT

246.0 + 9.7 221.2+7.9

183.4 + 11.4 a 238.3+ 7.9 b

-

150

P < 0.01 vs. respective saline, b p < 0.01 (ANOVA 2 × 2).

227 (F(1,28) = 3.0, P > 0.05). Haloperidol 0.5 m g / k g and sulpiride 100 m g / k g also significantly antagonized the effect of 8-OH-DPAT (F(1,28)= 9.9, P < 0.01 and F(1,44) = 8.6, P < 0.01, respectively). Neither methiothepin and 1-propranolol nor haloperidol and sulpiride induced any overt behavioural change or motor dysfunction which could have interfered with the effect of 8-OHD P A T on immobility time. This was borne out by the fact that, with one exception (haloperidol vs. vehicle), the immobility time of animals which had received the various pretreatments was not significantly different from that of the controls. Table 3 shows the effect of i.c.v, administration of 5,7-DHT on the reduction of immobility caused by a three-injection course of 8-OH-DPAT. The animals treated with 5,7-DHT showed an increased irritability to touch and slight loss of body weight (about 30 g) compared to vehicle-treated animals but no motor dysfunction which could have interfered with testing. The immobility time of rats which had received i.c.v. 5,7-DHT was not significantly different from that of vehicle-treated animals but the effect of 8-OH-DPAT was completely antagonized in the 5,7-DHT-treated animals. Factorial analysis of the data showed a highly significant interaction between 5,7-DHT and 8-OH-DPAT treatments (F(1,23) = 17.0, P < 0.01). The brain 5-HT levels of 5,7-DHT and vehicle-treated rats injected with saline and killed after testing were (values in n g / g _+ S.E.) vehicle351 + 16, 5,7-DHT = 94 _+ 9, P < 0.01 Student's t-test.

4. Discussion

At doses ranging from 0.125 to 0.500 m g / k g , a three-injection course of 8-OH-DPAT in 24 h significantly reduced the immobility time of rats. Single doses of 0.250 and 0.500 m g / k g also reduced immobility and the effect of 0.25 m g / k g was seen after single daily injections for 7 or 21 days. The effect could not be attributed to changes in the rats' general activity, as previously reported for stimulants such as d-amphetamine (Porsolt et al., 1978) since activity in the open-field was not increased by 8-OH-DPAT under conditions iden-

tical to those used in the forced swimming test. Stereotyped movements or changes in the animals' body temperature also could not account for the effects since they were not seen under the conditions in which immobility was measured. The fact that methiothepin, a drug showing high affinity for 5-HT 1 binding sites (Markstein et al., 1986), antagonized the effect of 8-OH-DPAT suggested that 5-HT a receptors are involved. Other effects of 8 - O H - D P A T were previously found to be antagonized by methiothepin (Tricklebank et al., 1984; Svensson, 1985). Metergoline, however, which also has high affinity for 5-HT 1 receptors (Garattini et al., 1986) did not modify the 8-OHD P A T effect on immobility, at a dose 10 times higher than that of methiothepin as previously found for other effects of 8-OH-DPAT (Svensson, 1985; Ahlenius and Larsson, 1984). The reason for the lack of effect of metergoline is not clear. The specificity of action of 8-OHD P A T on central 5-HT mechanisms (Middlemiss and Fozard, 1983; Hjorth et al., 1982; Feenstra et al., 1980) and the antagonism by ( - ) - p r o p r a n o l o l , a drug recently reported to block the action of 8-OH-DPAT on 5-HT cell bodies in the dorsal raphe (Sprouse and Aghajanian, 1986) weaken the possibility that an action of methiothepin on receptors other than 5-HT mediates its effects. However two dopamine receptor blocking agents such as haloperidol and sulpiride completely prevented the effect of 8-OH-DPAT as reported for various tricyclic and atypical antidepressants (Borsini et al., 1985; Pulvirenti and Samanin, 1986), suggesting that at least part of the effect of methiothepin could be mediated by its ability to block dopamine receptors (Lloyd and Bartholini, 1974). 8O H - D P A T seems to have no direct effect on central dopamine mechanisms (Feenstra et al., 1980). Reducing the activity of 5-HT neurons in raphe nuclei, e.g. by local injection of lysergic acid diethylamide, has been reported to facilitate dopamine-mediated behaviour (Fink and Oelssner, 1981). Thus it is likely that the activation of dopamine transmission is secondary to the reduced 5-HT activity caused by 8-OH-DPAT. This possibility is confirmed by the fact that 8-OHD P A T injected in the dorsal raphe nucleus significantly increases dopamine release in the caudate

228

nucleus as studied by intracerebral dialysis (Bettini et al., personal communication). More clear-cut evidence that 8-OH-DPAT reduces the immobility of rats by acting on 5-HT neurons is provided by the finding that an i.c.v. injection of 5,7-DHT, which depleted brain 5-HT by 70%, prevented the effect of 8-OH-DPAT. This also indicates that postsynaptic 5-HT receptors are not involved in the effect of systemically administered 8-OH-DPAT in the forced swimming test. Previous studies have shown that an action on midbrain raphe nuclei 5-HT neurons mediates the stimulatory effect of 8-OH-DPAT on eating (Bendotti and Samanin, 1986; Hutson et al., 1986) and that an injection of 0.5 ktg 8-OH-DPAT in the dorsal raphe, an area particularly rich in 5 - H T 1 A binding sites (Verge et al., 1985), reduces the immobility of rats in the forced swimming test (Cervo and Samanin, unpublished result). Together with recent evident that presynaptic receptors in nerve terminals are mainly of the 5-HTIB type (Engel et al., 1986), these findings suggest that the anti-immobility effect of 8-OHDPAT is mediated by activation of autoreceptors located on serotonergic cell bodies in midbrain raphe nuclei, particularly the nucleus raphe dorsalis. The fact that a single dose of 8-OH-DPAT reduced immobility and that no tolerance was observed after 21 days of treatment suggests that 5-HT1A receptor agonists could constitute a novel class of rapid-acting antidepressants. A similar suggestion was made recently by Kennett et al. (1987) who studied the effect of 8-OH-DPAT on behaviourai changes after acute uncontrollable stress.

Acknowledgements This work was supported by Gruppo Farmacologia e Chemioterapia del CNR, Unita' di Ricerca Neuropsicofarmacologica, and project 637 Regione Lombardia (Medicina Preventiva).

References Ahlenius, S. and K. Larsson, 1984, Failure to antagonize the 8-hydroxy-2-(di-n-propylamino)tetralin-induced facilitation

of male rat sexual behavior by the administration of 5-HT receptor antagonists, European J. Phan~acol. 99, 279. Ahlenius, S., K. Larsson, L. Svensson, S. Hjorth, A. Carlsson, P. Lindberg, H. Wikstrom, D. Sanchez, L.-E. Arvidsson, U. Hacksell and J.L.G. Nilsson, 1981, Effects of a new type of 5-HT receptor agonist on male rat sexual behavior, Pharmacol. Biochem. Behav. 15, 785. Baumgarten, H.G., A. Bjorklund, L. Lachenmayer and A. Nobin, 1973, Evaluation of the effects of 5,7-dihydroxytryptamine on serotonin and catecholamine neurons in the rat CNS, Acta Physiol. Scand. (Suppl.) 391. Bendotti, C. and R. Samanin, 1986, 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) elicits eating in free-feeding rats by acting on central serotonin neurons, European J. Pharmacol. 121, 147. Berge, O.-G., O.B. Fasmer, T. Flatmark and K. Hole, 1983, Time course of changes in nociception after 5,6-dihydroxytryptamine lesions of descending 5-HT pathways, Pharmacol. Biochem. Behav. 18, 637. Blackshear, M.A. and E. Sanders-Bush, 1982, Serotonin receptor sensitivity after acute and chronic treatment with mianserin, J. Pharmacol. Exp. Ther. 221, 303. Borsini, F., E. Nowakowska and R. Samanin, 1984, Effect of repeated treatment with desipramine in the behavioral 'despair' test in rats: Antagonism by 'atypical' but not 'classical' neuroleptics or antiadrenergic drugs, Life Sci. 34, 1171. Borsini, F., L. Pulvirenti and R. Samanin, 1985, Evidence of ,dopamine involvement in the effect of repeated treatment with various antidepressants in the behavioural 'despair' test in rats, European J. Pharmacol. 110, 253. De Montigny, C. and G.K. Aghajanian, 1978, Tricyclic antidepressants: Long-term treatment increases responsivity of rat forebrain neurons to serotonin, Science 202, 1303. Dourish, C.T., P.H. Hutson and G. Curzon, 1985, Low doses of the putative serotonin agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) elicit feeding in the rat, Psychopharmacology 86, 197. Engel, J.A., S. Hjorth, K. Svensson, A. Carlsson and S. Liljequist, 1984, Anticonflict effect of the putative serotonin receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), European J. Pharmacol. 105, 365. Feenstra, M.G.P., H. Rollema, D. Dijkstra, C.J. Grol, A.S. Horn and B.H.C. Westerink, 1980, Effect of non-catecholic 2-aminotetralin derivatives on dopamine metabolism in the rat striatum, Naunyn-Schmiedeb. Arch. Pharmacol. 313, 213. Fink, H. and W. Oelssner, 1981, LSD, mescaline and serotonin injected into medial raphe nucleus potentiate apomorphine hypermotifity, European J. Pharmacol. 75, 289. Fuxe, K., S.-O. Ogren, B.J. Everitt, L.F. Agnati, P. Eneroth, J.-A. Gustafsson, G. Jonsson, P. Skett and A.C. Holm, 1978, The effect of antidepressant drugs of the imipramine type on various monoamine systems and their relation to changes in behaviour and neuroendocrine function, in: Depressive Disorders, ed. S. Garattini (Schattauer Verlag, Stuttgart) p. 67. Garattmi, S., T. Mennini, C. Bendotti, R. Invernizzi and R. Samanin, 1986, Neurochemical mechanism of action of

229 drugs which modify feeding via the serotoninergic system, Appetite 7 (Suppl.), 15. Heel, R.C., P.A. Morley, R.N. Brogden, A.A. Carmine, T.M. Speight and G.S. Avery, 1982, Zimelidine: A review of its pharmacological properties and therapeutic efficacy in depressive illness, Drugs 24, 169. Hjorth, S., A. Carlsson, P. Lindberg, D. Sanchez, H. Wikstrom, L.-E. Arvidsson, U. Hacksell and J.L.G. Nilsson, 1982, 8-Hydroxy-2-(di-n-propylamino) tetralin, 8-OH-DPAT, a potent and selective simplified ergot congener with central 5-HT-receptor stimulating activity, J. Neural Transm. 55, 169. Hutson, P.H., C.T. Dourish and G. Curzon, 1986, Neurochemical and behavioural evidence for mediation of the hyperphagic action of 8-OH-DPAT by 5-HT cell body autoreceptors, European J. Pharmacol. 129, 347. Kennett, G.A., C.T. Dourish and G. Curzon, 1987, Antidepressant-like action of 5-HT1A agonists and conventional antidepressants in an animal model of depression, European J. Pharmacol. 134, 265. Ljungberg, T. and U. Ungerstedt, 1978, Classification of neuroleptic drugs according to their ability to inhibit apomorphine-induced locomotion and gnawing: Evidence for two different mechanisms of action, Psychopharmacology 56, 239. Lloyd, K.G. and G. Bartholini, 1974, The effect of methiothepin on cerebral monoamine neurons, Adv. Biochem. Psychopharmacol. 10, 305. Maj, J., H. Sowinska, L. Baran, L. Gancarczyk and A. Rawlow, 1978, The central antiserotonergic action of mianserin, Psychopharmacology 59, 79. Marcinkiewicz, M., D. Verg6, H. Gozlan, L. Pichat and M. Hamon, 1984, Autoradiographic evidence for the heterogeneity of 5-HT 1 sites in the rat brain, Brain Res. 291, 159. Markstein, R., D. Hoyer and G. Engel, 1986, 5-HT1A-receptor mediate stimulation of adenylate cyclase in rat hippocampus, Naunyn-Schmiedeb. Arch. Pharmacol. 333, 335. Middlemiss, D.N., L. Blakeborouge and S.R. Leather, 1977, Direct evidence for an interaction of/3-adrenergic blockers with the 5-HT receptor, Nature 267, 289. Middlemiss, D.N. and J.R. Fozard, 1983, 8-Hydroxy-2-(di-npropylamino)tetralin discriminates between subtypes of the 5-HT 1 recognition site, European J. Pharmacol. 90, 151. Murphy, D.L., I. Campbell and J.L. Costa, 1978, Current status of the indoleamine hypothesis of the affective disorders, in: Psychopharmacology: A generation of Progress, eds. M.A. Lipton, A. DiMascio and K.F. Killam (Raven Press, New York) p. 1235. Najorski, S.R. and A.L. Willcocks, 1983, Interactions of fladrenoceptor antagonists with 5-hydroxytryptamine receptor subtypes in rat cerebral cortex, Br. J. Pharmacol. 78, 107P. Ogren, S.O., K. Fuxe, T. Archer, G. Johansson and A.C. Holm, 1982, Behavioural and biochemical studies on the effects of acute and chronic administration of antidepressant drugs on central serotonergic receptor mechanisms, in: New Vistas in Depression, eds. S.Z. Langer, R. Takahashi, T. Segawa

and M. Briley (Pergamon Press, Oxford) p. 11. Pazos, A. and J.M. Palacios, 1985, Quantitative autoradiographic mapping of serotonin receptors in the rat brain. I. Serotonin-1 receptors, Brain Res. 346, 205. Pedersen, O.L., P. Kragh-Sorensen, M. Bjerre, K.F. Overo and L.F. Gram, 1982, Citalopram, a selective serotonin reuptake inhibitor: Clinical antidepressive and long-term effect. A phase II study, Psychopharmacology 77, 199. Peroutka, S.J. and S.H. Snyder, 1980, Long-term antidepressant treatment decreases spiroperidol-labeled serotonin receptor binding, Science 210, 88. Ponzio, F. and G. Jonsson, 1979, A rapid and simple method for the determination of picogram levels of serotonin in brain tissue using liquid chromatography with electrochemical detection, J. Neurochem. 32, 129. Porsolt, R.D., G. Anton, N. Blavet and M. Jalfre, 1978, Behavioural despair in rats: A new model sensitive to antidepressant treatments, European J. Pharmacol. 47, 379. Pulvirenti, L. and R. Samanin, 1986, Antagonism by dopamine, but not noradrenaline receptor blockers of the antiimmobility activity of desipramine after different treatment schedules in the rat, Pharmacol. Res. Commun. 18, 73. Romandini, S., E. Merlo Pich, E. Esposito, A.Z. Kruszewska and R. Samanin, 1986, The effects of intracerebroventricular 5,7-dihydroxytryptamine on morphine analgesia is time-dependent, Life Sci. 38, 869. Samanin, R., C. Bendotti, F. Miranda and S. Garattini, 1977, Decrease of food intake by quipazine in the rat: Relation to serotoninergic receptor stimulation, J. Pharm. Pharmacol. 29, 53. Samanin, R., S. Bernasconi and S. Garattini, 1975, The effect of nomifensine on the depletion of brain serotonin and catecholamines induced respectively by fenfluramine and 6-hydroxydopamine in rats, European J. Pharmacol. 34, 377. Samanin, R., T. Mennini, A. Ferraris, C. Bendotti, F. Borsini and S. Garattini, 1979, m-Chlorophenylpiperazine: A central serotonin agonist causing powerful anorexia in rats, Naunyn-Schmiedeb. Arch. Pharmacol. 308, 159. Sprouse, J.S. and G.K. Aghajanian, 1986, (-)-Propranolol blocks the inhibition of serotonergic dorsal raphe cell firing by 5-HT1A selective agonists, European J. Pharmacol. 128, 295. Svensson, L., 1985, Effect of 8-OH-DPAT, lisurine and some ergot-related compounds on the acoustic startle response in the rat, Psychopharmacology 85, 469. Tricklebank, M.D., C. Forler and J.R. Fozard, 1984, The involvement of subtypes of the 5-HT 1 receptor and of catecholaminergic systems in the behavioural response to 8-hydroxy-2-(di-n-propylamino)tetralin in the rat, European J. Pharmacol. 106, 271. Verge, D., G. Daval, A. Patey, H. Gozlan, S. E1 Mestikawy and M. Hamon, 1985, Presynaptic 5-HT autoreceptors on serotonergic cell bodies a n d / o r dendrites but not terminals are of the 5-HTIA subtype, European J. Pharmacol. 113, 463.