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The effect of the novel antidepressant tianeptine on the concentration of 5-hydroxytryptamine in rat hippocampal dialysates in vivo
A’eirropharmacology Vol. 30, No. 1, pp. 1-4, Printed in Great Britain. All rights reserved
002%3908/91$3.00+ 0.00 Copyright 0 1991Pergamon Press plc
1991
THE EFFECT OF THE NOVEL ANTIDEPRESSANT TIANEPTINE ON THE CONCENTRATION OF 5HYDROXYTRYPTAMINE IN RAT HIPPOCAMPAL DIALYSATES IN VIP’0 P. S. WHITTON,* G. S. SARNA,? M. T. G’CONNELL and G. CURZON$ Department of Neurochemistry, Institute of Neurology. I, Wakefield Street, London, WClN lPJ, U.K. (Accepted 6 September 1990) Summary-Tianeptine is a novel tricyclic antidepressant which, in marked contrast to other antidepressants, increases the uptake of 5-HT in brain tissue and blood platelet preparations ex uivo.In the present study it was shown, by microdialysis, that tianeptine (lOmg/kg i.p.), whether given as a single dose or as daily doses for 14 days, attenuated the K+-evoked rise of extracellular S-HT in the hippocampus in uivo. Tianeptine (10 mg/kg i.p.), given acutely, did not reduce basal levels of 5-HT in hippocampal dialysates in the presence of the 5-HT reuptake inhibitor, citalopram. These results suggest mutually opposing effects of tianeptine and citalopram on neuronal uptake of S-HT. Their significance in relation to the role of S-HT in the action of antidepressants is discussed. Key words-antidepressants,
5-HT, microdialysis, tianeptine.
These findings appear to challenge conventional ideas on the role of 5-HT in depressive illness. Therefore, the effect of tianeptine on the basal and K+-evoked extracellular concentration of 5-HT in the hippocampus was investigated in vivo, using microdialysis.
Much evidence points to serotonergic transmission being defective in depressive illness and increased by antidepressant drugs (Van Praag, 1982; Heninger, Charney and Sternberg, 1984; Blier, De Montigny and Chaput, 1987). Two particularly persuasive findings are the exacerbation of depression when the availability of tryptophan to the brain is rapidly decreased (Delgado, Charney, Price, Aghajanian, Landis and Heninger, 1990) and the prevention of the antidepressant effect of the monoamine oxidase (MAO) inhibitor tranylcypromine by the inhibitor of the synthesis of 5-hydroxytryptamine (5HT) pchlorophenylalanine (Shopshin, Friedman and Gershon, 1976). It is therefore of great interest that the novel tricyclic drug, tianeptine ([3-chloro-6-methyl-5, 5dioxo-6,l I-dihydro-(c,f)-dibenzo-( 1,2-thiazepine)-1 lyl)amino]-7-heptanoic acid, sodium salt) has antidepressant activity (DeFrance, Marey and Kamoun, 1988; Guelfi, Pichot and Dreyfus, 1989) as, ex olvo
METHODS Animals
Male Sprague-Dawley rats (28&350 g, Charles River, U.K.) were housed individually in a room with a 12 hr IightAark cycle (lights on 06 hr 00 min). Food (Labsure rat diet, Poole, Dorset) and water were freely available. The ambient temperature was maintained at 20°C f 1°C. Drug treatment
Rats were given tianeptine (10 mg/kg ip.), dissolved in 0.9% NaCl in a volume of 1 ml/kg. This dose of the drug has been frequently used in previous work on the disposition of 5-HT (Fattaccini et al., 1990; Mennini et al., 1987; Mocaer, Rettori and Kamoun, 1988). Injections were made between 10.00 and 12.00 hr. Drug or vehicle was given either as a single dose on the day of the dialysis experiment or for 14 days with the final dose on the day of the experiment.
evidence suggests that, unlike other tricyclic antidepressants, it does not inhibit the reuptake of amine transmitters in the central nervous system (Mennini, Mocaer and Garattini, 1987). On the contrary, both acute and chronic treatment with tianeptine increased uptake of 5-HT by hippocampal (Mennini et al., 1987) and cortical synaptosomes from the rat (Fattaccini, Bolanos-Jimenez, Gozlan and Hamon, 1990).
Intracerebral dialysis Present addresses: *Department of Pharmacology, School ‘of Pharmacy, Brunswick Square, London, WClN IAX, U.K. and tM.R.C. Cyclotron Unit, Hammersmith Hospital, 150 Du Cane Road, London, W12 OHS, U.K. $To whom correspondence should be addressed. PIlP w--A
Rats were anaesthetised with pentobarbitone (Sagatal, May and Baker, 60mg/kg, i.p.) and implanted with concentric dialysis probes into the ventral hippocampus, using coordinates according to I
P. S. WHITrON et al.
2
Paxinos and Watson (1982) (A, +3.2 mm from the intra-aural line; L, 4.75 mm from lamda, V, 8.0 mm below dura). Concentric dialysis probes (4 mm membrane length, 200 ,um diameter), were essentially as described by Hutson, Sarna, Kantamaneni and Curzon (1985) but with the internal glass capillary tubes replaced by vitreous silica tubing @GE, Milton Keynes, U.K.). The probe was implanted directly without the use of a guide assembly and perfusion started 18 hours later using artificial cerebrospinal fluid (CSF) (composition in mM: NaCl, 125; KCl, 2.5; MgCl,, 1.18; CaCl,, 1.26) at 0.25 pl/min (CMA Pumps Carnegie Medicin, Sweden). Samples of dialysate were collected every 20 min into tubes containing 10~1 of mobile phase (see below). In the experiments on the effect of tianeptine on K+-evoked release of 5-HT, four 20 min samples were collected before a 20 min period of stimulation with K+, during which 100mM KC1 was substituted for an equimolar concentration of NaCl in the artificial CSF. Two hours later the rats were injected with tianeptine or vehicle, as described above and 20min later a second period of K+-stimulation was imposed. Dialysate sampling was continued for a further 2 hr. In experiments on basal release of 5-HT, the perfusion medium contained 1.O FM of the 5-HT reuptake inhibitor, citalopram. This was done to raise basal levels of 5-HT in the dialysate from levels of about twice the detection limit of 2 fmol/sample, to values from which decreases could be more reliably measured. Four 20 min samples of dialysate were collected before injecting tianeptine or vehicle and sampling was continued for a further 3 hr. The 5-HT in the dialysate was determined by high pressure liquid chromatography, as described by Hutson, Sarna, O’Connell and Curzon (1989). Statistics
Potassium-evoked peaks of 5-HT, before and after injection of tianeptine, were compared using Wilcoxon’s non-parametric test for paired data. Results were also analysed by comparison of ratios of peak heights (after injection) to peak heights (before injection), using the Mann-Whitney U-test.
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Fig. I. Effect of acutely or chronically- ( x 14 days) administered tianeptine (10 mg/kg i.p.) on K+-evoked levels of S-HT in hippocampal dialysate in uiuo. Tianeptine (0) or 0.9% NaCl (0) was given, K+ was infused between 6O-80min and between 20&220min (n = 67). Results given as means + SE. Significant reduction of peak level of 5-HT by tianeptine *P < 0.05.
the pre-injection peak for the same animals. As the latter was somewhat larger than the corresponding peak for the control group in this experiment, the results were also expressed as ratios of post-injection to pre-injection peaks (Table 1). Tianeptine significantly decreased the ratio. Essentially similar results were obtained with rats which were treated chronically with tianeptine i.e. the final injection of the drug substantially and significantly decreased the rise of 5-HT in the dialysates on stimulation with K+. In both experiments, mean values for 5-HT in the period between injection of tianeptine and stimulation with K+ were slightly below values for animals injected with 0.9% NaCl but all these values were small and the differences were not significant.
RESULTS
Effects
of tianeptine
release
of 5-HT
(1Omglkg
in hippocampal
i.p.)
on K+-evoked
Table 1. Increase of 5-HT in dialysate (fmol/sample)
dialysates
The results in Figure 1 show that 20min stimulation with K+ rapidly and transiently increased 5-HT in hippocampal dialysates. After the intraperitoneal injection of 0.9% NaCl, a second period of stimulation with K+ caused a marginally larger rise of 5-HT. When tianeptine (10 mg/kg) was given instead of 0.9% NaCl, the subsequent peak for 5-HT, on stimulation with K+, was appreciably smaller than
evoked by K+
Pre-injection peak (Sl)
Post-injection peak (S2)
NaCl Tianeptine
17.2 f 3.4 21.7 i 2.7
20.4 k 2.7 12.8 i 2.5
1.29kO.14 0.55 f 0.05**
Chronic 0.9% NaCl Tianeptine
14.1 + 2.2 14.0 i 2.8
17.4 f 2.0 8.7 k 0.8
1.38 f 0.24 0.74 * 0.09’
Treatment
(SW(S1)
ACWe 0.9%
n = 6-7. Results given as means i SE. Basal levels of 5-HT (means of4pre-peak-values) were subtracted from the total peak height. *P < 0.05, **P < 0.01 vs corresponding controls.
Brain dialysate 5-HT after tianeptine E#ect of tianeptine (lOmg/kg i.p.) on basal concentration of 5-HT in hippocampal dialysates
The results in Figure 2 show that basal levels of S-HT in hippocampal dialysate from rats treated with citalopram, were about three times greater than in its absence (Fig. 1). Under these conditions, the temporal profiles of the concentration of 5-HT in diall/sates after injecting tianeptine or the 0.9% NaCl vehicle, were essentially similar, rising by about 40% over the 80min period after injection and returning to basal values over the next 120 min. Results (not shown) were similar after giving 40 mg/kg of the drug. DISCUSSION
The results show that the tricyclic antidepressant dr.ig tianeptine decreased the K’-evoked rise of exlracellular 5-HT in brain in vivo, as indicated by microdialysis. They thus agree well with previous ex vito evidence that the drug increases the uptake of 5-HT into synaptosomes in the brain of the rat (Mennini et al., 1987; Fattaccini et al., 1990) and human platelets (Mocaer et al., 1988). The in vivo eff’ects were comparable, whether tianeptine was gi\,en acutely or daily for 14 days i.e., they seemed largely dependent on the acute response to the last dose of the drug and were not affected by the previous dose, given 24 hr before. The latter result contrasts wil:h the ex vivo data of Mennini et al. (1987) who found increased uptake of 5-HT by synaptosomes prepared from the brains of rats, killed 72 hr after the end of chronic treatment with tianeptine. .4lthough tianeptine decreased the K+-evoked rise of 5-HT in the dialysate it did not, in a separate experiment, decrease basal level of 5-HT in the dialysate (Fig. 2). This may have been due to the presence of the 5-HT reuptake inhibitor, citalopram, which was used to enhance the very low basal levels of 5-HT (cf. Sharp, Bramwell and Grahame-Smith,
011 0
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I 240
(min)
Fig. 2. Effect of a single dose of tianeptine (lOmg/kg i.p.) on levels of 5-HT in hippocampal dialysate; 1.0 pm citalopram was added to the artificial CSF, with which the dialysis probe was infused. Tianeptine (0) or 0.9% NaCl (0) was given as indicated (n = 4-5). Results given as means k SE. No significant effect of the drug was observed.
3
1989; Hutson et al., 1989). It suggests that while tianeptine does not bind to 5-HT uptake sites, i.e. sites for 5-HT reuptake inhibitors (Mennini et al., 1987; Kato and Weitsch, 1988), it cannot bind effectively to its own site of action in their presence. The strong suggestion that injecting either tianeptine or vehicle increased 5-HT in the dialysate, to a comparable degree, suggests that the stress of the injection tends to increase release of 5-HT in the hippocampus. Similar results have been obtained on handling and tail-pinch (Kalen, Rosegren, Lindvall and Bjorklund, 1989). If tianeptine decreases extracellular concentrations of 5-HT in brain, it presumably also decreases synaptic concentrations of the amine. These results contrast strikingly with the effects of other tricyclic antidepressants, which increase synaptic 5-HT (Cowen, 1990) thus presumably correcting the deficit of 5-HT in brain in depressive illness, which is suggested by a large volume of evidence (see introduction). Although the concentration of 5-HT in the CSF has been reported to be raised in depression (e.g. Gjerris, Sorensen, Rafaelsen, Werdelin, Alling and Linnoila, 1987), a recent methodological study casts considerable doubt on these findings (Anderson, Mefford, Tolliver, Riddle, Ocame, Leckman and Cohen, 1990). How can the apparently paradoxical evidence that tianeptine, unlike other antidepressants, decreases 5-HT at postsynaptic receptors be explained in view of prevailing concepts on the neurochemical mechanisms involved in depression and its treatment? It could be that, despite preliminary evidence (Mocaer et al., 1988) that the uptake of 5-HT by platelets from subjects treated with tianeptine is increased, tianeptine at therapeutic does not substantially alter the disposition of 5-HT in the human brain. Another possibility is that the therapeutic effects are unrelated to the changes in 5-HT. While this hypothesis is difficult to study in humans, relevant animal experiments are feasible. For example, as behavioural deficits in a model of depression in the rat (Kennett, Dourish and Curzon, 1987; Curzon, 1989) are opposed by tianeptine (Curzon, Kennett, Sarna and Whitton, 1991), the relationship between this behavioural change and the effect of the drug on the disposition of 5-HT could be investigated. Another conceivable mechanism for therapeutic action of tianeptine is that it depends on changes at postsynaptic 5-HT receptors or on altered responses to their activation. However, the drug does not affect the binding of ligands to a wide range of 5-HT and other receptors (Mennini et al., 1987; Kato and Weitsch, 1988). The antidepressant properties of tianeptine could perhaps be explained if its effect on the disposition of 5-HT altered the responses of different types of 5-HT receptors to different degrees, so that the balance between different responses dependent on 5-HT, was altered. That this could conceivably have therapeutic effects is suggested by indications that the anti-
P. s.
4
WHITTONel al.
depressant action may depend on relationships between activation of 5-HT, and 5-HT, receptors (Deakin, 1989). Ack~aw~edgements-We thank Institut de Recherches Internationales Servier (I.R.I.S.) and the M.R.C. for financial support. REFERENCES
Anderson G. M., Mefford I. N., Tolliver T. J., Riddle M. A.. Ocame D. M.. Leckman J. F. and Cohen D. J. (19901 Seiotonin in human cerebrospinal fluid: A reasseskmeni Lije Sci. 46: 247-2.55. Blier P., De Montigny L. and Chaput Y. (1987) Modification of the serotonin system by antidepressant treatments: implications for the therapeutic response in major depression. J. clin. Psychopharmac. 7: 243-35s. Cowen P. J. (1990) A role for 5-HT in the action of antjdep~ssant drugs. Pharmac. Ther. 06: 43-51. Curzon G. (1989) S-Hydroxytryptamine and corticosterone in an animal model of depression. Prog. Neuropsychopharmac. biol. Psychiat. 13: 305-310.
Curzon G., Kennett G. A., Sarna G. S. and Whitton P. S. (1991) The effect of tianeptine and other antidepressants on a rat model of depression. Br. J. Psychiat. In press. Deakin J. F. W. (1989) 5-HT receptor subtypes in depression. In: 3ehaviourai Pharmacology of S-HT (Bevan P., Cools A. R. and Archer T., Eds), pp. 179-204. L. Erlbaum, Hillsdale, New Jersey. DeFrance R., Marey C. and Kamoun A. (1988) Antidepressant and anxiolytic activities of tianeptine: an overview of clinical trials. Clin. Neuropharmac. 11: Suppl.
2, S74-382. De&ado P. L., Charney D. S., Price L. H., Aghajanian G. K., Landis H. and Heninger G. R. (1990) Serotonin function and the mechanism of antidepressant action: reversal of antidepressant-induced remission by rapid depletion of plasma tryptophan. Arch. gen. Psychiat. 47: 41 l-418.
Fattaccini C. M., Bolonos-Jimenez H., Gozlan H. and Hamon M. (1990) Tianeptine stimulates uptake of S-hydroxytryptamine in viua in the rat brain. Neuropharmacotonv. 29: l-8.
GjerrisA., Sorensen A. S., Rafaelsen 0. J., Werdelin L., Alling A. and Linnoila M. (1987) 5-HT and 5-HlAA in cerebrospinal fluid in depression. J. A#& Disord. 12: 13-22.
Guelfi J. D., Pichot P. and Dreyfus J. F. (1989) Efficacy of tianeptine in anxious-depressed patients: results of a controlled multicenter trial versus amitriptyline. Neuropsychobiology 22: 41-48.
Heninger G. R., Chantey D. S. and Sternberg D. E. (1984) Serotonergic function in depression: prolactin response to intravenous tryptophan in depressed patients and healthy subjects. Arch. gen. Psychiat. 41: 398-402. Hutson P. H., Sarna G. S., Kantamaneni B. D. and Curzon G. (1985) Monitoring the effect of a tryptophan load on brain indole metabolism in freely moving rats by simultaneous cerebrospinal fluid sampling and brain dialysis. J. ~euroehem. 44: 1266-1273. Hutson P. H., Sarna G. S., O’Connell M. T. and Curzon G. (1989) Hippocampal 5-HT synthesis and release in viva is decreased by infusion of I-OHDPAT into the nucleus raphe dorsalis. Neurosci. Left. 100: 274-280. Kalen P., Rosegren E., Lindvall D. and Bjorklund A. (1989) Hippocampal noradrenaline release over 24 hours as measured by Ihe dialysis technique in freely moving rats: correlation to behavioural activity state, effect of handling and tail pinch. Eur. J. Neurosci. I: 181-188. Kato G. and Weitsch A. F. (1988) Neurochemical profile of tianeptine, a new antidepressant drug. Clin. Neuropharmat. 11: Suppl. 2, S43-SSO. Kennett G. A., Dourish C. T. and Curzon G. (1987) Antidepressant-like action of 5-HT,, agonists and conventional antidepress~ts in an animal model of depression. Eur. J. Pharmac. 134: 26.5-274. Mennini T., Mocaer E. and Garattini S. (1987) Tianeptine, a selective enhancer of serotonin uptake in rat brain. Naunyn-Schmiedebergs
Arch. Pharmac. 336: 478482.
Mocaer E., Rettori M. C. and Kamoun A. (1988) Pharmacological antidepressive effects and tianeptine-induced 5-HT uptake increase. C&Z. Neuropharmac. 11: Suppl. 2, S32-S42. Paxinos G. and Watson C. (1982) The Rat Brain. Academic Press, New York. Sharp T., Bramwell S. R. and Grahame-Smith D. G. (1989) S-HT, agonists reduce 5-hydroxytryptamine release in rat hippocampus in viva as determined by brain microdialysis Br. J. Pharmac. 96: 283-290.
Shopshin B., Friedman E. and Gershon S. (1976) Parachlorophenylalanine reversal of tranylcypromine effects in depressed patients. Arch. gen. Psychiat. 33: 811-819.
Van Praag H. M. (1982) Neurotransmitters disease: depression. Lancet ii: 1259-1264.
and CNS
002%3908/91$3.00+ 0.00 Copyright 0 1991Pergamon Press plc
1991
THE EFFECT OF THE NOVEL ANTIDEPRESSANT TIANEPTINE ON THE CONCENTRATION OF 5HYDROXYTRYPTAMINE IN RAT HIPPOCAMPAL DIALYSATES IN VIP’0 P. S. WHITTON,* G. S. SARNA,? M. T. G’CONNELL and G. CURZON$ Department of Neurochemistry, Institute of Neurology. I, Wakefield Street, London, WClN lPJ, U.K. (Accepted 6 September 1990) Summary-Tianeptine is a novel tricyclic antidepressant which, in marked contrast to other antidepressants, increases the uptake of 5-HT in brain tissue and blood platelet preparations ex uivo.In the present study it was shown, by microdialysis, that tianeptine (lOmg/kg i.p.), whether given as a single dose or as daily doses for 14 days, attenuated the K+-evoked rise of extracellular S-HT in the hippocampus in uivo. Tianeptine (10 mg/kg i.p.), given acutely, did not reduce basal levels of 5-HT in hippocampal dialysates in the presence of the 5-HT reuptake inhibitor, citalopram. These results suggest mutually opposing effects of tianeptine and citalopram on neuronal uptake of S-HT. Their significance in relation to the role of S-HT in the action of antidepressants is discussed. Key words-antidepressants,
5-HT, microdialysis, tianeptine.
These findings appear to challenge conventional ideas on the role of 5-HT in depressive illness. Therefore, the effect of tianeptine on the basal and K+-evoked extracellular concentration of 5-HT in the hippocampus was investigated in vivo, using microdialysis.
Much evidence points to serotonergic transmission being defective in depressive illness and increased by antidepressant drugs (Van Praag, 1982; Heninger, Charney and Sternberg, 1984; Blier, De Montigny and Chaput, 1987). Two particularly persuasive findings are the exacerbation of depression when the availability of tryptophan to the brain is rapidly decreased (Delgado, Charney, Price, Aghajanian, Landis and Heninger, 1990) and the prevention of the antidepressant effect of the monoamine oxidase (MAO) inhibitor tranylcypromine by the inhibitor of the synthesis of 5-hydroxytryptamine (5HT) pchlorophenylalanine (Shopshin, Friedman and Gershon, 1976). It is therefore of great interest that the novel tricyclic drug, tianeptine ([3-chloro-6-methyl-5, 5dioxo-6,l I-dihydro-(c,f)-dibenzo-( 1,2-thiazepine)-1 lyl)amino]-7-heptanoic acid, sodium salt) has antidepressant activity (DeFrance, Marey and Kamoun, 1988; Guelfi, Pichot and Dreyfus, 1989) as, ex olvo
METHODS Animals
Male Sprague-Dawley rats (28&350 g, Charles River, U.K.) were housed individually in a room with a 12 hr IightAark cycle (lights on 06 hr 00 min). Food (Labsure rat diet, Poole, Dorset) and water were freely available. The ambient temperature was maintained at 20°C f 1°C. Drug treatment
Rats were given tianeptine (10 mg/kg ip.), dissolved in 0.9% NaCl in a volume of 1 ml/kg. This dose of the drug has been frequently used in previous work on the disposition of 5-HT (Fattaccini et al., 1990; Mennini et al., 1987; Mocaer, Rettori and Kamoun, 1988). Injections were made between 10.00 and 12.00 hr. Drug or vehicle was given either as a single dose on the day of the dialysis experiment or for 14 days with the final dose on the day of the experiment.
evidence suggests that, unlike other tricyclic antidepressants, it does not inhibit the reuptake of amine transmitters in the central nervous system (Mennini, Mocaer and Garattini, 1987). On the contrary, both acute and chronic treatment with tianeptine increased uptake of 5-HT by hippocampal (Mennini et al., 1987) and cortical synaptosomes from the rat (Fattaccini, Bolanos-Jimenez, Gozlan and Hamon, 1990).
Intracerebral dialysis Present addresses: *Department of Pharmacology, School ‘of Pharmacy, Brunswick Square, London, WClN IAX, U.K. and tM.R.C. Cyclotron Unit, Hammersmith Hospital, 150 Du Cane Road, London, W12 OHS, U.K. $To whom correspondence should be addressed. PIlP w--A
Rats were anaesthetised with pentobarbitone (Sagatal, May and Baker, 60mg/kg, i.p.) and implanted with concentric dialysis probes into the ventral hippocampus, using coordinates according to I
P. S. WHITrON et al.
2
Paxinos and Watson (1982) (A, +3.2 mm from the intra-aural line; L, 4.75 mm from lamda, V, 8.0 mm below dura). Concentric dialysis probes (4 mm membrane length, 200 ,um diameter), were essentially as described by Hutson, Sarna, Kantamaneni and Curzon (1985) but with the internal glass capillary tubes replaced by vitreous silica tubing @GE, Milton Keynes, U.K.). The probe was implanted directly without the use of a guide assembly and perfusion started 18 hours later using artificial cerebrospinal fluid (CSF) (composition in mM: NaCl, 125; KCl, 2.5; MgCl,, 1.18; CaCl,, 1.26) at 0.25 pl/min (CMA Pumps Carnegie Medicin, Sweden). Samples of dialysate were collected every 20 min into tubes containing 10~1 of mobile phase (see below). In the experiments on the effect of tianeptine on K+-evoked release of 5-HT, four 20 min samples were collected before a 20 min period of stimulation with K+, during which 100mM KC1 was substituted for an equimolar concentration of NaCl in the artificial CSF. Two hours later the rats were injected with tianeptine or vehicle, as described above and 20min later a second period of K+-stimulation was imposed. Dialysate sampling was continued for a further 2 hr. In experiments on basal release of 5-HT, the perfusion medium contained 1.O FM of the 5-HT reuptake inhibitor, citalopram. This was done to raise basal levels of 5-HT in the dialysate from levels of about twice the detection limit of 2 fmol/sample, to values from which decreases could be more reliably measured. Four 20 min samples of dialysate were collected before injecting tianeptine or vehicle and sampling was continued for a further 3 hr. The 5-HT in the dialysate was determined by high pressure liquid chromatography, as described by Hutson, Sarna, O’Connell and Curzon (1989). Statistics
Potassium-evoked peaks of 5-HT, before and after injection of tianeptine, were compared using Wilcoxon’s non-parametric test for paired data. Results were also analysed by comparison of ratios of peak heights (after injection) to peak heights (before injection), using the Mann-Whitney U-test.
Acute
study
30 24
zP) E
J
t
Oo I
I
I
I
I
I
40
60
120
160
200
I
I
240
260
320
I 360
1-1 200 240
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L I 320
I 360
I
P \ z -
Chronic
study
24-
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I
120
160
Time
(min)
Fig. I. Effect of acutely or chronically- ( x 14 days) administered tianeptine (10 mg/kg i.p.) on K+-evoked levels of S-HT in hippocampal dialysate in uiuo. Tianeptine (0) or 0.9% NaCl (0) was given, K+ was infused between 6O-80min and between 20&220min (n = 67). Results given as means + SE. Significant reduction of peak level of 5-HT by tianeptine *P < 0.05.
the pre-injection peak for the same animals. As the latter was somewhat larger than the corresponding peak for the control group in this experiment, the results were also expressed as ratios of post-injection to pre-injection peaks (Table 1). Tianeptine significantly decreased the ratio. Essentially similar results were obtained with rats which were treated chronically with tianeptine i.e. the final injection of the drug substantially and significantly decreased the rise of 5-HT in the dialysates on stimulation with K+. In both experiments, mean values for 5-HT in the period between injection of tianeptine and stimulation with K+ were slightly below values for animals injected with 0.9% NaCl but all these values were small and the differences were not significant.
RESULTS
Effects
of tianeptine
release
of 5-HT
(1Omglkg
in hippocampal
i.p.)
on K+-evoked
Table 1. Increase of 5-HT in dialysate (fmol/sample)
dialysates
The results in Figure 1 show that 20min stimulation with K+ rapidly and transiently increased 5-HT in hippocampal dialysates. After the intraperitoneal injection of 0.9% NaCl, a second period of stimulation with K+ caused a marginally larger rise of 5-HT. When tianeptine (10 mg/kg) was given instead of 0.9% NaCl, the subsequent peak for 5-HT, on stimulation with K+, was appreciably smaller than
evoked by K+
Pre-injection peak (Sl)
Post-injection peak (S2)
NaCl Tianeptine
17.2 f 3.4 21.7 i 2.7
20.4 k 2.7 12.8 i 2.5
1.29kO.14 0.55 f 0.05**
Chronic 0.9% NaCl Tianeptine
14.1 + 2.2 14.0 i 2.8
17.4 f 2.0 8.7 k 0.8
1.38 f 0.24 0.74 * 0.09’
Treatment
(SW(S1)
ACWe 0.9%
n = 6-7. Results given as means i SE. Basal levels of 5-HT (means of4pre-peak-values) were subtracted from the total peak height. *P < 0.05, **P < 0.01 vs corresponding controls.
Brain dialysate 5-HT after tianeptine E#ect of tianeptine (lOmg/kg i.p.) on basal concentration of 5-HT in hippocampal dialysates
The results in Figure 2 show that basal levels of S-HT in hippocampal dialysate from rats treated with citalopram, were about three times greater than in its absence (Fig. 1). Under these conditions, the temporal profiles of the concentration of 5-HT in diall/sates after injecting tianeptine or the 0.9% NaCl vehicle, were essentially similar, rising by about 40% over the 80min period after injection and returning to basal values over the next 120 min. Results (not shown) were similar after giving 40 mg/kg of the drug. DISCUSSION
The results show that the tricyclic antidepressant dr.ig tianeptine decreased the K’-evoked rise of exlracellular 5-HT in brain in vivo, as indicated by microdialysis. They thus agree well with previous ex vito evidence that the drug increases the uptake of 5-HT into synaptosomes in the brain of the rat (Mennini et al., 1987; Fattaccini et al., 1990) and human platelets (Mocaer et al., 1988). The in vivo eff’ects were comparable, whether tianeptine was gi\,en acutely or daily for 14 days i.e., they seemed largely dependent on the acute response to the last dose of the drug and were not affected by the previous dose, given 24 hr before. The latter result contrasts wil:h the ex vivo data of Mennini et al. (1987) who found increased uptake of 5-HT by synaptosomes prepared from the brains of rats, killed 72 hr after the end of chronic treatment with tianeptine. .4lthough tianeptine decreased the K+-evoked rise of 5-HT in the dialysate it did not, in a separate experiment, decrease basal level of 5-HT in the dialysate (Fig. 2). This may have been due to the presence of the 5-HT reuptake inhibitor, citalopram, which was used to enhance the very low basal levels of 5-HT (cf. Sharp, Bramwell and Grahame-Smith,
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Fig. 2. Effect of a single dose of tianeptine (lOmg/kg i.p.) on levels of 5-HT in hippocampal dialysate; 1.0 pm citalopram was added to the artificial CSF, with which the dialysis probe was infused. Tianeptine (0) or 0.9% NaCl (0) was given as indicated (n = 4-5). Results given as means k SE. No significant effect of the drug was observed.
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1989; Hutson et al., 1989). It suggests that while tianeptine does not bind to 5-HT uptake sites, i.e. sites for 5-HT reuptake inhibitors (Mennini et al., 1987; Kato and Weitsch, 1988), it cannot bind effectively to its own site of action in their presence. The strong suggestion that injecting either tianeptine or vehicle increased 5-HT in the dialysate, to a comparable degree, suggests that the stress of the injection tends to increase release of 5-HT in the hippocampus. Similar results have been obtained on handling and tail-pinch (Kalen, Rosegren, Lindvall and Bjorklund, 1989). If tianeptine decreases extracellular concentrations of 5-HT in brain, it presumably also decreases synaptic concentrations of the amine. These results contrast strikingly with the effects of other tricyclic antidepressants, which increase synaptic 5-HT (Cowen, 1990) thus presumably correcting the deficit of 5-HT in brain in depressive illness, which is suggested by a large volume of evidence (see introduction). Although the concentration of 5-HT in the CSF has been reported to be raised in depression (e.g. Gjerris, Sorensen, Rafaelsen, Werdelin, Alling and Linnoila, 1987), a recent methodological study casts considerable doubt on these findings (Anderson, Mefford, Tolliver, Riddle, Ocame, Leckman and Cohen, 1990). How can the apparently paradoxical evidence that tianeptine, unlike other antidepressants, decreases 5-HT at postsynaptic receptors be explained in view of prevailing concepts on the neurochemical mechanisms involved in depression and its treatment? It could be that, despite preliminary evidence (Mocaer et al., 1988) that the uptake of 5-HT by platelets from subjects treated with tianeptine is increased, tianeptine at therapeutic does not substantially alter the disposition of 5-HT in the human brain. Another possibility is that the therapeutic effects are unrelated to the changes in 5-HT. While this hypothesis is difficult to study in humans, relevant animal experiments are feasible. For example, as behavioural deficits in a model of depression in the rat (Kennett, Dourish and Curzon, 1987; Curzon, 1989) are opposed by tianeptine (Curzon, Kennett, Sarna and Whitton, 1991), the relationship between this behavioural change and the effect of the drug on the disposition of 5-HT could be investigated. Another conceivable mechanism for therapeutic action of tianeptine is that it depends on changes at postsynaptic 5-HT receptors or on altered responses to their activation. However, the drug does not affect the binding of ligands to a wide range of 5-HT and other receptors (Mennini et al., 1987; Kato and Weitsch, 1988). The antidepressant properties of tianeptine could perhaps be explained if its effect on the disposition of 5-HT altered the responses of different types of 5-HT receptors to different degrees, so that the balance between different responses dependent on 5-HT, was altered. That this could conceivably have therapeutic effects is suggested by indications that the anti-
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depressant action may depend on relationships between activation of 5-HT, and 5-HT, receptors (Deakin, 1989). Ack~aw~edgements-We thank Institut de Recherches Internationales Servier (I.R.I.S.) and the M.R.C. for financial support. REFERENCES
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