European Journal o f Pharmacology - Molecular Pharmacology Section, 225 (1992) 273-274 273 © 1992 Elsevier Science Publishers B.V. All rights reserved 0922-4106/92/$05.00

EJPMOL 0108R

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Serotonin enhances the 13-adrenergic response in rat brain cortical slices

Didier Morin ~, Rosa Sapena a, Ro land Zini ~ and Jean-Paul Ti l lement ~

'~ D~partement de Pharmacologie and I, CNRS, Facuttd de Mddecine de Paris X1], F-94010 Cr~teil, France

Received 20 January 1992. accepted 29 JanuaE¢ i992

Serotonin increased the isoproterenot-induced cyclic AMP accumulation in rat brain cortical slices but did not itself stimulate the production of cyclic AMP in this area. This effect appeared to be inhibited by the 5-HT 2 receptor antagonist, ritanserin. This potentiation was not observed in the hippocampus. These data reinforce the hypothesis of a coupling between the noradrenergic and serotonergic systems, which may be involved in the mechanism of action of antidepressant drugs.

cAMP accumulation; 5-HT (5-hydroxytryptamine, serotolfin);/3-Adrenoceptors

Several studies have provided strong evidence for a linkage between the noradrenergic and the serotoner- gic systems in rat brain cortex. A reduction of the concentration of normetanephrine, a norepinephrine metabolite, was observed in rats with destroyed sero- tonergic neurons (Racagni et at., 1983). Furthermore the destruction of serotonergic terminals was shown to increase the density of /3-adrenoceptors (Stockemeier et al., 1985) and to prevent the down-regulatic~n of the same receptors after chronic administration of tricyclic antidepressants (Manier et al., 1987). It is reasonable to conceive that, while this has been disputed (Hensler et al., 1991), serotonin can influence the regulation of central /3-adrenoceptors.

The present results reinforce the idea of a func- tional linkage between the two systems as they demon- strated that serotonin enhances the isoproterenol- stimulated cyclic AMP accumulation.

The production of cyclic AMP was measured in rat cerebral cortical and hippocampal slices as described by Morin e t a l . (1991). Briefly, the slices (250× 250 txm) were made with a McIIwain tissue chopper and were rinsed in a Krebs-Ringer bicarbonate buffer. The slices were then incubated with 2/~M [14C]adenine, 0.5 mM 3-isobutyl-l-methyixanthine and the agonists were studied in a total volume of 250 /xt. When the 5-HT 2 antagonist, ritanserin, was used it was added 10 min prior to agonist addition. The reaction was stopped after 45 min at 37°C by adding 940 txl of methanol /

Correspondence to: D. Morin, D~partement de Pharmacologie, Fac- ult~ de M~decine Paris XII. 8, rue du G~n~ral Sarrail, F-94910 Cr~teil, France. Tel. 33 (1) 49.81.36.61; Fax 33 (1) 49.81.35.94.

chloroform (2: 1), 310 pA of chloroform and 310 ~1 of distilled water. The tubes were centrifuged (2000 rpm, 4°C, 10 rain) and [14C]cyclic AMP was isolated by sequential chromatography on Dowex and alumina columns (Morin et al., 1991).

Isoproterenol (1 ~tM) caused a 5.6-fold increase in the cyclic AMP response in the cortex and a 4-fold increase in the hippocampus (table 1). The addition of 100 ~M serotonin enhanced the production of cyclic AMP induced by isoproterenol in the cortex (+ 27%) although serotonin was ineffective alone. These results demonstrate that 5-HT receptors which arc not cou- pled to adenylate cyclase participate indirectly in the activation of the enzyme by adrenergic receptors in this area. The synergistic effect was antagonized in the presence of ritanserin (10 #M), which suggests that 5-HT2 receptors may be involved in this potentiation.

The rest.,!t~ eL~tained for hippocampus were differ- ent. Indeed 5-HT is able to stimulate the accumulation of cyclic AMP in this area. The stimulation elicited by 100 tzM 5-HT averaged 55% over basal activity. The stimulation produced by a combination of 100 p.M 5-HT and 1 p.M isoproterenol was greater than that obtained in the presence of isoproterenol alone but the increase did not reach significance (P = 0.0623). This stimulation was simply additive, corresponding to the sum of the stimulations observed with each agonist alone and was not inhibited in the presence of 10 tzM ritanserin. A synergistic interaction between adrenergic and 5-HT recegtors could thus not be detected in the rat hippocampus.

This functional interaction could be of great impo:- tance in the mecl~anism of action of antidepressants which are known to act on the two systems. It would be

274

TABLE 1

Effects ef isopro~erenol and 5-HT on [14C]cyclic AMP accumulation in rat brain slice'..

The concentrations of isopi'oterenol (ISO), ritanserin (RIT) and 5-HT were 1, 10 and 100 tzM respectively. Each value is the mean_+ S.E.M. ~f experiments done in quadruplicate with n (numbers in parentheses) animals.

[ 14ClCyclic AMP accumulation (cpm)

Cortex Hippocampus

Basal 51+_ 4 (17) 31-+ 2 (6) 5-HT 55+_ 5 (16) 48 ! 7 " (6) ISO 286+_ 9 (17) 125_+ 9 (6) ISO +5-HT 362+-16"** (17) 15~_+ 11 (6) ISO +5-HT+ RIT 280_+16 ** (5) 162_+16 (6)

* P < 0.05 versus basal value; ** P < 0.02 versus ISO+5-HT stimu- lation; *** P < 0.0005 versus ISO stimulation.

of g rea t in teres t to s tudy if this po t en t i a t i on r e s p o n s e

still exists or is modi f ied af te r ch ron ic admin i s t r a t i on o f

a n t i d e p r e s s a n t s act ing selectively on each of the two

m o n o a m i n e r g i c systems.

References

Hensler, J.G., G.A. Ordway, C. Gambarana, P. Areso and A. Frazer, 1991, Serotonin neurons do not influence the regulation of beta adrenoceptorr induced by either desipramine or isoproterenol, J. Pharmacol. Exp. Ther. 256, 656.

Manier, D.H., D.D. Gillespie, E. Sanders-Bush and F. Sulser, 1987, The serotonin/noradrenaline link in brain. I. The role of nor- adrenaline and serotonin in the regulation of density and func- tion of /3-adrenoceptors and its alteration by desipramine, Naunyn-Schmied. Arch. Pharmacol. 335, 109.

Morin, D., R. Zini, V. Querol-Ferrer, R. Sapena and J.P. Tillement, 1991, A method to measure simultaneously cyclic AMP and inositol phosphate accumulation in rat brain slices, J. Neu- rochem. 56, 1114.

Racagni, G., I. Mochetti, G. Calderini, A. Battistella and N. Brunello, 1983, Temporal sequence of changes in central noradrenergic system of rat after prolonged antidepressant treatment: receptor desensitization and neurotransmitter interactions, Neuropbarma- cology 22, 415.

Stockemeier, C.A., A.M. Martino and K.J. Kellar, 1985, A strong influence of serotonin axons on ~-adrenergic receptors in rat brain, Science 230, 323.

Acknowledgement

R, Sapena gratefully acknowledges Roussel UCLAF for their grant.