Modulation of Dialysate Levels of Dopamine, Noradrenaline, and Serotonin (5-HT) in the Frontal Cortex of Freely-Moving Rats by (-)-Pindolol Alone and in Association with 5-HT Reuptake Inhibitors: Comparative Roles b of -Adrenergic, 5-HT1A, and 5-HT1B Receptors Alain Gobert, Ph.D. and Mark J. Millan, Ph.D. (-)-Pindolol, which possesses significant affinity for 5-HT1A, involved. (-)-Pindolol potentiated the increase in FCX levels b 5-HT1B, and 1/2-adrenergic receptors (AR)s, dose- of 5-HT elicited by the 5-HT reuptake inhibitors, fluoxetine dependently increased extracellular levels of dopamine and duloxetine, and also enhanced their ability to elevate (DA) and noradrenaline (NAD) versus 5-HT, in dialysates FCX levels of DA—though not of NAD. In contrast to of the frontal cortex (FCX), but not accumbens and (-)-pindolol, betaxolol and ICI118,551 did not affect the striatum, of freely-moving rats. In distinction, the actions of fluoxetine, whereas both WAY100,635 and b preferential 1-AR antagonist, betaxolol, and the SB224,289 potentiated the increase in levels of 5-HT—but b preferential 2-AR antagonist, ICI118,551, did not increase not DA or NAD levels—elicited by fluoxetine. In basal levels of DA, NAD, or 5-HT. Further, they both dose- conclusion, (-)-pindolol modulates, both alone and together dependently and markedly blunted the influence of with 5-HT reuptake inhibitors, dopaminergic, adrenergic, (-)-pindolol upon DA and NAD levels. The selective 5-HT1A and serotonergic transmission in the FCX via a complex b receptor antagonist, WAY100,635, slightly attenuated the pattern of actions at 1/2-ARs, 5-HT1A, and 5-HT1B (-)-pindolol-induced increase in DA and NAD levels, while receptors. These findings have important implications for the selective 5-HT1B antagonist, SB224,289, was ineffective. clinical studies of the influence of (-)-pindolol upon the These data suggest that (-)-pindolol facilitates frontocortical actions of antidepressant agents. dopaminergic (and adrenergic) transmission primarily by [Neuropsychopharmacology 21:268–284, 1999] b activation of 1/2-ARs and, to a lesser degree, by stimulation © 1999 American College of Neuropsychopharmacology. of 5-HT1A receptors, whereas 5-HT1B receptors are not Published by Elsevier Science Inc. KEY WORDS: Pindolol; Selective 5-HT Reuptake inhibitor; From the Institut de Recherches Servier, Centre de Recherches de Croissy, Psychopharmacology Department, Croissy-sur-Seine, Paris, Depression; Antidepressant; Frontal cortex; Dialysis France. Address correspondence to: Alain Gobert, Institut de Recherches Although selective 5-HT reuptake inhibitors (SSRI)s Servier, Centre de Recherches de Croissy, Psychopharmacology present significant advantages to tricyclic antidepres- Department, 125, Chemin de Ronde, 78290—Croissy-sur-Seine, sant (AD)s in terms of their improved tolerance, they Paris, France. Received December 2, 1998; revised March 1, 1999; accepted share the delayed onset of action and limited efficacy of March 8, 1999. first-generation ADs (Frazer 1997). Correspondingly, NEUROPSYCHOPHARMACOLOGY 1999–VOL. 21, NO. 2 © 1999 American College of Neuropsychopharmacology Published by Elsevier Science Inc. 0893-133X/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0893-133X(99)00035-4 NEUROPSYCHOPHARMACOLOGY 1999–VOL. 21, NO. 2 Monoamines and Depression 269 there is considerable interest in strategies which may al- more, several questions remain concerning the mecha- low for a more rapid onset of therapeutic benefit (Blier nism(s) underlying the apparent ability of (-)-pindolol to and de Montigny 1994; Frazer 1997; Lucki et al. 1994; enhance the actions of AD agents. First, possibly depend- Nemeroff 1997). In this regard, it has been hypothesized ing upon the level of extracellular 5-HT, (-)-pindolol has that a desensitization of 5-HT1A autoreceptors (localized been shown to exert mixed agonist and antagonist actions on the dendrites of serotonergic neurones) underlies the at 5-HT1A autoreceptors (Clifford et al. 1998; Corradetti et lack of an immediate response to AD administration. al. 1998; Romero et al. 1996; Lejeune and Millan unpub- According to this hypothesis (Artigas et al. 1996; Blier et lished observations) (see Discussion). Second, serotonergic al. 1990), the delay to onset of action of SSRIs (and other neurones also bear inhibitory 5-HT1B receptors on their drugs inhibiting 5-HT uptake) may be explained by terminals (and, possibly, cell bodies) which may also their propensity to increase extracellular levels of 5-HT adapt upon chronic treatment with SSRIs, and (-)-pin- not only postsynaptically, but also presynaptically at dolol is also an antagonist at these sites in the rat (Adham inhibitory 5-HT1A autoreceptors localized on the den- et al. 1992; Assie and Koek 1996; Bourin et al. 1998; Hoyer drites of serotonergic cell bodies in raphe nuclei (David- and Schoeffter 1991; Millan et al. in press). Third, (-)-pin- son and Stamford 1995; Rutter et al. 1995). This simulta- dolol is a potent partial agonist at b-ARs (Frishman 1983; neous activation of 5-HT1A autoreceptors diminishes Hoffman and Lefkowitz 1996), a role of which has been the activity of serotonergic neurones, thereby braking implicated in depressive states (O’Donnell et al. 1994; the postsynaptic increase in 5-HT levels. Thiessen et al. 1990; Zohar et al. 1987). Fourth, 5-HT1A, b The progressive desensitization of 5-HT1A autorecep- 5-HT1B and -ARs may all play modulatory roles in con- tors is accompanied by a gradual reinforcement in sero- trolling the activity of dopaminergic and adrenergic pro- tonergic transmission and the development of therapeu- jections to the FCX (Gobert et al. 1998; Kelland and tic AD actions (Artigas et al. 1996). Correspondingly, the Chiodo 1996; Lejeune et al. 1998; Millan et al. 1997; Misu acute blockade of 5-HT1A autoreceptors should mimic and Kubo 1986; Murugaiah and O’Donnell 1995), and a processes of desensitization. Indeed, selective 5-HT1A re- perturbation in the activity of these pathways is impli- ceptor antagonists, such as WAY100,635, as well as the cated in the emotional and cognitive deficits of depres- 5-HT1A receptor ligand, (-)-pindolol, potentiate the influ- sive states (Caldecott-Hazard et al. 1991; Goodwin 1997; ence of SSRIs upon postsynaptic levels of 5-HT in corti- Leonard 1997; Zacharko and Anisman 1991). colimbic structures in rats (Arborelius et al. 1996; Artigas These observations raise the question of a potential in- et al. 1996; Galloway 1996; Gartside et al. 1995; Gobert et fluence of (-)-pindolol upon depressive states via an inter- al. 1997b; Hjorth 1996; Hjorth et al. 1996). Furthermore, action with dopaminergic and/or adrenergic mechanisms blockade of 5-HT1A autoreceptors potentiates certain be- in the FCX. Thus, herein, we characterized the potential havioural actions of SSRIs (Detke et al. 1996; Jackson et influence of (-)-pindolol, both alone and together with SS- al. 1997; Millan et al. 1998a; Mitchell and Redfern 1997; RIs, upon frontocortical dopaminergic, adrenergic, and se- Trillat et al. 1998). This intriguing hypothesis provides, rotonergic transmission. Inasmuch as (-)-pindolol pos- b thus, a rational, theoretical basis for clinical studies of the sesses significant affinity for 1/2-AR, 5-HT1A, and 5-HT1B co-administration of 5-HT1A autoreceptor antagonists receptors (Table 1) (vide-supra), we evaluated their respec- with AD agents. Indeed, as concerns therapeutic AD ac- tive roles by examining whether selective antagonists tions, co-treatment with (-)-pindolol and SSRIs has been at these sites either mimicked or blocked the effects of reported as beneficial in several—though not all—clini- (-)-pindolol. A preliminary account of some of these find- cal studies (Berman et al. 1997; DeBattista et al. 1998; ings has appeared in Abstract form (Lejeune et al. 1998). McAskill et al. 1998; Nemeroff 1997). There exist, however, conflicting experimental data concerning the desensitization of 5-HT1A autoreceptors METHODS upon their chronic stimulation by treatment with SSRIs or direct agonists (Artigas et al. 1996; Hjorth and Auerbach Male Wistars rats (Iffa Credo, l’Arbresle, France) of 1994; Kreiss and Lucki 1997; Le Poul et al. 1995). Further- 200–220 g were allowed free access to food and water Table 1. Binding Affinities of (-)-Pindolol at Cloned, Human (h), Native Rat (r), and Native guinea pig (gp) receptor subtypes b b h5-HT1A r5-HT1B gp5-HT1B h5-HT1B h5-HT1D r 1 r 2 (-)-Pindolol 8.2a 7.2b 5.9c ,5.0c 5.2c 9.2b 9.5b Values are pKis. Data are from Newman-Tancredi et al. (1998)a, Tsuchihashi et al. (1990)b, and Newman- Tancredi and Millan unpublished observationsc. 270 A. Gobert and M.J. Millan NEUROPSYCHOPHARMACOLOGY 1999–VOL. 21, NO. 2 and housed singly. Laboratory temperature was 21 6 mide}, 3HCl were synthesized by Servier chemists (G. 18C and humidity 60 6 5%. There was a 12h/12h light/ Lavielle and J.-L. Peglion). dark cycle (lights on at 7:30 a.m.). All animal use proce- dures conformed to international european ethical stan- dards (86/609-EEC) and the French National Commit- RESULTS tee (décret 87/848) for the care and use of laboratory Influence of (-)-Pindolol upon Resting Levels of animals. 5-HT, DA, and NAD in FCX as Compared to Rats were implanted under pentobarbital anaesthe- Striatum and Nucleus Accumbens sia (60 mg/kg, i.p.) with a guide cannula in the FCX or in both the nucleus accumbens and the contralateral Administration of vehicle did not influence extracellu- striatum as described (Gobert et al. 1998; Millan et al. lar levels of DA or 5-HT but elicited a mild and tran- 1997). Five days later, a concentric dialysis probe—4 sient increase in dialysate levels of NAD (see Gobert et mm length (FCX and striatum) and 2 mm length (ac- al. 1998; Millan et al. 1997) (Figure 1). As shown in Fig- cumbens), 0.24 mm, o.d.—was lowered into position ure 1, (-)-pindolol elicited a sustained and marked in- and perfused at 1 ml/min with a phosphate-buffered crease in levels of both DA and NAD.