SB 242 084, a selective serotonin2C receptor antagonist, increases dopaminergic transmission in the mesolimbic system
Introduction
It is now well established that the functional status of central dopamine (DA)-containing neurons is modulated by serotonin (5-HT)-containing neurons originating from the midbrain raphe nuclei. Anatomical studies have shown that serotonergic neurons innervate both the substantia nigra, pars compacta (SNc) and the ventral tegmental area (VTA) (Azmitia and Segal, 1978, Phillipson, 1979, Steinbusch, 1984, Hervé et al., 1987, Mori et al., 1987, Moukhles et al., 1996). In addition, terminal areas of the SNc and VTA, such as the striatum or the nucleus accumbens, receive an input from serotonergic neurons originating in the raphe nuclei (Azmitia and Segal, 1978). The existence of a functional relationship between 5-HT and DA neurons in the brain has been confirmed by a number of biochemical studies that showed either inhibition or facilitation of DA release in the presence of 5-HT agonists. Ennis et al. (1981) reported that the addition of 5-HT decreased DA release from striatal slices, thus suggesting an inhibitory action of 5-HT upon striatal DA release. However, a series of recent in vivo microdialysis studies has challenged this hypothesis, inasmuch as it has been found that 5-HT enhances striatal DA release (Benloucif and Galloway, 1991, Bonhomme et al., 1995), and this effect is apparently mediated by activation of 5-HT1B, as well as 5-HT4 striatal receptors (Benloucif and Galloway, 1991, Bonhomme et al., 1995, Steward et al., 1996). These biochemical data are not reconcilable with in vivo electrophysiological findings showing that the serotonergic system exerts an inhibitory action on the basal activity of midbrain DA-containing neurons. Thus, electrical stimulation of the dorsal raphe nucleus has been found to inhibit the spontaneous activity of dopaminergic neurons, both in the SNc and the VTA (Kelland et al., 1990, Kelland et al., 1993). Moreover, m-chlorophenylpiperazine (mCPP) and trifluoromethylphenylpiperazine (TFMPP), two mixed-5-HT2C/1B receptor agonists, reduced the basal firing rate of DA neurons in the VTA, whereas these neurons were stimulated by the 5-HT2C/2B/2A receptor antagonist mesulergine, suggesting that 5-HT control the function of DA neurons in the VTA by acting through these receptor subtypes (Prisco et al., 1994). These data are consistent with the distribution of 5-HT2C receptor mRNA. Thus, in situ hybridization histochemistry has shown that SN and VTA neurons express moderate levels of mRNA for 5-HT2C receptors (Molineaux et al., 1989, Mengod et al., 1990, Eberle-Wang et al., 1997). Moreover, significant levels of 5-HT2C receptor mRNA are present in terminal areas of the dopaminergic system such as the striatum and the nucleus accumbens (Mengod et al., 1990, Ward and Dorsa, 1996, Eberle-Wang et al., 1997). Therefore, it is conceivable that the disinhibitory action of mesulergine upon the mesolimbic dopaminergic system might be due to its capability to block 5-HT2C receptors. However, although mesulergine may represent a useful tool to investigate the role played by 5-HT2C receptors in the control of various functions in the central nervous system (Kennett and Curzon, 1988, Hoyer et al., 1994, Prisco et al., 1994), it is not an ideal ligand. Thus this drug binds to dopamine D2 receptors, its affinity being one order of magnitude lower than that for 5-HT2C/2A receptors (Van Wijngaarden et al., 1990). Nevertheless, recent studies have shown that SB 206 553 {5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f]indole}, a selective 5-HT2C/2B receptor antagonist which binds with high affinity (pKi 7.9) to 5-HT2C/2B receptors and has 100-fold selectivity over the 5-HT2A receptors (Kennett et al., 1996), increases the basal firing and the bursting activity of VTA dopaminergic neurons (Di Giovanni et al., 1997, Lejeune et al., 1997). Moreover, SB 206 553 enhances DA release in the rat frontal cortex and nucleus accumbens (Di Giovanni et al., 1997, Lejeune et al., 1997, Di Matteo et al., 1998). However, the data obtained with SB 206 553 does not allow us to draw any conclusions about the relative contribution of 5-HT2C and 5-HT2B receptor subtypes in the control of mesolimbic and nigro-striatal dopaminergic function. This problem might be circumvented by the use of SB 242 084 {6-chloro-5-methyl-1-[2-(2-methylpyridyl-3-oxy)-pyrid-5-yl carbamoyl] indoline}, a new potent and selective 5-HT2C receptor antagonist (pKi 9.0) which has 100- and 158-fold selectivity over the closely related cloned human 5-HT2B and 5-HT2A subtypes, respectively (Kennett et al., 1997). Thus, SB 242 084 represents a useful tool to evaluate the effect of selective blockade of central 5-HT2C receptors in the tonic control of nigro-striatal and mesolimbic dopaminergic function. Moreover, the effect of phasic stimulation of 5-HT2C receptors on these systems may be assessed by using RO 60-0175 [(S)-2-(chloro-5-fluoro-indol-1-yl)-1-methylethylamine 1:1 C4H4O4], a selective 5-HT2C agonist (Martin et al., 1998).
In the present study, electrophysiological techniques were used to investigate the effects of SB 242 084 and RO 60-0175 on the electrical activity of neurochemically-identified dopaminergic neurons in the SNc and the VTA. Moreover, neurochemical methods were used to study the effects of these drugs on in vivo DA release in the striatum and the nucleus accumbens of anesthetized rats. For this purpose, extracellular levels of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) were monitored by using intracerebral microdialysis coupled to HPLC with electrochemical detection, following the administration of SB 242 084 and RO 60-0175.2.
Section snippets
Animals
Male Sprague-Dawley rats (Consorzio Mario Negri Sud, Italy) weighing 300–350 g were used. Animals were kept at constant room temperature (21±2°C) and relative humidity (60%) with a 12-h light–dark cycle (dark from 20:00) and had free access to water and food. Procedures involving animals and their care were conducted in conformity with the institutional guidelines that are in compliance with national (D.L. n. 116, G.U., suppl. 40, 18 Febbraio 1992) and international laws and policies (EEC
Effects of SB 242 084 and RO 60-0175 on the basal activity of dopamine neurons in the SNc and the VTA
Treatment with SB 242 084 (160–640 μg/kg, i.v.; n=6–8) and RO 60-0175 (80–320 μg/kg, i.v.; n=6) did not induce any significant change in the basal firing activity of DA neurons in the SNc. The typical effects of SB 242 084 and RO 60-0175 are represented in Fig. 1, Fig. 2, respectively. Some DA neurons showed a slight excitatory response to SB 242 084, but the overall effect was not statistically significant. Neither SB 242 084 nor RO 60-0175 changed the bursting activity of SNc DA cells (not
Discussion
The results of this study confirm previous findings showing that the central 5-HT system elicits a tonic inhibitory control on mesolimbic DA activity and provide the first evidence of the selective involvement of 5-HT2C receptors in this effect. Consistent with these data, a recent report has shown that selective blockade of 5-HT2C receptors by SB 242 084 (10 mg/kg, i.p.) causes a marked increase in dialysate levels of DA in the rat prefrontal cortex, thus indicating that the mesocortical
Acknowledgements
This work was supported by the Italian National Research Council (Convenzione C.N.R.-Consorzio Mario Negri Sud). The authors thank Dr Guy A. Kennett (SmithKline Beecham Pharmaceuticals, Harlow, UK) for generously supplying SB 242 084. The authors are also grateful to Dr Eva-Maria Gutknecht (F. Hoffmann-LaRoche Ltd, Basel, Switzerland) for kindly supplying RO 60-0175.
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