Electrophysiological evidence of a role for 5-HT2C receptors in the feedback inhibition of 5-HT neuronal activity

Laura Boothman, Josephine Raley & Trevor Sharp, University Department of Pharmacology, Mansfield Road, Oxford, OX1 3QT, UK.

Recent studies indicate the presence of novel negative feedback mechanisms which operate alongside 5-HT1A autoreceptors to regulate the firing of midbrain 5-hydroxytryptamine (5-HT) neurones. These mechanisms utilize post-synaptic 5-HT receptors, and poly-synaptic inputs to 5-HT neurones, including inputs from local GABA neurones (Liu et al. 2000; Boothman et al. 2005) . The involvement of 5-HT2 receptors is evident in findings that 5-HT2 receptor agonists inhibit the firing of 5-HT neurones in the rat dorsal raph é nucleus (DRN) (Boothman et al. 2003) . Pharmacological analysis of this effect revealed a key role for 5-HT2A receptors, but also indicated the involvement of 5-HT2C receptors. The present study investigated the latter possibility using the 5-HT2C receptor agonist WAY 161503 (Cryan et al. 2000) .

Male Sprague-Dawley rats (280-300 g) were anaesthetised with chloral hydrate supplemented with saffan. Extracellular recordings of DRN 5-HT neurones were made using single barrel glass electrodes (2 M NaCl, 2 % pontamine sky blue, 10-16 M Ω ). 5-HT neurones were identified according to established electrophysiological and pharmacological criteria (e.g. Boothman et al. 2003) . After 5 min baseline recording drugs were injected via a lateral tail vein. Rats (n=6-8/group) received WAY 161503 (0.125, 0.25 and 0.5 mg/kg at 2 min intervals), or the 5-HT2C receptor antagonist SB 242084 (1.0 mg/kg) followed by WAY 161503. In some experiments, the antagonists SB 242084 (5‑HT2C; 0.5 mg/kg), ritanserin (5-HT2; 1 mg/kg) or picrotoxin (GABAA; 0.5-2.0 mg/kg) were administered after WAY 161503. Cell firing rates were determined for the final min of each post-drug interval (Spike2 software). Data were analysed by 1- or 2-way ANOVA with appropriate post-hoc tests.

WAY 161503 caused a dose-related inhibition of 5-HT cell firing (P<0.0001 versus pre-drug values). This effect was apparent at 0.125 mg/kg, and 0.5 mg/kg reduced firing to 21 % of pre-drug levels. SB 242084 pre‑treatment, which alone did not alter the rate of 5-HT cell firing, caused a rightward shift in the dose-response to WAY 161503 (P<0.01). The inhibitory action of WAY 161503 was reversed by SB 242084 (3/4 cells) and ritanserin (15/18 cells), and partially restored by picrotoxin (4/7 cells).

In summary, these data demonstrate that the 5-HT2C receptor agonist WAY 161503 inhibits the firing of DRN 5-HT neurones. Pharmacological analysis indicates that this effect is mediated by 5-HT2 receptors (ritanserin), specifically the 5-HT2C receptor subtype (SB 242084), and also indicates the involvement of GABA neurones (picrotoxin). These findings are consistent with a role for 5-HT2C receptors in feedback regulation of DRN 5-HT neurones, and accord with immunocytochemical evidence that 5-HT2C receptor agonists activate DRN GABA neurones (Raley et al., this meeting).

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Boothman L J, et al. (2005). Neuroreport 16(9): 891-896.
Cryan J F, et al. (2000). J Pharmacol Exp Ther 295(3): 1120-1126.
Liu R, et al. (2000). Brain Res  873(1): 34-45.

Funded by the EC 6 th Framework Programme (NEWMOOD; LMSH-CT-2004-503474).