Electrophysiological evidence for 5-HT2C receptor-mediated control of 5-HT cell firing in the anaesthetised rat

P. Quérée & T. Sharp, University Department of Pharmacology, Mansfield Road, Oxford, OX1 3 QT, UK.

The role of 5-HT1 autoreceptors in feedback regulation of central 5-hydroxytryptamine (5-HT) neurones is well established. Emerging data suggest that 5-HT neurones may also be controlled by the 5-HT2C receptor subtype. Specifically, 5-HT2C receptor agonist administration was reported to inhibit the firing of 5-HT neurones in the rat dorsal raphe nucleus (DRN) in vivo, an effect partially blocked by the 5-HT2B/C receptor antagonist SB 206553 (Boothman et al., 2003). More recently it was found that the putative selective 5-HT2C receptor agonist, WAY 161503, inhibited 5-HT cell firing, an effect reversed by the selective 5-HT2C receptor antagonist, SB 242084 (Boothman et al., 2006). The present study extended this pharmacological analysis using the 5-HT2C receptor agonists, RO-60-0175 and m -chlorophenylpiperazine (mCPP) (Nilsson, 2006).

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 stereotaxically implanted single barrel glass electrodes (2 M NaCl, 2 % pontamine sky blue, 10-16 MΩ). 5-HT neurones were identified using specific electrophysiological and pharmacological criteria (Boothman et al., 2003). After 3 min baseline recording, rats (6-7 per group) were injected i.v. with WAY 161503, RO-60-0175 or mCPP in accumulating doses given at 2 min intervals (0.125, 0.25, 0.5, 1.0 mg kg-1). SB 242084 (1.0 mg kg-1) was injected 2 min after the final dose, usually followed by the 5-HT1A agonist, 8-OH-DPAT ( 10µ g kg-1). Firing rate was determined for the final min of each post-drug interval (Spike2 software). Agonist and antagonist effects were analysed statistically using 1-way ANOVA with Dunnett’s test post-hoc (versus pre-drug values) and Student’s paired t-test (versus last dose of agonist), respectively.

WAY 161503 caused a dose-related inhibition of 5-HT cell firing to 4 % of pre-drug levels (P<0.0001), and this effect was reversed by SB 242084 in each case (p<0.05). RO-60-0175 also caused a dose-related inhibition of 5-HT cell firing (P<0.001) to 20 % of pre-drug levels, and this effect was reversed by SB 242084 in each case (p<0.05). mCPP caused a dose-related inhibition of 5-HT cell firing in 4 out of 7 cells recorded (P<0.01). For inhibited cells, mCPP reduced firing to 6 % of pre-drug levels and the effect was reversed by SB 242084 (p<0.05). The 3 cells that did not respond to mCPP were inhibited by 8-OH -DPAT.

In summary, these data show that the 5-HT2C receptor agonists WAY 161503, RO-60-0175 and mCPP each inhibit the firing of rat DRN 5-HT neurones in vivo, and that this effect is reversed by the selective 5-HT2C receptor antagonist SB 242084. The lack of sensitivity of some 5-HT neurones to mCPP may reflect a partial agonist or non-specific action of this drug (Nilsson, 2006). These findings add further support to the hypothesis that 5-HT2C receptors participate in the feedback regulation of central 5-HT neurones.

Boothman L.J. et al ., (2003). Br J Pharmacol, 139, 998-1004.
Boothman L.J. et al ., (2006). Br J Pharmacol, in press.
Nilsson B.M. (2006). J Med Chem, 49, 4023-4034.

 Supported by a research studentship from the States of Jersey (PQ) and EC FP6 Integrated Network (NEWMOOD, LMSH-CT-20046503474).