It is well recognized that presynaptic 5-HT autoreceptors control midbrain 5-HT neurones, however recent data also indicate a role for postsynaptic 5-HT receptors. Specifically, electrophysiological data indicate that 5-HT_1A and 5-HT_2A receptor agonists inhibit the firing of 5-HT neurones in the dorsal raphe nucleus (DRN) through the modulation of afferent inputs (Hajós et al, 1999; Boothman et al, 2003). The neural basis of these effects is uncertain but major afferent inputs to the DRN inhibit 5-HT neurones by activating local GABA neurones (Varga et al, 2003). The present study used fos immunohistochemistry to investigate whether neurones in the DRN are activated by 5-HT_1A and 5-HT_2A receptor agonists, and whether these neurones are GABAergic.

Male Sprague Dawley rats (230-240g, 6 per group) were injected s.c. with saline, WAY 100635 (5-HT_1A antagonist; 0.3 mg/kg) or SB 269970 (5-HT₇ antagonist; 10 mg/kg) followed 30 min later by either saline or 8-OH-DPAT (5-HT_1A/7 agonist; 1 mg/kg). In a separate study, rats were injected i.p. with vehicle, MDL 100907 (5-HT_2A antagonist; 0.5 mg/kg) or SB 242084 (5-HT_2C antagonist; 2 mg/kg) followed 30 min later by either saline or DOI (5-HT_2A/C agonist; 2 mg/kg). Two h later rats were killed by fixation-perfusion, and DRN sections were processed for Fos and glutamate decarboxylase (GAD) immunoreactivity. Immunopositive cells were counted in a blind fashion by microscopic observation, and analysed statistically using one-way ANOVA with post-hoc tests.

Compared to vehicle controls 8-OH-DPAT increased the number of Fos positive cells in the DRN by 233±29 % (p<0.001). This effect was reduced by both WAY 100635 and SB 269970 (p<0.05). The increase in Fos immunoreactivity evoked by 8-OH-DPAT occurred bilaterally with a distribution reminiscent of DRN GABA neurones. Co-localisation studies revealed that 8-OH-DPAT significantly increased the number of DRN neurones immunoreactive for both GAD and Fos, and that this effect was reduced by WAY 100635 (p<0.05) but not SB 269970. DOI also increased the number of Fos positive cells in the DRN (161±31%) and this effect was blocked by MDL 100907 (P<0.01) but not SB 242084. Furthermore, after DOI there was a marked increase in DRN neurones immunoreactive for both GAD and Fos.

In summary, the current findings indicate that both 8-OH-DPAT and DOI cause a marked increase in Fos immunoreactivity in GAD positive neurones in the DRN and these effects were predominantly mediated by 5-HT_1A and 5-HT_2A receptors, respectively. These data are consistent with the hypothesis that 5-HT_1A and 5-HT_2A receptor agonists inhibit DRN 5-HT neurones at least in part by activation of local GABA neurones. 

Boothman et al, 2003 Br J. Pharmacology 139(5) 998-1004.
Hajós et al, 1999 Br J. Pharmacology 126 (8):1741-50.
Varga et al, 2003 Eur J Neurosci. 17(2):280-6.

JR and LJB are funded by Wellcome Trust and MRC studentships, respectively.