The median raphe nucleus (MRN) and dorsal raphe nucleus (DRN) contain the cell bodies of 5‑HT neurones which innervate the forebrain. Although the regulation of 5‑HT neurones in the DRN is well established (Innis et al., 1988; Judge et al., 2004) relatively little is known of the regulation of MRN 5-HT neurones. Here we compared the sensitivity of MRN and DRN 5-HT neurones to 5-HT_1A receptor-mediated inhibition and ₁-adrenoceptor-mediated excitation using in vitro electrophysiology.

Brain slices (400 μm thick) containing the DRN and MRN were prepared from male hooded Lister rats (200-400 g). Slices were perfused with oxygenated artificial cerebrospinal fluid (aCSF) containing the
₁-adrenoceptor agonist phenylephrine (PE, 1 μM) at 36˚C. Extracellular recordings were made from presumed 5‑HT neurones in the DRN and MRN. Drugs were applied via the perfusion medium.

Putative 5-HT neurones (83 neurones from 25 animals) were identified on the basis of their location, slow and regular firing, and inhibitory response to 5-HT (10-50 μM). In the continuous presence of PE (1 μM), the basal firing rate was markedly lower in MRN neurones than in DRN neurones (0.52 ± 0.03 Hz; range 0.09 – 1.0 Hz; n = 41 vs 1.21 ± 0.07 Hz; range 0.3 – 2.34 Hz; n = 42; p<0.001, unpaired t-test). Brief (2 min) application of higher concentrations of PE (3, 6 and 10 μM) caused concentration-dependent excitation in both nuclei but the responses were strikingly smaller in the MRN than in the DRN. Thus, two-way ANOVA revealed a main effect of location (F_1,13 = 15 p < 0.005) and [PE] (F_2,26 = 43.1 p < 0.001) and a [PE]*location interaction (F_2,26 = 6.8 p < 0.05)>. Brief (2 min) application of 5-HT (10, 25 and 50 μM) caused a concentration-dependent inhibition in both nuclei. In all neurones tested the response to 5-HT was blocked by the 5-HT_1A receptor antagonist WAY 100635 (100 nM) (MRN n = 10; p < 0.001; DRN (n = 9; p < 0.001, paired t-test). The response to 5-HT in the MRN was not significantly different from that in the DRN when expressed as a percentage decrease. Thus, two-way ANOVA revealed a main effect of [5-HT] (F_2,54 = 101.7 p < 0.001) but not location (F_1,27 = 2.8 p = 0.11). However, when expressed as the number of spikes suppressed, the response to 5-HT in the MRN was significantly smaller than in the DRN. Two-way ANOVA revealed a main effect of location (F_1,27 = 9.6 p < 0.005) and [5-HT] (F_2,54 = 76.9 p < 0.001) and a [5-HT] *location interaction (F_2,54 = 21.5 p < 0.001). To examine whether the basal firing rate might have influenced the 5‑HT response, the responses of individual MRN neurones to 5-HT (10 μM) were tested under conditions of low basal firing rate (0.51 ± 0.09 Hz in presence of 1 μM PE) and high basal firing rate (1.08 ± 0.1 Hz in presence of 30 μM ₁-adrenoceptor agonist norepinephrine). The response expressed as spikes suppressed was not different between the low and high firing rate conditions (n = 8; p = 0.31, paired t-test).

The data indicate that 5-HT neurones in the MRN are less sensitive to ₁-adrenoceptor excitation than those in the DRN, resulting in differences in basal firing rate. When responses are considered as spikes suppressed, MRN 5-HT neurones also appear less sensitive to 5-HT_1A receptor inhibition than those in the DRN, a factor which cannot be accounted for by their lower basal firing rate.

Innis, RB et al. (1988) Brain Res. 459 :27-36.
Judge, SJ et al. (2004) Neurochem. Int. 45 :1057-1065.