Effects of 5-hydroxytryptamine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis in immature mice

Serotonin (5-hydroxytryptamine, 5-HT) is involved in the descending modulation of nociceptive transmission in the spinal dorsal horn. The trigeminal subnucleus caudalis (Vc; medullary dorsal horn) processes nociceptive input from the orofacial region, and 5-HT-containing axons are numerous in the superficial layers of the Vc. This study examined the actions of 5-HT on the substantia gelatinosa (SG) neurons of the Vc, using gramicidin-perforated patch-clamp recording in brainstem slice preparations from immature mice. In order to clarify the possible mechanisms underlying 5-HT actions in the SG of the Vc, the direct membrane effects of 5-HT and effects of 5-HT receptor subtype agonists were examined. 5-HT induced a hyperpolarization in the majority (64/115, 56%) of the SG neurons tested. Thirty nine (34%) SG neurons showed no response, and 12 (10%) neurons responded with depolarization. The hyperpolarizing response to 5-HT was concentration-dependent (0.1-30 μM; n = 7), not desensitized by repeated application (n = 22), and significantly attenuated by Ba²⁺ (K⁺ channel blocker; n = 8). The 5-HT-induced hyperpolarization was maintained in the presence of TTX (Na ⁺ channel blocker), CNQX (non-NMDA glutamate receptor antagonist), AP5 (NMDA glutamate receptor antagonist), picrotoxin (GABA_A receptor antagonist), and strychnine (glycine receptor antagonist), indicating direct postsynaptic action of 5-HT on SG neurons (n = 7). The 5-HT-induced hyperpolarizing effects were mimicked by 8-OH-DPAT (5-HT_1A receptor agonist) and α-methyl-5-HT (5-HT₂ receptor agonist) and blocked by WAY-100635 (5-HT_1A receptor antagonist) and ketanserin (5-HT ₂ receptor antagonist). Single-cell RT-PCR also revealed the presence of mRNA for 5-HT_1A and 5-HT_2C subtypes in the SG neurons. These results suggest that 5-HT acts directly on SG neurons and 5-HT-induced hyperpolarization is mediated, in part, by 5-HT_1A receptors and 5-HT₂ receptors, as well as by the activation of K⁺ channels, indicating an important role for 5-HT in the modulation of orofacial nociceptive processing at the level of the SG of the Vc in mice.