Modulation of afferent-evoked neurotransmission by 5-HT3 receptors in young rat dorsal horn neurones in vitro: a putative mechanism of 5-HT3 induced anti-nociception.

1. The in vitro hemisected spinal cord from young rat was used to investigate the mechanism of serotoninergic modulation of primary afferent-mediated synaptic transmission in the dorsal horn through activation of the 5-HT3 receptor. 2. Dorsal root-evoked excitatory post-synaptic potentials (DR-EPSPs) were recorded intracellularly from dorsal horn neurones. Extracellular recordings of the population primary afferent depolarization (PAD) and the dorsal root-evoked dorsal root reflex (DR-DRR) were made from segmental dorsal roots. 3. 5-Hydroxytryptamine (5-HT) and the selective 5-HT3 receptor agonist 1-(m-chloro-phenyl)-biguanide hydrochloride (m-ChPB) (10 and 50 microM) induced statistically significant reductions of the DR-EPSP amplitude (P<0.001) and duration (P<0.001) in the majority of dorsal horn neurones. The 5-HT3 receptor selective antagonists 3-Tropanyl-indole-3-carboxylate hydrochloride (Tropisetron, 10 microM) and N-(1-Azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dihydro-2H-1 ,4-benzoxazine-8-carboxamide (Y-25130, 10 microM) abolished m-ChPB-induced DR-EPSP attenuation and partially blocked the 5-HT effect. 4. m-ChPB (50 microM)-induced DR-EPSP amplitude and duration attenuation was retained in the presence of the GABA(A) receptor antagonist bicuculline (30 microM), the GABA(B) receptor antagonist saclofen (50 microM) and the opioid receptor antagonist naloxone (50 microM). 5. Both 5-HT and m-ChPB (10 and 50 microM) induced a PAD but the mean peak amplitude of 5-HT-induced PAD was significantly greater than PAD to m-ChPB (98.6+/-12 microV compared to 51.8+/-10 V for 50 microM of agonist, respectively). Tropisetron partially reduced 5-HT-induced PAD and abolished m-ChPB-induced PAD. 5-HT, but not m-ChPB, significantly (P<0.001) reduced the peak amplitude of the DR-DRR and this action of 5-HT was unaffected by Tropisetron or Y-25130. 6. These data provide experimental evidence for a putative cellular mechanism at the level of the dorsal horn for anti-nociceptive effects of 5-HT3 receptor activation. This 5-HT3-mediated modulation of sensory afferent transmission was evidently independent of inhibitory GABA- or opioid-dependent interneuronal pathways. The extent to which the 5-HT3 receptor could be involved in the operation of endogenous analgesia and sensory modulation by descending monoamine bulbo-spinal pathways is discussed.