Identification of 5-HT receptor subtypes enhancing inhibitory transmission in the rat spinal dorsal horn in vitro

BACKGROUND 5-hydroxytryptamine (5-HT) is one of the major neurotransmitters widely distributed in the CNS. Several 5-HT receptor subtypes have been identified in the spinal dorsal horn which act on both pre- and postsynaptic sites of excitatory and inhibitory neurons. However, the receptor subtypes and sites of actions as well as underlying mechanism are not clarified rigorously. Several electrophysiological studies have been performed to investigate the effects of 5-HT on excitatory transmission in substantia gelatinosa (SG) of the spinal cord. In the present study, to understand the effects of 5-HT on the inhibitory synaptic transmission and to identify receptor subtypes, the blind whole cell recordings were performed from SG neurons of rat spinal cord slices. RESULTS Bath applied 5-HT (50 μM) increased the frequency but not amplitudes of spontaneous inhibitory postsynaptic currents (sIPSCs) in 58% of neurons, and both amplitude and frequency in 23% of neurons. The frequencies of GABAergic and glycinergic mIPSCs were both enhanced. TTX (0.5 μM) had no effect on the increasing frequency, while the enhancement of amplitude of IPSCs was eliminated. Evoked-IPSCs (eIPSCs) induced by focal stimulation near the recording neurons in the presence of CNQX and APV were enhanced in amplitude by 5-HT. In the presence of Ba(2+) (1 mM), a potassium channel blocker, 5-HT had no effect on both frequency and amplitude. A 5-HT(2A) receptor agonist, TCB-2 mimicked the 5-HT effect, and ketanserin, an antagonist of 5-HT(2A) receptor, inhibited the effect of 5-HT partially and TCB-2 almost completely. A 5-HT(2C) receptor agonist WAY 161503 mimicked the 5-HT effect and this effect was blocked by a 5-HT(2C) receptor antagonist, N-desmethylclozapine. The amplitudes of sIPSCs were unaffected by 5-HT(2A) or 5-HT(2C) agonists. A 5-HT(3) receptor agonist mCPBG enhanced both amplitude and frequency of sIPSCs. This effect was blocked by a 5-HT(3) receptor antagonist ICS-205,930. The perfusion of 5-HT(2B) receptor agonist had no effect on sIPSCs. CONCLUSIONS Our results demonstrated that 5-HT modulated the inhibitory transmission in SG by the activation of 5-HT(2A) and 5-HT(2C) receptors subtypes located predominantly at inhibitory interneuron terminals, and 5-HT(3) receptors located at inhibitory interneuron terminals and soma-dendrites, consequently enhanced both frequency and amplitude of IPSCs.