Tyramine reduces glycinergic transmission by inhibiting presynaptic Ca ²⁺ channels in the rat trigeminal subnucleus caudalis

We have recently reported that tyramine acts on putative presynaptic trace amine receptors to inhibit glycinergic transmission in substantia gelatinosa (SG) neurons of the rat trigeminal subnucleus caudalis. However, it is still unknown how tyramine elicits presynaptic inhibition of glycine release. In the present study, therefore, we investigated cellular mechanisms underlying the tyramine-induced inhibition of glycinergic transmission in SG neurons using a conventional whole-cell patch clamp technique. Tyramine (100 μM) reversibly and repetitively decreased the amplitude of action potential-dependent glycinergic inhibitory postsynaptic currents (IPSCs), and increased the paired-pulse ratio. Pharmacological data suggest that the tyramine-induced decrease in glycinergic IPSCs was not mediated by the modulation of adenylyl cyclase, protein kinase A and C, or G-protein coupled inwardly rectifying K ⁺ channels. On the other hand, glycinergic IPSCs were mainly mediated by the Ca²⁺ influx passing through presynaptic N-type and P/Q-type Ca²⁺ channels. The tyramine-induced decrease in glycinergic IPSCs was completely blocked by ω-conotoxin GVIA, an N-type Ca²⁺ channel blocker, but not ω-agatoxin IVA, a P/Q-type Ca²⁺ channel blocker. The results suggest that tyramine acts presynaptically to decrease action potential-dependent glycine release onto SG neurons via the selective inhibition of presynaptic N-type Ca²⁺ channels. This tyramine-induced inhibition of glycinergic transmission in SG neurons might affect the process of orofacial nociceptive signals.