Presynaptic serotonin 5-HT1B/D receptor-mediated inhibition of glycinergic transmission to the frog spinal motoneurons

Endogenous monoamine 5-hydroxytryptamine (5-HT, serotonin) is a phylogenetically ancient neurotransmitter present in vertebrates. The functions of 5-HT in central nervous system are intensively studied; however, the presynaptic effects of 5-HT in frog spinal motoneurons are practically unexplored. We have previously shown that 5-HT decreases the frequency of glycinergic miniature inhibitory postsynaptic potentials (mIPSPs), but does not affect the frequency of GABAergic mIPSPs and increases the frequency of glutamatergic postsynaptic potentials. In the present study, using pharmacological methods and intracellular recordings in motoneurons from an adult frog’s isolated spinal cord, we aimed to identify the 5-HT receptor subtype responsible for inhibiting the release of glycine. Аn agonist of 5-HT 1A and 5-HT 7 receptors, 8-OH-DPAT, and a selective agonist of 5-HT 2 receptors, α-Ме-5-НТ, did not show any significant effect on inhibitory transmission, indicating that 5-HT 1A , 5-HT 2, and 5-HT 7 receptors are not involved in the modulation of glycine release in the adult frog spinal cord. An agonist of 5-HT 1B/D receptors sumatriptan decreased the frequency (but not the amplitude) of glycinergic mIPSPs similar to 5-HT. An antagonist of 5-HT 1,2 receptors, methysergide, abolished the effect of sumatriptan. Together our results suggest that 5-HT inhibits the release of glycine by activation of 5-HT 1B/D receptors.