Presynaptic inhibition by serotonin of glycinergic inhibitory synaptic currents in the rat brain stem.

1. With the use of a thin brain stem slice preparation, we recorded in visualized neonatal rat hypoglossal motoneurons unitary glycinergic inhibitory postsynaptic currents (IPSCs) that were evoked by extracellular stimulation of nearby interneurons. We found that 10 microM serotonin (5-HT) presynaptically inhibited this glycinergic synaptic transmission by 85.5%. 2. In the somata of presynaptic interneurons, 5-HT1A receptor activation potentiated inwardly rectifying K+ channels and inhibited voltage-activated calcium channels. 3. In contrast, the 5-HT1B receptor was primarily responsible for inhibition of evoked glycinergic IPSCs; a selective 5-HT1B receptor agonist, N-(3-trifluoromethylphenyl)piperazine (TFMPP, 10 microM), inhibited synaptic transmission by 97.3%. On the other hand, 5-HT1A receptor activation by (+)-8-OH-dipropylaminotetralin (8-OHDPAT, 1 microM) inhibited IPSCs by only 24.1%. A 5-HT1A antagonist, 1-(2-methyoxyphenyl)-4-[4-(2-phthalimido)-butyl]piperazine hydrobromide (NAN-190, 1 microM)...