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 mu M 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 mu M), inhibited synaptic transmission by 97.3%. On the other hand, 5-HT1A receptor activation by (+)-8-OH-dipropylaminotetralin (8-OHDPAT, 1 mu M) inhibited IPSCs by only 24.1%. A 5-HT1A antagonist, 1-(2-methyoxyphenyl)-4-[4-(2-phthalimido)-butyl]piperazine hydrobromide (NAN-190, 1 mu M), had no effect on synaptic inhibition by 5-HT. 4. In the presence of tetrodotoxin (TTX) as well as TTX with cadmium (50 mu M), we found that 5-HT1B receptor activation by TFMPP reduced the frequency of spontaneous miniature IPSCs (mIPSCs) without changing their mean amplitude. The results suggested that the 5-HT1B receptors activated at the presynaptic terminal inhibited synaptic transmission independent of inhibiting calcium influx through voltage-activated calcium channels. 5. These results indicate that activation of inwardly rectifying K+ channels and inhibition of voltage-activated calcium channels by 5-HT1A receptor activation do not constitute a main pathway for presynaptic inhibition by 5-HT of glycinergic synaptic transmission. Modulation of the transmitter release mechanism through 5-HT1B receptor activation may, at least in part, contribute to presynaptic inhibition by 5-HT.