Presynaptic inhibition of glutamatergic synaptic transmission to rat motoneurons by serotonin

1. In a brain stem slice preparation, we recorded glutamatergic excitatory postsynaptic currents (EPSCs) in hypoglossal motoneurons (HMs) evoked by extracellular stimulation in the reticular formation just ipsilateral to the hypoglossal motor nucleus (n. XII). Serotonin (5-HT) inhibited glutamatergic synaptic transmission in a dose-dependent fashion as indicated by a reduction in the evoked EPSC (eEPSC) peak amplitude to 46 +/- 2% (mean +/- SE, n = 26) of control (5-HT 10 mu M). This effect was not voltage dependent, as the eEPSC reversal potential was not altered (n = 5). Additionally, 5-HT decreased the rate of rise of the eEPSC to 41 +/- 2% of control (n = 14). Blockade of N-methyl-D-aspartate-receptor-channels by D(-) -2-amino-5-phosphonopentanoic acid (50 mu M) or of a alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptor-channels by 6,7-dinitro-quinoxaline (20 mu M) did not alter the relative reduction of the eEPSC amplitude by 5-HT (n = 7 and 3, respectively)