Abstract
The afferent fibers in the vagus nerve transmit visceral information encoded as varying firing frequency to the second-order neurons in the nucleus of the solitary tract (NTS) and dorsal motor nucleus of the vagus (DMX). The purpose of this study was to examine how the firing frequency affects transmission efficiency at these synapses. EPSCs evoked by the solitary tract (TS) stimulation were recorded from DMX and NTS neurons in the thick brainstem slices of young rats. TS stimulation at various frequencies (0.1–20 Hz) revealed distinct frequency-dependent responses in the EPSC amplitude among different types of neurons recorded. When stimulated at 20 Hz, NTS neurons and low-pass type DMX neurons exhibited marked amplitude reduction (<30% of the first EPSC) within 10 pulses, whereas high-fidelity (hi-fi) type DMX neurons presented only modest attenuation (>40%). These neurons exhibited distinct short-term plasticity as revealed by paired-pulse ratio (PPR) evaluation. Surprisingly, unlike the NTS neurons, PPRs in the low-pass DMX neurons were not significantly affected by changes in [Ca2+]o, suggesting distinct mechanisms for their short-term depression. These results indicate that the transmission efficiency between the visceral afferents and second-order neurons depends largely on the firing frequency mostly through distinct target cell-dependent presynaptic mechanisms, which might result in differential activation of distinct components in the solitary complex network. [J Physiol Sci. 2006;56 Suppl:S173]
