Substance P modulates taste responses in the nucleus of the solitary tract of the hamster.

The effects of substance (SP) microinjections on the electrophysiological response of gustatory neurons within the nucleus of the solitary tract (NST) were examined in hamsters following either anodal electrical or NaCl stimulation of the anterior tongue. For both types of stimulation, SP produced excitatory and suppressive effects on the activity of gustatory NST neurons, with excitatory effects being more common. In response to repetitive anodal stimulation of the tongue, the modulatory effect of SP lasted 30-400 s. In the presence of SP, the firing rate of 48% of the neurons was increased and that of 9% was decreased following NaCl stimulation. This dual action of SP could be due to direct excitation of teste-responsive neurons and to excitation of inhibitory local circuit neurons which, in turn, decrease the responsiveness of gustatory neurons.