| Given the prominence of inhibitory synaptic processes within the rostral portion of the nucleus of the solitary tract (NTS), it is interesting that little is known about the role of this inhibition in the neural representation of taste quality. Additionally, it is not fully understood how these processes are activated or modulated. However, recent data have implied that afferent sensory input normally activates inhibition within the gustatory NTS. Concentrating on a specific peripheral input to this nucleus, the current study is a more direct investigation of this hypothesis. Given that tetanic electrical activation of the rostral NTS was shown to potentiate inhibitory synaptic activity within this nucleus in vitro (Bradley & Grabauskas, 1998; Grabauskas & Bradley, 1998), we utilized an in vivo preparation that allowed us to deliver electrical tetanus to the chorda tympani (CT), a gustatory-related afferent input to the NTS, while the capability to record taste responses was preserved. Stimulating electrodes were implanted in the middle ear of urethane-anesthetized rats to facilitate the passage of current across the CT. Electrophysiological responses evoked by sucrose, NaCl, HCl, and quinine were recorded from single NTS units both prior to and immediately following tetanic electrical activation of the CT. Additionally, NTS field responses to paired pulse stimulation of the CT were recorded. It was found that tetanic electrical stimulation of the CT suppressed single unit taste responses in a stimulus specific manner. Across all neurons, quinine responses were the most dramatically suppressed following tetanus while those evoked by NaCl were least affected. Though not to the extent observed for quinine, suppression of post-tetanus sucrose and HCl responses was also noted. Gustatory NTS field responses evoked by paired pulse stimulation of the CT revealed evidence for feedback inhibition. These data are consistent with the idea that taste inputs to the NTS normally invoke inhibitory synaptic activity. As with other sensory systems, such inhibition may serve to facilitate contrast in the neural representation of different stimulus qualities. |
