Experimentally cross-wired lingual taste nerves can restore normal unconditioned gaping behavior in response to quinine stimulation

Abstract

Studies examining the effects of transection and regeneration of the glossopharyngeal (GL) and chorda tympani (CT) nerves on various taste-elicited behaviors in rats have demonstrated that the GL (but not the CT) nerve is essential for the maintenance of both an unconditioned protective reflex (gaping) and the neural activity observed in central gustatory structures in response to lingual application of a bitter substance. An unresolved issue, however, is whether recovery depends more on the taste nerve and the central circuits that it supplies and/or on the tongue receptor cell field being innervated. To address this question, we experimentally cross-wired these taste nerves, which, remarkably, can regenerate into parts of the tongue they normally do not innervate. We report that quinine-stimulated gaping behavior was fully restored, and neuronal activity, as assessed by Fos immunohistochemistry in the nucleus of the solitary tract and the parabrachial nucleus, was partially restored only if the posterior tongue (PT) taste receptor cell field was reinnervated; the particular taste nerve supplying the input was inconsequential to the recovery of function. Thus, PT taste receptor cells appear to play a privileged role in triggering unconditioned gaping to bitter tasting stimuli, regardless of which lingual gustatory nerve innervates them. Our findings demonstrate that even when a lingual gustatory nerve (the CT) forms connections with taste cells in a non-native receptor field (the PT), unconditioned taste rejection reflexes to quinine can be maintained. These findings underscore the extraordinary ability of the gustatory system to adapt to peripherally reorganized input for particular behaviors.