Special senses are really special: Evidence for a reciprocal, bilateral pathway between insular cortex and nucleus parabrachialis

Abstract

Nucleus parabrachialis (NPB) is implicated in gustation and a wide range of visceral and homeostatic functions. The nucleus was recently shown to be the major relay for second order gustatory and general visceral afferent (GVA) pathways from the nucleus of the solitary tract (NTS) to the gustatory-GVA division of the ventrobasal thalamus (VBcm) via the parabrachial nucleus (NPB). VBcm projects to the primary cortical sensory area for these modalities in the insular cortex (IC). In the present report anterograde and retrograde transport of either HRP or wheat germ agglutinin conjugated to HRP (WGA-HRP) revealed a strong, direct, reciprocal connection between NPB and IC. Inputs from and outputs to the contralaterai NPB were also seen. Thus, in addition to the thalamo-cortical (VBcm → IC) ascending gustatory-GVA pathway, there is a more direct route to primary cortex from third order neurons in NPB. With the exception of olfaction, all other ascending lemniscal sensory systems have an obligatory synaptic relay in the thalamus en route to their primary cortex. The present study demonstrates that the gustatory-GVA system has both; information from these modalities may reach their cortical area via a thalamic relay and via direct projections from NPB. Moreover, both olfactory and gustatory-GVA pathways impinge upon limbic structures without a thalamo-cortical relay. In these regards, the special senses—olfaction, gustation and general visceral afference—are really special. The strong, reciprocal projection from IC to NPB compliments our previous description [40] of a bilateral pathway from this same cortical area to NTS and several brainstem visceromotor nuclei. Taken together, these findings suggest that there is a well-organized cortical substrate for gustatory and visceral afference which may have a functionally important descending influence on both the afferent and motoneuronal aspects of gustatory-autonomic brainstem mechanisms.