DIFFERENTIAL PROJECTIONS FROM GUSTATORY RESPONSIVE REGIONS OF THE PARABRACHIAL NUCLEUS TO THE MEDULLA AND FOREBRAIN

The present study combined extracellular electrophysiology with antero grade and retrograde tracing techniques to determine efferent projecti ons from taste responsive sites within the parabrachial nucleus (PBN). Taste activity was recorded from two distinct regions of the PEN, the waist region consisting of the ventrolateral (VL) and central medial (CM) subnuclei, and the external region, consisting of the external me dial (EM) and external lateral (EL) subnuclei. Ascending and descendin g projections from these two regions differed. Small biotinylated dext ran injections placed in taste responsive sites in the waist area prod uced a prominent descending projection to the medullary parvocellular reticular formation, a projection nearly non-existent from the externa l region. Differences in ascending projections were more subtle. Proje ctions to the thalamus were bilateral in all cases, however, the waist region had a larger ipsilateral thalamic projection than the external region and the external region had a larger contralateral projection compared to the waist. Central nucleus of amygdala (CNA) projections f rom the waist area were primarily from posterior tongue responsive sit es in VL and terminated in the central medial and lateral CNA subnucle i; external region projections were distributed to the capsular region of CNA. Both the external and waist region projected to substantia in nominata (SI). Different efferent projections from the two gustatory r esponsive regions of the PBN may reflect functional specialization of PEN subnuclei. Descending projections from orally responsive sites in the waist area project to the lateral parvocellular reticular formatio n, a region implicated in brainstem circuitry underlying consummatory components of ingestive function. The external region, contains cells responsive to pain and oral aversive stimuli, but does not apparently contribute directly to local brainstem functions. Rather, forebrain pa thways appear critical to the expression of external region functions. (C) 1998 Elsevier Science B.V. All rights reserved.