Substance P, neurotensin, enkephalin, and catecholamine‐synthesizing enzymes: Light microscopic localizations compared with autoradiographic label in solitary efferents to the rat parabrachial region

The immunocytochemical localizations of substance P, neurotensin, enkephalin and the catecholamine‐synthesizing enzymes tyrosine hydroxylase and dopamine‐β‐hydroxylase were examined in the rat parabrachial region. The immunoreactivity for each marker was compared with the distribution of superimposed autoradiographic labeling of parabrachial afferents after unilateral injection of ³H‐amino acids into the caudal portion of the medial nucleus of the solitary tract (m‐NTS). Substance‐P‐ and neurotensinlike immunoreactivity (SPLI and NTLI, respectively) were localized primarily in varicose processes in the ventrolateral quadrant of the parabrachial region. The SPLI and NTLI were differentially localized with respect to each other; however, both peptides were detected in regions of the parabrachial containing dense autoradiographic label. In contrast, enkephalinlike immunoreactivity (ELI), tyrosine hydroxylase, and dopamine‐β‐hydroxylase were detected in processes and a few perikarya located outside the ventrolateral parabrachial region. The ELI was primarily in the dorsolateral, and the catecholamine‐synthesizing enzymes were primarily in the medial parabrachial regions which contained sparse autoradiographic labeling of transported amino acids. We conclude that in the rat parabrachial region, SPLI and NTLI are contained within two distinct populations of afferents which may originate from perikarya in the caudal m‐NTS, whereas ELI and the cate‐cholamines are more likely to be found in other afferents or possibly in intrinsic neurons.