Cholecystokinin‐, galanin‐, and corticotropin‐releasing factor‐like immunoreactive projections from the nucleus of the solitary tract to the parabrachial nucleus in the rat

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Abstract
The parabrachial nucleus (PB) is the main relay for ascending visceral afferent information from the nucleus of the solitary tract (NTS) to the forebrain. We examined the chemical organization of solitary‐parabrachial afferents by using combined retrograde transport of fluorescent tracers and immunohistochemistry for galanin (GAL), cholecystokinin (CCK), and corticotropin‐releasing factor (CRF).Each peptide demonstrated a unique pattern of immunoreactive staining. GAL‐like immunoreactive (‐ir) fibers were most prominent in the “waist” area, the inner portion of external lateral PB, and the central and dorsal lateral PB subnuclei. Additional GAL‐ir innervation was seen in the medial and external medial PB subnuclei. GAL‐ir perikarya were observed mainly rostrally in the dorsal lateral, superior lateral, and extreme lateral PB.CCK‐ir fibers and terminals were most prominent in the outer portion of the external lateral PB; some weaker labeling was also present in the central lateral PB. CCK‐ir cell bodies were almost exclusively confined to the superior lateral PB and the “waist” area, although a few cells were seen in the Kölliker‐Fuse nucleus.The distribution of CRF‐ir terminal fibers in general resembled that of GAL, but showed considerably less terminal labeling in the lateral parts of the dorsal and central lateral PB, and the external medial and KöUlliker‐Fuse subnuclei. The CRF‐ir cells were most numerous in the dorsal lateral PB and the outer portion of the external lateral PB; rostrally, scattered CRF‐ir neurons were seen mainly in the central lateral PB.After injecting the fluorescent tracer Fast Blue into the PB, the distribution of double‐labeled neurons in the NTS was mapped. GAL‐ir cells were mainly located in the medial NTS subnucleus; 34% of GAL‐ir cells were double‐labeled ipsilaterally and 7% contralaterally. Conversely, 17% of the retrogradely labeled cells ipsilaterally and 16% contralaterally were GAL‐ir. CCK‐ir neurons were most numerous in the dorsomedial subnucleus of the NTS and the outer rim of the area postrema. Of the CCK‐ir cells, 68% in the ipsilateral and 10% in the contralateral NTS were double‐labeled, whereas 15% and 10%, respectively, of retrogradely labeled cells were CCK‐ir. In the area postrema, 36% of the CCK‐ir cells and 9% of the Fast Blue cells were double‐labelled CRF‐ir neurons were more widely distributed in the medial, dorsomedial, and ventrolateral NTS subnuclei, but double‐labeled cells were mainly seen in the medial NTS. Of CRF‐ir cells in the NTS, 26% ipsilaterally and 8%contralaterally were retrogradely labeled by the PB injections. Conversely, of retrogradely labeled cells in the NTS, 4% ipsilaterally and 6% contralaterally were CRF‐ir.Our results suggest that the functional specificity of NTS afferents may be maintained by their selective termination in particular PB subnuclei. In addition, the neuropeptides found in these pathways may provide chemical coding for the relay of specific types of visceral sensory information to the PB.