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Neurons containing calcitonin gene‐related peptide in the parabrachial nucleus project to the central nucleus of the amygdala

The source, distribution, and morphology of axons displaying calcitonin gene‐related peptide (CGRP) immunoreactivity in the central amygdaloid nucleus of the adult rat were investigated with immunohistochemical techniques, both alone and in combination with retrograde transport of fluorescent tracers. An extremely dense plexus of CGRP‐immunoreactive axons is differentially concentrated within the lateral capsular and lateral central subdivisions of the central nucleus, and much lighter concentrations of labeled fibers are present in the rostral part of the medial subdivision. No immunoreactive neurons were observed in the central nucleus in any of the experimental animals. The immunoreactive axons characteristically form prominent pericellular terminal arborizations surrounding unlabeled neurons. The number of cells receiving this dense input increases at caudal levels of the central nucleus. Retrograde label of central nucleus neurons by dye transport from injections into the pontine parabrachial nucleus and the nucleus of the tractus solitarius combined with CGRP immunohistochemistry established that many neurons in the central nucleus which receive dense pericellular innervation from CGRP‐immunoreactive axons are projecting caudally to the parabrachial nucleus or, to a lesser extent, to the nucleus tractus solitarii. Central amygdaloid injections of rhodamine‐labeled microspheres or fluorogold followed by immunohistochemical localization of cellular CGRP immunoreactivity revealed that the central amygdaloid CGRP fiber plexus originates bilaterally from the parabrachial nucleus. These multipolar CGRP‐containing neurons are preferentially concentrated in the external medial and external lateral subnuclei, in the ventral aspect of the parabrachial nucleus. These results relating central amygdaloid CGRP to ascending and descending brainstem pathways, taken together with the extreme density of the fiber plexus, strongly suggest the relevance of the CGRP input to central nucleus function in cardiovascular and other autonomic regulation.