Calcitonin Gene-related Peptide-containing Pathways in the Rat Forebrain

The present study focuses on the topographical distribution of calcitonin gene-related peptide (CGRP)-containing cell bodies and fibers and their connections and pathways in the rat forebrain. We confirm previously reported CGRP projections from the perifornical area of the hypothalamus to the lateral septum, from the posterior thalamus to the caudate putamen and cerebral cortex, and from the parabrachial nuclei to the central extended amygdala, lateral hypothalamus, and ventromedial thalamus. Despite previous descriptions of CGRP in the central nervous system, important neuroanatomical aspects of the forebrain CGRP system remained obscure, which we addressed by using brain lesion techniques combined with modern immunohistology. We first report CGRP terminal fields in the olfactory-anterior septal region and also CGRP projections from the parabrachial nuclei to the olfactory-anterior septal region, the medial prefrontal cortex, the interstitial nucleus of the anterior commissure, the nucleus of the lateral olfactory tract, the anterior amygdaloid area, the posterolateral cortical amygdaloid nucleus, and the dorsolateral part of the lateral amygdaloid nucleus. In addition, we identified a CGRP cell group in the premamillary nuclei and showed that it projects to the medial CGRP layer of the lateral septum. CGRP fibers usually join other pathways rather than forming bundles. They run along the fornix from the hypothalamus, along the supraoptic decussations or the inferior thalamic peduncle-stria terminalis pathway from the posterior thalamus, and along the superior cerebellar peduncle, thalamic fasciculus, and ansa peduncularis from the parabrachial nuclei. This description of the forebrain CGRP system will facilitate investigation of its role in higher brain functions.