CALRETININ-CONTAINING PATHWAYS IN THE RAT FOREBRAIN

The anatomy of pathways containing the calcium binding protein calreti nin was investigated in the forebrain of the rat, using a combination of immunohistochemical and retrograde tract tracing techniques. Numero us well identified pathways do contain calretinin, whereas others do n ot. Pathways arising from the substantia nigra/ventral tegmental area, the dorsal raphe, the lateral mammillary nucleus, the supramammillary nucleus, the triangular septal and septo-fimbrial nuclei, several tha lamic nuclei, the parabrachial nucleus, the peripeduncular nucleus, th e medial amygdala contain at least some calretinin. The proportion of projection neurons containing calretinin ranged from 2% (dorsal raphe to caudate) to about 75% (triangular septal nucleus to habenula, media l amygdala to the ventromedial hypothalamus). More than 50% of the nig ro-striatal neurons contain calretinin immunoreactivity. In contrast, other pathways do not contain any calretinin immunoreactivity (for ins tance the pathways arising from cerebral cortex, locus coeruleus, chol inergic forebrain nuclei), although calretinin may be present in local neurons in these structures. The present study demonstrates that calr etinin is not associated specifically with projection neurons or local neurons, identified transmitter systems or functionnally related path ways in the forebrain of the rat.