ORGANIZATION OF PROJECTIONS FROM THE MEDIAL NUCLEUS OF THE AMYGDALA -A PHAL STUDY IN THE RAT

The organization of axonal projections from the four recognized parts of the medial amygdalar nucleus (MEA) were characterized with the Phae solus vulgaris leucoagglutinin (PHAL) method in male rats. The results indicate that the MEA consists of two major divisions, ventral and do rsal, and that the former may also consist of rostral and caudal regio ns. As a whole, the MEA generates centrifugal projections to several p arts of the accessory and main olfactory sensory pathways, and project ions to a) several parts of the intrahippocampal circuit (ventrally); b) the ventral striatum, ventral pallidum, and bed nuclei of the stria terminalis (BST) in the basal telencephaon; c) many parts of the hypo thalamus; d) midline and medial parts of the thalamus; and e) the peri aqueductal gray, ventral tegmental area, and midbrain raphe. The dorsa l division of the MEA (the posterodorsal part) is characterized by pro jections to the principal nucleus of the BST, and to the anteroventral periventricular, medial, and central parts of the medial preoptic, an d ventral premammillary hypothalamic nuclei. These hypothalamic nuclei project heavily to neuroendocrine and autonomic-related parts of the hypothalamic periventricular zone. The ventral division of the MEA (th e anterodorsal, anteroventral, and posteroventral parts) is characteri zed by dense projections to the transverse and interfascicular nuclei of the BST, and to the lateral part of the medial preoptic, anterior h ypothalamic, and ventromedial hypothalamic nuclei. However, dorsal reg ions of the ventral division provide rather dense inputs to the medial preoptic region and capsule of the ventromedial nucleus, whereas vent ral regions of the ventral division preferentially innervate the anter ior hypothalamic, dorsomedial, and ventral parts of the ventromedial n uclei. Functional evidence suggests that circuits associated with dors al regions of the ventral division may deal with reproductive behavior , whereas circuits associated with ventral regions of the ventral divi sion may deal preferentially with agonistic behavior. (C) 1995 Wiley-L iss, Inc.