THE RELATIONSHIP OF EFFERENT PROJECTIONS FROM THE AREA POSTREMA TO VAGAL MOTOR AND BRAIN-STEM CATECHOLAMINE-CONTAINING CELL GROUPS - AN AXONAL-TRANSPORT AND IMMUNOHISTOCHEMICAL STUDY IN THE RAT

The area postrema has been implicated as a major station for the proce ssing of visceral sensory information, involved primarily in eliciting rapid homeostatic reponses to fluid and nutrient imbalances. Yet the precise relationship of efferent projections from the area postrema to medullary motor and relay nuclei involved in such functions remains u nclear. In this study, axonal transport and immunohistochemical techni ques were used to investigate the relationship of efferent projections from the area postrema to vagal motor neurons and medullary catechola mine-containing cell groups in the rat. The results may be summarized as follows: (1) The area postrema gives rise to dense inputs to the co mmissural and medial parts of the nucleus of the solitary tract. Many of these projections are intimately associated with catecholamine-cont aining neurons in the A2 and C2 cell groups, including a particularly prominent input to a caudally placed cluster of adrenergic neurons (th e C2d cell group) in the dorsal aspect of the medial part of the nucle us of the solitary tract. (2) The area postrema provides a dense input to the external lateral part of the parabrachial nucleus. (3) The are a postrema does not project significantly to vagal motor neurons in ei ther the dorsal motor nucleus or the nucleus ambiguus, although the po ssibility for inputs to distal dendrites of dorsal vagal motor neurons cannot be excluded. (4) En route to the parabrachial nucleus, axons o f area postrema neurons traverse the regions of the A1, C1 and A5 cell groups, although these fibers make few arborizations, suggesting litt le functional contact. Together, these results suggest that sensory in formation received by the area postrema is dispatched to a restricted set of neurons in the commissural, medial, and dorsal parts of the nuc leus of the solitary tract, most probably including catecholamine-cont aining cells in the A2, C2, and C2d cell groups, and to the external l ateral portion of the parabrachial nucleus. The targets of area postre ma projections are, in turn, in a position to effect adaptive changes in the activities of hypothalamic neurosecretory neurons, vagal motor neurons, and limbic forebrain regions in response to perturbations in fluid and nutrient homeostasis.