PROJECTIONS OF MEDULLARY AND PONTINE NORADRENERGIC NEURONS TO THE HORIZONTAL LIMB OF THE NUCLEUS OF DIAGONAL BAND IN THE RAT

Recent investigations in the rat have implicated a noradrenergic inner vation to the horizontal nucleus of the diagonal band of Broca as a cr itical link in a neural circuit that conveys baroreceptor information centrally to inhibit the firing of vasopressin-secreting neurons in th e hypothalamic supraoptic nucleus. In this study we used small intra-d iagonal band injections of a retrograde tracer, rhodamine latex micros pheres, in combination with tyrosine hydroxylase histochemistry to ide ntify brainstem noradrenergic cells contributing to this innervation. In three cases where tracer injections were limited to the horizontal limb of the diagonal band, we observed 20-50 double-labelled neurons i psilaterally in the dorsal part of the locus coeruleus (A6) and the ca udal nucleus tractus solitarius (A2), and bilaterally in the caudal ve ntrolateral medulla (A1). Double-labelled neurons were also noted in t he ventral tegmental area (dopaminergic A10 cell group). Although all major brainstem noradrenergic cell groups contribute fibers to the hor izontal limb of the nucleus of diagonal band, data from physiological studies suggest that the noradrenergic A2 neurons in the nucleus tract us solitarius are the most likely pathway through which it receives th is baroreceptor information. (C) 1998 IBRO. Published by Elsevier Scie nce Ltd.