LOWER BRAIN-STEM CATECHOLAMINE AFFERENTS TO THE RAT DORSAL RAPHE NUCLEUS

A large body of data suggests that the activation of alpha(1) receptor s by a tonic noradrenergic input might be responsible for the tonic di scharge of the serotonergic neurons of the dorsal raphe nucleus (DRN). To test this hypothesis, it was necessary to determine the origin of the noradrenergic and adrenergic innervation of these neurons. For thi s purpose, we combined small iontophoretic injections of the sensitive retrograde tracer cholera toxin b subunit (CTb) in the different subd ivisions of the DRN with tyrosine hydroxylase immunohistochemistry. Af ter CTb injections in the ventral or dorsal parts of the central DRN, a small number of double-labeled cells was observed in the locus coeru leus (A6 noradrenergic cell group), the A5 noradrenergic group, the do rsomedial medulla (C3 adrenergic cell group), and the lateral paragiga ntocellular nucleus (C1 adrenergic cell group). After CTb injections i n the lateral wings or the dorsal part of the rostral DRN, a similar n umber of double-labeled cells was seen in C3. Slightly more double-lab eled cells were seen in A6 and A5. In addition, a substantial to large number of double-labeled cells appeared in C1, the commissural part o f the nucleus of the solitary tract (A2 noradrenergic cell group) and the caudoventrolateral medulla (Al noradrenergic cell group). These re sults indicate that the noradrenergic and adrenergic inputs to the DRN arise from all the catecholaminergic cell groups of the lower brainst em except the A7 noradrenergic group. They further reveal the existenc e of a topographical organization of these afferents to the different subdivisions of the DRN. (C) 1996 Wiley-Liss, Inc.