Potential disynaptic pathways connecting the fastigial pressor area and the paraventricular nucleus of the hypothalamus in the rat.

Efferents from the cardiovascular portion of the fastigial nucleus (rFN), whose electrical stimulation has been found to elicit secretion of vasopressin in the rat, have been traced by use of the anterograde fluorescent tracer Fast blue (FB). In the same animal the retrograde marker Diamidino yellow (DY) was injected in the paraventricular nucleus of the hypothalamus (PVH) to trace afferents to the neurosecretory cells of the hypothalamus. Frontal sections of the brainstem have been analyzed and the FB-labeled axons and terminals and the DY-labeled cells mapped. Both efferents from the rFN and cells projecting to the PVH were found in close contact in the region of the locus coeruleus and parabrachial nucleus (lateral division) and in the caudal ventrolateral medulla, dorsal to the lateral reticular nucleus. Those two regions may function as putative relay stations of disynaptic pathways linking the rFN to the PVH.