A disynaptic pathway from the central amygdaloid nucleus to the paraventricular hypothalamic nucleus via the parastrial nucleus in the rat.

The organization of projections from the central amygdaloid nucleus (CeA) to the paraventricuilar hypothalamic nucleus (PVH) has been studied in order to understand the anatomical substrates of amygdaloid modulation of endocrine and autonomic functions, and a hypothesis that the bed nucleus of the stria terminalis (BST) may act as a relay site between the CeA and PVH has been proposed. Using anterograde and retrograde tract-tracing methods, in the rat, we first indicated that neurons in the parastrial nucleus (PS), where projection fibers from the central amygdaloid nucleus (CeA) terminated, sent their axons to the paraventricular hypothalamic nucleus (PVH). We further demonstrated that the CeA terminals formed symmetrical synaptic contacts with somata and dendrites of the PVH-projecting PS neurons, and that the PS received CeA fibers predominantly from the lateral part and sent large numbers of projection fibers to almost all the subdivisions of the PVH. Using anterograde tracing combined with the postembedding immunogold method, we finally revealed that nearly all the CeA terminals in the PS were immunoreactive for gamma-aminobutyric acid. The present data suggest that output signals from the CeA are transmitted disynaptically to the PVH neurons via the PS neurons and modulate PVH neuron activity by way of disinhibition.