GABAERGIC NEURONS IN RAT NUCLEI OF SOLITARY TRACTS RECEIVE INHIBITORY-TYPE SYNAPSES FROM AMYGDALOID EFFERENTS LACKING DETECTABLE GABA-IMMUNOREACTIVITY

Gamma-aminobutyric acid (GABA) is a prominent inhibitory transmitter i n both the central nucleus of the amygdala (Ce) and the medial nuclei of the solitary tracts (mNTS). These regions are reciprocally connecte d by anatomical pathways mediating the coordinated visceral responses to emotional stress. To further determine whether GABA is present in t he amygdaloid efferents or their targets in the mNTS, we combined pero xidase labeling of Phaseolus vulgaris leucoagglutinin (PHA-L) or bioti nylated dextran amine (BDA) anterogradely transported from the Ce with immunogold-silver detection of antibodies against GABA in the rat mNT S. By light microscopy, peroxidase labeling for either PHA-L or BDA wa s seen in varicose processes, whereas immunogold-silver labeling for G ABA was detected in perikarya and processes throughout the rostrocauda l mNTS. The intermediate mNTS at the level of the area postrema, a reg ion receiving mainly cardiorespiratory and gastric visceral afferents, were examined by electron microscopy. In this region, anterograde lab eling was observed exclusively in unmyelinated axons and axon terminal s. These terminals lacked detectable GABA-immunoreactivity, but formed symmetric synapses that are associated with inhibition. The targets o f the anterogradely labeled terminals were medium-sized dendrites both with and without GABA-labeling. These dendrites often also received c onvergent input from terminals that were intensely GABA-immunoreactive . We conclude that visceral activation accompanying emotional response to stress is likely to involve inhibition of GABAergic neurons in the mNTS by non-GABA-containing amygdaloid efferents. Furthermore, our re sults indicate that the inhibition of these GABAergic neurons may be f urther augmented by release of GABA from other converging terminals in the mNTS. (C) 1996 Wiley-Liss, Inc.