EVIDENCE FOR A GABAERGIC INTERFACE BETWEEN CORTICAL AFFERENTS AND BRAIN-STEM PROJECTION NEURONS IN THE RAT CENTRAL EXTENDED AMYGDALA

The synaptic circuitry of the intrinsic GABAergic system of the centra l extended amygdala (CEA) in relation to efferent neurons and cortical afferents was examined in the present study. Neurons in the CEA proje cting to the dorsal vagal complex and the parabrachial complex were id entified by the retrograde transport of wheat germ agglutinin-horserad ish peroxidase (WGA-HRP). Postembedding GABA-immunocytochemistry revea led that GABA-immunoreactive (GABA-IR) terminals formed largely symmet rical synaptic contacts with the perikarya and proximal dendritic proc esses of almost all WGA-HRP-labeled neurons in the CEA. To determine t he relationship between cortical afferents and CEA GABAergic neurons, WGA-HRP was used to anterogradely label afferents from the insular cor tex in combination with postembedding immunogold detection of GABA. Co rtical afferents formed asymmetrical synaptic contacts predominantly o n small dendrites and dendritic spines. Many of the dendrites postsyna ptic to cortical terminals in the central nucleus were immunoreactive for GABA although only relatively few spines were GABA-IR. Combining p re-embedding GAD-immunocytochemistry with cortical lesions resulted in approximately 40% of degenerating terminals of insular cortical origi n in the central nucleus in contact with small, GAD-IR dendrites and s pines. The present results demonstrate that the neurons providing the major CEA outputs to the brainstem receive an extensive GABAergic inne rvation, strongly supporting our proposal that CEA efferent neurons ar e under strong tonic inhibition by intrinsic GABAergic neurons. Furthe r, our finding that the major cortical input to the central nucleus pr eferentially innervates intrinsic GABAergic neurons suggests that thes e neurons in the CEA may serve as an interface between the principal i nputs and outputs of this forebrain region. (C) 1994 Wiley-Liss, Inc.