A gabaergic projection from the central nucleus of the amygdala to the nucleus of the solitary tract: A combined anterograde tracing and electron microscopic immunohistochemical study

The central nucleus of the amygdala is involved in the modulation of autono mic, somatic and endocrine functions, as well as behavioural responses to s tressful stimuli. Anatomical and physiological studies have suggested that this nucleus sends projections to the nucleus of the solitary tract, the pr imary site of termination of vagal and glossopharyngeal afferent fibres in the brain stem. To determine the neurochemical nature of the amygdaloid inp ut to the nucleus of the solitary tract, anterograde tracing with biotinyla ted dextran amine was combined with post-embedding immunogold labelling for GABA and glutamate immunoreactivities and with pre-embedding labelling for the vesicular GABA transporter. Following injection of biotin dextran amin e into the central nucleus of the amygdala, anterogradely labelled axons an d varicosities were found throughout the rostrocaudal extent of the nucleus of the solitary tract, particularly in the medial, ventral and ventrolater al subnuclei. The anterogradely labelled terminals were found to make predo minantly symmetrical synaptic contacts with dendrites, and occasionally ont o cell bodies and dendritic spines, and to contain immunoreactivity for GAB A and for the vesicular GABA transporter. Immunolabelling of serial section s with antibodies to glutamate showed that none of these axon terminals con tained high enough densities of gold particle labelling to suggest that the y contained other than low metabolic levels of glutamate immunoreactivity. These results provide conclusive evidence for a GABAergic pathway from the central nucleus of the amygdala to the nucleus of the solitary tract, This GABAergic projection may provide a substrate for inhibition of lower brain stem visceral reflexes, including baroreflex inhibition, through which the central nucleus of the amygdala could participate in cardiovascular regulat ion related to emotional behaviour and the defence reaction. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.