EVIDENCE FOR A GABAERGIC PROJECTION FROM THE CENTRAL NUCLEUS OF THE AMYGDALA TO THE BRAIN-STEM OF THE MACAQUE MONKEY - A COMBINED RETROGRADE TRACING AND IN-SITU HYBRIDIZATION STUDY

The central nucleus of the amygdala is interconnected with a variety o f visceral and autonomic nuclei of the brainstem. These include the pa rabrachial nucleus, the nucleus of the solitary tract, the nucleus amb iguus and the dorsal motor nucleus of the vagus. Despite repeated atte mpts, neurochemical characterization of the major subcortical connecti ons of the central nucleus has not yet been accomplished. Based on ear lier immunohistochemical and in situ hybridization evidence indicating the presence of numerous GABAergic neurons in the macaque monkey cent ral nucleus, we predicted that a sizeable portion of the descending pr ojections may be GABAergic. We tested this hypothesis using a novel do uble labelling method with gold conjugated WGA-apoHRP as a retrograde tracer and in situ hybridization for detecting the mRNA that encodes t he enzyme glutamic acid decarboxylase (GAD67) as a marker for GABAergi c cells. Following WGA-apoHRP-gold injections into the brainstem, a la rge number of retrogradely labelled cells was observed in the medial a nd lateral divisions of the central nucleus. Of the retrogradely label led cells observed in the medial division of the central nucleus, appr oximately half were double-labelled for GAD67 mRNA; about 30% double l abelling was observed in the lateral division. These data support the view that a sizeable component of the central nucleus projection to th e brainstem is GABAergic.