Intra-amygdaloid projections of the lateral nucleus in the cat: PHA-L anterograde labeling combined with postembedding GABA and glutamate immunocytochemistry.

Research on the implication of the amygdala in classical fear conditioning suggests that the central amygdaloid nucleus is the output station of the amygdala for conditioned fear responses, while the lateral nucleus acts as the input nucleus, at least for auditory conditioned stimuli. However, the nature and locus of the plastic changes taking place between these two nuclei are unknown partly because the neurotransmitter(s) used by intra-amygdaloid projections of the lateral nucleus has not been identified. To address this issue in cats, anterograde tracing with Phaseolus vulgaris-leucoagglutinin (PHA-L) was combined with postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate. Two sectors can be recognized in the lateral nucleus of the cat: a shell located laterally along the external capsule, and a core. Iontophoretic injections of PHA-L in these two sectors revealed that they have nonoverlapping intra-amygdaloid targets with the exception of a common projection to the central lateral nucleus. The core projects mainly to itself and to the basomedial nucleus, whereas the shell contributes a massive projection to the basolateral nucleus. No projection of the lateral nucleus to the central medial nucleus was found. Electron microscopically, PHA-L-labeled axon terminals in the lateral, basomedial, basolateral, and central lateral nuclei as well as in the perirhinal and insular cortices formed asymmetric synapses (100%; n = 289) with dendritic spines (77-100%). Moreover, postembedding immunocytochemistry revealed that PHA-L-labeled axon terminals are immunoreactive for glutamate but not GABA. Since most amygdaloid projections to the brainstem originate in the central medial nucleus, these results suggest that intra-amygdaloid targets of the lateral nucleus are involved in the transmission of auditory conditioned stimuli to the central medial nucleus. Moreover, these findings imply that intra-amygdaloid projections of the lateral nucleus use glutamate but not GABA as a neurotransmitter.