Intra-amygdaloid projections of the basolateral and basomedial nuclei in the cat: Phaseolus vulgaris-leucoagglutinin anterograde tracing at the light and electron microscopic level.

The amygdaloid complex plays an essential role in auditory fear conditioning of the Pavlovian type. The available evidence suggests that the lateral nucleus is the input station of the amygdala for auditory conditioned stimuli, whereas the central medial nucleus is the output for conditioned fear responses. However, the intrinsic pathway transmitting auditory information about the conditioned stimulus from the lateral to the central medial nuclei is unknown as there are no direct projections between these nuclei. The present study was undertaken to determine if the main intra-amygdaloid targets of the lateral nucleus, namely the basomedial and basolateral nuclei, project to the central medial nucleus. To this end, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were performed in these nuclei. To rule out the possibility that the anterograde labeling reflected passing fibers merging with the major fiber bundles that course in and around the central medial nucleus, labeled terminals and varicosities were observed in the electron microscope. It was determined that the basolateral and basomedial nuclei have partially overlapping intraamygdaloid targets. They both project to the central medial nucleus, nucleus of the lateral olfactory tract and peri-amygdaloid cortex, but have limited projections to each other. Small Phaseolus vulgaris-leucoagglutinin injections in both nuclei gave rise to prominent intranuclear projections but only the basomedial nucleus was found to project to the lateral and anterior cortical nuclei. At the electron microscopic level, all labeled axon terminals and varicosities formed asymmetric synapses (n = 245) with dendritic spines (83%) or with dendritic shafts (17%). This is the first unambiguous demonstration that the basolateral and basomedial nuclei project to the central medial nucleus. Since these nuclei constitute the main intra-amygdaloid targets of the lateral nucleus, they represent likely candidates for the transmission of auditory conditioned stimuli to the central medial nucleus in auditory fear conditioning.