Axonal connections from posterior paralaminar thalamic neurons to basomedial amygdaloid projection neurons to the lateral entorhinal cortex in rats

Stimulation of amygdaloid nuclei and emotionally relevant stimuli are known to influence the induction and maintenance of long-term potentiation in the hippocampal formation and the formation of long-term declarative memories. Because the thalamic projection from the posterior paralaminar thalamic nuclei is an important sensory afferent projection to amygdaloid nuclei mediating the fast acquisition of fear-potentiated behavior, we were interested in verifying whether this projection establishes synaptic contacts on amygdala neurons that project to the hippocampal formation. Thalamic afferents were labeled with the anterograde tracer Phaseolus vulgaris leucoagglutinin and amygdalo-hippocampal neurons were identified by injection of the retrograde tracer Fluorogold into the lateral entorhinal cortex. A massive overlap of both projection systems was observed especially in the anterior basomedial nucleus of the amygdala. Light microscopic examination revealed that single anterogradely labeled boutons were in close apposition to retrogradely labeled neurons suggesting synaptic contacts. The occurrence of such synaptic contacts was confirmed with electron microscopy. However, despite the massive overlap of anterogradely labeled axons and retrogradely labeled neurons observed at the light microscopic level, electron microscopy revealed that only 10% of all labeled profiles make direct contacts on each other; anterogradely labeled boutons predominantly contacted unlabeled profiles but synapses with direct contact between labeled profiles were rare. Altogether the findings demonstrate that the thalamic connection with the basomedial nucleus of the amygdala may represent an anatomical substrate for modulating amygdala output to the hippocampal formation.