Basolateral amygdala efferents to the ventral subiculum preferentially innervate pyramidal cell dendritic spines

The basolateral amygdala and the ventral subiculum of the hippocampal formation are two of the major limbic-related regions within the brain, both of which project heavily to the nucleus accumbens. The nucleus accumbens is regarded as the limbic-motor interface, in view of these limbic afferent and its somatomotor and autonomic efferent connections. These afferent inputs have been suggested to converge monosynaptically on cells within the accumbens and are hypothesised to play a role in functions such as affective motivational behaviour. Convergence between inputs from the basolateral amygdala and the hippocampus at the level of the accumbens can be demonstrated with electrophysiological recording methods, but these do not conclusively preclude polysynaptic mechanisms. In fact there is a robust reciprocal projection between the basolateral amygdala and the hippocampus, synaptic details of which have not been fully investigated. We examined the synaptic input from the basolateral amygdala to the projection neurons of the subiculum, the spiny pyramidal neurons. We labelled the afferents from basolateral amygdala with a small injection of biotinylated dextran amine, and revealed the anterogradely labeled fibers within the subiculum. The labeled basolateral amygdala fibers were studied with electron microscopy to identify their postsynaptic target structures. With this technique we have demonstrated anatomically that the basolateral amygdala preferentially innervates spiny subiculum neurons, presumed pyramidal projection neurons, although some dendrites and possibly local circuit neurons may be targeted.