Hippocampal and amygdaloid interactions in the nucleus accumbens

Abstract

The nucleus accumbens, in view of its afferent and efferent fiber connections, appears to hold a key position for 'limbic' (e.g., hippocampal and amygdaloid) influences to reach somatomotor and autonomic brain structures, and it has therefore been considered as a limbic-motor interface. The nucleus accumbens can be subdivided into a shell and a core region, which both contain further inhomogeneities. The present account first summarizes the detailed topographical anatomical relationships of inputs from different dorso-ventral parts of the hippocampus and different rostrocaudal parts of the basal amygdaloid complex at the level of the accumbens. Subsequently, the electrophysiological characteristics of hippocampal and amygdaloid inputs in the accumbens are described. Interactions between hippocampal and amygdaloid inputs appear to exist primarily in the medial parts of both the shell and the core of the nucleus accumbens. In the short term, stimulating amygdaloid inputs appear to facilitate hippocampal throughput (heterosynaptic paired pulse facilitation), whereas stimulation of hippocampal inputs depresses amygdaloid throughput in a paired pulse paradigm. Tetanic stimulation of hippocampal inputs to the accumbens leads to a decremental long-term potentiation (LTP) of this fiber pathway (homosynaptic LTP) but, along a similar time range, to a depression of amygdaloid inputs (heterosynaptic long-term depression). The involvement of dopaminergic, GABAergic, and glutamatergic mechanisms in these interactions is discussed. Finally, it is suggested that the interactions between hippocampal and amygdaloid inputs at the level of the nucleus accumbens play a role in different aspects of associative learning.