Hippocampal and amygdaloid interactions in the nucleus accumbens

The nucleus accumbens, in view of its afferent and efferent fiber connectio ns, appears to hold a key position for "limbic" (e.g., hippocampal and amyg daloid) 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 cont ain further inhomogeneities. The present account first summarizes the detai led topographical anatomical relationships of inputs from different dorso-v entral parts of the hippocampus and different rostrocaudal parts of the bas al amygdaloid complex at the level of the accumbens. Subsequently, the elec trophysiological characteristics of hippocampal and amygdaloid inputs in th e 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 amygdaloi d inputs appear to facilitate hippocampal throughput (heterosynaptic paired pulse facilitation), whereas stimulation of hippocampal inputs depresses a mygdaloid throughput in a paired pulse paradigm. Tetanic stimulation of hip pocampal inputs to the accumbens leads to a decremental long-term potentiat ion (LTP) of this fiber pathway (homosynaptic LTP) but, along a similar tim e range, to a depression of amygdaloid inputs (heterosynaptic long-term dep ression). The involvement of dopaminergic, GABAergic, and glutamatergic mec hanisms in these interactions is discussed. Finally, it is suggested that t he interactions be tween hippocampal and amygdaloid inputs at the level of the nucleus accumbens play a role in different aspects of associative learn ing.