ELECTROPHYSIOLOGY OF THE HIPPOCAMPAL AND AMYGDALOID PROJECTIONS TO THE NUCLEUS-ACCUMBENS OF THE RAT - CONVERGENCE, SEGREGATION, AND INTERACTION OF INPUTS

The nucleus accumbens (Nacb) receives inputs from hippocampus and amyg dala but it is still unclear how these inputs are functionally organiz ed and may interact. The interplay between these input pathways was ex amined using electrophysiological tools in the rat, in vivo, under hal othane anesthesia. After fornix/fimbria stimulation (Fo/Fi, subicular projection fibers to the Nacb), mono- and polysynaptically driven sing le units were recorded in the medial shell/core regions of the Nacb an d in the ventromedial caudate putamen. Monosynaptically driven neurons by basolateral amygdala (BLA) stimulation were found in the medial sh ell/core and in the ventrolateral shell/core regions. in the areas of convergence (medial shell/core), paired activation of BLA followed by that of Fo/Fi resulted in an enhancement of the Fo/Fi response, wherea s stimulation in the reverse order, Fo/Fi followed by BLA, led to a de pression of the BLA response. In addition to these patterns of interac tions, the tetanization of the Fo/Fi to Nacb pathway caused a homosyna ptic decremental (long-term) potentiation in the Nacb, accompanied by a heterosynaptic (long-term) depression of the non-tetanized BLA to Na cb pathway. We postulate that the hippocampal inputs may close a ''gat e'' for the amygdala inputs, whereas the gate is opened for the hippoc ampus inputs by previous amygdalar activity. These opposite effects on the Nacb neuronal populations should be taken into account when inter preting behavioral phenomena, particularly with respect to the contras ting effects of the amygdala and the hippocampus in locomotion and pla ce learning.