An electrophysiological study of the neural projections from the hippocampus to the ventral pallidum and the subpallidal areas by way of the nucleus accumbens.

The integrative role of the ventral striatum in transmitting signals from the hippocampus to the ventral pallidal and subpallidal areas was investigated in urethane-anaesthetized rats using an extracellular single-unit recording technique. Neurones of the nucleus accumbens were first activated by single-pulse stimulation of the ventral subiculum of the hippocampus. Further tests were made to investigate whether these accumbens neurones could be activated antidromically by stimulation of either the ventral pallidal or subpallidal areas. More than 4 times as many accumbens neurones, activated by hippocampal stimulation, responded antidromically to stimulation of subcommissural ventral pallidum than to stimulation of the sublenticular subpallidal area. This observation suggests that the hippocampus has preferential inputs to accumbens efferent neurones which project monosynaptically to the ventral pallidum. Spontaneously active neurones in the ventral pallidum and subpallidal area were inhibited by stimulation of the ventral subiculum of the hippocampus. These inhibitory responses were reduced when glutamic acid diethyl ester, a glutamate antagonist, was microinjected into the medial accumbens, apparently blocking the hippocampal-accumbens glutamatergic synapses to both the ventral pallidal-directed and the subpallidal-directed accumbens efferents. This evidence suggests that signals from the hippocampus reach ventral pallidal and subpallidal regions by way of the nucleus accumbens. The presence of a projection from ventral pallidal and subpallidal regions to the brainstem mesencephalic locomotor region further supports the hypothesis that limbic (e.g. hippocampus) can influence somatomotor activities by way of the nucleus accumbens and its efferent projection to ventral pallidal and subpallidal regions.