The role of the direct perforant path input to the CA1 subregion of the dorsal hippocampus in memory retention and retrieval

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Abstract
Subregional analyses of the hippocampus have suggested a selective role for the CA1 subregion in intermediate/long‐term spatial memory and consolidation, but not short‐term acquisition or encoding processes. It remains unclear how the direct cortical projection to CA1 via the perforant path (pp) contributes to these CA1‐dependent processes. It has been suggested that dopamine selectively modulates the pp projection to CA1 while having little to no effect on the Schaffer collateral (SC) projection to CA1. This series of behavioral and electrophysiological experiments takes advantage of this pharmacological dissociation to demonstrate that the direct pp inputs to CA1 are critical in CA1‐dependent intermediate‐term retention and retrieval function. Here we demonstrate that local infusion of the nonselective dopamine agonist, apomorphine (10, 15 μg), into the CA1 subregion of awake animals produces impairments in between‐day retention and retrieval, sparing within‐day encoding of a modified Hebb‐Williams maze and contextual conditioning of fear. In contrast, apomorphine produces no deficits when infused into the CA3 subregion. To complement the behavioral analyses, electrophysiological data was collected. In anesthetized animals, local infusion of the same doses of apomorphine significantly modifies evoked responses in the distal dendrites of CA1 following angular bundle stimulation, but produces no significant effects in the more proximal dendritic layer following stimulation of the SC. These results support a modulatory role for dopamine in the EC‐CA1, but not CA3‐CA1 circuitry, and suggest the possibility of a more fundamental role for EC‐CA1 synaptic transmission in terms of intermediate‐term, but not short‐term spatial memory.