Activity of dentate granule cells during learning: differentiation of perforant path input.

Experiments were conducted which extended previous findings regarding the activity of the perforant path and its synaptic relationship to the granule cells of the dentate gyrus during conditioning. A differential conditioning paradigm was utilized in which rats were trained to respond to one of two different tone frequencies. Results demonstrated that (1) tone elicited averaged evoked potentials recorded from the perforant path terminal zone in the outer molecular layer of the dentate gyrus were similar for both the positive and negative tones regardless of frequency or reversal of the reinforcement condition; (2) extracellular unit discharge patterns of dentate granule cells were differentially associated with the positive and negative tones as demonstrated by post stimulus histograms (PSHs); (3) this differential pattern of unit discharges could be reversed following establishment of criterion differential behavioral responding after reversal of the reinforcement contingency between the two tone stimuli and (4) the differential unit discharge pattern was not present when behavioral responding was not differentiated to the two tone stimuli, e.g., immediately following reversal of the reinforcement contingency. The results are discussed within the context of other anatomically defined functional circuits within the hippocampus which could serve as the basis for alteration of the non differentiated excitatory perforant path input into a differential dentate granule cell discharge pattern for behaviorally relevant sensory stimuli.