Functional connectivity from CA3 to the ipsilateral and contralateral CA1 in the rat dorsal hippocampus

Anatomical advances have led to a reappraisal of the organization of hippocampal circuitry. However, it is not clear whether the functional connectivity is fully determined by the anatomical connectivity or whether it is significantly modified by feed-forward inhibition and modulatory inputs. Therefore, we have mapped CA1 responses evoked by stimulation of ipsilateral and contralateral CA3 in vivo. Population spike amplitude and threshold were plotted to produce response maps. All CA3 subregions projected diffusely to ipsilateral CA1. However, a pattern of maximal response emerged. Caudal CA3 stimulation evoked the maximal responses septally, while rostral CA3 responses were maximal temporally. The ipsilateral CA3 response maps were compared with those produced by stimulation at the homotopic point in the contralateral CA3. The CA1 areas of maximal functional connectivity were the same implying that there is convergence of the input to CA1 from homotopic CA3 sites in the two hippocampi. Although a response in CA1 was evoked widely, our results suggest that the functional connectivity is ordered, within and between the dorsal hippocampi, and that it is consistent with the recent anatomical data. The present findings allow more precise study of the propagation of normal and abnormal neuronal activity, within and between the dorsal hippocampi. Information on the site and speed of propagation of neuronal activity would be necessary for the development of a physiologically realistic model of hippocampal computation.