Electrical signs of an oligosynaptic visual projection to the rat hippocampus

In rats with pons transection photic or optic nerve stimulation elicits a response in dorsal hippocampus with approximately with same latency, amplitude and time course as the response in striate cortex. After optic nerve stimulation a positive polyphasic deflection with the last peak at 10 ms is followed by a larger biphasic major deflection with peaks at 15 and 25 ms and later, slower deflections at intervals of 125 ms. The polyphasic deflection is maximal in the inferior part of the hippocampus but does not reverse polarity; the other deflections reverse above and below an isoelectric point in the hilum of the dentate gyrus, a distribution attributable to depolarization in the molecular layers, perhaps also cell bodies, of that structure. The major deflection is more sensitive to changes in optic nerve stimulus strength than the response in striate cortex and is more resistant to reduction in amplitude during repetitive stimulation, following frequencies up to 50/s. The pathway between retina and hippocampus for all parts of the response is interrupted by lesions in the tectum. The major deflection is abolished by lesions in the posterior cingulum. In the posterior cingulum the pathway has fast and slow components in the lower and upper portions, respectively, associated with the first and second parts, respectively, of the major deflection. The pathway is not interrupted by lesions in the fornix, septal nuclei, anterior cingulum, anterior medial thalamus or medial midbrain ventral to the superficial tectum. There are complex interactions, up to several hundred milliseconds in duration, between the response to optic nerve stimulation and those elicited by stimulation in the cingulum, midbrain and thalamus. Tonic influences on the dentate gyrus from cingulum and tectum are described.