Postnatal hippocampal granule cell agenesis in the rat: effects on two types of rhythmical slow activity (RSA) in two hippocampal generators.

Hippocampal slow wave activity was studied in rats which were normal or had been subjected to denate gyrus granule cell agenesis by focal X-irradiation starting at birth (0-day group) or two days (2-day group) of age. X-irradiation reduced adult brain weight, abolished most (2-day) or all (0-day) granule cells in the lower (endal) blade of the dentate gyrus, and reduced granule cell density by up to 70% in the upper (ectal) blade of the dentate gyrus. X-irradiation did not affect pyramidal cells of the hippocampus proper. Tracking with microelectrodes in urethane anesthetized rats given eserine, sensory, or brain stimulation showed two foci of hippocampal rhythmical slow activity (theta or RSA), one in stratum oriens of CA1 and one in stratum moleculare of the dentate gyrus. These were opposite in phase by approximately 180 degrees and separated by a null zone and phase reversal point occurring at stratum radiatum. There were no significant differences in the amplitude, frequency, null, or phase reveral points in the normal or X-irradiated groups. However, the width of the RSA amplitude peak in the ectal blade was reduced, correlated with the reduction in the length of the ectal blade, and the RSA amplitude peak in the lower blade was absent, correlated with the absence of the lower blade. The fast activity recorded in the hilus of normal rats was absent in the X-irradiated groups. RSA recorded during spontaneous movement (walking) had identical amplitude, frequency, anatomical foci, and phase in both the hippocampus proper and dentate gyrus in normal and X-irradiated rats. Antimuscarinic, but not antinicotinic, agents abolished anesthesia-related RSA, but not movement-related RSA, in all groups of rats. The results are discussed in terms of their relevance to the two-generator hypothesis of RSA.