Ontogeny of paired-pulse responses in the dentate gyrus

This study examined the effect of normal development and vigilance state on the modulation of dentate granule cell activity in the freely moving rat at 15, 30 and 90 days of age across three vigilance states: quiet waking (QW), slow-wave sleep (SWS) and rapid eye movement (REM) sleep. Using paired-pulse techniques, the population spike amplitude of the first pulse (PSA1) and the paired-pulse index (PPI) were obtained for the dentate evoked field potentials elicited by the stimulation of the medial perforant path. The PSA1 values obtained for the 90-day old animals were significantly greater during SWS than during QW or REM sleep. PSA1 values obtained for the 15- and 30-day old animals, however, were found to be significantly different than those obtained for adult animals. Transmission through the first leg of the hippocampal trisynaptic circuit appeared to be more efficient during REM sleep at 15 days of age and then, in adulthood, to be greater during SWS. PPI values obtained at 15, 30 and 90 days of age, were altered by normal ontogeny, but not by vigilance state. PPI values were significantly different as a result of normal development with 15-day old animals exhibiting significantly less early and late inhibition, and little facilitation when compared to the 90-day old adult rats. These changes in the paired-pulse index (PPI) as well as those in PSA1, therefore, provide measures of the modulation of dentate granule cell activity as the animal matures.