The paired-pulse index: a measure of vigilance state-dependent changes during maturation

The response of the hippocampal dentate granule cell population to paired-pulse stimulation of the perforant path obtained during quiet waking (QW), slow-wave sleep (SWS) and REM sleep was examined in freely-moving rats at 15, 30, and 90 days of age. The response to paired-pulse stimulations applied over a range of interpulse intervals (IPIs) was used to construct a paired-pulse index (PPI) Indicative of the modulatory control exerted on dentate granule cell excitability. In comparison to 90 day old animals, PPIs obtained from 15-day old rats during QW indicated a markedly lower levels of early inhibition and facilitation and a relative lack of late inhibitory modulation. Values of the PPI obtained from 30 day old rats fell intermediate between the 15 day and adult groups. Adult rats showed an absence of facilitation and a decrease in late inhibition during REM sleep. Although no significant vigilance state-dependent changes in the PPI were obtained for 15or 30- day old animals, these results indicate the PPI is an effective measure of the normal maturation of the modulation of dentate granule cell activity.