Coordinate diurnal variation in the strength of startle elicitation and of startle modification in the rat

Abstract

In order to determine the effect of time of day on reflex modification, we measured the acoustic startle reflex (ASR) in rats (n = 4) to 120-dB tone bursts given alone to provide the ASR baseline, 100 msec after an inhibiting brief noise prepulse, or 10 to 20 sec after the onset of a facilitating noise background for 4–5 days in a test environment illuminated on a 12:12-h light:dark cycle. ASR amplitudes were greater in the dark, and sinusoids with a 24-h period starting at the light:dark transition accounted for about 80% of the variance in ASR amplitudes within each stimulus condition. The absolute differences in amplitude between baseline and either inhibited or facilitated ASR means were also greater in the dark, but relative measures of reflex modification (expressed as a proportion of the ASR baseline) were constant across time of day save for a slight compressive nonlinearity. The outcome for difference scores indicated that those neural mechanisms responsible for ASR elicitation and those responsible for ASR modification by external stimuli are both controlled in part by time of day, and the outcome for the relative response scores indicated further that the sensitivity of these separate reflex control mechanisms to the circadian process must be about equal.