Potentiation of acoustic startle behavior in the rat (Rattus norvegicus) at the onset of darkness.

In Experiment 1 (N = 8), a 20-ms light pulse, given at various times before a noise burst, inhibited reflex expression with a single trough at a lead time of 70 ms, whereas a dark pulse facilitated the reflex with two peaks at 40 and 160 ms. In Experiment 2 (N = 18) facilitation by dark onset had a single peak, and inhibition by light onset a single trough; thus, the double peak of the dark pulse may result because inhibition from light onset at the end of the dark pulse was briefly impressed on the facilitatory effect of dark onset. In Experiment 3 (N = 12), diazepam (2.5 mg/kg, but not 1 mg/kg) eliminated dark facilitation but not light inhibition. These diazepam data reveal a basic similarity, perhaps identity, of the mechanisms responsible for the effect of dark onset and those producing reflex facilitation by Pavlovian fear conditioning and prolonged background noise, because all are moderated by a GABAergic system.