Light-enhanced startle: further pharmacological and behavioral characterization

Abstract.

Rationale: Previous findings indicate that the acoustic startle response is elevated when rats are tested in bright light. The phenomenon is disrupted by the 5HT_1A partial agonist and the D₂ receptor antagonist buspirone, a compound that also blocks the effect on startle of conditioned fear, and it was suggested that light-enhanced startle reflects an anxious state produced by bright light. It was also suggested that pre-test handling may be necessary for light-enhanced startle. To characterize this phenomenon further, we evaluate here the sensitivity of light-enhanced startle to the anxiolytic compound chlordiazepoxide, to the noradrenergic beta-receptor antagonist propranolol, and to pre-test handling. Methods: Startle was measured for 20 min in the dark (phase I), followed 5 min later by a second test (phase II) either in the dark or the light. Immediately prior to testing, rats received IP injections of chlordiazepoxide (5, 10, or 20 mg/kg; experiment 1), propranolol (10 or 20 mg/kg; experiment 2) or saline. Using the minimally effective doses from the light-enhanced startle experiments, conditioned fear to a shock-paired cue was also evaluated. In a third experiment, rats were (a) removed from the test cages and briefly handled between phases I and II, (b) were not handled during this interval, or (c) were tested without the interposed delay. Results: Propranolol (10 and 20 mg/kg) and chlordiazepoxide (10 mg/kg) disrupted light-enhanced startle at doses comparable to those required to disrupt fear-potentiated startle to a shock-paired cue. There was no effect of handling. Conclusion: These results further characterize the pharmacology of light-enhanced startle, provide additional support for the view that the effects of light on startle reflect an influence of anxiety, and offer additional information concerning the procedural variables that influence this behavior.