EFFECTS OF CLONIDINE AND DIAZEPAM ON THE ACOUSTIC STARTLE RESPONSE AND ON ITS INHIBITION BY PREPULSES IN MAN

This experiment examined the effects of two sedative/anxiolytic drugs, diazepam and clonidine, on the eyeblink component of the acoustic sta rtle response in healthy volunteers. Twelve males (18-30 years), scree ned for normal hearing thresholds, participated in three sessions in w hich they received oral doses of placebo, diazepam 10 mg and clonidine 200 mu g according to a balanced double-blind protocol. Thirty-minute electromyographic recordings from the orbicularis oculi muscle of the right eye were carried out 120 min after ingestion of clonidine and 6 0 min after ingestion of diazepam. Subjects received 36 40-msec sound pulses (115 dB), separated by variable intervals (mean 25 sec); in 24 of the trials the pulse was preceded by a 40-msec prepulse (75 dB in 1 2 trials and 85 dB in 12 trials; prepulse-pulse interval, 120 msec). T he amplitude of the startle response was significantly reduced both by diazepam (mean +/- SEM: -43.9 +/- 7.4%)and by clonidine (-75.7 +/- 4. 7%). Under the placebo condition, the 75 and 85 dB prepulses inhibited the startle response by 38.6 +/- 6.5 and 70.3 +/- 2.9%, respectively. Neither drug significantly altered the degree of prepulse inhibition. Both drugs reduced self-rated alertness; clonidine reduced systolic b lood pressure and salivation. The results confirm the suppressant effe ct of clonidine on the startle response and show a qualitatively simil ar effect of diazepam; the results also demonstrate the insensitivity of prepulse inhibition of the startle response to doses of sedative dr ugs that are sufficient to attenuate the startle response itself.