STRYCHNINE EFFECTS ON ULTRASOUND ELICITED BEHAVIORS IN LISTER HOODED RATS

Previous studies have demonstrated that Lister hooded rats will exhibi t characteristic bursts of locomotion when exposed to a 20-kHz acousti c stimulus; this ultrasound-induced locomotion has been suggested as a potential model for panic attacks. Although ultrasound presentation r arely induces convulsions, the locomotor bursts exhibited resemble pn- convulsant running. The present studies examined the interactions betw een strychnine treatment and experimenter-presented ultrasounds on beh aviour in male Lister hooded rats. Strychnine was selected because it is a potent and effective convulsion-inducing agent which is not known to induce anxiety in humans. Behaviour in a circular arena (75 cm dia meter) was observed live, videotaped and traced electronically. In exp eriments 1 and 2, moderate (60 s) or relatively brief (15 s) exposure to an ultrasound stimulus (20 kHz, 98 dB, SPL) typically resulted in 5 - to 10-s bursts of locomotion in saline-treated subjects; strychnine treatment (0.5, 0.7, 1.0 mg/kg, injected IP, 10 min prior to testing) significantly increased this ultrasound-induced locomotion in a dose-d ependent manner. Experiment 3 demonstrated that the strychnine enhance ment of the ultrasound response was not different in naive animals whe n compared to those subjects which had received occasional strychnine and/or ultrasound treatment previously. Experiment 3 also demonstrated that strychnine treatment can cause at least modest running in subjec ts exposed to a 2 kHz tone (96 dB SPL). In experiment 4, exposure to t he 20 kHz, 98 dB ultrasound stimulus for a much longer period, 9 min, resulted in irregular cycles of bursts of locomotion, followed immedia tely by periods of relative inactivity in saline-treated animals; appr oximately 10% of these subjects exhibited tonic-clonic convulsions. No convulsions occurred in strychnine-treated subjects during the period 10-20 min post-injection in the absence of ultrasound exposure, in co ntrast, the frequency of occurrence of convulsions in strychnine-treat ed subjects (10-20 min post-injection) exposed to the ultrasound stimu lus was greater than 50%; these convulsions typically occurred at the end of a locomotor burst. The results of the present studies suggest t hat then may be a relationship between ultrasound-induced locomotor bu rsts and convulsant activity.