Effect of atropine on the onset of fear bradycardia in eastern cottontail rabbits Sylvilagus floridanus

Abstract

1. Biotelemetry transmitters were surgically implanted in the abdominal cavity of six mature eastern cottontail rabbits, Sylvilagus floridanus, and the effect of atropine on the rate of onset of fear bradycardia investigated. 2. Electrocardiogram records were obtained from demodulation of magnetically recorded data on a physiograph. Instantaneous heart rate was obtained and averaged for each second. Ten seconds of pre-stimulus data was obtained, followed by thirty seconds of data obtained while frightening the rabbit. 3. A marked fear bradycardia was present in all untreated and atropine-treated animals. 4. Mean heart rate values for untreated rabbits were 195 ± 2.9 and 125 ± 2.1 ( x ± SE) bpm during pre-stimulus and stimulus respectively. 5. Mean values for atropine-treated animals were 241 ± 1.1 and 162 ± 5.0 bpm during prestimulus and stimulus respectively. 6. Atropine reduced fear bradycardia from 36.1% to 33.8%, but reduced the rate of onset from −9.25 to −3.14 bpm/sec. 7. Fear bradycardia can occur without parasympathetic activity, but parasympathetic activity is necessary for the normal rapid onset of fear bradycardia.