EFFECT OF TONE-DEPENDENT FEAR CONDITIONING ON HEART-RATE AND BEHAVIOROF C57BL 6N MICE/

The effects of the temporal sequence of tone (conditioned stimulus [CS ]) and footshock (unconditioned stimulus [US]) during training (acquis ition) on tone-dependent retention were studied in mice. Freezing incr eased significantly as an associative behavioral response in mice subj ected to CS paired with US or after unpaired by 30 s in the memory tes t performed 24 hr after training. In the home cage of freely moving mi ce implanted with an electrocardiogram transmitter, CS triggered a str ong tachycardiac response in the memory test. Heart rate (HR) increase d from about 580 bpm to more than 750 bpm, and HR variability decrease d significantly. The inhibition of the HR increase by the nonspecific beta-adrenergic antagonist sotalol indicated the strong sympathetic co ntribution to the tachycardiac response. CS evoked a significant but m inor HR increase in mice subjected to either CS or US only or CS and U S unpaired by 60 s. Thus, HR and KR variability reflected associative learning.