EFFECTS OF EARLY CEREBELLAR REMOVAL ON THE CLASSICALLY-CONDITIONED BRADYCARDIA OF ADULT-RABBITS

The magnitude of classically conditioned bradycardia was studied in 18 -day-old and adult rabbits in which the cerebellar vermis had been sur gically removed on either the 5th or 18th postnatal day. In the condit ioning procedure, an auditory stimulus (5 s, 1000 Hz) served as condit ioned stimulus (CS) and a train of electric impulses applied to the ea r (100 Hz, 500 ms, 1.5 mA) was employed as the unconditioned stimulus (US). Heart rate (HR) responses developed in the operated animals duri ng the CS-alone (orientation), and CS-US paired presentations (conditi oning) were analyzed and compared with those developed in control anim als. In all the experimental groups, lesions were localized to the cor tex of lobules IV-VII and the underlying white matter, sparing the dee p cerebellar nuclei. None of the lesioned animals showed any behaviora l or somatomotor deficit. All the operated animals exhibited a normal base-line HR and a marked orienting response, both comparable with tho se of controls. In contrast, while the animals tested at 18 days showe d a normal pattern of conditioned bradycardia, at the age of 3 months the HR conditioned response differed significantly from that observed in control rabbits: the animals that received the earliest cerebellar lesion showed a conditioned bradycardia greater than that of controls, the rabbits lesioned on the 18th postnatal day exhibited a reduced br adycardic response. These results suggest that the timing of cerebella r vermis removal, at early stages of development, represents a crucial factor in the organization of the bradycardic response in the adult.