Exp Brain Res (1996) l 11:417-423 9 Springer-Verlag 1996 Brunello Ghelarducci 9 Domenico Salamone Alfredo Simoni 9 Laura Sebastiani Effects of early cerebellar removal on the classically conditioned bradycardia of adult rabbits Received: I 1 October 1995 / Accepted: 19 April 1996 Abstract The magnitude of classically conditioned bra- dycardia was studied in 18-day-old and adult rabbits in which the cerebellar vermis had been surgically removed on either the 5th or 18th postnatal day. In the condition- ing procedure, an auditory stimulus (5 s, 1000 Hz) served as conditioned stimulus (CS) and a train of elec- tric impulses applied to the ear (100 Hz, 500 ms, 1.5 mA) was employed as the unconditioned stimulus (US). Heart rate (HR) responses developed in the operat- ed animals during the CS-alone (orientation), and CS-US paired presentations (conditioning) were analyzed and compared with those developed in control animals. In all the experimental groups, lesions were localized to the cortex of lobules IV-VII and the underlying white mat- ter, sparing the deep cerebellar nuclei. None of the le- sioned animals showed any behavioral or somatomotor deficit. All the operated animals exhibited a normal base- line HR and a marked orienting response, both compara- ble with those of controls. In contrast, while the animals tested at 18 days showed 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 cer- ebellar lesion showed a conditioned bradycardia greater than that of controls, the rabbits lesioned on the 18th postnatal day exhibited a reduced bradycardic response. These results suggest that the timing of cerebellar vermis removal, at early stages of development, represents a crucial factor in the organization of the bradycardic re- sponse in the adult. Key words Heart rate responses 9 Cerebellar lesion 9 Neonates 9 Rabbit B. Ghelarducci(~) 9 D. Salamone. A. Simoni - L. Sebastiani Dipartimento di Fisiologiae Biochimica, Universit~ di Pisa, Via S.Zeno29/31, 1-56127 Pisa, Italy; Fax: +39 50-552183 Introduction In a previous study it has been shown that classically conditioned bradycardia elicited in rabbits by pairing a tone (conditioned stimulus, CS) to an electric shock (un- conditioned stimulus, US) does not emerge until the 18th postnatal day (Sebastiani et al. 1994). This result is quite surprising, since sensory systems (auditory and nocicep- rive) necessary for conditioned bradycardia, are mature before this age (Rubel 1978). Moreover, conditioned so- matomotor defence reflexes in response to an acoustic stimulus are known to be present, in the rabbit, between the 10th and 13th postnatal day (Volokhov 1968). These data indicate that rabbits are able to perform complex as- sociations between stimuli at early stages of develop- ment and that the failure of neonates to show heart rate (HR) conditioned responses cannot be ascribed to a gen- eral inability of neonatal rabbits to learn. In addition, the presence of a marked bradycardia following the US ob- served in our experiments in some of the neonatal rab- bits, suggests that the lack of HR conditioning cannot be due to an overall inability to decelerate the heart. This is in agreement with the findings reported by several au- thors that indicate that the baroreceptor reflex pathways are already functional at early stages of development (see Downing 1979 for a review). These studies then indicate that the processes mediat- ing specific HR responses to certain types of environ- mental stimulation are not present at birth and require several days to mature. In addition, since the sensorimo- tor systems specifically involved in the expression of the aversive responses are known to be functioning early in development, this delay could be ascribed to incomplete maturation of those central nervous system (CNS) re- gions involved in associative processes. Among these structures, a significant role may be exerted by the cere- bellar vermis, which has been identified as one of the brain structures that control classically conditioned bra- dycardia (Sebastiani et al. 1992; Supple and Kapp 1993; Supple et al. 1993); moreover, its maturation is not com- plete at birth (Altman 1982; Larsell 1970). The present