Role of the medial prefrontal cortex in the development of conditioned bradycardia in rabbits with lesions of the cerebellar vermis

The effects on the expression of conditioned bradycardia of pairing an earl y (fourth postnatal day) cerebellar vermal lesion with a lesion of the medi al prefrontal cortex (mPFC) were studied in adult New Zealand rabbits. In t he conditioning procedure, an auditory stimulus (5 s, 1000 Hz) served as a conditioning stimulus (CS) and a train of electrical impulses applied to th e ear (500 ms, 100 Hz, 1.5 mA) was used as the unconditioned stimulus (US). Heart rate (HR) responses exhibited by rabbits with the early double lesio n (PFCBs) during orientation (CS-alone) and conditioning (CS-US paired) wer e analyzed and compared with those shown by unoperated controls as well as by a group of animals in which a cerebellar lesion alone had been performed on the fourth postnatal day (CBs). In all the experimental groups vermal l esions were localized in the cortex of lobules V-VII and the underlying whi te matter. As for mPFC ablation, the lesioned area involved the agranular p recentral region (Brodmann's area 8), the anterior cingulate cortex (Brodma nn's area 24) and the prelimbic area (Brodmann's area 32). All the experime ntal animals had a normal baseline HR as well as a marked orientation respo nse, both comparable with those exhibited by controls. In contrast, while C B rabbits showed an increase in the amplitude of the conditioned bradycardi c response when compared with controls, the HR conditioned response of PFCB animals was comparable to that exhibited by controls. These results sugges t that, since the double lesion produces a conditioned bradycardia similar to that of the controls, the increase in the amplitude of this response obs erved after early cerebellar removal may depend on the mPFC which, in the a bsence of specific cerebellar circuits, is unable to produce a properly cal ibrated HR conditioned response.