CONDITIONED DIMINUTION OF THE UNCONDITIONED RESPONSE IN RABBIT EYEBLINK CONDITIONING - IDENTIFYING NEURAL SUBSTRATES IN THE CEREBELLUM AND BRAIN-STEM

Several models of Pavlovian conditioning assume that processing of an unconditioned stimulus (US) is diminished by a conditioned stimulus (C S) with which it had been paired. Two experiments evaluated the hypoth esis that US processing may be diminished by CS-dependent activation o f the cerebellum. Experiment 1 showed that electrical brain stimulatio n (EBS) of the cerebellar interpositus nucleus diminished the peak amp litude of the rabbit's unconditioned eyeblink response. This effect wa s bilateral, was systematically related to the intensity of EBS, maxim al 50 ms after the offset of EBS, and substantially reversed by naloxo ne. Experiment 2 showed that inactivating the contralateral red nucleu s with gamma-D-glutamylglycine blocked the decremental effect of inter positus stimulation. Implications for neural systems mediating the inh ibitory effects of cerebellar activation and the antinociceptive role of noradrenergic and opioid systems in Pavlovian conditioning phenomen a are discussed.