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Department of Anatomy and Developmental Biology, University College London, UK. N. Ramnani:n.ramnani@ucl.ac.uk

1. Studies show that reversible inactivation of the anterior interpositus nucleus (AIP) of the cerebellum with muscimol (a GABAA agonist) prevents acquisition of the classically conditioned nictitating membrane response (NMR) in the rabbit. Here, we have used reversible inactivations of the AIP with muscimol to investigate the role of the cerebellum in the extinction of this response. 2. Experimental subjects were implanted with cannulae targeted to the AIP, through which muscimol could be infused via an injector cannula. This experiment was divided into three phases lasting 4 days, separated by 3 day intervals. Experimental and unoperated control subjects received acquisition training in phase 1; in phases 2 and 3 they received extinction training. 3. Presentation of the conditioned stimulus (CS) alone in phase 2 produced normal extinction in control subjects. Muscimol inactivation of the AIP in experimental subjects during phase 2 prevented extinction of conditioned responses (CRs), shown by initial high CR frequency in the first post-drug session of phase 3, which then extinguished in a manner indistinguishable from controls in phase 2. 4. Our findings support the suggestion that similar cerebellar circuitry is engaged in acquisition and extinction of NMR conditioning.

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