Microinjections of local anesthetic into the pontine nuclei reduce the amplitude of the classically conditioned eyelid response.

Seventeen rabbits were implanted bilaterally with cannulae into the pontine nuclei, and were then trained in the classically conditioned eyeblink procedure. Nine of the rabbits were trained with a tone as a conditioned stimulus (CS), and eight were trained with both tone and a light CS. After each rabbit had learned the conditioned response (CR) well, testing began. One microliter of 5% lidocaine injected bilaterally significantly impaired the CR amplitude for both tone and light as a CS, although unconditioned response amplitude was not affected. Injection of 1 microliter of isotonic saline did not impair CR amplitude, suggesting that the impairment seen after lidocaine injection was due to the anesthetic action of lidocaine in the pons, and not to nonspecific effects distal to the site of injection. These results support and extend the lesion data which shows that the pontine nuclei and their mossy fiber projection to the cerebellum are necessary for eyeblink conditioning using a peripheral CS.