Inactivation of the superior cerebellar peduncle blocks expression but not acquisition of the rabbit ' s classically conditioned eyeblink response ( learning / memory / cerebellum )

The localization of sites of memory formation within the mammalian brain has proven to be a formidable task even for simple forms of learning and memory. Recent studies have demonstrated that reversibly inactivating a localized region of cerebellum, including the dorsal anterior interpositus nucleus, completely prevents acquisition of the conditioned eye-blink response with no effect upon subsequent learning without inactivation. This result indicates that the memory trace for this type of learning is located either (i) within this inactivated region ofcerebellum or (ii) within some structure(s) efferent from the cerebellum to which output from the interpositus nucleus ultimately projects. To distinguish between these possibilities, two groups of rabbits were conditioned (by using two conditioning stimuli) while the output fibers of the interpositus (the superior cerebellar peduncle) were reversibly blocked with microinjections of the sodium channel blocker tetrodotoxin. Rabbits performed no conditioned responses during this inactivation training. However, training after inactivation revealed that the rabbits (trained with either conditioned stimulus) had fully learned the response during the previous inactivation training. Cerebellar output, therefore, does not appear to be essential for acquisition of the learned response. This result, coupled with the fact that inactivation of the appropriate region of cerebellum completely prevents learning, provides compelling evidence supporting the hypothesis that the essential memory trace for the classically conditioned eye-blink response is localized within the cerebellum. Identification of the site or sites of memory formation and storage within the brain for any particular type of memory is an essential prerequisite for elucidating the cellular or subcellular mechanisms as well as the network level properties that mediate the acquisition, retrieval, and expression of that particular memory. Although much progress toward identifying potential sites of memory formation and storage has been achieved, definitive localization of a particular memory locus within the mammalian brain has remained frustratingly elusive. A major obstacle impeding this localization of memory traces is the necessity of first identifying the neural-anatomical circuitry essential for acquisition and expression of a particular learned response. For at least one form of learned behavioraversive, classically conditioned discrete skeletal movements, specifically, the classically conditioned eye-blink responsethe neural circuitry essential for acquisition and expression of the learned response has largely been identified (for review, see ref. 1). In brief, the results of lesion, recording, and stimulation studies indicate that the conditioned stimulus (CS) pathway includes sensory relay nuclei, the pontine nuclei, and mossy fiber projections, via the middle cerebellar peduncle, to The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. the cerebellum (2-4); that the unconditioned stimulus (US) pathway includes somatosensory relay nuclei, the inferior olive, and its climbing fiber projections, via the inferior cerebellar peduncle, to the cerebellum (5, 6); and that the conditioned response (CR) pathway includes the cerebellum, its projections via the superior cerebellar peduncle (SCP) from the interpositus nucleus to the red nucleus and red nucleus projections to premotor and motor nuclei (7-15). Reversibly inactivating a critical region of cerebellum encompassing the dorsal aspects of the anterior interpositus nucleus and overlying regions of cerebellar cortex during eye-blink conditioning with local cooling (16), microinjection of the y-aminobutyric acid agonist muscimol (17), or infusion of the sodium channel blocker lidocaine (18) completely prevents acquisition of the learned response with no effect upon the ability to acquire the CR in subsequent training without inactivation and with no effect upon the ability to perform the unconditioned response (UR). In marked contrast, inactivation of the contralateral red nucleus, a major efferent target of the cerebellar interpositus nucleus that is essential for performance of the CR (see above), prevents expression but not acquisition of the CR (17, 19). Infusions of lidocaine into white matter just ventral to the interpositus, which would presumably inactivate cerebellar output fibers, also prevent expression but not acquisition of the learned response (18). Inactivation of the motor nuclei (accessory abducens and facial) and surrounding regions of reticular formation, which mediate performance of the CR and the UR, prevents expression of both the CR and UR but has no effect on the ability to learn the CR (20, 21). Collectively, these results argue that the critical locus of memory formation and storage for the eye-blink CR is localized to a region of cerebellum encompassing the dorsal aspects of the anterior interpositus and regions of overlying cortex, because inactivation of this region of cerebellum completely prevents learning from occurring while inactivation of structures downstream from the cerebellum in the essential eye-blink circuit does not prevent acquisition of the CR but does prevent expression. It is well established that output fibers of the interpositus nucleus project, via the superior cerebellar peduncle, to several regions within the brain other than the red nucleus, including, but not limited to the thalamus, inferior olive, pontine nuclei, and the nucleus reticularis tegmenti pontis (22). Thus, it remains possible that one or more of the efferent targets of the cerebellum might be the locus of the memory trace for the eye-blink CR. If the memory trace were normally formed or stored within one or more of these sites efferent from the cerebellum, then the total abolition of both acquisition and expression of the eye-blink CR after appropriate lesions of the Abbreviations: CR, conditioned response; CS, conditioned stimulus; LRN, lateral reticular nucleus; SCP, superior cerebellar peduncle; TTX, tetrodotoxin; UR, unconditioned response; US, unconditioned stimulus. *To whom reprint requests should be addressed.