Neuronal activity was recorded from regions of the cerebellar cortex and dentate-interpositus nuclei during learning and/or performance of a classically conditioned nictitating membrane (NM-a third eyelid)/eyeblink response in the rabbit. It was found that neurons located within restricted portions of the ansiform lobule and anterior lobe cortical regions and of the dentate-interpositus nuclei respond in relation to the performance of the learned eyeblink response. Furthermore, chronic recordings from the dentate-interpositus nuclei revealed that these responses develop in close relation to the learning of the conditioned eyeblink response. Stimulation of the dentate-interpositus nuclei through the recording electrodes in some cases yielded eyelid closure and NM extension in both trained and untrained animals. Lesion of the axons of the dentate-interpositus nuclei (superior cerebellar peduncle), a manipulation which is known to abolish the learned eyeblink response, abolished the stimulation effect. We have previously reported that lesions of the dentate-interpositus nuclei cause abolition of the learned eyeblink response. In the present study, we report that lesions of the regions of cerebellar cortex projecting to the dentate-interpositus nuclei do not permanently abolish the conditioned response, although the amplitude-time course of the learned response could be affected. These results, together with results of other studies, demonstrate that the medial dentate and/or lateral interpositus nuclei are active during learning and performance of the conditioned eyeblink response, are capable of producing this learned response, and are essential for the learning and retention of the conditioned eyeblink response. Therefore, the medial dentate and/or lateral interpositus nuclei are a part of the essential neuronal circuit involved in the learning and production of the classically conditioned eyeblink response in the rabbit.

Converging lines of evidence from rabbits, rats, and humans argue for the crucial involvement of the cerebellum in classical conditioning of the eyeblink/nictitating membrane response in mammals. For example, selective lesions (permanent or reversible) of the cerebellum block both acquisition and retention of eyeblink conditioning. Correspondingly, electrophysiological and brain-imaging studies indicate learning-related plasticity in the cerebellum. The involvement of the cerebellum in eyeblink conditioning is also supported by stimulation studies showing that direct stimulation of the two major afferents to the cerebellum (the mossy fibers emanating from the pontine nucleus and climbing fibers originating from the inferior olive) can substitute for the peripheral conditioned stimulus (CS) and unconditioned stimulus (US), respectively, to yield normal behavioral learning. In the present study, we examined the relative contribution of the cerebellar cortex versus deep nuclei (specifically the interpositus nucleus) in eyeblink learning by using mutant mice deficient of Purkinje cells, the exclusive output neurons of the cerebellar cortex. We report that Purkinje cell degeneration (pcd) mice exhibit a profound impairment in the acquisition of delay eyeblink conditioning in comparison with their wild-type littermates. Nevertheless, the pcd animals did acquire a subnormal level of conditioned eyeblink responses. In contrast, wild-type mice with lesions of the interpositus nucleus were completely unable to learn the conditioned eyeblink response. These results suggest that both cerebellar cortex and deep nuclei are important for normal eyeblink conditioning.

Over the past 10 years, a number of laboratories have reported that classically conditioned skeletal muscle responses, such as conditioned nictitating membrane/eyelid responses, are critically dependent on activity in the cerebellum. For example, unilateral lesions of the cerebellar interpositus nucleus have been shown to prevent acquisition and abolish retention of the conditioned eyelid response on the side ipsilateral to the lesions without affecting conditioned responding (CR) on the contralateral side. Also, recording studies involving the interpositus nucleus have consistently revealed patterns of neuronal discharge that predict execution of the CR. The lesion and recording studies have generally been cited as evidence that plasticity in the cerebellum is critically involved in the learning and memory of classically conditioned responses. This interpretation was recently challenged by Welsh and Harvey (1989a), who claimed that cerebellar lesions simply produced a performance deficit and speculated that the role of the cerebellum was not in learning and memory processes associated with the CR but only in performance of the eye blink response. Presented here are three experiments that provide additional strong evidence for a critical role of the cerebellum in the learning and memory of the Pavlovian CR. These experiments include (1) demonstrations of complete and permanent CR abolition after appropriate interpositus lesions, (2) a failure to find systematic or persisting decrements in the unconditioned response amplitude (i.e., the eye blink reflex) after appropriate interpositus lesion, and (3) observations of differential effects on the CR and unconditioned response after lesions were placed in populations of motoneurons responsible for executing the eye blink response. These data are discussed in the context of performance versus learning issues; evidence presented here rules out the possibility that interpositus lesion abolition of the eye blink CR is simply due to lesion effects on performance.

It is now clear that there are a number of different forms or aspects of learning and memory that involve different brain systems. Broadly, memory phenomena have been categorized as explicit or implicit. Thus, explicit memories for experience involve the hippocampus-medial temporal lobe system and implicit basic associative learning and memory involves the cerebellum, amygdala, and other systems. Under normal conditions, however, many of these brain-memory systems are engaged to some degree in learning situations. But each of these brain systems is learning something different about the situation. The cerebellum is necessary for classical conditioning of discrete behavioral responses (eyeblink, limb f lexion) under all conditions; however, in the ''trace'' procedure where a period of no stimuli intervenes between the conditioned stimulus and the unconditioned stimulus the hippocampus plays a critical role. Trace conditioning appears to provide a simple model of explicit memory where analysis of brain substrates is feasible. Analysis of the role of the cerebellum in basic delay conditioning (stimuli overlap) indicates that the memories are formed and stored in the cerebellum. The phenomenon of cerebellar long-term depression is considered as a putative mechanism of memory storage.

Monoamine oxidase (MAO) catalyzes the oxidative deamination of a number of biogenic amines, including the key neurotransmitters serotonin (5-HT), norepinephrine (NE), and dopamine (DA) and the neuromodulator phenylethylamine (PEA). Two forms of MAO, designated "MAO A" and "MAO B," have been identified on the basis of biochemical properties and, subsequently, by cloning the relevant genes. Of the two, MAO A exhibits a higher affinity for 5-HT and NE and for the inhibitor clorgyline (Johnston 1968), whereas MAO B has a higher affinity for PEA, benzylamine, and the inhibitor deprenyl (Knoll and Magyar 1972). DA is a substrate for both MAO A and MAO B. Although most tissues express both isoenzymes, human placenta and fibroblasts express predominantly MAO A, and platelets and lymphocytes express only MAO B (for review, see Shih et al. 1999).The ability of the MAOs to catabolize neurotransmitters has made these enzymes attractive candidates in the study of neurological diseases and psychiatric and behavioral traits. Indeed, even before the genes for MAO A and B were cloned, the role of MAO B in psychiatric disorders was widely studied. Platelets, which are easily obtained and lack MAO A expression, were the cell type of choice for much of this work. Low platelet MAO B activity has been associated with bipolar disorder, suicidal behavior, and alcoholism (Devor et al. 1993), as well as with sensation seeking and poor impulse control (Oreland 1993;Holschneider and Shih 1998). However, much of this biochemical work may need to be revisited, given the recent finding that smoking inhibits both MAO A activity and MAO B activity (Fowler et al. 1996a(Fowler et al. , 1996b. For instance, after correcting for the effect of smoking, Simpson et al. (1999) determined that MAO