Acute and chronic effects of climbing fiber lesions on cerebellar cyclic guanosine monophosphate

Climbing Fibres as a Source of Nitric Oxide in the Cerebellum

Exposure of adult rat cerebellar slices to a moderately raised K+ concentration (15 mM) caused a large (30-fold) rise in the levels of cyclic GMP. Excitatory amino acid antagonists failed to inhibit this response, nor could it be mimicked by agonists active at a number of other transmitter receptors. It was, however, inhibited by the nitric oxide (NO) synthase antagonist, l-methylarginine (IC50=10 microM), and also by tetrodotoxin (1 microM) implying that underlying the cyclic GMP response was an action potential-dependent formation of NO. Prelesioning of climbing fibres resulted in a loss of approximately 50% of the response to K+ but failed to influence the effects of glutamate receptor agonists or the NO-donor, nitroprusside. These findings point to a new mechanism for the formation of NO in the central nervous system and suggest that, in the cerebellum, climbing fibres are a source of NO.

Cerebellar long-term depression: characterization, signal transduction, and functional roles

Cerebellar Purkinje cells exhibit a unique type of synaptic plasticity, namely, long-term depression (LTD). When two inputs to a Purkinje cell, one from a climbing fiber and the other from a set of granule cell axons, are repeatedly associated, the input efficacy of the granule cell axons in exciting the Purkinje cell is persistently depressed. Section I of this review briefly describes the history of research around LTD, and section II specifies physiological characteristics of LTD. Sections III and IV then review the massive data accumulated during the past two decades, which have revealed complex networks of signal transduction underlying LTD. Section III deals with a variety of first messengers, receptors, ion channels, transporters, G proteins, and phospholipases. Section IV covers second messengers, protein kinases, phosphatases and other elements, eventually leading to inactivation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolone-propionate-selective glutamate receptors that mediate granule cell-to-Purkinje cell transmission. Section V defines roles of LTD in the light of the microcomplex concept of the cerebellum as functionally eliminating those synaptic connections associated with errors during repeated exercises, while preserving other connections leading to the successful execution of movements. Section VI examines the validity of this microcomplex concept based on the data collected from recent numerous studies of various forms of motor learning in ocular reflexes, eye-blink conditioning, posture, locomotion, and hand/arm movements. Section VII emphasizes the importance of integrating studies on LTD and learning and raises future possibilities of extending cerebellar research to reveal memory mechanisms of implicit learning in general.

Reduction of GABAA and GABAB receptor densities in the cerebellar cortex from 3-acetylpyridine-induced ataxic rats

We measured GABAA and GABAB receptor densities in the cerebellar cortex from 3-acetylpyridine-induced ataxic rats using receptor autoradiography. GABAA and GABAB receptor densities were significantly reduced both in the molecular layer and the granule cell layer. Reduction of GABA receptor densities may be induced by loss of GABA receptors on the degenerated climbing fibers or by secondary or compensatory changes of neuronal activities in the cerebellum.