Neuroscience. From molecules to memory in the cerebellum

Eyeblink conditioning in patients with hereditary ataxia: a one-year follow-up study

Delay eyeblink conditioning was examined in patients with genetically-defined heredoataxias and age-matched control subjects. 24 patients with spinocerebellar ataxia type 6 (SCA6), type 3 (SCA3), and Friedreich's ataxia (FRDA) participated. SCA6 affects primarily the cerebellum, whereas extracerebellar involvement is common in SCA3 and FRDA. Testing was performed in three sessions six months apart. Severity of ataxia was defined based on the International Ataxia Cooperative Rating Scale (ICARS). As expected, cerebellar patients were significantly impaired in eyeblink conditioning compared to controls. Signs of retention and further learning across sessions were present in controls, but not in the cerebellar patients. In addition, findings of disturbed timing of conditioned responses were observed. Both onsets and peaks of the conditioned responses (CRs) occurred significantly earlier in cerebellar patients. Shortened CR responses were most prominent in patients with primarily cerebellar cortical disease (SCA6). In the group of all cerebellar patients, the SCA3 and the FRDA group correlations between learning deficits and clinical findings were weak. Moderate-to-strong correlations were found in the SCA6 patients. There was no significant change, however, in clinical ataxia scores and CR incidence across the three sessions. In summary, impaired learning of conditioned eyeblink responses is a stable finding across multiple sessions in patients with degenerative cerebellar disorders. Eyeblink conditioning may be a useful measure of cerebellar impairment in patients with hereditary ataxias that primarily affect the cerebellum (such as SCA6). In other heredoataxias (such as SCA3 and FRDA), extracerebellar involvement not assessed by ICARS likely contributes to eyeblink conditioning abnormalities.

Cerebellar ataxia and Purkinje cell dysfunction caused by Ca2+-activated K+ channel deficiency

Malfunctions of potassium channels are increasingly implicated as causes of neurological disorders. However, the functional roles of the large-conductance voltage- and Ca(2+)-activated K(+) channel (BK channel), a unique calcium, and voltage-activated potassium channel type have remained elusive. Here we report that mice lacking BK channels (BK(-/-)) show cerebellar dysfunction in the form of abnormal conditioned eye-blink reflex, abnormal locomotion and pronounced deficiency in motor coordination, which are likely consequences of cerebellar learning deficiency. At the cellular level, the BK(-/-) mice showed a dramatic reduction in spontaneous activity of the BK(-/-) cerebellar Purkinje neurons, which generate the sole output of the cerebellar cortex and, in addition, enhanced short-term depression at the only output synapses of the cerebellar cortex, in the deep cerebellar nuclei. The impairing cellular effects caused by the lack of postsynaptic BK channels were found to be due to depolarization-induced inactivation of the action potential mechanism. These results identify previously unknown roles of potassium channels in mammalian cerebellar function and motor control. In addition, they provide a previously undescribed animal model of cerebellar ataxia.

Temporal properties of cerebellar-dependent memory consolidation

Classical conditioning of the nictitating membrane response in rabbits is a well defined model of cerebellar-dependent motor memory. This memory undergoes a period of consolidation after the training session, when it is sensitive to reversible inactivations of the cerebellar cortex, but not of the cerebellar nuclei, with the GABA(A) receptor agonist muscimol. Here, the temporal properties of this cerebellar cortex-dependent consolidation were examined using delayed infusions of muscimol in cortical lobule HVI. Cortical infusions delayed by 5 or 45 min after a conditioning session produced significant and very similar impairments of consolidation, but infusions delayed by 90 min produced little or no impairment. Behavioral measures indicate that the muscimol infusions produced significant effects after approximately 30 min and they lasted for a few hours. So, over a time window beginning approximately 1 hr after the end of the training session and closing 1 hr after that, intracortical activity is critical for consolidation of this motor memory.

Reversing cerebellar long-term depression

The discovery of a postsynaptically expressed form of cerebellar parallel fiber-Purkinje cell long-term potentiation (LTP) raises the question whether this is the long-sought resetting mechanism for long-term depression (LTD). Extracellular monitoring of PC spikes enables stable prolonged recordings of parallel fiber-Purkinje cell synaptic efficacy. LTD, saturated by repeated induction protocols, can be reversed by a single round of postsynaptic LTP or nitric oxide (NO), enabling LTD to be reinduced. Conversely, after postsynaptic LTP has been saturated, one round of LTD permits fresh postsynaptic LTP. By contrast, after saturation of LTD, induction of presynaptic LTP or application of forskolin leaves LTD still saturated. Likewise, presynaptic LTP cannot be reversed by LTD. Therefore postsynaptic LTP mediated by NO without postsynaptic Ca2+ elevation, unlike presynaptic LTP mediated by cAMP, is a true counterbalance to LTD mediated by coincidence of NO plus postsynaptic Ca2+