Action of ethanol and zolpidem on gamma-aminobutyric acid responses from cerebellar Purkinje neurons: relationship to beta-adrenergic receptor input

The observation that cerebellar Purkinje cells contain type-I benzodiazepine-sensitive GABA(A) receptors is consistent with findings in the present work that the majority of Purkinje neurons are sensitive to enhancement of GABA by the type-1 benzodiazepine agonist, zolpidem. Previous work has demonstrated a relation between zolpidem and ethanol enhancement of GABA responses in several brain regions, but had not tested Purkinje neurons. Therefore, given that a majority of Purkinje neurons were found to be sensitive to zolpidem, ethanol would have been expected to enhance GABA responses from this cell type. However, in agreement with earlier electrophysiological studies, ethanol enhanced GABA inhibitory responses from only a small proportion of these cerebellar Purkinje neurons. Rather than enhancement of GABA, local application of ethanol either inhibited or did not affect responses to GABA from a majority of cerebellar-Purkinje neurons. Nonetheless, as previously reported, a portion of the Purkinje neurons initially insensitive to ethanol enhancement of GABA became sensitive to this action of ethanol with co-application of the beta-adrenergic agonist, isoproterenol. Thus, these results collectively implicate a beta-adrenergic input dependency for ethanol enhancement of GABA from some, but not all, cerebellar Purkinje neurons sensitive to zolpidem. Because a beta-adrenergic input did not allow ethanol enhancement of GABA from all Purkinje neurons, future studies should explore the possibility that other auxiliary neural inputs to zolpidem-sensitive cerebellar Purkinje neurons may be required for ethanol enhancement of GABA responsiveness when a beta-adrenergic input does not have this action. Likewise, knowing that the action of zolpidem can predict ethanol enhancement of GABA in other brain regions, the present findings suggest that a future determination be made concerning whether zolpidem-sensitive neurons in these other regions of brain require a beta-adrenergic or an alternative neural input for ethanol enhancement of GABA responses.

Ataxia and altered dendritic calcium signaling in mice carrying a targeted null mutation of the calbindin D28k gene

Intracellular calcium-binding proteins are abundantly expressed in many neuronal populations. Previous evidence suggests that calcium-binding proteins can modulate various neuronal properties, presumably by their action as calcium buffers. The importance of calcium-binding proteins for nervous system function in an intact integrated system is, however, less clear. To investigate the physiological role of a major endogenous calcium-binding protein, calbindin D28k (calbindin) in vivo, we have generated calbindin null mutant mice by gene targeting. Surprisingly, calbindin deficiency does not affect general parameters of development and behavior or the structure of the nervous system at the light microscopic level. Null mutants are, however, severely impaired in tests of motor coordination, suggesting functional deficits in cerebellar pathways. Purkinje neurons, the only efferent of the cerebellar cortex, and inferior olive neurons, the source of the climbing fiber afferent, have previously been shown to express calbindin. Correlated with this unusual type of ataxia, confocal calcium imaging of Purkinje cells in cerebellar slices revealed marked changes of synaptically evoked postsynaptic calcium transients. Their fast, but not their slow, decay component had larger amplitudes in null mutant than in wild-type mice. We conclude that endogenous calbindin is of crucial importance for integrated nervous system function.