Encoding sensory input requires the expression of postsynaptic ion channels to transform key features of afferent input to an appropriate pattern of spike output. Although Ca 2+ -activated K + channels are known to control spike frequency in central neurons, Ca 2+ -activated K + channels of intermediate conductance (KCa3.1) are believed to be restricted to peripheral neurons. We now report that cerebellar Purkinje cells express KCa3.1 channels, as evidenced through single-cell RT-PCR, immunocytochemistry, pharmacology, and single-channel recordings. Furthermore, KCa3.1 channels coimmunoprecipitate and interact with low voltage-activated Cav3.2 Ca 2+ channels at the nanodomain level to support a previously undescribed transient voltage-and Ca 2+ -dependent current. As a result, subthreshold parallel fiber excitatory postsynaptic potentials (EPSPs) activate Cav3 Ca 2+ influx to trigger a KCa3.1-mediated regulation of the EPSP and subsequent after-hyperpolarization. The Cav3-KCa3.1 complex provides powerful control over temporal summation of EPSPs, effectively suppressing low frequencies of parallel fiber input. KCa3.1 channels thus contribute to a high-pass filter that allows Purkinje cells to respond preferentially to high-frequency parallel fiber bursts characteristic of sensory input.C entral neurons receive an enormous number of spontaneously active synaptic inputs, but exhibit the capacity to differentiate features of sensory input from background noise. Cerebellar Purkinje cells are contacted by up to ∼150,000 parallel fibers from granule cells, of which only a subset will convey sensory information at any given time. The activation of a peripheral receptive field is transmitted to the cerebellar cortex by mossy fibers in the form of high-frequency spike bursts (1). The resulting temporal summation of excitatory postsynaptic potentials (EPSPs) generates a similar high-frequency burst in granule cells (2). Purkinje cells should then also possess the means to respond effectively to bursts of parallel fiber input that convey sensory information compared with background activity.Postsynaptic membrane excitability can be controlled by activation of K + channels. There are two established types of Ca 2+ -activated K + (KCa) channels in CNS neurons: small conductance (SK, KCa2.x) and big conductance (BK, KCa1.1) (3, 4). A third class of intermediate conductance (KCa3.1, SK4, IK1) KCa channel is thought to be expressed only in microglia and endothelial cells in the CNS (3, 5, 6). KCa3.1 channels are gated by calmodulin in a similar manner to KCa2.x channels but are insensitive to block by apamin and tetraethylammonium (TEA) (6-8). Instead, the KCa3.1 α-subunit, encoded by the gene KCNN4, has specific residues that bind charybdotoxin and 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (TRAM-34) (5-7, 9, 10).In cerebellar Purkinje cells, KCa1.1 and KCa2.2 channels are activated during a spike by high voltage-activated (HVA) P-type Ca 2+ channels (11). In contrast, low voltage-activated (LVA) Cav3 (T-type) Ca 2+ channe...

The expression pattern and functional roles for calcium-activated potassium channels of the KCa2.x family and KCa1.1 have been extensively examined in central neurons. Recent work indicates that intermediate conductance calcium-activated potassium channels (KCa3.1) are also expressed in central neurons of the cerebellum and spinal cord. The current study used immunocytochemistry and GFP linked to KCNN4 promoter activity in a transgenic mouse to determine the expression pattern of KCa3.1 channels in rat or mouse neocortex, hippocampus, thalamus, and cerebellum. KCa3.1 immunolabel and GFP expression were closely matched and detected in both excitatory and inhibitory cells of all regions examined. KCa3.1 immunolabel was localized primarily to the somatic region of excitatory cells in cortical structures but at the soma and over longer segments of dendrites of cells in deep cerebellar nuclei. More extensive labeling was apparent for inhibitory cells at the somatic and dendritic level with no detectable label associated with axon tracts or regions of intense synaptic innervation. The data indicate that KCa3.1 channels are expressed in the CNS with a differential pattern of distribution between cells, suggesting important functional roles for these calcium-activated potassium channels in regulating the excitability of central neurons.

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