K+ channel principal subunits are by far the largest and most diverse of the ion channels. This diversity originates partly from the large number of genes coding for K+ channel principal subunits, but also from other processes such as alternative splicing, generating multiple mRNA transcripts from a single gene, heteromeric assembly of different principal subunits, as well as possible RNA editing and posttranslational modifications. In this chapter, we attempt to give an overview (mostly in tabular format) of the different genes coding for K+ channel principal and accessory subunits and their genealogical relationships. We discuss the possible correlation of different principal subunits with native K+ channels, the biophysical and pharmacological properties of channels formed when principal subunits are expressed in heterologous expression systems, and their patterns of tissue expression. In addition, we devote a section to describing how diversity of K+ channels can be conferred by heteromultimer formation, accessory subunits, alternative splicing, RNA editing and posttranslational modifications. We trust that this collection of facts will be of use to those attempting to compare the properties of new subunits to the properties of others already known or to those interested in a comparison between native channels and cloned candidates.

Four mammalian Kv3 genes have been identified, each of which generates, by alternative splicing, multiple protein products differing in their C-terminal sequence. Products of the Kv3.1 and Kv3.2 genes express similar delayed-rectifier type currents in heterologous expression systems, while Kv3.3 and Kv3.4 proteins express A-type currents. All Kv3 currents activate relatively fast at voltages more positive than -10 mV, and deactivate very fast. The distribution of Kv3 mRNAs in the rodent CNS was studied by in situ hybridization, and the localization of Kv3.1 and Kv3.2 proteins has been studied by immunohistochemistry. Most Kv3.2 mRNAs (approximately 90%) are present in thalamic-relay neurons throughout the dorsal thalamus. The protein is expressed mainly in the axons and terminals of these neurons. Kv3.2 channels are thought to be important for thalamocortical signal transmission. Kv3.1 and Kv3.2 proteins are coexpressed in some neuronal populations such as in fast-spiking interneurons of the cortex and hippocampus, and neurons in the globus pallidus. Coprecipitation studies suggest that in these cells the two types of protein form heteromeric channels. Kv3 proteins appear to mediate, in native neurons, similar currents to those seen in heterologous expression systems. The activation voltage and fast deactivation rates are believed to allow these channels to help repolarize action potentials fast without affecting the threshold for action potential generation. The fast deactivating current generates a quickly recovering after hyperpolarization, thus maximizing the rate of recovery of Na+ channel inactivation without contributing to an increase in the duration of the refractory period. These properties are believed to contribute to the ability of neurons to fire at high frequencies and to help regulate the fidelity of synaptic transmission. Experimental evidence has now become available showing that Kv3.1-Kv3.2 channels play critical roles in the generation of fast-spiking properties in cortical GABAergic interneurons.

Kv3.1 and Kv3.2 K(+) channel proteins form similar voltage-gated K(+) channels with unusual properties, including fast activation at voltages positive to -10 mV and very fast deactivation rates. These properties are thought to facilitate sustained high-frequency firing. Kv3.1 subunits are specifically found in fast-spiking, parvalbumin (PV)-containing cortical interneurons, and recent studies have provided support for a crucial role in the generation of the fast-spiking phenotype. Kv3.2 mRNAs are also found in a small subset of neocortical neurons, although the distribution of these neurons is different. We raised antibodies directed against Kv3.2 proteins and used dual-labeling methods to identify the neocortical neurons expressing Kv3.2 proteins and to determine their subcellular localization. Kv3.2 proteins are prominently expressed in patches in somatic and proximal dendritic membrane as well as in axons and presynaptic terminals of GABAergic interneurons. Kv3.2 subunits are found in all PV-containing neurons in deep cortical layers where they probably form heteromultimeric channels with Kv3.1 subunits. In contrast, in superficial layer PV-positive neurons Kv3.2 immunoreactivity is low, but Kv3.1 is still prominently expressed. Because Kv3.1 and Kv3.2 channels are differentially modulated by protein kinases, these results raise the possibility that the fast-spiking properties of superficial- and deep-layer PV neurons are differentially regulated by neuromodulators. Interestingly, Kv3. 2 but not Kv3.1 proteins are also prominent in a subset of seemingly non-fast-spiking, somatostatin- and calbindin-containing interneurons, suggesting that the Kv3.1-Kv3.2 current type can have functions other than facilitating high-frequency firing.

Voltage-gated K ϩ channels of the Kv3 subfamily have unusual electrophysiological properties, including activation at very depolarized voltages (positive to Ϫ10 mV) and very fast deactivation rates, suggesting special roles in neuronal excitability. In the brain, Kv3 channels are prominently expressed in select neuronal populations, which include fast-spiking (FS) GABAergic interneurons of the neocortex, hippocampus, and caudate, as well as other high-frequency firing neurons. Although evidence points to a key role in high-frequency firing, a definitive understanding of the function of these channels has been hampered by a lack of selective pharmacological tools. We therefore generated mouse lines in which one of the Kv3 genes, Kv3.2, was disrupted by gene-targeting methods. Whole-cell electrophysiological recording showed that the ability to fire spikes at high frequencies was impaired in immunocytochemically identified FS interneurons of deep cortical layers (5-6) in which Kv3.2 proteins are normally prominent. No such impairment was found for FS neurons of superficial layers (2-4) in which Kv3.2 proteins are normally only weakly expressed. These data directly support the hypothesis that Kv3 channels are necessary for high-frequency firing. Moreover, we found that Kv3.2 Ϫ/Ϫ mice showed specific alterations in their cortical EEG patterns and an increased susceptibility to epileptic seizures consistent with an impairment of cortical inhibitory mechanisms. This implies that, rather than producing hyperexcitability of the inhibitory interneurons, Kv3.2 channel elimination suppresses their activity. These data suggest that normal cortical operations depend on the ability of inhibitory interneurons to generate high-frequency firing. Approximately 10 -20% of the neurons in the cerebral cortex are inhibitory GABAergic interneurons. These cells play a critical role in a number of important functions, including the gating and processing of sensory information, the establishment and plasticity of sensory receptive fields, the synchronization of cortical circuits, the generation of rhythms, and the limiting of seizure activity (Fairen et al., 1984;Gilbert, 1993;Jones, 1993;Amitai and Connors, 1995;Keller, 1995;Singer and Gray, 1995;Freund and Buzsaki, 1996;Jefferys et al., 1996;Steriade, 1997).Cortical GABAergic interneurons represent a heterogenous population of cells with subtypes differing in morphological appearance, expression of specific markers such as calcium-binding proteins or neuropeptides, firing patterns, synaptic properties, and axonal connectivity (Jones, 1975;Somogyi et al., 1984;Hendry et al., 1989;Freund and Buzsaki, 1996;Cauli et al., 1997;Gonchar and Burkhalter, 1997;Kawaguchi and Kubota, 1997;Gupta et al., 2000).The largest group of neocortical inhibitory interneurons (ϳ50%) consists of cells that contain the calcium-binding protein parvalbumin (PV). These neurons are characterized by a "fast-spiking" firing pattern, i.e., the ability to fire long trains of very brief action potentials at high frequen...

Histamine-containing neurons of the tuberomammilary nucleus project to the hippocampal formation to innervate H1 and H2 receptors on both principal and inhibitory interneurons. Here we show that H2 receptor activation negatively modulates outward currents through Kv3.2-containing potassium channels by a mechanism involving PKA phosphorylation in inhibitory interneurons. PKA phosphorylation of Kv3.2 lowered the maximum firing frequency of inhibitory neurons, which in turn negatively modulated high-frequency population oscillations recorded in principal cell layers. All these effects were absent in a Kv3.2 knockout mouse. These data reveal a novel pathway for histamine-dependent regulation of high-frequency oscillations within the hippocampal formation.