Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Activation of an apical

The generation of an excitatory receptor current in mammalian olfactory sensory neurons (OSNs) involves the sequential activation of two distinct types of ion channels: cAMP-gated Ca 2ϩ -permeable cation channels and Ca

Ca v 1.3 (␣1D) L-type Ca2ϩ channels have been implicated in substantia nigra (SN) dopamine (DA) neuron pacemaking and vulnerability to Parkinson's disease. These effects may arise from the depolarizing current and cytoplasmic Ca 2ϩ elevation produced by Ca v 1.3 channels at subthreshold membrane potentials. However, the assumption that the Ca 2ϩ selectivity of Ca v 1.3 channels is essential has not been tested. In this study the properties of SN DA neuron L-type Ca 2ϩ channels responsible for driving pacemaker activity in juvenile rat brain slices were probed by replacing native channels blocked with the dihydropyridine nimodipine with virtual channels generated by dynamic clamp. Surprisingly, virtual L-type channels that mimic native and recombinant Ca v 1.3 channels supported pacemaker activity even though dynamic clamp currents are not carried by Ca 2ϩ . This effect is specific because pacemaker activity could not be restored by tonic current injection, virtual nonselective leak channels or virtual NMDA receptors, which share with L-type channels a negative slope conductance region in their current-voltage (I-V) curve. Altering virtual channels showed that the production of pacemaker activity depended on the characteristic voltage dependence of DA neuron L-type channels, while activation kinetics and reversal potential were not critical parameters. Virtual L-type channels also supported slow oscillatory potentials and enhanced firing rate during evoked bursts. Thus, Ca v 1.3 channel voltage dependence, rather than Ca 2ϩ selectivity, drives pacemaker activity and amplifies bursts in SN DA neurons.

Recent evidence suggests that Cl(-) ion channels are important for retinal integrity. Bestrophin Cl(-) channel mutations in humans are genetically linked to a juvenile form of macular degeneration, and disruption of some ClC Cl(-) channels in mice leads to retinal degeneration. In both cases, accumulation of lipofuscin pigment is a key feature of the cellular degeneration. Because Cl(-) channels regulate the ionic environment inside organelles in the endosomal-lysosomal pathway, retinal degeneration may result from defects in lysosomal trafficking or function.

Bursting activity by midbrain dopamine neurons reflects the complex interplay between their intrinsic pacemaker activity and synaptic inputs. Although the precise mechanism responsible for the generation and modulation of bursting in vivo has yet to be established, several ion channels have been implicated in the process. Previous studies with nonselective blockers suggested that ether-a-go-go-related gene (ERG) K+ channels are functionally significant. Here, electrophysiology with selective chemical and peptide ERG channel blockers (E-4031 and rBeKm-1) and computational methods were used to define the contribution made by ERG channels to the firing properties of midbrain dopamine neurons in vivo and in vitro. Selective ERG channel blockade increased the frequency of spontaneous activity as well as the response to depolarizing current pulses without altering spike frequency adaptation. ERG channel block also accelerated entry into depolarization inactivation during bursts elicited by virtual NMDA receptors generated with the dynamic clamp, and significantly prolonged the duration of the sustained depolarization inactivation that followed pharmacologically evoked bursts. In vivo, somatic ERG blockade was associated with an increase in bursting activity attributed to a reduction in doublet firing. Taken together, these results show that dopamine neuron ERG K+ channels play a prominent role in limiting excitability and in minimizing depolarization inactivation. As the therapeutic actions of antipsychotic drugs are associated with depolarization inactivation of dopamine neurons and blockade of cardiac ERG channels is a prominent side effect of these drugs, ERG channels in the central nervous system may represent a novel target for antipsychotic drug development.

Packaging by the vesicular monoamine transporter (VMAT) is essential for mood-controlling serotonin transmission, but has not been assayed during activity. Here, two-photon imaging of the fluorescent serotonin analog 5,7-dihydroxytryptamine and three-photon imaging of endogenous serotonin were used to study vesicular packaging as it supports release from the soma of serotonin neurons. Glutamate receptor activation in dorsal raphe brain slice evoked somatic release that was mediated solely by vesicle exocytosis. This release was accompanied by VMAT-mediated serotonin depletion from the nucleus, a large compartment free of monoaminergic degradation pathways that has not been implicated in neurotransmission previously. Finally, while some monoamine packaged at rest was held in reserve, monoamine packaged during stimulation was released completely. Hence, somatic vesicles loaded by VMAT during activity rapidly undergo exocytosis. In the absence of active zones and with limited neurotransmitter reuptake, somatic release by serotonin neurons is supported by recruitment from a large pool of extra-vesicular serotonin in the nucleus and cytoplasm, and preferential release of the newly packaged transmitter. KeywordsVMAT2; vesicular transport; multiphoton microscopy; somatodendritic release; fluorescent false neurotransmitter; SERT Dorsal raphe nucleus (DR) serotonin neurons project widely throughout the forebrain to control mood and behavior. In addition to synaptic release from nerve terminals, DR neurons also release serotonin (5-hydroxytryptamine) extra-synaptically from somatodendritic compartments in a paracrine manner to regulate activity (Bunin and Wightman, 1998;Adell et al., 2002;De-Miguel and Trueta, 2005). Similar to synaptic release, somatodendritic release in serotonin neurons is mediated by calcium-induced vesicle exocytosis (de Kock et al., 2006), suggesting a critical role of the transport of serotonin by the vesicular monoaminergic transporter (VMAT). In fact, VMAT alterations in the DR have been implicated in the pathology and pharmacotherapy of psychiatric disorders (Cordeiro et al., 2002;Schwartz et al., 2003). However, VMAT dependent packaging during release has not been assayed directly in vivo.In principle, optical assays of neurotransmitter dynamics could be used to study activitydependent monoamine packaging and release in DR serotonin neurons. 5,7-dihydroxytryptamine (dHT) is a pH sensitive, fluorescent serotonin analog (Schlossberger, 1978;Vaney, 1986;Kim et al., 2000) that accumulates in serotonin neurons through the serotonin transporter (SERT) (Bjorklund et al., 1974) and can be detected in serotonin neuron vesicles (Gershon and Sherman, 1982). Prolonged exposure to high concentrations of dHT is *corresponding author (elevitan@pitt.edu, 412-648-9486 NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript toxic, but this effect is inhibited by preventing its chemical and enzymatic oxidation (Bjorklund et al., 1975;Silva et al., 1988). Indeed, acute dHT uptake identif...

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