The Ca 2؉ channel ␣1A-subunit is a voltage-gated, pore-forming membrane protein positioned at the intersection of two important lines of research: one exploring the diversity of Ca 2؉ channels and their physiological roles, and the other pursuing mechanisms of ataxia, dystonia, epilepsy, and migraine. ␣1A-Subunits are thought to support both P-and Q-type Ca 2؉ channel currents, but the most direct test, a null mutant, has not been described, nor is it known which changes in neurotransmission might arise from elimination of the predominant Ca 2؉ delivery system at excitatory nerve terminals. We generated ␣1A-deficient mice (␣1A ؊͞؊ ) and found that they developed a rapidly progressive neurological deficit with specific characteristics of ataxia and dystonia before dying Ϸ3-4 weeks after birth. P-type currents in Purkinje neurons and P-and Q-type currents in cerebellar granule cells were eliminated completely whereas other Ca 2؉ channel types, including those involved in triggering transmitter release, also underwent concomitant changes in density. Synaptic transmission in ␣1A ؊͞؊ hippocampal slices persisted despite the lack of P͞Q-type channels but showed enhanced reliance on N-type and R-type Ca 2؉ entry. The ␣1A ؊͞؊ mice provide a starting point for unraveling neuropathological mechanisms of human diseases generated by mutations in ␣1A. The ␣ 1A -subunit, the most abundant ␣ 1 -subunit in vertebrate brain (1), mediates Ca 2ϩ influx across presynaptic and somatodendritic membranes, thereby triggering fast neurotransmitter release and other key neuronal responses (2-5). Because of its high expression levels in the brain, the ␣ 1A -subunit was the first representative of its subclass to be isolated by cDNA cloning (1, 6). This predominantly neuronal subclass also includes ␣ 1B (N-type Ca 2ϩ channel) and ␣ 1E [possibly R-type Ca 2ϩ channel (7-9)] and is referred to as ABE or Ca V 2. There is no information to date on the behavioral or electrophysiological consequences of deleting a member of the ABE subfamily.␣ 1A Transcripts are widely distributed in rat (10) and human brain (11), most prominently in cell body layers in cerebellum and hippocampus. At the subcellular level, ␣ 1A immunoreactivity has been found in cell bodies, dendrites, and presynaptic terminals (12). Less clear has been the role of ␣ 1A in supporting Ca 2ϩ channel components defined by biophysical and pharmacological criteria. In either Xenopus oocytes (13, 14) or HEK293 cells (15), expression of ␣ 1A -subunits along with ancillary ␣ 2 ͞␦-and ␤-subunits generated currents with properties closely resembling the Q-type current found in cerebellar granule cells (8) and much less the P-type current first described in cerebellar Purkinje neurons by Llinás and colleagues (16,17). Unlike native P-type channels (18), the expressed currents showed pronounced inactivation during sustained depolarizations and responded to -agatoxin IVA (-Aga-IVA) at half-blocking doses of Ϸ100 nM, not Ϸ1 nM (13). Various explanations for the discrepancies have been advanced...

Functional presynaptic vesicles have been subdivided into readily releasable (RRP) and reserve (RP) pools. We studied recycling properties of RRP vesicles through differential retention of FM1-43 and FM2-10 and by varying the time window for FM dye uptake. Both approaches indicated that vesicles residing in the RRP underwent rapid endocytosis (tau approximately 1s), whereas newly recruited RP vesicles were recycled slowly (tau approximately 30 s). With repeated challenges (hypertonic or electrical stimuli), the ability to release neurotransmitter recovered 10-fold more rapidly than restoration of FM2-10 destaining. Finding neurotransmission in the absence of destaining implied that rapidly endocytosed RRP vesicles were capable of reuse, a process distinct from repopulation from the RP. Reuse would greatly expand the functional capabilities of a limited number of vesicles in CNS terminals, particularly during intermittent bursts of activity.

Several human channelopathies result from mutations in alpha1A, the pore-forming subunit of P/Q-type Ca2+ channels, conduits of presynaptic Ca2+ entry for evoked neurotransmission. We found that wild-type human alpha1A subunits supported transmission between cultured mouse hippocampal neurons equally well as endogenous mouse alpha1A, whereas introduction of impermeant human alpha1A hampered the effect of endogenous subunits. Thus, presynaptic P/Q-type channels may compete for channel type-preferring "slots" that limit their synaptic effectiveness. The existence of slots generates predictions for how neurotransmission might be affected by changes in Ca2+ channel properties, which we tested by studying alpha1A mutations that are associated with familial hemiplegic migraine type 1 (FHM1). Mutant human P/Q-type channels were impaired in contributing to neurotransmission in precise accord with their deficiency in supporting whole-cell Ca2+ channel activity. Expression of mutant channels in wild-type neurons reduced the synaptic contribution of P/Q-type channels, suggesting that competition for type-preferring slots might support the dominant inheritance of FHM1.

Transmission at the mouse neuromuscular junction normally relies on P͞Q-type channels, but became jointly dependent on both Nand R-type Ca 2؉ channels when the P͞Q-type channel ␣1A subunit was deleted. R-type channels lay close to Ca 2؉ sensors for exocytosis and I K(Ca) channel activation, like the P͞Q-type channels they replaced. In contrast, N-type channels were less well localized, but abundant enough to influence secretion strongly, particularly when action potentials were prolonged. Our data suggested that active zone structures may select among multiple Ca 2؉ channels in the hierarchy P͞Q>R>N. The ␣1A؊͞؊ neuromuscular junction displayed several other differences from wild-type: lowered quantal content but greater ability to withstand reductions in the Ca 2؉ ͞Mg 2؉ ratio, and little or no paired-pulse facilitation, the latter findings possibly reflecting compensatory mechanisms at individual release sites. Changes in presynaptic function were also associated with a significant reduction in the size of postsynaptic acetylcholine receptor clusters. neuromuscular junction ͉ ␣1A knockout ͉ paired-pulse facilitation ͉ SNX-482 ͉ Ca 2ϩ -activated potassium channel

We show that alpha and betaCaMKII are inversely regulated by activity in hippocampal neurons in culture: the alpha/beta ratio shifts toward alpha during increased activity and beta during decreased activity. The swing in ratio is approximately 5-fold and may help tune the CaMKII holoenzyme to changing intensities of Ca(2+) signaling. The regulation of CaMKII levels uses distinguishable pathways, one responsive to NMDA receptor blockade that controls alphaCaMKII alone, the other responsive to AMPA receptor blockade and involving betaCaMKII and possibly further downstream effects of betaCaMKII on alphaCaMKII. Overexpression of alphaCaMKII or betaCaMKII resulted in opposing effects on unitary synaptic strength as well as mEPSC frequency that could account in part for activity-dependent effects observed with chronic blockade of AMPA receptors. Regulation of CaMKII subunit composition may be important for both activity-dependent synaptic homeostasis and plasticity.

Many neurons of the central nervous system display multiple high voltage-activated Ca 2؉ currents, pharmacologically classified as L-, N-, P-, Q-, and R-type. Of these current types, the R-type is the least understood. The leading candidate for the molecular correlate of R-type currents in cerebellar granule cells is the ␣ 1E subunit, which yields Ca 2؉ currents very similar to the R-type when expressed in heterologous systems. As a complementary approach, we tested whether antisense oligonucleotides against ␣ 1E could decrease the expression of R-type current in rat cerebellar granule neurons in culture. Cells were supplemented with either antisense or sense oligonucleotides and whole-cell patch clamp recordings were obtained after 6-8 days in vitro. Incubation with ␣ 1E antisense oligonucleotide caused a 52.5% decrease in the peak R-type current density, from ؊10 ؎ 0.6 picoamperes͞picofarad (pA͞pF) (n ‫؍‬ 6) in the untreated controls to ؊4.8 ؎ 0.8 pA͞pF (n ‫؍‬ 11) (P < 0.01). In contrast, no significant changes in the current expression were seen in sense oligonucleotide-treated cells (؊11.3 ؎ 3.2 pA͞pF). The specificity of the ␣ 1E antisense oligonucleotides was supported by the lack of change in estimates of the P͞Q current amplitude. Furthermore, antisense and sense oligonucleotides against ␣ 1A did not affect R-type current expression (؊11.5 ؎ 1.7 and ؊11.7 ؎ 1.7 pA͞pF, respectively), whereas the ␣ 1A antisense oligonucleotide significantly reduced whole cell currents under conditions in which P͞Q current is dominant. Our results support the hypothesis that members of the E class of ␣ 1 subunits support the high voltage-activated R-type current in cerebellar granule cells.Individual nerve cells in the vertebrate nervous system express several types of voltage-gated Ca 2ϩ channel (1-4), as many as five or six channel types distinguishable in some neurons (5). These channels work together to support fundamental cellular activities such as membrane excitation, neurotransmitter release, neurite outgrowth, and gene expression (6, 7). Considerable advances have been made in the understanding of the relationship between channel types, defined by their biophysical and pharmacological characteristics, and their underlying ␣ 1 subunits isolated by biochemistry and molecular biology (8)(9)(10)(11)(12). It is clear that L-type currents are supported by dihydropyridine-sensitive ␣ 1C or ␣ 1D subunits (13,14) and N-type currents are generated by -conotoxin-GVIA-sensitive ␣ 1B subunits (15). Likewise, P-and Q-type currents are likely to arise from -Aga-IVA and -CTx-MVIIC-sensitive ␣ 1A subunits (16-21).Among the major categories of Ca 2ϩ channels uncovered so far, R-type channels were the most recently defined and remain the least well-understood. R-type currents were first identified in rat cerebellar granule neurons (22, 23) and were found to be pharmacologically and kinetically distinguishable from L-, N-, P-, and Q-type currents in the same cells (5). The importance of R-type channels for dendritic Ca 2ϩ entr...

J. Neurochem. (2011) 118, 224–236. Abstract Physical exercise produces a variety of psychophysical effects, including altered pain perception. Elevated levels of centrally produced endorphins or endocannabinoids are implicated as mediators of exercise‐induced analgesia. The effect of exercise on the development and persistence of disease‐associated acute/chronic pain remains unclear. In this study, we quantified the physiological consequence of forced‐exercise on the development of diabetes‐associated neuropathic pain. Euglycemic control or streptozotocin (STZ)‐induced diabetic adult male rats were subdivided into sedentary or forced‐exercised (2–10 weeks, treadmill) subgroups and assessed for changes in tactile responsiveness. Two weeks following STZ‐treatment, sedentary rats developed a marked and sustained hypersensitivity to von Frey tactile stimulation. By comparison, STZ‐treated diabetic rats undergoing forced‐exercise exhibited a 4‐week delay in the onset of tactile hypersensitivity that was independent of glucose control. Exercise‐facilitated analgesia in diabetic rats was reversed, in a dose‐dependent manner, by naloxone. Small‐diameter (< 30 μm) DRG neurons harvested from STZ‐treated tactile hypersensitive diabetic rats exhibited an enhanced (2.5‐fold) rightward (depolarizing) shift in peak high‐voltage activated (HVA) Ca2+ current density with a concomitant appearance of a low‐voltage activated (LVA) Ca2+ current component. LVA Ca2+ currents present in DRG neurons from hypersensitive diabetic rats exhibited a marked depolarizing shift in steady‐state inactivation. Forced‐exercise attenuated diabetes‐associated changes in HVA Ca2+ current density while preventing the depolarizing shift in steady‐state inactivation of LVA Ca2+ currents. Forced‐exercise markedly delays the onset of diabetes‐associated neuropathic pain, in part, by attenuating associated changes in HVA and LVA Ca2+ channel function within small‐diameter DRG neurons possibly by altering opioidergic tone.

The expansion of polyglutamine tracts encoded by CAG trinucleotide repeats is a common mutational mechanism in inherited neurodegenerative diseases. Spinocerebellar ataxia type 6 (SCA6), an autosomal dominant, progressive disease, arises from trinucleotide repeat expansions present in the coding region of CACNA1A (chromosome 19p13). This gene encodes alpha(1A), the principal subunit of P/Q-type Ca(2+) channels, which are abundant in the CNS, particularly in cerebellar Purkinje and granule neurons. We assayed ion channel function by introduction of human alpha(1A) cDNAs in human embryonic kidney 293 cells that stably coexpressed beta(1) and alpha(2)delta subunits. Immunocytochemical analysis showed a rise in intracellular and surface expression of alpha(1A) protein when CAG repeat lengths reached or exceeded the pathogenic range for SCA6. This gain at the protein level was not a consequence of changes in RNA stability, as indicated by Northern blot analysis. The electrophysiological behavior of alpha(1A) subunits containing expanded (EXP) numbers of CAG repeats (23, 27, and 72) was compared against that of wild-type subunits (WT) (4 and 11 repeats) using standard whole-cell patch-clamp recording conditions. The EXP alpha(1A) subunits yielded functional ion channels that supported inward Ca(2+) channel currents, with a sharp increase in P/Q Ca(2+) channel current density relative to WT. Our results showed that Ca(2+) channels from SCA6 patients display near-normal biophysical properties but increased current density attributable to elevated protein expression at the cell surface.