A novel type of calcium channel sensitive to omega-agatoxin-TK in cultured rat cerebral cortical neurons

Control of Glutamate Transport by Extracellular Potassium: Basis for a Negative Feedback on Synaptic Transmission

Glutamate and K+, both released during neuronal firing, need to be tightly regulated to ensure accurate synaptic transmission. Extracellular glutamate and K+ ([K+]o) are rapidly taken up by glutamate transporters and K+-transporters or channels, respectively. Glutamate transport includes the exchange of one glutamate, 3 Na+, and one proton, in exchange for one K+. This K+ efflux allows the glutamate binding site to reorient in the outwardly facing position and start a new transport cycle. Here, we demonstrate the sensitivity of the transport process to [K+]o changes. Increasing [K+]o over the physiological range had an immediate and reversible inhibitory action on glutamate transporters. This K+-dependent transporter inhibition was revealed using microspectrofluorimetry in primary astrocytes, and whole-cell patch-clamp in acute brain slices and HEK293 cells expressing glutamate transporters. Previous studies demonstrated that pharmacological inhibition of glutamate transporters decreases neuronal transmission via extrasynaptic glutamate spillover and subsequent activation of metabotropic glutamate receptors (mGluRs). Here, we demonstrate that increasing [K+]o also causes a decrease in neuronal mEPSC frequency, which is prevented by group II mGluR inhibition. These findings highlight a novel, previously unreported physiological negative feedback mechanism in which [K+]o elevations inhibit glutamate transporters, unveiling a new mechanism for activity-dependent modulation of synaptic activity.

A voltage-gated calcium channel regulates lysosomal fusion with endosomes and autophagosomes and is required for neuronal homeostasis

Autophagy helps deliver sequestered intracellular cargo to lysosomes for proteolytic degradation and thereby maintains cellular homeostasis by preventing accumulation of toxic substances in cells. In a forward mosaic screen in Drosophila designed to identify genes required for neuronal function and maintenance, we identified multiple cacophony (cac) mutant alleles. They exhibit an age-dependent accumulation of autophagic vacuoles (AVs) in photoreceptor terminals and eventually a degeneration of the terminals and surrounding glia. cac encodes an α1 subunit of a Drosophila voltage-gated calcium channel (VGCC) that is required for synaptic vesicle fusion with the plasma membrane and neurotransmitter release. Here, we show that cac mutant photoreceptor terminals accumulate AV-lysosomal fusion intermediates, suggesting that Cac is necessary for the fusion of AVs with lysosomes, a poorly defined process. Loss of another subunit of the VGCC, α2δ or straightjacket (stj), causes phenotypes very similar to those caused by the loss of cac, indicating that the VGCC is required for AV-lysosomal fusion. The role of VGCC in AV-lysosomal fusion is evolutionarily conserved, as the loss of the mouse homologues, Cacna1a and Cacna2d2, also leads to autophagic defects in mice. Moreover, we find that CACNA1A is localized to the lysosomes and that loss of lysosomal Cacna1a in cerebellar cultured neurons leads to a failure of lysosomes to fuse with endosomes and autophagosomes. Finally, we show that the lysosomal CACNA1A but not the plasma-membrane resident CACNA1A is required for lysosomal fusion. In summary, we present a model in which the VGCC plays a role in autophagy by regulating the fusion of AVs with lysosomes through its calcium channel activity and hence functions in maintaining neuronal homeostasis.

A voltage-gated calcium channel required for neurotransmitter release also regulates the fusion of neuronal lysosomes with endosomes and autophagosomes, thereby helping to maintain cellular homeostasis.

Autophagy is a cellular process used by cells to prevent the accumulation of toxic substances. It delivers misfolded proteins and damaged organelles by fusing autophagosomes—organelles formed by a double membrane that surrounds the “debris” to be eliminated—with lysosomes. How this fusion process is regulated during autophagy, however, remains to be established. Here, we analyze this process in flies and mice, and find that loss of different subunits of a specific type of Voltage Gated Calcium Channel (VGCC) leads to defects in lysosomal fusion with autophagosomes in neurons. It was already known that VGCCs control calcium entry at synaptic terminals to promote the fusion of synaptic vesicles with the plasma membrane, and that mutations in the subunits of VGCCs in humans cause neurological diseases. Our data indicate that defects in autophagy and lysosomal fusion are independent of defects in synaptic vesicle fusion and neurotransmitter release, and we show that a specific VGCC is present on lysosomal membranes where it is required for lysosomal fusion with endosomes and autophagosomes. These observations suggest that the fusion events required in autophagy rely on mechanisms similar to those that trigger the fusion of synaptic vesicles with the presynaptic membrane.

Suppression of potassium channels elicits calcium-dependent plateau potentials in suprachiasmatic neurons of the rat

By using whole-cell recordings in acute and organotypic hypothalamic slices, we found that following K+ channel blockade, sustained plateau potentials can be elicited by current injection in suprachiasmatic neurons. In an attempt to determine the ionic basis of these potentials, ion-substitution experiments were carried out. It appeared that to generate plateau potentials, calcium influx was required. Plateau potentials were also present when extracellular calcium was replaced by barium, but were independent upon an increase in the intracellular free calcium concentration. Substitution of extracellular sodium by the impermeant cation N-methyl-D-glucamine indicated that sodium influx could also contribute to plateau potentials. To gain some information on the pharmacological profile of the Ca++ channels responsible for plateau potentials, selective blocker of various types of Ca++ channel were tested. Plateau potentials were unaffected by isradipine, an L-type Ca++ channel blocker. However, they were slightly reduced by omega-conotoxin GVIA and omega-agatoxin TK, blockers of N-type and P/Q-type Ca++ channels, respectively. These data suggest that R-type Ca++ channels probably play a major role in the genesis of plateau potentials. We speculate that neurotransmitters/neuromodulators capable of reducing or suppressing potassium conductance(s) may elicit a Ca++-dependent plateau potential in suprachiasmatic neurons, thus promoting sustained firing activity and neuropeptide release.

The role of Ca2+ channels in the repetitive firing of striatal projection neurons

Blockade of L-type Ca2+ channels results in a decrease in firing frequency of neostriatal neurons. In contrast, N- and P/Q-types of Ca2+ channel cooperate to tune firing pattern, since both of these channel types have to be blocked to enhance firing frequency. Parameters of the intensity-frequency plot were differentially modified by Ca2+ channel antagonists: while L-type Ca2+ channel block reduced the dynamic range by about 80%, block of N- and P/Q-types of Ca2+ channel generated a steeper intensity-frequency plot. These effects are explained in terms of the sustained depolarization and the afterhyperpolarizing potential known to be dependent upon L- and N-, P/Q-types of Ca2+ channels, respectively.

High-affinity inhibition of glutamate release from corticostriatal synapses by omega-agatoxin TK

To know which Ca(2+) channel type is the most important for neurotransmitter release at corticostriatal synapses of the rat, we tested Ca(2+) channel antagonists on the paired pulse ratio. omega-Agatoxin TK was the most effective Ca(2+) channel antagonist (IC(50)=127 nM; maximal effect=211% (with >1 microM) and Hill coefficient=1.2), suggesting a single site of action and a Q-type channel profile. Corresponding parameters for Cd(2+) were 13 microM, 178% and 1.2. The block of L-type Ca(2+) channels had little impact on transmission, but we also tested facilitation of L-type Ca(2+) channels. The L-type Ca(2+) channel agonist, s-(-)-1,4 dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644 (5 microM)), produced a 45% reduction of the paired pulse ratio, suggesting that even if L-type channels do not participate in the release process, they may participate in its modulation.

Ca2+ channels that activate Ca2+-dependent K+ currents in neostriatal neurons

It is demonstrated that not all voltage-gated calcium channel types expressed in neostriatal projection neurons (L, N, P, Q and R) contribute equally to the activation of calcium-dependent potassium currents. Previous work made clear that different calcium channel types contribute with a similar amount of current to whole-cell calcium current in neostriatal neurons. It has also been shown that spiny neurons possess both "big" and "small" types of calcium-dependent potassium currents and that activation of such currents relies on calcium entry through voltage-gated calcium channels. In the present work it was investigated whether all calcium channel types equally activate calcium-dependent potassium currents. Thus, the action of organic calcium channel antagonists was investigated on the calcium-activated outward current. Transient potassium currents were reduced by 4-aminopyridine and sodium currents were blocked by tetrodotoxin. It was found that neither 30 nM omega-Agatoxin-TK, a blocker of P-type channels, nor 200 nM calciseptine or 5 microM nitrendipine, blockers of L-type channels, were able to significantly reduce the outward current. In contrast, 400 nM omega-Agatoxin-TK, which at this concentration is able to block Q-type channels, and 1 microM omega-Conotoxin GVIA, a blocker of N-type channels, both reduced outward current by about 50%. These antagonists given together, or 500 nM omega-Conotoxin MVIIC, a blocker of N- and P/Q-type channels, reduced outward current by 70%. In addition, the N- and P/Q-type channel blockers preferentially reduce the afterhyperpolarization recorded intracellularly. The results show that calcium-dependent potassium channels in neostriatal neurons are preferentially activated by calcium entry through N- and Q-type channels in these conditions.

Role of glutamate receptors and voltage-dependent calcium channels in glutamate toxicity in energy-compromised cortical neurons

We have examined the effect of glutamate receptor antagonists and voltage-dependent calcium channel blockers on the neuronal injury induced by the combination of a low concentration of N-methyl-D-aspartate (NMDA) or kainate and energy compromise resulting from the use of glucose-free incubation buffer. Toxicity induced by NMDA or kainate was enhanced in the glucose-free buffer. NMDA-or non-NMDA-receptor antagonists added to the glucose-free buffer at the same time inhibited the neuronal cell death induced by each agonist. An NMDA-receptor antagonist, MK-801, but not non-NMDA-receptor antagonists, inhibited the toxicity when added to the culture medium after exposure of the cells to the agonists. P/Q-type calcium channel blockers, omega-agatoxin IVA and omega-agatoxin TK, and an N-type calcium channel blocker, omega-conotoxin GVIA, significantly attenuated the neuronal injury, although an L-type calcium channel blocker, nifedipine, showed little neuroprotective effect. A combination of calcium channel blockers of the three subtypes showed the most prominent neuroprotective effect. These observations suggest that the overactivation of NMDA and non-NMDA receptors and consequent activation of the voltage-dependent calcium channels lead to neuronal cell death in energy-compromised cortical neurons.

Voltage-activated calcium channels involved in veratridine-evoked [3H]dopamine release in rat striatal slices

The present study explored the role of different sub-types of voltage-activated Ca2+ channels (VACCs) in mediating veratridine-evoked [3H]dopamine (DA) release from rat striatal slices. The release of [3H]DA evoked by veratridine (25 microM) decreased by 50.6+/-2.9% (n=8) in the absence of calcium and was completely abolished by 1 microM tetrodotoxin. The L-type Ca2+ channel blockers nifedipine (10 microM), nitrendipine (10 microM), diltiazem (10 microM) and verapamil (10 microM) did not modulate this release. Similarly, [3H]DA release was affected neither by the N-type VACC blocker omega-conotoxin-GVIA (1 microM) nor by the selective P-type channel blockers omega-agatoxin-IVA and omega-agatoxin-TK at low nM concentrations (30 nM), indicating no involvement of N- and P-type Ca2+ channels. In contrast, higher concentrations of omega-agatoxin-IVA that would also inhibit Q-type VACCs, blocked the release of [3H]DA by 27.9+/-8.1% (n=5) and 37.5+/-13.6% (n=3) at 0.3 and 1 microM, respectively. In addition, application of the Q-type Ca2+ channel blocker omega-conotoxin-MVIIC (0.01-3 degrees M) reduced [3H]DA release in a concentration-dependent manner, with maximum inhibition of 35.3+/-4.1% at 3 microM (n=5). On the basis of these results, it is concluded that the Ca2+ channels that participate in veratridine-evoked [3H]DA release are Q-type Ca2+ channels.