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Results for “Calcium Channels, Q-Type”

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PubMed1996

Voltage-sensitive Ca2+ channels involved in nicotinic receptor-mediated [3H]dopamine release from rat striatal synaptosomes.

The potent nicotinic agonist anatoxin-a elicits mecamylamine-sensitive [3H]dopamine release from striatal synaptosomes, and this action is both Na+ and Ca2+ dependent and is blocked by Cd2+. This suggests that stimulation of presynaptic nicotinic receptors results in Na+ influx and local depolarisation that activates voltage-sensitive Ca2+ channels, which in turn provide the Ca2+ for exocytosis. Here we have investigated the subtypes of Ca2+ channels implicated in this mechanism. [3H]-Dopamine release evoked by anatoxin-a (1 microM) was partially blocked by 20 microM nifedipine, whereas KCl-evoked release was insensitive to the dihydropyridine. However, a 86Rb+ efflux assay of nicotinic receptor function suggested that nifedipine has a direct effect on the receptor, discrediting the involvement of L-type channels. The N-type Ca2+ channel blocker omega-conotoxin GVIA (1 microM) blocked anatoxin-a-evoked [3H]dopamine release by 60% but had no significant effect on 86Rb+ efflux; release evoked by both 15 and 25 mM KCl was inhibited by only 30%. The P-type channel blocker omega-agatoxin IVA (90 nM) also inhibited KCl-evoked release by approximately 30%, whereas anatoxin-a-evoked release was insensitive. The Q-type channel blocker omega-conotoxin MVIIC (1 microM) had no effect on either stimulus. These results suggest that presynaptic nicotinic receptors on striatal nerve terminals promote [3H]dopamine release by activation of N-type Ca2+ channels. In contrast, KCl-evoked [3H]dopamine release appears to involve both N-type and P-type channels.

Animals↗

PubMed1997

The amyloid beta-protein of Alzheimer's disease increases acetylcholinesterase expression by increasing intracellular calcium in embryonal carcinoma P19 cells.

One of the characteristic changes that occurs in Alzheimer's disease is the loss of acetylcholinesterase (AChE) from both cholinergic and noncholinergic neurons of the brain. However, AChE activity is increased around amyloid plaques. This increase in AChE may be of significance for therapeutic strategies using AChE inhibitors. The aim of this study was to examine the effect of amyloid beta-protein (A beta), the major component of amyloid plaques, on AChE expression. A beta peptides spanning residues 1-40 or 25-35 increased AChE activity in P19 embryonal carcinoma cells. A peptide containing a scrambled A beta(25-35) sequence did not stimulate AChE expression. To examine the possibility that the increase in AChE expression was mediated by an influx of calcium through voltage-dependent calcium channels (VDCCs), drugs acting on VDCCs were tested for their effects. Inhibitors of L-type VDCCs (diltiazem, nifedipine, and verapamil), but not N- or P- or Q-type VDCCs, resulted in a decrease in AChE expression. Agonists of L-type VDCCs (maitotoxin and S(-)-Bay K 8644) increased AChE expression. As L-type VDCCs are known to be modulated by cyclic AMP-dependent protein kinase, the effect of the adenylate cyclase activator forskolin was also examined. Forskolin stimulated AChE expression, an action that was blocked by the L-type VDCC antagonist nifedipine. The A beta(25-35)induced increase in AChE expression was mediated by an L-type VDCC, as the effect was also blocked by nifedipine. The results suggest that the increase in AChE expression around amyloid plaques could be due to a disturbance in calcium homeostasis involving the opening of L-type VDCCs.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1995

Regional difference of high voltage-activated Ca2+ channels in rat CNS neurones.

The diversity of high voltage-activated (HVA) Ca2+ channels in rat CNS neurones was investigated with the nystatin perforated patch recording configuration. The neurones were freshly dissociated from rat substantia nigra, ventromedial hypothalamus, tuberomammillary nucleus, nucleus tractus solitarius, hippocampal CA1 region and cerebellum. Five different types of HVA Ca2+ channels were distinguished pharmacologically; dihydropyridine sensitive L-type, omega-conotoxin-GVIA sensitive N-type, omega-agatoxin-IVA sensitive P-type, omega-conotoxin-MVIIC sensitive Q-type, and R-type which is insensitive to these organic Ca2+ antagonists. The results showed clearly that the five subtypes of HVA Ca2+ channels differ considerably in their distribution among various CNS regions.

Animals↗

PubMed1995

Two types of calcium channels sensitive to omega-agatoxin- TK in cultured rat hippocampal neurones.

We characterized the electrophysiological properties of calcium channels in cultured rat hippocampal neurones using omega-agatoxin-TK (omega-Aga-TK) and compared them with those of the P-type channel and the BI (alpha 1A) channel which resembles the Q-type channel. Two types of omega-Aga-TK-sensitive calcium channels were detected in hippocampal neurones. The first type showed slow inactivation, high sensitivity to omega-Aga-TK and low reversibility from omega-Aga-TK-induced block, resembling the P-type channel. The second type showed fast inactivation, low sensitivity to omega-Aga-TK and high reversibility from omega-Aga-TK-induced block. These results suggest that the second type of calcium channel (Q-type-like) plays a prominent role in the hippocampal synaptic transmission.

Agatoxins↗

PubMed1996

Opioid inhibition of Ca2+ channel subtypes in bovine chromaffin cells: selectivity of action and voltage-dependence.

Bovine chromaffin cells possess a mixture of high-voltage-activated Ca2+ channel subtypes: L-type, dihydropyridine-sensitive channels, and N-, P- and Q-types, omega-conotoxin MVIIC-sensitive channels. In these cells, we studied the reversible, naloxone-antagonized inhibition of Ba2+ currents by the opioid agonist met-enkephalin (IC50 = 272 nM). This inhibition could be resolved into a voltage-dependent and a voltage-independent component. The first was revealed by its slow Ba2+ current activation kinetics at 0 mV and by the current facilitation induced by short prepulses to +90 mV. The second was estimated as the residual inhibition persisting after the facilitation protocol. The two inhibitory components varied markedly from cell to cell and each contributed to about half of the total inhibition. Replacement of internal GTP by GDP-beta-S or cell pretreatment with pertussis toxin completely abolished the voltage-dependent inhibition by opioids, partially preserving the voltage-independent component. The opioid-induced inhibition was not selective for any Ca2+ channel subtype, being not prevented after the addition of specific Ca2+ channel antagonists. However, when separately analysing the contribution of each channel type to the voltage-dependent and voltage-independent modulation, a clear-cut distinction could be achieved. The voltage-independent inhibition was effective on all Ca2+ channel subtypes but predominantly on L-type Ca2+ channels. The voltage-dependent process was abolished by omega-conotoxin-MVIIC, but unaffected by nifedipine, and was thus sharply restricted to non-L-type channels (N-, P- and Q-types). Our data suggest a functionally distinct opioid receptor-mediated modulation of L- and non-L-type channels, i.e. of the two channel classes sharing major control of catecholamine secretion from bovine chromaffin cells.

Animals↗

PubMed1997

Multiple types of Ca2+ channels in mouse motor nerve terminals.

We measured neurotransmitter release and motor nerve terminal currents in mouse phrenic nerve-diaphragm and triangularis sterni preparations, to evaluate the role of Ca2+-channel subtypes in regulating transmitter release. Saturated concentrations of either omega-agatoxin IVA [omega-Aga-IVA (0.3 microM), a blocker of P-type Ca2+ channels] or omega-conotoxin MVIIC [omega-CTx-MVIIC (2 microM), a P- and Q-type Ca2+-channel blocker], inhibited nerve-evoked muscle contractions and the amplitude of endplate potentials respectively. In contrast, combined treatment with nifedipine (50 microM, a blocker of L-type Ca2+ channels) plus omega-conotoxin GVIA [omega-CTx-GVIA (2 microM), a blocker of N-type Ca2+ channels] did not elicit inhibitory effects on nerve-evoked muscle contractions, endplate potentials or nerve terminal waveforms. Because of the non-linear relationship between endplate potentials and Ca2+ signals, a small decrease in presynaptic Ca2+ entry can significantly reduce the amplitude of the endplate potential. Thus, we applied 3,4-diaminopyridine (3,4-DAP, a K+-channel blocker) or high Ca2+ (10 mM) to accelerate and amplify the endplate potentials and Ca2+ currents. The endplate potentials amplified by 3,4-DAP or by high Ca2+ correspondingly proved to be quite resistant to both omega-Aga-IVA and omgea-CTx-MVIIC; omega-Aga-IVA exerted only a partial inhibitory effect on endplate potentials, and the omega-Aga-IVA-resistant component was further inhibited by omega-CTx-MVIIC. The component that was resistant to the two toxins could be completely blocked by the non-selective Ca2+ channel blocker Cd2+ (300 microM). A combination of the two toxins had no significant effects on either spontaneous transmitter release or postsynaptic resting membrane potentials of the diaphragm preparation and the Na+ and K+ waveforms of the triangularis sterni preparations. This finding suggests a preferential inhibitory effect at a presynaptic site. Measuring the Ca2+ currents in the triangularis sterni also revealed partial inhibition by omega-CTx-MVIIC with further incomplete inhibition by omega-Aga-IVA. Cd2+ (300 microM) abolished the toxin-resistant component of the Ca2+ current. In contrast, a combination of nifedipine (50 microM) with omega-CTx-GVIA (2 microM) was without inhibitory effect. We conclude that multiple types of Ca2+ channels, i.e. omega-Aga-IVA-sensitive, omega-CTx-MVIIC-sensitive and toxin-resistant Ca2+ channels, coexist in mouse motor nerve terminals.

Animals↗

PubMed1997

Role of Q-type Ca2+ channels in vasopressin secretion from neurohypophysial terminals of the rat.

1. The nerve endings of rat neurohypophyses were acutely dissociated and a combination of pharmacological, biophysical and biochemical techniques was used to determine which classes of Ca2+ channels on these central nervous system (CNS) terminals contribute functionally to arginine vasopressin (AVP) and oxytocin (OT) secretion. 2. Purified neurohypophysial plasma membranes not only had a single high-affinity binding site for the N-channel-specific omega-conopeptide MVIIA, but also a distinct high-affinity site for another omega-conopeptide (MVIIC), which affects both N- and P/Q-channels. 3. Neurohypophysial terminals exhibited, besides L- and N-type currents, another component of the Ca2+ current that was only blocked by low concentrations of MVIIC or by high concentrations of omega-AgaIVA, a P/Q-channel-selective spider toxin. 4. This Ca2+ current component had pharmacological and biophysical properties similar to those described for the fast-inactivating form of the P/Q-channel class, suggesting that in the neurohypophysial terminals this current is mediated by a 'Q'-type channel. 5. Pharmacological additivity studies showed that this Q-component contributed to rises in intraterminal Ca2+ concentration ([Ca2+]i) in only half of the terminals tested. 6. Furthermore, the non-L- and non-N-component of Ca(2+)-dependent AVP release, but not OT release, was effectively abolished by the same blockers of Q-type current. 7. Thus Q-channels are present on a subset of the neurohypophysial terminals where, in combination with N- and L-channels, they control AVP but not OT peptide neurosecretion.

Animals↗

PubMed1998

Heterogeneous presynaptic Ca2+ channel types triggering GABA release onto medial preoptic neurons from rat.

1. Voltage-dependent Ca2+ channels triggering GABA release onto neurons from the medial preoptic nucleus of rat were investigated. Acutely dissociated neurons with adherent functional synaptic terminals were investigated by tight-seal whole-cell recordings from the postsynaptic cells. 2. Spontaneous current events similar to miniature postsynaptic currents were recorded. They were blocked by bicuculline (100 microM), showed a roughly unimodal amplitude distribution and a reversal potential consistent with a Cl- current, and were therefore attributed to GABAA receptors activated by synaptically released GABA. 3. Application of 140 mM KCl, expected to depolarize presynaptic terminals, evoked currents that were ascribed to a more massive release of GABA. The KCl-induced synaptic currents were abolished in Ca2+-free solutions and showed a roughly hyperbolic relation to external Ca2+ concentration with half-saturation at 0.15 mM. They further depended on the concentration of applied KCl in a way expected for high-threshold Ca2+ channels. 4. The KCl-evoked synaptic currents were completely blocked by 200 microM Cd2+, but only partially blocked by 200 microM Ni2+. The KCl-evoked synaptic currents were insensitive to the L-type Ca2+ channel blocker nifedipine (10 microM). However, the synaptic currents were sensitive to either 1 microM omega-conotoxin GVIA, 25 nM omega-agatoxin IVA or 1 microM omega-conotoxin MVIIC. 6. It was concluded that, in many presynaptic terminals, the Ca2+ influx triggering GABA release onto medial preoptic neurons is mainly mediated by one predominant type of high- threshold Ca2+ channel that may be either of N-, P- or Q-type. 7. It was further concluded that terminals with similar predominant channel types often were clustered on the same postsynaptic cell.

Animals↗

PubMed1998

Lambert-Eaton syndrome antibodies inhibit acetylcholine release and P/Q-type Ca2+ channels in electric ray nerve endings.

1. The types of voltage-dependent calcium channels (VDCCs) present in the cholinergic terminals isolated from the electric organ of the ray, Narke japonica, were characterized on the basis of their pharmacological sensitivity to specific antagonists. Inhibition of these channel types by autoantibodies from patients with the Lambert-Eaton syndrome (LES) was then studied to determine the specificity of the pathogenic IgG. 2. In normal untreated synaptosomal preparations, maximal doses of N- and P and/or Q-type Ca2+ channel antagonists, omega-conotoxin GVIA and omega-agatoxin IVA, inhibited depolarization-evoked ACh release by 47 % and 43 %, respectively. Calciseptine, an L-type VDCC antagonist, caused a 20 % reduction in the release. This indicates that the exocytotic release process is predominantly mediated by N- and P/Q-type VDCCs. 3. LES IgG or sera caused an inhibition of ACh release by 39-45 % in comparison with the control antibody-treated preparations. The ionomycin-induced ACh release, however, was not altered by the antibodies. Additionally, the same LES antibodies inhibited whole-cell calcium currents (ICa) in bovine adrenal chromaffin cells. Thus, the pathogenic antibodies exert their action on VDCCs present in the synaptosomes. 4. The efficacy of three Ca2+ channel antagonists in blocking ACh release was determined in preparations pretreated with LES IgG. omega-Agatoxin IVA produced only an additional 3-5 % reduction in release beyond that obtained with LES antibodies. Despite the pretreatment with LES IgG, omega-conotoxin GVIA and calciseptine inhibited the release to nearly their control levels. 5. These results indicate that LES antibodies mainly downregulate P/Q-type Ca2+ channels which contribute to presynaptic transmitter release from the cholinergic nerve terminals of electric organ. 6. The present findings are consistent with the hypothesis that P/Q-type VDCCs at the neuromuscular junction are the target of LES antibodies and that their inhibition by the antibodies produces the characteristic neuromuscular defect in this disease.

Acetylcholine↗

PubMed1997

Different contributions of L- and Q-type Ca2+ channels to Ca2+ signals and secretion in chromaffin cell subtypes.

In this study, we investigated the contribution of different subtypes of voltage-dependent Ca2+ channels to changes in cytosolic free Ca2+ ([Ca2+]i) and secretion in noradrenergic and adrenergic bovine chromaffin cells. In single immunocytochemically identified chromaffin cells, [Ca2+]i increased transiently during high K+ depolarization. Furnidipine and BAY K 8644, L-type Ca2+ channel blocker and activator, respectively, affected the [Ca2+]i rise more in noradrenergic than in adrenergic cells. In contrast, the Q-type Ca2+ channel blocker omega-conotoxin MVIIC inhibited the [Ca2+]i rise more in adrenergic cells. omega-Agatoxin IVA (30 nM), which blocks P-type Ca2+ channels, had little effect on the [Ca2+]i signal. The N-type Ca2+ channel blocker omega-conotoxin GVIA similarly inhibited the [Ca2+]i rise in both cell types. The effects of furnidipine, BAY K 8644, and omega-conotoxin MVIIC on K+-evoked norepinephrine and epinephrine release paralleled those effects on [Ca2+]i signals. However, omega-conotoxin GVIA and 30 nM omega-agatoxin IVA did not affect the secretion of either amine. The data suggest that, in the bovine adrenal medulla, the release of epinephrine and norepinephrine are preferentially controlled by Q- and L-type Ca2+ channels, respectively. P- and N-type Ca2+ channels do not seem to control the secretion of either catecholamine.

Animals↗

PubMed1989

Anoxia on slow inward currents of immature hippocampal neurons.

1. The effects of brief anoxia (2-4 min) on membrane currents--especially the tetrodotoxin (TTX)-insensitive, Cd2+-sensitive slow inward currents, presumed to be Ca2+ currents--were studied by single-electrode voltage clamp in CA1 and CA3 neurons in submerged hippocampal slices from adult and newborn Wistar rats (PN1-13). 2. In mature neurons, anoxia had no effect on Q-type inward relaxations, but slowly activating C-type outward currents were depressed. The most striking change was the suppression of Ca inward currents (especially the slowly inactivating L-type, by greater than 95%). This effect of anoxia was not sensitive to the N-methyl-D-aspartate (NMDA) receptor blocker, D-aminophosphonovalerate. Anoxia also reversibly abolished the NMDA-evoked inward current. 3. In neurons from newborn animals (PN1-6), Q-type inward relaxations and postanoxic outward currents were very small or undetectable. The slow inward (Ca) currents were smaller than in mature cells, but they showed a clearer separation between low-threshold, fast-inactivating and high-threshold, slowly inactivating currents. Both types of current were more resistant to anoxia (mean depression of L-type was by only 53.3 +/- 5.6%, mean +/- SE). 4. In such immature neurons, the NMDA-evoked inward currents were also more resistant to anoxia. 5. By PN7-13, increasing maturation was reflected in 1) larger voltage-dependent inward currents, 2) increasingly evident Q-type relaxations and postanoxic outward currents, and 3) near-complete blockade of inward currents by anoxia (at PN11-13, mean depression of L-type currents was by 98.5 +/- 1.5%).

Aging↗

PubMed1996

Pharmacological dissection of high-voltage-activated Ca2+ current types in acutely dissociated rat supraoptic magnocellular neurons.

1. We used pharmacological blockers and whole cell voltage-clamp recordings from acutely dissociated magnocellular neurons from the rat supraoptic nucleus to test for the presence of L-, N-, P-, Q-, and R-type calcium currents. 2. We found clear evidence for L-type (dihydropyridine sensitive), N-type (blocked by omega-conotoxin (GVIA), and P-type (blocked by omega-agatoxin IVA) currents. Nifedipine blocked 28 +/- 2% (mean +/- SE) of the current, omega-conotoxin GVIA blocked 26 +/- 4%, and omega-agatoxin IVA (25-30 nM) blocked 17 +/- 4%. In addition, omega-conotoxin MVIIC blocked about half (54 +/- 2%) of the current remaining after L-, N-, and P-type currents were blocked, suggesting the presence of Q-type currents. 3. About 20% of the whole cell current was unblocked by the combination of all four blockers, suggesting the presence of a fifth current type. This residual current inactivated slowly at -10 mV.

Animals↗

PubMed1997

Dihydropyridine- and neurotoxin-sensitive and -insensitive calcium currents in acutely dissociated neurons of the rat central amygdala.

The central amygdala (CeA) is an area involved in emotional learning and stress, and identification of Ca2+ currents is essential to understanding interneuronal communication through this nucleus. The purpose of this study was to separate and characterize dihydropyridine (DHP)- and neurotoxin-sensitive and -resistant components of the whole cell Ca2+ current (ICa) in acutely dissociated rat CeA neurons with the use of whole cell patch-clamp recording. Saturating concentrations of nimodipine (NIM, 5 microM), a DHP antagonist, blocked 22% of ICa: this NIM-sensitive (L-type) current was recorded in 68% of CeA neurons. The DHP agonist Bay K 8644 (5 microM) produced a 36% increase in ICa in a similar proportion of CeA neurons (70%). omega-Conotoxin GVIA (CgTx GVIA, 1 microM) in saturating concentrations inhibited 30% of ICa, whereas omega-agatoxin IVA (Aga IVA, 100 nM), in concentrations known to block P-type currents, did not affect ICa. Higher concentrations of Aga IVA (1 microM) alone reduced ICa by 34%, but in the presence of NIM (5 microM) and CgTx GVIA (1 microM) blocked only 18% of ICa. omega-Conotoxin MVIIC (CgTx MVIIC, 250 nM) reduced ICa by 13% in the presence of CgTx GVIA (1 microM). Application of NIM (5 mM), CgTx GVIA (1 microM); and Aga IVA (1 microM) blocked approximately 67% of ICa. A similar portion (63%) of Ca2+ current was blocked with CgTx MVIIC (250 nM) in the presence of NIM (5 microM) and CgTx GVIA (1 microM). The current resistant to NIM and the neurotoxins represented 37% of ICa, whereas in neurons not having L-type currents the resistant current made up approximately 53% of ICa (49 +/- 2%, mean +/- SE). The resistant current activated at around -40 mV and peaked at approximately 0 mV with half-activation and -inactivation potentials of -17 and -58 mV and slopes for activation and inactivation of -5 and 13 mV, respectively. The resistant current was sensitive to Cd2+ (IC50 = 2.5 microM) and Ni2+ (IC50 = 86 microM), was larger in Ca2+ than in Ba2+ (ratio = 1.31:1), and showed a moderate rate of decay. In summary, our results show that the high-voltage-activated calcium current in rat CeA neurons is composed of at least four pharmacologically distinct components: L-type current (NIM sensitive, 22%), N-type current (CgTx GVIA sensitive, 30%), Q-type current [Aga IVA (1 microM) and CgTx MVIIC sensitive, approximately 13-18%], and a resistant current (Non-L, -N, and -Q current, 33 approximately 37%), amounting to 37-53% of the total current. The resistant current has some electrophysiological and pharmacological characteristics in common with doe-1, alpha 1E, and R-type calcium currents, but remains unclassified.

Amygdala↗

PubMed1997

Neurotensin and substance P inhibit low- and high-voltage-activated Ca2+ channels in cultured newborn rat nucleus basalis neurons.

Inhibition of Ca2+ currents by the excitatory neurotransmitters neurotensin and substance P was investigated in cultured nucleus basalis neurons with the use of the whole cell patch-clamp technique. The whole cell Ca2+ current, elicited from a holding potential of -80 mV by a step pulse to 0 mV and measured at 100 ms, was inhibited 67.9% by neurotensin and 57.6% by substance P. Low-voltage-activated (LVA) Ca2+ current, elicited by a step pulse to -40 mV from a holding potential of -90 mV, was inhibited by both neurotensin (26.2%) and substance P (24.1%). High-voltage-activated Ca2+ currents were separated with the use of the Ca2+ channel antagonists. Nimodipine (3 microM) inhibited 24.2% of the whole cell Ca2+ current elicited by a step to 0 or +10 mV and measured at 100 ms. Under the same conditions, omega-conotoxin (omega-CgTx)-GVIA (0.5 microM) inhibited 46.4%, omega-CgTx-GVIA + nimodipine 58.7%, and omega-CgTx-MVIIC (5 microM) + nimodipine 75.7% of the current. Omega-Agatoxin (omega-Aga)-IVA (100 nM) did not produce any effect. Neurotensin inhibition of the whole cell Ca2+ current was attenuated by each of these treatments except for the omega-Aga-IVA treatment, which did not change the neurotensin effect. In contrast, neither the omega-Aga-IVA nor the nimodipine treatment had any effect on the substance-P-induced inhibition; the rest of the treatments attenuated the substance-P-induced response. Thus the data indicate that nucleus basalis neurons express LVA as well as L-, N-, and Q-type, but not the P-type, Ca2+ currents. N- and Q-type HVA Ca2+ currents, as well as LVA Ca2+ currents, are inhibited by both neurotensin and substance P. In contrast, L-type current is inhibited by neurotensin but not by substance P. In addition, a fraction of the total whole cell current was resistant to all Ca2+ channel antagonists and thus may correspond to the R-type Ca2+ current. This residual current was inhibited by both neurotensin and substance P. The inhibition of the whole cell Ca2+ current produced by both neurotransmitters was voltage independent, because a large depolarization (+70 mV) was not able to relieve either effect. In cells loaded with 0.1 mM guanosine 5'-[gamma-thio]triphosphate, response to both neurotensin and substance P became irreversible, indicating that the effects of both neurotransmitters were mediated through G proteins. However, pertussis toxin did not affect either the neurotensin or the substance P response.

Animals↗

PubMed1998

Adenosine A1 receptors mediate retinotectal presynaptic inhibition: uncoupling by C-kinase and role in LTP during regeneration.

Presynaptic adenosine receptors inhibit transmitter release at many synapses and are known to exist on retinotectal terminals. In this paper we show that adenosine decreases retinotectal field potentials by approximately 30% and investigate the mechanism. First, as judged by the effects of specific calcium channel blockers, retinotectal transmission was mediated almost exclusively by N-type calcium channels, which are known to be modulated by adenosine A1 receptors. Transmission was completely blocked by either omega-Conotoxin GVIA (-100%, N-type blocker) or omega-Conotoxin MVIIC (-99%, N-, P- and Q-type blocker) and was not significantly affected by omega-Agatoxin IVA [+1.7 +/- 9. 3% (SE), P-,Q-type blocker], but was augmented slightly by nifedipine(+9.3 +/- 2.1%, L-type blocker). Second, the adenosine inhibition was presynaptic, as indicated by a 43% increase in paired-pulse facilitation. Third, the selective A1 agonist cyclohexyl adenosine (CHA) at 50 nM caused a 21% decrease in amplitude and the selective A2 agonist N6-[2-(3, 5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) at 100 nM caused a 24% increase. Fourth, the selective A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) alone produced an increase in the field potential, suggesting a tonic inhibition mediated by endogenous adenosine. Fifth, pertussis toxin eliminated adenosine inhibition implicating Gi or Go protein coupling. Sixth, C-kinase activation eliminated the A1-mediated inhibition. In regenerating projections, adenosine also caused a decrease in transmission (-30 +/- 12%), but after induction of long-term potentiation (LTP) via trains of stimuli or via treatment with the phosphatase inhibitor okadaic acid, the adenosine response was converted to an augmentation. Because LTP is associated with C-kinase activation, this is consistent with C-kinase uncoupling the A1 receptor from inhibiting N-type Ca2+ channels. This uncovers the A2-mediated augmentation as demonstrated in normals with DPMA. Such an effect could account in part for the LTP of immature synapses and the change from rapidly fatiguing to robust synaptic transmission.

Adenosine↗

PubMed1998

Biophysical and pharmacological characterization of voltage-dependent Ca2+ channels in neurons isolated from rat nucleus accumbens.

The nucleus accumbens (NA) has an integrative role in behavior and may mediate addictive and psychotherapeutic drug action. Whole cell recording techniques were used to characterize electrophysiologically and pharmacologically high- and low-threshold voltage-dependent Ca2+ currents in isolated NA neurons. High-threshold Ca2+ currents, which were found in all neurons studied and include both sustained and inactivating components, activated at potentials greater than -50 mV and reached maximal activation at approximately 0 mV. In contrast, low-threshold Ca2+ currents activated at voltages greater than -64 mV with maximal activation occurring at -30 mV. These were observed in 42% of acutely isolated neurons. Further pharmacological characterization of high-threshold Ca2+ currents was attempted using nimodipine (Nim), omega-conotoxin-GVIA (omega-CgTx) and omega-agatoxin-IVA (omegaAga), which are thought to identify the L, N, and P/Q subtypes of Ca2+ currents, respectively. Nim (5-10 muM) blocked 18%, omegaCgTx (1-2 muM) blocked 25%, and omegaAga (200 nM) blocked 17% of total Ca2+ current. Nim primarily blocked a sustained high-threshold Ca2+ current in a partially reversible manner. In contrast, omegaCgTx irreversibly blocked both sustained and inactivating components. omegaAga irreversibly blocked only a sustained component. In all three of these Ca2+ channel blockers, plus 5 muM omega-conotoxin-MVIIC to eliminate a small unblocked Q-type Ca2+ current (7%), a toxin-resistant high-threshold Ca2+ current remained that was 32% of total Ca2+ current. This current inactivated much more rapidly than the other high-threshold Ca2+ currents, was depressed in 50 muM Ni2+ and reached maximal activation 5-10 mV negative to the toxin-sensitive high-threshold Ca2+ currents. Thus NA neurons have multiple types of high-threshold Ca2+ currents with a large component being the toxin-resistant "R" component.

Animals↗

PubMed1998

Specificity in the interaction of HVA Ca2+ channel types with Ca2+-dependent AHPs and firing behavior in neocortical pyramidal neurons.

Intracellular recordings and organic and inorganic Ca2+ channel blockers were used in a neocortical brain slice preparation to test whether high-voltage-activated (HVA) Ca2+ channels are differentially coupled to Ca2+-dependent afterhyperpolarizations (AHPs) in sensorimotor neocortical pyramidal neurons. For the most part, spike repolarization was not Ca2+ dependent in these cells, although the final phase of repolarization (after the fast AHP) was sensitive to block of N-type current. Between 30 and 60% of the medium afterhyperpolarization (mAHP) and between approximately 80 and 90% of the slow AHP (sAHP) were Ca2+ dependent. Based on the effects of specific organic Ca2+ channel blockers (dihydropyridines, omega-conotoxin GVIA, omega-agatoxin IVA, and omega-conotoxin MVIIC), the sAHP is coupled to N-, P-, and Q-type currents. P-type currents were coupled to the mAHP. L-type current was not involved in the generation of either AHP but (with other HVA currents) contributes to the inward currents that regulate interspike intervals during repetitive firing. These data suggest different functional consequences for modulation of Ca2+ current subtypes.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1996

Pituitary adenylate cyclase-activating polypeptide causes rapid Ca2+ release from intracellular stores and long lasting Ca2+ influx mediated by Na+ influx-dependent membrane depolarization in bovine adrenal chromaffin cells.

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been reported to increase intracellular Ca2+ concentrations ([Ca2+]i) and catecholamine release in adrenal chromaffin cells. We measured [Ca2+]i with fura-2 and recorded ion currents and membrane potentials with the whole cell configuration of the patch-clamp technique to elucidate the mechanism of PACAP-induced [Ca2+]i increase in bovine adrenal chromaffin cells. PACAP caused [Ca2+]i to increase due to Ca2+ release and Ca2+ influx, and this was accompanied by membrane depolarization and inward currents. The Ca2+ release was suppressed by ryanodine, an inhibitor of caffeine-sensitive Ca2+ stores, but was unaffected by cinnarizine, an inhibitor of inositol trisphosphate-induced Ca2+ release. Ca2+ influx and inward currents were both inhibited by replacement of extracellular Na+, and Ca2+ influx was inhibited by nicardipine, an L-type Ca2+ channel blocker, or by staurosporine, a protein kinase C (PKC) inhibitor, but was unaffected by a combination of omega- conotoxin-GVIA, omega-agatoxin-IVA, and omega-conotoxin- MVIIC, blockers of N-, P-, and Q-type Ca2+ channels. Moreover, 1-oleoyl-2-acetyl-sn-glycerol, a PKC activator, induced inward currents and Ca2+ influx. These results indicate that PACAP causes both Ca2+ release, mainly from caffeine-sensitive Ca2+ stores, and Ca2+ influx via L-type Ca2+ channels activated by membrane depolarization that depends on PKC-mediated Na+ influx.

Adrenal Glands↗