Investigations of the roles of dihydropyridine and omega-conotoxin-sensitive calcium channels in mediating depolarisation-evoked endogenous dopamine release from striatal slices

Role of voltage-sensitive Ca2+ channels in the in vivo dopamine release induced by the organophosphorus pesticide glufosinate ammonium in rat striatum

The possible role of voltage-sensitive calcium channels (VSCC) activation in the glufosinate ammonium (GLA)-induced dopamine release was investigated using selective VSCC blockers and the dopamine levels were measured by HPLC from samples obtained by in vivo cerebral microdialysis. While pretreatment with 10 μM flunarizine (T-type VSCC antagonist) or nicardipine (L-type VSCC antagonist) had no statistically significant effect on dopamine release induced by 10 mM GLA, pretreatment with 100 μM of both antagonists, or 20 μM ω-conotoxin MVIIC (non-selective P/Q-type VSCC antagonist) significantly decreased the GLA-induced dopamine release over 72.2%, 73%, and 70.2%, respectively. Administration of the specific antagonist of neuronal N-type VSCCs, the ω-conotoxin GVIA (20 μM), produced an almost complete blockade of in vivo dopamine release induced by GLA. These results show that GLA-induced dopamine release could be produced by the activation of a wide range of striatal VSCC located at the synaptic terminals and axons of striatal dopaminergic neurons, especially N-type VSCC.

The organ-protective effect of N-type Ca(2+) channel blockade

The six subtypes of voltage-dependent Ca(2+) channels (VDCCs) mediate a wide range of physiological responses. N-type VDCCs (NCCs) were originally identified as a high voltage-activated Ca(2+) channel selectively blocked by omega-conotoxin (ω-CTX)-GVIA. Predominantly localized in the nervous system, NCCs are key regulators of neurotransmitter release. Both pharmacological blockade with ω-CTX-GVIA and, more recently, mice lacking CNCNA1B, encoding the α1B subunit of NCC, have been used to assess the physiological and pathophysiological functions of NCCs, revealing in part their significant roles in sympathetic nerve activation and nociceptive transmission. The evidence now available indicates that NCCs are a potentially useful therapeutic target for the treatment of several pathological conditions. Efforts are therefore being made to develop effective NCC blockers, including both synthetic ω-CTX-GVIA derivatives and small-molecule inhibitors. Cilnidipine, for example, is a dihydropyridine L-type VDCC blocking agent that also possesses significant NCC blocking ability. As over-activation of the sympathetic nervous system appears to contribute to the pathological processes underlying cardiovascular, renal and metabolic diseases, NCC blockade could be a useful approach to treating these ailments. In this review article, we provide an overview of what is currently known about the physiological and pathophysiological activities of NCCs and the potentially beneficial effects of NCC blockade in several disease conditions, in particular cardiovascular diseases.

Midbrain dopaminergic neurons generate calcium and sodium currents and release dopamine in the striatum of pups

Midbrain dopaminergic neurons (mDA neurons) are essential for the control of diverse motor and cognitive behaviors. However, our understanding of the activity of immature mDA neurons is rudimentary. Rodent mDA neurons migrate and differentiate early in embryonic life and dopaminergic axons enter the striatum and contact striatal neurons a few days before birth, but when these are functional is not known. Here, we recorded Ca²⁺ transients and Na⁺ spikes from embryonic (E16–E18) and early postnatal (P0–P7) mDA neurons with dynamic two-photon imaging and patch clamp techniques in slices from tyrosine hydroxylase-GFP mice, and measured evoked dopamine release in the striatum with amperometry. We show that half of identified E16–P0 mDA neurons spontaneously generate non-synaptic, intrinsically driven Ca²⁺ spikes and Ca²⁺ plateaus mediated by N- and L-type voltage-gated Ca²⁺ channels. Starting from E18–P0, half of the mDA neurons also reliably generate overshooting Na⁺ spikes with an abrupt maturation at birth (P0 = E19). At that stage (E18–P0), dopaminergic terminals release dopamine in a calcium-dependent manner in the striatum in response to local stimulation. This suggests that mouse striatal dopaminergic synapses are functional at birth.

Dopamine release in the basal ganglia

Dopamine (DA) is a key transmitter in the basal ganglia, yet DA transmission does not conform to several aspects of the classic synaptic doctrine. Axonal DA release occurs through vesicular exocytosis and is action potential- and Ca²⁺-dependent. However, in addition to axonal release, DA neurons in midbrain exhibit somatodendritic release by an incompletely understood, but apparently exocytotic, mechanism. Even in striatum, axonal release sites are controversial, with evidence for DA varicosities that lack postsynaptic specialization, and largely extrasynaptic DA receptors and transporters. Moreover, DA release is often assumed to reflect a global response to a population of activities in midbrain DA neurons, whether tonic or phasic, with precise timing and specificity of action governed by other basal ganglia circuits. This view has been reinforced by anatomical evidence showing dense axonal DA arbors throughout striatum, and a lattice network formed by DA axons and glutamatergic input from cortex and thalamus. Nonetheless, localized DA transients are seen in vivo using voltammetric methods with high spatial and temporal resolution. Mechanistic studies using similar methods in vitro have revealed local regulation of DA release by other transmitters and modulators, as well as by proteins known to be disrupted in Parkinson's disease and other movement disorders. Notably, the actions of most other striatal transmitters on DA release also do not conform to the synaptic doctrine, with the absence of direct synaptic contacts for glutamate, GABA, and acetylcholine (ACh) on striatal DA axons. Overall, the findings reviewed here indicate that DA signaling in the basal ganglia is sculpted by cooperation between the timing and pattern of DA input and those of local regulatory factors.

Control of extracellular dopamine at dendrite and axon terminals

Midbrain dopamine neurons release dopamine from both axons and dendrites. The mechanism underlying release at these different sites has been proposed to differ. This study used electrochemical and electrophysiological methods to compare the time course and calcium dependence of somatodendritic dopamine release in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) to that of axonal dopamine release in the dorsal striatum. The amount of dopamine released in the striatum was approximately 20-fold greater than in cell body regions of the VTA or SNc. However, the calcium dependence and time to peak of the dopamine transients were similar. These results illustrate an unexpected overall similarity in the mechanisms of dopamine release in the striatum and cell body regions. To examine how diffusion regulates the time course of dopamine following release, dextran was added to the extracellular solution to slow diffusion. In the VTA, dextran slowed the rate of rise and fall of the extracellular dopamine transient as measured by fast-scan cyclic voltammetry yet did not alter the kinetics of the dopamine-dependent IPSC. Dextran failed to significantly alter the time course of the rise and fall of the dopamine transient in the striatum, suggesting a more influential role for reuptake in the striatum. The conclusion is that the time course of dopamine within the extracellular space of the VTA is dependent on both diffusion and reuptake, whereas the activation of D(2) receptors on dopamine neurons is primarily limited by reuptake.

Behavioral and neurochemical characterization of mice deficient in the N-type Ca2+ channel alpha1B subunit

N-type voltage-dependent calcium channels (VDCCs) play an important role in neurotransmission, synaptic plasticity, and brain development. They are composed of several subunits named alpha(1), alpha(2), delta, beta and gamma. The alpha(1) subunit is essential for channel functions and determines fundamental channel properties. Since N-type VDCC are critically involved in the release of neurotransmitters and clinical relevance, we predicted that alpha(1) subunit KO mice would show several alterations in behavior. In the present study, we investigated neuronal functions in mice lacking the alpha(1B) (Ca(V)2.2) subunit of the N-type calcium channels. Ca(V)2.2(-/-) mice exhibited a significant increase in locomotion on an activity wheel during the dark phase. Furthermore, when challenged with apomorphine, mutant mice showed enhanced locomotor activity. Cognitive functions were examined using a Y-maze task for short-term memory and a passive avoidance task for long-term memory. The Y-maze revealed no differences in spontaneous alternation behavior between mutant and wild-type mice. The passive avoidance test revealed that the latency time in mutant mice was significantly decreased. The mutant mice showed prepulse inhibition deficits reminiscent of the sensorimotor gating deficits observed in a large majority of schizophrenic patients. Decreases in baseline levels of dopamine and serotonin within the striata and frontal cortices of mutant mice were also observed. These results suggest that Ca(2+) in the central nervous system modulates various neurophysiological functions, such as locomotor activity, long-term memory, and sensorimotor gating through the alpha(1B) subunit of the N-type calcium channels.

Voltage-operated Ca2+ channels regulate dopamine release from somata of dopamine neurons in the substantia nigra pars compacta

Dopamine (DA) neurons release DA not only from axon terminals at the striatum, but from their somata and dendrites at the substantia nigra pars compacta (SNc). Released DA may auto-regulate further DA release or modulate non-DA cells. However, the actual mechanism of somatodendritic DA release, especially the Ca(2+) dependency of the process, remains controversial. In this study, we used amperometry to monitor DA release from somata of acutely isolated rat DA neurons. We found that DA neurons spontaneously released DA in the resting state. Removal of extracellular Ca(2+) and application of blockers for voltage-operated Ca(2+) channels (VOCCs) suppressed the frequency of secretion events. Activation of VOCCs by stimulation with K(+)-rich saline increased the frequency of secretion events, which were also sensitive to blockers for L- and T-type Ca(2+) channels. These results suggest that Ca(2+) influx through VOCCs regulates DA release from somata of DA neurons.

Modified behavioral characteristics following ablation of the voltage-dependent calcium channel beta3 subunit

Voltage-dependent calcium channels are important for calcium influx and the ensuing intracellular calcium signal in various excitable membranes. The beta subunits of these channels modify calcium currents through pore-forming alpha1 subunits of the high-voltage- activated calcium channels. In the present study, beta3 subunit-null mice were used to investigate the importance of the beta3 subunit of the voltage-dependent calcium channel, which couples with the CaV2.2 (alpha1B) subunit to form the major component of neuronal N-type calcium channels in the brain. Western blot analysis revealed a significant decrease in N-type calcium channels in beta3 subunit-null mice, while protein levels of other high-voltage-activated calcium channel alpha1 subunits were unchanged. Immunoprecipitation analysis with an anti-CaV2.2 antibody showed that reshuffling of the assembly of N-type channels had occurred in the beta3 subunit-null mice. Ablation of this subunit resulted in modified nociception, decreased anxiety, and increased aggression. The beta3 subunit-null mice also showed impaired learning ability. These results suggest the importance of voltage-dependent calcium channels and the key role of the beta3 subunit in memory formation, nociceptive sensory transduction, and various neurological signal transduction pathways.

Limited regulation of somatodendritic dopamine release by voltage-sensitive Ca channels contrasted with strong regulation of axonal dopamine release

The mechanism underlying somatodendritic release of dopamine (DA) appears to differ from that of axon-terminal release. Specifically, somatodendritic DA release in the substantia nigra pars compacta (SNc) persists in low extracellular Ca2+ concentrations that are insufficient to support axonal release in striatum, suggesting that limited Ca2+ entry is necessary to trigger somatodendritic release. Here, we compared the role of voltage-dependent Ca2+ channels in mediating DA release in striatum versus SNc using specific blockers of N-, P/Q-, T-, R- and L-type Ca2+ channels individually and in combination. Release of DA evoked by a single stimulus pulse in the dorsal striatum and SNc of guinea-pig brain slices was monitored in real time using carbon-fiber microelectrodes with fast-scan cyclic voltammetry. Single-pulse evoked DA release was shown to be independent of regulation by concurrently released glutamate or GABA acting at ionotropic receptors in both regions. Under these conditions, striatal DA release was completely prevented by an N-type channel blocker, omega-conotoxin GVIA (100 nm), and was decreased by 75% by the P/Q-type channel blocker omega-agatoxin IVA (200 nm). Blockade of T-type channels with Ni2+ (100 microm) or R-type channels with SNX-482 (100 nm) decreased axonal release in striatum by 25%, whereas inhibition of L-type channels with nifedipine (20 microm) had no effect. By contrast, none of these Ca2+-channel blockers altered the amplitude of somatodendritic DA release in the SNc. Even a cocktail of all blockers tested did not alter release-signal amplitude in the SNc, although the duration of the release response was curtailed. The limited involvement of voltage-dependent Ca2+ channels in somatodendritic DA release provides further evidence that minimal Ca2+ entry is required to trigger the release process, compared with that required for axon-terminal release.

N-type calcium channel alpha1B subunit (Cav2.2) knock-out mice display hyperactivity and vigilance state differences

Differential properties of voltage-dependent Ca2+ channels have been primarily ascribed to the alpha1 subunit, of which 10 different subtypes are currently known. For example, channels that conduct the N-type Ca2+ current possess the alpha1B subunit (Cav2.2), which has been localized, inter alia, to the piriform cortex, hippocampus, hypothalamus, locus coeruleus, dorsal raphe, thalamic nuclei, and granular layer of the cortex. Some of these regions have been previously implicated in metabolic and vigilance state control, and selective block of the N-type Ca2+ channel causes circadian rhythm disruption. In this study of Cav2.2-/- knock-out mice, we examined potential differences in feeding behavior, spontaneous locomotion, and the sleep-wake cycle. Cav2.2-/- mice did not display an overt metabolic phenotype but were hyperactive, demonstrating a 20% increase in activity under novel conditions and a 95% increase in activity under habituated conditions during the dark phase, compared with wild-type littermates. Cav2.2-/- mice also displayed vigilance state differences during the light phase, including increased consolidation of rapid-eye movement (REM) sleep and increased intervals between non-REM (NREM) and wakefulness episodes. EEG spectral power was increased during wakefulness and REM sleep and was decreased during NREM sleep in Cav2.2-/- mice. These results indicate a role of the N-type Ca2+ channel in activity and vigilance state control, which we interpret in terms of effects on neurotransmitter release.

Effects of voltage-sensitive calcium channel blockers on extracellular dopamine levels in rat striatum

Various subtypes of voltage-sensitive calcium channels (VSCCs) support the release of dopamine (DA) in the central nervous system. Using in vivo microdialysis, we investigate the influence of these subtypes of calcium channels on dopaminergic terminals in the rat striatum. L-type (nifedipine-sensitive), N-type (omega-conotoxin GVIA-sensitive), or N- and P/Q-type (omega-conotoxin MVIIC-sensitive) Ca2+ channels were blocked using selective antagonists injected locally, and K+-evoked DA release was measured in freely moving animals. K+ (100 mM) induced a massive increase of basal DA extracellular levels (930%) and was without significant effect on extracellular levels of DA metabolites DOPAC and HVA, and on the serotonin metabolite 5HIAA. Omega-conotoxin GVIA (1 microM) and omega-conotoxin MVIIC (1 microM) significantly reduced the K+-evoked DA release by 55 and 62%, respectively. The simultaneous application of the two conotoxins at the same concentration reduced K+-evoked DA release by 66%. Nifedipine (10 microM) had no significant effect on K-evoked DA release, while neomycin, a nonspecific VSCC blocker, produced a highly significant decrease when applied at 250 and 500 microM (56 and 75%, respectively). The compounds. however, had no effect on basal DA release and on the levels of extracellular DOPAC, HVA, and 5HIAA. These results suggest that under high and persistent conditions of membrane depolarization (15 min, 10 mM K+), striatal DA release is mainly mediated by N-type VSCCs.

Functional disorders of the sympathetic nervous system in mice lacking the alpha 1B subunit (Cav 2.2) of N-type calcium channels

N-type voltage-dependent Ca(2+) channels (VDCCs), predominantly localized in the nervous system, have been considered to play an essential role in a variety of neuronal functions, including neurotransmitter release at sympathetic nerve terminals. As a direct approach to elucidating the physiological significance of N-type VDCCs, we have generated mice genetically deficient in the alpha(1B) subunit (Ca(v) 2.2). The alpha(1B)-deficient null mice, surprisingly, have a normal life span and are free from apparent behavioral defects. A complete and selective elimination of N-type currents, sensitive to omega-conotoxin GVIA, was observed without significant changes in the activity of other VDCC types in neuronal preparations of mutant mice. The baroreflex response, mediated by the sympathetic nervous system, was markedly reduced after bilateral carotid occlusion. In isolated left atria prepared from N-type-deficient mice, the positive inotropic responses to electrical sympathetic neuronal stimulation were dramatically decreased compared with those of normal mice. In contrast, parasympathetic nervous activity in the mutant mice was nearly identical to that of wild-type mice. Interestingly, the mutant mice showed sustained elevation of heart rate and blood pressure. These results provide direct evidence that N-type VDCCs are indispensable for the function of the sympathetic nervous system in circulatory regulation and indicate that N-type VDCC-deficient mice will be a useful model for studying disorders attributable to sympathetic nerve dysfunction.

Inhibitory effects of intravenous anaesthetic agents on K+-evoked norepinephrine and dopamine release from rat striatal slices: possible involvement of P/Q-type voltage-sensitive Ca2+ channels

The role of the voltage-sensitive Ca2+ channel (VSCC) as a target for anaesthetic action remains controversial. In this study we characterized the VSCC subtypes involved in K+-evoked norepinephrine and dopamine release from rat striatal slices and used this model system to examine the effects of a range of i.v. anaesthetics on release. Nifedipine (L-channel-selective), omega-conotoxin GVI(A) (N-channel-selective), omega-agatoxin IV(A) (P-channel-selective), omega-conotoxin MVIIc (P/Q-channel-selective) and Cd2+ (non-selective), along with alphaxalone, propofol and ketamine, were used in various combinations. Omega-Agatoxin IV(A), omega-conotoxin MVIIc and Cd2+ fully (100%) inhibited norepinephrine and dopamine release. Clinically achievable concentrations of alphaxalone inhibited norepinephrine and dopamine release, with concentrations producing 25 and 50% inhibition (IC25 and IC50) of the maximum of 2.1 and 7.8 microM respectively for norepinephrine and 2.9 and 7.2 microM for dopamine. The effects of propofol were observed at the top of the clinical range and those of ketamine exceeded this range. In addition, IC50 values for alphaxalone in the presence and absence of nifedipine and omega-conotoxin GVI(A) did not differ from the control. Our data suggest that clinically achievable concentrations of alphaxalone and propofol inhibit norepinephrine and dopamine release, which is mediated predominantly through P/Q-type VSCCs, suggesting a role for these channels in anaesthetic action.

Differential recruitment of N-, P- and Q-type voltage-operated calcium channels in striatal dopamine release evoked by 'regular' and 'burst' firing

This study used the peptides omega-conotoxin GVIA, omega-agatoxin IVA and omega-conotoxin MVIIC, singly and in combination, to investigate the relative involvement of N-, P- and Q-type voltage-operated calcium channels in the control of striatal dopamine release. Electrically stimulated dopamine release was measured by fast cyclic voltammetry at carbon fibre microelectrodes in rat striatal slices. The contribution of these channel subtypes was compared in dorsolateral and medial neostriatum for 'regular' (discrete) and 'burst' stimulation modalities. In dorsolateral neostriatum, a role for N-, P- and Q-type channels was demonstrated for discrete stimulations, whilst at least one other unidentified channel was also involved in dopamine release on 'burst' stimulations. Similarly, in the medial axis of the neostriatum, N-, P- and Q-type channels were involved in dopamine release for discrete stimulations, and N-, Q- and at least one other channel type for 'burst' stimulations. However, blockade of P-type channels had no effect on dopamine release for 'burst' stimulations in the medial axis. In both regions and stimulation paradigms, N-type channels played a greater role than P/Q-type channels. In the medial axis of the neostriatum there was a smaller contribution by N- and P-type channels and the unidentified component, but a greater Q-type contribution to DA release. 'Burst' stimulations induced a lesser involvement of N- and P-type channels than discrete stimulations, and a greater role of the unidentified component. In summary, this study suggests that there is heterogeneity in the distribution of functional voltage-operated calcium channel subtypes in the neostriatum, and differences in subtype recruitment for different firing patterns.

Possible involvement of L-type voltage-gated calcium channels in release of dopamine in the striatum of irradiated rats

The object of this study was to determine the effect of exposure to gamma radiation on potassium chloride (KCl)-stimulated release of dopamine (DA) in the striatum of the rat. In addition, the effect of some calcium channel blockers [nicardipine, a blocker of the L-type voltage-gated N-type VGCC; Omega-agatoxin TK, a selective blocker of P-type VGCC; and nickel chloride (NiCl(2)), which preferentially blocks the T-type VGCC] on KCl-stimulated release of DA in the striatum in sham-irradiated and irradiated rats was determined. Exposure of rats to 1-10 Gy (60)Co gamma rays had no significant effect on KCl-stimulated release of DA in the striatum in comparison to sham-irradiated animals. Administering 100, 300 and 500 nM of Omega-agatoxin TK or 50, 100 and 200 nM of Omega-conotoxin GVIA significantly decreased the release of DA stimulated by KCl in both irradiated and sham-irradiated animals in a dose-dependent manner. However, 10, 30 and 50 microM of nicardipine decreased the release of DA in irradiated animals but not in sham-irradiated animals. It is unknown why doses of 5-20 microM NiCl(2) had no effect on the release of DA in sham-irradiated and irradiated animals. The results demonstrate that the doses of radiation used in this study had no effect on release of DA in the striatum. Multiple calcium channel types coexist to regulate release of DA. P- and N-type VGCCs are involved in release of DA in sham-irradiated and irradiated animals, whereas only L-type VGCCs are involved in release of DA in irradiated animals.

The effects of verapamil and diltiazem on N-, P- and Q-type calcium channels mediating dopamine release in rat striatum

1. The putative inhibitory effects of verapamil and diltiazem on neuronal non-L-type Ca2+ channels were studied by investigating their effects on either K+- or veratridine-evoked [3H]-dopamine ([3H]-DA) release in rat striatal slices. Involvement of N-, P- and Q-type channels was identified by sensitivity of [3H]-DA release to omega-conotoxin GVIA (omega-CTx-GVIA), omega-agatoxin IVA (omega-Aga-IVA) and omega-conotoxin MVIIC (omega-CTx-MVIIC), respectively. 2. KCl (50 mM)-evoked [3H]-DA release was abolished in the absence of Ca2+, and was insensitive to dihydropyridines (up to 30 microM). It was significantly blocked by omega-CTx-GVIA (1 microM), omega-Aga-IVA (30 nM) and was confirmed to be abolished by omega-CTx-MVIIC (3 microM), indicating involvement of N-, P- and Q-type channel subtypes. 3. Verapamil and diltiazem inhibited K+-evoked [3H]-DA release in a concentration-dependent manner. The inhibitory effects of verapamil or diltiazem (each 30 microM) were fully additive to the effect of omega-CTx-GVIA (1 microM), whereas co-application with omega-Aga-IVA (30 nM) produced similar effects to those of omega-Aga-IVA alone. 4. As shown previously, veratridine-evoked [3H]-DA release in Ca2+ containing medium exclusively involves Q-type Ca2+ channels. Here, diltiazem (30 microM) did not inhibit veratridine-evoked [3H]-DA release, whereas verapamil (30 microM) partially inhibited it, indicating possible involvement of Q-type channels in verapamil-induced inhibition. However, verapamil (30 microM) inhibited this release even in the absence of extracellular Ca2+, suggesting that Na+ rather than Q-type Ca2+ channels are involved. 5. Taken together, our results suggest that verapamil can block P- and at higher concentrations possibly N- and Q-type Ca2+ channels linked to [3H]-DA release, whereas diltiazem appears to block P-type Ca2+ channels only.

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.

Effects of drugs interfering with sodium channels and calcium channels on the release of endogenous dopamine from superfused substantia nigra slices

The importance of voltage-dependent sodium channels and different types of voltage-sensitive calcium channels for depolarisation-induced release of endogenous dopamine from dendrites and cell bodies in superfused guinea pig substantia nigra slices was investigated. The stimulatory effect of veratridine (10 microM) on dopamine release was only marginally attenuated in Ca(2+)-free medium but was completely blocked by tetrodotoxin (1 microM) and by the dopamine reuptake inhibitor GBR 12909 (10 microM). Low extracellular concentration of Na+ stimulated the dopamine release. Potassium-evoked dopamine release was completely Ca(2+)-dependent, not blocked by GBR 12909 and partially blocked by tetrodotoxin. Nifedipine (20 microM), omega-conotoxin GVIA (0.5 microM), penfluridol (5 microM), and Ni2+ (20 microM) had no effect, amiloride (1 mM) attenuated and neomycin (350 microM), and omega-agatoxin IVA (1 microM) almost totally blocked the potassium-induced dopamine release. The results suggest that veratridine released dopamine mostly by reversing the dopamine transporter. High concentrations of potassium induced release of nigral dopamine by opening of voltage-sensitive calcium channels of P/Q type but not L-type, N-type and probably not T-type. The depolarisation evoked by high concentrations of potassium seems to open voltage-sensitive calcium channels both by the depolarisation induced by potassium per se and by the secondary depolarisation induced by opening of voltage-dependent sodium channels.

Involvement of N- and P/Q- but not L- or T-type voltage-gated calcium channels in ischaemia-induced striatal dopamine release in vitro

Calcium influx and transmitter efflux are central events in the neuropathological cascade that occurs during and following cerebral ischaemia. This study explored the role of voltage-gated calcium channels (VGCCs) in ischaemia-induced striatal dopamine (DA) release in vitro. Slices (350 microm thickness) of rat neostriatum were superfused (400 ml/h) with an artificial cerebrospinal fluid (aCSF) at 34 degrees C and subjected to episodes of 'ischaemia' by reduction of the glucose concentration from 4 to 2 mM and gassing with 95% N2/5% CO2. DA release was monitored with fast cyclic voltammetry at implanted carbon fibre microelectrodes. The time to onset, time to peak, rate and magnitude of DA release were measured. Non-selective blockade of VGCCs with a high concentration of Ni2+ (2.5 mM), markedly delayed (P < 0.01) and slowed (P < 0.05) DA release but preferential blockade of T-type VGCCs with a lower concentration (200 microM) had no effect. DA release was also unaffected by selective antagonism of L-type VGCCs with nimodipine and nicardipine (10 microM each). Selective blockade of N-type VGCCs with omega-conotoxin GVIA (100 nM) delayed DA release (P < 0.05) but did not affect its rate or magnitude. Blockade of P- and possibly Q-type VGCCs with omega-agatoxin IVA (up to 200 nM) both delayed (P < 0.05) and slowed (P < 0.05) DA release. Preferential blockade of P- type VGCCs with neomycin (500 microM) also delayed (P < 0.05) and slowed (P < 0.05) DA release. These findings suggest that N-, P- and possibly Q- but not L- or T-type VGCCs mediate ischaemia-induced DA release. Although it is not possible to say, on the basis of these results, that the effects are directly upon the dopamine terminals, these calcium channels nevertheless constitute promising targets for therapeutic intervention.

Effects of nimodipine and nifedipine upon behavior and regional brain monoamines in the rat

The effects of single and repeated (9 times) administration of two dihydropyridines (DHPs), nimodipine (NIM) and nifedipine (NIF) (5 mg/kg per 12 h and 2.5 mg/kg per 12 h, IP), on the behavior of male adult rats in the holeboard and in the plus-maze, were investigated. Besides, the effects of repeated administration of the drugs on the levels of dopamine (DA), serotonin (5-HT), and their respective major metabolites in several regions of the central nervous system (CNS) were also assessed. The effects of single and repeated administration of the drugs were similar. Both DHPs caused a significant decrease in general motor activity which was evident in both tests and more marked, with the higher doses. The two exploratory parameters measured in the holeboard, i.e. head-dipping frequency and duration, were dissociated under pharmacological treatment. The drug-treated animals did not show an increased emotionality in the holeboard. However, in the plus-maze, NIF (5 mg/kg) and to a lesser extent NIM, appeared to induce some anxiety-related responses which may be secondary, at least in part, to the depressing effect on activity and exploration. Repeated administration of NIM and NIF caused an increase in striatal DA and DOPAC levels, whilst no effects were found on serotonergic system in any of the regions of the CNS analyzed.

omega-Agatoxin IVA identifies a single calcium channel subtype which contributes to the potassium-induced release of acetylcholine, 5-hydroxytryptamine, dopamine, gamma-aminobutyric acid and glutamate from rat brain slices

The voltage-dependent calcium channels (VDCCs) involved in K(+)-induced transmitter release have been studied. A maximally effective concentration of the N-type VDCC inhibitor, omega-conotoxin GVIA (GVIA) blocked the release of 5-HT (30%), DA (30%) and ACh (60%) but not that of GABA or glutamate. The O, P and Q-type VDCC inhibitor, omega-agatoxin IVA (Aga IVA, 1 microM), blocked 100% of GABA and glutamate, 70% of DA and about 50% of 5-HT and ACh release. The slopes of the inhibiton curves indicate that it acts on the same, single type of VDCC in all cases. omega-Conotoxin MVIIC (MVIIC) completely inhibited the release of all the transmitters. It is concluded that a single GVIA-insensitive type of VDCC is involved in the K(+)-induced release of all the transmitters and, in addition, N-type VDCCs, with a higher affinity for GVIA than MVIIC, are required for the release of 5-HT, DA and ACh. The non-N-type VDCC is not the O-type as it is not blocked by low (< 10 nM) concentrations of MVIIC. Further resolution of this VDCC into P or Q-type requires more selective antagonists.

Inhibitory effect of MK-801 on amantadine-induced dopamine release in the rat striatum

In the present study, we examined the effect of amantadine on extracellular dopamine levels in the rat striatum using an in vivo microdialysis. Perfusion of amantadine (0.1-1 mM) through the microdialysis probe caused an increase both in extracellular dopamine and glutamate levels in rat striatum. Amantadine was found to increase extracellular dopamine concentration in Ca(2+)-dependent manner, but the effect was not abolished by omega-conotoxin. Although intraperitoneal administration of MK-801 [(+)-5-methyl-10, 11-dihydroxy-5H-dibenzo (a,d)cyclohepten-5,10-imine] alone could not significantly alter the concentration of dopamine, it attenuated amantadine-induced increase in dopamine level. These findings suggest that an interaction between dopaminergic and glutamatergic neurotransmission is an important component in the regulation of striatal dopamine levels.

The role of various calcium and potassium channels in the regulation of somatodendritic serotonin release

We prepared slices from midbrain containing the raphe nuclei and from hippocampus of rats. The brain slices were loaded with [3H]serotonin and superfused in order to measure the release of radioactivity at rest and in response to electrical stimulation. No difference was observed in the resting and stimulated fractional release of tritium in the somatodendritic and axon terminal parts of serotonergic neurons. The selective 5-HT1A receptor agonist 8-OH-DPAT decreased the electrically induced tritium efflux from raphe nuclei slices preloaded with [3H]serotonin, and this inhibition was reversed by the 5-HT1A receptor antagonist (+)WAY-100135. The 5-HT1B receptor agonist CGS-12066B but not 8-OH-DPAT, inhibited the stimulation-evoked tritium efflux from hippocampal slices after labeling with [3H]serotonin. The electrical stimulation-evoked tritium efflux in raphe nuclei slices incubated with [3H]serotonin was completely external Ca(2+)-dependent, and omega-conotoxin GVIA and Cd2+, but not diltiazem, inhibited the tritium overflow. In raphe nuclei slices 4-aminopyridine enhanced the electrical stimulation-induced tritium release in a concentration-dependent manner. The inhibition of tritium efflux by 8-OH-DPAT was abolished with 4-aminopyridine. Glibenclamide or tolbutamide proved to be ineffective. These data indicate that (1) different 5-HT receptor subtypes (5-HT1A and 5-HT1B) regulate dendritic and axon terminal 5-HT release; (2) serotonin release from the dendrites may be regulated by the voltage-sensitive N-type Ca2+ channels; (3) the 5-HT1A receptor-mediated inhibition of serotonin release may be due to opening of voltage-sensitive K+ channels.

ATP increases extracellular dopamine level through stimulation of P2Y purinoceptors in the rat striatum

The effect of ATP on release of dopamine (DA) from rat striatum was studied using in vivo microdialysis. ATP increased the striatal extracellular levels of DA dose-dependently. These analogs produced an increase in DA according to this order of potency: 2-methylthio ATP > ATP > or = alpha,beta-methylene ATP > ADP > AMP > adenosine. Adenosine 5'-[beta, gamma imido]-triphosphate had a more prolonged effect on the increase in DA level than ATP. The ATP-induced increase in DA was inhibited by adding suramin, a nonselective P2 purinoceptor antagonist, and reactive blue 2, a P2Y purinoceptor antagonist, but not inhibited by xanthine amine congener, an adenosine receptor antagonist. Pertussis toxin reduced the increase in DA produced by ATP, which suggests that the P2 purinoceptor may be coupled with a G-protein in the rat striatum. Results suggest that P2Y purinoceptors may involve an ATP-induced increase in DA. The ATP-induced release of DA was tetrodotoxin-sensitive, Ca(2+)-dependent and was abolished by omega-conotoxin GVIA, indicating that the opening of voltage-sensitive Na+ channel and the Ca2+ influx through the N-type voltage-dependent calcium channel are both required for the ATP-induced increase in DA. The ATP-induced increase in DA is presumably due to the release of DA via the stimulation of P2Y purinoceptors in the rat striatum.

Phosphorylation- and voltage-dependent inhibition of neuronal calcium currents by activation of human D2(short) dopamine receptors

1. Activation of human D2(s) dopamine receptors with quinpirole (10 nM) inhibits omega-conotoxin GVIa-sensitive, high-threshold calcium currents when expressed in differentiated NG108-15 cells (55% inhibition at +10 mV). This inhibition was made irreversible following intracellular dialysis with the non-hydrolysable guanosine triphosphate analogue GTP-gamma-S (100 microM), and was prevented by pretreatment with pertussis toxin (1 microgram ml-1 for 24 h). 2. Stimulation of protein kinase C with the diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol (100 microM), also attenuated the inhibition of the sustained calcium current but did not affect the receptor-mediated decrease in rate of current activation. Similarly, okadaic acid (100 nM), a protein phosphatase 1/2A inhibitor, selectively occluded the inhibition of the sustained current. 3. The depression of calcium currents by quinpirole (10 nM) was enhanced following intracellular dialysis with 100 microM cyclic adenosine monophosphate (cyclic AMP, 72.8 +/- 9.8% depression), but was not mimicked by the membrane permeant cyclic GMP analogue, Sp-8-bromoguanosine-3',5':cyclic monophosphorothioate (100 microM). 4. Inhibition of calcium currents was only partly attenuated by 100 ms depolarizing prepulses to +100 mV immediately preceding the test pulse. However, following occlusion of the sustained depression with okadaic acid (100 nM) the residual kinetic slowing was reversed in a voltage-dependent manner (P < 0.05). 5. Thus pertussis toxin-sensitive G-proteins liberated upon activation of human D2(short) dopamine receptors inhibited high-threshold calcium currents in two distinct ways. The decrease in rate of calcium current activation involved a voltage-dependent pathway, whereas the sustained inhibition of calcium current involved, in part, the voltage-resistant phosphorylation by cyclic AMP-dependent protein kinases and subsequent dephosphorylation by protein phosphatases 1/2A.

Properties of the voltage-gated calcium channels mediating dopamine and acetylcholine release from the isolated rat retina

We examined the properties of voltage-gated calcium channels mediating endogenous dopamine (DA) and acetylcholine (ACh) release in the isolated rat retina. Application of 30 mM KCl elicited the release of DA and ACh, and these releases were abolished in Ca(2+)-free medium. The high K(+)-evoked DA release was largely blocked by both of omega-agatoxin IVA and omega-conotoxin MVIIC, P- and Q-type calcium channel antagonists, and partly blocked by isradipine, and L-type calcium channel antagonist, and omega-conotoxin GVIA, an N-type calcium channel antagonist. omega-Agatoxin IVA at a small dose, sufficient to block P-type channels alone, was however without effect. On the other hand, the high K(+)-evoked ACh release was partly blocked by omega-agatoxin IVA and omega-conotoxin MVIIC, but was resistant to isradipine and omega-conotoxin GVIA. Flunarizine, a non-selective T-type calcium channel antagonist, did not inhibit the release of DA and ACh. Cd2+ markedly blocked the release of both DA and ACh, Co2+ and Ni2+ slightly blocked the release of DA, and the release of ACh was not blocked by these two divalent cations. These results suggest that the high K(+)-evoked release of retinal DA is largely mediated by omega-agatoxin IVA and omega-conotoxin MVIIC sensitive calcium channels (probably Q-type channels), while the release of retinal ACh is largely mediated by as yet uncharacterized Cd2+ sensitive calcium channels. The properties of voltage-gated calcium channels involved in the release of ACh in the rat retina differ from those of DA.

Isradipine, a calcium channel blocker, attenuates the ischemia-induced release of dopamine but not glutamate in rats

This study was designed to investigate the role of the L-type voltage sensitive calcium channel blocker, isradipine, in the ischemia-induced release of neurotransmitters. Male spontaneously hypertensive rats were subjected to cerebral ischemia for 60 min by bilateral carotid artery occlusion, and recirculated for 120 min. Isradipine (0.25 mg/kg n = 6) or vehicle (n = 6) was administered subcutaneously at 20 min before ischemia. In the striatum, cerebral blood flow was determined by the hydrogen clearance method and concentrations of extracellular dopamine and glutamate were measured by in vivo brain dialysis technique. Extracellular dopamine in the vehicle-treated group increased by 180-fold from the basal level, and glutamate by 24-fold during cerebral ischemia. Isradipine significantly attenuated the ischemic release of dopamine to 33-34% (P < 0.05) of the vehicle group, while it did not affect glutamate release. It is suggested that the release mechanism of dopamine and glutamate during cerebral ischemia may be different, especially in the dependence on the L-type calcium channels.

Dopaminergic function in rat brain after oral administration of calcium-channel blockers or haloperidol. A microdialysis study

Microdialysis technique was used to study the effects of both acute and repeated oral administration of calcium-channel blockers (flunarizine, cinnarizine, verapamil, nifedipine and nicardipine) in dopaminergic function in rat brain and to compare them to the effects of haloperidol. Acute flunarizine, nicardipine or haloperidol increased extracellular levels of dopamine (DA) or metabolites. After repeated (18 days) administration, nicardipine, nifedipine, verapamil or haloperidol increased and flunarizine decreased extracellular striatal levels of dopamine or metabolites. Chronic treatment with calcium-channel blockers or haloperidol failed to block K(+)-evoked release of dopamine. This suggests that the calcium-channel blockers used in this study do not influence calcium entry necessary for DA release. An acute challenge with haloperidol caused either no change or a decrease in extracellular levels of DA or metabolites after repeated administration of calcium-channel blockers or haloperidol. This is considered to be due to the lesser response of dopaminergic neurons because of treatment. A neuroleptic-like mechanism of action together with a decrease in firing activity and/or a reduced dopamine re-uptake of dopaminergic neurons are considered.

Regulation of tyrosine hydroxylase in olfactory bulb cultures: selective inhibition of depolarization-induced increase by endogenous opioids

Regulation of tyrosine hydroxylase (TH) by second messenger pathway activators was examined in rat olfactory bulb cell cultures. The number of TH-immunoreactive neurons was increased 2-3-fold by 36 h treatments with forskolin (Fsk, 10(-6) M) or phorbol myristate acetate (PMA, 10(-7) M), but was not significantly increased by a depolarizing concentration of KCl (45 mM). In contrast, KCl increased media [Met5]enkephalin (ME) immunoreactivity 2-fold in these cultures, equivalent to stimulation with Fsk or PMA. The possibility was examined that ME or another opioid produced by the cultures selectively inhibited the TH response to KCl. Pretreatment with the opioid receptor antagonist naloxone (10(-6) M) greatly increased the number of TH-immunoreactive neurons observed in response to KCl treatment, but had no effect on basal or Fsk-stimulated TH immunostaining, nor on basal or stimulated ME release. The increase in TH-immunoreactivity observed with combined KCl plus naloxone treatment was prevented by pretreating the cultures with the calcium channel blocker nimodipine (10(-6) M), which had no effect on Fsk stimulation or basal TH immunostaining. These data suggest that endogenous opioids selectively inhibit KCl-stimulated Ca2+ entry and thus TH induction in olfactory bulb cell cultures. These cultures offer a simple model system for further study of TH regulation in dopaminergic neurons.

Release of secretoneurin and noradrenaline from hypothalamic slices and its differential inhibition by calcium channel blockers

Secretoneurin is a newly discovered peptide found in high concentrations in brain. We have studied the release of secretoneurin and noradrenaline from superfused hypothalamic slices from rat brain. Both electrical stimulation and potassium induced depolarisation released secretoneurin and noradrenaline from these slices in a calcium-dependent manner. Electrical stimulation caused a preferential release of noradrenaline when compared to the secretion elicited by high potassium. The time course of secretoneurin release was more protracted than that of noradrenaline. The calcium channel blocker omega-conotoxin inhibited only the electrically induced release of noradrenaline, whereas nifedipine inhibited only that of secretoneurin. These results establish that secretoneurin is secreted from neurons. Inhibition of this release by nifedipine is consistent with the concept that secretion from large dense core vesicles occurs at sites different from that of small vesicles and depends on calcium influx via L-type calcium channels.

Effects of ion channel blockers and phorbol ester treatments on [3H]dopamine release and neurotensin facilitation of [3H]dopamine release from rat mesencephalic cells in primary culture

In this work, we tested the effect of ion channel blockers and of phorbol ester treatments on [3H]dopamine ([3H]DA) release and neurotensin (NT)-induced facilitation of [3H]DA release from cultures of rat fetal mesencephalic cells. The potassium channel blockers tetraethylammonium and 4-aminopyridine increased basal [3H]DA release and decreased K(+)-evoked [3H]DA release, whereas apamin was without effect. K(+)-evoked [3H]DA release was decreased by omega-conotoxin and nifedipine, totally suppressed by cadmium, and unaffected by amiloride. These results show the differential sensitivity of [3H]DA release to blockade of various ion channels and suggest the involvement of N-type, L-type, and non-L-non-N-type, but not T-type, voltage-sensitive calcium channels in K(+)-evoked release. Phorbol 12-myristate 13-acetate increased both spontaneous and K(+)-evoked [3H]DA release, suggesting a modulatory action of protein kinase C on DA release in this system. Unexpectedly, however, the effects of the phorbol ester were not counteracted by the protein kinase C inhibitors H7, staurosporine, or polymyxin B. NT-induced facilitation of K(+)-evoked [3H]DA release was insensitive to most of the ion channel blockers, except cadmium (64% decrease in NT effect), suggesting that the corresponding potassium and calcium channels were not involved in the effect of NT on [3H]DA release in this system. The NT effect was totally suppressed by phorbol ester treatments, indicating a possible desensitization of the corresponding transduction mechanisms after protein kinase C activation.

Multiple Ca2+ channel types coexist to regulate synaptosomal neurotransmitter release

The regulation of excitation-secretion coupling by Ca2+ channels is a fundamental property of the nerve terminal. Peptide toxins that block specific Ca2+ channel types have been used to identify which channels participate in neurotransmitter release. Subsecond measurements of [3H]-glutamate and [3H]dopamine release from rat striatal synaptosomes showed that P-type channels, which are sensitive to the Agelenopsis aperta venom peptide omega-Aga-IVA, trigger the release of both transmitters. Dopamine (but not glutamate) release was also controlled by N-type, omega-conotoxin-sensitive channels. With strong depolarizations, where neither toxin was very effective alone, a combination of omega-Aga-IVA and omega-conotoxin produced a synergistic inhibition of 60-80% of Ca(2+)-dependent dopamine release. The results suggest that multiple Ca2+ channel types coexist to regulate neurosecretion under normal physiological conditions in the majority of nerve terminals. P- and N-type channels coexist in dopaminergic terminals, while P-type and a omega-conotoxin- and omega-Aga-IVA-resistant channel coexist in glutamatergic terminals. Such an arrangement could lend a high degree of flexibility in the regulation of transmitter release under diverse conditions of stimulation and modulation.

The non-dihydropyridine L-type voltage-sensitive calcium channel activator FPL 64176 enhances K(+)-evoked efflux of [3H]norepinephrine from rat neocortical slices

The non-dihydropyridine FPL 64176 (methyl-2,5-dimethyl-4-(2-phenylmethyl)benzoyl-[1-H]pyrrole-3-carboxy la te) was tested for an interaction with neuronal L-type voltage-sensitive calcium channels (L-VSCCs) by using a [3H]isradipine ([3H]ISR) binding assay, and for its ability to enhance K(+)-evoked [3H]norepinephrine ([3H]NE) release from rat neocortical slices. The classical L-VSCC activator, the dihydropyridine (DHP) BAY K 8644, was also used for comparative purposes. FPL 64176 and BAY K 8644 both produced a similar concentration-dependent enhancement of 15 mM K(+)-evoked [3H]NE release which could be completely blocked by the L-VSCC blocker ISR (0.1 microM). FPL 64176, in contrast to BAY K 8644, was a very weak inhibitor of [3H]ISR binding to L-VSCCs. These findings indicate that FPL 64176 is a novel non-dihydropyridine L-VSCC activator, most probably by acting on a site different from the DHP binding site.

Calcium channels at the adrenergic neuroeffector junction in the rabbit ear artery

Neurotransmitter release is dependent on influx of Ca2+ through voltage-operated calcium channels (VOCCs). These channels may be divided into L, N, T and P subtypes. To investigate the subtypes of VOCC involved in transmitter release from adrenergic nerves in the isolated rabbit ear artery, the effects of some subtype selective VOCC antagonists were examined on contractile responses induced by electrical field stimulation (EFS), and exposure to an isosmolar (low Na+, normal Cl- content) or a hyperosmolar (normal Na+, high Cl- content) 60 mM K+ solution. Tetrodotoxin (TTX) and the L channel blocker nimodipine were present in the latter experiments to inhibit sodium-dependent action potential discharge and the direct contractile effect of K+ depolarization on the smooth muscle cells. Prazosin abolished the contractile effect of EFS, indicating that the response was elicited by activation of adrenergic nerves. The EFS-induced contractions were concentration-dependently inhibited by the N channel blocker omega-conotoxin (pIC50 = 9.0) and the proposed L channel blocker T-cadinol (pIC50 = 4.5), while nimodipine and the T channel blocker tetramethrin had no effect. The isosmolar and hyperosmolar K+ solutions induced a prazosin-sensitive contraction, amounting to 46% and 10% of the response to 10(-5) M noradrenaline (NA), respectively. omega-Conotoxin inhibited the contractile response to the hyperosmolar K+ solution, but not that to the isosmolar K+ solution. T-cadinol preferentially inhibited the response to the hyperosmolar K+ solution. Tetramethrin had no effect on contractions induced by either type of K+ solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Isradipine attenuates the ischemia-induced release of dopamine in the striatum of the rat

We examined the effect of isradipine, a blocker of L-type voltage-sensitive Ca2+ channels (VSCCs), on the ischemia-induced release of dopamine in the rat striatum. Perfusion of 200 micrograms/ml isradipine in the striatum did not alter extracellular dopamine concentrations monitored by microdialysis. However, a marked increase (145-fold) in dopamine level during forebrain ischemia, developed by bilateral carotid artery occlusion, was attenuated significantly by 37% by isradipine whereas the intensity of ischemia, monitored by striatal blood flow, was unchanged. These results suggest that isradipine attenuates the ischemia-induced release of dopamine via blockade of L-type VSCCs on dopaminergic neurons.

Involvement of calcium channels in depolarization-evoked release of adenosine from spinal cord synaptosomes

The potential involvement of L- and N-type voltage-sensitive calcium (Ca2+) channels and a voltage-independent receptor-operated Ca2+ channel in the release of adenosine from dorsal spinal cord synaptosomes induced by depolarization with K+ and capsaicin was examined. Bay K 8644 (10 nM) augmented release of adenosine in the presence of a partial depolarization with K+ (addition of 6 mM) but not capsaicin (1 and 10 microM). This augmentation was dose dependent from 1 to 10 nM and was followed by inhibition of release from 30 to 100 nM. Nifedipine and nitrendipine inhibited the augmenting effect of Bay K 8644 in a dose-dependent manner, but neither antagonist had any effect on release of adenosine produced by K+ (24 mM) or capsaicin (1 and 10 microM). omega-Conotoxin inhibited K(+)-evoked release of adenosine in a dose-dependent manner but had no effect on capsaicin-evoked release. Ruthenium red blocked capsaicin-induced release of adenosine but had no effect on K(+)-evoked release. Although L-type voltage-sensitive Ca2+ channels can modulate release of adenosine when synaptosomes are partially depolarized with K+, N-type voltage-sensitive Ca2+ channels are primarily involved in K(+)-evoked release of adenosine. Capsaicin-evoked release of adenosine does not involve either L- or N-type Ca2+ channels, but is dependent on a mechanism that is sensitive to ruthenium red.

Comparison of the effects of voltage-sensitive calcium channel antagonism on the electrically stimulated release of dopamine and norepinephrine in vivo

IN vivo electrochemistry was used to monitor the effects of several voltage-sensitive calcium channel (VSCC) antagonists (e.g. divalent metal ions, diltiazem and omega-conotoxin GVIA (omega-CT) on the electrically evoked release of dopamine (DA) in the striatum and norepinephrine (NE) in the thalamus of the anesthetized rat. The results suggest that the N-type voltage-sensitive calcium channel is the primary VSCC involved in the electrically stimulated release of DA in the striatum, whereas stimulated release of NE in the thalamus was only partially dependent on N-type VSCC. In addition, DA release appears to be more sensitive to VSCC antagonism than does NE release with the in vivo application used in this study.

Omega-conotoxin GVIA inhibits release of noradrenaline from rat hippocampal slices in the absence of extracellular calcium

Slices of hippocampus of the rat, preincubated with [3H]noradrenaline ([3H]NA), were used to investigated the effects of omega-conotoxin GVIA (omega-CTX) on the release of [3H]NA evoked by 3,4-diaminopyridine (3,4-DAP, 200 microM), veratridine (0.7 microM) or monensin (0.01 microM) in the absence of extracellular CA2+. The 3H outflow, evoked by 3,4-DAP or veratridine, was inhibited by tetrodotoxin (TTX) or omega-CTX but the 3H outflow evoked by monensin was neither affected by TTX nor by omega-CTX in Ca(2+)-free medium, containing 1 mM EGTA. The release response to 3,4-DAP or veratridine was also blocked by omega-CTX in a concentration-dependent manner in Ca(2+)-free medium, containing 2.5 mM Mg2+ and the blockade was still complete after washing for 20 min with omega-CTX-free medium. The findings suggest that, under these conditions, the toxin might also block sodium channels.

Effects of stimulus intensity on the inhibition by omega-conotoxin GVIA and neomycin of K(+_-evoked [3H]norepinephrine release from hippocampal brain slices and synaptosomal calcium influx

The effects of various K+ concentrations on the inhibition of [3H]norepinephrine release from rat hippocampal brain slices and evoked synaptosomal 45Ca2+ influx by omega-conotoxin GVIA (omega-CgTx) and neomycin were examined. K+ (15-75 mM) caused a concentration-dependent release of [3H]norepinephrine that was greater than 90% dependent on extracellular calcium. The ability of omega-CgTx to inhibit [3H]norepinephrine release was optimal at 25 mM K+ and was reduced substantially at higher concentrations of K+. omega-CgTx maximally inhibited [3H]norepinephrine release by 49% (15 mM K+), 58% (25 mM K+), 22% (50 mM K+), and 12% (75 mM K+). In contrast, neomycin caused a concentration-dependent and virtually complete inhibition of [3H]norepinephrine release at all concentrations of K+, with IC50 values of 210 microM (15 mM K+), 150 microM (25 mM K+), 450 microM (50 mM K+), and 1500 microM (75 mM K+). omega-CgTx (1 microM) had little effect (10% or less inhibition) on hippocampal synaptosomal 45Ca2+ influx at any concentration of K+, whereas 3 mM neomycin caused at least 75% inhibition of 45Ca2+ influx, with the largest inhibition (96%) occurring at 25 mM K+. The results suggest that increasing stimulus intensity decreases the contribution of N-type voltage-sensitive calcium channels (VSCC) in mediating K(+)-evoked release of [3H]norepinephrine. The comparative absence of omega-CgTx-sensitive synaptosomal 45Ca(2+)-influx sites suggests that N-type calcium channels are a small subset of channels in rat hippocampal synaptosomes. The demonstration that neomycin can inhibit omega-CgTx-sensitive and -insensitive neurotransmitter release and calcium influx suggests that neomycin may block N-type VSCC as well as non-N-type VSCC.

Effects of L- and N-type Ca2+ channel antagonists on excitatory amino acid-evoked dopamine release

In the present study we tested the effect of dihydropyridine (DHP) Ca2+ channel antagonists and of omega-conotoxin GVIA on [3H]dopamine (DA) release evoked by the activation of excitatory amino acid (EAA) receptors in cultures of fetal rat ventral mesencephalon, in order to investigate the role of voltage-sensitive L- and N-type Ca2+ channels in these EAA-mediated processes. Micromolar concentrations (10-30 microM) of DHP L-type Ca2+ channel antagonists inhibited [3H]DA release evoked by N-methyl-D-aspartate (NMDA), kainate, quisqualate or veratridine. [3H]DA release evoked by the L-type Ca2+ channel agonist, Bay K 8644, was inhibited by lower concentrations (0.1-1 microM) of the DHP antagonist, nitrendipine, than was the release evoked by EAAs. The DHP antagonist, (+)-PN 200-110, was more potent than (-)-PN 200-110 in inhibiting [3H]DA release evoked by Bay K 8644, but the two stereoisomers were equipotent in inhibiting NMDA-evoked release. These results indicate that activation of L-type Ca2+ channels is able to evoke [3H]DA release. However activation of L-type channels is not involved in EAA-induced [3H]DA release and therefore inhibition of EAA-induced [3H]DA release by micromolar concentrations of DHPs must be mediated by actions other than inhibition of L-type Ca2+ channels. omega-Conotoxin GVIA (3 microM) had no effect on [3H]DA release evoked by Bay K 8644, indicating that the toxin may selectively inhibit N-type channels in this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of voltage-sensitive calcium channel antagonists on the release of 5-hydroxytryptamine from rat hippocampus in vivo

The effect of calcium channel antagonists on the release of 5-hydroxytryptamine from the hippocampus of the chloral hydrate-anaesthetised rat was studied using the technique of intracerebral microdialysis. As the basal concentration of 5-hydroxytryptamine was close to the limit of detection of the HPLC method (8 fmol), the 5-hydroxytryptamine reuptake inhibitor, fluoxetine (10 microM), was included in the perfusion fluid. The L-type voltage-sensitive calcium channel antagonists, PN200-110, diltiazem, and verapamil, all passed through the dialysis membrane, giving a recovery of 20-30%. The N-type voltage-sensitive calcium channel antagonist, omega-conotoxin, penetrated less readily (12% recovery). The dihydropyridine, PN200-110, adhered to the probe, resulting in an effective concentration at the membrane 30% of that in the perfusion fluid. The concentration of 5-hydroxytryptamine in the dialysate samples was reduced by 60% in the absence of calcium. The L channel antagonists had little effect on the release of 5-hydroxytryptamine, which was inhibited, in a dose-dependent manner, to a maximum of 40% by omega-conotoxin. It is concluded that, under physiological conditions, the release of 5-hydroxytryptamine from the rat hippocampus is dependent on the entry of calcium through N-type voltage-sensitive calcium channels, although another calcium channel may also be involved.

A toxin (Aga-GI) from the venom of the spider Agelenopsis aperta inhibits the mammalian presynaptic Ca2+ channel coupled to glutamate exocytosis

Venom of the funnel web spider Agelenopsis aperta was fractionated and screened for activity against the mammalian presynaptic, voltage-dependent Ca2+ channel coupled to glutamate exocytosis. A purified toxin (Aga-GI) from this venom inhibits glutamate exocytosis evoked by elevated potassium or by 4-aminopyridine but is without effect on ionomycin-evoked release. At the same time a partial inhibition of the depolarisation-evoked elevation of cytoplasmic free Ca2+ is seen. The toxin does not inhibit 4-aminopyridine- or potassium-evoked depolarisation, or block Ca(2+)-dependent, potassium-evoked [3H]noradrenaline release. The results indicate that the venom contains a toxin capable of inhibiting the presynaptic voltage-dependent Ca2+ channel coupled to glutamate exocytosis in the mammalian central nervous system. This channel is resistant to block by either omega-conotoxin GVIA or nifedipine. Thus Aga-GI is a novel tool with which to probe this elusive neuronal calcium channel.