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Pharmacological evidence for an omega-conotoxin, dihydropyridine-insensitive neuronal Ca2+ channel
P M Lundy1, R Frew, T W Fuller
1Biomedical Defence Section, Defence Research Establishment Suffield, Ralston, Alberta, Canada.
Abstract:
Inactivation of N-type voltage-sensitive Ca2+ channels (VSCC) with omega-conotoxin (omega-CgTx) in tissue obtained from chicken brain produces a concentration dependent (0.01-0.1 microM) inhibition of K(+)-stimulated Ca2+ influx (delta K+), the rise in [Ca2+]i and acetylcholine (ACh) release. In identical preparations from rat brain, Ca2+ influx and the rise in [Ca2+]i were only marginally affected by much higher (1-10 microM) concentrations of omega-CgTx. The release of ACh, however, was inhibited to the same degree with similar amounts of omega-CgTx as those used in chicken brain. An L-type VSCC inhibitor failed to affect any of these parameters alone, or to augment the effect of omega-CgTx. The results suggest that almost all the VSCC in chicken brain are of the N type and that these channels regulate neurotransmitter release. In rat brain, on the other hand, Ca2+ channels resistant to N- or L-type blockers account for almost 75% of the measurable Ca2+ influx and rise in [Ca2+]i. The conspicuous dissociation between the regulation of Ca2+ influx and ACh release demonstrated in rat brain by using omega-CgTx, suggest that neurotransmitter release is governed by only a small proportion of strategically located N-type, omega-CgTx sensitive, VSCC in the presynaptic terminal.
Insights
N-type voltage-sensitive calcium channels (VSCC) are crucial for neurotransmitter release in chicken brains. In rat brains, however, these channels play a smaller role in calcium influx but are critical for acetylcholine release.
Area of Science:
- Neuroscience
- Neuropharmacology
- Molecular Biology
Background:
- Voltage-sensitive calcium channels (VSCC) are critical for neurotransmitter release.
- N-type VSCC are implicated in regulating neuronal excitability and synaptic transmission.
Purpose of the Study:
- To investigate the role of N-type VSCC in regulating calcium influx and acetylcholine release in chicken and rat brains.
- To compare the contribution of N-type VSCC to these processes across different species.
Main Methods:
- Using omega-conotoxin (omega-CgTx), a specific N-type VSCC blocker.
- Measuring K(+)-stimulated Ca2+ influx, intracellular Ca2+ ([Ca2+]i) rise, and acetylcholine (ACh) release in brain tissue preparations.
- Employing an L-type VSCC inhibitor for comparative analysis.
Main Results:
- In chicken brain, omega-CgTx (0.01-0.1 microM) significantly inhibited Ca2+ influx, [Ca2+]i rise, and ACh release, indicating N-type VSCC dominance.
- In rat brain, higher omega-CgTx concentrations (1-10 microM) only marginally affected Ca2+ influx and [Ca2+]i rise, but potently inhibited ACh release.
- L-type VSCC blockers had no significant effect alone or in combination with omega-CgTx.
Conclusions:
- Chicken brain VSCC are predominantly N-type and directly regulate neurotransmitter release.
- In rat brain, a significant portion of Ca2+ influx is mediated by non-N-type, non-L-type channels.
- Neurotransmitter release in rat brain is governed by a small subset of strategically located, N-type VSCC, demonstrating a dissociation between Ca2+ influx and release regulation.