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Inhibition of transmitter release from sympathetic nerve endings by omega-conotoxin
J A Brock1, T C Cunnane, R J Evans
1University Department of Pharmacology, Oxford, United Kingdom.
Abstract:
1. The effects of the calcium channel blocker, omega-conotoxin, on sympathetic neuroeffector function in the guinea-pig vas deferens have been investigated using a combination of mechanical and electrophysiological recording techniques. 2. Biphasic contractions evoked by electrical field stimulation were irreversibly abolished by omega-conotoxin (10-100 nmol/L). 3. Electrically evoked excitatory junction potentials and currents were irreversibly blocked by omega-conotoxin (10-100 nmol/L). Spontaneous excitatory junction potentials and currents were unaffected by this treatment. 4. omega-Conotoxin did not block impulse propagation in the nerve terminals. However, in three of four experiments omega-conotoxin caused a decrease in the size of the nerve terminal impulse. 5. These findings support the suggestion the omega-conotoxin acts prejunctionally to inhibit sympathetic neuroeffector by interfering with depolarization-secretion coupling.
Insights
Omega-conotoxin, a calcium channel blocker, irreversibly inhibits sympathetic neuroeffector function in guinea-pig vas deferens. It blocks electrically evoked responses by interfering with nerve terminal depolarization and secretion coupling.
Area of Science:
- Neuropharmacology
- Autonomic Nervous System Physiology
Background:
- Sympathetic neuroeffector function is crucial for regulating smooth muscle activity.
- Calcium channels play a vital role in neurotransmitter release at neuroeffector junctions.
Purpose of the Study:
- To investigate the prejunctional effects of omega-conotoxin on sympathetic neuroeffector function.
- To elucidate the mechanism by which omega-conotoxin modulates neurotransmission in the guinea-pig vas deferens.
Main Methods:
- Mechanical recording of muscle contractions.
- Electrophysiological techniques to record junction potentials and currents.
- Electrical field stimulation of sympathetic nerves.
Main Results:
- Omega-conotoxin (10-100 nmol/L) irreversibly abolished biphasic contractions.
- Electrically evoked excitatory junction potentials and currents were irreversibly blocked.
- Spontaneous events and nerve impulse propagation remained unaffected, though nerve terminal impulse size decreased.
Conclusions:
- Omega-conotoxin acts prejunctionally to inhibit sympathetic neuroeffector function.
- The mechanism involves interference with depolarization-secretion coupling at the nerve terminal.
- This highlights the role of specific calcium channels in sympathetic neurotransmission.