Bretylium abolishes neurotransmitter release without necessarily abolishing the nerve terminal action potential in sympathetic terminals
Article first published online: 29 JAN 2009
2008 British Pharmacological Society
British Journal of Pharmacology
Volume 153, Issue 4, pages 831–839, February 2008
How to Cite
Brain, K. L. and Cunnane, T. C. (2008), Bretylium abolishes neurotransmitter release without necessarily abolishing the nerve terminal action potential in sympathetic terminals. British Journal of Pharmacology, 153: 831–839. doi: 10.1038/sj.bjp.0707623
- Issue published online: 29 JAN 2009
- Article first published online: 29 JAN 2009
- (Received September 17, 2007, Revised October 18, 2007, Accepted October 23, 2007)
- vas deferens;
Background and purpose:
The antidysrhythmic bretylium is useful experimentally because it selectively abolishes neurotransmitter release from sympathetic peripheral nerve terminals. Its mechanism of action seemed settled, but recent results from optical monitoring of single terminals now suggests a new interpretation.
Orthograde transport of a dextran-conjugated Ca2+ indicator to monitor Ca2+ in nerve terminals of mouse isolated vas deferens with a confocal microscope. In some experiments, local neurotransmitter release was detected by monitoring neuroeffector Ca2+ transients (NCTs) in adjacent smooth muscles, a local measure of purinergic transmission. Sympathetic terminals were identified with catecholamine fluorescence (UV excitation) or post-experiment immunohistochemistry.
Bretylium (10 μM) abolished NCTs at 60/61 junctions over the course of 2 h, indicating effective abolition of neurotransmitter release. However, bretylium did not abolish the field stimulus-induced Ca2+ transient in most nerve terminals, but did increase both action potential delay (by 2±0.4 ms) and absolute refractory period (by 4±2 ms). Immunohistochemistry demonstrated that 85–96% of terminals orthogradely filled with a dextran-conjugated fluorescent probe contained Neuropeptide Y (NPY). A formaldehyde–glutaraldehyde-induced catecholamine fluorescence (FAGLU) technique was modified to allow sympathetic terminals to be identified with a Ca2+ indicator present. Most terminals contained catecholamines (based on FAGLU) or secrete ATP (as NCTs in adjacent smooth muscle cells are abolished).
Conclusions and implications:
Bretylium can inhibit neurotransmitter release downstream of Ca2+ influx without abolishing the nerve terminal action potential. Bretylium-induced increases in the absolute refractory period permit living sympathetic terminals to be identified.
British Journal of Pharmacology (2008) 153, 831–839; doi:10.1038/sj.bjp.0707623; published online 10 December 2007