Depolarization initiates phasic acetylcholine release by relief of a tonic block imposed by presynaptic M2 muscarinic receptors

J Neurophysiol. 2005 Jun;93(6):3257-69. doi: 10.1152/jn.01131.2004. Epub 2005 Feb 9.

Abstract

The role of presynaptic muscarinic autoreceptors in the initiation of phasic acetylcholine (ACh) release at frog and mouse neuromuscular junctions was studied by measuring the dependency of the amount (m) of ACh release on the level of presynaptic depolarization. Addition of methoctramine (a blocker of M2 muscarinic receptors), or of acetylcholinesterase (AChE), increased release in a voltage-dependent manner; enhancement of release declined as the depolarizing pulse amplitude increased. In frogs and wild-type mice the slope of log m/log pulse amplitude (PA) was reduced from about 7 in the control to about 4 in the presence of methoctramine or AChE. In M2 muscarinic receptor knockout mice, the slope of log m/log PA was much smaller (about 4) and was not further reduced by addition of either methoctramine or AChE. The effect of a brief (0.1 ms), but strong (-1.2 microA) depolarizing prepulse on the dependency of m on PA was also studied. The depolarizing prepulse had effects similar to those of methoctramine and AChE. In particular, it enhanced release of test pulses in a voltage-dependent manner and reduced the slope of log m/log PA from about 7 to about 4. Methoctramine + AChE occluded the prepulse effects. In knockout mice, the depolarizing prepulse had no effects. The cumulative results suggest that initiation of phasic ACh release is achieved by depolarization-mediated relief of a tonic block imposed by presynaptic M2 muscarinic receptors.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetylcholine / metabolism*
  • Acetylcholinesterase / pharmacology
  • Animals
  • Diamines / pharmacology
  • Dose-Response Relationship, Drug
  • Dose-Response Relationship, Radiation
  • Electric Stimulation / methods
  • In Vitro Techniques
  • Linear Models
  • Mice
  • Mice, Knockout
  • Neuromuscular Junction / metabolism
  • Neuromuscular Junction / radiation effects
  • Parasympatholytics / pharmacology
  • Presynaptic Terminals / drug effects
  • Presynaptic Terminals / metabolism*
  • Presynaptic Terminals / radiation effects
  • Rana ridibunda
  • Receptor, Muscarinic M2 / deficiency
  • Receptor, Muscarinic M2 / physiology*
  • Synaptic Transmission / drug effects
  • Synaptic Transmission / physiology
  • Synaptic Transmission / radiation effects
  • Time Factors

Substances

  • Diamines
  • Parasympatholytics
  • Receptor, Muscarinic M2
  • Acetylcholinesterase
  • Acetylcholine
  • methoctramine