Presynaptic effects of muscarine on ACh release at the frog neuromuscular junction

Authors

  • I. Slutsky,

    1. The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem, Israel
    Search for more papers by this author
  • H. Parnas,

    1. The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem, Israel
    Search for more papers by this author
  • I. Parnas

    1. The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem, Israel
    Search for more papers by this author

Corresponding author I. Parnas: The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem, Israel. Email: ruthy@vms.huji.ac.il

Abstract

  • 1Presynaptic effects of muscarine on neurotransmitter release were studied at the frog neuromuscular junction, using focal depolarization of the presynaptic terminal to different levels.
  • 2Muscarine (10 μM) had a dual effect on ACh release: concomitant inhibition and enhancement of release at the same patch of presynaptic membrane.
  • 3These two effects were maximal at low depolarizing pulses and diminished as depolarization increased.
  • 4At low depolarizing pulses, atropine (1 μM) enhanced release, suggesting that ACh in the synaptic cleft causes a net tonic inhibition of ACh release.
  • 5In the presence of the M2 antagonist methoctramine (1 μM), muscarine (10 μM) enhanced ACh release.
  • 6In the presence of the M1 antagonist pirenzepine (10 μM), muscarine (10 μM) produced stronger inhibition.
  • 7These results show that the M2 receptor is responsible for inhibition of ACh release, while the M1 receptor is responsible for its enhancement.
  • 8The inhibitory effect of muscarine did not depend on extracellular [Ca2+]. Enhancement of release was abolished at low extracellular [Ca2+].
  • 9The muscarine inhibitory effect was not associated with a reduction of Ca2+ current, while release enhancement was associated with an increase of Ca2+ current.

Ancillary