Hideyuki Nishiwaki (1,2), Noriko Saitoh (1), Hideaki Nishio (1), Tadayoshi
Takeuchi (1,3) and Fumiaki Hata (1,3,*)
(1) Department of Veterinary Pharmacology, College of Agriculture and
(3) Department of Molecular Physiology and Biochemistry, Research Institute
for Advanced Science and Technology, Osaka Prefecture University, Sakai
599-8531, Japan
(2) Kawanishi Pharma Research Institute, Nippon Boehringer Ingelheim Co.,
Ltd., Kawanishi 666-0131, Japan
(*) To whom correspondence should be addressed (1).
Abstract: The relationship between muscarinic autoinhibition
and the inhibitory effect of morphine on acetylcholine (ACh) release was
investigated in a longitudinal muscle with myenteric plexus (LMMP) preparation
of guinea pig ileum. Morphine (10 microM) inhibited spontaneous and evoked
ACh release by electrical field stimulation (EFS) at 1 Hz but not at 10
Hz. Atropine (1 microM) did not affect the resting ACh release, but it significantly
increased EFS-evoked release, suggesting activation of muscarinic autoreceptors
by ACh released during EFS. Only when the autoinhibition was weakened by
atropine, morphine exhibited an inhibitory effect on the EFS-evoked release
at 10 Hz. Bethanechol (300 microM) inhibited the EFS-evoked release at 1
Hz but not 10 Hz, suggesting that muscarinic autoreceptors are partially
or almost fully activated at 1 or 10 Hz stimulation, respectively. After
bethanechol treatment, morphine did not exhibit its inhibitory effect on
the EFS-evoked release at 1 Hz. Naloxone (1 microM) increased spontaneous
and EFS-evoked ACh release at 1 Hz but not at 10 Hz. Following treatment
with atropine, naloxone also increased ACh release at 10-Hz stimulation.
These results suggest that morphine and an endogenous opioid inhibit ACh
release from LMMP preparations when muscarinic autoinhibition mechanism
does not fully work. This inhibitory effect of morphine is discussed in
relation to the calcium sensitivity of the preparations in ACh release.
Keywords: Muscarinic autoinhibition, Morphine, Acetylcholine release,
Myenteric plexus, Ileum (guinea pig)