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: Relationship between activation of opioid receptors
and muscarinic autoinhibition in acetylcholine (ACh) release from the myenteric
plexus was studied in longitudinal muscle myenteric plexus (LMMP) preparations
of guinea pig ileum. A mu-receptor agonist, [D-Ala2, N-Me-Phe4,
Gly5-ol] enkephalin (DAMGO), at a concentration of 1 microM inhibited
the ACh release evoked by electrical field stimulation (EFS) at 1 Hz but
not at 10 Hz. After the muscarinic autoreceptors were blocked with atropine
(1 microM), DAMGO inhibited EFS-evoked ACh release also at 10 Hz. After
the autoreceptors were potently activated with muscarine (200 microM), the
inhibitory effect of DAMGO at 1 Hz was abolished. A kappa-receptor agonist,
U-50,488, at 1 microM inhibited the EFS-evoked ACh release both at 1 and
10 Hz. U-50,488 inhibited ACh release regardless of the presence of atropine
or muscarine. A delta-agonist, enkephalin [D-PEN2.5] (PDPDE),
did not show any significant effect. On the other hand, a selective mu-receptor
antagonist, cyprodime, increased ACh release evoked by EFS at 1 Hz, but
not at 10 Hz. After the autoreceptors were blocked, cyprodime increased
EFS-evoked ACh release also at 10 Hz. The selective kappa-receptor antagonist,
nor-binaltorphimine, did not affect ACh release in the absence or presence
of atropine. The results suggest that endogenous opioid(s) inhibits ACh
release by activating mu-, but not kappa- and delta-receptors in the LMMP
of guinea pig ileum and that the inhibitory effect of endogenous opioid(s)
in the ACh release is important when muscarinic autoinhibition mechanism
does not fully work.
Keywords: mu-Receptor, Muscarinic autoinhibition, Acetylcholine release,
Myenteric plexus, Ileum (guinea pig)