Evidence for a glutamatergic neural pathway in the myenteric plexus

The objective of this study was to determine whether L-glutamate (L-Glu) may serve as a neurotransmitter candidate in the guinea pig myenteric plexus. We observed that [3H]Glu and gamma-[3H]aminobutyric acid were synthesized from [3H]glutamine and released from neurons of the myenteric plexus during K+ and 1,1-dimethyl-4-phenylpiperazinium-evoked depolarization in a concentration-dependent manner. Muscle tension studies performed on ileal longitudinal muscle-myenteric plexus (LM-MP) preparations revealed that L-Glu [mean effective dose (ED50) 2.5 x 10(-5) M] produced concentration-dependent contractions, which were unaffected by hexamethonium but abolished by tetrodotoxin, atropine, and magnesium, suggesting that L-Glu acts via N-methyl-D-aspartate (NMDA)-type receptors that stimulate a cholinergic neural pathway unaffected by ganglionic blockade. In addition, L-Glu (ED50 4 x 10(-5) M) and NMDA (ED50 2 x 10(-4) M) stimulated concentration-dependent release of [3H]acetylcholine (ACh) from LM-MP sections, which was inhibited by tetrodotoxin, magnesium, and the NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid (AP-5). L-Glu-mediated release of [3H]ACh was enhanced by theophylline (10-6 M) and 3-isobutyl-1-methylxanthine (1 mM) and was significantly reduced by the adenylate cyclase inhibitor, 2',5'-dideoxyadenosine (10(-4) M) and somatostatin-14 (10(-6) M), which inhibits adenosine 3',5'-cyclic monophosphate (cAMP)-dependent cholinergic transmission in the myenteric plexus. These studies suggest that L-Glu may serve as an excitatory neurotransmitter in the myenteric plexus via its action on NMDA-type receptors, which are coupled to cAMP-dependent release of ACh.