Evidence for a glutamatergic modulation of the cholinergic function in the human enteric nervous system via NMDA receptors

Abstract

Several reports suggest that enteric cholinergic neurons are subject to a tonic inhibitory modulation, whereas few studies are available concerning the role of facilitatory pathways. Glutamate, the main excitatory neurotransmitter in the central nervous system (CNS), has recently been described as an excitatory neurotransmitter also in the guinea-pig enteric nervous system (ENS). The present study aimed at investigating the presence of glutamatergic neurons in the ENS of the human colon. At this level, the presence of ionotropic glutamate receptors of the NMDA type, and their possible interaction with the enteric cholinergic function was also studied. In the human colon, l-glutamate and NMDA concentration dependently enhance spontaneous endogenous acetylcholine overflow in Mg 2+-free buffer, both effects being significantly reduced by the antagonists, (±)-2-amino-5-phosphonopentanoic acid (±AP5) and 5,7-diCl-kynurenic acid. In the presence of Mg 2+, the facilitatory effect of l-glutamate changes to inhibition, while the effect of NMDA is significantly reduced. In addition, morphological investigations reveal that glutamate- and NR1-immunoreactivities are present in enteric cholinergic neurons and glial cells in both myenteric and submucosal plexus. These findings suggest that, as described for the guinea-pig ileum, glutamatergic neurons are present in enteric plexuses of the human colon. Modulation of the cholinergic function can be accomplished through NMDA receptors.