The effects of excitatory amino acids on isolated gut segments of the rat.

Glutamate and aspartate are excitatory neurotransmitters in both central and peripheral nervous systems, acting on ionotropic and metabotropic receptors. In our study we have examined the effects of glutamate, aspartate, N-methyl-d-aspartate (NMDA), kainic acid and (+/-)-1-aminocyclopentane-cis-1,3-dicarboxylic acid (ACPD) on tone and spontaneous activity of isolated rat gastric fundus, jejunum, ileum, ascending colon and rectum. Both glutamate and aspartate produced concentration-dependent tonic contractions of rat fundus and rectum; the other gut segments used in the study were not responsive. While only NMDA and kainic acid produced concentration-dependent tonic contractions of isolated rat gastric fundus, all three type-selective agonists of glutamate receptors (NMDA, kainic acid and ACPD) produced tonic contractions of isolated rat rectum. The results of our study suggest that glutamate and aspartate in rat gastric fundus activate excitatory intrinsic neurons through only ionotropic receptors (NMDA and non-NMDA receptors), while the same action in rat rectum is mediated through both ionotropic and metabotropic receptors.