CHARACTERIZATION OF METABOTROPIC GLUTAMATE RECEPTOR-MEDIATED NITRIC-OXIDE PRODUCTION IN-VIVO

We tested the hypothesis that stimulation of metabotropic glutamate re ceptors (mGluRs) increases nitric oxide (NO) production in the hippoca mpus in vivo. Microdialysis probes were placed bilaterally into the CA (3) region of the hippocampus of adult Sprague-Dawley rats under pento barbital anesthesia. Probes were perfused for 5 h with artificial cere brospinal fluid (CSF) containing 3 mu M [C-14]-L-arginine. Recovery of [C-14]-L-citrulline in the effluent was used as a marker of NO produc tion. In nine groups of rats, increases in [C-14]-L-citrulline recover y were compared between right- and left-sided probes perfused with var ious combinations of the selective mGluR agonist, trans-(1S,3R)-1-amin o-1,3-cyclopentanedicarboxylic acid (ACPD); the mGluR antagonist, (+/- )-alpha-methyl-4-carboxyphenylglycine (MCPG); the NO synthase inhibito r, N-nitro-L-arginine (LNNA); the ryanodine sensitive calcium-release channel inhibitor dantrolene, the non-N-methyl-D-aspartate (NMDA); rec eptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); the NMDA receptor antagonist 0,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imin e (MK-801); and the Na+ channel blocker, tetrodotoxin. Recovery of [C- 14]-L-citrulline during perfusion with artificial CSF progressively in creased to 90 +/-21 fmol/min (+/-SD) over 5 h. Perfusion in the contra lateral hippocampus with 1 mM ACPD augmented [C-14]-L-citrulline recov ery to 250+/-81 fmol/min. Perfusion of 1 mM nitroarginine + ACPD inhib ited [C-14]-L-citrulline recovery compared to that with ACPD alone. Pe rfusion with 1 mM MCPG + ACPD attenuated ACPD enhanced [C-14]-L-citrul line recovery. Perfusion of 1 mM dantrolene + ACPD inhibited the ACPD- evoked increase in [C-14]-L-citrulline recovery. Perfusion of 1 mM MCP G or dantrolene without ACPD did not decrease [C-14]-L-citrulline reco very as compared to CSF alone. ACPD-enhanced [C-14]-L-citrulline recov ery was not attenuated by CNQX, MK-801, or tetrodotoxin (TTX). Using a n indirect method of assessing NO production in vivo, these data demon strate that mGluR stimulation enhances NO production in rat hippocampu s. Inhibition with dantrolene suggests that calcium-induced calcium re lease amplifies the inositol triphosphate-mediated calcium signal asso ciated with mGluR stimulation, thereby resulting in augmented calcium- dependent NO production.