Characterization of metabotropic glutamate receptor-mediated nitric oxide production in vivo

We tested the hypothesis that stimulation of metabotropic glutamate receptors (mGluRs) increases nitric oxide (NO) production in the hippocampus in vivo. Microdialysis probes were placed bilaterally into the CA₃ region of the hippocampus of adult Sprague-Dawley rats under pentobarbital anesthesia. Probes were perfused for 5 h with artificial cerebrospinal fluid (CSF) containing 3 μM [¹⁴C]-L-arginine. Recovery of [¹⁴C]-L-citrulline in the effluent was used as a marker of NO production. In nine groups of rats, increases in [¹⁴C]-L-citrulline recovery were compared between right- and left-sided probes perfused with various combinations of the selective mGluR agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD); the mGluR antagonist, (±)-α-methyl-4-carboxyphenylglycine (MCPG); the NO synthase inhibitor, N-nitro-L-arginine (LNNA); the ryanodine sensitive calcium-release channel inhibitor dantrolene, the non-N-methyl-D-aspartate (NMDA); receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801); and the Na⁺ channel blocker, tetrodotoxin. Recovery of [¹⁴C]-L-citrulline during perfusion with artificial CSF progressively increased to 90 ± 21 fmol/min (± SD) over 5 h. Perfusion in the contralateral hippocampus with 1 mM ACPD augmented [¹⁴C]-L-citrulline recovery to 250 ± 81 fmol/min. Perfusion of 1 mM nitroarginine + ACPD inhibited [¹⁴C]-L-citrulline recovery compared to that with ACPD alone. Perfusion with 1 mM MCPG + ACPD attenuated ACPD enhanced [¹⁴C]-L-citrulline recovery. Perfusion of 1 mM dantrolene + ACPD inhibited the ACPD-evoked increase in [¹⁴C]-L-citrulline recovery. Perfusion of 1 mM MCPG or dantrolene without ACPD did not decrease [¹⁴C]-L-citrulline recovery as compared to CSF alone. ACPD-enhanced [¹⁴C]-L-citrulline recovery was not attenuated by CNQX, MK-801, or tetrodotoxin (TTX). Using an indirect method of assessing NO production in vivo, these data demonstrate that mGluR stimulation enhances NO production in rat hippocampus. Inhibition with dantrolene suggests that calcium induced calcium release amplifies the inositol triphosphate-mediated calcium signal associated with mGluR stimulation, thereby resulting in augmented calcium dependent NO production.