Antinociceptive effect of a group II metabotropic glutamate receptor antagonist in the thalamus of monoarthritic rats.

ABSTRACT

Adult rats were rendered monoarthritic (MA) by injection of 50 microl of complete Freund's adjuvant (CFA) into the tibiotarsal joint. The ankle-bend (AB) test of nociception was performed in those animals before and during 60 min after the stereotaxic injection of 2 microl of either saline (controls) or (2S)-alpha-ethylglutamic acid (EGLU, 80 nmol in 2 microl), a group II metabotropic glutamate receptors (mGluR) antagonist, in the reticular thalamic nucleus (Rt) contralateral to the arthritic joint. AB scores reached near maximum values before the stereotaxic injections (18.7+/-0.8), and remained constant throughout the entire experimental period in the control group, denoting marked allodynia. In the EGLU-treated group, AB scores gradually decreased after EGLU injection, with minimum values at 10 min (7.7+/-1.6), recovering to scores near maximum at 60 min (19.7+/-0.3). The data point to an activation of group II mGluR by noxious inputs in the Rt of MA rats, suggesting their participation in inhibiting local gamma-aminobutyric acid (GABA)ergic inhibitory neurones.