Activity-dependent modifications of spinal nociceptive circuitry that may be linked to inflammation and sensitization have been described but their mechanisms are poorly understood. Involvement of Group I metabotropic glutamate receptors (mGluRs) coupled to second messenger processes are implicated (Gerber et al. 2000; Karim et al. 2001). Here, we have compared the effects of protein kinase C (PKC) and extracellular signal-regulated kinase (ERK) inhibitors on Group I mGluR agonist modulation of afferent-evoked neurotransmission in substantia gelatinosa (SG) neurons and on Group I mGluR-induced Fos expression in rat spinal dorsal horn in vitro.For electrophysiology, lumbar spinal cord slices were prepared from 12-15 day old rats after irreversible anaesthesia (Urethane, 2g/Kg i.p.). The Group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 100 µM, 5 min) elicited a significant inhibition of EPSC amplitude (31 ± 4 % [mean ± S.E.M.] inhibition from control, n = 6, P < 0.01, Student’s t test) in a population of SG neurons. In the presence of the PKC inhibitor 2-[1-(3-Dimethylaminopropyl)-1H-indolyl-3-yl]-3-(indol-3-yl)-maleimide (GF109203X, 1 µM, 15 min) DHPG-induced inhibition was attenuated (10 ± 4 % inhibition from control, n = 7, n.s.). The ERK inhibitor 2-(2-Amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD98059, 50 µM, 15 min) had no significant effect on DHPG-induced inhibition (24 ± 6 % inhibition from control, n = 6, P < 0.05). In a separate study, DHPG-evoked (100 µM, 1 hr incubation) Fos expression in dorsal horn in vitro was evaluated in the presence or absence of the PKC inhibitor GF109203X (1 µM) or the ERK inhibitor PD98059 (50 µM). DHPG-evoked Fos expression in the dorsal horn was significantly reduced by GF109203X (n = 6, P < 0.05), and to a greater extent by PD98059 (n = 6, P < 0.01).These data suggest differential involvement of second messenger cascades under conditions of brief versus sustained Group I mGluR activation. The former short term adaptive response is apparently linked mainly to PKC activation whereas the latter long term adaptive response is linked to both PKC and the ERK signal transduction cascades. Selective signalling through Group I mGluRs and these signal transduction cascades could be manifest in nociceptive neuroplasticity and persistent pain sensitization phenomena.