In the decerebrated rabbit, electrical stimulation of the toes evokes reflexes in the knee flexor semitendinosus (ST) and the ankle flexor tibialis anterior (TA) that are facilitated for several minutes after application of mustard oil to the toe tips. The present experiments have investigated the roles of glutamate N-methyl-D-aspartate (NMDA), and group I metabotropic (mGlu1 and mGlu5) receptors in mediating mustard oil-induced sensitization of flexor reflexes.
Experiments were performed on rabbits decerebrated under halothane (2-3 %) nitrous oxide anaesthesia. Reflexes were evoked by electrical stimulation of the skin at the base of the toes and recorded from the ipsilateral TA and ST muscle nerves, to be averaged and integrated by computer. A total of 100 ml 20 % mustard oil in paraffin oil was applied to the tips of either the two lateral or medial toes and the effects on the reflexes recorded. At least 1 h after the mustard oil stimulus one of three drugs was given intrathecally: dizocilpine (NMDA antagonist, 1 mg, n = 7); 7-(hydroxylimino)cyclopropa(b)chromen-1a-carboxylate ethyl ester (CPCCOEt, mGlu1 antagonist, 1-3 mg, n = 8); or 2-methyl-6-(phenylethynyl)pyridine (MPEP, mGlu5 antagonist, 0.2-1 mg, n = 10). After a further 30-40 min, mustard oil was applied to the toes that had not received it previously. Dizocilpine and MPEP were dissolved in Ringer solution, whereas CPCCOEt was dissolved in DMSO. In experiments with this drug, the first mustard oil stimulus was preceded by an intrathecal injection of 100 ml DMSO. Experiments were terminated by I.V. injection of KCl solution.
In the control states before dizocilpine, CPCCOEt and MPEP respectively, mustard oil significantly (Friedman’s ANOVA, P < 0.01) enhanced TA reflexes to median peak values of 186, 172 and 176 % and ST reflexes to medians of 163, 158 and 214 % of pre-stimulus levels. Median duration of effect was 63, 37 and 63 min for TA responses and 55, 45 and 29 min for ST reflexes. None of the drugs or vehicles had any significant effect on either reflex response per se (Wilcoxon tests, P > 0.05). After dizocilpine, mustard oil failed to increase the TA reflex (Friedman’s ANOVA, P > 0.1) and the duration of enhancement of ST reflexes was significantly reduced, to a median of 3 min (Wilcoxon test, P < 0.05). After CPCCOEt, the peak facilitation of TA reflexes was reduced (Wilcoxon, P < 0.05) but no other significant effects were observed. After MPEP, mustard oil induced changes that were statistically indistinguishable from controls.
These data show that glutamate NMDA receptors make a major contribution to the development of central sensitization of spinal reflexes in the rabbit, but that Group I metabotropic receptors have little role to play in generating the sensitized state.
This work was supported by BBSRC. Dizocilpine was a gift from Merck, Sharp and Dohme.
All procedures accord with current UK legislation.