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 tachykinin NK₁ and NK₃ 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 µl 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: L-733,060 (selective NK₁ antagonist, 0.3 mg intrathecal, n = 6); SR 142801 (selective NK₃ antagonist, 1 mg kg^-1 I.V., n = 7); or ZD-6021 (non-selective antagonist of NK₁, NK₂ and NK₃ receptors, Rumsey et al. 2001, 0.3 mg intrathecal, n = 9). After a further 30-40 min, mustard oil was applied to the toes that had not received it previously. SR 142801 dissolved 1 % DMSO/Ringer solution whereas L-733,060 and ZD-6021 were dissolved in DMSO. In experiments with these two drugs, the first mustard oil stimulus was preceded by an intrathecal injection of 100 µl DMSO. Experiments were terminated using an overdose of KCl.

In the control states before L-733,060, SR 142801 and ZD-6021, respectively, mustard oil significantly (Friedman’s ANOVA, P < 0.01) enhanced TA reflexes to median peak values of 230, 370 and 165 % and ST reflexes to medians of 154, 265 and 152 % of pre-stimulus levels. Median duration of effect was 63, 63 and 53 min for TA responses and 37, 23 and 37 min for ST reflexes. The effects of mustard oil after L-733,060 and SR 142801 were statistically indistinguishable from controls (Wilcoxon tests, P > 0.1). After ZD-6021, peak enhancement and duration of increase of the TA reflex were significantly less than in controls (median increase to 122 % of pre-mustard levels, median duration 17 min, Wilcoxon tests, P < 0.05). Peak enhancement of the ST reflex was also decreased (Wilcoxon, P < 0.01), but the decrease in duration (to a median of 11 min) was not significant (Wilcoxon, P > 0.1).

As there are few, if any, NK₂ receptors in the spinal cord (Zerari et al. 1998), the present data show that both NK₁ and NK₃ receptors contribute to sensitization of reflexes in rabbit, but that simultaneous blockade is required to show an effect.

This work was supported by the BBSRC. ZD-6021 and SR 142801 were gifts of AstraZeneca and Sanofi Recherche, respectively.