Organization of N-methyl-D-aspartate (NMDA) receptor-mediated sensitization of spinal reflexes following plantar incision in the anaesthetized rabbit

John Harris, Katharine Dobson, Kelly Iles, Deborah Reeve. University of Nottingham, Nottingham, United Kingdom.

In anaesthetized or decerebrated rabbits, reflexes in the knee flexor semitendinosus (ST) and the ankle flexor tibialis anterior (TA) are facilitated for several minutes after application of mustard oil (MO) to the toe tips, whereas only ST responses are enhanced if MO is applied to the heel. Functionally this is appropriate, as the toes-TA reflex would shift weight to the heel. In spinally transected animals however, both flexor reflexes are enhanced following MO application to either site suggesting sensitization of TA responses following noxious insult at the heel is controlled by descending pathways (Harris & Clarke, 2003). The present experiments have investigated whether a more intense stimulus, plantar incision (Zahn & Brennan, 1999) can affect this descending control and the involvement of glutamate NMDA receptors in sensitization in this model.

Experiments were performed on 17 rabbits anaesthetized by a continuous i.v. infusion of pentobarbitone (between 21 and 50 mg kg-1 h-1). Reflexes were evoked in the left ST and TA muscles by electrical stimulation of the plantar skin near the two central toes and recorded as compound EMG signals which were averaged and integrated by computer. Following control recordings, in 9 animals a conditioning stimulus (a 5-6 mm incision through the overlying skin) was applied to either the metatarsophalangeal (MT) joints or the heel, and the effect on reflexes recorded. After at least an hour, an incision was then made at the site not cut previously. In a second group of animals (n = 8), intrathecal administration of the NMDA receptor antagonist dizocilpine (0.5 mg and 0.25 mg), preceded the first and second incisions, respectively. Experiments were terminated using an overdose of anaesthetic.

In non-dizocilpine pretreated animals, MT incision significantly (Friedman’s ANOVA, P < 0.0001) enhanced toes-ST and toes-TA reflexes to median peak values of 284% and 563% of pre-stimulus levels, respectively. Median duration of effect was 55 min for TA responses and 63 min for ST reflexes. In the presence of dizocilpine, MT incision still increased both reflex responses but median peak amplitude and duration were significantly (Mann-Whitney tests, P < 0.05) reduced; values for toes-ST and toes-TA were 122% and 133% of pre-incision levels for 7 and 11 mins respectively. Incision at the heel in non-drug treated animals also significantly (Friedman’s ANOVA, P < 0.0001) enhanced ST and TA reflexes to median peak values of 166% and 176% of control levels respectively and lasted for 19 min in both cases. Pre-administration of dizocilpine abolished the facilitatory effect of heel incision on both flexor responses.

The present data indicate that glutamate NMDA receptors are involved in the sensitization of spinal flexor reflexes following plantar incision. Plantar incision appears to be sufficiently intense to cause functionally inappropriate increases in reflex responses (albeit relatively transiently) in spinally intact animals.

This work was supported by the BBSRC.