Evidence for the participation of glutamate in reflexes involving afferent, substance P-containing nerve fibres in the rat.

1. Responses mediated, either peripherally or centrally, by substance P-containing primary afferent C-fibres were investigated in the rat following impairment of axonal transport by colchicine (120 micrograms kg-1, i.p., daily for 3 days), and after treatment with the tachykinin antagonist SR-140333 (10-100 micrograms kg-1, i.v.) or the N-methyl-D-aspartate (NMDA) antagonist MK-801 (100 micrograms kg-1). 2. Peripheral effects mediated by afferent C-fibres were measured by plasma protein extravasation (Evans blue method), following antidromic stimulation of the sciatic nerve, topical application of mustard oil and, as control, i.v. injection of substance P. SR-140333 (100 micrograms kg-1) reduced the effects by 86%, 75% and 74%, respectively. Colchicine reduced the effects of the first two stimuli by 31% and 33% and, as expected not the effect of substance P. The increase of paw skin temperature following capsaicin i.v. was inhibited by SR-140333, but not by colchicine. MK-801 had no effect on the plasma protein extravasation following antidromic sciatic nerve stimulation or on the rise of paw skin temperature induced by capsaicin i.v., thus excluding an effect of MK-801 on peripheral terminals of afferent neurones. 3. Depressor reflexes, which are known to be mediated by capsaicin-sensitive afferent neuones, such as those elicited (A) by a stimulating dose of 30 ng capsaicin i.a., (B) by distension of the ascending colon or (C) by afferent sciatic nerve stimulation were studied. Colchicine significantly reduced depressor reflexes A and B, but had no effect on reflex C. None of the reflexes was affected by SR-140333. MK-801 significantly inhibited all three reflexes. 4. Capsaicin, injected either i.v. (200 micrograms kg-1) or into the nucleus caudatus/putamen (i.c., 30 micrograms), induced an increase in paw skin temperature and a decrease in colon temperature. The rise in fore paw skin temperature (delta t = 2.3 +/- 0.4 degrees C) evoked by capsaicin i.v. was almost completely blocked by SR-140333 (100 micrograms kg-1, i.v.), but no inhibition was observed with MK-801, indicating that capsaicin had brought about a release of substance P from peripheral nerve terminals. Colchicine did not influence heat dissipation induced by i.v. capsaicin. 5. When capsaicin was injected i.c., the rise in paw skin temperature in colchicine- and SR-140333-pretreated groups did not differ from that of the control group. MK-801 totally prevented the heat loss reaction to i.c. capsaicin administration. Colchicine did not change the effects of i.v. or i.c. injected capsaicin: this excludes the involvement of a mechanism dependent on axonal transport of neurotransmitters. 6. The reduction of axonal transport by colchicine reduced plasma extravasation induced by mustard oil and antidromic sciatic nerve stimulation (peripheral functions) and depressor reflexes evoked by i.a. capsaicin and colon distension (central functions). It can be argued that afferent stimulation of the sciatic nerve includes the stimulation of A-fibres, which might be less sensitive to colchicine. SR-140333 was effective only on peripherally mediated responses. 7. The recent evidence for the concomitant release of glutamate and substance P from central terminals of afferent C-fibres, known to mediate reflexes abolished after capsaicin treatment allows the following conclusions: (a) the inhibition by MK-801 indicates an essential role for glutamate in the central transmission of these reflexes; (b) tachykinin antagonists such as SR-140333 do not affect these responses when administered systemically. Centrally released substance P could be involved in functions of the CNS other than those investigated here unless the access of neurokinin antagonists to their receptors in the CNS is insufficient.