EVIDENCE FOR THE PARTICIPATION OF GLUTAMATE IN REFLEXES INVOLVING AFFERENT, SUBSTANCE P-CONTAINING NERVE-FIBERS IN THE RAT

1 Responses mediated, either peripherally or centrally, by substance P -containing primary afferent C-fibres were investigated in the rat fol lowing impairment of axonal transport by colchicine (120 mu g kg(-1), i.p., daily for 3 days), and after treatment with the tachykinin antag onist SR-140333 (10-100 mu g kg(-1), i.v.) or the N-methyl-D-aspartate (NMDA) antagonist MK-801 (100 mu g kg(-1)). 2 Peripheral effects medi ated by afferent C-fibres were measured by plasma protein extravasatio n (Evans blue method), following antidromic stimulation of the sciatic nerve, topical application of mustard oil and, as control, i.v. injec tion of substance P. SR-140333 (100 mu g kg(-1)) reduced the effects b y 86%, 75% and 74%, respectively. Colchicine reduced the effects of th e 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 sciati c nerve stimulation or on the rise of paw skin temperature induced by capsaicin i.v., thus excluding an effect of MK-801 on peripheral termi nals of afferent neurones. 3 Depressor reflexes, which are known to be mediated by capsaicin-sensitive afferent neuones, such as those elici ted (A) by a stimulating dose of 30 ng capsaicin i.a., (B) by distensi on of the ascending colon or (C) by afferent sciatic nerve stimulation were studied. Colchicine significantly reduced depressor reflexes A a nd 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 Ca psaicin, injected either i.v. (200 mu g kg(-1)) or into the nucleus ca udatus/putamen (i.c., 30 mu g), induced an increase in paw skin temper ature and a decrease in colon temperature. The rise in fore paw skin t emperature (Delta t=2.3+/-0.4 degrees C) evoked by capsaicin i.v. was almost completeley blocked by SR-140333 (100 mu g kg(-1), i.v.), but n o inhibition was observed with MK-801, indicating that capsaicin had b rought about a release of substance P from peripheral nerve terminals. Colchicine did not influence heat dissipation induced by i.v. capsaic in. 5 When capsaicin was injected i.c., the rise in paw skin temperatu re in colchicine- and SR-140333-pretreated groups did not differ from that of the control group. MK-801 totally prevented the heat loss reac tion to i.c. capsaicin administration. Colchicine did not change the e ffects of i.v. or i.c. injected capsaicin: this excludes the involveme nt of a mechanism dependent on axonal transport of neurotransmitters. 6 The reduction of axonal transport by colchicine reduced plasma extra vasation induced by mustard oil and antidromic sciatic nerve stimulati on (peripheral functions) and depressor reflexes evoked by i.a. capsai cin and colon distension (central functions). It can be argued that af ferent stimulation of the sciatic nerve includes the stimulation of A- fibres, which might be less sensitive to colchicine. SR-140333 was eff ective 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 af ter capsaicin treatment allows the following conclusions: (a) the inhi bition by MK-801 indicates an essential role for glutamate in the cent ral transmission of these reflexes; (b) tachykinin antagonists such as SR-140333 do not affect these responses when administered systemicall y. Centrally released substance P could be involved in functions of th e CNS other than those investigated here unless the access of neurokin in antagonists to their receptors in the CNS is insufficient.