Masakazu Yoshimura

Alteration in sensitivity of ionotropic glutamate receptors and tachykinin receptors in spinal cord contribute to development and maintenance of nerve injury-evoked neuropathic pain

Allodynia or hyperalgesia induced by peripheral nerve injury may be involved in changes in the sensitivity of neurotransmitters at the spinal cord level. In order to clarify the functional role of neurotransmitters in peripheral nerve injury, we used rats with nerve injury induced by chronic constriction of the sciatic nerve (CCI rat model) and estimated the effects of the intrathecal injection of drugs known to affect glutamate and tachykinin receptors. In sham-operated rats, the NMDA receptor agonist NMDA and AMPA-kinate receptor agonist RS-(5)-bromowillardin reduced withdrawal latency. The non-competitive NMDA receptor antagonist MK-801, competitive NMDA receptor antagonist AP-5 and AMPA-kinate receptor antagonist NBQX increased withdrawal latency. Substance P (SP) increased the withdrawal latency but only transitorily. The NK1 receptor antagonist RP67580 increased withdrawal latency, but the NK2 receptor antagonist SR48968 did not show an effect. In CCI rats, RS-(5)-bromowillardin reduced withdrawal latency, but NMDA did not show an effect. NBQX increased withdrawal latency, while MK-801 and AP-5 showed little or no effect. SP reduced withdrawal latency, and both RP67580 and SR48968 increased it. These results indicate that the alteration in sensitivity of ionotropic glutamate receptors and tachykinin receptors in the spinal cord contribute to development and maintenance of nerve injury-evoked neuropathic pain.

Influence of painful chronic neuropathy on neurogenic inflammation

&NA; The effect of topical application of capsaicin cream on neurogenic inflammation was investigated in a neuropathic pain model in rat. The neuropathic state was induced by loose ligation of the sciatic nerve with a chromic gut suture. A marked thermal hyperalgesia was observed in response to heat stimulation applied to the operated side from 3 days through 2 weeks, followed by a gradual return to the control level 35 days after surgery. In sham‐operated animals, topical application of capsaicin cream to both sides of the hind paw, the instep and sole, as well as antidromic stimulation of the sciatic nerve led to a significant increase in the amounts of Evans blue and substance P (SP) released into the perfusates. This stimulus‐induced extravasation was significantly suppressed by pretreatment with RP67580, an NK1 antagonist. On day 7 after ligation, capsaicin‐ and antidromic stimulation‐induced extravasation were significantly reduced. At this time, both amount of SP released immediately after application of capsaicin and during antidromic stimulation were almost similar to that in sham‐operated rats, whereas the basal amount of SP release significantly increased in ligated animals. In particular a major release of SP was detected immediately after the start of the perfusion compared with that in sham‐operated rats. Plasma extravasation evoked by SP (10−4 M) applied to the subcutaneous perfusate was significantly less in ligated than in sham‐operated rats. These results suggest that nerve injury with chronic pain may produce increase in basal SP release into the peripheral tissues, and then such enhanced SP release cause reduction of SP‐induced extravasation.

Effect of nitric oxide on substance P release from the peripheral endings of primary afferent neurons

To elucidate the interaction between nitric oxide (NO) and substance P (SP) in neurogenic inflammatory responses, we studied the effect of drugs related to nitric oxide (NO) on the levels of Evans blue and SP released into perfusate from the subcutaneous space in the rat instep. Noxious heat stimulation (47 degrees C for 30 min) caused an increase in SP release in parallel with plasma extravagation. N(omega)-nitro-L-arginine methylester (L-NAME: 100 mg/kg, once daily) given intraperitoneally (i.p.) five times (chronic treatment) significantly suppressed the heat-evoked SP release and Evans blue leakage. FK409 (10(-4) M), which evokes a release of NO, applied locally caused a remarkable increase in the SP release into the perfusate. These results suggest that heat-induced NO generation causes SP release from the peripheral endings of small-diameter primary afferent neurons.

Interaction between nitric oxide and substance P on heat-induced inflammation in rat paw

To elucidate the interaction between nitric oxide (NO) and substance P (SP) in neurogenic inflammatory responses, we measured the change in the degree of Evans blue leakage and NO levels in perfusate from the subcutaneous space in the rat instep following noxious heat stimulation (47 degrees C for 30 min). Furthermore, the effects of drugs affecting nitric oxide synthase were examined. Noxious heat stimulation caused on an increase in NOx, or NO2- and NO3- into the perfusate in parallel with plasma extravasation. Nw-nitro-L-arginine methylester (L-NAME: 100 mg/kg once daily.) intraperitoneally (i.p.) given five times (chronic treatment) significantly suppressed the increase in Evans blue extravasation induced by heat stimulation, whereas acute treatments with L- and D-NAME (100 mg/kg once, i.p.) did not show any significant effect. NO release induced by heating also was significantly suppressed by chronic pretreatment with L-NAME, but not by acute treatment. SP (10(-5) M) applied into the perfusate caused a remarkable increase in the NOx release into the perfusate. Intra-arterial injection of RP67580 (1 mg/kg) on the perfused side, but not SR48968 (1 mg/kg), significantly attenuated the increases in Evans blue leakage and NOx release during heat stimulation. These results suggest that heat-induced SP release from the peripheral endings of small-diameter afferent fibers causes NO generation through NK-1R, and that this gas act to elicit or enhance inflammatory responses.