To search for new and improved therapies for pain, a series of experiments was undertaken to examine the pharmacological mechanisms underlying the hyperalgesia following peripheral nerve or tissue injury. We tested the possibility that nerve growth factor (NGF) may block behavioral signs of hyperalgesia and allodynia in rats receiving chronic constriction injury (CCI) of the sciatic nerve. From post-operative day (POD) 1-3, thermal and mechanical hyperalgesias on the ipsilateral hindpaw developed. When NGF infusion via an osmotic pump was started immediately after the CCI, thermal hyperalgesia was significantly reduced or abolished from POD 5 up to the end of the test period (POD 42), when compared with rats receiving vehicle or no infusion. CCI-induced mechanical hyperalgesia was also abolished by NGF infusion. Delayed infusion of NGF (POD 4) failed to block thermal hyperalgesia. Infusion of NGF had no significant effect on paw withdrawal latency of the rats that had no CCI. Our preliminary results showed that, chronic infusion of a nitric oxide synthase antagonist also significantly attenuated thermal hyperalgesia in CCI rats. We also studied the involvement of N- methyl-D-aspartate (NMDA) receptors in the mechanical hyperalgesia that follows tissue inflammation. Following inflammation of the rat hindpaw, a state of mechanical hyperalgesia was induced. The intrathecal (i.t.) administration of the NMDA receptor antagonists significantly increased mechanical threshold and reduced response duration in the inflamed hindpaw, but had no effect on the non-inflamed paw. Substance P has been proposed as an important mediator for nociception at the first sensory synapse. Selective non-peptide substance P receptor antagonists WIN 51708 and CP 96,345, when injected i.t., produced a maximum of 80% and 84% reduction of behavioral hyperalgesia in inflamed rats, respectively. CP 96,345 was found 10 times less potent than WIN 51708. By using drugs in combination, it may be possible to affect a more clinically effective analgesia than by using single agents alone. Our results suggest that, in inflammation-induced hyperalgesia, the combination of an opioid agonist and an NMDA receptor antagonist produces a supra-additive antihyperalgesic effect; a combination of NMDA and substance P receptor antagonists produces an additive effect. These have therapeutic value in the treatment of chronic pain.
