Mechanisms underlying the hyperalgesia induced by a single systemic injection of nerve growth factor (NGF) in adult rats were studied in vivo. A single dose of NGF initiated a prolonged thermal hyperalgesia to a radiant heat source within minutes that lasted for days. Animals which had been pretreated with the mast cell degranulating compound 48/80 or either one of two specific 5-hydroxytryptamine receptor antagonists (ICS 205-930 and methiothepin) also developed an NGF-induced thermal hyperalgesia, but onset was delayed by more than 3 h. In the presence of ICS 205-930 or methiothepin the early component NGF-induced hyperalgesia was reversed and the animals responded with an initial hypoalgesia to the thermal stimuli. Whereas these results indicate a peripheral mechanism for the initial thermal hyperalgesia, the later phase (7 h-4 days after NGF) appeared to be centrally maintained, since it could be selectively blocked by the non-competitive NMDA receptor antagonist MK-801. In contrast to the almost immediate thermal hyperalgesia following a single injection of NGF, a significant mechanical hyperalgesia began only after a 7 h latency. This NGF-induced mechanical hyperalgesia was not blocked by any of the treatments that attenuated the thermal hyperalgesia, indicating that a separate mechanism may be involved. Additional electrophysiological experiments showed that NGF-induced hyperalgesia was not maintained by an increased amount of spontaneous activity in C-fibres. A final result showed that endogenous release of NGF in a model of acute inflammation (complete Freund's adjuvant-induced inflammation) may be involved in the development of thermal hyperalgesia, since it could be blocked by concomitant treatment with anti-NGF antisera. These data indicate that NGF-induced thermal and mechanical hyperalgesia are mediated by different mechanisms. The rapid onset component of thermal hyperalgesia is due to a peripheral mechanism involving the degranulation of mast cells, whereas the late component involves central NMDA receptors. In contrast, the NGF-induced mechanical hyperalgesia seems to be independent of mast cell degranulation or central NMDA receptor sites.

1 The effects of bradykinin on nociceptors have been characterized on a preparation of the neonatal rat spinal cord with functionally connected tail maintained in vitro. Administration of bradykinin to the tail activated capsaicin-sensitive peripheral fibres and evoked a concentration-dependent (EC50= 130 nM) depolarization recorded from a spinal ventral root (L3-L5). 2 The response to bradykinin was unaffected by the peptidase inhibitors, bestatin (0.4 mM), thiorphan(1 gM), phosphoramidon (1 ELM) and MERGETPA (10 gM) or by the presence of calcium blocking agents, cadmium (200 gM) and nifedipine (10 4M).3 Inhibition of cyclo-oxygenase with indomethacin (1-5I1M), aspirin (1-1O M) and paracetamol (10-50ILM) consistently attenuated responses to bradykinin. 4 The effect of bradykinin was mimicked by the phorbol ester PDBu, an activator of protein kinase C. The response to bradykinin was attenuated following desensitization to PDBu but desensitization to bradykinin did not induce a cross-desensitization to PDBu. The protein kinase C inhibitor staurosporine (10-500 nM) consistently attenuated the effects of PDBu and bradykinin.5 Bradykinin responses were reversibly enhanced by dibutyryl cyclic AMP (100 gM). However dibutyryl cyclic GMP (0.5 mM) and nitroprusside (10 EM) produced prolonged block of responsiveness to bradykinin. Prolonged superfusion with pertussis toxin did not affect responses to bradykinin. 6 The B,-receptor agonist des Arg9-bradykinin (10-100 AM) was ineffective alone or after prolonged exposure of the tail to lipopolysaccharide (100 ng ml-') or epidermal growth factor (100 ng ml-') to induce B, receptors. The BI-receptor antagonist, des Arg9 Leu8-bradykinin (10 JM 7 These data show that bradykinin produces concentration-dependent activation of peripheral nociceptors in the neonatal rat tail. The responses were unaffected by calcium channel block and were partially dependent on the production of prostanoids. Bradykinin-evoked responses were consistent with the activation of protein kinase C-dependent mechanisms. Cyclic GMP-dependent mechanisms may be involved in bradykinin-receptor desensitization whereas cyclic-AMP dependent mechanisms increase fibre excitability and facilitate bradykinin-induced responses. The effects of bradykinin were mediated by a B2 receptor.

1. Perfusion of the receptive field (RF) of C- or A delta-fiber nociceptors with nerve growth factor (NGF) in an in vitro preparation of the rat saphenous nerve with functionally attached skin induced a significant decrease in heat threshold without changing mechanical or cold sensitivity. 2. NGF-induced thermal sensitization was absent after saline perfusion and in skin taken from mast-cell depleted animals, hence confirming a role for mast cells in NGF-induced thermal hyperalgesia. 3. Neurotrophin-5 (NT-5) also induced a small but significant reduction in heat threshold without affecting mechanical sensitivity. It is speculated that NT-5 exerts its action either directly on the trkA receptor, as with NGF or alternatively through trkB receptors located on sympathetic efferents or on small diameter afferents.

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