Attenuation of activity in an endogenous analgesia circuit by ongoing pain in the rat

Analgesic efficacy varies depending on the pain syndrome being treated. One reason for this may be a differential effect of individual pain syndromes on the function of the endogenous pain control circuits at which these drugs act to produce analgesia. To test this hypothesis, we examined the effects of diverse (i.e., ongoing inflammatory, neuropathic, or chronic widespread) pain syndromes on analgesia induced by activation of an opioid-mediated, noxious stimulus-induced endogenous pain control circuit. This circuit was activated by subdermal capsaicin injection at a site remote from the site of nociceptive testing. Analgesia was not affected by carrageenan-induced inflammatory pain or the early phase of oxaliplatin neuropathy (a complication of cancer chemotherapy). However, the duration of analgesia was markedly shorter in the late phase of oxaliplatin neuropathy and in alcoholic neuropathy. A model of fibromyalgia syndrome produced by chronic unpredictable stress and proinflammatory cytokines also shortened analgesia duration, but so did the same stress alone. Therefore, since chronic pain can activate neuroendocrine stress axes, we tested whether they are involved in the attenuation of analgesic duration induced by these pain syndromes. Rats in which the sympathoadrenal axis was ablated by adrenal medullectomy showed normal duration pain-induced analgesia in groups with either late-phase oxaliplatin neuropathy, alcoholic neuropathy, or exposure to sound stress. These results support the suggestion that pain syndromes can modulate activity in endogenous pain control circuits and that this effect is sympathoadrenal dependent.