The adverse interaction between smoking and chronic pain has been known for decades. A variety of chronic pain conditions – ranging from headache to low back pain to fibromyalgia – markedly exacerbate smoking prevalence and intensity in packs per day among multiple patient populations. In patients seeking pain treatment, the prevalence of smoking approaches 50%, compared to less than 20% in the general population. Perhaps not surprisingly, the relationship is bidirectional: not only does persistent pain increase rates and intensity of smoking, but smoking also appears to exacerbate both the intensity and associated impairment of chronic pain. In fact, smoking appears to place individuals at risk for developing a chronic pain condition and may also facilitate the transition from acute to chronic pain. The growing body of literature documenting these associations has led to the proposition of a positive feedback loop: individuals smoke in part to cope with their pain, but smoking actually worsens the pain. Despite the strong evidence for the existence of this adverse interaction, the mechanisms responsible for it remain poorly understood.
A number of preclinical and clinical studies have documented that nicotinic acetylcholine receptor (nAChR) agonists, e.g., nicotine, have analgesic efficacy in the acute pain setting, such as that produced experimentally in the research laboratory or experienced by patients postoperatively. In contrast, the role of nAChR activation in modulating chronic pain is less well characterized. The experiments described in this thesis determine whether persistent pain diminishes the antinociceptive (analgesic) efficacy of an α4β2 nAChR agonist in the rostral ventromedial medulla (RVM), a key brainstem pain modulatory nucleus, and subsequently begin to elucidate the mechanisms by which persistent pain elicits this plasticity.
The complete Freund’s adjuvant (CFA) model of chronic pain was employed to test the hypothesis that persistent inflammatory injury diminishes the antinociceptive efficacy of the selective and potent α4β2 nAChR agonist epibatidine in key brainstem pain modulatory nuclei. Paw withdrawal latency to a noxious heat stimulus was used to evaluate the anti-hyperalgesic and antinociceptive effects of epibatidine microinjected in the RVM or periaqueductal gray (PAG) of male rats. The effects of epibatidine were assessed both in uninjured animals and in animals at different times after intraplantar CFA injection. Interestingly, pretreatment with an α4β2-selective antagonist demonstrated that the antinociceptive effects of epibatidine in naïve rats were mediated by α4β2 nAChRs in the RVM but not in the PAG. While the antinociceptive effects of epibatidine in the RVM were abolished after two weeks of inflammatory pain, the anti-hyperalgesic effects remained unchanged. Surprisingly, epibatidine no longer appeared to be acting primarily at α4β2 nAChRs as early as four hours after injury. Persistent inflammation did not alter the anti-hyperalgesic or antinociceptive effects of epibatidine in the PAG.
Radioligand binding studies were conducted to test the most parsimonious hypothesis that a global reduction in α4β2 nAChR number or binding affinity during persistent injury was in part responsible for the decreased efficacy of epibatidine in the RVM after intraplantar CFA. Saturation binding using [3H]-epibatidine in membrane homogenates prepared from RVM and PAG tissue revealed no differences in receptors between saline- and CFA-treated rats at any time after injury, suggesting that a whole-nucleus reduction in nAChRs could not explain the observed behavioral phenomena.
To query functional changes with greater resolution, whole-cell patch clamp electrophysiology was employed to begin assessing the consequences of nAChR activation by nicotine at the level of the neuron. Initial studies performed in the locus coeruleus demonstrated that all neurons responded to nicotine with an inward current that desensitized with continued exposure to the drug. Neurons in the RVM exhibited significantly more heterogeneity in their response to nicotine: desensitizing inward currents were seen in some; sustained outward currents in others; inward currents followed by outward currents in a third population; and still others had no response to nicotine exposure. The sustained outward currents persisted in the presence of the sodium channel blocker tetrodotoxin, were not blocked by an α4β2 nAChR-selective antagonist, and appeared to be mediated by G protein-coupled receptors and potassium channels.
Taken together, the present results demonstrate that persistent inflammatory injury produces adaptive changes in nicotinic signaling in the RVM such that the antinociceptive effects of epibatidine activation are abolished in a time-dependent manner. These changes cannot be attributed to a whole-nucleus reduction in α4β2 nAChRs. However, nicotinic signaling in the RVM is complex, and small alterations in the pre- or postsynaptic actions of nicotine may have significant ramifications for the overall nociceptive sensitivity of an animal. The data presented here suggest that plasticity in nicotinic signaling within the bulbospinal pain modulatory pathways may in part explain the adverse interaction between smoking and chronic pain observed in humans.