Analgesic Effects Mediated by Alpha 7 Nicotinic Acetylcholine Receptors: A Potential Non‐Opioid Treatment for Pain

Nicotinic acetylcholine receptors (nAChRs) are expressed abundantly throughout the nervous system. A wide variety of neurobehavioral processes involve nAChRs, including analgesic effects in chronic pain models. However, nAChR subunit composition and cellular mechanisms underlying nAChR mediated analgesia are not well understood. Ventrolateral periaqueductal gray (vlPAG) is an important target for analgesic drugs in the descending pain modulatory pathway. Thus, we investigated the nAChRs in vlPAG and tested their antinociceptive effects in an animal model of pain. Adult male Sprague Dawley rats (> 8weeks) were used for all the experiments. Whole cell recordings were performed from backlabeled vlPAG neurons that project to the RVM in a brain slice preparation. nAChR expressing cells were identified by focal application of ACh. The contribution of the α7 nAChR subtype was tested with selective antagonists. In addition, μ‐opioid receptors (MOR) expression was assessed using selective MOR agonists. Further, we tested the behavioral relevance of these physiological effects using the formalin assay while modulating α7 nAChR activity using both systemic and focal (within the vlPAG) pharmacological interventions. We found that 63% of vlPAG‐RVM projection neurons expressed functional α7 nAChRs and were largely non‐overlapping with those expressing MORs. Nicotine enhanced spontaneous inhibitory postsynaptic current (sIPSC) frequency onto vlPAG neurons that lack α7 nAChRs (12/19), but less so in α7 expressing neurons (7/24 neurons, Fisher's test p<0.05). However, nicotinic modulation of spontaneous excitatory postsynaptic current (EPSC) frequency was similar between α7+ and α7‐neurons. Pharmacological testing indicated that these presynaptic effects of nicotine were mediated by β2‐containing nAChRs. In the in vivo behavioral assay, both systemic and focal administration of the α7 nAChR‐selective agonist, PHA‐543613 reduced nocifensive responses to the formalin injection. Furthermore, focal α7 antagonist pretreatment blocked PHA‐543613‐induced antinociception via either systemic or focal administration methods. We then tested a combination of sub‐maximal doses of the α7 agonist and morphine, which produced additive antinociceptive effects. Interestingly, focal administration of the α7 antagonist blocked the antinociceptive effects of this drug combination. The α7 antagonist did not affect morphine induced antinociception when administered alone at a maximal effective concentration. Our study demonstrates two populations of vlPAG‐RVM projection neurons that differentially express α7 nAChRs and MORs. Our working model is that α7‐expressing neurons contribute to a pain inhibiting pathway, while MOR‐expressing neurons enhance pain signaling. The additive antinociceptive effects of combining sub‐maximal doses of α7 and MOR agonists supports the idea of independent mechanisms and these observations may lead to novel treatment strategies to limit opioid drug use for the relief of chronic pain. Support or Funding Information This study was funded by a Frank Family Fellowship to I.U., a Gates Foundation Fellowship to B.M., and the NIH grants DA07255 to C.A.D., DA019695, DA036978, and DA015918 to D.S.M.