The role of neuronal nitric oxide on current vocalization thresholds

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Neuronal nitric oxide synthase (NOS1) is a constitutive isoform of NOS predominantly expressed in the brain and spinal cord. The end-product of NOS1, nitric oxide (NO), has been shown to be an important messenger in nociceptive signal transduction pathways and to play an important role in nociception. In rats, NOS1 is upregulated in the spinal cord after the injection of capsaicin into the animals paw. The increased responses to noxious chemical stimulus to the paw of rats were blocked by inhibitors of NOS1. Mice genetically deficient in NOS1 have reduced late phase carrageenan-induced thermal hyperalgesia and NOS1 inhibitors inhibited inflammation-induced hyperalgesia in wild type mice. Also, in diabetic rats with diabetes-induced hyperalgesia, the expression of NOS1 is decreased compared to controls. Therefore, there is evidence to indicate that NOS1 plays an important role in nociception. However, the role of NOS1 in the transmission of pain is not entirely understood and it may vary according to the types of injury that produce pain. We are studying wild type and NOS1 genetically deficient mice and using a non injurious and neurospecific method to measure current vocalization threshold a surrogate measure of pain tolerance thresholds. We define vocalization as the pain avoiding behavior end point of this study and measured current vocalization threshold to electrical stimulus delivered at different frequencies. In order to study the role of NOS1 in the transmission of noxious stimulus via specific nerve fibers, we deliver electrical stimuli to the skin (tail) at increasing intensities at different frequencies, 5-Hz, 250-Hz and 2000-Hz that specifically stimulate C, A delta, and A beta fibers respectively. The intensity of the electrical stimulus that elicits pain avoiding behavior (here defined as vocalization) is defined as current vocalization threshold for each frequency. Mice deficient in NOS1 have higher vocalization thresholds compared with wild type mice. We are therefore investigating the mechanisms of such findings, and exploring whether NO and/or NOS1 itself are responsible for the changes in current vocalization thresholds found at baseline. We have demonstrated that exogenous sources of NO are not sufficient to reduce vocalization thresholds in NOS1-deficient mice and that the expression of inflammatory cytokines in the brain is increased in NOS-1 deficient mice.