Neuronal nitric oxide synthase mRNA and protein distribution along the trigeminal pathway: a possible role in mechanism of headache

ABSTRACT

Nitric oxide [NO] is produced in the nervous system by the activity of the neuronal nitric oxide synthase [nNOS]. Nitric oxide is involved in nociceptive processing and is an important molecule in the regulation of cranial blood flow and arterial diameters. Thus it is a key molecule in the generation of headache. Headaches are hypothesised to be due to neural activation in the trigeminal pathway resulting in vasodilatation of meningeal blood vessels leading to pain. In this study we investigated the cellular source of NO in structures known to be involved in headache by studying the expression of nNOS mRNA and protein along the trigeminal pathway. Fifteen Wistar adult male rats weighing 300-400g were killed by decapitation under brief anesthesia. The dura mater, trigeminal ganglia and trigeminal nerves were removed and processed for in situ hybridization to study nNOS mRNA expression and for immunohistochemistry to study nNOS protein localization. In situ hybridization showed nNOS mRNA expression in all divisions of the trigeminal ganglia, exclusively in neuronal cell bodies. Immunohistochemistry showed nNOS localization in the trigeminal ganglia both in neuronal cell bodies and neuronal process. Neuronal NOS was also localized in trigeminal nerves and in the dura mater. In the dura mater nNOS immunoreactivity was exclusively localized in meningeal nerve fibers but neither in dural vessels nor in dural cells. These data provide evidence that nNOS mRNA and protein are expressed in neuronal but not vascular structures known to be involved in pathogenesis of headache. Therefore, nNOS could be contributing to the production of dural NO which is known to be a key factor in generation of headache