IntroductionNeurovascular headaches, such as migraine and cluster headache, are very disabling, common disorders. Despite much progress in research during recent years both at the therapeutic level with the development of triptans and at the level of the general understanding of the basic mechanisms, there is still much to learn about these disorders [1].Nociceptive fibers on intracranial vessels such as the superior sagittal sinus (SSS) and the middle meningeal and middle cerebral arteries [2] are central in the understanding of the pathogenesis of neurovascular headache. These vessels are innervated by myelinated Aδ-and unmyelinated C-fibers from the trigeminal ganglion [3]. The system consists of bipolar neurons with cell bodies that reside in the trigeminal ganglion and project to the dura mater and the SSS as well as the major cerebral arteries at the base of the brain, while the central projection is to the trigeminal nucleus caudalis (TNC) in the caudal medulla oblongata and in the cervical spinal cord at the C1 and C2 levels [4]. J Headache Pain (2003) 4:156-163 DOI 10.1007 Immunoreactivity of NOS, CGRP, PACAP, SP and VIP in the trigeminal nucleus caudalis and in the cervical spinal cord C1 and C2 of the cat Abstract The aim of the present study was to map the presence of neurotransmitters and nitric oxid synthase (NOS) in the trigeminal nucleus caudalis and cervical spinal cord of cat. Immunocytochemistry was used to map the presence of neuronal NOS, calcitonin gene-related peptide (CGRP), pituitary adenylate cyclase activating peptide (PACAP), substance P (SP) and vasoactive intestinal peptide (VIP). The superior sagittal sinus (SSS) was stimulated electrically for 2h (0.3 Hz, 0.25 ms duration, 120 V) to examine if this would lead to depletion of the neurotransmitters. Numerous SP-and CGRP-positive nerve fibers (but no cells) were found in the trigeminal nucleus caudalis (TNC) and in Rexed lamina I and II°of the dorsal horn at C1/C2 level of the spinal cord. Moderate amounts of PACAP-and NOS-positive nerve fibers were also seen in these areas while there were only few VIP-positive fibers. There was no observable difference in the amount of immunoreactivity of NOS or the neuropeptides between stimulated and control cats. The stimulation resulted in marked c-fos immunoreactivity in the same regions of the TNC. The bipolar trigeminal neurons project to the TNC and C1/C2 areas with fibers containing neuropeptides and NOS. Stimulation of the SSS for 2 h was not sufficient to cause noticeable depletion.