Distribution of nitric oxide synthase‐immunoreactive interneurons in the spinal trigeminal nucleus

The spinal trigeminal nucleus is involved in the transmission of orofacial sensory information. Neither the distribution of the neuromessenger, nitric oxide, within the trigeminal system nor the possible relationship of this simple gas with trigeminothalamic neurons has been carefully studied. Using immunocytochemical (against nitric oxide synthase) and histochemical (NADPH‐diaphorase staining) techniques, we have found that nitric oxide neurons and processes are more prominent in the nucleus caudalis and the dorsomedial aspect of the nucleus oralis than in other spinal trigeminal regions. To study the relationship of nitric oxide to trigeminothalamic neurons and intertrigeminal interneurons of the spinal trigeminal nucleus, spinal trigeminal neurons were retrogradely labeled with fluorogold by thalamic injections or by injections into the junction of the nucleus interpolaris and nucleus caudalis. Medullary sections were subsequently processed with NADPH‐diaphorase histochemistry. None of the diaphorase‐stained neurons in the spinal trigeminal nucleus was found to contain fluorogold; however, some diaphorase‐stained processes were found in close proximity to trigeminothalamic neurons. Following spinal trigeminal nucleus injections, many diaphorasestained neurons were found to contain fluorogold, especially in the nucleus caudalis, suggesting that nitric oxide‐containing neurons in the spinal trigeminal nucleus are intertrigeminal interneurons. Collectively, these data indicate that nitric oxide is most prominent in interneurons located in nucleus caudalis and that these interneurons give rise to processes that appose trigeminothalamic neurons, raising the possibility that they may indirectly influence orofacial nociceptive processing at the level of the spinal trigeminal nucleus via nitric oxide production. © 1994 Wiley‐Liss, Inc.