Nitric oxide synthase neurons in the rodent spinal cord: distribution, relation to Substance P fibers, and effects of dorsal rhizotomy

The indirect immunofluorescent method was employed to investigate the distr ibution of neuronal nitric oxide synthase-like immunoreactivity(nNOS-LI) in the spinal cord of the golden hamster and to compare it to data obtained f rom rats. Immunoreactive neurons were found throughout the cervico-sacral e xtent in the dorsal horn (mainly in laminae I-III) and in the preganglionic autonomic regions, i.e., the sympathetic intermediolateral nucleus (IML), lateral funicle (LF), intercalated region (IC), the area surrounding the ce ntral canal (CA), and the sacral preganglionic parasympathetic cell group. While the distribution of immunoreactive cells was generally similar in bot h species, some differences were observed. For example in the hamster LF. a higher percentage of stained neurons was seen than in the IML, while the s ituation was rather inverse in the rat. In order to study the coincidence o f nNOS-LI in the population of preganglionic sympathetic neurons (PSN) that innervate the superior cervical ganglion (SCG); these were identified by r etrograde axonal transport of fluoro-gold (FG) following unilateral injecti on into the SCG. PSN were localized ipsilateral to the injection site mainl y in the IML and LF of spinal segments C7-Th4. The portion of double-labele d neurons of the IML were lower in hamster (17% in C7, 34% in C8) of FG-lab eled cells) than in rat (47%, in C8, 77% in Th2), while in the LF of segmen ts C8-Th2 in both species the majority of FG-neurons contained nNOS. While only very few double-labeled neurons were detected in the IC in hamster and rat, a striking difference was observed in the CA, where no double-labeled neurons were found in hamster, but up to 50% in rat. Double immunofluoresc ence detection of nNOS and substance P (SP) showed that in both the autonom ic regions and the dorsal horn, SP-LI fibers and puncta were present in clo se spatial relationship to nNOS-LI cell bodies. These results were basicall y identical in the hamster and rat. Unilateral transection of the dorsal ro ots of segments C6-Th2 in rats resulted in a clear reduction of SP-LI struc tures in the dorsal horn 5 days after rhizotomy, but not in the autonomic r egions. Compared to the unlesioned side. the numbers of nNOS-LI neurons in the superficial laminae of the dorsal horn were reduced to 32-46%; in the l esioned segments. and to 53% and 87%. respectively, in the two segments cra nial to the rhizotomized segments but remained unchanged caudally to the le sion. Numbers of nNOS-LI cell bodies in the autonomic regions were not alte red following dorsal root transection. The present study provides data on t he widespread distribution of nNOS in the spinal cord of golden hamster and describes the partial coincidence of the enzyme in PSN. The effects of dor sal rhizotomy on nNOS-LI neurons in the dorsal horn reveal that primary-eff erent fibers provide a stimulatory influence on neurons of the dorsal horn to generate the gaseous neuroactive substance, nitric oxide. (C) 2001 Elsev ier Science B.V. All rights reserved.