NITRIC-OXIDE SYNTHASE IS FOUND IN SOME SPINOTHALAMIC NEURONS AND IN NEURONAL PROCESSES THAT APPOSE SPINAL NEURONS THAT EXPRESS FOS INDUCED BY NOXIOUS-STIMULATION

To determine if nitric oxide (NO) and Fos immunoreactivity induced by noxious stimulation were colocalized in spinothalamic neurons, double- staining immunocytochemical techniques were combined with retrograde n euroanatomical tracing procedures. Initial studies on three rats demon strated that Fos and nitric oxide synthase (NOS), the synthesizing enz yme for nitric oxide, did not coexist in spinothalamic tract neurons. However, some spinothalamic neurons were found to contain NOS and some NOS immunoreactive processes were found to appose Fos containing neur ons. Thus the remainder of the study: (1) analyzed the relationship of NOS positive neuronal processes with Fos stained neurons using a Fos immunocytochemical technique in combination with either NOS immunofluo rescence or NADPH-diaphorase histochemistry; and (2) quantitated the n umber of NOS containing cells that project to the thalamus using a com bined immunofluorescent-retrograde tracing procedure. Both NOS-like im munoreactive (NOS IR) neuronal processes and NADPH-diaphorase positive neuronal processes in the dorsal horn of the lumbar spinal cord were found to appose Fos positive neurons located in laminae I and II of th e dorsal horn. Approximately 40% of Fos-labeled cells in these superfi cial laminae were found to be in apposition to or in close proximity t o NOS labeled neuronal processes. Examination of spinal cord sections for NOS-containing spinothalamic tract neurons revealed that lamina X was the only spinal cord region containing such double-labeled neurons . Further quantification revealed that approximately 10% of NOS positi ve neurons in lamina X were double-labeled with Fluorogold. These find ings support the hypothesis that nitric oxide is involved in nocicepti ve events occurring in the spinal cord in response to a peripheral nox ious stimulus and further indicate that nitric oxide may contribute to the central transmission of spinothalamic information.