DISTRIBUTION OF NADPH-DIAPHORASE STAINING AND LIGHT-INDUCED FOS EXPRESSION IN THE RAT SUPRACHIASMATIC NUCLEUS REGION SUPPORTS A ROLE FOR NITRIC-OXIDE IN THE CIRCADIAN SYSTEM

Nitric oxide serves as a messenger molecule in some neuronal systems t hat use glutamate as a transmitter and it has been shown that glutamat e mediates the transmission of photic signals by retinal ganglion cell axons terminating in the hypothalamic suprachiasmatic nucleus, site o f the circadian pacemaker in rodents. Recent experiments have demonstr ated that pharmacological treatments which block nitric oxide synthesi s by nitric oxide synthase prevent glutamate-induced phase shifts of t he cell firing rhythm in suprachiasmatic nucleus slice preparation in vitro; similar treatments were found to inhibit light transmission to the suprachiasmatic nucleus as well as light-induced phase shifts in a ctivity rhythms in vivo, implicating nitric oxide in circadian light s ignalling in vivo. There is limited information, however, about the pr esence and function of nitric oxide synthase-containing neurons within retinorecipient regions of the rodent suprachiasmatic nucleus. In the present study we used NADPH-diaphorase histochemistry and immunostain ing for the nuclear phosphoprotein Fos to assess the co-distribution o f nitric oxide synthase-containing neurons and light-responsive cells in the rat suprachiasmatic nucleus region. A strong convergence betwee n NADPH-diaphorase-stained cell bodies and fibres and cells that expre ssed Fos in response to photic stimulation was noted in the anterior p eriventricular nucleus, suprachiasmatic preoptic nucleus, retrochiasma tic area, the inter-suprachiasmatic nucleus region, and the dorsal asp ect of the optic chiasm, below the suprachiasmatic nucleus. A similar convergence between NADPH-diaphorase-stained fibres and Fos-immunoreac tive cells was noted inside the suprachiasmatic nucleus, but the numbe r of NADPH-diaphorase-stained elements found in this region was substa ntially low compared with that found in,retinorecipient regions border ing the nucleus. In many cases both inside and outside the suprachiasm atic nucleus, the Fos-immunoreactive cells appeared to make direct con tact with NADPH-diaphorase-stained cells or fibres, but no co-locaiiza tion of Fos immunoreactivity and NADPH-diaphorase histochemical activi ty within individual cells was detected. Extensive co-distribution of NADPH-diaphorase-stained cells and fibres and cells that express Fos i n response to photic stimulation in the suprachiasmatic nucleus region is in line with the hypothesis that nitric oxide participates in the mechanism mediating circadian light signalling in the suprachiasmatic nucleus. However, lack of co-localization of the two markers to indivi dual cells rules out the possibility that retinorecipient cells in the suprachiasmatic region synthesize and release nitric oxide when photi cally-activated. Instead, the results support the possibility that pho tic stimulation triggers nitric oxide synthesis in nitric oxide syntha se-containing neurons located near the photically-activated cells.