Blocking NMDA receptors or nitric oxide production disrupts light transmission to the suprachiasmatic nucleus.

Retinal stimulation with a brief pulse of light (200 lx, 3 min) stimulated heart rate in dark-adapted urethane-anaesthetized rats. This effect was inhibited by prior infusion of a competitive blocker of N-methyl-D-aspartate (NMDA) receptors, (+-)-3-(2-carboxypiperazin-4-yl)-propyl-L-phosphonic acid (CPP, 20 nmol) into the hypothalamic suprachiasmatic nucleus (SCN) region. Furthermore, this inhibition of the stimulatory effect of light on heart rate was mimicked by prior infusion in the SCN region of a competitive blocker of nitric oxide (NO) production from L-arginine, NG-nitro-L-arginine methyl ester (40 nmol), or a blocker of the soluble guanylate cyclase. Methylene blue (20 nmol). None of these effects was seen when infusions were made in a region located 2 mm dorsal to the SCN or when a non-visual stimulus (tail pinch) was used to stimulate heart rate. These results point to a functional link between activation of an NMDA receptor coupled NO/cGMP signalling pathway and light transmission to the SCN.