Following guanylyl cyclase and friends through hamster circadian entrainment

G.A. FERREYRA, A. MURAD, M.F. RUBIO, P.V. AGOSTINO AND D.A. GOLOMBEK.

Natal-RN (Brasil)

Simposio; VI Latin American Symposium of Chronobiology; 2001

The cGMP-signal transduction pathway appears to play an important role in circadian photic entrainment. We have previously shown that SCN NOS activity, whose product can activate the soluble isoform of guanylyl cyclase (GC), is rhythmic and regulated by light, and inhibition of NOS, GC or PKG in vivo blocks light-induced phase shifts at CT18. cGMP levels vary in the SCN under L:D or D:D cycles (with minimal levels at ZT/CT=20 and maximal at ZT/CT=24-04); 5 min. light pulses increase cGMP levels at CT18 but not at CT14. Here we show that guanylyl cyclase (GC) and cGMP-phosphodiesterase (PDE) activity follow the cGMP rhythm in the SCN. NO donors significantly increased SCN GC activity at both ZT4 and ZT18, suggesting that total GC levels are about the same at these time points. However, incubation with the NOS substrate arginine increased GC activity and cGMP production in a dose- and time-dependent manner. Glutamate had a complex effect on GC activity, suggesting a dual modulation of the enzyme, with a negative PKG-mediated feedback loop after 10-min incubations. We also found SCN expression of heme oxygenase, whose product carbon monoxide (CO) is also known to co-activate GC activity. These results suggest a complex pathway that mediate light induce phase advances, which includes NOS activation (probably related to CaM kinase regulation) and a rapid GC-cGMP induction.