Calcium-sensitive particulate guanylyl cyclase as a modulator of cAMP in olfactory receptor neurons

The second messengers cAMP and inositol-1,4,5-triphosphate have been implicated in olfaction in various species. The odorant-induced cGMP response was investigated using cilia preparations and olfactory primary cultures. Odorants cause a delayed and sustained elevation of cGMP. A component of this cGMP response is attributable to the activation of one of two kinetically distinct cilial receptor guanylyl cyclases by calcium and a guanylyl cyclase-activating protein (GCAP). cGMP thus formed serves to augment the cAMP signal in a cGMP-dependent protein kinase (PKG) manner by direct activation of adenylate cyclase. cAMP, in turn, activates cAMP-dependent protein kinase (PKA) to negatively regulate guanylyl cyclase, limiting the cGMP signal. These data demonstrate the existence of a regulatory loop in which cGMP can augment a cAMP signal, and in turn cAMP negatively regulates cGMP production via PKA. Thus, a small, localized, odorant-induced cAMP response may be amplified to modulate downstream transduction enzymes or transcriptional events.