CAMP-gated Na+ channels

Substrate for cAMP PDE (activates PKA, opens cAMP-gated Na+ channels binds to Dictyostelium cAMP receptor)... 

This is achieved through the action of olfactory neurons. These hnk the olfactory bulb of the brain and the nasal cavity. Each olfactory neuron expresses one of 500-700 different kinds of odorant receptor (OR). ORs are G protein-coupled receptors. Binding of an odorant to a specific OR triggers activation of the Ga protein that is associated with the OR. The Ga protein activates adenyl cyclase, and the cAMP consequently opens a cAMP-gated ion channel. The channel allows entry of Na+ and Ca + into the neuron, thus triggering transmission of a nerve impulse to the brain. Our perception of particular smells is based on combinatorial activation of different ORs by different odorants in a sample that might have a complex mixmre of odorants. 

The increased intracellular Ca2+ concentration also has an inhibitory effect, which eventually shuts down the signal. The ion binds to calmodulin (CaM) lowering the ligand sensitivity of the cAMP-gated channels and activating the activity of a phosphodiesterase (PDE). Ca2+ is finally extruded by a Na+/Ca2+ exchanger. 

A potent aaivator of the cAMP, cyclic nucleotide-gated (CNG) Na+/K+ channel in olfactory cells is NO (nitric oxide), which is many times more potent than cAMP. However, NO is present only in developing olfactory neurons, where it seems to react directly with cysteine SH-groups in the P-subunit of the channel. It was speculated, therefore, that NO might be a channel activator early in development or in regenerating olfactory neurons (see ref. 91). 

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