Cyclic nucleotide-gated channels in identified human olfactory receptor neurons

Patch-clamp recordings revealed the presence of a non-desensitizing cyclic nucleotide-gated channel on human olfactory receptor neurons and a fast-desensitizing non-specific cation channel activated by nucleotides on human supporting cells. Cyclic nucleotide-gated channels on olfactory receptor neurons showed selective channel activation by cAMP (K-1/2 = 5 mu M) and cGMP (K-1/2 = 2 mu M), a unitary conductance of similar to 20 pS, a reversal potential of single-channel currents close to 0 mV, a linear current-voltage relationship over the range of -80 to 80 mV and a strong extracellular but a weaker intracellular blocking effect of Ca2+. The channel activity outlasted the cyclic nucleotide pulses for hundreds of milliseconds when higher agonist concentrations (>50 mu M cAMP) were applied. The duration of the response was longer than in cyclic nucleotide-gated channels from other species studied so far. The plateau duration and the decay remained constant for pulses with a length of 50-150 ms, whereas pulses shorter than 50 ms successively reduced the time required by shortening the plateau phase. A larger difference for the K-1/2 values of cAMP (K-1/2 = 22 mu M) and cGMP (K-1/2 = 2.5 mu M) were found for a small group (n = 3) of cyclic nucleotide-gated channels, pointing to the selective expression of the alpha-subunit in a small subgroup of olfactory receptor neurons.