The Role of Second Messengers in Ion Channel Regulation 
Richard Axel's group, we have been studying the 
properties of a CNG channel cloned from olfac- 
tory neurons of catfish. Our goal is to understand 
how the binding of cyclic nucleotides leads to 
channel activation and how the channel itself 
may participate in adaptation. 
The gene for the catfish olfactory channel is 
"highly homologous to olfactory channel genes 
from other species and to the gene for the photo- 
receptor channel. The channel is expressed selec- 
tively in olfactory neurons, supporting its role in 
olfactory signal transduction. Like the rat olfac- 
tory CNG channel, the catfish channel is also acti- 
vated directly by both cAMP and cGMP. Unlike 
the rat channel, however, the catfish channel 
does not discriminate between cAMP and cGMP, 
suggesting a structural difference in the cyclic 
nucleotide-binding sites. Using genetic engineer- 
ing, we are trying to define the structural bases 
for these differences. 
We have also used single-channel recording to 
measure the unitary currents that flow through an 
open CNG channel. We find that the probability 
of a channel being open increases with rising cy- 
clic nucleotide concentration. When the cloned 
channel is expressed in frog oocytes, it requires 
relatively high concentrations of cAMP or cGMP 
(around 50 micromolar) to become activated. 
Surprisingly, the same channel studied in its na- 
tive environment, the catfish olfactory neuron, 
requires 20-fold lower cyclic nucleotide concen- 
trations for activation. This difference in cyclic 
nucleotide sensitivity could mean that the oo- 
cytes fail to process the channel correctly (for 
example, by not phosphorylating it properly) or 
that they lack some important channel-regulating 
protein. Defining the factors responsible for the 
discrepancy between the cloned and native chan- 
nels is an important goal. 
In other work, we are focusing on the role of 
the channel in odor adaptation. When the olfac- 
tory CNG channel is activated, it allows calcium 
ions to enter the olfactory neuron from the out- 
side medium. In photoreceptors, a similar influx 
of calcium underlies visual adaptation. We find 
that intracellular calcium greatly reduces the re- 
sponse of the olfactory channel to cyclic nucleo- 
tides. This effect appears to result from a shift in 
the dose-response curve for channel activation to 
higher concentrations of cyclic nucleotides. The 
inhibitory effect of calcium occurs at physiologi- 
cal calcium levels. Moreover, the effect is not due 
to a direct action of calcium on the channel but 
rather appears to involve an intermediate regula- 
tory protein that is loosely associated with the 
channel. Thus calcium acts as a negative feedback 
regulator of olfactory responses. 
Thus the olfactory system provides a useful 
model for studying neuronal signal transduction 
and neuronal plasticity. Studies on the molecular 
bases of these phenomena should provide us with 
insight into many of the basic mechanisms con- 
trolling nerve cell behavior. 
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