Widespread expression of olfactory cyclic nucleotide-gated channel genes in rat brain: Implications for neuronal signalling

The cyclic nucleotides cAMP and cGMP are important intracellular messengers involved in a wide variety of signal transduction events in the nervous system. It has been proposed that cAMP/cGMP elicit some of their effects through direct gating of a novel class of Ca²⁺-permeable ion channels that are termed cyclic nucleotide-gated (CNG) channels. Previous studies have identified the expression of a gene encoding one major CNG channel subtype, the olfactory receptor neuron, subunit, in the brain [El- Husseini et al. (1995) NeuroReport 6:1331-1335; Kingston et al. (1996a) Proc. Natl. Acad. Sci. U.S.A. 93:10440-10445; Bradley et al. (1997) J. Neurosci. 17:1993-2085]. We, therefore, proposed that the actions of cAMP/cGMP on neurons in the brain might occur through the activation of these CNG channels. To determine how widespread such a function might be, the regional and cellular distribution of the olfactory CNG channel subunit has been examined in detail. Primers for multiple portions of the olfactory CNG channel were used in polymerase chain reaction (PCR) to amplify cDNA reverse- transcribed from several brain regions. The identities of PCR products were confirmed with Southern blots and by sequencing. In situ hybridization experiments demonstrated localization of CNG channel mRNA in discrete neuronal populations throughout the brain. In agreement with previous work, relatively strong hybridization signals are present in neuronal cell bodies of the cerebellum, olfactory bulb, cerebral cortex, and brainstem. Additionally, somewhat lesser signals are found in thalamus, hypothalamus, midbrain, and spinal cord while no hybridization signal was detectable in the caudate nucleus. This surprisingly wide distribution throughout the rat brain strengthens the hypothesis that CNG channels may influence numerous processes as downstream effectors of cyclic nucleotide cascades. Interestingly, the distribution of CNG channels is very similar to that of the nitric oxide/cGMP system, suggesting that one function of CNG channels in the brain could be to link diffusible messengers to elevated Ca²⁺ entry into neurons.