Altered calmodulin response to light in the suprachiasmatic nucleus of PAC1 receptor knockout mice revealed by proteomic analysis.

In mammals circadian rhythms are generated by a light-entrainable oscillator located in the hypothalamic suprachiasmatic nucleus (SCN). Light signals reach the SCN via a monosynaptic neuronal pathway, the retinohypothalamic tract, originating in a subset of retinal ganglion cells. The nerve terminals of these cells contain the classical neurotransmitter glutamate and the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP), and there is evidence that these two transmitters interact to mediate photoentrainment of the oscillator in the SCN. To elucidate light-provoked PACAP receptor signaling we used proteomic analysis. Wild-type mice and mice lacking the PAC1 receptor (PAC1-/-) were light stimulated at early night, and the SCN was examined for proteins that were differentially expressed using two-dimensional gel electrophoresis and identification by tandem mass spectrometry. The most striking finding, which was subsequently confirmed by Western blotting, was a significant reduction of calmodulin (CaM) in wild-type mice as compared with PAC1-/- mice. Analysis at the mRNA level by quantitative in situ hybridization histochemistry was inconclusive, indicating that a translational mechanism might be involved. The findings indicate that PAC1 receptor signaling in the SCN in response to light stimulation induces a down-regulation of CaM expression and that CaM is involved in the photic-entrainment mechanism.