Cellular Levels of Messenger Ribonucleic Acids Encoding Vasoactive Intestinal Peptide and Gastrin‐Releasing Peptide in Neurons of the Suprachiasmatic Nucleus Exhibit Distinct 24‐Hour Rhythms

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Abstract
There is strong evidence supporting the view that the Suprachiasmatic nucleus (SCN) functions as a circadian clock; however, the neural and molecular events underlying SCN function remain unclear. A specific subpopulation of neurons within the ventrolateral aspect of the SCN that contains three peptides, vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI) and gastrin‐releasing peptide (GRP), play an important role in SCN function. VIP‐containing neurons of the SCN receive synapses from photic projections, and co‐injection of all three peptides mimics the phase‐delaying effects of light on circadian activity rhythms. In principle, the signaling potential of a neuron containing several transmitters may be affected by the concentration ratio of co‐released factors; hence, one mechanism by which VIP/PHI/GRP‐containing neurons could influence SCN function is by changing the concentration ratio of these peptides throughout the light‐dark cycle. The present study was performed to examine this possibility. Relative cellular levels of mRNA encoding both VIP/PHI and GRP were determined within the SCN every 4 h in rats housed in a 14 h light: 10 h dark cycle. Quantitative in situ hybridization revealed a statistically significant (P<0.005) 24‐h profile of changes in VIP/PHI mRNA that peaked during the dark phase, and a significant (P<0.005) 24‐h profile of changes in GRP mRNA that peaked during the light phase. These data support the interpretation that cellular levels of mRNAs encoding VIP/PHI and GRP within the SCN exhibit distinct profiles of changes throughout the light‐dark cycle. Further, these findings are consistent with the hypothesis that the concentration ratio of VIP and PHI to GRP changes over the light‐dark cycle, and that this may be an important mechanism by which circadian rhythms are generated or entrained.