Daily and circadian fluctuation in 2-deoxy[¹⁴C]-glucose uptake in circadian and visual system structures of the chick brain: Effects of exogenous melatonin

Previous studies show that several structures of the house sparrow visual system are metabolically rhythmic, as determined by 2-deoxy[¹⁴C]glucose (2DG) uptake, and that these metabolic rhythms depend upon rhythmic melatonin in this species. In many species of birds, high affinity binding of 2[¹²⁵I] iodomelatonin is widespread in the brain, especially in visual system structures. The present study asks whether 2DG uptake is similarly rhythmic in the chick brain and whether exogenous melatonin administration affects 2DG uptake. Chicks were injected with 2DG and sacrificed 1 h later. Their brains were removed and processed for 2DG autoradiography. Chicks were injected during the late day with melatonin or saline prior to the 2DG injection and brain processing. We found that the visual suprachiasmatic nucleus showed both daily and circadian differences in 2DG uptake. Six of seven visual structures displayed daily uptake changes, while only two structures showed circadian fluctuations. Melatonin affected daytime 2DG uptake within visual suprachiasmatic nucleus and ectostriatum only. These results indicate that the chick circadian system is involved in the regulation of energy metabolism in the visual system but that the role for pineal melatonin in that regulatory process is a subtle one.