Diurnal Rythems in the Rat Main Olfactory Bulb

Circadian rhythms are 24-hour daily rhythms in transcription, translation, hormone release, electrical activity, and other behaviors. Rhythms are primarily generated in different brain areas and tissues by input from the suprachiasmatic nucleus of the hypothalamus; however, there are some brain areas and tissues that are rhythmic without input from the suprachiasmatic nucleus, called independent oscillators. Many circadian rhythms are measured by the transcription and translation of a select group of genes that oscillate over circadian cycles, called clock genes. The olfactory bulb of the rat was discovered to be an independent oscillator by virtue of oscillations of the clock gene Period1 and electrical activity. The group that showed the olfactory bulb was an independent oscillator also showed that rats show diurnal variations in odor sensitivity, but did not examine any molecular mechanisms for how variations in electrical activity or odorant sensitivity were generated. We hypothesized that connexins and AMPA receptors fluctuate in order to eventually generate electrical rhythms. First, tissue was extracted from different animals at different times of the day and examined by quantitative PCR and immunoblotting to see if specific non-clock messenger RNAs and proteins fluctuated across the day. Some genes that affect synaptic transmission did fluctuate across the day. This research was followed by HPLC analysis to determine if monoamine neurotransmitters, which have a wealth of functions in the olfactory bulb, fluctuated across the day, as these too could affect synaptic transmission and generate electrical oscillations. Serotonin and dopamine were found to fluctuate, which would presumably affect synaptic transmission in the olfactory bulb. Finally, melatonin could be a message by which olfactory rhythms are timed, and melatonin receptors have been previously reported in the olfactory bulb. Olfactory bulb tissue was examined for the presence of melatonin receptor mRNAs and melatonin synthesis enzyme mRNAs, and all were observed in the olfactory bulb. We examined if these genes fluctuated across the light/dark cycle and found that some did. In addition, we treated olfactory bulb slices and cell cultures with iodomelatonin to determine if melatonin receptor protein was expressed, and we observed responses in rhythmic genes to iodomelatonin. Finally, whole-cell patch-clamp electrophysiology was performed on olfactory bulb cells, whose K+ currents decreased in response to iodomelatonin. Together, we have identified multiple rhythms in the olfactory bulb that can affect synaptic transmission, and identified melatonin receptor activity in the bulb.