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G.Y. Ko, Y. Liu, R.F. Lyon, M. Roper, S.E. Dryer, M.L. Ko; Protein Expression and Somatostatin Modulation of Calcium Channels Are Under Circadian Control in Chick Retina Photoreceptors . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5423.
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The L–type voltage–dependent calcium channels (VDCCs) are essential in neurotransmitter release in vertebrate retina. The synthesis and release of melatonin from photoreceptors is under circadian control and also dependent on L–type VDCCs (Ivanova and Iuvone, 2003). Somatostatin has been shown to inhibit L–type VDCCs in neurons and epithelial cells. The purpose of this study was to investigate the circadian regulation of L–type VDCCs, as well as whether the somatostatin modulation of VDCCs was under circadian control.
Chick embryos (E10) were entrained under 12:12 hr light–dark (LD) cycles in ovo for 6 days and kept in constant darkness (DD) for 1 day. On the second day of DD, retinas were dissected at different circadian time points and prepared for western immunoblotting for detection of the α subunit of L–type VDCCs. Retinas from E10 were cultured and entrained under LD cycles in vitro for 5 days and kept in DD for another day. On the second day of DD, either perforated patch clamp recordings were performed from cultured chick retinal photoreceptors, or cells were harvested for western immunoblotting. All–trans retinol, which promotes photoreceptor survival, was present in cultures. Thus, the photoreceptor population in culture was at least 80%.
The protein expression of the α subunit of L–type VDCCs (Cav1.2) and the VDCC current ampllititudes are greater during the subjective night (CT 16–22) than the subjective day (CT 4–10) in chick cones free–running on the second day of DD. The VDCC rhythm is under the control of the MAP kinase Erk, since treatment with the Erk inhibitor PD 98059 for 2 hr inhibits the VDCC rhythm. Acute perfusion of somatostatin (SS–14) increases the VDCC current amplitude during the subjective day but decreases the current during the subjective night. Treatment with SS–14 for 2 hr also decreases the expression of L–type VDCC α subunit during the subjective night but not the subjective day.
The protein expression of L–type VDCCs is under circadian control, and Erk is part of the output pathway that regulates the VDCC rhythm. The somatostatin modulation of L–type VDCCs is also under circadian control. This result is rather unusual, since somatostatin enhances the VDCC currents during the subjective day but inhibits the currents during the subjective night on the same type of cells.
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