Light inhibits the release of both [Met5]enkephalin and [Met5]enkephalin-containing peptides in chicken retina, but not their syntheses.
The levels of native and cryptic [Met5]enkephalin in the chicken retina were found to vary during a 12:12 h light-dark cycle, both rising in the light and falling during the dark. Such variations could conceivably arise from (a) changes in the rate of release and subsequent degradation of native and/or cryptic [Met5]enkephalin, (b) changes in the rate of proenkephalin A synthesis, or (c) changes in the rate of proenkephalin A processing. Measurement of the rate of release of native and cryptic [Met5]enkephalin in vitro indicated that the increased rate of release of both of these forms of [Met5]enkephalin during the dark quantitatively accounted for the fall in their retinal levels during the dark. This indicated that the biosynthesis of proenkephalin A was not activated during the light-dark cycle. Molecular weight fractionation of retinal extracts also supported this idea, since the pool of high molecular weight precursors did not vary in size, suggesting that processing was not modulated during the light-dark cycle. Instead, the fall in both cryptic and native [Met5]enkephalin during the dark was due to their increased rate of release together with a rate-limiting conversion of high molecular weight [Met5]enkephalin-containing peptides to low molecular weight [Met5]enkephalin-containing peptides. The enkephalinergic cells of the retina seem to cope with physiological variations in demand by accumulating a large pool of peptide during periods of low stimulation (light), so that when stimulation and release is high (dark), the decrease in pool levels does not compromise the function of the cells and their postsynaptic targets.
Dowton
M
M
Department of Biology, University of Wollongong, NSW, Australia.
Boelen
M K
MK
Morgan
I G
IG
Chubb
I W
IW
D000818:Animals; D000831:Animals, Newborn; D002645:Chickens; D002851:Chromatography, High Pressure Liquid; D046508:Culture Techniques; D003624:Darkness; D004744:Enkephalin, Methionine / Q000378:metabolism*; D008027:Light; D008970:Molecular Weight; D010455:Peptides / Q000378:metabolism*; D012160:Retina / Q000378:metabolism*
D010455:Peptides; D004744:Enkephalin, Methionine