Journal of Neuroscience, Vol 5, 851-856, Copyright © 1985 by Society for Neuroscience
Opioid pathways in an avian retina. I. The content, biosynthesis, and release of Met5-enkephalin
YY Su, CB Watt and DM Lam
By means of an enzyme-linked immunosorbent assay, the concentration of
enkephalin-immunoreactive substances was estimated to be about 25 nM in the
chicken retina. The biosynthesis of 3H-Met5-enkephalin in this retina was
studied by a pulse-chase incubation technique. Isolated retinas were
incubated with 0.2 ml of oxygenated Ringer's solution containing 40 microCi
of [3H]methionine and trasylol, a peptidase inhibitor, for 30 min at room
temperature. The tissue was then rinsed three times in large volumes of
Ringer's solution and incubated in the same solution containing unlabeled
methionine (100 micrograms/ml) and trasylol for at least another hour. The
products synthesized were extracted in acetic acid and assayed by high
performance liquid chromatography (HPLC) and immunoassay. A peak of
radioactivity that comigrated with Met5-enkephalin on HPLC and
cross-reacted with antibodies against enkephalins was detected. The level
of 3H-Met5- enkephalin radioactivity increased approximately 10-fold as the
chase- incubation period increased from 0 to 120 min, suggesting that, as
in other tissues, Met5-enkephalin may be synthesized as part of a larger
precursor. The newly synthesized Met5-enkephalin could be released by
depolarization of the retina with high extracellular K+ concentration.
Furthermore, this K+-stimulated release was greatly suppressed by 5 mM Co2+
in the medium, suggesting that this release is Ca2+-dependent and may be
synaptically mediated.