Central regulation of photosensitive membrane turnover in the lateral eye of Limulus, II: octopamine acts via adenylate cyclase/cAMP-dependent protein kinase to prime the retina for transient rhabdom shedding.
Why photoreceptors turn over a portion of their photoreceptive membrane daily is not clear; however, failure to do so properly leads to retinal degeneration in vertebrates and invertebrates. Little is known about the molecular mechanisms that regulate shedding and renewal of photoreceptive membrane. Photoreceptive cells in the lateral eye of the horseshoe crab Limulus turn over their photoreceptive membrane (rhabdom) in brief, synchronous burst in response to dawn each morning. Transient rhabdom shedding (TRS), the first phase of rhabdom turnover in Limulus, is triggered by dawn, but requires a minimum of 3-5 h of overnight priming from the central circadian clock (Chamberlain & Barlow, 1984). We determined previously that the clock primes the lateral eye for TRS using the neurotransmitter octopamine (OA) (Khadilkar et al., 2002), and report here that OA primes the eye for TRS through a G(s)-coupled, adenylate cyclase (AC)/cyclic adenosine 3',5'-monophosphate (cAMP)/cAMP-dependent protein kinase (PKA) signaling cascade. Long-term intraretinol injections (6-7 h @ 1.4 microl/min) of the AC activator forskolin, or the cAMP analogs Sp-cAMP[s] and 8-Br-cAmp primed the retina for TRS in eyes disconnected from the circadian clock, and/or in intact eyes during the day when the clock is quiescent. This suggests that OA primes the eye for TRS by stimulating an AC-mediated rise in intracellular cAMP concentration ([cAMP]i). Co-injection of SQ 22,536, an AC inhibitor, or the PKA inhibitors H-89 and PKI (14-22) with OA effectively antagonized octopaminergic priming by reducing the number of photoreceptors primed for TRS and the amount of rhabdom shed by those photoreceptors compared with eyes treated with OA alone. Our data suggest that OA primes the lateral eye for TRS in part through long-term phosphorylation of a PKA substrate.
Runyon
Scott L
SL
Department of Biology, Biological Foundations of Behavior Program, Franklin & Marshall College, Lancaster, Pennsylvania 17604-3003, USA.
Washicosky
Kevin J
KJ
Brenneman
Randall J
RJ
Kelly
Jeremy R
JR
Khadilkar
Rashmi V
RV
Heacock
Kevin F
KF
McCormick
Shaelan M
SM
Williams
Kelly E
KE
Jinks
Robert N
RN
D000067956:Adenylyl Cyclase Inhibitors; D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006737:Horseshoe Crabs / Q000502:physiology*; D007425:Intracellular Membranes / Q000502:physiology; D008027:Light; D009655:Octopamine / Q000502:physiology*; D009799:Ocular Physiological Phenomena; D017956:Photoreceptor Cells, Invertebrate / Q000502:physiology*; D012160:Retina / Q000187:drug effects / Q000201:enzymology*; D015290:Second Messenger Systems / Q000502:physiology
D000067956:Adenylyl Cyclase Inhibitors; D004791:Enzyme Inhibitors; D009655:Octopamine; D005576:Colforsin; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D000262:Adenylyl Cyclases