G. Gaus et al., THE EFFECT OF NEUROPEPTIDES FROM LIMULUS ON ITS CIRCADIAN-RHYTHM IN RETINAL SENSITIVITY, Journal of comparative physiology. A, Sensory, neural, and behavioral physiology, 180(2), 1997, pp. 137-142
Retinal responses of the Limulus lateral eyes to light are greater at
night than during the day. A circadian clock in the brain of the horse
shoe crab controls these rhythmic changes of light sensitivity. The in
crease in sensitivity (as measured by the amplitude of the electroreti
nogram) is mediated at least in part by octopamine that is released fr
om efferent axons terminating in the visual cells. Earlier studies ind
icate that certain factors in Limulus hemolymph can act in conjunction
with octopamine. More recently, five neuropeptides (LP1-LP4 and Lip-H
P) had been isolated from acetone extracts of the Limulus central nerv
ous system using HPLC fractionation and radioimmunoassay with antisera
against FMRFamide-like peptides for detection. Presently, we have inj
ected into the Limulus lateral eye these five peptides and observed ch
anges in retinal sensitivity. Injection during daytime had no immediat
e effect on that daytime electroretinogram but decreased the electrore
tinogram amplitude for the entire subsequent night (12 h). However, up
on injection at night, we observed an immediate but only transitory de
crease in electroretinogram amplitude for about 1 h without effect on
the subsequent daytime electroretinogram. We suggest that the peptides
act antagonistically to octopamine and are highly dependent upon the
activity state of the efferent nerve terminals.