THE EFFECT OF NEUROPEPTIDES FROM LIMULUS ON ITS CIRCADIAN-RHYTHM IN RETINAL SENSITIVITY

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.