Sinus gland participation on the phase relationship between the circadian rhythms of the response to light (electroretinogram, ERG) of the visual photoreceptors in the crayfish was studied. Chronic ERG recordings were simultaneously obtained from both eyestalks in intact and sinus-gland-deprived animals during free-running Circadian oscillators allow various functions either to coincide or to avoid coincidence, providing an organism with internal temporal order. This phenomenon is known as internal synchronization (Moore-Ede and Sulzman, '81) and can be analyzed under at least 2 different sets of experimental conditions, namely, free-running conditions and conditions in which the oscillators have been synchronized by external periodic signals. From a comparison of the results obtained under each set of conditions, one would expect to gather information about the factors that determine the ability of the circadian oscillators to confer upon each function that they regulate its characteristic adaptive behavior.In crayfish, a circadian rhythm in the amplitude of the photoreceptors' electrical response to light (as measured by an electroretinogram or ERG) has been found in recordings from the eyestalk (Arkhiga et al., '73). Simultaneous recording of the ERG from both eyestalks has provided evidence that in intact animals, the circadian rhythm of the amplitude of the ERG in one eye maintains a tight phase relationship with that in the other eye '87). Within the eyestalk itself there exists an important part of the neuroendocrine system of the crayfish, namely, the X organ-sinus gland complex. The X organ is located in the medulla terminalis; it is a tightly clustered group of neurosecretory cells from which neurosecretions migrate 0 1992 WILEY-LISS, INC. along axons to be stored in a sinus gland, a neurohemal organ that is located in the medulla externa. In other crustacean species (Abramowitz, '37; Gorgels-Kallen and Voorter, ,851, it is known that there is a periodic (circadian) release of neurosecretions from the sinus gland into the systemic circulation.These findings led us to study the role of the sinus gland in the internal synchronization phenomenon.From the present study it appears that the sinus gland is part of the oscillator system that underlies the circadian rhythm of the ERG and that it is also an indispensable element for the establishment of a reciprocal interaction between the circadian oscillators of the left and right eyestalks, not only under free-running conditions but also during entrainment produced by external periodic signals.
MATERIALS AND METHODS
AnimalsAdult crayfkh Procambarus clarki or Procambarus digueti were used. Before experimentation animals were maintained on a 24-hour cycle that consisted of 12 hr light and 12 hr darkness (LD 12:12); the light was turned on at 0700 hours and turned off at 1900 hours. The temperature was 16°C.
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