EFFECT OF CONSTANT AND FLUCTUATING TEMPERATURE ON DAILY MELATONIN PRODUCTION BY EYECUPS FROM RANA-PEREZI

We analysed the effect of daily temperature cycles in relation to cons tant temperature on day/night melatonin synthesis in frog eyecups in c ulture. Eyecups were cultured for 24 h under 12L:12D photoperiod and t wo thermal regimes, constant temperature (25, 15 and 5 degrees C) and thermoperiod (WL/CD, thermophase coinciding with photophase and cryoph ase coinciding with scotophase; and CL/WD, cryophase coinciding with p hotophase and thermophase coinciding with scotophase). A negative corr elation between ocular serotonin N-acetyltransferase activity and cult ure temperature for both diurnal and nocturnal activities has been obs erved. This effect of increased ocular activity at low temperature is more pronounced than the well-known stimulatory effect of darkness, an d it does not depend on the photoperiod phase. The lack of interaction s between the phase of photoperiod and culture temperature indicates t hat the effects of both factors are independent. Nighttime temperature is the key factor in determining the amplitude of the melatonin rhyth m in the Rana perezi retina. However, daytime temperature can not coun teract the inhibitory effect of light on ocular melatonin synthesis.