Evidence for an endogenous clock in the retina of rainbow trout: I. Retinomotor movements, dopamine and melatonin.

The purpose of the present study was to investigate patterns of circadian rhythmicity in the retina of a salmonid fish, the rainbow trout (Oncorhynchus mykiss). In a first attempt to show both morphological and neurochemical variations during light/dark conditions (LD) and during continuous darkness (DD), we investigated retinomotor movements and the associated regulatory transmitters, dopamine and melatonin. All parameters studied showed patterns of rhythmicity in LD and DD, clearly indicating the presence of an endogenous clock in the rainbow trout eye. The most salient variations of all parameters studied were found at the transitions from light to dark and vice versa in LD and from subjective day to subjective night and vice versa in DD. The amplitudes of rhythms compared between LD and DD were similar in pigment index, whereas a clear reduction was found for cone index, dopamine and melatonin.