Responses of synaptic ribbons in pineal photoreceptors under normal and experimental lighting conditions.

The size of synaptic ribbons (SR) in photoreceptor cells of the goldfish pineal organ was quantified over 24-h light:dark cycles of long (16:8) and short (10:14) photoperiods during summer and winter months, respectively. The amplitude of both rhythms was similar with peak values occurring toward the latter part of the photophase or early dark. When fish were entrained to the long photoperiod and exposed to continual light, SR size continued to increase during the expected dark time. The effect of extending the photoperiod into the expected dark time was diminished when fish were entrained to a short photoperiod and presented with 6 h of darkness at the end of the 24-h period. The size increase in response to environmental lighting is believed to reflect a greater demand for either vesicle attachment sites or neurotransmitter storage sites since vesicles (neurotransmitter) have been hypothesized to accumulate in the synaptic pedicles during inhibition by light. From a comparative standpoint it is noteworthy that synaptic ribbons (vesicle-crowned rods) in mammals react in a similar manner to both normal and experimental lighting conditions.