Biological role of the pineal. Responses to the environmental photoperiod.

The pineal of vizcacha (Lagostomus maximus maximus) in specimens collected during winter, has the cytological characteristic of an active organ. In this state the animal is adapted to a prolonged dark environment and its reproductive organs are inactive. During summer, the vizcacha displays its sexual annual cycle of reproduction and the pineal is apparently inactive. These seasonal pineal changes may be related to the natural photoperiod and the gradual exposure to twilight during the long daylight summer period. This photic information may be expressed as changes in the sympathetic tone of the pineal. The vizcacha and rat's pineals may also influence on the pituitary pars intermedia secretion to respond as photoreceptors in the mechanism of adaptation to the natural photoperiod.