A circannual cycle in pinealocyte synaptic ribbons in the hibernating and seasonally reproductive 13-lined ground squirrel (Spermophilus tridecemlineatus).

Pinealocyte synaptic ribbons (SR) in the 13-lined ground squirrel were quantified at monthly intervals over a one-year period spanning the hibernating and reproductive cycles of these animals. SR numbers were high (70-90/20,000 microns2) during periods of activity and reproductive quiescence (May-October). With the onset of hibernation (November-December) there was a rapid 6 to 7-fold reduction in SR frequency. During arousal (January-February) and sexual maturation (March-April) there was a gradual increase in SR frequency. The winter decline in SR frequency is consistent with the decline in pineal melatonin during hibernation and supports the hypothesis that pinealocyte SR play an important role in the neurotransduction of melatonin biosynthesis.