Pinealocyte synaptic ribbons and neuroendocrine function.

A comparative study of pinealocyte synaptic ribbons (SR) revealed two predominant populations exhibiting either a rod/ribbon shape (SRr) or a spherical/punctate shape (SRsp). Species-specific differences were found in the abundance of SR, the ratio of SRr/SRsp, and the occurrence of SR in ribbon fields. The close topographical relationship of SR to the plasma membrane and the numerical changes that occurred with changes in metabolism of the pinealocytes suggest that SR have important vesicle-mediated interactions with the cell membrane. Experiments designed to clarify the relationship between SR and pineal neuroendocrine function revealed a positive correlation between SR numbers and indole intermediates during pineal development in the rat, and increased SR frequency after denervation of the rat pineal gland or administration of the beta-adrenergic agonist, isoproterenol. These data are consistent with the hypothesis that SR function is linked to receptor mechanisms regulating indoleamine production in the pineal gland.