OBJECTIVE: The pineal gland is part of the circadian clock system and is under the predominant influence of the endogenous oscillator located in the suprachiasmatic nucleus. A polysynaptic pathway involving hypothalamus, spinal cord and sympathetic system regulates the so far best-studied aspect of its neuroendocrine output, i.e., the synthesis and secretion of melatonin. This parameter increases dramatically at night upon sympathetic activation in rats and many other mammals including man. In addition, parasympathetic, trigeminal, diencephalic and other sites or mechanisms connect the gland, mainly via its stalk, to the nervous system. However, their function for pineal metabolic or morphological features are hardly known. An interesting ultrastructural attribute of the pineal gland are ribbon synapses. These presynaptic structures in pinealocytes are composed of a ribbon and vesicles. They are thought to regulate and facilitate multivesicular release, and display a circadian rhythm with higher levels at night paralleling melatonin synthesis but regulated differently.
METHODS: To gain more insight into the roles of both, the non-sympathetic ("central") innervation and the regulation of pineal ribbon synapses, a surgical transection of the pineal stalk was conducted in rats and the number of synaptic ribbons (SR) were determined by electron microscopy from experimental, sham-operated and control animals.
RESULTS: The transection resulted in normal daytime levels but diminished the nocturnal increase of SR numbers when compared to controls or sham-operated rats.
CONCLUSION: These data provide first evidence that the central innervation of this neuroendocrine organ plays an important role in SR (up)regulation, and that this pathway is antagonistic to the sympathetic innervation.