The response to isoproterenol of synaptic ribbon numbers in the rat pineal gland changes during postnatal development

In the mammalian pineal gland synaptic ribbons (SRs) are dynamic organelles of pinealocytes undergoing a day/night rhythm, with small numbers during daytime and significantly higher numbers at night, similar to the formation of the pineal hormone melatonin. Whereas the day/night rhythm of melatonin synthesis is adrenergically regulated, data on the adrenergic regulation of SR numbers in the rat pineal gland are at variance. While some authors have demonstrated that isoproterenol (ISO) stimulates SR numbers, others could not find any effect. To clarify the issue, we carried out identical experiments in two age groups. It was found that in male Sprague-Dawley rats, administration of the beta-adrenergic agonist ISO (2.5 or 10 mg/kg body weight) increased SR profile numbers in one age group (3 weeks), but not in the other (8 weeks). These findings resolve the apparent discrepancy reported in the literature, by showing that the response to ISO of SR profile numbers is apparently lost during postnatal differentiation of the pineal gland.