Pinealocyte dense-cored vesicles and synaptic ribbons: a correlative ultrastructural-biochemical investigation in...

Dense-cored vesicles (DCV) and synaptic ribbons (SR) were quantified in the pineal gland of the rat (Sprague-Dawley) and mouse (Sasco/ICR strain), and day/night differences in frequency of these organelles correlated with levels of indoles determined by high performance liquid chromatography (HPLC). There were significant day/night differences in levels of serotonin (5HT), 5-hydroxyindole acetic acid (5HIAA), N-acetyl-5HT, and melatonin in the rat gland. Melatonin and N-acetyl-5HT were not detectable in the mouse gland sampled every 4 h over the light:dark cycle. The concentrations of 5HT and 5HIAA (ng/microgram protein) were similar in light-adapted rats and mice, but these indoles did not exhibit a circadian rhythm in the mouse gland. Correlative ultrastructural/biochemical results suggest that DCV do not contain physiologically important stores of 5HT since 1) the mouse gland contains the same number of DCV as the rat during the daytime, but only one-tenth the levels of 5HT, 2) day/night 5HT levels do not vary in the mouse gland, but there is a significant nocturnal decline in DCV numbers, and 3) 5HT levels in the rat gland decline at night when DCV numbers increase. Numbers of SR were significantly elevated at night in the rat and mouse, and the frequency of this organelle was similar in both species. However, ribbon-type SR predominated in rat pinealocytes, whereas SR in the mouse were almost exclusively spherical in shape. Day/night differences in SR numbers in the mouse gland suggest that cellular mechanisms regulating the frequency of this organelle do not involve factors related to indole metabolism. Because of the lack of photoperiodic effects on indole metabolism in the mouse pineal gland, this species is a potentially important model to study the functional relationship of pinealocyte organelles to cyclical changes in pineal products other than indoles (e.g., peptide/protein factors).