Changes in vasopressin-converting aminopeptidase activity in the rat pineal gland during summer: relationship to vasopressin contents

Biochemistry of vasopressin fragments

Vasopressin (VP) undergoes a step-wise aminopeptidase conversion process in the brain, leading to accumulation of several metabolites. Some of these metabolites, in particular [pGlu4,Cyt6]VP 4-9 and 4-8, show behavioral effects comparable to VP, but are more potent and selective than VP. Most data favor the existence of a separate receptor for the VP metabolites distinct of the classical VP and oxytocin receptors, although its identity has remained obscure thus far. The characterization of this receptor is a major challenge to understand how the brain VP system generates and regulates divers central functions.

Neurohypophysial hormones and circadian rhythm

In conclusion, neurohypophysial hormone levels in human plasma and in the plasma, pituitary, and hypothalamus of the rat show clear rhythms over the 24-hour period. These are not true circadian rhythms, as they may be modified by a number of factors, including the stage of the estrous cycle and hydrational status. There is evidence to suggest that the patterns observed may in part reflect the action of the pineal product melatonin. Pinealectomy influences the pattern of hormone release, while melatonin and its analogues inhibit hormone release, although the major circulating metabolite has no effect.

The role of the pineal in the control of the daily patterns of neurohypophysial hormone secretion

Plasma concentrations of neurohypophysial hormones show clear rhythms over 24 hr which can be suppressed by exposure to constant light, an observation consistent with pineal involvement. A study has therefore been performed on the changes in the hormone levels in the hypothalamus, posterior pituitary, and plasma over 24 hr in control, pinealectomised, and sham pinealectomised animals to determine if the pineal could play a role. Water intake, urine excretion, packed cell volume, plasma osmolality, and electrolytes were also monitored. Pinealectomy had little effect on fluid balance, but after 8 weeks for oxytocin and 2 weeks for vasopressin the morning values (0700-0800) for the circulating concentrations of the hormones were significantly higher in the pinealectomized group compared with the combined sham operated and unoperated groups (pineal intact). By contrast, the pituitary vasopressin was significantly lower in the pinealectomised group. The increase in plasma oxytocin and vasopressin seen over the hours of daylight and accompanying fall in plasma osmolality seen in the pineal intact group were absent in the pinealectomised group. Similarly, the evening fall in pituitary hormone concentrations and increase in hypothalamic hormone content were absent in the pinealectomised animals. After 10 days of exposure to constant light, the fall in plasma osmolality in the pineal-intact animals over the day was no longer significant; instead a significant increase in plasma osmolality and sodium was seen in the pinealectomised group. Exposure to constant light, while altering the patterns of neurohypophysial activity in the pineal intact group, had little effect on the pinealectomised animals.

Immunocytochemical and circadian biochemical analysis of neuroactive amino acids in the pineal gland of the rat: effect of superior cervical ganglionectomy

Semiquantitative immunocytochemistry by immuno-gold techniques revealed differences in the spatial distribution of glutamate, glutamine, and taurine within the pineal gland, with greatest labeling over pinealocytes, glia, and endothelia, respectively. At the subcellular level, glutamate labeling tended to be highest over pinealocyte synaptic ribbons and mitochondria, and lowest over lipid inclusions. Pineal levels of glutamate, glutamine and taurine, as measured by high performance liquid chromatography, did not vary over a light: dark cycle. Superior cervical sympathetic denervation, which abolishes pineal melatonin synthesis, resulted in a nearly 50% reduction in pineal glutamate levels, but had no effect on levels of glutamine and taurine. Other amino acids (alanine, arginine, aspartate, serine) were reduced by 23%-33% following sympathectomy. These data suggest an important role for glutamate in pinealocyte function(s) possibly related to the noradrenergic innervation of the gland.