Isolation and identification of vasopressin- and oxytocin-immunoreactive substances from bovine pineal gland. Presence of N alpha-acetyloxytocin

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Potential role of kallikrein in diurnal rhythms and perivascular distribution in rat pineal glands

Kallikrein hydrolyzes various biologically active peptides, other than kininogens, such as vasoactive intestinal polypeptide (VIP), in vitro. Since kallikrein and VIP have been immunohistochemically shown to be present in the perivascular areas of the pineal gland, this study was designed to determine their topographic proximity in these glands, using immunohistochemical and immunoelectron microscopic double staining methods. Furthermore, since this gland is well-known to have a circadian rhythm, the kallikrein content was measured every 4 h, using a synthetic substrate, Pro-Phe-Arg-MCA, and an enzyme-linked immunosorbent assay (ELISA) to determine whether kallikrein has a circadian rhythm. The immunoreactivities of kallikrein and VIP were highly localized in the perivascular extracellular spaces and were virtually identical in distribution. The kallikrein content changed every 4 h and was high under light and low under dark conditions. The change was more evident when the synthetic substrate was used, and this rhythm was subtle on ELISA. VIP is also said to have a circadian rhythm in the pineal glands, being low under light and high under dark conditions, i.e., opposite to that of kallikrein. Since kallikrein degrades VIP in vitro, it is reasonable to speculate that pineal gland kallikrein is involved in the processing of VIP and possibly other biologically active peptides in the perivascular areas with a discernible circadian rhythm.

Pharmacological characterisation of oxytocin binding sites in the ovine pineal gland

Both oxytocin (OT) and [Arg8]vasopressin (AVP) are found within the ovine pineal gland and may function to modulate melatonin secretion. However, the receptors which mediate the actions of these peptides have yet to be characterised. Preliminary studies of ovine pineal microsomal cell membranes showed binding of [3H]OT (79+/-9 fmol/mg) 10 times greater than binding of [3H]AVP (8+/-3 fmol/mg). Saturation studies using either [3H]OT or the selective OT receptor ligand [125I]d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH2(9)]-vasotocin (OTA) revealed high affinity, single site kinetics (Kd = 1.72+/-0.32 nM; Bmax = 68+/-18 fmol/mg). Binding of [3H]AVP was more effectively displaced by OT than AVP, suggesting that binding may be due to cross-reaction with the OT binding site. Displacement of [3H]OT using a range of selective agonists and antagonist analogues revealed pharmacological characteristics similar to [3H]OT binding sites in the ovine and rat uterus. These data show that the ovine pineal expresses a high density of OT binding sites which may participate in the regulation of melatonin secretion.

The mammalian pineal gland and reproduction. Controversies and strategies for future research

Evidence for a role of the pineal gland and its major hormonal product, melatonin, in mammalian reproduction has accumulated for over three decades. In sorting through the masses of data pertaining to this issue, certain facts are becoming established. For example, the pineal gland is clearly involved in seasonal reproductive cycles via transduction of daylength (more properly nightlength) information in the form of plasma melatonin rhythms. Specific melatonin receptors are found in the hypothalamus and pituitary pars tuberalis of most of the mammalian species examined thus far. Melatonin's mode of action on the reproductive axis is quite variable but may, in many cases, involve modulation of gonadotropin-releasing hormone secretion from the median eminence. Clinical evidence continues to support the idea that melatonin may play a role in the timing of puberty.

Characterization of immunoreactive AVP in the ovine hypothalamo-pituitary axis

The aims of these studies were to determine the precise molecular nature of immunoreactive (IR-) vasopressin (AVP) in the ovine hypothalamo-pituitary axis and to examine a possible role for glucocorticoids in regulating both AVP processing and levels in this axis. The IR-AVP in extracts of paraventricular nucleus, median eminence, portal blood, and anterior and neurointermediate pituitary elutes as a single peak on two distinct HPLC solvent systems, suggesting that AVP is processed identically in these tissues. Identical profiles were also found in extracts from pituitaries and sheep subjected to chronic (10 days) glucocorticoid treatment, or hypothalamo-pituitary disconnection. The latter findings confirm that in sheep, like the rat, AVP is synthesized and processed in the anterior pituitary and is not sequestered from extrapituitary sources.

Immunoreactive arginine vasopressin in human fetal and neonatal skeletal muscle

We provide evidence for the presence of arginine vasopressin (AVP) in human fetal and neonatal skeletal muscle using a combination of specific RIA, tangential flow ultrafiltration and reverse-phase HPLC separation. The IR-AVP concentrations are negatively correlated with gestational age (r = -0.75, P less than 0.0001) and range from 1 to 10 pmol/g wet wt at 15 weeks gestation to 0.04 pmol/g wet wt at term. This IR-AVP substance is of low molecular weight (less than 3000 mol. wt), elutes in the same position as standard AVP after HPLC separation and is detected by four different anti-AVP antisera.

Isolation and structure elucidation of bovine pineal arginine vasopressin: arginine vasotocin not identified

A large number of reports have demonstrated the presence of neurohypophysial hormone-like peptides in mammalian pineal glands and an antigonadotropic function has been ascribed to pineal arginine vasotocin (AVT). We have undertaken large scale purification of bovine pineal neurohypophysial hormone-like substances which demonstrate mouse mammary milk-ejection activity (ME-activity) in vitro. Peptides with ME-activity were extracted from more than 5 kg of bovine pineal glands. ME-activity containing peptides were found in both high (Mr approximately 10,000-15,000) and low (Mr approximately 500-1000) Mr species from Sephadex G-25 chromatography of 0.2 N acetic acid extracts. After ultrafiltration in 5% formic acid, the neurohypophysial hormone-like peptides were localized to an ultrafiltration Mr 500-1000 retentate. A homogeneous peptide, which shared an identical retention time (RT) and amino acid sequence with synthetic 8-arginine vasopressin (AVP), was isolated by serial semipreparative high performance liquid chromatography. On the other hand, the non-mammalian nonapeptide AVT was not identified.

N alpha-acetyl-[Arg8]vasopressin antagonizes the behavioral effect of [Cyt6]vasopressin-(5-9), but not of vasopressin

It has been found recently that N alpha-acetyl-[Arg8]vasopressin (Ac-VP) is present in the brain of rats. The physiological significance of this peptide is as yet unknown. Therefore, the central nervous system effects of this peptide were investigated, namely, its effects on passive avoidance behavior, exploratory behavior and body temperature. The interaction of Ac-VP with the central nervous system effects of vasopressin (VP) was also studied. Ac-VP had a slight agonistic effect on passive avoidance behavior, i.e. it facilitated passive avoidance behavior at a dose 100 times higher than that of VP. Relatively low doses (3-10 ng) of Ac-VP attenuated passive avoidance behavior, which suggests that Ac-VP interfered with an endogenous compound involved in the control of passive avoidance responding. Ac-VP was also able, albeit in higher doses (30 ng), to competitively antagonize the effect of [Cyt6]VP-(5-9), a highly potent, putative endogenous metabolite of vasopressin in the rat brain. This antagonism could be due to an interaction of Ac-VP with sites other than the V1 vasopressin receptor. Ac-VP had no significant influence on other central nervous system effects of the hormonally active nonapeptide VP, such as exploratory behavior and body temperature. These effects were readily antagonized by the V1 vasopressin receptor antagonist d(CH2)5Tyr(Me)VP. Ac-VP may be competitive antagonist of behaviorally active vasopressin metabolite(s) in the brain.

N-acetyl-vasopressin- and N-acetyl-oxytocin-like substances: isolation and characterization in the rat neurointermediate pituitary and presence in the brain

Abstract Post-translational modifications of vasopressin and oxytocin in pituitary and brain were investigated in view of recent evidence that oxytocin is partly N(alpha)-acetyfated in the bovine pineal gland. Two peptides were isolated from the neurointermediate lobe of the rat pituitary gland and characterized as N(alpha)-acetyl-vasopressin and N(alpha)-acetyl-oxytocin, based on chromatographic and immunological properties as well as the blocked N-terminus. In the neurointermediate pituitary the acetylated forms represented approximately 1% of the vasopressin and oxytocin contents. These two peptides were also detected in some, but not all, investigated brain areas. The highest degree of acetylation was found in the pineal gland. In all regions acetylation of oxytocin was more abundant than that of vasopressin. The data indicate that acetylation of vasopressin and oxytocin generally occurs as a post-translational modification. They support the concept that acetylation may represent a mechanism aimed to control bioactivity of the neurohypophyseal hormones.

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

Vasopressin (VP)-converting aminopeptidase (VP-AP) activity and VP contents were measured in single rat pineal glands during the summer of two successive years. The peptidase activity decreased significantly in August. The lowest activity (+/- SEM) of 0.18 +/- 0.02 pmol.hour-1 was recorded on August 14, compared to the basal activity of 0.25 +/- 0.01 pmol.hour-1 in July and September of 1986. The change with similar percentage occurred in the same period of 1987. The specific activity of the enzyme in the crude homogenate, 15,000 g pellet and supernatant fraction of rat pineal glands, exhibited the same pattern of variations. The decrease in peptidase activity coincided with the previously reported dramatic rise in pineal VP content in early August which was confirmed in this series of experiments. Another peptidase, the so-called gamma-endorphin generating endopeptidase (gamma-EGE) activity, and beta-endorphin-related peptides in the pineal gland did not change in this period. The results show that the variations of pineal VP contents and VP-AP activity during summer are not general for other peptides and peptidases. The coincidence of opposite changes in VP content and VP-AP activity of the pineal gland may indicate a role of the peptidase activity to regulate the VP content.

Circadian variations of vasopressin level and vasopressin-converting aminopeptidase activity in the rat pineal gland

Vasopressin levels and vasopressin-converting aminopeptidase activity were measured in the rat pineal gland during the 24 hr light-dark cycle. A rhythmic variation in peptide levels and peptidase activity occurred. At the onset of light at 6.00 hr, the peptidase displayed a significant, short-lasting (approximately 3 hr) increase of about 35% in activity, while a decrease of 28% in pineal vasopressin levels was observed. The changes in peptidase activity and peptide level were not triggered by light per se, since they persisted to occur at the same time point in animals which were not exposed to light, indicating the circadian nature of the rhythmicity. These changes were specific to the pineal gland, since other tissues, like hippocampus and pituitary gland, did not show these daily variations. The data suggest a relationship between vasopressin levels and vasopressin-converting aminopeptidase activity.