The depression in rat pineal melatonin production after saline injection at night may be elicited by corticosterone

A hind-leg subcutaneous saline injection into rats at night elicits a decrease in N-acetyltransferase (NAT) activity and melatonin content of the pineal gland. The decrement in pineal melatonin production after saline injection is prevented by adrenalectomy. The present studies were undertaken to determine what factor(s) from the adrenal gland cause(s) the drop in pineal melatonin production after saline injection at night. In the first study, groups of intact and adrenal-demedullated male rats were given a saline injection at 23.10 h (3 h, 10 min after lights off) and their pineals were collected 15 or 30 min later. Pineal NAT activity was depressed in both intact and adrenal-demedullated rats at 15 min postinjection as compared to their respective control animals. Pineal melatonin levels exhibited a drop in intact animals at 15 min and in adrenal-demedullated rats at 30 min. In a second study, hypophysectomy was found to prevent the drop in nocturnal pineal NAT activity and melatonin levels normally associated with a hind leg injection of saline. Finally, in a third experiment, groups of hypophysectomized rats were injected i.p. with corticosterone at 23.10 h and killed 10, 25 or 40 min postinjection. Corticosterone injection in hypophysectomized rats produced a response similar to that caused by saline injection in intact animals: NAT activity was depressed at 10 min and melatonin content was lowered at 25 min. These results suggest that the adrenal-mediated depression in melatonin synthesis after saline injection at night in rats may be elicited by an adrenal cortical hormone (corticosterone) and apparently does not involve the release of factors from the adrenal medulla.     

Twenty-four-hour pineal melatonin synthesis in the vasopressin-deficient Brattleboro rat

The diurnal time course of pineal melatonin synthesis was analyzed simultaneously in the arginine vasopressin (AVP)-deficient Brattleboro rat (BB), the Long-Evans (LE) and Sprague-Dawley (SD) rat by means of radioenzymatic determination of the rate-limiting enzyme serotonin-N-acetyltransferase (NAT) and the melatonin content over a period of 24 h. While all 3 strains displayed a distinct day—night rhythm of melatonin synthesis (low day-time, high night-time values), BB rats generally exhibited lower NAT values as compared to LE and SD rats, though reaching a significant difference at 02.00 h only. Twenty-four-hour melatonin content was characterized by distinct nocturnal maxima in LE and SD rats, while BB rats showed a plateau-like nocturnal time course. Electrophysiological and pharmacological findings in SD rats point to an inhibitory influence of AVP upon pineal melatonin synthesis. The lack of AVP obviously does not result in disinhibition of pineal melatonin synthesis but rather in a different time course of pineal melatonin content. This might either be due to strain differences or to yet unknown compensatory mechanisms in BB rats.     

Arginine vasotocin stimulates glucocorticoid secretion in male rats

1. 1. The endocrine effects of the putative pineal peptide arginine vasotocin (AVT) were compared with those of the pineal indoleamines melatonin and N-acetylserotonin and passive immunization against these indoleamines in male rats injected at 1200 hr or 2400 hr.

2. 2. A pharmacologie dose of AVT markedly enhanced both day and night serum gluoocorticoid levels but lowered prolactin levels at both time points.

3. 3. Melatonin decreased night prolactin and corticosterone levels whereas passive immunization against circulating melatonin and N-acetylserotonin elevated night levels of these hormones.

4. 4. Preliminary evidence that AVT (10 μM) produces a guanosine triphosphate (GTP)-dependent stimulation of adenylate cyclase activity in the hypothalamus, anterior pituitary and adrenal gland suggests that the endocrine effects of this peptide may involve alterations in cAMP levels at one or more of these sites.

Influence of light irradiance on hydroxyindole-O-methyltransferase activity, serotonin-N-acetyltransferase activity, and radioimmunoassayable melatonin levels in the pineal gland of the diurnally active Richardson's ground squirrel

When Richardson's ground squirrels were kept under light:dark cycles of 14:10 h there was no nocturnal rise in pineal hydroxyindole-O-methyltransferase (HIOMT) activity. Conversely, the 10 h dark period was associated with large nocturnal rises in both pineal serotonin-N-acetyltransferase (NAT) activity and radioimmunoassayable melatonin levels. The nighttime rises in pineal NAT and melatonin were not suppressed by the exposure of the animals to a light irradiance of 925 mu W/cm2 during the normal dark period. On the other hand, when the light irradiance was increased to 1850 mu W/cm2 the rise in pineal NAT activity was eliminated while the melatonin rise was greatly reduced. When ground squirrels were acutely exposed to a light irradiance of 1850 mu W/cm2 for 30 min beginning at 5.5 h after lights out, pineal NAT activity and melatonin levels were reduced to daytime values within 30 min. The half-time (t 1/2) for each constituent was less than 10 min. Exposure to a light irradiance of either 5 s or 5 min (beginning at 5.5 h into dark period) was equally as effective as 30 min light exposure in inhibiting pineal NAT activity and melatonin levels. When animals were returned to darkness after a 30 min exposure to a light irradiance of 1850 mu W/cm2 at night, both pineal NAT activity and melatonin levels were restored to high nighttime levels within 2 h of their return to darkness. The results indicate that the pineal gland of the wild-captured, diurnal Richardson's ground squirrel is 9000 X less sensitive to light at night than is the pineal gland of the laboratory raised, nocturnal Syrian hamster.     

Melatonin synthesis in the pineal gland of the Richardson's ground squirrel (Spermophilus richardsonii): Influence of age and insulin-induced hypoglycemia

Summary The nocturnal rises in pineal N-acetyltransferase (NAT) activity and melatonin levels were compared in young (25–35 days old) and adult (at least 1 year old) Richardson's ground squirrels. When expressed as NAT activity per pineal gland, the nighttime rise in the activity of this enzyme was less in young than in the adult animals; conversely, the melatonin content of the pineal glands of young animals was higher at one point (4 a.m., 8 hours after darkness onset) when compared to that in adult squirrels. When data were expressed relative to total protein, the NAT and melatonin rhythms in the pineals of young and adult animals were very similar. The effect of insulin-induced hypoglycemia on both daytime and nighttime NAT and melatonin levels in the pineal gland of the Richardson's ground squirrel was also assessed. Low daytime levels of these constituents were not influenced by the administration of 10 units insulin, a treatment which caused a marked drop in circulating glucose levels. At night, when pineal NAT and melatonin levels were high insulin injection had a very modest stimulatory effect on NAT activity (one point was elevated above saline injected controls) while melatonin levels remained unchanged by the treatment. These findings in the ground squirrel in reference to insulin-induced hypoglycemia, and Stressors in general, appear to differ from those in the rat where stress can have a substantial influence or both low daytime and high nighttime levels of pineal NAT and melatonin.     

The pineal complex in Roman high avoidance and Roman low avoidance rats

Summary Previous studies have shown that the pineal gland of Roman high avoidance (RHA/Verh) rats is larger than that of Roman low avoidance rats (RLA/Verh). In the present study measurement of enzyme activities (serotonin-N-acetyl-transferase, hydroxyindole-O-methyltransferase) revealed that pineals of RHA/Verh rats are twice as active in melatonin production than pineals of RLA/Verh rats. Indoleamine content was also higher in RHA/Verh rats, whereas noradrenaline content was the same in both lines. When values were expressed per mg protein, these differences disappeared except for N-acetyl-serotonin and noradrenaline which were higher or lower in RHA/Verh rats, respectively. Both lines had higher serum levels of melatonin during the dark phase than during the light phase. However, RHA/Verh rats had increased serum levels as compared to RLA/Verh rats during both day and night. Morphometric analysis of the deep and superficial part of the pineal complex revealed, that the volumes of both parts are enlarged in RHA/Verh rats. Electron microscopic studies of pineals collected during day- and nighttime showed higher numbers of synaptic ribbons per unit area in pineals of RHA/Verh rats. In pineals collected during June synaptic ribbons displayed a day/night rhythm in RHA/Verh rats only, whereas in glands of both lines collected during November no daily changes were found. These results show that closely related but divergently selected rat lines may differ in pineal ultrastructure and pineal function.     

Studies on pineal melatonin levels in a diurnal species,the Eastern chipmunk (Tamias striatus): Effects of light at night,propranolol administration or superior cervical ganglionectomy

Summary Five experiments were carried out on the control of melatonin levels in the pineal gland of a diurnal species, the Eastern chipmunk (Tamias striatus). We confirmed that the exposure of chipmunks to fluorescent white light of 3,981–4,304 lux during the normal dark period does not prevent the rise in pineal melatonin levels normally associated with darkness. Also, the administration of propranolol (20mg/kg) at 8 p.m. did not block the rise in pineal melatonin in animals exposed to either dark or light at night. Similarly, if chipmunks received propranolol 4 hours into the dark phase, pineal melatonin levels were not depressed 2 hours later. When animals were superior cervical ganglionectomized, however, the pineal content of melatonin remained low regardless of whether the animals were exposed to darkness or light at night. The exposure of chipmunks acutely to light at midnight (4 hours after darkness onset) had only a slight depressive effect on pineal melatonin 30 min later; by comparison, when chipmunks were acutely exposed to light at 3 a.m. (7 hours after darkness onset) daytime pineal melatonin levels were reached within 15 min after light onset. These findings in a diurnal species, the Eastern chipmunk, differ markedly when compared to previously reported observations on nocturnal laboratory rodents.     

Castration Reduces the Nocturnal Rise of Pineal Melatonin Levels in the Male Rat by Impairing its Noradrenergic Input

The effects of castration and testosterone treatment on pineal day-night rhythms were studied in male rats. Bilateral gonadectomy was performed at 21 days of age. Testosterone propionate was given subcutaneously to castrated animals in a dose of 10 μg/100 g body weight during two consecutive days before sacrifice. Animals were killed 40 days after gonadectomy at four different times of a 12:12 h light-dark cycle (1600, 2400, 0400 and 0800h). Tyrosine hydroxylase activity was measured in individual pineals by means of high-performance liquid chromatography determination of L-DOPA formed. Pineal levels of norepinephrine, dopamine, 5-hydroxytryptamine and 5-hydroxyindole acetic acid were determined by high-performance liquid chromatography with amperometric detection, while pineal melatonin content was measured by radioimmunoassay. Castration abolished the day-night rhythms of pineal tyrosine hydroxylase activity and norepinephrine content, both by elevating their daytime levels and by blocking their nocturnal rise. In addition, gonadectomy drastically modified pineal indoleamine metabolism by increasing daytime levels of both 5-hydroxytryptamine and 5-hydroxyindole acetic acid, and by reducing the nocturnal elevation of pineal melatonin content. Testosterone treatment was unable to prevent the effect of orchidectomy on pineal rhythms of tyrosine hydroxylase activity, 5-hydroxytryptamine or 5-hydroxyindole acetic acid content, however it partially restored the day-night pineal rhythms of both norepinephrine and melatonin content. These results are indicative of a possible participation of reproductive hormones in the control of pineal rhythmic activity in the male rat. Apparently, since gonadectomy abolished the nocturnal rise of both pineal tyrosine hydroxylase activity and norepinephrine content, the primary site of action of reproductive hormones could be at the level of the superior cervical ganglion.     

Suprachiasmatic control of melatonin synthesis in rats: inhibitory and stimulatory mechanisms

The suprachiasmatic nucleus (SCN) controls the circadian rhythm of melatonin synthesis in the mammalian pineal gland by a multisynaptic pathway including, successively, preautonomic neurons of the paraventricular nucleus (PVN), sympathetic preganglionic neurons in the spinal cord and noradrenergic neurons of the superior cervical ganglion (SCG). In order to clarify the role of each of these structures in the generation of the melatonin synthesis rhythm, we first investigated the day- and night-time capacity of the rat pineal gland to produce melatonin after bilateral SCN lesions, PVN lesions or SCG removal, by measurements of arylalkylamine N-acetyltransferase (AA-NAT) gene expression and pineal melatonin content. In addition, we followed the endogenous 48 h-pattern of melatonin secretion in SCN-lesioned vs. intact rats, by microdialysis in the pineal gland. Corticosterone content was measured in the same dialysates to assess the SCN lesions effectiveness. All treatments completely eliminated the day/night difference in melatonin synthesis. In PVN-lesioned and ganglionectomised rats, AA-NAT levels and pineal melatonin content were low (i.e. 12% of night-time control levels) for both day- and night-time periods. In SCN-lesioned rats, AA-NAT levels were intermediate (i.e. 30% of night-time control levels) and the 48-h secretion of melatonin presented constant levels not exceeding 20% of night-time control levels. The present results show that ablation of the SCN not only removes an inhibitory input but also a stimulatory input to the melatonin rhythm generating system. Combination of inhibitory and stimulatory SCN outputs could be of a great interest for the mechanism of adaptation to day-length (i.e. adaptation to seasons).     

Melatonin, pituitary function and stress in humans

(1) The nocturnal rise in plasma melatonin concentration continued through two cycles with continuous light occlusion by blindfolds in normal subjects. A daytime nap did not disturb the rhythm. (2) The plasma melatonin rhythm was present in patients with pituitary-adrenal and pituitary-gonadal failure, but appeared diminished or absent in patients bearing lesions at different points in the pineal afferent pathway. (3) In other patients and subjects who showed normal response of cortisol, growth hormone or prolactin after stimuli including hypoglycemia, pneumoencephalography, exercise or administration of -dopa during the daytime, there was no stimulation of plasma melatonin concentration. (4) Melatonin was not correlated with prolactin in blood or cerebrospinal fluid of patients with a wide range of plasma prolactin levels. (5) The adult human melatonin rhythm is relatively independent from pituitary, gonadal, and adrenal function, but may rely on a neural pathway similar to that controlling the rhythm in lower animals. The human melatonin rhythm may represent the output of a stable oscillator with a signal relatively free from acute perturbation by sleep, darkness, or stress sufficient to cause changes in other hormones.     

Magnetic field effects on pineal gland melatonin synthesis: comparative studies on albino and pigmented rodents

Previous investigations have shown that the inhibitory effects of an earth-strength magnetic field on albino rat pineal melatonin synthesis is dependent on optic input. The possibility that ocular pigmentation might play a role in mammalian magnetosensitivity was explored in the present study by comparing hooded rat and golden hamsters with albino rats. Pineal melatonin synthesis, i.e. N-acetyl-transferase activity and melatonin content, was utilized as a parameter for assessing magnetosensitivity. In both rat strains nocturnal pineal melatonin synthesis was markedly inhibited following a single 30-min magnetic field stimulus consisting of a 50 degree rotation of the earth's field horizontal component. However, golden hamsters did not respond to the same magnetic stimulus, indicating a species-specific magnetosensitivity that is apparently independent of ocular pigmentation. Possible reasons for these differences are discussed.     

Factors influencing pituitary-adrenal rhythmicity: its ontogeny and circadian variations in stress responsiveness.

(1) The controlling mechanism for circadian pituitary-adrenal periodicity is clearly of central neural origin. (2) Ontogenetic modification of the pituitary-adrenal rhythm was investigated in order to determine critical periods in development during which the presence of excess ACTH or corticosteroid would affect normal maturation of the rhythm. (3) Neonatal treatment with ACTH on days 7–9 or 17–19, but not at other times postnatally, suppressed the circadian plasma corticosteroid rhythm in adult rats. (4) Treatment with corticosterone on day 18, but not on days 3, 6 or 12, had similar persistent effects. (5) Thus, brief exposure to high circulating levels of ACTH or corticosterone during two critical neonatal periods which correspond to specific stages in the development of hypothalamic and forebrain structures may affect the normal development of central structures involved in circadian regulation. (6) Development of the pituitary-adrenal rhythm in the female rat is closely related to the onset of puberty. When puberty is delayed, i.e. by neonatal implantation of corticosterone or a sham implant on day 4, maturation of the full amplitude of the circadian rhythm is correspondingly delayed. (7) The role of the hippocampus and septum in the circadian periodicity of stress-induced pituitary-adrenal responsiveness was investigated. (8) Electrical stimulation of hippocampus in freely-behaving chronically-implanted adult male rats prolonged the plasma corticosterone response to 10-min restraint stress in the morning, but was without effect in the afternoon. (9) Septal stimulation in the afternoon abolished the plasma corticosterone response to the stress, whereas stimulation in the morning was less effective. (10) These differential effects obtained at the trough and peak of the pituitary-adrenal cycle suggest that hippocampus and septum may play a role in producing the diminished stress responsiveness normally observed at the peak of the cycle.     

Diurnal variation in glucose utilization in the pineal body of the monkey

We employed the quantitative 2-[14C]-deoxyglucose method (Sokoloff's method) to measure glucose utilization in the pineal gland of pubescent monkeys. Glucose utilization in the pineal was 80-110% higher in the nocturnal, awake animals compared to the rate of both groups studied in the nocturnal, awake animals with both eyes open and with light deprivation for three hours. Short term visual deprivation during the day was without effect on pineal glucose utilization. The diurnal variations in melatonin levels in blood and CSF, higher at night than during the day, are the result of corresponding changes in the rate of production and elaboration of melatonin in the pineal gland. The release of norepinephrine from the postganglionic fiber of the superior cervical ganglia controls the production of melatonin in the pineal by regulating the activity of serotonin-N-acetyltransferase. It was reported that electrical stimulation of SCG via sympathetic trunk increased the levels of serotonin-N-acetyltransferase in the pineal and that it also increased glucose utilization in the pineal. It is believed that metabolic increase in the pineal reflects increased activity in sympathetic terminals distributed throughout the gland which stimulate its increase in hormone production. The present results indicate that there is an elevation of pineal metabolic rate at a time when blood and CSF levels of melatonin are known to be elevated Our finding that short-term light deprivation during the day did not affect the pineal metabolic rate is consistent with the result by Reppert et al (1981) in which they found that exposure to darkness during the day does not result in an increase in CSF melatonin.     

Effect of neonatal pinealectomy on circadian periodicity of adrenocortical activity

Summary Newborn male and female rats aged between 24 and 48 hours were pinealectomized (Px) or sham-operated (Sh). Animals were sacrificed at 45 and 60 days of age, at the trough and peak of the adrenocortical diurnal cycle. Blood was collected for corticosterone determinations and the adrenal glands weighed. Adrenal gland weight was greater at 45 days (p<0.01) in Px rats than in Sh rats. This difference in adrenal gland weight disappeared in the group sacrificed 60 days post-operatively. On the other hand, the diurnal plasma rhythm of corticosterone remained intact in male and female rats with or without pineal at 45 and 60 days. Furthermore, the trough and peak plasma corticosterone levels were not different in Px 45 and 60 days post-operatively.These data do not support the assumption of the important role of the pineal in the newborn rat for the maintenance of a normal circadian rhythm of the pituitary-adrenal system.     

Nocturnal headache associated with melatonin deficiency due to a pineal gland cyst

The cyclic nature of some of headache disorders is closely related to melatonin, which is secreted by the pineal gland. We report a 29-year-old male patient with a 2.5-year history of headaches that woke him in the middle of the night. These headaches were pulsatile and continued until sunrise. During these attacks he also suffered from allodynia over the scalp, bilateral conjunctival hyperemia, and nervousness. His brain MRI showed a 5mm by 4mm neuroepithelial cyst in the pineal gland. The peak plasma melatonin level that was measured at 2 am was 28 pg/mL. The patient underwent oral melatonin treatment (6 mg/day). After 1 month he experienced a 70% reduction in his symptoms. When the melatonin dosage was increased to 10mg/day he became headache-free, and 5 months after the treatment began, had no complaints. His 5-month follow-up plasma melatonin level at 2 am was 61 pg/mL. To our knowledge this is the first report of a patient with nocturnal headache associated with a low level of melatonin due to a neuroepithelial cyst in the pineal gland.     

Neuroendocrine probes as biological markers of affective disorders: new directions

Numerous endocrine abnormalities are found in depressive illness and, among these, several have been proposed as useful markers in diagnosis, prediction of treatment response, monitoring treatment outcome or in understanding of etiology. This paper reviews five endocrine systems--the hypothalamic-pituitary-adrenal axis, hypothalamic-pituitary-thyroid axis, growth hormone regulation, prolactin regulation and pineal function, in which such abnormalities have been reported. The dexamethasone suppression test (DST) results are affected by a variety of other diseases and confounding conditions. Furthermore, variability in dexamethasone availability has recently been shown to be an important factor, influencing post-DST cortisol levels. Refined tests, taking into account all these factors, or alternative tests of hypothalamic-pituitary-adrenal function may lead to improved clinical utility. Pineal function is now the focus of considerable investigation. Low nocturnal output of melatonin is found in unipolar and bipolar affective disorder and is normalized by treatment with antidepressant drugs which block re-uptake of noradrenaline. These findings support the hypothesis of noradrenergic abnormality in depression. In seasonal affective disorder there is evidence for a phase delay in the melatonin rhythm which may be a key factor in the seasonal disorder. Effective light therapy causes a phase advance in the abnormal melatonin rhythm. Whether the normalization of the melatonin rhythm is instrumental in producing the antidepressant effect is yet to be determined. There are wide spread neuroendocrine abnormalities in depressive illness. These abnormalities encompass many different pituitary hormones, as well as the pineal.(ABSTRACT TRUNCATED AT 250 WORDS)