Sensitive assay for serotonin N-acetyltransferase activity in rat pineal

Status of dopamine in bovine pineal glands and the stimulation of N-acetyltransferase activity by D2-dopaminergic receptor agonists in the rat pineal glands in culture

In a previous study, we identified in the bovine pineal gland two [3H]spiroperidol-binding sites with KD values of 0.18 and 2.1 nM and Bmax values of 37 and 630 fmol/mg protein, respectively. In this study, the status of dopamine in the bovine pineal glands was delineated further by measuring the relative concentrations of dopamine and norepinephrine and the relative concentrations of serotonin and melatonin. Furthermore, the presence of 4.0 +/- 0.6 micrograms/dopamine/gm tissue encouraged us to delineate the effects of select dopaminergic receptor agonists and antagonists on the synthesis of melatonin in vivo and on the activity of N-acetyltransferase in the rat pineal gland in culture. The acute administration of haloperidol (3 mg/kg intraperitoneally [ip]) or sulpiride (200 mg/kg ip) increased the concentration of melatonin in the pineal gland from 160.6 +/- 8.18 to 327.6 +/- 45.43 and 306.5 +/- 40.53 pg/gland, respectively. Dopamine exhibited dual effects on the activity of N-acetyltransferase, inhibiting the basal activity at 0.1 microM and stimulating it at 10 microM, and the later effect was blocked by propranolol. D2-dopaminergic receptor agonists such as bromocriptine (4.0 microM) or LY-171555 (10.0 microM) partially attenuated the norepinephrine-induced stimulation of N-acetyltransferase, and these attenuating effects were reversed by D2-dopaminergic antagonists such as haloperidol (10 microM) or domperidone (10 microM). The results of these studies are interpreted to indicate that for the synthesis of melatonin, the pineal D2-dopaminergic receptors may function independently from those of the beta 1-adrenergic receptor sites. Furthermore, the said D2-dopaminergic receptor are amenable to down regulation since the activity of N-acetyltransferase remained unaltered (0.0717 vs. 0.0729 nmol/gland/h) following chronic treatment (4 mg/kg ip/day for 30 days) with bromocriptine.

Effects of EGTA and calmodulin, neutral thiol proteinases and protein kinase C inhibitors on loss of chicken pineal serotonin N-acetyltransferase activity

This paper shows that the loss of chicken pineal serotonin N-acetyltransferase activity in crude homogenates of the pineal gland during preincubation at 37 degrees C is a complex process which seems to involve protein kinase C, calmodulin and calcium-activated neutral protease. All three compounds are strongly related to free calcium levels, and hence EGTA effectively prevents this loss of activity. It is proposed that the loss of serotonin N-acetyltransferase activity in crude chicken pineal homogenates is due to a series of molecular events, probably triggered by loss of the calcium gradient present in the intact gland by the homogenization process, leading to rapid serotonin N-acetyltransferase deactivation. In these homogenates two calmodulin inhibitors, a protein kinase C inhibitor and a neutral thiol proteinase inhibitor, and EGTA were found to markedly reduce the rate of serotonin N-acetyltransferase deactivation.

Stimulation of serotonin-N-acetyltransferase activity in the pineal gland of the mongolian gerbil (Meriones unguiculatus) by intracerebroventricular injection of vasoactive intestinal polypeptide

There is ever-increasing evidence that intrapineal peptides have an important role in the modulation of pineal melatonin synthesis. In the pineal gland of the Mongolian gerbil (Meriones unguiculatus), we have previously shown the presence of VIP-immunoreactive nerve fibers as well as pinealocytic VIP receptors. To assess the functional significance of these findings, 10 microliters of a 1 microM or 1 nM solution of VIP were injected into the lateral ventricle of gerbils over a period of 10 min. Animals were killed 1.5 hr after injection, and the superficial pineal glands were excised and assayed for N-acetyltransferase (NAT) activity. Injection of the 1 microM VIP solution stimulated the NAT activity to values four times the control values. The results are compatible with an in vivo influence on the pineal gland indole metabolism of the nonsympathetic VIP-containing nerve fibers via VIP-receptors present in the gland.

Extraretinal mechanisms mediate light-induced changes in neonatal rat pineal gland N-acetyltransferase activity

Nocturnal light-induced changes in pineal gland N-acetyltransferase activity were investigated in intact or enucleated 4-, 6-, or 8-day-old maternally isolated rats maintained at different ambient temperatures. Nighttime enzyme activity was increased markedly in rats maintained in 23 degrees C compared to 35 degrees C environments. Four hours of nocturnal light significantly reduced N-acetyltransferase activity in intact rats at all ages studied when the animals were maintained at 35 degrees C. Shorter duration (1 min) nocturnal light reduced N-acetyltransferase activity in intact rats 6 days of age. However, the enzyme activity suppression observed after the 4 hr of nocturnal light was still present in 4-day-old rats (but not older animals) even when the younger animals had been made blind by bilateral orbital enucleation. Taken together, these data suggest that an extraretinal mechanism may mediate light-induced changes in some aspects of pineal gland indoleamine biochemistry in newborn rats.

Xylamine enhances pineal gland N-acetyltransferase activity in vitro

1. The action of N-2'-chloroethyl-N-ethyl-2-methyl benzylamine (xylamine) on rat pineal gland sympathetic innervation was examined. 2. This alkylating agent caused a concentration-dependent increase in pineal gland N-acetyltransferase (NAT) activity in neurologically intact pineal glands that was suppressed in glands previously subjected to bilateral superior cervical ganglionectomy. 3. Xylamine-induced elevations in NAT activity were attenuated by beta-noradrenergic antagonist drugs but not by alpha-noradrenergic antagonist drugs. 4. Since pineal gland uptake of radiolabelled norepinephrine (NE) was impaired by xylamine, the drug may increase pineal gland NAT activity by inhibiting NE reuptake into the presynaptic nerve terminal, thereby increasing the amount of the neurotransmitter available to stimulate pinealocyte beta-noradrenoceptors.

Catecholaminergic mechanisms mediate hypothermia-induced elevations in pineal gland N-acetyltransferase in neonatal rats

The newly born of many mammalian species are ectothermic, and it is possible that biochemical processes important for the metabolism of endocrine hormones might vary with alterations in the environmental temperature. Temperature-induced fluctuations in pineal gland N-acetyltransferase activity were investigated in 4-, 12-, and 20-day-old rats placed for 4 hr in 23 or 34 degrees C environments. Enzyme activity increased dramatically in ectothermic 4- and 12-day-old animals exposed to the 23 degrees C environment, but not in endothermic 20-day-old rats. The elevations in daytime pineal gland NAT activity seen in cold-exposed animals were absent in rats previously subjected to chemical sympathectomy induced by 6-hydroxydopamine, or in animals treated with the beta-noradrenoceptor antagonist drug propranolol. Catecholaminergic nerves and beta-noradrenoceptors known to be important for light-induced changes in mammalian pineal gland biochemistry also appear essential for environmental temperature-dependent elevations in neonatal pineal N-acetyltransferase activity.

Effects of steroids on serotonin-N-acetyltransferase activity of pineals in organ culture

Treatment of neonatal, but not adult, rats with glucocorticoids decreases the rise in pineal serotonin N-acetyltransferase activity upon stimulation with beta agonists. Pineals in organ culture and exposed to steroids also show a dose-dependent decrement in response to beta agonists which increases with steroid exposure time. Pineals from neonatal and adult animals are equally sensitive. The effects of steroids on pineals in organ culture appear to be reversible, and the order of potency of different steroids differs from that observed when steroids are administered in vivo. Both in vitro and in vivo steroids appear to act at a site after cyclic AMP generation. Hydroxyindole-O-methyltransferase activity in the adult pineal does not appear affected by steroid exposure.

Pineal indoleamine metabolism in pyridoxine-deficient rats

Pyridoxine deficiency causes physiologically significant decrease in brain serotonin (5-HT) due to decreased decarboxylation of 5-hydroxytryptophan (5-HTP). We have examined the effect of pyridoxine deficiency on indoleamine metabolism in the pineal gland, a tissue with high indoleamine turnover. Adult male Sprague-Dawley rats were fed either a pyridoxine-supplemented or pyridoxine-deficient diet for 8 weeks. Pyridoxine deficiency did not alter the pattern of circadian rhythm of pineal 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), N-acetylserotonin (NAS), and melatonin. However the levels of these compounds were significantly lower in the pineal glands of pyridoxine-deficient animals. Pineal 5-HTP levels were consistently higher in the pyridoxine-deficient animals and a conspicuous increase was noticed at 22.00 h. Increase in pineal NAS and melatonin levels caused by isoproterenol (5 mg/kg at 17.00 h) were significantly lower (P less than 0.05) in the pyridoxine-deficient animals. Treatment of pyridoxine-deficient rats with pyridoxine restored the levels of pineal 5-HT, 5-HIAA, NAS, and melatonin to values seen in pyridoxine-supplemented control animals. These results suggest that 5-HT availability could be an important factor in the regulation of the synthesis of pineal NAS and melatonin.

Effects of aggressive encounters on pineal melatonin formation in male gerbils (Meriones unguiculatus, Cricetidae)

Pineal N-acetyl-transferase activity and radioimmunoassayable melatonin levels were determined in adult male gerbils subjected to aggressive encounters using the intruder-model. In the first experiment, a single encounter of 3 min was applied in the afternoon to intact and to animals with sympathetically denervated pineal organs. Compared with controls, both stressed groups demonstrated a drastic decrease in N-acetyl-transferase activity followed by a slow recovery. In both groups there also occurred a marked change in pineal melatonin content: in intact animals pineal melatonin levels were elevated immediately after the encounter; thereafter, melatonin values decreased. In animals bearing denervated pineal organs melatonin levels fell as a consequence of the encounter. In a second experiment, intact gerbils experienced four daily encounters of 1 min for one week. Thereafter the nocturnal formation of melatonin was studied. In comparison with untreated controls, the repeatedly stressed animals demonstrated a temporal delay in the rise of both N-acetyl-transferase activity and melatonin. Since the pineal organ is able to transduce events of the social environment into an endocrine message--as set forth by both our experiments--the pineal organ might play an important role within central processing of social stress.

Neither the pituitary gland nor the sympathetic nervous system is responsible for eliciting the large drop in elevated rat pineal melatonin levels due to swimming

Since the pineal gland is an end organ of the sympathetic nervous system, stress might increase the synthesis of its hormone, melatonin. The stress of a 10 min swim, which elicits a marked rise in circulating catecholamines, causes a dramatic depression of high pineal melatonin levels at night within 15 min after swimming onset. N-acetyltransferase (NAT) activity is unaffected by the treatment at 15 or 30 min after swimming onset. Within 90 min after initiation of a 15 min swim, high nighttime pineal melatonin levels are restored while NAT values remain elevated. The swimming-induced reduction in high pineal melatonin levels is not influenced by either hypophysectomy, superior cervical ganglionectomy, prazosin (alpha 1-adrenergic receptor blocker) pretreatment, yohimbine (alpha 2-adrenergic receptor blocker) pretreatment, or reserpine (amine depletor) pretreatment. These results indicate that neither hormones secreted from the pituitary gland nor catecholamines secreted from the sympathetic nerves are involved in eliciting the dramatic reduction in elevated pineal melatonin levels in the rat.

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 02.99 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.

12-Hydroperoxyeicosatetraenoic acid (12-HPETE) and 15-HPETE stimulate melatonin synthesis in rat pineals

The role of arachidonic acid metabolites in norepinephrine (NE)-induced N-acetyltransferase (NAT) activity and melatonin release was examined from 6 h-incubations of rat pineal glands. A cyclooxygenase inhibitor, indomethacin (5 x 10(-8) - 5 x 10(-6) M) was ineffective on melatonin release, in the presence of absence of NE (5 x 10(-6) M) while a lipoxygenase inhibitor, nordihydroguaiaretic acid (5 x 10(-7) -5 x 10(-5) M) had an inhibitory effect. Among the lipoxygenase metabolites, 12-hydroperoxyeicosatetraenoic acid (12-HPETE) and 15-HPETE stimulated both NAT activity and melatonin release in a dose-dependent manner, with a maximal effect occurring at 10(-6) M, while 5-HPETE or hydroxy derivatives of these compounds (12-HETE, 15-HETE and 5-HETE) were ineffective. These results indicate that 12-HPETE and 15-HPETE can be involved in NE-induced melatonin release.

Effects of short-term cold exposure on pineal biosynthetic function in rats

In light of recent studies demonstrating stress-induced changes in pineal indoleamine metabolism, we tested the effect of acute cold stress on pineal biosynthetic function. Adult male rats were subjected to 30, 60, or 120 min of cold exposure (Ta = 2 degrees C) during either the light or dark phase of the daily photoperiodic cycle. Controls were kept at room temperature (22 +/- 2 degrees C). Animals were killed by decapitation and pineals were analyzed by radioimmunoassay for melatonin content and by radioenzymeassay for the activity of N-acetyltransferase (NAT). Cold exposure during the day elicited no significant changes in pineal indoleamine metabolism. Exposure to cold for 1 hr during the second hour after lights off slightly increased pineal melatonin content, without a concomitant change in NAT activity. Rats exposed to 2 hr of cold beginning 2 hr after lights off, however, displayed a 50% reduction in NAT activity, whereas pineal melatonin content remained unchanged. The paradoxical response of pineal NAT activity and melatonin content are not uncommon when rats are exposed to adverse stimuli.

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.

Responses of the mammalian retina to experimental alteration of the ambient magnetic field

The detection of earth strength magnetic fields by rodents has been demonstrated previously by numerous physiological and behavioral techniques. This phenomenon appears to require input from the eyes. In an effort to better understand this phenomenon retinal melatonin synthesis and catecholamine contents were assayed in rats exposed at night to an alteration of the ambient magnetic field. In normal animals both dopamine and norepinephrine levels in the retina were reduced by this stimulus, while retinal melatonin synthesis was unaffected. Animals that had lost their intact photoreceptors as a result of 8 weeks of previous constant light exposure did not show a catecholamine response to the magnetic stimulus. These results support the view that the mammalian retina participates in the relaying of magnetic information into the central nervous system.

Melatonin biosynthesis in Drosophila: its nature and its effects

Drosophila melanogaster homogenates incubated with tritiated 5-hydroxytryptophan, 5-hydroxytryptamine, or N-acetylserotonin have the ability of converting them into labelled indolamines, including melatonin. All these compounds were characterized by two-dimensional thin-layer chromatography by comparison with nonradiolabelled standards, and melatonin by mass spectrometry and radioimmunoassay. On the other hand, the injection of pharmacological doses of melatonin into 2-day-old female flies diminishes the mating speed and the oviposition rate.

Response of pineal serotonin N-acetyltransferase activity in male guinea pigs exposed to light pulses at night

Serotonin N-acetyltransferase (NAT), which is crucial for the formation of melatonin, undergoes a typical day/night rhythm in the pineal gland with low levels during daytime and high levels at night. Short pulses of light given at night have been shown to rapidly depress NAT activity in some species, but not in others, the reasons for this difference being unclear. As diurnality and nocturnality of the experimental animals may play a role and since diurnally active animals have been little investigated in this respect, in the present study the diurnally active guinea pig was investigated. Male guinea pigs kept under a lighting regimen of LD 12:12 (lights off at 1700 hrs) were killed between 1200 or 1300 hrs and between 0000 and 0200 hrs, at night in the dark or after exposure to 10 or 45 min of light. The results obtained show that the day/night difference of NAT activity is about 2-fold. 10 min or 45 min of light given at night significantly depress pineal NAT activity. Re-exposure to darkness for 1 hr of animals previously given light for 10 min leads to restoration of NAT activity. These findings together with data from the literature suggest that it does not appear to be the activity pattern (diurnality versus nocturnality) of an animal nor the amplitude of the day/night difference of pineal NAT activity that account for the suppressibility or non-suppressibility of pineal NAT activity by light at night.

Magnetic field effects on pineal N-acetyltransferase activity and melatonin content in the gerbil--role of pigmentation and sex

The ambient geomagnetic field influences a variety of biological phenomena. Electrical and biochemical parameters of the rodent pineal gland are influenced by the alteration of weak magnetic fields (MF), the magnetic receptor probably residing in the retina. However, open questions concern the role of retinal pigmentation as well as species- and sex-specific differences in MF perception. We therefore exposed male and female naturally pigmented and albino Mongolian gerbils, as well as Sprague-Dawley (SD) rats to a 60 degrees rotation of the horizontal component of the ambient MF. Alteration of nocturnal pineal melatonin content and N-acetyltransferase (NAT) activity were utilized as a parameter for assessing magneto-sensitivity. In pigmented gerbils, MF exposure resulted in no significant changes in pineal melatonin synthesis. In contrast, albino gerbils and SD rats exhibited--regardless of sex--significant decreases in pineal NAT activity and melatonin content following MF exposure. These results suggest that in rodents hypopigmentation appears to favor magnetoperception. The available evidence indicates that the pigmentation of the retina could play a crucial role.

Rapid adjustment of the pineal N-acetyltransferase rhythm to change from long to short photoperiod in the Japanese quail (Coturnix coturnix japonica)

The dynamics of adjusting the pineal N-acetyltransferase rhythm from long to short photoperiod was assessed in the Japanese quail (Coturnix coturnix japonica). The transition from LD 16:8 to LD 8:16 was accomplished by symmetrical prolongation of the dark period. In LD 16:8, the period of elevated nocturnal activity lasted approximately 7 hours. During the first prolonged night, the evening N-acetyltransferase rise advanced by almost 3 hours relative to the rise in LD 16:8 and occurred at the same time as during the 3rd, 7th, and 14th day after the transition. The morning N-acetyltransferase decline did not shift during the first long night; during the third night it was delayed relative to the decline in LD 16:8 by more than 2 hours and occurred at the same time as during the 7th and 14th night following the LD 16:8 to LD 8:16 transition. Three, 7, and 14 days after the transition, the period of elevated N-acetyltransferase activity lasted approximately 12 hours. Hence extension of the N-acetyltransferase rhythm profile proceeded first into the evening and then only into the morning hours, and it was accomplished within 2 to 3 days.

Intra-arterially administered vasopressin inhibits nocturnal pineal melatonin synthesis in the rat

1. In order to investigate the possible involvement of arginine-vasopressin (AVP) in the inhibition of nocturnal pineal melatonin synthesis following electrical stimulation of the hypothalamic paraventricular nuclei, adult male rats received injections of 5 micrograms/100 g body weight of the peptide during either day- or night-time. Following survival times of 30 or 120 min, animals were killed and the activity of the melatonin synthesis enzyme N-acetyltransferase (NAT) was determined. 2. At night, NAT activity was significantly decreased 30 and 120 min following AVP injection. 3. During the daytime, NAT activity was unchanged following AVP administration. 4. It is suggested that pineal melatonin synthesis may be affected by PVN stimulation not only via neural pathways but possibly also by PVN-released blood-borne AVP.

Day-to-day variation in pineal serotonin N-acetyltransferase activity in stressed and non-stressed male Sprague-Dawley rats

Previous studies involving physical-immobilization stress in laboratory rats have yielded inconsistent results with respect to melatonin synthesis in the pineal gland. As melatonin formation undergoes circadian and infradian rhythms, the aim of the present study was to examine whether stress experiments exhibit day-to-day variation. Toward this end, groups of male Sprague-Dawley rats were stressed by physical immobilization on eight consecutive days, respectively, or left relatively undisturbed, and killed. The pineal gland was rapidly dissected out and serotonin N-acetyltransferase (NAT) activity and melatonin levels were measured. NAT activity was significantly depressed on experimental days 1, 3 and 5, and slightly depressed on day 7. In addition, both in control and experimental animals NAT activity exhibited statistically significant differences between experimental days. Pineal melatonin levels were less variable. On experimental days 3 and 6 immobilization led to a significant increase of pineal melatonin levels. These results show that day-to-day variation is an important factor that influences the outcome of stress experiments and represent another example that NAT activity and pineal melatonin levels do not always show corresponding changes.

Atypical 24-hour rhythms of serotonin N-acetyltransferase activity in the rat pineal gland

Previous long-term studies have shown that in the pineal gland of rats melatonin synthesis is subject to infradian rhythms with periods between 4 and 7 days. Since in these studies melatonin-related parameters were measured at one timepoint of a 24-hr cycle only, the aim of the present investigation was to extend these experiments by more frequent sampling, to characterize the infradian rhythmicity in more detail. Male Sprague-Dawley rats kept under a light schedule of LD 12:12 (lights on at 0700) were killed at 6-hr intervals on 8 consecutive days. After decapitation the pineal gland was rapidly dissected out, followed by measurements of one of the melatonin-forming enzymes, serotonin N-acetyltransferase (NAT) activity. It was found that pineal NAT activity exhibited the well known day/night rhythm, i.e. low activity during daytime and strikingly enhanced activity at night, during the first 4 days of the experiment. On the fifth night (from Saturday to Sunday) an unusually high NAT peak occurred at 2400 hr, followed by two atypical 24-hr cycles. In the first cycle the midnight and 0600 hr values were equal and in the second cycle the 0600 hr value was significantly higher than the midnight value. To investigate whether the unusually high NAT peak was a single event or not, four additional short-term experiments were carried out at 2400 hr on 4 consecutive weekends, from Friday to Monday. In each of the four 4-day experiments a distinctly higher peak of NAT activity was found on Saturday, but with time the peaks became less prominent.(ABSTRACT TRUNCATED AT 250 WORDS)

Elevated daytime rat pineal and serum melatonin levels induced by isoproterenol are depressed by swimming

Isoproterenol (1 mg/kg) was subcutaneously injected into adult male rats during the day to stimulate pineal N-acetyltransferase (NAT) activity and pineal and serum melatonin levels. Two hours after isoproterenol administration when levels of each of these variables had increased significantly, the experimental animals swam for 10 min in 22 degrees C water. At 15 min after swimming onset, pineal and serum melatonin levels were highly significantly depressed compared to those in control animals that did not swim. The high NAT level was not influenced by swimming. In a second study, isoproterenol injected rats swam for either 1, 3, 6 or 10 min and were sampled 15 min after the onset of swimming. The reduction in the elevated pineal melatonin in these animals was correlated with the length of the swim, i.e., as the duration of swim increased the percent reduction in pineal melatonin also increased. Neither pineal NAT nor hydroxyindole-O-methyltransferase (HIOMT) activities were influenced by swimming. The results suggest that elevated pineal and serum melatonin induced by isoproterenol can be depressed with no effect on the activity of the enzymes which convert serotonin to melatonin.

Synergistic action of postsynaptic alpha-adrenergic receptor stimulation on vasoactive intestinal polypeptide-induced increases in pineal N-acetyltransferase activity

The alpha-adrenergic agonists phenylephrine and methoxamine, at concentrations that have little effect on pineal N-acetyltransferase activity, markedly enhance stimulation of this enzyme by vasoactive intestinal polypeptide (VIP). This augmentation can be blocked by the alpha 1-adrenergic antagonists phenoxybenzamine and prazosin and, at 10 but not 1 microM, by the alpha 2-antagonist yohimbine. The time course for VIP stimulation is not altered by concomitant alpha-adrenergic stimulation. Augmented activity does not require concomitant alpha-adrenergic stimulation, but alpha-adrenergic agonists must be present for augmentation to be maintained. Phorbol 12,13-diacetate or -dibutyrate but not 4 alpha-phorbol can substitute for phenylephrine, a finding suggesting that protein kinase C is involved in the augmentation. These results are, in general, analogous to alpha-adrenergic magnification of N-acetyltransferase induction by beta-adrenergic agonists.

Dopamine mediates the light-evoked suppression of serotonin N-acetyltransferase activity in retina

The possible role of dopamine in the light-induced suppression of serotonin N-acetyltransferase (NAT) activity in retinas of the African clawed frog (Xenopus laevis) was investigated using an in vitro eye cup preparation. The nocturnal increase of retinal NAT activity was significantly inhibited by either light exposure or exogenous dopamine. Spiperone, a dopamine receptor blocker, antagonized this inhibitory effect of light on NAT activity, but had no effect in darkness. The effect of spiperone required the presence of cyclic nucleotide phosphodiesterase inhibitors, 3-isobutylmethylxanthine (IBMX), papaverine, or Ro 20-1724. Under the conditions employed in this study, neither spiperone nor the phosphodiesterase inhibitors significantly affected NAT activity when added alone. This observation suggests a synergistic interaction between the dopaminergic antagonists and the phosphodiesterase inhibitors. Other dopamine receptor blockers, including haloperidol, cis-flupenthixol, clozapine and metoclopramide, increased NAT activity of light-exposed retinas incubated in the presence of IBMX. SCH 23390, a D1-selective dopamine receptor antagonist, did not increase NAT activity, nor did the alpha- and beta-adrenergic receptor antagonists tested. The effect of spiperone and IBMX on NAT activity was blocked by apomorphine and by the D2-dopamine receptor agonist LY 171555, but not by the D1-receptor agonist SKF 38393-A. The concentration of 3,4-dihydroxyphenylacetic acid was higher in light-exposed retinas than in dark-adapted retinas, suggesting that light exposure increases dopamine metabolism in Xenopus retina. The results presented in this paper suggest that dopamine, released in response to light exposure and acting on D2-dopamine receptors, is partially responsible for the light-induced suppression of the nocturnal increase in retinal NAT activity.

Adjustment of the rat pineal N-acetyltransferase rhythm to eight-hour shifts of the light-dark cycle: advance of the cycle disturbs the rhythm more than delay

After an 8-h delay of a light-dark (LD) cycle by lengthening of one light period, the rat pineal N-acetyltransferase (NAT) rhythm adjusted to the delay shift almost within one cycle. After an 8-h advance of the LD cycle by shortening of one dark period, the NAT rhythm adjusted to the advance shift within 5 cycles only; during the first 2-3 cycles the rhythm was abolished.

Entrainment of the circadian rhythm in the rat pineal N-acetyltransferase activity by prolonged periods of light

Entertainment of the circadian rhythm in the pineal N-acetyltranferase activity by prolonged periods of light was studied in rats synchronized with a light:dark regime of 12:12 h by observing phase-shifts in rhythm after delays in switching off the light in the evening or after bringing forward of the morning onset of light. When rats were subjected to delays in switching off the light of up to 10 h and then were released into darkness, phase-delays of the evening N-acetyltransferase rise during the same night corresponded roughly to delays in the light switch off. However, phase-delays of the morning decline were much smaller. After a delay in the evening switch off of 11 h, no N-acetyltransferase rhythm was found in the subsequent darkness. The evening N-acetyltransferase rise was phase-delayed by 6.2 h at most 1 day after delays. Phase-delays of the morning N-acetyltransferase decline were shorter than phase-delays of the N-acetyltransferase rise by only 0.7 h to 0.9 h at most. Hence, 1 day after delays in the evening switch off, the period of the high night N-acetyltransferase activity may be shortened only slightly. The N-acetyltransferase rhythm was abolished only after a 12 h delay in switching off the light. Rats were subjected to a bringing forward of the morning light onset and then were released into darkness 4 h before the usual switch off of light. In the following night, the morning N-acetyltransferase decline, but not the evening rise, was phase advanced considerably. Moreover, when the onset of light was brought forward to before midnight, the N-acetyltransferase rise was even phase-delayed. Hence, 1 day after bringing forward the morning onset of light, the period of the high night N-acetyltransferase activity may be drastically reduced. When rats were subjected to a 4 h light pulse around midnight and then released into darkness, the N-acetyltransferase rhythm in the next night was abolished. The data are discussed in terms of a two-component pacemaker controlling the N-acetyltransferase rhythm. It is suggested that delays in the evening switch off of light may disturb the N-acetyltransferase rhythm the next day only a little, as the morning component may adjust to phase-delays of the evening component almost within one cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Dynamics of discrete entrainment of the circadian rhythm in the rat pineal N-acetyltransferase activity during transient cycles

To elucidate entrainment of a pacemaker controlling the N-acetyltransferase (NAT) rhythm in the rat pineal gland, we studied the phase response curves (PRCs) of this rhythm. We exposed 50- to 60-day-old male Wistar rats maintained in a light-dark cycle (LD 12:12) to a 1-min light pulse at different times before midnight or at various times throughout the whole night. We then released them into constant darkness and studied the morning NAT decline during the night when rats were pulsed before midnight, as well as the evening NAT rise and the morning decline after 4 days following the pulses. The PRC for the first NAT decline and the PRCs for the NAT rise and decline after 4 days were compared with published transient PRCs (Illnerová and Vanĕcek, 1982b), in order to obtain a complete picture of the dynamics of the NAT rhythm entrainment during the transient cycles. Phase delays in the NAT rise due to a pulse before midnight were complete (i.e., identical to those of day 4) on day 1. Phase delays in the NAT decline were almost complete on day 1, while incomplete phase delays were observed on day 0. Phase advances in the NAT rise and decline due to a pulse past midnight had different dynamics: Advances in the decline were complete on day 1, while advances in the rise were absent on day 1 and much smaller than in the decline on day 4. The results are discussed in terms of a two-component (E-M) pacemaker controlling the NAT rhythm. The NAT rise may reflect the phase of the E-component, while the decline reflects the M-component. Phase delays of the E-component are accomplished within one cycle, and so are phase advances of the M-component. However, although delays of E already result in delays of M one cycle after the pulse, it takes several transient cycles before advances of M begin to induce advances of E.

Circadian rhythms of pineal N-acetyltransferase activity in the Djungarian hamster, Phodopus sungorus, in response to seasonal changes of natural photoperiod

The aim of this study was to describe the regular annual pattern of the daily melatonin synthesis in Djungarian hamsters, Phodopus sungorus sungorus. The hamsters were maintained from birth in natural photoperiodic conditions and in bimonthly intervals the day/night rhythms of pineal N-acetyltransferase (NAT) were measured. Analysis of the circadian profiles of NAT activity showed that the duration of elevated melatonin synthesis closely reflects the duration of the scotophase throughout the seasons. Thus the duration of elevated melatonin seems to represent a direct humoral signal transmitting the photoperiodic message. The duration of the nightly melatonin pulse appears to be influenced mainly by the time of dawn rather than by the time of dusk. Additional information about the time of year might be encoded in the total amount of melatonin synthesized per day, whereas the amplitude of the nightly melatonin peak seems to be of minor importance.

Pineal N-acetyltransferase, pineal and serum melatonin response to isoproterenol stimulation in underfed male rats

Adult male rats were subjected to 4 weeks of 50% food restriction under lighting regimen of 14 h light and 10 h dark. The pineal response to isoproterenol (ISO) was determined. In the time-course study, animals were injected with 0.5 mg/Kg ISO subcutaneously (SC) and killed at different times up to 180 min post injection. In the dose-response study, various doses of ISO (0.2 mg/Kg to 5.0 mg/Kg) were injected intraperitoneally (IP) and animals were killed 120 min post injection. Body weight, pineal N-acetyltransferase (NATase), pineal and serum melatonin (MT) were determined. After 4 weeks of restricted feeding, body weight was reduced by 40%. In the time-course study, peak pineal NATase occurred 120 min post injection in the ad libitum fed animals. By contrast, the food restricted animals showed a gradual increase of pineal NATase up to 180 min post injection. In the dose-response study, the ad libitum fed animals demonstrated a dose dependent increase of pineal NATase up to 5 mg/kg dose. The food restricted animals, however, achieved their maximal pineal NATase at 1 mg/Kg dose with no further increment at 5 mg/Kg dose. These differences in responsiveness were also reflected in pineal and serum MT levels. These results indicate that underfed animals have abnormal pineal NATase, pineal and serum MT responses to ISO stimulation.

Action spectrum of the retinal mechanism mediating nocturnal light-induced suppression of rat pineal gland N-acetyltransferase

The spectral properties of the retinal mechanism mediating the inhibitory effects of nocturnal light on pineal gland N-acetyltransferase (NAT) activity were determined. Pineal gland NAT activity declined linearly in albino rats exposed to different irradiances of a 460 or 580 nm monochromatic light during the middle of the dark phase of the cycle. The difference in sensitivity to the test lights is that predicted for a photopigment having peak absorbance at 495 nm, suggesting the inhibition of pineal gland N-acetyltransferase activity is mediated by the photopigment found in rat rods.

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

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.

Neuropeptide Y effects on pineal melatonin synthesis in the rat

Neuropeptide Y (NPY)-like immunoreactivity is present in the rodent pineal gland. To elucidate possible effects on pineal melatonin synthesis NPY (5 nmol/kg body wt.) was injected into the common carotid artery of male rats. Activities of the melatonin biosynthetic enzymes, serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were determined by means of radioenzymatic methods. Intact light-exposed animal showed increased NAT activity at day- and at nighttime. Blinded animals showed a more than 10-fold reduction of NAT activity after nocturnal NPY injections. HIOMT activity was only slightly influenced at either time. These results are discussed in terms of the possible actions of NPY at the pineal sympathetic neuroeffector sites.

Electrical stimulation of the hypothalamic nucleus paraventricularis mimics the effects of light on pineal melatonin synthesis

In an attempt to clarify further the role of the hypothalamic paraventricular nuclei (PVN) in the control of pineal function, the effects of 2 min electrical stimulation of these nuclei were investigated in acutely blinded, adult, male Sprague-Dawley rats. Pineal serotonin-N-acetyltransferase (NAT) activity, melatonin content and catecholamine levels were measured by means of radio-enzymatic, radioimmunoassay and high-performance liquid-chromatography methods, respectively. All three pineal parameters underwent significant declines following brief PVN stimulation during the night time. These observations lend credence to the view that the neural pathways transmitting light information to the sympathetic innervation controlling pineal melatonin synthesis.

A simple and rapid method for the purification of ovine pineal arylalkylamine N-acetyltransferase

A two-step chromatographic procedure has been developed for the purification of ovine pineal arylalkylamine N-acetyltransferase (EC 2.3.1.87), based on the principles of disulfide exchange and anion exchange. The enzyme from 20 ovine pineal glands can be purified about 500-fold in a day; recovery is about 5%. Polyacrylamide gel electrophoretic analysis of the final preparation shows four major bands; one appears to be arylalkylamine N-acetyltransferase.

Injections of alpha-melanocyte stimulating hormone affect pineal serotonin, melatonin and N-acetyltransferase activity

To determine if exogenously administered alpha-melanocyte stimulating hormone (alpha-MSH) affects nighttime pineal N-acetyltransferase activity, pineal levels of 5-hydroxytryptophan, serotonin and melatonin, and plasma prolactin levels, adult male hamsters were injected at 1900 hr (lights out 2000-0600 hr) with two doses of the peptide and killed at 0300 hr. The low dose of alpha-MSH (200 ng) produced a significant fall in pineal serotonin, pineal NAT activity and plasma prolactin values. The high dose of the peptide (20 micrograms) increased circulating prolactin titers and pineal serotonin levels and caused a concomitant decrease in pineal melatonin levels.

Multiple forms of arylalkylamine N-acetyltransferases in the rat pineal gland: purification of one molecular form

Rat pineal serotonin N-acetyltransferase (EC 2.3.1.87) activity is isolated in two molecular forms (Mr approximately equal to 10,000 and 95,000) by high performance size exclusion liquid chromatography in the presence of ammonium acetate (0.1 M, pH 6.5). In the presence of sodium citrate (0.1 M, pH 6.5), however, it is eluted as a single peak of intermediate size (Mr approximately equal to 30,000). A highly enriched preparation of one of the molecular forms has been obtained by a two-step purification procedure involving disulfide-exchange and anion-exchange chromatography. The N-acetyltransferase in 250 pineal glands obtained from isoproterenol-treated rats can be purified about 80-fold in 1 day; recovery is about 3%. Polyacrylamide gel electrophoresis of the final preparation indicates that a single major band (Mr approximately equal to 11,000) is present; this appears to be serotonin N-acetyltransferase.

Adrenalectomy prevents changes in rat pineal melatonin content and N-acetyltransferase activity induced by acute insulin stress

The activity of N-acetyltransferase (NAT) and the content of melatonin (MEL) in the rat pineal have been shown to be sensitive to several types of stressors. This study was designed to assess the role of the adrenals in mediating the effect of one such stressor, insulin-induced hypoglycemia, on pineal synthetic activity. Intact and bilaterally adrenalectomized (ADX) adult male rats were kept under light:dark cycles of 14:10 (lights on 0600 h) and injected intraperitoneally with 10 IU insulin at 1300 h, and groups (n = 8) were killed 2, 3, or 4 h postinjection. Plasma catecholamines were assayed by means of high performance liquid chromatography and radioimmunoassay was used to assess pineal NAT activity and MEL content. All injected groups were rendered hypoglycemic by insulin administration. Compared to uninjected controls, plasma epinephrine in hypoglycemic intact rats rose after 2 h, whereas epinephrine did not change in hypoglycemic ADX animals. The increase in epinephrine in intact animals was correlated with a rise in NAT activity at 2 h. Moreover, pineal MEL content at 2, 3, and 4 h was significantly greater than control values. In contrast, no changes in pineal biosynthetic function were found in ADX rats. This differential response by intact and ADX rats suggests that an adrenal product (possibly epinephrine) is responsible for mediating the stimulatory effects of acute insulin-induced hypoglycemic stress on the rat pineal.

Depression in rat pineal N-acetyltransferase activity and melatonin content produced by a hind leg saline injection is time and darkness dependent

It has recently been shown that a 1.5-ml subcutaneous saline injection into the dorsal aspect of the hind limb induces a dramatic and rapid fall in N-acetyltransferase activity and melatonin content of the rat pineal gland at night. Since many studies have shown the opposite response to stress during the day, the first experiment was undertaken to test whether the timing of the saline injection at night influences the response of the pineal gland. In the present studies, rats were kept under light:dark (LD) cycles of 14:10 with lights out daily at 2000 h. Groups of rats were then given a saline injection at one of the following times: 2315, 0015, 0115, 0215, or 0315. Early in the dark phase (2315 and 0015) the saline injection depressed both the N-acetyltransferase (NAT) activity and the melatonin content of the pineal. As the animals were treated later in the dark period, the response became more blunted and, finally, disappeared. In the second experiment, animals that were kept in light during the usual dark period showed no pineal response when subjected to a hind leg injection of saline at either 2315 or 0315. Additionally, no response was seen in the two pineal parameters when rats had darkness onset delayed by 4 h (to 2400) and were then treated with saline at 0410. The results of these studies indicate that the pineal response to saline injection is time dependent. Also, if the nighttime rise in melatonin is suppressed by light exposure, a saline injection has no further inhibitory effect on pineal NAT activity or melatonin levels.

Pineal indoles: significance and measurement

Despite intensive investigation, particularly over the past fifteen years, many aspects of pineal function with respect to mammalian physiology remain obscure. Much of this work is reviewed and particular attention focussed on indole metabolism within the pineal gland. Emphasis is placed on the development of new analytical techniques with special reference to high performance liquid chromatography coupled with electrochemical detection. The growth in knowledge regarding pineal indole synthesis which can be attributed to the use of this technique is discussed. The possibility that pineal indoles other than melatonin may function as hormones or neuromodulators is considered. A functional role for 5-hydroxytryptophol as a neuromodulator, possibly associated with diffuse neuroendocrine function (amine precursor, uptake and decarboxylation, APUD) is suggested.

Pineal melatonin: circadian rhythm and variations during the hibernation cycle in the ground squirrel, Spermophilus lateralis

Variations in pineal melatonin content throughout a 24-hour period and during different phases of the hibernation bout cycle were studied in the golden-mantled ground squirrel (Spermophilus lateralis). In addition to pineal melatonin, the circadian variation in the activities of pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were also investigated in summer animals maintained at 22 +/- 2 degrees C, on a light:dark (L:D) schedule of 12:12 hr for 1 month (lights on at 08.00 hr). Pineal glands were collected from six animals in each group at 1200, 1600, 2000, 2400, 0200, 0400, and 0800 hr. Changes in pineal melatonin content during the hibernation bout cycle were investigated in ground squirrels housed at 4 +/- .05 degrees C in relative darkness (1.9-3.4 lux; 10:14 LD). Pineal glands were obtained between 12:00 and 18:00 hr from 30 animals during one of three phases of the cycle (deep hibernation, euthermic interbout, and entrance into hibernation). Pineal melatonin was also measured for comparison in six winter euthermic animals that were housed at 22 +/- 2 degrees C, on a L:D schedule of 10:14 hr. Melatonin was measured in individual pineal glands by radioimmunoassay. The daily melatonin rhythm in S. lateralis was characterized by a marked increase in pineal melatonin during the dark phase, in which peak nighttime values were nearly 20-fold greater than daytime basal levels. The daily rhythm for NAT activity paralleled the changes in melatonin, showing a peak activity at 0200 hr that was 45 times greater than mean daytime values.(ABSTRACT TRUNCATED AT 250 WORDS)

Retarded growth of the suprachiasmatic nucleus and pineal body in dw and lit dwarf mice

The suprachiasmatic nucleus (SCN) and the pineal body in 3 types of inherited hormone-deficient mice, the dw, lit and hyt mice were examined by morphological, morphometric and biochemical techniques. In the dw and lit mice the SCN was underdeveloped. In the ventral part of the SCN, where most of the retinal fibers appeared to terminate, both cell number and cell size were decreased, although the size of the SCN was unaltered. In addition, the pineal bodies of both mice were morphologically underdeveloped and showed low levels of N-acetyltransferase activity. In contrast, the hyt SCN was comparable to the normal controls in every respect. The hyt pineal was well developed and showed levels of enzyme activity comparable to the controls. However, in all the deficient mice, the optic nerve appeared to be normal in morphological and biochemical studies. These results suggest that the underdevelopment of the pineal body, the reduced levels of spontaneous locomotion and the indistinct diurnal periodicity of the dw and lit mice might be related to the retarded neuronal growth of the SCN, and that growth hormone likely is indispensable for the development of the SCN.

Adrenal-mediated depression of N-acetyltransferase activity and melatonin levels in the rat pineal gland

N-acetyltransferase (NAT) is believed to be the rate-limiting enzyme in the synthesis of melatonin from serotonin in the pineal gland. Norepinephrine released from sympathetic nerve endings within the pineal gland stimulates NAT activity and, therefore, melatonin synthesis. When an animal is subjected to a stressful stimulus, it would be expected that the increase in plasma stimulus, it would be expected that the increase in plasma catecholamines originating from the adrenal medulla and/or the sympathetic nervous system would result in a stimulation of pineal NAT activity. Adult male rats were given a 1.5cc injection of physiological saline subcutaneously into the back leg. Compared to non-injected controls, animals stressed in this manner were shown to have significantly lower pineal melatonin content 10 min after the saline injection late in the light phase of the light/dark cycle (at 18.30 h-lights on at 07.00 h). To test this more thoroughly, a time course study was conducted during the dark phase (at 02.00 h-5 hours after lights out) when pineal NAT activity and melatonin levels are either increasing or elevated. NAT activity and melatonin levels in the pineal were significantly depressed in stressed animals as compared to controls by 10 min after the saline injection, and remained so until 60 min after injection. By 90 min they had returned to control values. In the next study the nighttime response of the pineal to stress was compared in intact and adrenalectomized rats. Adrenalectomy prevented the changes in NAT activity and melatonin content associated with the saline injection. Some factor, such as a catecholamine or corticosterone from the adrenal, seems to be eliciting the response in the pineal to the saline injection. It is not known if the factor is acting centrally or directly on the pineal gland.

Developmental appearance of light-dark entrainment in the rat

Previous studies show that the developing circadian system is entrained by a maternal signal during the prenatal and early postnatal period. The present study investigated the developmental onset of retina-mediated, light-dark entrainment. Juvenile rats were exposed to phase-shifts in the light-dark cycle at different ages and the pineal N-acetyltransferase rhythm was monitored on postnatal day 10. The results show that retina-mediated, light-dark entrainment begins by postnatal day 6 and overrides maternal entrainment by postnatal day 8.

Dopamine receptor-mediated inhibition of serotonin N-acetyltransferase activity in retina

The possible involvement of catecholamines in the regulation of serotonin N-acetyltransferase (NAT) activity in retina of the African clawed frog was investigated using an in vitro eye cup preparation. Dopamine (10 microM) and norepinephrine (50 microM) had no significant effect on NAT activity of eye cups incubated in the light. However, dopamine inhibited the increase of retinal NAT activity that occurs in eye cups incubated in darkness; the ED50 for dopamine was 0.3 microM. The effect of dopamine on NAT activity was mimicked by the dopamine receptor agonists apomorphine and bromocriptine, but not by agonists of alpha 1-, alpha 2- or beta-adrenergic receptors. Dopamine-mediated inhibition of NAT activity was antagonized by spiroperidol and by alpha-flupenthixol, but not by beta-flupenthixol, phentolamine or timolol. Benztropine, an inhibitor of dopamine reuptake, also decreased NAT activity in eye cups incubated in the dark. The inhibitory effect of benztropine was antagonized by spiroperidol, suggesting that it was mediated by an increase in the extracellular concentration of endogenous dopamine. These studies indicate that the regulation of NAT activity in the retina is subject to modulation by a dopamine receptor-mediated mechanism and suggest that dopamine may play a role in the inhibition of NAT activity by light.