Aldo Gonzalez-Brito

N-Acetyltransferase activity, hydroxyindole-O-methyltransferase activity, and melatonin levels in the Harderian glands of the female Syrian hamster: Changes during the light: Dark cycle and the effect of 6-parachlorophenylalanine administration

The activities of NAT and HIOMT and the melatonin content of the Harderian glands of female Syrian hamsters were studied. When hamsters were kept under a light:dark cycle of 14:10 (lights on at 06.00 h), NAT activity exhibited a sharp, short term rise at one hour after lights on. Simultaneously, the activity of HIOMT, which forms melatonin, exhibited a rapid decline. Melatonin levels, like HIOMT activity, also showed a precipitous drop at one hour after light onset. After the respective changes, both NAT and HIOMT activity reverted back to night time levels. Melatonin levels remained depressed for several hours but by 1400 h (8 hours after lights on), nighttime melatonin values were re-established. Treatment of female hamsters with PCPA, a trytophan hydroxylase inhibitor, led to depressed levels of Harderian melatonin without affecting the activities of either NAT or HIOMT.

Characterization and measurement of [125I]iodopindolol binding in individual rat pineal glands: existence of a 24-h rhythm in β-adrenergic receptor density

A simple procedure has been developed that permits measurement of beta-receptors in membrane preparations from individual rat pineal glands using [125I]iodopindolol ([125I]PIN). [125I]PIN binding to pineal membranes was stereospecific and saturable. Scatchard analysis of saturation isotherms yielded a Kd of 147.3 +/- 54 pM and a Bmax of 11.1 +/- 1.5 fmol/pineal gland. Binding was linear suggesting that [125I]PIN binds to a single population of pineal beta-adrenergic receptors. This procedure was used to evaluate 24-h variations in density of pineal [125I]PIN binding sites in male rats maintained in a 14:10 h light:dark cycle. Binding remained uniformly low during the daytime, increased slightly prior to lights off and peaked after 6 h of darkness decreasing abruptly 2 h later, before lights on. In animals maintained in light at night, the number of binding sites also increased, but did not exhibit the darkness-related decrease. The results demonstrate that beta-adrenergic receptors defined via [125I]PIN binding can be measured in tissue samples equivalent to less than one pineal gland. Moreover, the technique can be used in studies concerning the noradrenergic regulation of pineal function.

5α-Dihydrotestosterone administration converts indolamine metabolism and porphyrin content of the female syrian hamster harderian gland to the male type

Abstract The effects of ovariectomy and exogenous androgen administration on the indole and porphyrin metabolism of Syrian hamster Harderian glands were studied. Ovariectomy alone had no effect on any of the parameters analyzed. The administration of either testosterone or 5α-dihydrotestosterone increased the activity of N-acetyltransferase in the Harderian glands. However, androgen treatment failed to change the activity of hydroxyindole-O-methyltransferase. Melatonin content of the glands dropped 20 days after treatment with testosterone and 10 days after the administration of 5α-dihydrotestosterone. The porphyrin content of the Harderian glands was dramatically depressed after the administration of either androgen. It is concluded that the Harderian glands of Syrian hamsters are under an androgenic control involving 5α-dihydrotestosterone.

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

The activity of N‐acetyltransferase (NAT) 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 bilaterally adrenalectomized (ADX) adult male rats were kept under light:dark cycles of 14:10 (lights on 0600 h) injected intraperitoneally with 10 IU insulin at 1300 h, groups (n = 8) were killed 2, 3, or 4 h postinjection. Plasma catecholamines were assayed by means of high performance liquid chromatography radioimmunoassay was used to assess pineal NAT activity 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, 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 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.

Nocturnal increase of type II thyroxine 5'-deiodinase activity in the Syrian hamster harderian gland is abolished by light exposure and induced by isoproterenol.

Abstract The presence of type II 5′-deiodinase activity in the Syrian hamster Harderian gland was investigated. This enzyme exhibited an increase of its activity after animals entered the normal dark phase, with maximal activity occurring at 04.00 hr (8 hr after lights off). The nocturnal increase was prevented by maintaining the animals in light during the night. Isoproterenol subcutaneously injected every 2 hr (1.0 mg/kg body wt) from 20.00 hr through 0.400 hr to animals exposed to light during the normal dark period mimicked the effect of darkness, i.e., with this treatment an increase in 5′-deiodinase activity with maximal peak values at 02.00 hr was observed. The results show that 5′-deiodinase activity in the Syrian hamster Harderian gland exhibits a nyctohemeral profile dependent on β-adrenergic activation of the gland.

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.

Differential responses of rat pineal thyroxine type II 5′-deiodinase and N-acetyltransferase activities to either light exposure, isoproterenol, phenylephrine, or propranolol

Summary1.Compared to pinealN-acetyl transferase (NAT) activity, which exhibited a dramatic drop following acute light exposure at night, nocturnal rat pineal thyroxine type II 5′-deiodinase (5′-D) activity was minimally influenced by the same light exposure. The injection of cycloheximide, a potent inhibitor of protein synthesis, although it did curtail the rise in NAT activity for at least 2 hr, did not elicit decreases in the activities of either 5′-D or NAT enzymes. Propranolol, aβ-adrenergic blocker, either delayed the continued nocturnal rise in 5′-D activity when injected at 0000 hr or slightly enhanced the fall in 5′-D activity when injected at 0200 hr. These results suggest that interruption of the synthesis of proteins is responsible for the slow deterioration of 5′-D activity induced by either light or propranolol.2.The slight fall in 5′-D activity induced by light at night was prevented by isoproterenol; phenylephrine, however, did not prevent the fall and the effect of isoproterenol + phenylephrine was similar to that obtained with isoproterenol alone. On the other hand, the light-inhibited NAT activity recovered after the injection of isoproterenol; phenylephrine did not elicit any effect, but the injection of both isoproterenol and phenylephrine simultaneously caused a greater NAT response than that induced by isoproterenol alone.3.When injected during the day, phenylephrine had no effect on either pineal 5′-D or NAT activities; however, the injection of either isoproterenol alone or isoproterenol + phenylephrine elicited 5-fold and 10-fold increases in nocturnal, light-suppressed 5′-D and NAT activities, respectively. During the day, phenylephrine did not potentiate the effects of isoproterenol on NAT activity as it did at night. When the effects of isoproterenol on the 5′-D activity were compared to rats exposed to light during the day and at night, the activity of 5′-D reached a higher level at night than during the day.

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.

ß-Adrenergic stimulation prior to darkness advances the nocturnal increase of Syrian hamster pineal melatonin synthesis

Pineal glands of male Syrian hamsters stimulated in vivo with isoproterenol (ISO) for 4 h before the onset of darkness showed a 4-h advance in the timing of the nighttime increases in both N-acetyltransferase activity and melatonin levels. When ISO (1 mg/kg) was administered every 2 h to animals kept in light during the night, a significant increase in melatonin synthesis was observed after 4-6 h. The results suggest that the Syrian hamster pineal gland can respond in vivo to continuous beta-adrenergic stimulation, but a lag period of 4-6 h is required before there is an increase in melatonin synthesis.

Forskolin, an activator of adenylate cyclase activity, promotes large increases in N-acetyl transferase activity and melatonin production in the Syrian hamster pineal gland only during the late dark period

The exposure of organ cultured pineal glands of Syrian hamsters to forskolin, an adenylate cyclase activator, caused marked increases in serotonin N-acetyltransferase activity and melatonin content in a dose-related manner (1-100 microM) when glands were collected in the second half of the dark period. However, addition of forskolin to glands collected anytime during the light period or at the beginning of the dark period failed or only modestly stimulated either pineal N-acetyltransferase activity or melatonin levels. Similar results were obtained with isoproterenol. The results suggest that intrapinealocyte regulatory mechanisms may determine the nocturnal rise in the Syrian hamster pineal gland.