Beta- and alpha-adrenergic receptors are involved in regulating type II thyroxine 5'-deiodinase activity in the rat Harderian gland

Non-invasively assessing disturbance and stress in laboratory rats by scoring chromodacryorrhoea

In rats, like many rodents, Harderian glands next to the orbits secrete porphyrins, lipids and other compounds. High levels of secretion lead to chromodacryorrhoea (red or "bloody" tears), often taken as a sign of stress or disease. Here, we developed a scoring system for recording chromodacryorrhoea in a quantitative way, and investigated whether the low-level, transient Harderian secretions of normal, healthy rats correlate with low to moderate levels of stress or disturbance. Rather than exposing our subjects (24 Lister Hoodeds, housed in 11 single-sex cages) experimentally to stressors, we made opportunistic use of three likely sources of low-level stress within the unit: 1) building maintenance work, taking several hours and involving several potential stressors; 2) visits by unfamiliar humans, and the other mild sources of disturbance normal in an animal unit; and 3) social status within the cage. The mean daily chromodacryorrhoea score increased most with the severe disturbance of building maintenance work (F1,9 = 602.67, p < < 0.0001), and also increased--though to a lesser extent--with the mild disturbance of visitors and similar (F1,9 = 8.77, p = 0.008), while being the subordinate member of a cage-group had a smaller effect still (F1,6 = 7.86, p = 0.03). Individual rats scored consistently across treatment conditions, and there was also significant inter-observer reliability between independent scorers. We therefore suggest that scoring chromodacryorrhoea could be a simple, practical and non-invasive way of sensitively assessing the impact on rats of housing, husbandry, or procedures.

A physiological role for protoporphyrin IX photodynamic action in the rat Harderian gland?

AIMS

The lipid-secreting exocrine Harderian gland contains a large amount of porphyrins (mainly protoporphyrin IX, PPIX) in the glandular cells, the physiological significance of which is rather poorly understood.

METHODS

In the present study, the possibility of using Fura-2 to measure intracellular calcium ([Ca2+]c) changes in these cells was assessed.

RESULTS

It was found that when Fura-2-loaded cells were excited by light at 340/380 nm, [Ca2+]c increased spontaneously, indicating a photodynamic action powered by light at 340/380 nm. In contrast, with the visible spectrum calcium probe Fluo-3 (lambda(ex) = 475 nm), carbachol at 10 microm induced [Ca2+]c increase; [Ca2+]c did not change without carbachol stimulation. Brief illumination with light at 340/380 nm induced a large [Ca2+]c increase in Fluo-3-loaded cells. Photodynamic stimulation of [Ca2+]c increase was confirmed with an exogenous photosensitizer sulphonated aluminium phthalocyanine (SALPC) and visible light (>580 nm). The wavelength-dependence of the [Ca2+]c increase correlates well with the excitation spectrum of the isolated Harderian glandular cells.

CONCLUSION

These data suggest that PPIX present in rat Harderian glandular cells plays the role of a photosensitizer which upon activation by UVA and blue components of daylight and subsequent singlet oxygen generation, triggers [Ca2+]c increase and secretory response. The PPIX photodynamic action may also play a potential role in photic entrainment of the central circadian clock.

Nocturnal increases in the triiodothyronine/thyroxine ratio in the rat thymus and pineal gland follow increases of type II 5'-deiodinase activity

The type II 5'-deiodinase (5'D-II) is regulated by the light-dark cycle in some tissues in which the enzyme is present. This prompted us to investigate putative influences of light-dark cycle on thyroid hormone concentrations in these tissues. The results revealed the following facts: (a) Deiodinase activity in the rat thymus exhibits a nyctohemeral profile with peak values late at night and basal values during the day. The thyroid hormone concentrations in the thymus also show a 24 h rhythm with an increase in the triiodothyronine/thyroxine (TT3/TT4) ratio at night. (b) The content of thyroid hormones in the pineal gland exhibits, like in the thymus, nyctohemeral variations with increase values in the TT3/TT4 ratio during the dark period coinciding with the maximal enzyme activity. (c) Other tissue, like the anterior pituitary, in which 5'D-II, activity does not exhibit a diurnal variation, the concentration of thyroid hormones does not show modifications. In conclusion, the nocturnal increase of 5'D-II activity produces an increase of T3 concentration and a decrease of T4 concentration in both thymus and pineal gland. Therefore, these diurnal changes in 5'D-II activity is a mean by which the cell can regulate the intracellular availability of the most active thyroid hormone T3.

Northern analysis of type II iodothyronine deiodinase mRNA in rat Harderian gland

It has been known that type II iodothyronine deiodinase activity is present in rat Harderian gland and the activity is significantly increased by isoproterenol administration. We have performed Northern analyses to study whether the transcript for type II iodothyronine deiodinase is expressed in rat Harderian gland and whether the isoproterenol stimulation of type II iodothyronine deiodinase activity in rat Harderian gland is due to the change in its mRNA level. Northern analyses have demonstrated that type II iodothyronine deiodinase mRNA, approximately 7.5 kb in size, is expressed in rat Harderian gland, and the mRNA levels as well as the deiodinase activities are greater in hypothyroid rats than those in euthyroid rats. Type II iodothyronine deiodinase mRNA levels and the deiodinase activities in Harderian gland were increased by isoproterenol administration, and the increase in the mRNA levels preceded that in the deiodinase activities. These results indicate that 7.5 kb transcript for type II iodothyronine deiodinase is expressed in rat Harderian gland and beta-adrenergic stimulation of type II iodothyronine deiodinase activity is due at least in part to the increase in its mRNA level.

Different experimental conditions which regulate type II 5'-deiodinase mRNA in rat Harderian gland

In the present study, we describe the modifications in the expression of type II 5'deiodinase activity (5'D) in Xenopus laevis oocytes by injection of polyadenylated (poly A) mRNA from hypothyroid rat Harderian gland. The time-course study showed that the expression of the enzyme was dependent on time. Thus, enzyme activity was observed in oocytes 6 and 12 hours after the injection with poly A mRNA, reaching a maximal value at 24 hours. The activity was partially inhibited by 6-n-propyl-thiouracil, completely inhibited by iopanoic acid and exhibited a higher affinity for the T4 (Km=1.5 nM) than rT3 (Km=20 nM). The expression of the enzyme was modified in different experimental conditions: (a) exhibited diurnal variations with maximal peak values at night, (b) was inhibited by light at night and, (c) was activated by isoproterenol. On the other hand, we have also identified, for the first time, the size of mRNA capable of inducing 5'D in rats.

Binding of [125I]iodocyanopindolol by rat harderian gland crude membranes: kinetic characteristics and day-night variations

The Harderian glands are innervated by sympathetic fibers originating in the superior cervical ganglia. The aim of this study is to characterize the beta-adrenergic receptors in the rat Harderian gland. The characteristics of beta-adrenergic receptors were determined in crude membrane preparations from rat Harderian gland, using [125I]iodocyanopindolol ([125I]CYP) as radioligand. The binding of the ligand to the receptor is rapid, reversible, saturable, specific and dependent on time, temperature and membrane concentration. At 30 degrees C, stoichiometric data suggest the presence of one binding site with a Kd value of 0.29 nM and Bmax of 32 pmol/L. The interaction shows a high degree of specificity for beta-adrenergic agonists and blockers, as suggested by competitive displacement experiment with isoproterenol (IC50 = 19.1 nM), propranolol (IC50 = 28.1 nM), and norepinephrine (IC50 = 96.3 nM). Clonidine, yohimbine, methoxamine, and prazosin are ineffective at concentrations up to 1 microM. In the other hand, binding of [125I]CYP by Harderian gland membranes exhibits day-night variations. Binding values are low during the daytime and increase progressively late in the evening to reach a maximum at 2200 h (2 h after the onset of dark period), but decreased to the end of the dark period (0600 h). In conclusion, the results presented in this paper show the functional and pharmacological characterization of beta-adrenergic receptors in the rat Harderian gland. This neurotransmitter may play a physiological role at this level regulating, at least, processes such as a thyroid hormone metabolism.

Sexual differences in 5'-deiodinase activity in the Harderian gland of Syrian hamsters and the effect of pinealectomy: regulation by androgens

Sexual differences on thyroxin 5'-deiodinase (5'-D) in the Harderian gland of Syrian hamsters were investigated. We compared the 24-h profile of 5'-D activity in male and female hamsters, observing a clear rhythm in males but not in females. Female values were always significantly higher than male ones. After pinealectomy day/night variations in male 5'-D activity at the time points studied were abolished, results that are in correlation with serum thyroid hormones. We also studied the regulation by androgen of the enzyme activity. Basal 5'-D activity increased in castrated males and levels fell when animals were implanted with testosterone or its product 5 alpha-dihydrotestosterone (DHT). Female 5'-D activity was also inhibited by androgens. As only the addition of DHT in the presence of epitestosterone, an inhibitor of the conversion of testosterone on DHT, in castrated males was able to decrease 5'-D activity to the control animal levels, we suggest a probable direct effect of DHT by itself.

Characterization of binding sites for beta-adrenergic agonists and vasoactive intestinal peptide in the rat harderian gland

Vasoactive intestinal peptide (VIP) receptors and beta-adrenergic receptors were investigated in rat Harderian gland membranes using 125I-VIP and 125I-cyanopindolol (125I-CYP), respectively, as ligands. The receptor bindings were rapid, reversible, saturable, specific, and dependent on time, temperature, and membrane concentration. The stoichiometric data suggested the presence of two classes of VIP receptors with Kd values of 0.36 and 65.37 nM and binding capacities of 323 and 39,537 fmol VIP/mg protein, respectively. The interaction showed a high degree of specificity, as suggested by competitive displacement experiments with several peptides structurally or not structurally related to VIP as follows: VIP > helodermin > rGRF > PHI > > secretin. Glucagon, somatostatin, insulin, and pancreastatin were ineffective at concentrations up to 1 microM. However, the stoichiometric data suggest the presence of one class of binding sites for 125I-CYP. The Kd for the single site was 290 pM with a binding capacity of 32 pmol/L. The pharmacological characterization of 125I-CYP binding to membranes showed that only isoproterenol, a beta-adrenergic agonist, and norepinephrine, an alpha beta-adrenergic agonist, was as effective as propranolol in inhibiting 125I-CYP binding to Harderian gland membranes. However, alpha 1- and alpha 2-adrenergic agonists and blockers such as methoxamine, prazosin, clonidine, and yohimbine were shown to be ineffective. These results demonstrate the presence of specific VIP and beta-adrenergic receptors in the Harderian gland and suggest a role for VIP and beta-adrenergic agonists in the physiology of this gland.

Lipid secretory mechanisms in the mammalian harderian gland

The mammalian Harderian glands are lipid-secreting glands. In an unstimulated condition, the glandular cells frequently exocytose the lipid materials; however, no intracellular calcium ion ([Ca2+]c) changes are detectable. Cholinergic (muscarinic) secretagogues induce secretory activity and increase of [Ca2+]c. A G-protein activator, sodium fluoride, enhances the secretory activity and increase of [Ca2+]c. Removal of extracellular calcium ions inhibits the secretion enhanced by cholinergic stimulation. Under pharmacologic stimulation, glandular cells may show an apocrine-like secretory pattern. Cholinergic stimulation also induces contraction of the myoepithelial cells covering glandular end pieces; however, the reduction in volume of glandular end pieces is not prominent. Catecholamines have no effect on the release of lipid materials. These results indicate the involvement of G-proteins linking with muscarinic receptors and Ca2+ dynamics (increase of [Ca2+]c and Ca2+ influx) in lipid secretion by glandular cells and in contraction of myoepithelial cells of mammalian Harderian glands. However, the increase of [Ca2+]c in Harderian glands was less when compared with other cells--for instance, those which secrete protein.

Beta- and alpha-adrenergic mechanisms are involved in regulating type II thyroxine 5'-deiodinase in rat thymus

The role of adrenergic receptors in regulation of rat thymus type II thyroxine 5'-deiodinase (5'-D) activity was investigated. Our results show that norepinephrine, an alpha- and beta-adrenergic agonist elicited an increase in thymus 5'-D activity. Isoproterenol, beta-adrenergic agonist, also increased the enzyme activity, being less effective than norepinephrine. Moreover, alpha-adrenergic agonists, i.e., methoxamine, an alpha1-agonist, and clonidine, an alpha2-agonist, did not increase 5'-D activity. The effect of isoproterenol was potentiated by methoxamine, but the potentiating effect was observed only at doses of isoproterenol which induce submaximal activation of the enzyme. Administration of propranolol, beta-adrenergic blocker, and prazosin, an alpha1-adrenergic blocker, inhibited significantly the activation of the enzyme induced by norepinephrine. However, yohimbine, an alpha2-adrenergic blocker, had small effect. These results show, in hypothyroid rats, a clear regulation by adrenergic mechanisms of 5'-D activity in the thymus, where alpha- and beta-adrenergic receptors might be involved.

G-protein activation enhances Ca(2+)-dependent lipid secretion of the rat harderian gland

We studied the secretory mechanism of the Harderian gland of rats. After perfusion with HEPES-buffered Ringer's solution containing NaF (10 mM) with AlCl3 (10 microM), a G-protein activator, the glandular cells of the Harderian gland showed massive exocytosis and apocrine-like protrusions on the luminal surface. Some of the secretory vacuoles aggregated within the cytoplasm, and large vacuoles were formed. Contraction of the myoepithelial cells covering the glandular endpieces caused a narrowing of the glandular lumina, which contained cytoplasmic fragments, and deformation of the basal contour of the glandular end-pieces. The basal regions of the glandular cells also bulged between the myoepithelial cells. Secretory vacuoles were also discharged to the lateral cell surface, and the intercellular spaces were dilated. The enhanced secretory activities of the glandular cells and the contraction of the myoepithelial cells were similar to those in rats stimulated with 10 microM carbachol (CCh). However, dilatation of the endoplasmic reticulum in glandular cells (type A cells), which leads to the formation of small vesicles, was observed in those glands stimulated by NaF+AlCl3, but not in those stimulated by CCh. Removal of Ca2+ from the perfusing HR or addition of EDTA (0.5 mM) diminished and inhibited NaF+AlCl3- or CCh-enhanced secretory activity of the glandular cells and also allayed the deformation of glandular cells caused by myoepithelial cell contraction. The present results demonstrate the involvement of G-proteins and Ca(2+)-influx in the lipid secretion of glandular cells and in the contraction of myoepithelial cells of the Harderian gland in rats.

Expression of type II thyroxine 5'-deiodinase from rat harderian gland in Xenopus laevis oocytes

The presence of isoenzymes mediating the conversion of thyroxine to 3,5,3'-triiodothyronine has been studied according to characteristic kinetics and physiological regulation. In this paper, we report the expression of type II 5'-deiodinase (5'D) activity in oocytes of Xenopus laevis. Oocytes injected with total RNA extracted from rat Harderian gland, and then incubated up to five days demonstrated a progressive increase in 5'D activity, reaching a maximal value at 24 h; then, 5'D activity remained almost stable for an additional period of four days. Characteristics of the enzyme activity expressed by oocytes included its inhibition by iopanoic acid, but not by propylthiouracil, and its increase during beta-adrenergic agonist treatment and hypothyroidism. The expressed activity manifests characteristics typical of the type II isoenzyme. Deiodinating activity in oocytes also exhibited diurnal variations. In this study, 5'D activity expressed in oocytes exhibited low values when animals were killed during the day, and high values when animals were killed at night. Maximal values were reached 3-4 h before the nocturnal peak of 5'D activity in Harderian gland crude homogenates. Results suggest that the in vivo activation of 5'D by isoproterenol, hypothyroidism, or dark exposure may be caused by an increase in the synthesis and/or maturation of the RNA expressing the enzyme.

Carbamylcholine-induced morphological changes and spatial dynamics of [Ca2+]c in Harderian glands of guinea pigs: calcium-dependent lipid secretion and contraction of myoepithelial cells

To determine whether lipid-secreting cells have cytosolic Ca2+ concentration ([Ca2+]c)-related secretory mechanisms, morphological changes and intracellular calcium dynamics of Harderian glands of guinea pigs stimulated by secretagogue were studied by electron microscopy and Fura-2/AM digital image analysis. Control glandular cells contained large lipid vacuoles that were bordered by multi-layered membranes. Rough-surfaced endoplasmic reticulum, mitochondria, and smooth-surfaced endoplasmic reticulum may be involved in lipid vacuole formation. Myoepithelial cells surrounded alveoli. After carbamylcholine (CCh, 10(-6), 10(-5), and 10(-3) M) stimulation, lipid materials within the membranous structures were frequently discharged by an exocytotic mechanism. Conspicuous deformation of glandular cells caused by vigorous contraction of myoepithelial cells was observed in isolated alveoli after 10(-6) M CCh stimulation, whereas the deformities of glandular tissues perfused via vessels were small even after 10(-3) M CCh stimulation. Connective tissue between glandular alveoli inhibited unbridled myoepithelial-cell contraction. Fura-2/AM digital imaging analysis revealed that CCh stimulation caused an increase in [Ca2+]c in isolated alveoli. The morphological reactions and changes in [Ca2+]c were prevented by atropine. When extracellular calcium ions were absent, enhanced extrusion of lipid vacuoles, myoepithelial-cell contraction, and a rise in [Ca2+]c after CCh stimulation were not observed. Nicotine and catecholamines had no effect on the secretion or on the dynamics of [Ca2+]c. It can be concluded that acetylcholine elicits exocytosis in glandular cells and contraction of the myoepithelial cells of Harderian glands, accompanied by an increase in [Ca2+]c. The dynamics of [Ca2+]c of the gland alveoli are mostly dependent on extracellular Ca2+.

Thyroxine type II 5'-deiodinase activity in pineal and Harderian gland is enhanced by hypothyroidism but is independent of serum thyroxine concentrations during hyperthyroidism

1. This paper studies the effect of thyroid status on 5'-D activity in pineal gland, Harderian gland, brown adipose tissue (BAT), pituitary gland, brain frontal cortex (BFC), and cerebellum. 2. Hypothyroidism clearly increased diurnal 5'-D activity in Harderian gland, BAT, pituitary gland, BFC, and cerebellum. In pineal gland, diurnal values of 5'-D activity were not affected by hypothyroidism. 3. Hypothyroidism in adult rats clearly enhanced nocturnal increase of 5'-D activity in pineal and Harderian gland. Congenital hypothyroidism also enhanced the nocturnal increase of 5'-D activity in pineal gland. 4. Hyperthyroidism inhibited 5'-D activity in pituitary gland, BFC, and cerebellum. A small inhibition, although significant, was found in BAT. 5. In pineal and Harderian gland, hyperthyroidism did not inhibit either the basal diurnal values of the enzyme or the nocturnal increase of its activity. 6. Results suggest that, in tissues where 5'D-activity is regulated by adrenergic mechanisms, mostly pineal gland and Harderian gland, the enzyme activity is independent of serum T4 concentrations during hyperthyroidism.

Potentiating effect of phenylephrine on isoproterenol activation of thyroxine type II deiodinase in the pineal gland of adult rats

In the present study we show, for the first time, that phenylephrine (PHE), an alpha-adrenergic receptor agonist, potentiates the effect of isoproterenol (ISO), a beta-adrenergic agonist, in activating pineal type II5'-deiodinase (5'-D) activity. The potentiating effect of PHE was observed only at doses of ISO which induce submaximal activation of the enzyme. However, at doses which lead to maximal activation of the enzyme, PHE was ineffective. The results suggest that not only beta-, but also alpha-adrenergic receptors, are involved in the sympathetic noradrenergic regulation of pineal 5'-D activity in the adult rat.

Thyroxine 5'-deiodinase type II activity in chick pineal and Harderian gland: nyctohemeral rhythmicity and its regulation by noradrenergic input

Circadian rhythmicity of type II thyroxine 5'-deiodinase (5'-D) activity was studied in the pineal gland and Harderian glands of chicks. Only Harderian 5'-D activity showed a nyctohemeral rhythmicity with a maximal peak during the day time (1300), while no rhythm of enzyme activity was found in the pineal gland. Besides type II 5'-D activity, we found high basal levels of the type I isoenzyme in both glands; this activity was specifically suppressed by the addition of 6-n-propyl-thiouracil (PTU). However, day-night differences in Harderian 5'-D activity were maintained even after the addition of PTU. This activity was not affected for either continuous light exposure or darkness during the day. 5'-D activity seems to be regulated by the noradrenergic input, since the enzymatic activity was stimulated by a beta-adrenergic agonist, isoproterenol, and by the alpha-adrenergic agonist, phenylephrine, in both pineal and Harderian glands. Both drugs affected 5'-D activity in the Harderian gland by stimulating the enzyme activity over basal levels.

Adrenergic regulation of type II 5'-deiodinase circadian rhythm in rat harderian gland

This paper reports on the regulation of the nyctohemeral profile of type II thyroxine 5'-deiodinase (T45'D) activity in the rat harderian gland. Harderian gland T45'D activity exhibits a nighttime increase with maximal values late in the dark period (0200-0400 h) and basal values during the daytime. The nocturnal rise of the deiodinating activity was prevented by either exposure of animals to light at night, injecting the animals with both alpha- and beta-adrenergic receptor blockers, or bilateral superior cervical ganglionectomy (SCGx). However, adrenalectomy did not affet the enzyme activity in the harderian gland. In brown adipose tissue (BAT), where thyroid hormone metabolism is extremely dependent on alpha 1-adrenergic stimulation by blood-circulating catecholamines, adrenalectomy significantly decreased T45'D activity. Deiodinating activities in brain frontal cortex (BFC) and pituitary gland were unaffected by adrenalectomy. Unlike in the harderian gland, SCGx did not modify the T45'D activity in either BAT, BFC, or the pituitary gland. The results suggest that elevated plasma catecholamines are not required for harderian gland T45'D activation and that the nyctohemeral profile of the enzyme activity in the harderian gland is dependent on the noradrenergic input from the superior cervical ganglia.

Iodothyronine 5'-deiodinating activity in the pineal gland

1. The presence of an iodothyronine 5'-deiodinating activity has been described in the pineal gland of various rodents, and it has been identified as a type II 5'-deiodinase isoenzyme since it is relatively insensitive to inhibition by propylthiouracil and its activity increases during hypothyroidism. 2. 5'-Deiodinase activity in the rat pineal gland follows a nyctohemeral profile, exhibiting basal values during the day and maximal values at night. The nocturnal increase is dependent on the noradrenergic input from the superior cervical ganglia, and both in vivo and in vitro studies show that beta-adrenergic receptors are primarily involved in the activation of the enzyme. 3. Day-night differences in rat pineal 5'-deiodinase activity are found beginning at 2 weeks of age, with rhythms increasing in amplitude until maximal differences are reached in adult animals. During the maturation of the rhythm, changes in regulation of enzyme activation are observed. Thus, during the first 2-3 weeks of age, alpha-adrenergic receptors appear to be as important as beta-adrenergic receptors in regulating the deiodinating activity of the pineal. However, in adults, no role of alpha-adrenergic receptors has been described. 4. Although regulation of 5'-deiodinase activity in the pineal gland is well established, few data are available concerning the physiological significance of the enzyme in the gland. Of the studies that have been performed, those attempting to demonstrate a relationship between pineal 5'-deiodinase activity and other pineal rhythms, e.g. those of melatonin production and N-acetyltransferase activity, indicates that the latter rhythms do not rely on the cyclic production of T3. The alternate possibility that the 5'D rhythm depends on the cyclic production of melatonin remains to be examined.