Cold-induced increment in rat adrenal gland type II deiodinase is corticosterone dependent

Corticosteroid dependent and independent effects of a cannabinoid agonist on core temperature, motor activity, and prepulse inhibition of the acoustic startle reflex in Wistar rats

RATIONALE

There are inconsistent reports on the effects of cannabinoid agonists on prepulse inhibition of the startle reflex (PPI) with increases, decreases, and no effects. It has been hypothesized that the conflicting observations may be as a result of modulation of the effects of cannabinoid agonists by the regulation of corticosteroid release.

OBJECTIVE

The purpose of the present study was to determine the effects of CP55940, a cannabinoid agonist, and metyrapone, a corticosteroid synthesis inhibitor on core temperature, motor activity, the startle reflex, and PPI.

METHODS

Startle responses were measured in 64 male Wistar rats while varying startling stimulus intensities, analogous to dose-response curves. A stimulus potency measure (ES(50)) and a response measure, the maximal achievable response (R (MAX)) were derived from the stimulus-response curves.

RESULTS

CP55940 reduced core temperature and motor activity; these effects were potentiated by metyrapone. CP55940 increased R (MAX) of startle in the absence of a prepulse by a corticosteroid-dependent mechanism but decreased it when metyrapone was administered before CP55940, a corticosteroid-independent mechanism. The inverse of stimulus potency (ES(50)) was not affected by either drug alone but was increased by the combined drugs. CP55940 increased the prepulse motor gating effects and decreased the prepulse sensory gating effects of the same prepulses but only when given after metyrapone.

CONCLUSIONS

The most parsimonious interpretation of these effects is that CP55940 has some effects through corticosteroid-dependent actions and opposite effects by corticosteroid-independent actions. These two putative sites of actions affect stimulus gating opposite to their effects on response gating.

Stress and thyroid gland

The review highlights the effects of acute and chronic stress on thyroid metabolism. Special attention is paid to the influence of stress and the direct effects of glucocorticoids on the thyroid status, the activities of thyrocyte iodine uptake, oxidation and organification as well as peripheral metabolism of thyroid hormones (deposition and transport of thyroid hormones, deiodinase activities in different tissues). The role of stress in the development of thyroid pathology is analysed and charestiristic features of thyroid function alterations during impaired functioning of the pitiutary-adrenal system are established. The mechanisms of the stress-induced impairments in thyroid functions are of interest for further research, taking into consideration serious consequences of thyroid deficiency for the body, even in subclinical thyroid insufficiency.

Daily variations in type II iodothyronine deiodinase activity in the rat brain as controlled by the biological clock

Type II deiodinase (D2) plays a key role in regulating thyroid hormone-dependent processes in, among others, the central nervous system (CNS) by accelerating the intracellular conversion of T4 into active T3. Just like the well-known daily rhythm of the hormones of the hypothalamo-pituitary-thyroid axis, D2 activity also appears to show daily variations. However, the mechanisms involved in generating these daily variations, especially in the CNS, are not known. Therefore, we decided to investigate the role the master biological clock, located in the hypothalamus, plays with respect to D2 activity in the rat CNS as well as the role of one of its main hormonal outputs, i.e. plasma corticosterone. D2 activity showed a significant daily rhythm in the pineal and pituitary gland as well as hypothalamic and cortical brain tissue, albeit with a different timing of its acrophase in the different tissues. Ablation of the biological clock abolished the daily variations of D2 activity in all four tissues studied. The main effect of the knockout of the suprachiasmatic nuclei (SCN) was a reduction of nocturnal peak levels in D2 activity. Moreover, contrary to previous observations in SCN-intact animals, in SCN-lesioned animals, the decreased levels of D2 activity are accompanied by decreased plasma levels of the thyroid hormones, suggesting that the SCN separately stimulates D2 activity as well as the hypothalamo-pituitary-thyroid axis.

Regulation of type II deiodinase expression by EGF and glucocorticoid in HC11 mouse mammary epithelium

Thyroid hormones are important for mammary gland growth and development. The iodothyronine deiodinases play a key role in thyroid hormone metabolism. We have showed that type II 5'-deiodinase (5'D2) activity and mRNA are present in the mouse mammary gland and that their levels are reduced in the lactating gland. To investigate the regulatory mechanism of mouse 5'D2 gene (mdio2) expression in mammary epithelium, we employed the HC11 cell line, which is derived from mouse mammary epithelial cells and retains the ability to express differentiated function. HC11 cells were treated with combinations of insulin, glucocorticoid (GC, dexamethasone), prolactin, and epidermal growth factor (EGF), and 5'D2 activity and the D2-to-GAPDH mRNA ratio were measured by (125)I(-) release from (125)I-labeled thyroxine and semiquantitative RT-PCR, respectively. EGF increased both 5'D2 activity and mRNA levels about twofold. GC reduced both 5'D2 activity and mRNA in a dose-dependent manner, and their levels were decreased to approximately one-tenth and one-fifth, respectively, of control levels. These data demonstrated that mdio2 expression in HC11 cells is upregulated by EGF mainly at the pretranslational level and downregulated by GC at both pre- and posttranslational levels. Furthermore, we showed that GC reduced the promoter activity of the 627- bp 5'-upstream region of the mdio2/luciferase chimeric reporter gene, suggesting that GC exerts its effect, at least in part, at the transcriptional level.