Abraham Thomas

CONCENTRATIONS OF LEPTIN IN THE SERUM OF PREGNANT, LACTATING, AND CYCLING RATS AND OF LEPTIN MESSENGER RIBONUCLEIC ACID IN RAT PLACENTAL TISSUE

Leptin concentrations were measured in the serum of cycling, pregnant, and lactating Sprague-Dawley rats. Serum leptin concentrations did not vary significantly during the estrous cycle. In contrast, as gestation advanced, serum leptin concentrations increased significantly, p < 0.0001. Following delivery, leptin concentrations declined and remained stable during lactation. Leptin messenger ribonucleic acid (mRNA) was identified in the visceral adipose tissue and placenta of rats sacrificed on days 14 and 21 of pregnancy. The relative abundance of placental leptin mRNA increased approximately 4 to 5 fold from day 14 to 21 of gestation. The pattern of elevated leptin concentrations in the serum of late pregnant rats is similar to that reported in pregnant women, therefore the rat may be a useful model for the study of leptin during pregnancy. The increase in leptin in the serum of late pregnant rats, as well as an increase in placental mRNA, raises the possibility that leptin may serve a physiological role for the late parturient rat and/or its young.

The duration of estradiol and progesterone exposure prior to progesterone withdrawal regulates oxytocin mrna levels in the paraventricular nucleus of the rat

The nonapeptide oxytocin (OT) is important for milk ejection during lactation, uterine contractility at parturition, and the onset of maternal behavior. Sequential exposure to estradiol (E2) and progesterone (P) followed by P withdrawal increases OT mRNA in the paraventricular nucleus (PVN), and to a lesser degree the supraoptic nucleus (SON), of the rat 48 hours after the P is removed. Although increases in PVN OT mRNA are not accompanied by changes in posterior pituitary OT peptide content, the PVN contains OT neurons that project to both the posterior pituitary (magnocellular group) and extra pituitary sites (parvocellular groups). Steroid-induced increases in OT mRNA occur in both the magnocellular and the parvocellular regions of the PVN. The latter are believed to contribute to CNS release of OT which may be important for certain behaviors including the onset of maternal behavior. The same steroid sequence that increases PVN OT mRNA also induces maternal behavior in virgin ovariectomized rats. Exposure of animals to E2 and P for 2 weeks resulted in the shortest latency to the onset of maternal behavior in ovariectomized rats, whereas exposure for 6 days was associated with a longer latency. In this study we questioned if the duration of E2 and P exposure prior to P withdrawal is an important regulator of PVN OT mRNA levels. We compared OT mRNA levels in the PVN of virgin ovariectomized rats administered no steroid or sequential E2 and P for 2 weeks versus 6 days. On day 1 animals received steroid-filled or empty capsules followed by P-filled or empty capsules on day 3. In one steroid-treated group, E2 and P were continued for 6 days and in the other group for 14 days prior to P removal. Animals were sacrificed 48 hours after P removal. Levels of OT mRNA were compared among 6 day and 2 week steroid-treated animals and sham-treated animals. The relative abundance of OT mRNA was significantly increased, P < 0.05, in animals receiving the 2-week, but not the 6-day, steroid treatment compared to sham-treated animals. Pituitary OT peptide content was not significantly different among the three groups. We conclude that the duration of steroid exposure may be an important regulator of the level of OT mRNA in the PVN of the rat.

Sequential estrogen and progesterone (P) followed by P withdrawal increases the level of oxytocin messenger ribonucleic acid in the hypothalamic paraventricular nucleus of the male rat.

We previously reported that alterations in gonadal steroid hormones can influence oxytocin (OT) messenger ribonucleic acid (mRNA) expression in the hypothalamic paraventricular nucleus (PVN) of the steroid-treated ovariectomized rat, the pregnant rat, and the lactating rat. Specifically, OT mRNA in the PVN of the female rat was increased by sequential estrogen and progesterone (P) followed by withdrawal of P. In this study we investigated whether this same steroid paradigm increases OT mRNA levels in the castrate rat. Castrate rats were administered either sequential estrogen and P followed by P withdrawal or no steroid treatment prior to sacrifice. PVN OT mRNA was measured by Northern blot hybridization and pituitary OT peptide content by radioimmunoassay (RIA). The steroid-treated rats had increased OT mRNA levels compared to the sham treated rats (p < 0.04), but pituitary OT peptide content was not significantly altered. We conclude that sequential estrogen and P followed by P withdrawal increases PVN OT mRNA in the castrate, as well as the ovariectomized, rat.

Differential regulation of oxytocin and vasopressin messenger ribonucleic acid levels by gonadal steroids in postpartum rats

We previously reported that sequential estradiol and progesterone exposure followed by progesterone withdrawal increases oxytocin (OT), but not arginine vasopressin (AVP), messenger ribonucleic acid (mRNA) in the hypothalamic paraventricular nucleus (PVN) of the rat. Substitution of testosterone for progesterone and subsequent testosterone withdrawal in the estrogen-primed rat increases PVN AVP mRNA levels. At the end of pregnancy (day 21), rats are exposed to high estrogen and declining progesterone and testosterone concentrations. Coincident with these changes in circulating gonadal steroid hormones in OT and AVP mRNAs. If progesterone levels are sustained at term, OT levels are attenuated and if testosterone is sustained, AVP mRNA levels are attenuated. Immediately postpartum, however, OT and AVP mRNA levels decline compared to term levels. To further determine the role of estrogen in the regulation of OT and AVP mRNAs, we performed two experiments. In the first experiment, we administered estrogen during the peripartum period to determine if estrogen supplementation prevents the relative attenuation of OT and AVP mRNAs that is seen after parturition. Day 18 pregnant rats were given estradiol-filled or empty capsules and sacrificed on day 2 lactation. By Northern analysis, significant differences in PVN AVP, but not OT, mRNA were found between the estrogen- and sham-treated lactational animals, P < 0.02. In the second experiment, we determined if sustaining estrogen after progesterone is removed in steroid-treated ovariectomized rats is essential for the increase in OT mRNA. Ovariectomized rats were given either empty capsules or sequential estradiol- and progesterone-filled capsules and both were sustained for 12 days. When progesterone-filled capsules were removed, estradiol-filled capsules were either removed or left in place, and the animals were sacrificed 48 h later. PVN OT mRNA was analyzed by Northern blot hybridization. OT mRNA was increased in both of the steroid-treated groups to the same degree, compared to sham-treated animals, P = 0.04. In summary, estrogen supplementation during early lactation prevents the attenuation of PVN AVP, but not OT, mRNA after parturition. In the estrogen-primed ovariectomized rat, it is not necessary to sustain estrogen to see the effects of progesterone withdrawal upon PVN OT mRNA.

Hyponatraemia and the Syndrome of Inappropriate Antidiuretic Hormone Secretion Associated with Drug Therapy in Psychiatric Patients

SummaryHyponatraemia, the most common electrolyte disorder of all hospitalised patients, is particularly common in psychiatric patients. Hyponatraemia is generally defined as a serum sodium level of less than 135 mmol/L. Certain psychotropic medications may predispose to hyponatraemia, yet a causative role for most has not been firmly established and their effect is most likely to be idiosyncratic. Certain factors such as age, schizophrenia and a history of hyponatraemia or polydipsia should alert the clinician to the need for closer follow-up.Although the majority of cases of hyponatraemia associated with psychotropic medications occur soon after initiation of the medication, some may occur much later. Thus, it is imperative to check a serum sodium level whenever patients who are receiving psychotropic medications have a marked change in their underlying disease, significant increases in body weight, seizures or other symptoms of hyponatraemia.Immediate treatment of hyponatraemia includes discontinuation of psychotropic drugs associated with hyponatraemia whenever possible, and treatment should be tailored to the underlying cause. Rapidity of correction should be determined by the chronicity of the hyponatraemia and whether the patient is symptomatic from the hyponatraemia. Strict adherence to guidelines for correction should be observed to prevent brain damage from pontine and extrapontine myelinolysis. Treatment of chronic hyponatraemia is best focused on the underlying psychiatric disorder. Overall, adherence to guidelines for early diagnosis and appropriate treatment of hyponatraemia will prevent mortality and reduce morbidity.

Administration of long term estradiol and progesterone followed by progesterone withdrawal does not alter the plasma oxytocin secretory response to cholecystokinin or the pituitary oxytocin content in ovariectomized rats

The hormone oxytocin (OT) is important for several pre- and postpartum events, including uterine contractions at parturition, the induction of maternal behavior, and milk ejection during nursing. During late pregnancy, OT mRNA is increased in the paraventricular nucleus (PVN) due to high estrogen and declining progesterone levels. Administration of sequential estrogen and progesterone to, followed by withdrawal of progesterone from, an ovariectomized rat also increases OT mRNA. However, pituitary OT peptide is not affected. In the present experiment, we determined if this steroid exposure alters peripheral OT secretion during a provocative stimulus to OT release, such as cholecystokinin (CCK). Adult ovariectomized Sprague-Dawley rats were implanted on day 1 with either estrogen or empty silastic capsules, on day 3 with progesterone or empty capsules, and on day 14 progesterone or empty capsules were removed. Forty-eight hrs after removal of the progesterone capsules, plasma OT was measured before and after i.v. injection of 10 micrograms/kg of CCK. At the completion of the study, pituitary glands were removed and OT peptide was measured. No significant differences were found between the sham and hormone-treated animals either in their basal or CCK-stimulated plasma OT levels or their pituitary content of OT peptide. Although sequential exposure to estradiol and progesterone followed by withdrawal of progesterone has been shown previously to increase PVN OT mRNA, neither pituitary OT immunoreactivity nor basal and CCK-stimulated release of plasma OT is affected by this treatment. Although the mechanism of this steroid effect is not yet understood, our observations suggest a unique action of gonadal steroids upon PVN OT neurons.

THYROIDECTOMY DOES NOT ALTER HYPOTHALAMIC OXYTOCIN AND VASOPRESSIN EXPRESSION IN CHRONICALLY HYPERNATREMIC RATS

Sustained hyperosmolality increases the levels of hypothalamic oxytocin (OT) and arginine vasopressin (AVP) messenger ribonucleic acids (mRNAs). Gonadectomy is known to abolish this response (12,18). In this study we investigated whether thyroidectomy would alter OT and AVP mRNA levels in the hypothalamic paraventricular nucleus (PVN) of the hyperosmotically stimulated rat. Male Sprague-Dawley rats underwent thyroidectomy (hypothyroid) or sham thyroidectomy (euthyroid) at 7 weeks of age. Three weeks later hypothyroid and euthyroid animals were administered 2% NaCl (6-11 days) or tap water and sacrificed at the end of the experiment. Northern blot hybridization was used to assess size and levels of hypothalamic OT and AVP mRNAs. Hypothyroid rats had significantly lower levels of serum thyroxine (T4) than their euthyroid cohorts (P < 0.0001). Both the euthyroid and the hypothyroid animals receiving 2% NaCl developed hypernatremia and increased the levels and the size of OT and AVP mRNAs compared to their tap water cohorts. We conclude that in contrast to gonadectomy, thyroidectomy does not alter the level of OT and AVP mRNAs in the hypothalamus of chronically hypernatremic male rats.

Progesterone regulates hypothalamic oxytocin mRNA levels through gamma aminobutyric acid.

Oxytocin (OT) messenger ribonucleic acid (mRNA) levels increase in the hypothalamic paraventricular nucleus (PVN) during late pregnancy, the second and third weeks of lactation, and with interruption of nursing for 48 h.1,2 Sequential exposure to estradiol (E2) and progesterone (P) followed by a decline in P accounts for these increases.1,2 This steroid regimen also increases PVN OT mRNA levels in virgin ovariectomized rats.1 The site of action and mechanism by which P influences OT mRNA is not understood, but P is known to exert effects within the CNS through its receptor or by binding to the gamma aminobutyric acid (GABA) A receptor and potentiating GABA tone.3 To determine if alterations in GABA tone influence steroid-induced increases in OT mRNA, we administered diazepam, a benzodiazepine agonist, to virgin, ovariectomized rats receiving this steroid regimen.