Effects of hyperprolactinemia on plasma glucose and prolactin in rats exposed to ether stress

The effects of inhalation anaesthetics on common clinical pathology parameters in laboratory rats

Effects of common anaesthetics such as ether, methoxyflurane, isoflurane, carbon dioxide (at 100%, 80% or 60% admixed with O(2)) on toxicity and clinical pathology parameters in rats were investigated. Ether, methoxyflurane and 100% CO(2) induced toxicity in some animals. Erythrocyte, haemoglobin and haematocrit were reduced in females by 100% CO(2), methoxyflurane and isoflurane. Glucose was increased by 60% CO(2), 80% CO(2), ether, isoflurane and methoxyflurane in males. Chloride was reduced by isoflurane and all CO(2) concentrations in females. Serum proteins were reduced by isoflurane and methoxyflurane. Sodium, inorganic phosphate, calcium and magnesium were reduced by methoxyflurane and isoflurane, but increased by all CO(2) concentrations. Potassium was reduced by ether, methoxyflurane or isoflurane. Triiodothyronine and thyroxine were reduced by all anaesthetics. Prolactin was reduced by methoxyflurane, but raised by ether and isoflurane. Erythrocyte cholinesterase (E-ChE) activity is markedly reduced (20-40%) after anaesthesia with all CO(2) concentrations in both sexes. E-ChE was unaffected by ether, methoxyflurane, or isoflurane. Serum and brain cholinesterase activities were not affected. E-ChE inhibition correlated with decreased blood pH, suggesting that this was caused by acidosis. This is of practical relevance in the risk assessment of cholinesterase inhibitors.

CONCLUSIONS

Clinical pathology data were affected by all anaesthetics. CO(2)/O(2) (80%/20%) and isoflurane are the most suitable anaesthetics. If E-ChE activity is to be determined, isoflurane is the anaesthetic of choice.

Ovarian steroids but not the locus coeruleus regulate stress-induced prolactin secretion in female rats

Stress has been proposed to stimulate prolactin release if its prestress levels are low, or to inhibit it if they are elevated, but the role of ovarian-steroid fluctuations in the prolactin stress response is not yet clearly understood. Because the noradrenergic nucleus locus coeruleus has been implicated in stress responses and generation of prolactin surges in female rats, the present study aimed to evaluate stress-induced prolactin secretion under different hormonal conditions, determining the effect of locus coeruleus lesion on each response. Blood samples were withdrawn from a jugular vein catheter 5 and 2 min before and 2, 5, 10, 15 and 30 min after ether stress in male rats, female rats during the oestrous cycle and ovariectomised rats treated with oil (OVX), oestradiol (OVE) or oestradiol plus progesterone (OVEP). Increased Fos immunoreactivity demonstrated locus coeruleus activation following ether stress. Ether stress increased both low (at 16.00 h in males, in OVX and on dioestrous and at 11.00 h on pro-oestrous and oestrous) and high plasma prolactin (at 16.00 h on oestrous and in OVE), but it decreased elevated prolactin levels during the afternoon on pro-oestrous and in OVEP. Locus coeruleus lesion prevented prolactin surges during the afternoon on pro-oestrous, oestrous, OVE and OVEP but did not modify either pattern (i.e. increase or decrease) or degree of prolactin stress response under any condition studied. The present data therefore suggest that oestradiol and progesterone modulate stress-induced prolactin secretion, regardless of its prestress levels. Moreover, the locus coeruleus is probably not involved in prolactin response to stress and most likely has a specific role in prolactin surges induced by ovarian steroids.

Effect of the pretreatment with prolactin on the distribution of immunoreactive beta-endorphin through different ovarian compartments in immature, superovulated rats

Beta-endorphin and prolactin (PRL) are natural inhibitors of ovulation via central and peripheral mechanisms, but their possible interactions within the ovary are still unknown. The aims of the present study were to determine the gene expression and the topographic distribution of beta-endorphin, and the possible changes evoked by the pretreatment with PRL on the ovarian beta-endorphin localization in immature, superovulated rats. Prepuberal female Wistar rats weighing 60-70 g were superovulated with 20 IU equine gonadotrophins and, 48 h later, 20 IU human chorionic gonalotropin (hCG). Four hours after the hCG injection, the rats received either 200 microg rat PRL .i.p. (n = 12) or saline vehicle (n = 10). In the following morning the rats were killed and their ovaries were quickly removed. Beta-endorphin localization was assessed by immunohistochemistry and proopiomelanocortin (POMC) mRNA was detected by reverse transcription polymerase chain reaction (RT-PCR). Beta-endorphin was expressed mostly in the corpora lutea and perivascular stroma, but a weak to moderate immunostaining was also present in the theca cells and some granulosa cells of tertiary/antral and preovulatory follicles. The main differences observed in the distribution of ovarian beta-endorphin between the two groups were a more intense immunostaining in the granulosa cells of antral follicles, corpus luteum and stroma of PRL-treated rats. POMC gene transcripts were detected in 2/5 samples from the control group and in 3/7 samples from the PRL-treated group. Thus, the expression of beta-endorphin in tertiary/antral follicles is enhanced by PRL treatment in immature, superovulated rats, providing a putative mechanism by which PRL could inhibit the ovarian response to induced ovulation.

Targeted deletion of the PRL receptor: effects on islet development, insulin production, and glucose tolerance

PRL and placental lactogen (PL) stimulate beta-cell proliferation and insulin gene transcription in isolated islets and rat insulinoma cells, but the roles of the lactogenic hormones in islet development and insulin production in vivo remain unclear. To clarify the roles of the lactogens in pancreatic development and function, we measured islet density (number of islets/cm(2)) and mean islet size, beta-cell mass, pancreatic insulin mRNA levels, islet insulin content, and the insulin secretory response to glucose in an experimental model of lactogen resistance: the PRL receptor (PRLR)-deficient mouse. We then measured plasma glucose concentrations after ip injections of glucose or insulin. Compared with wild-type littermates, PRLR-deficient mice had 26-42% reductions (P < 0.01) in islet density and beta-cell mass. The reductions in islet density and beta-cell mass were noted as early as 3 wk of age and persisted through 8 months of age and were observed in both male and female mice. Pancreatic islets of PRLR-deficient mice were smaller than those of wild-type mice at weaning but not in adulthood. Pancreatic insulin mRNA levels were 20-30% lower (P < 0.05) in adult PRLR-deficient mice than in wild-type mice, and the insulin content of isolated islets was reduced by 16-25%. The insulin secretory response to ip glucose was blunted in PRLR-deficient males in vivo (P < 0.05) and in isolated islets of PRLR-deficient females and males in vitro (P < 0.01). Fasting blood glucose concentrations in PRLR-deficient mice were normal, but glucose levels after an ip glucose load were 10-20% higher (P < 0.02) than those in wild-type mice. On the other hand, the glucose response to ip insulin was normal. Our observations establish a physiologic role for lactogens in islet development and function.

Prolactin release during exercise in normal and adrenodemedullated untrained rats submitted to central cholinergic blockade with atropine

To study the role of the central cholinergic system in pituitary prolactin (PRL) release during exercise we injected atropine (5 x 10(-7) mol) into the lateral cerebral ventricle of intact or adrenodemedullated (ADM) untrained rats, at rest or submitted to exercise on a treadmill (18 m x min(-1), 5% grade) until exhaustion. The rats were implanted with chronic jugular catheters for blood sampling and with unilateral intracerebroventricular (icv) cannulas placed in the right lateral ventricle. Blood prolactin concentrations were measured before and every 10 min after the start of exercise for a period of 60 min. After the animals started running, plasma prolactin levels rose rapidly in both normal and ADM rats, reaching near maximum at 10 min. Close to exhaustion (19.8 +/- 2.9 min for intact rats and 23.5 +/- 4.1 min for ADM) they were still high, remained increased until 30 min, and returned to preexercise levels at 40 min. Icv injections of atropine decreased the time to exhaustion by 67% in intact rats and by 96.2% in ADM and also reduced the exercise-induced PRL release in both intact (50%) and ADM rats (90%). The results showed that prolactin release induced by exercise was dependent on the exercise workload and could be observed as early as after 10 min of running, remaining increased until 30 min. These data indicate that adrenodemedullation does not affect prolactin secretion induced by exercise, although adrenodemedullated rats proved to be more sensitive to the reducing effect of central cholinergic blockade on their maximal capacity for exercise.

Bromocriptine-induced dissociation of hyperglycemia and prolactin response to restraint

The present study investigated the effects of immobilization (restraint stress) on rat chronically treated with a D(2) receptor agonist (bromocriptine, 0.4 mg/100 g body weight, injected daily intraperitoneally (ip) for 2 weeks) on plasma glucose, prolactin, and insulin levels. During restraint, the plasma prolactin of vehicle-treated (VEH) rats increased rapidly, reaching a peak at 10 min (57.9 +/- 8.1 ng/ml, P < .01). In contrast, restraint failed to induce any significant change in the plasma prolactin levels of bromocriptine-treated (BR) rats. The hyperglycemic response to immobilization was 97% higher (P < .05) in BR rats than in VEH rats. Our data demonstrate that prolactin secretion and hyperglycemia in response to restraint can be dissociated by chronic treatment with BR, which also increased the hyperglycemic response to immobilization probably due to central D(2) dopaminergic activity.

Paradoxical effect of imipramine in hyperprolactinemic female rats exposed to the forced swimming test

This study was designed to investigate the effect of hyperprolactinemia, with high or low estrogen levels, on the response to imipramine in the forced swimming test. Three groups of female rats were studied: (1) ovariectomized controls, with low serum prolactin (PRL) and estrogen levels, (2) ovariectomized, estrogen-treated rats, with high PRL and high estrogen levels, and (3) pituitary-grafted rats, with high PRL and low estrogen levels. The hyperprolactinemic groups did not show significant behavioral changes in the forced swimming test preceded by saline injection. Imipramine decreased the immobility time by 37.5% in ovariectomized controls but not in the pituitary-grafted group, and there was an increment of 48.4% in immobility time following imipramine administration in the estrogen-treated group (p<0.05). This paradoxical response to imipramine was significantly correlated with serum PRL (r = 0.59, p<0.01) but not with estradiol levels. These findings suggest that, at least in female rats submitted to the forced swimming model, PRL may induce reversed behavioral effects in response to imipramine, independently of circulating estrogen levels.

Evidence for sexual differences in the preoptic area regulation of blood glucose in rats

The effect of noradrenaline (NA) injection (20 or 40 nmol) into the preoptic area (POA) on plasma glucose and insulin was studied in male and female rats. The rats were implanted with chronic jugular catheters for blood sampling and unilateral intracerebral cannulas placed just above the POA. Blood samples were taken before and at 5, 10, 15, 30 and 60 min after NA injection. As early as 5 min after NA injection, plasma glucose levels rose rapidly in both male and female rats, reaching a peak at 15 min poststimulus. NA injection into the POA caused a dose-dependent hyperglycemic response in both male and female rats, although the response was more intense and longer lasting in females than in males. However, NA injection into the POA induced an increase in plasma insulin concentration in male but not in female rats. In addition, the increase in plasma glucose induced by 40 nmol NA injection in males preceded that of insulin. Plasma levels of glucose after POA injection of NA were already significantly elevated (p < 0.01) within the first experimental interval (5 min), whereas a plasma insulin increase were first detected 15 min post injection. We conclude that, when administered locally into the POA, NA can activate the sympathetic outflow expressed by a neurally mediated hyperglycemia which is more intense in females than in males. These data demonstrate that the POA has a sexually differentiated function in the regulation of glycemia.

Blood glucose and prolactin in hyperprolactinemic rats exposed to restraint and surgical stress

The effects of chronic hyperprolactinemia on plasma prolactin (PRL) and glucose were investigated in male rats submitted to two different types of stress: restraint (60 min in a plastic tube) or surgery (laparotomy under ether anesthesia). Hyperprolactinemia was induced by grafting one homologous pituitary gland under the kidney capsule. Restraint stress induced a marked increase of plasma PRL of control rats with a peak at 15 min (increase of 403%), but did not change the PRL levels of hyperprolactinemic rats. Plasma glucose levels of both groups were elevated by restraint stress at 5 min (control, 26%; grafted, 63%), and remained above basal levels during the whole experimental period. However, at 15 min the hyperglycemic response of the grafted rats was higher than that of control rats (p<0.05). Surgical stress induced a 204% increase of plasma PRL at 5 min in the control group, but failed to induce alterations of PRL in the hyperprolactinemic group. Plasma glucose was remarkably elevated at 15 min both in control (138%) and grafted (124%) rats after surgery, producing a hyperglycemic response much more intense than that induced by restraint. Grafted rats presented hyperglycemia during all the experimental period, whereas control rats showed glycemia similar to basal levels by the end of the experiment. In conclusion, different responses are induced depending on the type of stress: more intense PRL secretion is induced by restraint and higher hyperglycemia by surgery. Chronic hyperprolactinemia induced a higher (restraint) or longer lasting (surgery) hyperglycemic response in the rat, adding new evidence for a diabetogenic effect of PRL.