Two experiments were performed to study the effects of bromocriptine and α-ergocryptine on oxytocin secretion in lactating rats. In both experiments, after overnight separation from their litters, rats were injected with either vehicle alone or ergot alkaloid plus vehicle; 4 h later the litters were returned.

In the first experiment the mothers were conscious. Treatment did not affect suckling behaviour, number of stretch reactions or litter weight gain in the first 30 min. Oxytocin injection before the second 30 min period of suckling caused no extra milk to be obtained. In the second experiment the mothers were anaesthetized with ethyl carbamate (1·1 g/kg body weight) at the time of the ergot alkaloid or vehicle injection. Changes in intramammary pressure were recorded during suckling. Ergot alkaloids altered neither the number of milk ejections caused by suckling, nor the proportion of milk ejections equivalent to 0·2 mu. or more oxytocin.

In both experiments treatment with ergot alkaloids suppressed secretion of prolactin. It is concluded that (a) in suppressing lactation, bromocriptine and α-ergocryptine do not inhibit oxytocin secretion as well as prolactin secretion, and that (b) prolactin secretion is not a necessary concomitant of oxytocin secretion.

Osmoregulation is critical for survival in all vertebrates, yet the endocrine regulation of this metabolically expensive process is not fully understood. Specifically, the function of leptin in the regulation of energy expenditure in fishes, and among ectotherms, in general, remains unresolved. In this study, we examined the effects of acute salinity transfer (72 h) and the effects of leptin and cortisol on plasma metabolites and hepatic energy reserves in the euryhaline fish, the tilapia (Oreochromis mossambicus). Transfer to 2/3 seawater (23 ppt) significantly increased plasma glucose, amino acid, and lactate levels relative to those in the control fish. Plasma glucose levels were positively correlated with amino acid levels (R ²=0.614), but not with lactate levels. The mRNA expression of liver leptin A (lepa), leptin receptor (lepr), and hormone-sensitive and lipoprotein lipases (hsl and lpl) as well as triglyceride content increased during salinity transfer, but plasma free fatty acid and triglyceride levels remained unchanged. Both leptin and cortisol significantly increased plasma glucose levels in vivo, but only leptin decreased liver glycogen levels. Leptin decreased the expression of liver hsl and lpl mRNAs, whereas cortisol significantly increased the expression of these lipases. These findings suggest that hepatic glucose mobilization into the blood following an acute salinity challenge involves both glycogenolysis, induced by leptin, and subsequent gluconeogenesis of free amino acids. This is the first study to report that teleost leptin A has actions that are functionally distinct from those described in mammals acting as a potent hyperglycemic factor during osmotic stress, possibly in synergism with cortisol. These results suggest that the function of leptin may have diverged during the evolution of vertebrates, possibly reflecting differences in metabolic regulation between poikilotherms and homeotherms.

ABSTRACT

The effects of morphine dependence and abrupt opiate withdrawal on the release of oxytocin and corticotrophin-releasing factor-41 (CRF-41) into hypophysial portal vessel blood in rats anaesthetized with urethane were investigated. Adult female Sprague–Dawley rats were made dependent upon morphine by intracerebroventricular infusion of morphine for 5 days; abrupt opiate withdrawal was induced by injection of the opiate antagonist naloxone. The basal concentrations of oxytocin in portal or peripheral plasma from morphine-dependent rats did not differ significantly from those in control, vehicle-infused rats. In rats in which the pituitary gland was not removed after stalk section, the i.v. injection of naloxone hydrochloride (5 mg/kg) resulted in a large and sustained increase in the concentration of oxytocin in both portal and peripheral plasma in control and morphine-dependent rats. The i.v. injection of naloxone resulted in a threefold increase in the secretion of oxytocin into portal blood in acutely hypophysectomized rats infused with morphine, but did not alter oxytocin secretion in vehicle-infused hypophysectomized rats. The concentration of oxytocin in peripheral plasma in both vehicle- and morphine-infused hypophysectomized rats was at the limit of detection of the assay and was unchanged by the administration of naloxone. There were no significant differences in the secretion of CRF-41 into portal blood in vehicle- or morphine-infused hypophysectomized rats either before or after the administration of naloxone. These data show that, as for oxytocin release from the neurohypophysis into the systemic circulation, the mechanisms which regulate oxytocin release into the portal vessel blood can also be made morphine dependent. The lack of effect of morphine or naloxone on the release of CRF-41 or other stress neurohormones suggests that the effect of opiate dependence and withdrawal is selective for oxytocin and is not simply a non-specific response to 'stress'.

Journal of Endocrinology (1990) 124, 141–150

ABSTRACT

The involvement of the region anterior and ventral to the third ventricle (AV3V region) in the control of oxytocin release was investigated using electrical stimulation and electrolytic lesioning techniques in the rat. Electrical stimulation (0·5 mA, 50 Hz, 15–25 s) of the AV3V region of lactating rats evoked a reproducible rise in intramammary pressure equivalent to that induced by 0·25–0·5 mu. oxytocin (i.v.). Increases in circulating concentrations of oxytocin, as determined by specific radioimmunoassay, confirmed that AV3V stimulation released oxytocin in both lactating and non-lactating rats. The increases in plasma oxytocin concentration evoked by electrical stimulation of the AV3V region were dependent upon intensity and frequency of stimulation, and electrode position. A significant (P<0·05) increase followed stimulation at 25 Hz and 0·1 mA, and a maximal response was obtained with 50 Hz and 1·0 mA. Stimulation of the area in and around the nucleus medianus produced the greatest rise in oxytocin secretion.

The milk-ejection reflex was not abolished after acute electrolytic ablation of the AV3V region in urethane-anaesthetized lactating rats, but electrolytic lesion of the AV3V region prevented the increase in plasma oxytocin concentration which normally followed an osmotic stimulus (1 ml 1·5 mol NaCl/l, i.p.),

These studies provide evidence that the AV3V region is a major source of excitatory afferents to oxytocin neurones; this input is essential for the osmoresponsiveness of these neurones but plays little role in the control of such neurones during reflex milk ejection.

J. Endocr. (1987) 114, 253–261

ABSTRACT

The region anterior and ventral to the third ventricle (AV3V) region is a major source of excitatory afferents to the magnocellular neuroendocrine system, and is essential for the osmotically regulated release of oxytocin. We investigated whether this input has a similarly essential role in parturition. Rats were implanted with a guide cannula in the AV3V region on days 9–18 of pregnancy. Following the birth of the third pup, rats were anaesthetized briefly with ether and either given an electrolytic AV3V lesion or a sham procedure was carried out. In eight AV3V-lesioned rats the mean (± s.e.m.) median interbirth interval following the lesion was 6·3 ± 1·2 min compared with 5·2 ± 0·6 min in 11 sham-lesioned rats. All rats completed delivery of their litters. The mean plasma concentration of oxytocin was unchanged following the sham procedure (pre-sham 17·1±2·8 pmol/l, n = 8; 15 min post-sham 18·1±2·7 pmol/l, n = 8; 30 min post-sham 19·2 ± 3·5 pmol/l, n = 8). In AV3V-lesioned rats, plasma oxytocin was significantly raised following the lesion (pre-lesion 14·6±1·3 pmol/l, n = 7; post-lesion 58·3 ± 9·8 pmol/l, n = 7) and was still higher than the sham-treated group after 30 min (55·8 ± 9·9 pmol/l). Thus there was no significant difference in the time-course of parturition between AV3V-lesioned rats and sham-lesioned rats, and no evidence that the lesion impaired the release of oxytocin. Furthermore, in rats given an AV3V lesion on the morning of the expected day of delivery, parturition was neither delayed nor disrupted, suggesting that the AV3V region does not contribute to the mechanisms controlling the onset of parturition.

Journal of Endocrinology (1989) 121, 109–115

Treatment of rats with bromocriptine between days 5 and 8 after the post-partum mating resulted in suppression of serum prolactin levels and caused luteal regression. Although this treatment led to embryonic resorption when suckling was prevented by removing litters soon after birth, the diapausing embryos of animals nursing a litter of eight pups were unaffected by the treatment. These results suggest that the high levels of prolactin and progesterone in the circulation during lactation are not responsible for maintenance of the diapausing state.

SUMMARY

The effects of clomiphene citrate were studied in nine normal men, in three patients with partial panhypopituitarism, and in four patients with isolated gonadotrophin deficiency. The administration of this drug to the normal subjects in a dosage of 3 mg/kg/day for 10 days resulted in a mean rise in serum luteinizing hormone (LH) of 107%, in plasma 17β-hydroxyandrogens (17-OHA) of 114%, and in plasma total cortisol of 86%. The rise of testosterone concentration in normal subjects was associated with a doubling of the non-protein bound fraction, and also with increased binding of testosterone to sex hormone-binding globulin (SHBG). In contrast, plasma non-protein bound and urinary unconjugated cortisol remained unchanged. The percentage of plasma cortisol not bound to protein fell, indicating that the rise in total plasma cortisol was secondary to increased protein binding. This was confirmed by finding increased binding of both cortisol and testosterone to their specific binding globulins at 1 °C, due apparently to increased concentrations of SHBG and corticosteroid-binding globulin after clomiphene administration. All the responses to clomiphene were prevented by simultaneous administration of fluoxymesterone in two normal subjects.

All the hypopituitary patients showed no rise of LH, 17-OHA or cortisol. The hypogonadotrophic patients, however, showed a normal total cortisol rise.

It is suggested that clomiphene has two actions. First, it interferes with the hypothalamic feed-back mechanisms for testosterone, resulting in increased LH secretion, and secondly it has an oestrogen-like effect in stimulating production of steroid-binding globulins.

Leptin, critical in regulation of energy metabolism, is also important for normal bone growth, maturation and turnover. Compared to wild type (WT) mice, bone mass is lower in leptin-deficient ob/ob mice. Osteopenia in growing ob/ob mice is due to decreased bone accrual, and is associated with reduced longitudinal bone growth, impaired cancellous bone maturation and increased marrow adipose tissue (MAT). However, leptin deficiency also results in gonadal dysfunction, disrupting production of gonadal hormones which regulate bone growth and turnover. The present study evaluated the role of increased estrogen in mediating the effects of leptin on bone in ob/ob mice. Three-month-old female ob/ob mice were randomized into one of the 3 groups: (1) ob/ob + vehicle (veh), (2) ob/ob + leptin (leptin) or (3) ob/ob + leptin and the potent estrogen receptor antagonist ICI 182,780 (leptin + ICI). Age-matched WT mice received vehicle. Leptin (40 µg/mouse, daily) and ICI (10 µg/mouse, 2×/week) were administered by subcutaneous injection for 1 month and bone analyzed by X-ray absorptiometry, microcomputed tomography and static and dynamic histomorphometry. Uterine weight did not differ between ob/ob mice and ob/ob mice receiving leptin + ICI, indicating that ICI successfully blocked the uterine response to leptin-induced increases in estrogen levels. Compared to leptin-treated ob/ob mice, ob/ob mice receiving leptin + ICI had lower uterine weight; did not differ in weight loss, MAT or bone formation rate; and had higher longitudinal bone growth rate and cancellous bone volume fraction. We conclude that increased estrogen signaling following leptin treatment is dispensable for the positive actions of leptin on bone and may attenuate leptin-induced bone growth.

SUMMARY

Preliminary observations of the responsiveness of the fat-pad to crystal-line insulin have shown a straight-line response to the log dose from 5 to 250 μu./ml. incubate, in respect of either ¹⁴CO₂ or ¹⁴C-fat production from ¹⁴C-1-glucose in the incubate when the response is expressed as

The response in this same metameter to added normal serum has shown a similar slope for additions of serum from 0·0625 to 0·45 ml./ml. incubate.

In experiments on fasting dogs, venous serum has been sampled from pancreatic, hepatic and peripheral veins and assayed for insulin-like activity (i.l.a.) with and without added antibody, i.e. for total and 'atypical' i.l.a. Pancreatic venous serum contained about five times as much total i.l.a. as the other sites, but only 10% 'atypical' i.l.a., while the others contained 55—70% 'atypical' i.l.a.

Similar experiments on dogs after intravenous administration of glucose showed a five- to ninefold rise in total i.l.a. in all sites, but only three- to fourfold rise in 'atypical' i.l.a.

The 'atypical' i.l.a. resembled insulin in stimulating both ¹⁴CO₂ and ¹⁴C-fat production and in being neutralized by cysteine, yet it could not be attributed to the added antiserum (which assayed alone could only produce < 3–12% of this activity).

Leptin, the protein product of the ob gene, is essential for normal bone growth, maturation and turnover. Peripheral actions of leptin occur at lower serum levels of the hormone than central actions because entry of leptin into the central nervous system (CNS) is limited due to its saturable transport across the blood–brain barrier (BBB). We performed a study in mice to model the impact of leptin production associated with different levels of adiposity on bone formation and compared the response with well-established centrally mediated actions of the hormone on energy metabolism. Leptin was infused (0, 4, 12, 40, 140 or 400 ng/h) for 12 days into 6-week-old female ob/ob mice (n = 8/group) using sc-implanted osmotic pumps. Treatment resulted in a dose-associated increase in serum leptin. Bone formation parameters were increased at EC₅₀ infusion rates of 7–17 ng/h, whereas higher levels (EC₅₀, 40–80 ng/h) were required to similarly influence indices of energy metabolism. We then analyzed gene expression in tibia and hypothalamus at dose rates of 0, 12 and 140 ng/h; the latter dose resulted in serum leptin levels similar to WT mice. Infusion with 12 ng/h leptin increased the expression of genes associated with Jak/Stat signaling and bone formation in tibia with minimal effect on Jak/Stat signaling and neurotransmitters in hypothalamus. The results suggest that leptin acts peripherally to couple bone acquisition to energy availability and that limited transport across the BBB insures that the growth-promoting actions of peripheral leptin are not curtailed by the hormone’s CNS-mediated anorexigenic actions.