Mary L. Forsling

Oxytocin release following osmotic activation of oxytocin neurones in the paraventricular and supraoptic nuclei

1. Recordings were made from a total of 35 antidromically identified neurones in the paraventricular (PV) and supraoptic (SO) nuclei of urethane‐anaesthetized lactating rats. During recording plasma osmotic pressure was raised by 12 m‐osmole/kg by injection of hypertonic solutions of NaCl, LiCl, or mannitol.

Low-dose MDMA (“ecstasy”) induces vasopressin secretion

p=0·025, one tail). Mean initial cortisol concentration was 331·4 nmol/L (range 208·8–603·4 nmol/L), which increased, although not significantly (p>0·05), to 377·2 nmol/L (range 268·4–583·3 nmol/L) at 2 h. Street “ecstasy” frequently contains more than 100 mg of MDMA. In this study, a single relatively small dose caused an acute rise in AVP concentration at a time of day when it would not be expected to change. The rise in AVP was accompanied by a small fall in plasma sodium concentrations. The hyponatraemic illness experienced by some users is thus likely to be linked to the drug’s ability to stimulate secretion of AVP. Hence, if fluid intake is excessive, even a relatively small dose of MDMA could lead to symptoms of hyponatraemia. The rise in AVP does not seem to be part of a generalised stress response because there was no significant change in plasma cortisol concentration. It therefore seems that MDMA-induced hyponatraemia is unlikely to be due to a rare and idiosyncratic reaction, but results from a pharmacological effect compounded by excessive fluid ingestion. Animal studies show that MDMA stimulates the output of serotonin by serotonergic neurones, and AVP secretion is regulated by serotonergic pathways. The message is that those who take “ecstasy” or similar drugs may be at risk of hyponatraemia and should, therefore, avoid drinking fluid in excess of the body’s requirements. This may be difficult for users to estimate because MDMA reduces perception of thirst and impairs judgment.

The clearance and antidiuretic potency of neurohypophysial hormones in man, and their plasma binding and stability

1. Bio‐assay techniques have been used to measure plasma levels of neurohypophysial hormones in man, following either a single injection or a continuous infusion.

Vasopressin and oxytocin secretion in response to the consumption of ecstasy in a clubbing population

Despite the common use of MDMA (ecstasy) in the UK, the mechanism underlying associated potentially fatal cerebral oedema is unclear. We used a new experimental approach working directly with clubbers to perform a study on 30 (17 male) experienced clubbers (mean 6.6 years of clubbing). Pre and post-clubbing measurements were performed to compare plasma levels of pituitary hormones (vasopressin, oxytocin), plasma and urine osmolality, urinary pH, and plasma sodium and urea. Ecstasy consumption was con.rmed by using urinary drug screening preand post-clubbing. MDMA was detected in the urine samples of 17 subjects, three of which tested positive during pre-clubbing tests. Mean plasma vasopressin concentration increased in the MDMA group (1.28 ± 0.29 to 1.43 ± 0.41 pmol/l), but fell in other participants (1.23 ± 0.42 to 1.16 ± 0.0.34 pmol/l). Similarly, mean plasma oxytocin concentrations increased after ingestion of MDMA (2.02 ± 0.29 to 2.43 ± 0.24 pmol/l), but fell in the group that did not use MDMA (2.17 ± 0.36 pmol/l to 1.89 ± 0.37 pmol/l). There was a significant group by time interaction for plasma osmolality and plasma sodium (p= 0.001 and p= 0.003, respectively) and between change in urinary osmolality (p< 0.001) and MDMA use, with the pattern of change being consistent with the induction of inappropriate vasopressin secretion (also known as SIADH) by MDMA. This report demonstrates SIADH in ecstasy-using ‘clubbers’, which has important clinical implications.

Interleukin‐1β and lnterleukin‐6 Stimulate Neurohypophysial Hormone Release in vitro

Interleukin‐1 (IL‐1) and interleukin‐6 (IL‐6) have been reported to stimulate the release of corticotrophin‐releasing hormone (CRN) in vitro, the response being antagonized by the cyclo‐oxygenase inhibitor, indomethacin. The effects of cytokines on the other major ACTH‐releasing hormone, vasopressin (AVP), and the other neurohypophysial hormone, oxytocin, have been little studied, and the published data are conflicting. We have therefore used a previously validated rat hypothalamic expiant model to evaluate whether IL‐1β and IL‐6 can directly activate the AVP and oxytocin neurosecretory system. In addition, we have also investigated the effects of inhibition of cyclo‐oxygenase (CO) and lipoxygenase (LO) activities on the stimulated release of AVP and oxytocin by means of a series of antagonists, including a specific LO pathway inhibitor. The static rat hypothalamic incubation system used involves fresh hypothalamic expiants with consecutive 20‐min incubations, and estimation of AVP and oxytocin concentrations in the medium by specific and sensitive radioimmuno‐assays. It was found that IL‐1β produced a dose‐dependent increase in the release of AVP and oxytocin at doses of 10 and 100 U/ml (P<0.005). Only at the higher dose of 100 U/ml was IL‐6 able to increase significantly AVP and oxytocin release (P<0.05). These stimulatory effects of IL‐1β and IL‐6 were blocked by cyclo‐oxygenase inhibitors, indomethacin (28 μM) and ibuprofen (100 nM), but not by the lipooxygenase inhibitor, BW A4C (10 μg/ml), suggesting that prostaglandins are involved in this process. Thus, cytokines are clearly able to modulate the neurohypophysial system in vitro, the effects probably being mediated by cyclo‐oxygenase products.

Daily patterns of secretion of neurohypophysial hormones in man: effect of age

The neurohypophysial hormone vasopressin contributes to control of urine output and, while urine flow shows a clear daily rhythm, there has been debate as to whether this is true of neurohypophysial hormones. A study was performed on fifteen adult males, with a mean age of 25 years, over a 24 h period, nine blood samples being taken at regular intervals for the determination of neurohypophysial hormones and indices of fluid balance. Samples were taken via an indwelling cannula so that sleep was undisturbed. A daily variation in the plasma concentrations of oxytocin and vasopressin was demonstrated with concentrations reaching a nadir in the late afternoon. Concentrations of both hormones peaked at 02.00 h. Vasopressin concentrations were inversely correlated with packed cell volume, indicating that the altered hormone release was affecting fluid retention. Consistent with this was the observation that the relationship of plasma osmolality to vasopressin depended on the time of day. To determine the effect of ageing, a similar study was performed on nine healthy elderly subjects with a mean age of 70 years. The nocturnal peak of vasopressin was markedly attenuated, while oxytocin release was similar to that in the younger group. These observations confirm the existence of a daily rhythm in the plasma concentrations of neurohypophysial hormones and indicate that the amplitude of the vasopressin change decreases with age.

The effect of 3,4-methylenedioxymethamphetamine (MDMA, ?ecstasy?) and its metabolites on neurohypophysial hormone release from the isolated rat hypothalamus

Methylenedioxymethamphetamine (MDMA, “ecstasy”), widely used as a recreational drug, can produce hyponatraemia. The possibility that this could result from stimulation of vasopressin by MDMA or one of its metabolites has been investigated in vitro. Release of both oxytocin and vasopressin from isolated hypothalami obtained from male Wistar rats was determined under basal conditions and following potassium (40 mM) stimulation. The results were compared with those obtained for basal and stimulated release in the presence of MDMA or metabolites in the dose range 1 μM to 100 pM (n=5 – 8) using Students t‐test with Dunnetts correction for multiple comparisons. All compounds tested affected neurohypophysial hormone release, HMMA (4‐hydroxy‐3‐methoxymethamphetamine) and DHA (3,4‐dihydroxyamphetamine) being more active than MDMA, and DHMA (3,4‐dihydroxymethamphetamine) being the least active. The effect on vasopressin release was greater than that on oxytocin. In the presence of HMMA the ratio test:control for basal release increased for vasopressin from 1.1±0.16 to 2.7±0.44 (s.e.m., P<0.05) at 10 nM and for oxytocin from 1.0±0.05 to 1.6±0.12 in the same hypothalami. For MDMA the ratio increased to 1.5±0.27 for vasopressin and to 1.28±0.04 for oxytocin for 10 nM. MDMA and its metabolites can stimulate both oxytocin and vasopressin release in vitro, the response being dose dependent for each drug with HMMA being the most potent.

Diurnal rhythms in neurohypophysial function

The neurohypophysial hormones oxytocin and vasopressin show daily rhythms of secretion with elevated hormone release during the hours of sleep. This pattern can be modulated by ovarian steroids and alters with age. The pattern appears to be due in part to the nocturnal increase in melatonin secretion, which stimulates hormone release in man, while being inhibitory in the rat. Pinealectomy alters both the 24 h pattern of neurohypophysial hormone release in the rat and the firing rate of magnocellular supraoptic nucleus neurones. There is also a reduced hormone release in response to hypovolaemia and raised plasma sodium concentration compared to sham operated animals, with a smaller increase in neuronal activity, as determined by immediate‐early gene expression. The normal responses can be restored by nocturnal administration of melatonin. Melatonin also influences the neurohypophysial hormone response in the human to known stimuli of release, such as raised plasma osmolality, exercise and insulin‐induced hypoglycaemia. Recent studies have revealed that not only does the release of vasopressin and oxytocin vary over each 24 h, but the respective renal and pregnant uterine responses also show diurnal variations.

Activation of heme oxygenase and consequent carbon monoxide formation inhibits the release of arginine vasopressin from rat hypothalamic explants. Molecular linkage between heme catabolism and neuroendocrine function

Heme oxygenase (HO)-catalyzed degradation of cellular heme moieties generates biliverdin and equimolar amounts of carbon monoxide (CO), which has been implicated as a possible modulator of neural function. Technical difficulties preclude direct measurements of CO within intact nervous tissues; hence, alternative procedures are needed to monitor the formation and possible biologic functions of this gas. In the present study rat hypothalamic explants were found to generate 114 +/- 5 or 127 +/- 11 pmol biliverdin/hypothalamus/1 h (n = 3) upon incubation with 1 or 10 microM hemin, respectively. Ten micromolar zinc-protoporphyrin IX (Zn-PP-IX), a known inhibitor of HO, significantly decreased the degradation of 10 microM hemin from 127 +/- 11 to 26 +/- 11 pmol biliverdin/hypothalamus/1 h (n = 3; P < 0.01). Biliverdin was the principal product of HO-dependent heme degradation, as its possible conversion into bilirubin was precluded by hemin-dependent inhibition of biliverdin reductase. Basal or hemin-supplemented hypothalamic incubations were also shown to generate sizable amounts of propentdyopents (PDPs), reflecting HO-independent degradation pathways which do not liberate CO and cannot be inhibited by Zn-PP-IX. Plotting the ratio of biliverdin to PDPs thus provided an index of the efficiency with which hemin was degraded through biochemical pathways involving CO. Under the experimental conditions of our study, the biliverdin/PDPs ratio varied from 0 to 32 or 15%, depending on the absence or presence of 1 or 10 microM hemin respectively: this suggested that the formation of CO was most efficient at 1 microM hemin. Under these defined conditions, 1 microM hemin was also found to inhibit the release of arginine vasopressin (AVP) evoked by depolarizing solutions of KCl. A series of experiments showed that the effect of hemin was counteracted by Zn-PP-IX, and also by tin-mesoporphyrin IX, which is even more selective in inhibiting HO; it was also attenuated in the presence of the gaseous scavenger ferrous hemoglobin. Furthermore, the inhibition of AVP release could be reproduced by omitting hemin and by incubating hypothalami under CO, whereas treatment with biliverdin had no effect. This suggested that the release of AVP was suppressed by HO degradation of hemin, yielding CO as a modulator of hypothalamic function. These observations may be relevant to diseases characterized by inappropriate secretion of AVP and enzymatic disturbances affecting the synthesis of heme and the formation of CO through the HO pathway (e.g., acute intermittent porphyria or lead intoxication).

Melatonin administration and pituitary hormone secretion.

The relationship between the pineal gland and pituitary function remains controversial, while the role of melatonin in the adaptation of the organism to the light‐dark cycle of the environment is becoming increasingly recognized. The aim of this study was to investigate the effect of a manipulation of the melatonin rhythm on pituitary hormone secretion in man.