Stimuli for the release of neurohypophysial hormones

Relevance of cortisol and copeptin blood concentration changes in an experimental pain model

The effect of pain and analgesics on stress biomarkers is not well studied. We evaluated the effect of acute pain and analgesics on serum cortisol and copeptin in an experimental pain model in healthy volunteers. Healthy volunteers presented at 8 a.m. for an experimental pain stimulation. Cortisol and copeptin levels were measured before, during and after electrophysiological stimulation, first before and then during opioid delivery. Difference in biomarker levels compared to baseline levels was calculated, and potential influencing factors were evaluated by linear regression analysis. Cortisol decreased by 13% during the 10 min of rest at baseline, but copeptin did not change significantly. Cortisol had a median decrease of −24% or −83 nmol/l (−44 to −124 nmol/l, p = 0.0002) during the electrophysiological stimulation training session, while the median difference for copeptin was −22% or −1.01 pmol/l (−2.35 to 0.08 pmol/l, p = 0.0003). After administration of opioids, cortisol did not decrease but increased by 3% (p = 0.043), indicating an increasing opioids effect on cortisol. This effect was not visible for copeptin (median change −0.003 pmol/l (−0.50 to 0.24), p = 0.45). In this experimental pain model performed in the morning, moderate pain did not have a relevant effect on cortisol or copeptin levels, whereas opioids led to a discrete peak of cortisol.

Clinicaltrials.gov identifier: NCT01975753 (registered on November 5, 2013, before start of recruitment).

Oxytocin polyuria and polydipsia is blocked by NaCl administration in food-deprived male rats

We examined the effects of NaCl injections on the polydipsia and polyuria induced by subcutaneous oxytocin (OT) administration in food-deprived male rats. During the first 12 h of the treatment day, both food deprivation and OT administration increased urine excretion but reduced water intake, water balance (fluid intake minus urine volume) and body weight. OT treatment enhanced urine excretion and the reduction in water balance and body weight without reducing the water intake of food-deprived animals. Analysis of the physiological effects of OT administration showed increases in urinary sodium concentration, sodium excretion and a reduced plasma sodium concentration. During the second 12 h, OT increased both urine excretion and water intake in food-deprived but not in ad lib.-fed rats. However, hypertonic NaCl administration at the start of this second 12-h period blocked the polyuric and polydipsic responses observed in the OT/deprived group but increased the water intake of the ad lib. groups. After the whole 24-h period, animals treated with OT showed a water balance and body weight change matching those observed in Control animals. Although the recording time period is a critical factor to demonstrate the effect of peripheral OT administration on water intake, the results obtained suggest that the polyuric and polydipsic responses observed in food-deprived animals depend on the negative sodium and water balance induced by the natriuretic effect of OT and the unavailability of sodium. These OT-induced deficits can be counteracted by the administration of hypertonic NaCl solutions or simply by the intake of standard food.

Oxytocin, water intake, and food sodium availability in male rats

This study examined the effect of subcutaneous administration of the neurohormone oxytocin on water intake of ad lib-fed (with or without sodium availability in the diet) and food-deprived animals. Results of the first experiment showed that oxytocin increased water intake and urine excretion in food-deprived but not in ad lib-fed animals. However, oxytocin treatment did not modify the reduced water "balance" (fluid intake minus urine volume) resulting from food deprivation or the daily food intake (Experiment 1). The dose-dependent polydipsic effect of oxytocin on food-deprived rats was always preceded by an increase in sodium and fluid urine excretion (Experiment 2). Oxytocin also increased the water intake of animals fed ad lib with a low sodium diet (Experiment 3). These results suggest that the effect of oxytocin on water intake is dependent on the presence or absence of sodium in the diet and that the excretion of sodium is the main mechanism of oxytocinergic polydipsia in food-deprived male rats.

Visceral sensory inputs to the endocrine hypothalamus

Interoceptive feedback signals from the body are transmitted to hypothalamic neurons that control pituitary hormone release. This review article describes the organization of central neural pathways that convey ascending visceral sensory signals to endocrine neurons in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus in rats. A special emphasis is placed on viscerosensory inputs to corticotropin releasing factor (CRF)-containing PVN neurons that drive the hypothalamic-pituitary-adrenal axis, and on inputs to magnocellular PVN and SON neurons that release vasopressin (AVP) or oxytocin (OT) from the posterior pituitary. The postnatal development of these ascending pathways also is considered.

Control of neurosecretory vasopressin cells by noradrenergic projections of the caudal ventrolateral medulla

Activation of noradrenergic afferents arising from the A1 cell group of the caudal VLM excites neurosecretory AVP cells of both the supraoptic and paraventricular nuclei, thus stimulating the release of this potent vasoconstrictor into the circulation. Although this effect is mimicked by application of alpha 1-adrenoreceptor agonists to AVP cells, the excitatory effects of A1 afferents may not be mediated by activation of post-synaptic alpha 1-receptors. Evidence has also been obtained that the actions of A1 afferents are not dependent upon the release of excitatory amino acids or NPY, although the latter is co-stored with NA in A1 cells and potentiates the actions of low concentrations of NA on AVP cells. Although a projection to AVP and OXY neurosecretory cells from the A2 NA cell group of the NTS has been established, this projection does not appear to contribute directly to the control of SON AVP cell activity. Rather, NTS stimulation excites SON AVP cells via a relay projection through the A1 cell group. This pathway is likely to correspond to that involved in the stimulatory effects of haemorrhage and caval constriction on AVP secretion, although it is uncertain whether the effects of these particular stimuli are contingent upon unloading of arterial baroreceptors and atrial stretch receptors, as commonly presumed, or upon the activation of other receptors such as ventricular mechanoreceptors or chemoreceptors. On balance, current evidence suggests that the A1 projection is unlikely to be critically involved in mediating the effects of arterial baroreceptor, arterial chemoreceptor, or atrial stretch receptor activation on AVP cells.

Hormone-releasing stimuli do not alter Met-enkephalin levels in the rat neurohypophysis

The levels of met-enkephalin in the rat neurohypophysis were measured by radioimmunoassay in four groups of animals. The groups were a control untreated group, a sham operated group, a group subjected to 1% body weight haemorrhage which is an acute hormone releasing stimulus, and a group subjected to four days 2% saline imbibition which is a chronic hormone releasing stimulus. In none of the groups was the met-enkephalin level significantly different from the control value of 1.90 +/- 0.18 ng/gland (n = 10), suggesting that there was no depletion of met-enkephalin concomitant with the depletion of neurohypophysial hormones in the rat.

Effects of hormone-releasing stimuli on the area of the perivascular space in the neural lobe of the rat

Neural lobes from rats subjected to neurohypophysial hormone-releasing stimuli were examined electron microscopically following fixation in 4% tannic acid in 2.5% glutaraldehyde. This fixation allowed the delineation of the perivascular space in the neural lobe tissue. Measurement of the area of the perivascular space showed that it was significantly increased in the rats subjected to vagal stimulation and intraarterial calcium ions compared to the control rats. The rats which had been subjected to haemorrhage as a hormone-releasing stimulus did not show any significant change in the area of the perivascular space. The significance of these findings in relation to hormone release is discussed.

Electrophysiological differentiation of oxytocin- and vasopressin-secreting neurones

Antidromically identified neurosecretory cells of the paraventricular (p. v.) and supraoptic (s. o.) nuclei of the hypothalamus were recorded in lactating rats under urethane anaesthesia during reflex milk ejection (m. e.) and haemorrhage. Eighty one p. v. and s. o. neurones were studied. Their background firing rates ranged from < 0.1 to 6.3 spikes/s and three distinct patterns of activity were encountered: slow irregular (73%), fast continuous (10%) and phasic (17%). Forty units (49%) displayed a brief (2-4 s) high-frequency discharge (30-60 spikes/s) correlated with suckling-induced m. e., and these were classified as m. e. (oxytocin-secreting) neurones. The remainder of the cells showed no activation at this time and were classified as non-m. e. neurones. Ten m. e. neurones tested through haemorrhage (5 ml of blood) showed a gradual acceleration of firing rates, reaching a maximum of 3.7 ± 0.7 spikes/s (mean + s. e.) about 20 min after blood withdrawal. The firing pattern of the m. e. neurones therefore changed from a slow irregular to a fast continuous type. By contrast, 11 non-m. e. neurones tested with the same procedure showed a rapid activation reaching a maximum of 6.4 ±0.6 spikes/s by the fourth minute. Non-m. e. neurones which were initially of the slow irregular type, first became fast continuous and later evolved into a highly characteristic phasic pattern of activity which was never induced in the m. e. neurones. After the blood was replaced, all the cells returned to their original firing pattern. In a parallel series of experiments, plasma samples taken 5 and 25 min after haemorrhage showed a ten-fold elevation in antidiuretic activity. A slight but non-significant increase in m. e. activity was also observed. Thus p. v. and s. o. neurosecretory cells may be electrophysically differentiated into two functionally distinct populations: (1) oxytocin releasing neurones which show a high-frequency discharge before m. e. induced by suckling, and (2) vasopressin-releasing neurones which adopt a phasic pattern of firing during vasopressin release induced by haemorrhage. We suggest that the rate of vasopressin secretion into the circulation largely depends on the proportion of vasopressin neurones firing phasically, their firing rates within the phases and the duration and degree of synchronization of the phases.

Electrical activity in the supraoptic and paraventricular nuclei associated with neurohypophysial hormone release

1. Unit recordings were made from the hypothalamus in anaesthetized male rats using steel or glass micro-electrodes.2. Stimuli which are known to release vasopressin and oxytocin (electrical stimulation of the central end of the severed right vagus nerve and intracarotid injection of CaCl(2) solution) also excite units in the supraoptic and paraventricular nuclei. In addition, these units are excited by intracarotid injections of carbachol, acetylcholine and NaCl (5%) which are less effective stimuli for vasopressin release.3. These stimuli also excite units from which potentials can be evoked by stimulation of the pituitary stalk and which are likely to be neurosecretory neurones. Neurosecretory neurones can conduct electrical impulses like other less specialized nerve cells. Estimations of conduction velocity range between 0.4 and 1.3 m/sec.4. The proportion of units in the supraoptic and paraventricular nuclei which are excited and the degree of excitation corresponds approximately to the amount of vasopressin released by the stimuli. In addition, excitation of the supraoptic nucleus seems to be more directly associated with vasopressin release and excitation of the paraventricular nucleus with oxytocin release.