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G Leng

Publications and source records attributed to G Leng.

At least 145 records · Page 8Linked to original sources
PubMed1988

Hypothalamic opioid mechanisms controlling oxytocin neurones during parturition.

The influences of opioids on oxytocin secretion and parturition were investigated in the rat. Morphine, administered centrally or peripherally, severely delays the course of established parturition. This delay is accompanied by reduced plasma oxytocin levels and is overcome by treatment either with the opioid antagonist naloxone, or by infusion of oxytocin. An endogenous opioid regulatory mechanism inhibiting oxytocin secretion becomes activated immediately prior to and during parturition. This mechanism does not operate earlier in pregnancy or during normal lactation and is not seen in nonpregnant animals. Naloxone acutely speeds up the course of established parturition, an effect accompanied by greatly elevated plasma oxytocin levels. The mechanisms underlying opioid regulation of oxytocin neurones were investigated at two sites. Precipitated withdrawal from chronic morphine treatment causes hypersecretion of oxytocin. This response is mediated by greatly enhanced electrical activity in the perikarya of oxytocin neurones indicating the presence of opioid receptors on oxytocin neurones and/or on their afferent input. Opioid receptors are also present in the neurohypophysis where they exert direct and noradrenaline mediated effects on secretion from oxytocin terminals in vitro.

Animals↗
PubMed1988

The role of the AV3V region in the control of magnocellular oxytocin neurons.

The AV3V region is important in the control of body fluid and Na+ regulation and projects to the supraoptic and paraventricular nuclei. Oxytocin from the neurohypophysis mediates milk ejection and is involved in parturition, but has also been recently implicated as a candidate natriuretic hormone. We have studied the role of the AV3V region in the control of magnocellular oxytocin neurons in rats. Electrical stimulation of the AV3V region increased the firing rate of supraoptic oxytocin neurons and evoked a concomitant release of oxytocin. Acute electrolytic AV3V lesions silenced supraoptic neurons and abolished their excitation by hyperosmotic stimulation. The lesions also abolished osmotically-induced release of oxytocin. Re-activation of supraoptic neurons by local glutamate restored their osmoresponsiveness to about 50% normal. Thus, while supraoptic neurons are directly osmosensitive, the AV3V region is essential for their normal osmoresponsiveness. Electrolytic AV3V lesions did not affect suckling-induced oxytocin secretion or, in conscious rats, the release of oxytocin secretion during parturition. Thus the AV3V region is not involved in the activation of oxytocin neurons during suckling or parturition.

Action Potentials↗
PubMed1988

Naloxone excites oxytocin neurones in the supraoptic nucleus of lactating rats after chronic morphine treatment.

1. Lactating rats were implanted with a cannula in a lateral cerebral ventricle to deliver morphine (up to 50 micrograms/h) chronically from a subcutaneous osmotically driven mini-pump. After infusion of morphine for 5 days the rats were anaesthetized with urethane and prepared with ventral surgery for recording the electrical activity of single, antidromically identified neurones in the supraoptic nucleus. 2. A single I.V. injection of naloxone (5 mg/kg) in these rats provoked a long-lasting, large increase in intramammary pressure, but in control rats had negligible effects. Concentrations in plasma of oxytocin, measured by radioimmunoassay in samples of femoral arterial blood, rose from 44.7 +/- 2.5 to 1072.1 +/- 89.5 pg/ml (means +/- S.E.M.) 6 min after naloxone in the morphine-treated rats. In control rats, the concentration of oxytocin in plasma rose only from 42.1 +/- 2.9 to 125.1 +/- 28.2 pg/ml after naloxone. 3. Naloxone produced a transient increase in arterial blood pressure in morphine-treated but not control rats. Concentrations in plasma of vasopressin, measured by radioimmunoassay in samples of femoral arterial blood, rose in morphine-treated rats from 7.4 +/- 2.4 to 29.2 +/- 3.7 pg/ml after naloxone, but did not rise significantly in control rats. 4. Naloxone (1-5 mg/kg) produced a prompt and prolonged increase in the discharge rate of each of ten continuously active (putative oxytocin) cells recorded from ten morphine-treated rats. The discharge rate of the six cells tested at the highest dose (5 mg/kg) increased by an average of 6.3 Hz (360%) within 5 min, and the firing rate remained elevated for at least 30 min; the discharge rate of six continuously active supraoptic neurones recorded in control rats was not affected by naloxone. 5. The firing activity of five phasic (putative vasopressin) supraoptic neurones in morphine-treated rats was increased for at least 30 min by the injection of naloxone; these increases were the result of a raised intraburst firing rate with no change in burst duration or frequency. One phasic neurone was inhibited for 15 min, and one phasic neurone was unaffected. 6. The excitatory effects of naloxone on neurones in the supraoptic nucleus of morphine-treated rats were not explained by changes in blood pressure or osmolarity and did not depend on suckling or a cholinergic pathway. 7. The concentrations of oxytocin in plasma and the operation of the milk-ejection reflex were similar in the controls and morphine-treated rats, prior to naloxone. These findings indicate tolerance to initial inhibitory effects of morphine on oxytocin secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1988

Effects of raised extracellular potassium on the excitability of, and hormone release from, the isolated rat neurohypophysis.

1. Single neurohypophyses from male rats were maintained in an in vitro perifusion chamber. Ion-sensitive microelectrodes were introduced into the tissue to measure changes in [K+]o and [Ca2+]o during electrical stimulation. 2. Electrical stimulation at 6 Hz for 1 min and 30 Hz for 12 s raised [K+]o by 5.4 +/- 0.4 and 13.5 +/- 0.5 mM (mean +/- S.E.M., n = 8) respectively. To investigate the effects of raised [K+]o on the excitability of the neurosecretory terminals, stimulations were repeated in media of altered K+ concentration. The increase in [K+]o evoked by 6 Hz stimulation was elevated in 10 mM-K+ medium (133% of that in 5 mM-K+ medium) and reduced in 0 mM-K+ medium and in 25 mM-K+ medium. Thus it appeared that stimulus-induced changes in [K+]o might enhance the excitability of the tissue during electrical activation. 3. To test this hypothesis, we measured the field potential responses evoked by 0.5 Hz stimulation in media of different K+ concentrations. The size of the field potential was enhanced in 10 mM-K+ medium and depressed in 0 mM-K+ medium and in 25 mM-K+ medium. 4. Electrical stimulation (6 Hz, 1 min) decreased [Ca2+]o by 10.9 +/- 1.8% (n = 6). This decrease was absent in the presence of 1 microM-tetrodotoxin or 1 mM-cadmium. Again, the [Ca2+] response to stimulation was enhanced in 10 mM-K+ medium and depressed in 0 mM-K+ medium or 25 mM-K+ medium. 5. The release of vasopressin and oxytocin evoked by stimulation at 6 or 30 Hz from isolated neurohypophyses was measured by radioimmunoassay in a separate series of experiments. Stimulation at 30 Hz for 1 min released 5- to 6-fold more hormone than stimulation at 6 Hz for 5 min. Release evoked by 6 Hz stimulation was enhanced in 15 mM-K+ medium and depressed in 25 mM-K+ medium. 6. We conclude that the rise in [K+]o that accompanies high-frequency activation of axons and terminals in the neurohypophysis contributes to the facilitation of hormone release with increasing frequencies of stimulation, and in particular to the efficiency of the milk-ejection burst discharge of oxytocin neurones for evoking oxytocin release.

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PubMed1988

Spike propagation and conduction failure in the rat neural lobe.

1. Single units were recorded from the rat hypothalamo-neurohypophysial system in vivo to test the hypothesis that action potential conduction failure might contribute to the relative inefficiency of neurohypophysial hormone release at low frequencies of stimulation, and following prolonged stimulation. 2. Recordings were made from the cell bodies of supraoptic neurones which project to the neural lobe of the pituitary. Stimuli applied to the neural lobe evoked antidromic action potentials (in ten of forty cells) at times when the axonal membrane at the site of stimulation should have been refractory following the passage of a spontaneous, orthodromically conducted action potential. This observation suggests that failure of orthodromic action potentials may occur intermittently in the neural lobe. 3. Recordings from single units in the neural lobe showed similar spontaneous patterns of activity to those seen from cell bodies in the supraoptic nucleus. 4. Stimuli applied to the neural stalk evoked orthodromically conducted spikes in these single units: evoked spikes followed stimulation faithfully at 50-80 Hz for 1 s or at 20 Hz for 1 min. Such stimulation was accompanied by a reduction in spike height and a prolongation of latency. 5. Comparable changes were seen in the latency and amplitude of evoked potentials recorded from the neural lobe with low-resistance electrodes. 6. Stalk stimulation at 50 Hz for 1 s was accompanied by a reduction in the threshold for initiation of action potentials, suggesting an increase in the excitability of neural lobe axons. 7. We conclude that, during low-frequency activation, spike failure occurs intermittently in neurohypophysial axons, and that changes in the excitability of the axons during activation at high frequencies may contribute to the facilitation of neurohypophysial hormone release that occurs with increasing frequencies of stimulation.

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PubMed1988

Endogenous opioid actions and effects of environmental disturbance on parturition and oxytocin secretion in rats.

Blood samples were taken from conscious, chronically-catheterized rats during parturition for measurement of oxytocin by specific radioimmunoassay. After the birth of the 3rd pup, rats were allowed to remain in their nesting cage (undisturbed rats) or were transferred for 45 min to a glass bowl (disturbed rats); at the time of transfer, rats were given an i.v. injection of the opioid antagonist naloxone or saline vehicle. Subsequent parturition was prolonged in saline-treated disturbed rats, but not in naloxone-treated disturbed rats. Parturition was significantly hastened in naloxone-treated undisturbed rats. Naloxone injections were followed by a large rise in plasma oxytocin concentrations in disturbed and undisturbed rats. We conclude, from a statistical analysis of the relationship within experimental groups between plasma oxytocin concentration and speed of parturition, that the effects of disturbance and of naloxone upon parturition may be accounted for, at least in part, by their effects upon oxytocin release. However, the effects of disturbance on parturition may not be mediated entirely by activation of opioid pathways. Naloxone did not potentiate oxytocin release in non-pregnant rats, or on Day 1 post partum, but did potentiate oxytocin release on Day 22 of pregnancy even in rats before the onset of parturition. Endogenous opioid pathways regulating oxytocin release therefore appear to be active during late pregnancy and during parturition itself.

Animals↗
PubMed1987

Extracellular potassium changes in the rat neurohypophysis during activation of the magnocellular neurosecretory system.

1. Potassium-sensitive microelectrodes were used to measure extracellular [K+] in the isolated rat neurohypophysis maintained in vitro. Electrical stimulation of the neurohypophysial stalk (20 Hz 5 s) increased the inferred extracellular [K+] by 9.2 +/- 0.4 mM (mean +/- S.E. of mean; n = 21). 2. Veratridine (10 microM) enhanced the response to stalk stimulation, and at a higher concentration (50 microM) increased extracellular [K+] in the absence of stimulation. By contrast, tetrodotoxin (1 microM) blocked the [K+] increase completely and reversibly in each of five experiments, indicating that the increase was a consequence of action potential generation. 3. At the end of brief periods of stimulation, the raised extracellular [K+] returned to pre-stimulation levels within 30 s. In the presence of ouabain (100 microM), the recovery was slower: the half-decay time was extended by 150-300% in each of three experiments. 4. Replacement of calcium in the medium with cobalt, cadmium or magnesium reduced the amplitude of the [K+] increase by 26-30%, indicating that the [K+] increase was largely independent of events subsequent to evoked release of hormone and/or transmitters. 5. Potassium-sensitive microelectrodes were placed in the neurohypophysis of rats anaesthetized with urethane. Electrical stimulation of the pituitary stalk (50 Hz, 5 s) produced transient voltage increases of 7.6 +/- 0.9 mV (mean +/- S.E. of mean of seven experiments). These voltage increases were similar in magnitude to the response of the electrodes to the addition of 7.6 +/- 1.0 mM-K+ to rat plasma. 6. In seven lactating rats, the suckling of a litter of hungry pups evoked periodic reflex milk ejections, as detected by increases in intramammary pressure. Potassium-sensitive microelectrodes in the neurohypophysis recorded transient voltage increases prior to each milk ejection (0.4-5.5 mV). Each increase preceded an increase in intramammary pressure by 12-30 s. 7. Thus synchronized high-frequency activation of magnocellular neurones can produce large changes in extracellular [K+]. The implications of these findings for stimulus-secretion coupling in the neurohypophysis are discussed in the light of previous reports that hormone release from the neurohypophysis is highly dependent on the frequency and pattern of electrical stimulation.

Animals↗
PubMed1987

Stress-induced disruption of parturition in the rat may be mediated by endogenous opioids.

Plasma samples were obtained before and during parturition from conscious rats implanted chronically with a jugular cannula. Rats were either allowed to remain in their nesting cage throughout parturition, or were moved immediately following the birth of the second or third pup into an empty glass chamber. The time-course of parturition was prolonged for mother rats which were moved in mid-parturition to this unfamiliar environment. However, in rats given an i.v. injection of the opioid antagonist naloxone at the time of transfer, parturition followed a normal time-course, and plasma oxytocin levels were significantly higher than in animals injected with saline. Thus our hypothesis is that stress activates opioid pathways which delay parturition by inhibiting oxytocin release. Opioid-mediated mechanisms may similarly be responsible for some problems in human parturition.

Animals↗
PubMed1987

Control of magnocellular oxytocin neurones by the region anterior and ventral to the third ventricle (AV3V region) in rats.

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 less than 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.

Animals↗
PubMed1986

Regulation of the milk ejection reflex in the rat.

Extracellular recordings were made from neurones in or near the supraoptic nucleus in suckled lactating rats under urethane anaesthesia to investigate the mechanism by which the firing of oxytocin cells is synchronized during reflex milk ejection. Cells synaptically driven but not antidromically activated by neural stalk stimulation, which thus probably receive an afferent input from supraoptic neurones, were classified as 'regular' or 'bursters' on the basis of their spontaneous electrical activity. The majority (twelve out of eighteen) of synaptically excited cells (o.d.+) were bursters and the majority of inhibited (o.d.-) cells (eleven out of nineteen) were regular, but only one o.d.+ burster showed any change of activity (inhibition) before milk ejection. Putative oxytocin cells in suckled lactating rats showed a firing pattern between milk-ejection bursts which could not be distinguished from that of putative oxytocin cells in male animals. The mode interspike interval between milk ejections was 47.1 +/- 3.1 ms (mean +/- S.E. of mean) compared with 47.3 +/- 3.3 ms in male rats, and fewer than 1.4% of interspike intervals were less than 20 ms in duration. By contrast, within milk-ejection bursts 40% of interspike intervals were in the range 8-20 ms. Short trains (10 or 20) of pulses applied to the neural stalk at regular (5 min) intervals, in an attempt to simulate the initial part of the milk ejection burst, failed to trigger bursts. In only 2 of 150 tests was the interval between train and milk-ejection burst less than 10 s, and after the pulse train all but one cell showed reduced activity for 1-3 s. The trains of pulses were however not without effect: they significantly (P less than 0.01) enhanced the chance of a milk-ejection burst occurring within the next 2.5 min. Our observation that pulse trains do not trigger bursts suggests that local positive feed-back mechanisms are not responsible for orchestrating the activation of oxytocin cells during the milk-ejection reflex. Moreover, because spontaneous tiring pattern is the same in lactating and non-lactating rats, we found no evidence that the anatomical changes in the synaptic organization within the supraoptic nuclei in lactation have any influence on the firing of oxytocin cells. It is likely, however, since pulse trains alter the timing of milk ejections, that oxytocin released locally in the region of the supraoptic nucleus can influence reflex milk ejection.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1986

Vasopressin, but not oxytocin, is released in response to water deprivation in conscious goats.

Plasma samples obtained at 4-h intervals from goats for at least 24 h before and then during 24 h of deprivation of water were analysed by radioimmunoassay for vasopressin and oxytocin concentrations. The samples were also analysed for osmolality and sodium concentration. The differential effect of night/day versus day/night deprivation was also studied. During the two periods before the two deprivations osmolality varied in a regular manner, with low values occurring at 08.00 h. Sodium concentration followed osmolality, whereas vasopressin did not vary during the period before deprivation. During deprivation vasopressin increased along with osmolality and sodium concentration, with the beginning of the increase occurring after the morning feed. Oxytocin levels did not increase during the period of deprivation. These results do not support the hypothesis of general release of neurohypophysial hormones in response to osmotic stimuli but instead indicate there are species variations with respect to hormonal response to water deprivation.

Animals↗
PubMed1985

Neurohypophysial opioids and oxytocin secretion: source of inhibitory opioids.

When electrical stimuli are applied to the neural stalk of the pituitary, oxytocin, vasopressin, and probably several opioid peptides also contained in nerve terminals in the gland are released: one action of the released opioids appears to be to inhibit oxytocin release by an action that has been likened to pre-synaptic inhibition. Thus, when Clarke et al. (1979) stimulated the neural stalk following intravenous injection of the opioid antagonist naloxone, they observed that the evoked oxytocin release was potentiated. In the present study we confirm this result and show that oxytocin release evoked by stimulation of the supraoptic nucleus is similarly potentiated by naloxone. This finding is consistent with the hypothesis that the opioid responsible for inhibition of oxytocin release coexists with either oxytocin or vasopressin. We further report that the specific delta-receptor antagonist ICI 174864 does not potentiate oxytocin release either in vivo or in vitro. Thus, it seems unlikely that the enkephalins, putative delta-receptor agonists present in neurohypophysial fibres, are the opioids responsible for the observed inhibition of oxytocin release.

Animals↗
PubMed1985

Effects of opioid agonists and antagonists on oxytocin and vasopressin release in vitro.

The rat neurohypophysis contains both opioid receptors and substantial amounts of endogenous opioid peptides. Inhibitory influences of opioids on the secretion of both oxytocin and vasopressin have been described. We have examined the effects of a range of opioid agonists and antagonists with differing relative selectivities towards opioid receptor subclasses on the secretion of oxytocin and vasopressin from the isolated neurohypophysis. Oxytocin and vasopressin release evoked by brief periods of electrical stimulation in control experiments was compared to evoked release in the presence of test compounds. Oxytocin release was depressed approximately 25% by the delta-agonist (D-Ala2, D-Leu5)-enkephalin but not affected by putative kappa-agonists or by beta-endorphin. The use of opioid antagonists revealed a strong inhibition of oxytocin secretion by endogenous opioids released during electrical stimulation. Naloxone, relatively mu-selective, enhanced oxytocin secretion by up to 90% with a half-maximal effect at approximately 10(-6) M. MR2266, a relatively kappa-selective antagonist also enhanced oxytocin secretion but displayed agonist-like activity at high concentrations. ICI 154129, a delta-selective antagonist, was without effect on oxytocin secretion. Vasopressin release was unaffected by any of the agonists tested and not potentiated by antagonists at a range of stimulation frequencies. The data do not support the suggestion of an inhibitory endogenous opioid influence over vasopressin secretion within the neurohypophysis but indicate that an endogenous opioid peptide, possibly acting via mu- or kappa rather than delta-receptors, strongly suppresses the secretion of oxytocin.

Animals↗
PubMed1985

Supraoptic neurones in Brattleboro rats respond normally to changes in plasma osmotic pressure.

Brattleboro rats homozygous for hypothalamic diabetes insipidus (DI rats) were anaesthetized with urethane. Extracellular recordings were made from antidromically identified neurones of the supraoptic nucleus. About half (77 out of 153) of the neurones recorded in DI rats showed phasic patterns of discharge activity similar to those which are characteristic of vasopressin-secreting neurones in normal rats during hyperosmotic stimulation. Significantly fewer neurones showed phasic activity in DI rats which had been pretreated with vasopressin tannate at a dose which significantly reduced urine volume, water intake and plasma osmolality. Acute systemic hyperosmotic stimulation, induced by an i.p. injection of 1 ml 1.5 M-NaCl, increased the discharge rate of each of 14 neurones from DI rats by 1-5 spikes/s. Hypo-osmotic stimulation, induced by an intragastric injection of 10 ml tap water, reduced the discharge rate of each of four neurones from DI rats by 50% or more. We conclude that supraoptic neurones in DI rats respond normally to acute systemic osmotic stimuli despite the total absence of vasopressin in these rats and despite their chronically disturbed water balance. This implies that the osmoreceptor mechanism which drives the supraoptic nucleus does not adapt substantially during prolonged disturbance of water balance and functions outside the normal physiological range of plasma osmolality, and that the reported alteration of noradrenergic innervation of the neurones in DI rats does not affect their osmotic responsiveness.

Action Potentials↗
PubMed1985

Central opioids: a possible role in parturition?

Pregnant rats were implanted with subcutaneous minipumps to deliver either naloxone or saline. The time-course of subsequent parturition was different between the two groups: the interval between successive births was significantly shorter for the naloxone-treated rats. This supports the hypothesis that the opioid innervation of the neurohypophysis, which is known to influence oxytocin release profoundly, has a physiological role in parturition. To test the further hypothesis that this role is particularly important in a stressful environment, pregnant rats, again implanted with minipumps, were regularly transferred, at 15-min intervals beginning with the birth of the first pup, between their normal cage and the unfamiliar environment of a glass observation chamber. No difference was noted between the time-courses of parturition for the naloxone- and saline-treated groups, although the time-courses were markedly altered from those observed in rats not subjected to an unfamiliar environment. We conclude that opioid modulation of oxytocin release may play a role in 'spacing' the delivery of successive births during normal parturition.

Animals↗
PubMed1984

Altered baroreceptor inputs to the supraoptic nucleus of the Brattleboro rat.

In Brattleboro rats, many more supraoptic neurones are baroresponsive than in normal rats. This may reflect the more widespread noradrenergic innervation of the supraoptic nucleus in Brattleboro rats. If so, the present data suggest that baroreceptor influences on vasopressin secretion are mediated by a noradrenergic pathway and that the altered responsiveness reflects the altered innervation.

Action Potentials↗
PubMed1984

Complex action potential waveform recorded from supraoptic and paraventricular neurones of the rat: evidence for sodium and calcium spike components at different membrane sites.

Magnocellular neurones of the supraoptic and paraventricular nuclei display a complex waveform when recorded extracellularly. The present work has examined the possible reasons for the complexity of this waveform by making extracellular recordings from antidromically identified neurones in the anaesthetized rat and extra- and intracellular recordings from similar neurones in the hypothalamic slice preparation, where external solutions can be changed. In extracellularly recorded units, action potentials had two positive going components. The first of these was abolished by tetrodotoxin. The second, slower component was abolished when external Ca++ concentration was lowered, but enhanced in magnitude and duration when Ba++ was placed in the external medium. The second component was sensitive to electrode movement and was not observed to the same degree when intracellular recordings were made. In light of these observations and the known morphology of these neurones, it is suggested that the magnocellular neurone action potential is comprised of a fast Na+-dependent component and a slower component dependent on Ca++ entry which may originate from a part of the cell other than the soma. The most likely site for this Ca++ component to occur is at the cell dendrite.

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