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Biomedical subjects

R E Dyball

Publications and source records attributed to R E Dyball.

At least 19 recordsLinked to original sources
PubMed1992

Naloxone potentiates the release of oxytocin induced by systemic administration of cholecystokinin without enhancing the electrical activity of supraoptic oxytocin neurones.

Studies performed in conscious female rats confirmed that iv injection of cholecystokinin octapeptide (CCK; 20 mu/kg) increased the circulating concentration of oxytocin but not that of vasopressin, and confirmed that the stimulation of oxytocin release was markedly facilitated after iv administration of naloxone (1 mg/kg), indicating attenuation of oxytocin release by endogenous opioids. To investigate the site of action of the endogenous opioids, the electrical activity of putative oxytocin neurones in the supraoptic nucleus was recorded in urethane-anaesthetised female rats. Oxytocin neurones responded to CCK injection with an increase in firing rate lasting 5-15 min, but this response was not facilitated by prior injection of naloxone. The results suggest that the opioid influence upon CCK-induced oxytocin release operates at the level of the neurosecretory terminals in the neurohypophysis rather than centrally. Since CCK does not elevate vasopressin release, it appears unlikely that dynorphin, the opioid peptide co-existing with vasopressin, is responsible in these circumstances for the cross-inhibition of oxytocin release. It is suggested that products of proenkephalin A, the met-enkephalin precursor present in the supraoptic nucleus and in the neurohypophysis itself, may be active in the regulation of oxytocin release.

Anesthesia↗
PubMed1992

Mechanisms of vasopressin secretion.

The magnocellular vasopressin system of the rat has been studied intensively in recent years. This review outlines the electrophysiological characteristics of vasopressin neurons, the characteristics of stimulus-secretion coupling in the neural lobe, and describes some of the major features of the neural regulation of this system which underlie physiological regulation of vasopressin release by osmoregulatory stimuli. The major afferent pathways to the magnocellular system are now well characterised. Those involved in osmoregulation have been mapped using expression of the primary response gene c-fos as a marker for neuronal activation.

Afferent Pathways↗
PubMed1992

Expansion of plasma volume by intragastric isotonic saline inhibits supraoptic neurones in rats.

To determine whether an increase in plasma volume might directly influence supraoptic neurones, single cell extracellular recordings were made from magnocellular neurones of the supraoptic nucleus in urethane-anaesthetized rats as plasma volume was expanded by intragastric injection of isotonic saline. Continuous ratemeter records taken before, during and after intragastric injections of 10 ml isotonic saline showed that the firing rate of putative vasopressin cells was reduced by 2.21 spikes/s (P < 0.02; n = 9; paired t-test) after 50 min. Putative oxytocin cells, after an initial increase in firing rate which lasted approximately 30 min, showed a decrease of 0.98 spikes/s (P < 0.02; n = 6; paired t-test). A population of 93 control cells of both types had a median firing rate of 4.69 spikes/s, a comparable group of 65 cells recorded 1 h after intragastric injection had a median firing rate of 3.15 spikes/s and another group of 68 cells recorded 1 h after a second injection had a median firing rate of 2.5 spikes/s. These differences were significant (P < 0.04 and P < 0.01; Mann-Whitney U test). The haematocrit of plasma samples taken from five similarly anaesthetized control animals was 49.7%. One hour after one intragastric injection the value was significantly (P < 0.02; paired t-test) reduced to 46.7% and 1 h after a second injection it was further reduced to 42.1% (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1979

Phasic firing enhances vasopressin release from the rat neurohypophysis.

1. Isolated rat neural lobes were incubated in vitro and electrically stimulated to release vasopressin. The released vasopressin was assayed using a radioimmunoassay and there was a reasonably good correlation (r = 0.81) between results obtained with this assay and those obtained by bioassay with the rat blood pressure method.2. Regular stimulation at frequencies of 5, 10 and 20 Hz released progressively more vasopressin and the release could be blocked by addition of tetrodotoxin to the incubation medium.3. Stimulation with pulse patterns derived from tape recordings of phasically firing units in the supraoptic nucleus of dehydrated rats released more vasopressin than the same number of pulses regularly spaced in time. In the range 2-8 pulses/sec vasopressin release was related to the pulse frequency within the bursts (r = 0.90) and the number of short (< 100 msec) interpulse intervals (r = 0.92). Vasopressin released per pulse increased over the frequency range 3-6 pulses/sec, but above 6 pulses/sec vasopressin release per pulse tended to diminish.4. We conclude that phasic firing of vasopressin neurosecretory cells may enhance vasopressin release in vivo and that an important factor in determining release is the number of short interspike intervals.

Action Potentials↗
PubMed1978

Effects of veratridine on Ca fluxes and the release of oxytocin and vasopressin from the isolated rat neurohypophysis.

Uptake of radioactive calcium, 45Ca efflux, and hormone release from the isolated rat neurohypophysis were monitored in vitro after the addition of veratridine to the incubation medium. Veratridine dramatically increased hormone release, but the release was not sustained and had declined by about 90% after 2 h. Removal of external Na+ prevented hormone release as did addition to the incubation medium of tetrodotoxin or the calcium antagonists D600 and Mn2+ ions. Veratridine increased 45Ca uptake into the isolated neurohypophysis and the increase could be prevented by addition of tetrodotoxin or D600 to the medium. Efflux of 45Ca was not changed by addition of veratridine. The results underline the importance of both Na+ and Ca+2 channels in the regulation of secretion of neurosecretory products.

Animals↗
PubMed1978

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.2. Nine PV neurones (mean firing rate 4.2 +/- 1.0 (S.E.) spikes/sec) were classified as oxytocin cells because they gave a burst of activity before reflex milk-ejections. None of these showed a bursting (phasic) firing pattern. Ten PV neurones (mean firing rate 1.8 +/- 0.2 spikes/sec) fired phasically either before or after injection of hypertonic NaCl and were classified as vasopressin cells. The remaining six PV cells (mean firing rate 1.6 +/- 0.9 spikes/sec) showed no bursts of firing related to milk ejection and did not fire phasically.3. Increasing plasma osmotic pressure by injection of hypertonic NaCl increased the mean firing rate of PV oxytocin cells to 7.0 +/- 1.0 spikes/sec. Vasopressin cells in the PV nucleus were much less responsive and the mean firing rate after injection was 2.9 +/- 0.4 spikes/sec. The third group of PV neurones was unresponsive.4. Plasma oxytocin concentration (determined by radioimmunoassay) increased from 2.1 +/- 0.3 muu./ml. in the control period to 10.9 +/- 2.8 muu./ml. 30 min after I.P. injection of 1 ml. 1.5 M-NaCl and to 14.8 +/- 2.8 muu./ml. following injection of a second 1 ml. 1.5 M-NaCl.5. The responses of oxytocin and vasopressin neurones in the SO nucleus to an increase in plasma osmotic pressure following injections of hypertonic solutions of LiCl or mannitol were similar to those observed when plasma osmotic pressure was raised by NaCl.6. It may be concluded that both oxytocin and vasopressin cells in the neurohypophysical system are responsive to the osmotic pressure of the blood plasma rather than to Na(+) or Cl(-) concentration, that osmotic activation of oxytocin cells releases sufficient oxytocin to increase its plasma concentration, and that there may be a functional difference between the SO and PV nuclei.

Action Potentials↗
PubMed1977

Characterization of the responses of oxytocin- and vasopressin-secreting neurones in the supraoptic nucleus to osmotic stimulation.

1. Extracellular action potentials were recorded from forty antidromically identified single units in the supraoptic nucleus of lactating, urethane-anaesthetized female rats. The activity was monitored both during reflex milk ejection and during an increase of 10-15 m-osmole/kg in plasma osmotic pressure induced by intraperitoneal injection of 1 ml. of 1.5 M-NaCl solution.2. About half (eighteen) the cells showed a burst of activity before reflex milk ejection and were dubbed oxytocin cells. Oxytocin cells responded to a hypertonic injection with a smooth sustained threefold increase in firing rate.3. The remainder (twenty-two) showed no burst of activity before reflex milk ejection and were dubbed vasopressin cells. Vasopressin cells doubled their firing rate as plasma osmotic pressure increased. Neither cell type increased its firing rate after injections of isotonic NaCl.4. A phasic firing pattern was rarely seen in slow firing vasopressin cells (< 2 spikes/sec) but was seen in almost all vasopressin cells (twelve out of fourteen) firing between 3 and 8 spikes/sec. Above 8 spikes/sec, some vasopressin cells fired continuously. Phasic firing was only once encountered in an oxytocin cell.5. The firing rate of both oxytocin and vasopressin cells decreased when plasma osmotic pressure was reduced 10-15 m-osmole/kg by an intragastric water load of 10 ml.6. Hypothalamic cells lying just outside the supraoptic nucleus did not show a consistent response to injection of hypertonic NaCl.7. Clearly, both oxytocin and vasopressin cells are osmoresponsive, but phasic firing is characteristic of stimulated vasopressin cells. Thus, osmotic activation allows discrimination between oxytocin- and vasopressin-secreting neurones.

Action Potentials↗