Blood levels of oxytocin and vasopressin during suckling in the rabbit and the problem of their independent release

Blunted HPA axis response in lactating, vasopressin-deficient Brattleboro rats

Adaptation to stress is a basic phenomenon in mammalian life that is mandatorily associated with the activity of the hypothalamic-pituitary-adrenal (HPA) axis. An increased resting activity of the HPA axis can be measured during pregnancy and lactation, suggesting that these reproductive states lead to chronic load in females. In this study, we examined the consequences of the congenital lack of vasopressin on the activity of the HPA axis during lactation using vasopressin-deficient Brattleboro rats. Virgin and lactating, homozygous vasopressin-deficient rats were compared with control, heterozygous rats. In control dams compared with virgins, physiological changes similar to those observed in a chronic stress state (thymus involution, adrenal gland hyperplasia, elevation of proopiomelanocortin mRNA levels in the adenohypophysis, and resting plasma corticosterone levels) were observed. In vasopressin-deficient dams, adrenal gland hyperplasia and resting corticosterone level elevations were not observed. Corticotropin-releasing hormone (Crh) mRNA levels in the hypothalamic paraventricular nucleus were elevated in only the control dams, while oxytocin (OT) mRNA levels were higher in vasopressin-deficient virgins and lactation induced a further increase in both the genotypes. Suckling-induced ACTH and corticosterone level elevations were blunted in vasopressin-deficient dams. Anaphylactoid reaction (i.v. egg white) and insulin-induced hypoglycemia stimulated the HPA axis, which were blunted in lactating rats compared with the virgins and in vasopressin-deficient rats compared with the controls without interaction of the two factors. Vasopressin seems to contribute to the physiological changes observed during lactation mimicking a chronic stress state, but its role in acute HPA axis regulation during lactation seems to be similar to that observed in virgins. If vasopressin is congenitally absent, OT, but not the CRH, compensates for the missing vasopressin; however, the functional restitution remains incomplete.

Oxytocin stimulates secretory processes in lactating rabbit mammary epithelial cells

Oxytocin plays a major role in lactation mainly by its action on milk ejection via the contraction of myoepithelial cells. The effect of oxytocin on milk production and the presence of oxytocin receptors on different epithelial cells suggest that this hormone may play a role in mammary epithelial cells. To determine precisely the various roles of oxytocin, we studied localization of oxytocin receptors in lactating rabbit and rat mammary tissue and the influence of oxytocin on secretory processes in lactating rabbit mammary epithelial cells. Immunolocalization of oxytocin receptors on mammary epithelial cells by immunofluorescence and in mammary tissue by immunogold in addition to in situ hybridization showed that lactating rat and rabbit mammary epithelial cells expressed oxytocin receptors. Moreover, oxytocin bound specifically to epithelial cells. To determine whether oxytocin had an effect on lactating rabbit mammary epithelial cells, isolated mammary fragments were incubated in the presence or absence of 10(-6) i.u. ml(-1) of oxytocin. After 1 min of incubation with oxytocin, the morphology of epithelial cells and the localization of caseins and proteins associated with the secretory traffic suggested a striking acceleration of the transport leading to exocytosis, whereas the contraction of myoepithelial cells was only detectable after 7 min. Addition of 10(-8) g ml(-1) of atosiban before the addition of oxytocin prevented the oxytocin effect on secretory processes and on myoepithelial cell contraction. Addition of 10(-6) i.u. ml(-1) of vasopressin to the incubation medium did not mimic the stimulating effect of oxytocin on secretory traffic. These results show that lactating rabbit and rat mammary epithelial cells express oxytocin receptors and that oxytocin binds to these receptors. They strongly suggest that oxytocin has a dual effect on lactating mammary tissue: an acceleration of the intracellular transfer of caseins in mammary epithelial cells followed by the contraction of myoepithelial cells.

Effect of photoperiod on the mechanical response of the pregnant rabbit uterus to oxytocin

We present findings suggesting that photoperiod is important in determining the sensitivity of the late-pregnant rabbit uterus to oxytocin (OT). Longitudinal myometrial strips were taken from term-pregnant and estrous rabbits and mounted in an organ bath for isometric myographic recording at different times during a 16:8-h light-dark cycle (lights on 0600–2200; n = 5/group), and the strength of contractions was registered in response to the application of OT or KCl. Strength of contractions was dose dependent and was up to 200 times greater at doses three to four orders of magnitude lower in tissue taken from pregnant animals during the light phase (0700 and 1300) than during the dark phase (2400 and 0400). Strips from nonpregnant estrous females also showed greater sensitivity and contractile force when taken in the light (0700) than in the dark (0400), although the differences were not significant. Consistent with the influence of photoperiod on uterine sensitivity to OT, strips taken from two groups of pregnant females ( n = 5/group) maintained on a light-dark cycle advanced 12 h showed significantly greater sensitivity and force in response to OT during the new subjective light than during the new subjective dark phase. The photoperiod-dependent contractile response to OT was specific and not simply the result of a change in general mechanical properties of the muscle, because administration of KCl resulted in dose-dependent contractions of similar magnitude in both the light and dark phase. These results are consistent with the fact that rabbits, like other nocturnal mammals, typically give birth during the day.

A cholinergic link in the reflex release of vasopressin by hypotension in the rat

Inhalation of amyl nitrite in the water-loaded rat under ethanol anaesthesia produced a brief fall of blood pressure followed by a prolonged antidiuretic response. The antidiuretic response to amyl nitrite was accompanied by increased urinary excretion of vasopressin, it was blocked by a specific vasopressin antagonist and by a barbiturate and it was absent in the Brattleboro rat with congenital diabetes insipidus. These results show that the antidiuretic response to the hypotension induced by amyl nitrite is due to the release of vasopressin and that this release is mediated by a neuroendocrine reflex acting through the brain stem. Carbachol and nicotine produced an antidiuretic response on injection into a lateral cerebral ventricle (i. vent.). Carbachol was almost ineffective, but nicotine much more effective, when injected into the cisterna magna (i.cist.) from which in the rat there is no access to the ventricles. Carbachol therefore acts at a site reached from the ventricles, possibly the paraventricular nucleus. Nicotine acts at a more distal site reached from the subarachnoid space. This site may correspond with the nicotine-sensitive area on the ventral surface of the brain stem which has been described in the cat. Atropine blocked the antidiuretic response to carbachol but not that to amyl nitrite. Hexamethonium blocked the antidiuretic response to amyl nitrite as well as that to nicotine and was more effective on i.cist. than i.vent. injection. These results reveal a cholinergic link with a nicotinic but not a muscarinic receptor in the neural pathways controlling the release of vasopressin in response to hypotension. A hypothetical model is presented in which the release of vasopressin is stimulated by a pathway arising from chemoreceptors and inhibited by a second pathway arising from stretch- and baroreceptors. Hypotension acts by suppressing the normally predominant inhibitory pathway and stimulating the excitatory pathway. Hexamethonium is presumed to block transmission at a synapse in the excitatory pathway at the ventral surface or, less probably, at the paraventricular and supraoptic nuclei.

Activity of putative oxytocin neurones during reflex milk ejection in conscious rabbits

Extracellular electrical recordings were taken from twenty antidromically identified paraventricular neurones in unanaesthetized, unrestrained rabbits. Neuronal activity was correlated with nursing behaviour of the doe and responses of the young during suckling. Magnocellular neurones were divided into two groups on the basis of their activity in suckling. Group 1 (n = 14) showed several discrete bursts of high-frequency activity whilst neurones in group 2 (n = 6) did not. Neurones in group 1 showed 5-9 bursts of high-frequency activity in suckling. Each burst lasted 1-4 s and represented a 3-10-fold rise in the discharge of the cell. These units were classified as oxytocinergic, as their stereotyped activation preceded bouts of sucking behaviour of the young indicative of milk ejection. All fourteen cells continued to show intermittent bursts of neurosecretory activity for up to 20 min after nursing terminated. This pattern of discharge followed grooming behaviour of the doe. In contrast, neurones in group 2 (n = 6) showed no high-frequency activity in suckling. They showed a significant fall in their discharge frequency compared with pre-suckling values (P less than 0.05; Student's t test) and a significant (P less than 0.05) lengthening of the modal interspike interval. They were classed as potential vasopressin-producing cells. Control recordings were taken from thirty-two neurones which could not be antidromically driven. The recording sites were shown histologically to be in the lateral hypothalamic area. These cells showed a significant fall in their discharge frequency (P less than 0.05) and a significant increase (P less than 0.01) in the modal interval during suckling. Cross-correlation studies of the activity, recorded from one electrode, of groups of neurones clustered around a single hypothalamic neurone suggest that bursting discharge from the putative oxytocin neurones in suckling is accompanied by the synchronous activation of some of the surrounding magnocellular units.

The milk ejection reflex in the pig

1. The milk ejection reflex in response to suckling was studied in conscious sows by continuous recording of intramammary pressure, radioimmunoassay of plasma concentrations of neurohypophysial hormones, and observation of the behaviour of the sows and piglets.2. A regular pattern of nursing, suckling and milk ejection was observed. The mean duration of the suckling period was 6.3 min. Over 144 suckling periods, 113 milk ejections were recorded. Each milk ejection was characterized by a sudden rise in intramammary pressure reaching 20-49 mmHg, and lasting 8-41 sec. Milk ejections occurred only once per suckling period, at a mean interval of 44.3 min.3. Each milk ejection occurred with a mean latency of 2.4 min from the onset of a period of initial massage of the udders by the piglets, and was coincident with a period of quiet suckling when the piglets were consuming milk. The onset of nursing was signalled by the sows grunting in a rhythmic manner. In most cases, the frequency of grunts, at first low, increased suddenly 23 sec before milk ejection.4. During eighteen suckling periods leading to milk ejection, neurohypophysial hormone assays performed on serial blood samples showed an increase in plasma concentration of oxytocin up to 30 sec before milk ejection. The concentration of lysine-vasopressin did not rise above basal levels.5. In 21.4% of the suckling periods, no rise in intramammary pressure was observed. In these ;incomplete sucklings', the sow usually failed to grunt rapidly, and the piglets obtained no milk. For three of these periods, hormone assay showed no increase in oxytocin or vasopressin concentrations in blood.6. Oxytocin given intravenously produced variations in intramammary pressure which depended on the dose and the rate of injection. Rapid injections of 25-50 m-u. oxytocin, caused milk ejections similar to those induced by suckling. When oxytocin was administered at different rates, the faster the injection, the shorter the latency and the higher the amplitude of the response. Plasma concentrations of oxytocin after injection of 25 m-u. were similar to those observed during reflex milk ejection.7. Trains of electrical pulses were applied to the posterior pituitary of four anaesthetized sows. At frequencies of stimulation above 10 Hz, a rise in intramammary pressure and an increase in plasma oxytocin and vasopressin concentrations were observed. At frequencies of stimulation of 30-50 Hz, the response of the mammary gland and the time course of the variations in oxytocin plasma concentrations were similar to those observed during natural reflex milk ejection.8. It is concluded that reflex milk ejections during suckling in the pig are caused by the intermittent and spurt-like release of about 25 m-u. oxytocin, without concomitant vasopressin release. It is postulated that the release of oxytocin is probably precipitated by a brief and massive activation of oxytocin-secreting neurones in the hypothalamus. Central mechanisms controlling the intermittent release of oxytocin are discussed.

Effect of modification of fluid intake in the puerperium on serum prolactin levels and lactation

Twenty-one puerperal women were divided into two groups, those with high (4 L/day) and those with low (800 mL/day) fluid intake. Basal and post-suckling serum prolactin levels and the production rate of breast milk were similar in each group. Osmotic influences on prolactin secretion appear to be unimportant in human physiology, and there is no basis for prescribing changes in fluid intake for the control of human lactation.

Water deprivation in lactating rats: changes in nucleolar dry mass of paraventricular and supraoptic neurones

The separation of function between and within the paraventricular (PV) and supraoptic (SO) nuclei was investigated in the rat. Nucleolar dry mass of PV and SO neurones was measured to detect increased synthetic activity after water deprivation for 3 days, lactation for 8 days or water deprivation during days 5 to 8 of lactation. Lactation or water deprivation increased nucleolar dry mass in both PV and SO neurones. These stimuli caused similar nucleolar changes in PV neurones, but water deprivation caused greater changes in SO neurones than lactation. The effects of lactation and water deprivation were additive for both SO and PV neurones. Furosemide was used to intensify the dehydration stimulus to determine whether such intensification could have caused the greater nucleolar changes when lactation and water deprivation were combined. For PV neurones this was not the case, but remained a possibility for SO neurones. Measurements of serum osmolality in the experimental groups were ranked as follows: water deprivation + furosemide > lactation + water deprivation > water deprivation > lactation = virgin control. Loss of body weight was similar in the first two groups but less during water deprivation alone. Although milk yield fell, milk was obtained by the litters of lactating animals throughout the period of water deprivation.

Changes in nucleolar dry mass of neurones of the paraventricular and supraoptic nuclei in the rat during pregnancy and lactation

Interference microscopy was used to measure the dry mass of nucleoli in unfixed nuclei isolated from neurones of the paraventricular (PV) and supraoptic (SO) nuclei of female rats. Changes in nucleolar dry mass during pregnancy and lactation have been interpreted as reflecting changes in rates of synthesis of ribosomes and protein in these neurones. Measurements were made on a total of 6580 nucleoli from 135 rats. At the end of pregnancy nucleolar dry mass of both PV and SO neurones was increased compared with virgin female rats. Nucleolar dry mass of PV neurones but not SO neurones increased further during lactation. This change was biphasic, with a nadir at 2 weeks post partum. After day 5 post partum, nucleolar dry mass of PV and SO neurones was increased only in rats sucking pups. Adjustment of litter size to 10 or 22 to 24 pups on the first day post partum did not affect nucleolar changes in PV and SO neurones. Nucleolar changes were less when only one pup was nursed. The results are discussed in relation to oxytocin secretion induced by the suckling stimulus and the synthetic response of PV and SO neruones to increased secretion.

Rhythmic patterns of discharge in hypothalamic neurosecretory neurons of cats and dogs

Recent studies in rats suggest that vasopressin- and oxytocin-secreting neurons in supraoptic nuclei and paraventricular nuclei of the hypothalamus show two different patterns of activity: one a "bursting" or rhythmic pattern and the other, irregular continuous discharges. This possibility was investigated in cats and dogs anesthetized with chloralose or Nembutal by recording electrical activity of single supraoptic and paraventricular neurons. Only some of the "identified" neurosecretory cells showed rhythmic, intermittent discharges ("bursting" cells in rats); the majority showed an irregular continuous firing pattern. Furthermore, the pattern of discharge sometimes changed from one ot the other during long periods of observation. This occurred without apparent stimulus in certain instances; in others, the rhythmic firing was associated with fluctuation in blood pressure and heart rate and was likely to be caused by changes in baroreceptor activity. Possible origins of rhythmic discharges and the physiological importance of such patterns in terms of hormone secretion are discussed.

The milk ejection of the rat, as a stimulus and a response to the litter

A finding that the milk ejection of the rat is stimulated effectively only if at least half the pups are suckled is confirmed. This relationship, however, is found if the mother is anaesthetized but not if the mother is conscious. If the mother is conscious, the milk obtained by individual hungry pups is unaffected by the number of other hungry pups suckled. This is true whether the other pups of the litter are removed, or whether they are present but satiated. After the milk ejection, pups tend to leave the mother's nipple. Milk yield decreases with each milk ejection of a series, but getting less milk does not make the pups more likely to leave the nipple in search of another.

Immunocytochemical study of the hypothalamo-neurohypophysial system. II. Distribution of neurophysin, vasopressin and oxytocin in the normal and osmotically stimulated rat

Antisera, with cross reactive antibodies removed by affinity chromatography, were used in the immunoperoxidase-bridge technique to study the distribution of oxytocin and vasopressin together with neurophysin in the hypothalamo-neurohypophysial system of the rat. The hormones were demonstrated in different areas of the supraoptic nucleus (SON) and paraventricular nucleus (PVN), in neurosecretory fibres of the hypothalamo-neurohypophysial tract, median eminence, and in nerve terminals of the neurohypophysis. Intact normal and rats with hereditary hypothalamic diabetes insipidus (Brattleboro strain), and rats dehydrated by the administration of oral hypertonic saline were studied. In dehydrated rats the hormone concentration in the neurons, and the number of neurons containing hormone varied according to the time of dehydration stress. The observations support the hypotheses that: 1) oxytocin and oxytocin-neurophysin, and vasopressin and vasopressin-neurophysin are synthesised in different neurons and are transported along different axons; 2) the SON and PVN are functionally indistinguishable in that neurons containing oxytocin or vasopressin are present in both nuclei; and 3) the two types of neurons respond to osmotic stimulation in a way that is qualitatively the same but quantitatively different.

Nucleolar proliferation and cell size changes in rat supraoptic neurons following osmotic and volemic challenges

Subcutaneous injections of isotonic saline induced nucleolar proliferation in supraoptic neurons in animals sacrificed approximately 5 min postinjection. The magnitude of this proliferation was sustained 4 and 8 hr postinjection. Polyethylene glycol (PG) injections depleted blood volume 4 and 8 hr after the injection, but the percentage of SON cells with multiple nucleoli in these animals was not different from saline-injected controls. The anterior (SOa) portion of the SON in rats given 2% NaCl to drink instead of water for three days contained more cells with multiple nucleoli than controls. This effect was enhanced after five days ingestion, and accompanied by a similar response in the tuberal portion of SON (SOt). Rehydration for ten days after three days of 2% NaCl intake brought the percentage of cells with multiple nucleoli down to control levels. Cell area in SON cells paralleled nucleolar responses during dehydration and rehydration. The results demonstrate the sensitivity of nucleolar proliferation in SON to environmental changes ranging from osmotic to neurogenic stress.

Excitation of phasically firing hypothalamic supraoptic neurones by carotid occlusion in rats

1. The activity of supraoptic neurones has been recorded extracellularly during bilateral occlusion of the common carotid arteries in anaesthetized rats. 2. Experiments in lactating rats showed that occlusion liberated sufficient amounts of neurohypophysial hormones to cause a rise in intramammary pressure 15-25 s after the onset of occlusion. 3. Ninety-one percent of the phasic neurones (defined as those showing bursts of activity alternating with periods of silence) were activated by carotid occlusion less than 10 s after the onset of occlusion. Most randomly firing neurones were inhibited or were unaffected. 4. The activation of phasic neurones is unlikely to be just a nonspecific effect, because in the same animals, phasic neurones were excited whilst random neurones were not. 5. Moreover, in phasic neurones, statistical analysis shows (a) that the intervals during which an occlusion was performed were significantly shorter than the intervals between spontaneously occurring bursts, and (b) that this activation was followed by a period of reduced firing probability. 6. The results are discussed with reference to the correlation of supraoptic neuronal activity with hormone release. The possibility is considered of relating the tendency of some supraoptic neurones to fire in bursts with the secretion of vasopressin.

Electrophysiological evidence for the activation of supraoptic neurones during the release of oxytocin

1. Antidromically identified supraoptic (SO) units were recorded in lactating rats anaesthetized with urethane (1.1 g/kg), and their activity was studied during milk ejection evoked by the suckling of the young. Fifty-eight SO units were recorded through 174 milk ejections. Each milk ejection was the result of a neurohypophysial release of an oxytocin pulse of 0.5-1.5 m-u.2. Fifty-five of the SO units displayed background activity and three were silent. The firing rates ranged from 0 to 15.4 spikes/sec, the distribution was exponential with 26% of the units firing at < 1 spike/sec. Sixteen (28%) of the SO units displayed a phasic pattern of activity characterized by periods of activity (6-132 sec) and silence (4-71 sec).3. Twenty-five of the non-phasic units displayed a large and stereotyped acceleration in spike activity some 10.5-17.4 sec before the rise in intramammary pressure at milk ejection. Units accelerated to rates between 9-66 spikes/sec, an increase of about thirtyfold (median) on background activity. The response was brief (0.9-4.7 sec) and was followed by a period of after-inhibition.4. It was concluded from studies of double recordings and observations of multi-unit activity that all the responsive units were synchronously activated. The mean latency of 13.3 sec between the onset of the neurosecretory response and milk ejection was similar to that observed following the experimental release of endogenous oxytocin by electrical stimulation of the neurohypophysis (50 pulses/sec for 2-4 sec).5. Four of the phasically active units were correlated with the oxytocin release for milk ejection. Three of these units displayed a burst of activity superimposed on the terminal portion of an active phase, some 10.2-14.7 sec before milk ejection. The fourth unit, recorded in conjunction with a responsive non-phasic unit, consistently switched from silence to activity coincident with the onset of the SO activation.6. The remaining SO units and a further ten units that were not antidromically activated by neurohypophysial stimulation displayed no change in activity during either the period of neurosecretory activation or the period of after-inhibition.7. This activation of the SO neurones, in the formulation of oxytocin release and milk ejection, is the same as that we have previously observed in recordings from the paraventricular (PV) region, and the proportion of neurones displaying the response is similar: 48% in the SO nuclei, 58% in the PV nuclei. We conclude, since the SO nuclei contain 80% of the neurosecretory cells that project to the neurohypophysis, that the SO nuclei are as important, if not more so, than the PV nuclei in the control of oxytocin release.

The distribution of vasopressin and oxytocin in the hypothalamoneurohypophysial system of the guinea-pig

1. The ratio of the content of vasopressin to that of oxytocin (V/O ratio) was estimated in the supraoptic nucleus (SON), paraventricular nucleus (PVN) and posterior pituitary gland (PIT) of guinea-pigs.2. Extracts were assayed for antidiuretic activity to estimate vasopressin and for milk-ejecting activity to estimate oxytocin. In assays for milk-ejecting activity, trypsin was used to inactivate vasopressin in the extracts.3. The mean V/O ratios in the SON, PVN and PIT were 28, 8.5 and 7.0 respectively in male guinea-pigs, 6.8, 7.4 and 6.9 in non-lactating females, and 5.1, 3.3 and 6.6 in lactating females.4. The distribution of the hormones within the hypothalamus is discussed in relation to their independent release in response to electrical stimulation of the SON and PVN.

Subcellular organization of neurophysins, oxytocin, (8-lysine)-vasopressin and adenosine triphosphatase in porcine posterior pituitary lobes

Posterior pituitary lobes from young pigs were fractionated by differential and sucrose-density-gradient centrifugation. The distributions of oxytocin and [8-lysine]-vasopressin were measured by bioassay and the distributions of neurophysin-I and -II by radioimmunoassays specific for each of these two proteins. Most of the hormone and neurophysin applied to the density gradient was localized in particles with the density expected of neurosecretory granules. However, the neurosecretory granules were separated into two bands (D and E). A close statistical correlation between the distributions of [8-lysine]-vasopressin and neurophysin-I, and of oxytocin and neurophysin-II on the gradients, suggested that in vivo porcine neurophysin-I binds [8-lysine]-vasopressin within one population of granules and porcine neurophysin-II binds oxytocin within another type of granule. However, there was no significant separation of oxytocin and vasopressin in fractions D and E. The molar ratios of hormones and neurophysins indicated that there was insufficient of either neurophysin to bind the [8-lysine]-vasopressin in the granule fractions or in the whole gland. Polyacrylamide-gel electrophoresis showed that only bands corresponding in mobility to porcine neurophysins-I, -II and -III were present in large amounts in the whole gland and in the granule fractions. The component with the mobility of neurophysin-III was, however, relatively enriched in whole young glands and granule fractions compared with adult gland extracts. It is suggested that the vasopressin that cannot be assigned to neurophysin-I may occur in (a) vesicles containing vasopressin but no neurophysin, (b) vesicles containing vasopressin and a protein that cannot be quantified by the radioimmunoassays used, such as porcine neurophysin-III, or (c) normal vasopressin-neurophysin granules which have accumulated extra vasopressin. Band E of the gradient was rich in adenosine triphosphatase activity, whereas band D possessed very little of this enzyme.

The hypothalamic neurosecretory pathways for the release of oxytocin and vasopressin in the cat

1. The neurones of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) were stimulated electrically in lactating cats under chloralose anaesthesia. Milk-ejection responses were used to monitor the release of oxytocin and vasopressin and both hormones were assayed in samples of blood collected during stimulation. The position of the tip of the stimulating electrode was confirmed from brain sections stained selectively for cystine-rich neurosecretory material.2. A previous finding that stimulation of the SON in the cat releases vasopressin without oxytocin was confirmed.3. Stimulation of the PVN caused both hormones to be released. The ratio of their concentrations in blood was variable; this suggests release from separate neurones.4. Both hormones were also released on stimulation of the median eminence but not of the zone lying vertically between this structure and the PVN. No neurosecretory material was detected in this zone. These findings argue against the existence of a direct or medial paraventriculo-hypophysial pathway running downwards along the wall of the third ventricle.5. Study of sections from unstimulated brains confirmed that the tractus paraventricularis cinereus of Greving which runs ventro-laterally from the PVN towards the SON, represents the principal efferent pathway for neurosecretory fibres from the PVN.6. The results are discussed in relation to the problem of the independent release of oxytocin and vasopressin in response to physiological stimulation of the neurohypophysis.

Some pharmacological properties of a synthetic oxytocin analogue [1-N-carbamoyl-hemicystine-2-O-methyltyrosine]-oxytocin (carbamoyl-methyloxytocin), an antagonist to the neurohypophysial hormones

1. A synthetic oxytocin analogue, [1-N-carbamoyl-hemicystine-2-O-methyltyrosine]-oxytocin (carbamoyl-methyloxytocin), has been tested as an antagonist to the actions of oxytocin and vasopressin on the uterus, the mammary gland and blood pressure.2. The analogue inhibited the response of the isolated rat uterus to both oxytocin and vasopressin without itself stimulating the uterus to contract. The responses to equipotent doses of oxytocin and vasopressin were inhibited equally. There was little or no inhibition of the response to bradykinin. carbachol, angiotensin or 5-hydroxytryptamine with doses of the analogue up to 160 times that required to inhibit the response to oxytocin by 50%. The analogue caused a parallel displacement of the log dose-response curve for oxytocin; the pA(2) value (2 min contact) varied from 6.4 to 7.1 according to the ionic composition of the solution in the organ bath.3. The analogue inhibited the response of the rat uterus in situ to oxytocin but not to angiotensin or 5-hydroxytryptamine. It did not stimulate the uterus.4. When, in certain experimental conditions, spontaneous activity occurred in the isolated uterus or the uterus in situ, this activity was unaffected by the analogue but the increase in amplitude and frequency of contractions caused by oxytocin was inhibited. The regular rhythm of contractions induced in the quiescent uterus by the intravenous infusion of oxytocin was interrupted by intravenous injections of the analogue.5. The response of the isolated strip of rat mammary gland to the analogue depended on whether or not magnesium was present in the bath solution. In the presence of this ion, the analogue generally caused an increase in tension; in its absence, it acted as a pure antagonist. As on the isolated uterus, oxytocin and vasopressin were equally inhibited, and the analogue caused a parallel displacement of the log dose-response curve for oxytocin. With 0.9 mM Ca and 1.0 mM Mg, the mean pA(2) value (2 min contact) was 6.28 +/- 0.08 (S.E.)6. In the lactating rat, the analogue inhibited the milk-ejection response to oxytocin and vasopressin but not that to acetylcholine, bradykinin or 5-hydroxytryptamine. A milk-ejection response to the analogue itself was seen occasionally with retrograde arterial but not with intravenous injections.7. The analogue inhibited the avian depressor response to oxytocin and the rat pressor response to vasopressin.8. On all assay preparations, the degree of inhibition caused by the analogue was dependent on the dose, and the inhibition could be surmounted by increasing the dose of agonist. Recovery usually occurred within 15 min. These features, together with the parallel displacement of the dose-response curve for oxytocin on the isolated uterus and mammary strip, and the equal inhibition of the responses to oxytocin and vasopressin, suggest that carbamoyl-methyl-oxytocin acts as a specific competitive inhibitor of the neurohypophysial hormones.9. The structure-activity relationships of analogues of oxytocin having substituents in the terminal amino and phenolic hydroxyl groups, and some practical applications of the carbamoyl-methyl analogue, are discussed.

Independent release of oxytocin and vasopressin during parturition in the rabbit

1. Oxytocin and vasopressin were assayed in samples of blood collected from seven conscious rabbits during parturition.2. Oxytocin was detected in the blood in ten out of fourteen samples collected during the expulsion of one or more foetuses. Four samples contained 6-100 muu./ml., three 100-200 muu./ml. and three 200-500 muu./ml.3. Vasopressin was detected in six blood samples collected during the delivery of foetuses but in only one experiment did the amount exceed that found in the corresponding control sample collected before or after delivery.4. When both hormones were detected in the same blood sample, the ratio of oxytocin to vasopressin varied from 5:1 to at least 26:1.5. It is concluded that, while oxytocin may not be essential for parturition in the rabbit, stretching of the birth canal during the expulsion of foetuses normally acts as a stimulus for the reflex release of oxytocin from the neurohypophysis and that oxytocin is released independently of vasopressin.