Oxytocin disappears rapidly from the circulation; half-lives in the circulation for the intravenously injected hormone of 40 sec. to 4 min. have been reported in a variety of species. In connexion with the interpretation of blood levels of oxytocin during suckling (Folley & Knaggs, 1966) the following experiments were carried out to determine the half-life of exogenous oxytocin in the circulation of the sow. Although the results are limited they may be worthy of report since no previous information for the sow is available.

Two experiments were performed on an adult sow (no. 2) weighing 102 kg. which had just lost a premature litter. In the first, the whole experiment was carried out while the sow was maintained under the anaesthetic (cyclopropane/oxygen) given for the insertion of a jugular cannula (for cannulation technique see Folley & Knaggs, 1966). One i.u. oxytocin (Pitocin, Parke, Davis and Co.) in 1 ml. 0·9%

SUMMARY

When the afferent pathway of the milk-ejection reflex, which we had previously reported, was surgically severed bilaterally in the mid-brain of the lactating rabbit, the reflex release of oxytocin in response to suckling was blocked for up to 11 days; unilateral severance did not block the reflex. The position and discrete nature of the pathway were also further substantiated by electrical stimulation experiments in acute studies in the anaesthetized rabbit. Some animals, however, did not release oxytocin in response to stimulation of the pathway. Furthermore, whereas stimulation of this reflex pathway in the guinea-pig brain at intervals of a few minutes evokes release of oxytocin after each stimulation, in the present study the release of oxytocin in the rabbit in response to repeated electrical stimulation was either progressively attenuated or did not occur at all after the initial release. There appears, therefore, to be a powerful overriding central inhibitory mechanism in the rabbit which can prevent release of oxytocin, even when the appropriate stimulus for release is applied.

SUMMARY

Oxytocin has been assayed in the jugular vein blood of goats during parturition; for comparison a few measurements were also made during pregnancy. The hormone was extracted from blood plasma by gel filtration, followed by lyophilization and then assayed in the lactating guinea-pig by the increase in intramammary pressure after intra-arterial injection.

No oxytocin could be detected in the blood during pregnancy and it was found in only one of eight goats studied during the first stage of labour. The hormone was present in appreciable quantities in blood taken during the second stage of labour, and in general, the concentration rose to a maximum when the head presented. In cases of twin births oxytocin was usually present in the blood during the birth of the second kid but at a concentration lower than during delivery of the first. After expulsion of the kid the blood oxytocin titre diminished rapidly, suggesting that secretion of oxytocin ceased as soon as the kid was born. In three experiments the total release of oxytocin during a considerable portion (2·7–11·0 min.) of the second stage labour was estimated as 223–726 m-u.

The results are consistent with the view that oxytocin is not essential for the induction of labour. Rather the hormone is released in response to stimuli arising from distension of the vagina and vulva, and by virtue of its contractile effect on the uterus assists parturition.

The half-life of intravenously injected oxytocin in the lactating goat was found to be 1 min. 22 sec. After storage of lyophilized blood extracts at −15° for 5 months milk-ejection activity had declined by only 27%.

SUMMARY

Milk-ejection activity (oxytocin) was assayed in the external jugular vein blood of cows, goats and sows during milking or suckling. The hormone was extracted from blood plasma by gel filtration, followed by lyophilization and then assayed in the lactating guinea-pig by the increase in intramammary pressure after intra-arterial injection.

Serial blood samples were taken during machine milking of cows which had been accustomed to the milking routine and blood sampling procedure. The stimulus most consistently followed by a transient appearance of oxytocin in the jugular vein blood was the application of the teat cups. In some cases, there was a second release of oxytocin, later in the milking process, unrelated to any apparent stimulus. There was no evidence in these experiments of any conditioning of oxytocin release to visual, auditory or olfactory stimuli associated with the milking routine.

Serial blood samples were taken during hand-milking of goats which had been accustomed to the milking routine and blood sampling procedure. Oxytocin was detected in jugular vein blood in only a minority of experimental milkings. Irrespective of whether oxytocin was found in the jugular blood the milk yield at the experimental milking did not differ appreciably from the value to be expected from comparable milkings for the preceding week.

Serial blood samples were obtained from sows during suckling, the animals having been accustomed to the presence of the experimenter and the blood sampling procedure. In most of the experimental sucklings oxytocin was found in the blood. The occurrence of the hormone was transient and it was usually released just before the milk-ejection phase. In most of the cases in which no hormone was detected in the blood the piglets obtained no milk as judged by their behaviour.

The results suggest that in the cow and sow the milk-ejection reflex is a neurohormonal reflex involving the release of oxytocin. On the other hand, in the goat, provided the animals are carefully hand-milked, normal milk yields can be obtained without the release of oxytocin.

SUMMARY

Rabbits in pentobarbitone anaesthesia were implanted bilaterally with a pair of monopolar electrodes in the brainstem. Approximately 10 days after the operation, pseudopregnancy was induced by i.v. injection of human chorionic gonadotrophin, and 1 week later the rabbits received electrical stimulation with square-wave pulses through the implanted electrodes for two periods of 30 min. daily for 11 days. At autopsy on the following day the mammary glands were inspected for occurrence of lactogenesis and sites of electrode tips in the brain were determined histologically. Lactogenesis, indicating release of prolactin, occurred when electrical stimulation had been applied to sites in the lateral mesencephalic tegmentum and further forward in a region medio-ventral to the medial geniculate body. Passing rostrally, the pathway moved medially and then forwards in association with the extreme rostral central grey matter, and was traced as far forward as the posterior hypothalamus where sites were found close to, but not involving, the mammillo-thalamic tracts.

When compared with our previous studies on the afferent path of the milk-ejection reflex in this species, the ascending path for release of both oxytocin and prolactin appears to be the same in the mesencephalon. However, whereas the ascending path for oxytocin release bifurcates on each side into dorsal and ventral paths which reunite in the posterior hypothalamus, that for prolactin release appears to follow only the dorsal path, since stimulation of the subthalamus, through which the ventral path passes, was ineffective. It is proposed that the pathway traced in the present study represents the mesencephalic and posterior diencephalic route by which impulses initiated by the suckling stimulus attain the hypothalamus to evoke release of prolactin from the adenohypophysis.

SUMMARY

The effect of various types of surgical damage to the forebrain on the release of oxytocin in response to electrical stimulation of the discrete ascending milk-ejection reflex pathway in the mid-brain was investigated in 99 anaesthetized lactating guinea-pigs. Oxytocin release was measured by comparison of experimental milk-ejection responses with the response to i.v. injection of known amounts of synthetic oxytocin.

Removal of the entire telencephalon, including cerebral cortex, hippocampi, amygdalae and forebrain rostral to the hypothalamus, did not affect the subsequent release of oxytocin after electrical stimulation of the pathway in the mid-brain, from which it was concluded that the reflex pathway within the forebrain is entirely diencephalic. Transection of the hypothalamus immediately rostral to the paraventricular (PV) nuclei was without effect, while transection immediately caudal to the PV nuclei blocked the release of oxytocin. Destruction of the PV nuclei by a radiofrequency lesion which spared the supraoptic (SO) nuclei blocked the release of oxytocin. Undercutting both PV nuclei so as to isolate them from the ventral hypothalamus blocked the release of oxytocin. Undercutting the PV nucleus ipsilateral to the stimulated side of the mid-brain blocked the release of oxytocin, while undercutting the contralateral PV nucleus had no effect. The PV nuclei, therefore, lie on the ascending path of the milk-ejection reflex, the SO nuclei do not, and, from the mid-brain forwards, the ascending pathway remains uncrossed.

The course of the reflex pathway was traced rostrally from the mesodiencephalic junction by making narrow transverse knife-cuts and determining which cuts reduced or blocked the release of oxytocin after mid-brain stimulation. At this level, the pathway on each side of the brain is represented by separate dorsal and ventral paths and in the present study it was found that the ventral path is more important than the dorsal path in terms of oxytocin release. The ventral path passes forward in the medial forebrain bundle, in the far-lateral hypothalamus, while the dorsal path enters the posterior hypothalamus dorsally in the periventricular region at the top of the third ventricle and impinges on the thalamic reuniens nucleus. Shortly afterwards the dorsal path swings abruptly in the lateral direction to join the ventral path in the lateral hypothalamus. The reunited pathway then moves forward in this position until it is level with the PV nuclei, where it swings dorsomedially to relay with the lateral tip of the ipsilateral PV nucleus, and in doing so intermingles with the descending neurosecretory fibres from this nucleus.

SUMMARY

Rabbits were implanted unilaterally with a bipolar electrode in the forebrain, and approximately 10 days later pseudopregnancy was induced by i.v. injection of human chorionic gonadotrophin. One week later the rabbits began receiving electrical stimulation with square-wave pulses through the implanted electrodes for two periods of 15 min daily for 11 days. At autopsy on the 12th day the mammary glands were inspected for the occurrence of lactogenesis and sites of electrode tips in the brain were determined histologically. In a preceding study a prolactin-release path, believed to be that normally activated by the suckling stimulus, had been traced from the mid-brain as far rostral as the posterior hypothalamus, and in the present work, lactogenesis, indicating release of prolactin, occurred after electrical stimulation of this same region in the posterior hypothalamus, between the third ventricle and the mammillo—thalamic tract. Further rostrally, effective stimulation sites were found in the medio-dorsal hypothalamus, in the farlateral hypothalamus within the medial forebrain bundle, and in the lateral and medial preoptic area. Sites extended caudally from the last area to the medial anterior hypothalamus. Passing rostrally from the lateral preoptic area, effective sites occurred in, and ventral to, the external capsule, in the claustrum and in the adjacent orbitofrontal cortex which yielded the two maximum lactogenic responses observed in the stimulated group. The role of these rostral structures, in particular the orbitofrontal cortex, is not yet clear, nor is the final mechanism mediating prolactin release, although the results suggest that the ascending pathway for prolactin release approaches the medial hypothalamus by way of the preoptic area. The presence of control electrodes in the brains of pseudopregnant rabbits was found to cause lactogenesis when their tips were in structures associated with prolactin release, as determined from the stimulated group of animals. It was concluded, therefore, that electrical stimulation may have been unnecessary in this work and that the physical irritation caused by the presence of the electrode tip in appropriate neuronal systems may be a sufficient stimulus per se to be used for tracing prolactin-release pathways in the brain.

The direct estimation of oxytocin in blood during suckling in women has only been reported by two groups of workers, who obtained conflicting results. Hawker & Robertson (1957) and Hawker, Walmsley, Roberts, Blackshaw & Downes (1961) failed to detect a change in the oxytocin content of peripheral venous blood during suckling. However, Coch, Fielitz, Brovetto, Cabot, Coda & Fraga (1968) demonstrated the presence of oxytocin (12–25 μu./ml. plasma) in internal jugular venous blood collected from women during suckling. The present study investigates the possible release of oxytocin during suckling and coitus in man by direct estimation of the hormone in the blood. No previous estimation during human coitus has been reported, but in domestic animals an increase in the oxytocin content of the blood has been claimed after mating (Walmsley, 1963) and after artificial stimulation of the genitalia (Fitzpatrick, 1957; Roberts & Share, 1968).

Three suckling experiments, with the same

SUMMARY

The position of the pathway for the release of oxytocin in the mid-brain was ascertained by exploration of a transverse stereotaxic plane (A4) in 23 anaesthetized goats. Electrical stimulation was applied between a monopolar electrode and an indifferent electrode in the scalp. Oxytocin release was monitored by simultaneous collection of blood samples during stimulation from a catheter in a jugular vein. The blood samples were extracted by the Sephadex G-25 or fuller's earth method and assayed for oxytocin content on the lactating guinea-pig preparation.

Oxytocin release occurred occasionally after stimulation of certain sites in the tectum, central grey and reticular formation. Regular releases of oxytocin, however, were only obtained after stimulation of a pathway which was compact and lay in the lateral tegmentum of the mid-brain in association with the spinothalamic tract. The position of this pathway corresponds to that described previously for the afferent pathway of the milk-ejection reflex in the mid-brain of the guinea-pig and rabbit. In these three species therefore, the impulses concerned in oxytocin release appear to ascend through the mid-brain in the spinothalamic tract.

SUMMARY

Discrete portions of the afferent path of the milk-ejection reflex have been explored in the brain of the lactating guinea-pig. Both intramammary pressure and arterial blood pressure were recorded to detect release of oxytocin and vasopressin. It was found that the milk-ejection responses which occurred after electrical stimulation of the pathway in the midbrain and hypothalamus were caused by the release of oxytocin without detectable release of vasopressin.

A mixture of oxytocin and vasopressin, in the ratio of approximately 3:1, was released only after electrical stimulation of the rostral tuberal region of the hypothalamus adjacent to the pituitary stalk.

It is concluded that the afferent path in the brain of the guinea-pig studied is concerned with the preferential release of oxytocin from the neurohypophysis and that it is the pathway of the milk-ejection reflex.