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B. T. PICKERING and H. HELLER

SUMMARY

Two peptides with uterotonic activity have been isolated from the pituitary gland of a holocephalian elasmobranch fish: Hydrolagus collei. One of them had an amino acid composition compatible with that of oxytocin itself, and also had the pharmacological properties of this hormone. The other peptide which was present in much smaller amounts was basic by chromatography and had the pharmacological characteristics of [8-arginine]-oxytocin. It was not completely purified because of the small amount available, but its amino acid composition was in accord with that of vasotocin. The implications of the presence of oxytocin in such a primitive fish on the phylogeny, and hence probably the evolution, of neurohypophysial hormones are discussed.

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T. Engstrom, A. Atke and H. Vilhardt

ABSTRACT

Binding of [3H]oxytocin to purified myometrial plasma membranes was unaffected by continuous infusion of bradykinin over 5 days in rats pretreated with oestradiol 2 days before collection of tissue. In contrast, oxytocin treatment resulted in a 76% decrease in maximal binding of [3H]oxytocin and thereby in oxytocin receptor concentration without affecting the dissociation constant. The K M value (molar concentration giving half maximal contraction) of isolated uterine strips stimulated with oxytocin was increased and maximal contractile responses were reduced following oxytocin infusions.

The binding of [3H]bradykinin to purified plasma membranes was influenced by treatment with both oxytocin and bradykinin. Bradykinin infusions down-regulated the bradykinin receptor concentration by 19%, while the receptor affinity remained unchanged. Maximal contraction (Emax) values of isolated strips stimulated with bradykinin exhibited a slightly attenuated response and K M values were significantly enhanced. Long-term treatment with oxytocin down-regulated myometrial bradykinin receptors by 31%. In addition, oxytocin infusions caused Emax to decrease and K M to increase in experiments with isolated uterine strips stimulated with bradykinin.

It is concluded that the down-regulation of oxytocin and bradykinin receptors following prolonged exposure to oxytocin may result from changes in a common pathway for intracellular peptide receptor processing. Likewise, the increased K M values of isolated myometrial strips (despite unchanged dissociation constants) suggest that prolonged oxytocin treatment affects the coupling between receptor activation and contractile response.

J. Endocr. (1988) 118, 81–85

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A. S. McNEILLY, M. J. MARTIN, T. CHARD and I. C. HART

Neurophysin and the octapeptide hormones oxytocin and vasopressin are synthesized in the hypothalamus and stored in the posterior lobe of the pituitary gland. It has recently been shown that the release of both oxytocin and vasopressin or of vasopressin alone, in response to potent stimuli, is accompanied by a simultaneous release of neurophysin into the circulation (Burton, Forsling & Martin, 1971; McNeilly, Legros & Forsling, 1972). However, it has yet to be shown that neurophysin can be released at the same time as a specific release of oxytocin. This situation occurs in animals during both parturition (Folley & Knaggs, 1965) and lactation (Folley & Knaggs, 1966; McNeilly, 1972). The present report describes the simultaneous release of oxytocin and neurophysin during parturition in the goat.

Serial blood samples (approx. 10 ml each) were taken from an indwelling jugular cannula during the whole of labour in two pedigree British Saanen goats. Samples

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I. C. A. F. ROBINSON and J. M. WALKER

The use of agarose-bound neurophysin for the extraction of oxytocin from biological fluids is described. Oxytocin can be extracted from plasma, urine and cerebrospinal fluid with a high rate of recovery and samples varying widely in volume and oxytocin concentration can be tested by the method.

Columns can be used to extract and concentrate dilute samples, or to help identify small amounts of neurohypophysial hormones by affinity chromatography. The oxytocin can be eluted from the column directly into the buffer used for subsequent bioassay. The composition of the final extract is constant and independent of the composition of the sample. The specificity of the binding is high. It is suggested that the method has many advantages over others in current use.

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G. S. KNAGGS

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%

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A. S. McNEILLY and I. C. HART

It has been suggested that in certain circumstances oxytocin may stimulate the release of prolactin in the rat (Benson & Folley, 1956) and goat (Bryant, Greenwood & Linzell, 1968). In order to test this hypothesis in a physiological situation the changes in blood levels of oxytocin and prolactin were determined in a series of samples taken during parturition in the goat, where oxytocin levels are known to be high (Folley & Knaggs, 1965; McNeilly, Martin, Chard & Hart, 1972).

The cannulation and blood sampling technique has been described previously (McNeilly et al. 1972). Jugular blood samples were taken continuously during the whole of labour in six pedigree British Saanen goats and all plasma samples were stored at −20 °C until assay. Oxytocin was

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D. F. SWAAB, F. NIJVELDT and C. W. POOL

Netherlands Central Institute for Brain Research, Ijdijk 28, Amsterdam-0, The Netherlands

(Received 3 July 1975)

The 'classical' view on the distribution of oxytocin and vasopressin producing cells suggests that the paraventricular nucleus (PVN) is predominantly or entirely responsible for oxytocin production and the supraoptic nucleus (SON) synthesizes mainly vasopressin. However, recent hormone assays and electrophysiological studies indicate the presence of each hormone in both nuclei (for references see Burford, Dyball, Moss & Pickering, 1974). We report an immunofluorescence study in the SON and the magnocellular part of the PVN (Bodian & Maren, 1951) using antibodies to oxytocin (produced in our laboratory) and to vasopressin (produced by Drs Hollemans, Schellekens and Touber), purified by absorption with arginine-vasopressin and oxytocin respectively (Swaab & Pool, 1975).

Frontal cryostat serial sections of glyoxal-prefixed hypothalami from five male Wistar rats weighing 200 g were studied. Out of each group of six sections, the first was

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J. RUSSELL and R. R. CHAUDHURY

Although the release of oxytocin from the neurohypophysis has been demonstrated both after physiological stimuli such as suckling (Fitzpatrick, 1961) and after administration of pharmacological agents such as nicotine (Bisset & Walker, 1953), very little work appears to have been carried out to investigate how quickly oxytocin is repleted at the posterior pituitary gland after release. In the present investigation the oxytocin content of the pituitary gland was determined at 2, 10, 30 and 120 min after intravenous administration of nicotine to rats. The effect of previous administration of pheniramine, an antihistamine, on the oxytocin-releasing action of nicotine was studied in a further series of experiments.

Male albino rats (150–200 g) were used. Nicotine bitartarate at a dose of 1 mg/kg was injected slowly into the tail vein of the rat. Groups of rats were killed by decapitation at 2, 10, 30 and 120 min after the administration of nicotine.

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A. M. BURTON, D. V. ILLINGWORTH, J. R. G. CHALLIS and A. S. McNEILLY

SUMMARY

Oxytocin levels in pregnant and parturient guinea-pigs were studied by means of a sensitive and specific radioimmunoassay. Oxytocin was released from the maternal pituitary in substantial amount only during the expulsive phase of labour, when the mean concentration in carotid arterial blood in five animals was 503 pg/ml plasma (range 96–2900 pg/ml). Oxytocin was not found in the plasma of the first born at the moment of birth, but was usually detected in amounts ranging from 96 to 455 pg/ml in those born subsequently. The mean half-time of oxytocin in the maternal circulation during late pregnancy was 62 ± 7·5 (s.e.m., n = 5) s. In-vivo experiments showed that the placenta was permeable to oxytocin in both directions.

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Y-F Wang, H Negoro and K Honda

Abstract

Unilateral knife cuts were performed in the midbrain of lactating rats and the activities of oxytocin neurones were recorded extracellularly from the supraoptic nuclei (SON) in order to investigate the location of the neural mechanism responsible for the synchronization of milk-ejection bursts of oxytocin neurones in different magnocellular nuclei of the hypothalamus. The lesions involved the mesencephalic lateral tegmentum, the intermedial tegmentum and the central grey. Ninety-six SON neurones were antidromically activated by neurohypophyseal stimulation and were also identified as oxytocin neurones, which included 17 pair-recorded neurones. First, the response of oxytocin neurones recorded from the unilateral SON to bilateral or unilateral suckling was tested. During bilateral suckling, not only the oxytocin neurones recorded from the SON on the intact side (n=34) but also those recorded from the SON on the lesioned side (n=58) displayed milk-ejection bursts. When only the nipples ipsilateral to the lesion were suckled (ipsilateral suckling), bursts were induced in most of the oxytocin neurones on the intact (83·3%, n=12) and lesioned side (88·9%, n=27). In contrast, none of the oxytocin neurones (n=37) produced bursts and none of the rats tested (n=23) showed milk ejections during contralateral suckling. Secondly, some characteristics of the bursts of pair-recorded neurones during bilateral suckling and their response to different modes of suckling were investigated. When oxytocin neurones on both sides displayed milk-ejection bursts, they were always well synchronized but the mean burst amplitude of the neurones on the lesioned side (55·6 ±4·9 spikes, n=43) was significantly (P<0·05) lower than that of the neurones on the intact side (65·7 ±5·6 spikes, n=43). Late-recruited neurones were observed in 6 pairs of oxytocin neurones, and these mainly occurred in the neurones on the lesioned side (5/6). In 5 pair-recorded oxytocin neurones, bursts could also be induced synchronously by ipsilateral suckling but not by contralateral suckling. Thus it is very likely that the major mechanism synchronizing the milk-ejection bursts of oxytocin neurones in the bilateral SON is located in the region rostral to the midbrain.

Journal of Endocrinology (1995) 144, 463–470