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The first oestrus and ovulation in the rat generally occur 24 hr. after vaginal opening. Prepuberal ovariectomy delays vaginal opening when performed between the 22nd and 40th day after birth and adrenalectomy in spayed rats delays it further (Wade & Haselwood, 1941). Although, according to Pfeiffer (1936), neonatal castration does not impede vaginal opening, information is desirable on the relationship between the age of vaginal opening and the presence of the ovary for variable periods of time after birth. Vaginal opening sometimes has been taken as the clearest index of the onset of puberty (Ramaley & Gorski, 1967), but this may not be true under experimental conditions, as is the case with the precocious vaginal opening observed following administration of testosterone propionate (Colombo, 1968).
The influence of the ovary during the prepuberal period on the maturation of the genital tract has been studied in Wistar rats. The experimental animals were
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ABSTRACT
Electrical stimulation of the XIII thoracic nerve (the 'mammary nerve') causes milk ejection and the release of prolactin and other hormones. We have analysed the route of the suckling stimulus at the level of different subgroups of fibres of the teat branch of the XIII thoracic nerve (TBTN), which innervates the nipple and surrounding skin, and assessed the micromorphology of the TBTN in relation to lactation.
There were 844 ± 63 and 868 ± 141 (s.e.m.) nerve fibres in the TBTN (85% non-myelinated) in virgin and lactating rats respectively. Non-myelinated fibres were enlarged in lactating rats; the modal value being 0·3–0·4 μm2 for virgin and 0·4–0·5 μm2 for lactating rats (P > 0·001; Kolmogorov–Smirnov test). The modal value for myelinated fibres was 3–6 μm2 in both groups. The compound action potential of the TBTN in response to electrical stimulation showed two early volleys produced by the Aα- and Aδ-subgroups of myelinated fibres (conduction velocity rate of 60 and 14 m/s respectively), and a late third volley originated in non-myelinated fibres ('C') group; conduction velocity rate 1·4 m/s).
Before milk ejection the suckling pups caused 'double bursts' of fibre activity in the Aδ fibres of the TBTN. Each 'double burst' consisted of low amplitude action potentials and comprised two multiple discharges (33–37 ms each) separated by a silent period of around 35 ms. The 'double bursts' occurred at a frequency of 3–4/s, were triggered by the stimulation of the nipple and were related to fast cheek movements visible only by watching the pups closely.
In contrast, the Aα fibres of the TBTN showed brief bursts of high amplitude potentials before milk ejection. These were triggered by the stimulation of cutaneous receptors during gross slow sucking motions of the pup (jaw movements). Immediately before the triggering of milk ejection the mother was always asleep and a low nerve activity was recorded in the TBTN at this time. When reflex milk ejection occurred, the mother woke and a brisk increase in nerve activity was detected; this decreased when milk ejection was accomplished. In conscious rats the double-burst type of discharges in Aδ fibres was not observed, possibly because this activity cannot be detected by the recording methods currently employed in conscious animals.
During milk ejection, action potentials of high amplitude were conveyed in the Aα fibres of the TBTN. During the treading time of the stretch reaction (SR), a brisk increase in activity occurred in larger fibres; during the stretching periods of the SR a burst-type discharge was again observed in slow-conducting afferents; when the pups changed nipple an abrupt increase in activity occurred in larger fibres.
In summary, the non-myelinated fibres of the TBTN are increased in diameter during lactation, and the pattern of suckling-evoked nerve activity in myelinated fibres showed that (a) the double burst of Aδ fibres, produced by individual sucks before milk ejection, could be one of the conditions required for the triggering of the reflex, and (b) the nerve activity displayed during milk-ejection action may result, at least in part, from 'non-specific' stimulation of cutaneous receptors.
J. Endocr. (1988) 118, 471–483
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Instituto de Neurobiologia, Serrano 665, Buenos Aires, Argentina
(Received 9 June 1977)
Changes in the levels of catecholamines in the hypothalamus during the oestrous cycle have been reported by Donoso, Stefano & Biscardi (1966) and Stefano & Donoso (1967). Shaar & Clements (1974) found that lower concentrations of dopamine and noradrenaline than those found in the hypothalamus can inhibit the release of prolactin from the rat pituitary gland. We have studied the levels of noradrenaline and dopamine in the median eminence around the time of the spontaneous preovulatory release of hormones.
Adult female Holtzman rats showing at least three consecutive 4 day cycles, were killed by decapitation at 09.30, 13.00 and 17.30 h on the days of dioestrus I and pro-oestrus. After decapitation, blood was collected into centrifuge tubes and the brains were immediately removed and frozen on solid CO2. Serum was separated by centrifugation and stored at −20 °C
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The neurointermediate lobe of the pituitary gland of male rats was assayed for choline acetyltransferase (CAT) activity. Precise microsample punches were obtained from neurohypophysial tissue, pure pars intermedia tissue and from the junction area between them.
The level of CAT activity (pmol/h per μg protein) in the neurohypophysis, pars intermedia and junction area were 0·390 ± 0·038 (s.e.m.), 0·228 ± 0·042 and 1·824 ± 0·268 respectively. These values show an uneven distribution of CAT in the neurointermediate lobe. The hypothesis of a cholinergic system located in the junction area has been advanced.
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SUMMARY
The effect of milk reabsorption upon lactation was studied by giving daily intraperitoneal injections of milk to lactating mice during the first 5 days of lactation.
The weight gain of the young of mice injected with mouse or cow milk was similar but was less than the gain of the young of the non-injected control mice. The weight gain of the young of the mothers receiving lipid-free cow milk was similar to that of the controls, whereas that of the young of mothers receiving lipid-free cow milk with added lipids was again less than in the control group. There was no difference in litter mortality rate between the controls and the group treated with lipid-free cow milk, nor between the groups treated with mouse, cow and lipid-free cow milk with added lipids; the mortality, however, in the three latter groups was higher than in the two former ones. The injection of milk decreased milk yield. The effect of the injection of milk was observed within 24 h of the first injection.
The results suggest that milk secretion is impaired by the injection of milk. The effect may be ascribed to the lipid fraction of milk or to some unknown chemical factor within it. The rapid onset of the effect suggests that it is not mediated by an immunological reaction.
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ABSTRACT
The effects of serotonin (5-HT) on the release of gonadotrophins and LH-releasing hormone (LHRH) were examined in an in-vitro perifusion system using median eminences and/or anterior pituitaries obtained from male or pro-oestrous female rats. Animals were killed by decapitation between 12.00 and 13.00 h. A serial double-chamber perifusion system was employed. Three types of experiments were performed. In the first, median eminences were placed in the first chamber and one anterior pituitary in the second chamber. In the second group, only the anterior pituitary was perifused. In the third group, only five median eminences were perifused. In the first and second experiments, LH, FSH and prolactin were determined in the perifusion efflux by radioimmunoassay (RIA). In the third experiment, LHRH was determined by RIA.
Addition of 5-HT (final concentrations 0·06, 0·6 and 6·0μmol/l) into the first chamber containing the median eminences stimulated the release of LH and FSH from the pituitary, but did not affect the levels of prolactin in the effluent in the same experiment (prooestrous rats). The stimulatory effect of 5-HT was blocked by the addition of cyproheptadine (1 μmol/l) in the perifusion fluid. The introduction of 5-HT (0·6 μmol/l) into the tube connecting the first and second chambers did not modify the release of LH, nor did 5-HT added to the pituitaries perifused alone.
Injection of 5-HT into the first chamber (median eminences), containing tissue samples from male rats, stimulated LH release, but to a significantly (P< 0·001) lower degree than that found when samples from pro-oestrous females were used (P< 0·0001).
When median eminences from pro-oestrous rats were perifused alone, injection of 5-HT produced an immediate release of LHRH which peaked during the first 10 min of collection and lasted for 30 min; in these experiments, a clear relationship existed between dose of 5-HT and release of LHRH (P<0·02). The stimulatory effect of 5-HT was blocked by the addition of cyproheptadine (5 μmol/l) or methiothepin (5 μmol/l).
These results demonstrate that 5-HT stimulates gonadotrophin release by acting directly on LHRH terminals in the median eminence from pro-oestrous rats. Furthermore, the effect of 5-HT on LHRH release was dose dependent and was nullified by 5-HT receptor blockers (cyproheptadine and methiothepin).
J. Endocr. (1986) 111, 309–315
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ABSTRACT
The presence of dopamine in the lactotroph cell, as well as in isolated prolactin secretory granules, was demonstrated by means of an histochemical reaction for electron microscopy. Biochemical assays further confirmed the presence of dopamine in the secretory granules.
Autoradiographic preparations examined by light microscopy showed dopamine internalization in dispersed anterior pituitary cells.
Isolated anterior pituitary lactotroph cells incorporated more [3H]dopamine than a fraction containing other anterior pituitary cells.
J. Endocr. (1985) 104, 23–28