Dolores Aguayo

Central Effects of Catecholamines upon Mammary Contractility in Rats Are Neurally Mediated

We injected, i.e., intracerebroventricularly (ICV) or systematically, small amounts of adrenaline (ADR), noradrenaline (NA), isoproterenol (ISOP) and dopamine (DA) in urethane-anesthetized lactating rats, and determined the effects on isometrically recorded intramammary pressure (IMP) responses to exogenous oxytocin (OXY). While centrally administered ADR, NA and DA provoked increased IMP responses to OXY, the beta-adrenergic agonist ISOP induced the opposite effect. These effects were reversible, dose related and also occurred in hypophysectomized rats. However, when injected systematically, all adrenergic agonists but DA depressed IMP responses to OXY. Further experiments showed that central effects of catecholamines were exerted by regulating ductal tone, through the direct innervation of the mammary glands. Thus, whereas complete blockage of these effects occurred after selective denervation of the mammary glands, increased ductal tone resulted from ICV administration of ISOP. Finally, evidence was also obtained that antagonistic alpha- and beta-adrenergic mechanisms may interact with each other to regulate milk ejection, and with afferent signals from the mammary glands. Thus, beta-adrenergic inhibition upon IMP was counteracted by either NA administration or by activation of ductal mechanoreceptors. Together, these results suggest that regulation of milk ejection may involve neurally mediated influences on mammary contractility. Such actions would interact closely with afferents from the mammary gland influencing ductal tone.

Differential Effects of Thyrotropin-Releasing Hormone on in vitro Release of in vivo or in vitro Newly Synthesized and Mature Prolactin by Lactating Rat Adenohypophyses

We studied the basal and thyrotropin-releasing hormone (TRH)-stimulated in vitro release of newly synthesized and mature prolactin (PRL) by lactating rat pituitary fragments. Newly synthesized PRL was defined as that resulting from either in vivo pulse labelling with [3H]-leucine (injected i.v. 10 min before removing the pituitary glands for in vitro incubation); or in vitro labelled by a 5-min preincubation period of adenohypophyseal fragments. Mature PRL was defined as that labelled in vivo 8 h before incubation. Medium PRL was quantified by polyacrylamide-gel-electrophoresis densitometric and liquid scintillation techniques. Under basal conditions, both total (unlabelled) and 8-hour labelled (mature) PRLs showed an in vitro release pattern characterized by an initial period of faster and higher secretion that lasted for 30-60 min followed by a lower secretion rate that extended from 60 to 240 min of incubation. Secretion of newly synthesized PRLs was detected since the first 30 min, but did not reach maximum levels until 90-120 min of incubation. Accordingly, PRL release during the first 30-60 min of incubation consisted primarily of mature PRL, whereas secretion during the rest of incubation included the release of newly synthesized hormone. These results confirm previous data on the sequential, rather than preferential, release of different age PRLs. TRH (0.35 microM) stimulated (about 30% above control) the release of both total and mature PRLs. No effect could be detected upon the release of in vivo labelled new PRL, while the same dose of TRH strongly stimulated (about 100% above control) the release of in vitro labelled new PRL.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Acute Increases in Suckling Frequency upon Food Intake and Milk Secretion in the Rabbit

Abstract The normal once-a-day frequency of suckling in rabbits was altered on Day 10 (early) and Day 30 (late lactation) by the addition of either one (8 hr after) or two extra sucklings (8 and 16 hr after) the daily suckling. One additional suckling significantly increased milk yield on Day 10 in comparison with the average 4-day milk yield before and after Day 10 whereas two additional sucklings decreased the increment. Either one or two additional sucklings on Day 30 significantly accelerated the already decreasing daily milk yields characteristic of late lactation. Not only was less milk secreted on Day 30 than on Day 10, as expected, but it was also secreted in a less consistent manner throughout the 24-hr period. The lower milk yields in late lactating rabbits were not due to depressed food intake; paradoxically the quantity of food ingested per gram of milk produced was greater at this time. These data suggest that suckling may activate mechanisms in late lactation in the rabbit which offset the stimulatory effects of suckling upon milk secretion.

Ultrastructure and motility of the caudal epididymis spermatozoa from the volcano mouse (Neotomodon alstoni alstoni Merriam, 1898)

The volcano mouse Neotomodon alstoni alstoni is a genus endemic to the higher elevations of the Mexican transvolcanic belt. In the present study we examined for the first time the morphological features of the spermatozoa taken from the caudal epididymis of this species by transmission and scanning electron microscopy. Spermatozoan motility was studied in sucrose and bicarbonate solutions; vitality and morphology were observed by light microscopy. Transmission electron microscopy shows that the head of spermatozoon is asymmetric and possesses a large and curved hook. The axoneme of the spermatozoan tail is highly developed at fibers 1, 5, and 6. Absolute and relative measurements of the length of the head, the midpiece, and the rest of the tail were also obtained. N. alstoni alstoni spermatozoa were hyperactive in the presence of 290 mM sucrose and 10 and 20 nM bicarbonate solutions exhibited high motility (180-190 microm/sec), and high flagellum beating frequency (10-12 Hz). In contrast, the spermatozoa in 310 mM sucrose solution showed scarce motility (13.5 +/- 3.8 microm/sec) and low beating frequency (1.5 +/- 0.4 Hz). It is proposed that the volcano mouse spermatozoa possess some features very similar to other mammalian spermatozoa and that bicarbonate triggers caudal epididymal sperm motility of this species. J. Exp. Zool. 287:316-326, 2000.

Prolactin and propranolol prevent the suckling-induced inhibition of lactation in rabbits.

The normal once-a-day frequency of suckling in the rabbit was increased on day 31 (late lactation) by the addition of two extra sucklings (8 and 16 hr after) the daily suckling. In confirmation of previous data, two additional sucklings significantly decreased milk yield acutely on day 31 in comparison with the average 4-day milk yield before and after day 31. The decrease in milk secretion after the two additional sucklings was prevented by a single injection of 3 mg prolactin (given 24 hr before the two extra sucklings) and/or by injections of the beta-adrenergic-blocking drug, propranolol (100 micrograms/kg b. wt. given 30 min before each additional suckling). Since prolactin secretion is decreased in these species and the mammary gland is less responsive to the hormone during late lactation, the present results suggest that in addition to these factors, suckling-induced activation of sympathetic influences may contribute to the decline in milk production at this stage of lactation. Taken together, these results suggest that suckling may regulate lactation in the rabbit through antagonistic mechanisms at different stages of lactation.

Effect of posterior pituitary lobectomy on in vivo and in vitro secretion of prolactin in lactating rats.

The effect of removing the posterior and neuro-intermediate lobes (PLX) of the pituitary gland of lactating rats was determined on both suckling-induced release and transformation of prolactin (PRL), and on regionalization of PRL release. Sixteen hours, or 1 or 4 d after either PLX or sham surgery, acute (15-min) suckling was applied. Also, regionalization of PRL release was analyzed by incubating the central and peripheral regions of APs from nonsuckled rats. Plasma PRL was analyzed by radioimmunoanalysis (RIA), whereas anterior pituitary (AP) PRL content and in vitro released PRL were determined by polyacrylamide gel electrophoresis. Plasma PRL increased 25- to 30-fold after suckling in intact and sham, and 10- to 15-fold in 1- and 4-d PLX rats, but no change occurred on either 16-h PLX nonsuckled and suckled rats. Also, PRL transformation occurred in intact, sham, and 4-d PLX suckled rats, but not in 16-h sham, or in 16-h and 1-d PLX suckled rats. Finally, the higher secretion of PRL shown in vitro by the central region of APs from intact and sham was not observed in APs from PLX rats. These results show that PLX transiently depresses the suckling-regulated PRL transformation and release. Likewise, influences from the posterior and/or neuro-intermediate lobes may determine regionalization of PRL release.

Effect of short-term food deprivation and prolactin upon milk yield in the lactating rabbit☆

Abstract Lactating rabbits in early lactation were deprived of food but not water for 4 or 8 hr either before or after the daily suckling on day 11 or from the 4th–8th hr after the daily suckling on day 11; rabbits in late lactation similarly were deprived of food for 8 hr after the daily suckling on day 31. In each instance, food deprivation caused a significant reduction in the amount of milk secreted during the first 8 hr following the daily suckling, but had no effect upon that secreted during the subsequent 16 hr. Prolactin (1–6 mg/12 hr) for 48 hr before food withdrawal did not ameliorate the effect of food deprivation upon milk secretion in early lactation, but fully restored milk secretion to fed control levels in rabbits during late lactation. These data support the concept that food intake is a short-term influence upon milk secretion and suggest a reduced availability of PRL as a primary causal factor in the reduced milk secretion which is characteristic of late lactation in the rabbit.