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Concentrations of prolactin in serum increased 1.8- to 7.8-fold within 6 weeks after duration of daily light was shifted from 8 to 16 hr. In comparison with cool-white fluorescent light, this increment in concentration of prolactin was similar when a light source that simulates natural sunlight (Vita-Lite), or incandescent, high-pressure sodium, or mercury vapor lamps were used. Regardless of light source, increasing the duration of daily light from 8 to 16 hr during warmer months (Aug) resulted in a greater magnitude of increase in mean prolactin concentrations as compared with increases observed during cooler months (Feb). We conclude that in prepubertal bulls, secretion of prolactin increases similarly when duration of light from lamps with different spectral properties is increased from 8 to 16 hr daily.

Prolactin in sera of four cows subjected to 30 min of continuous milking increased from 28 ± 9 ng/ml before milking to a peak of 290 ± 29 ng/ml at 20 min, and averaged 229 ± 32 ng/ml at 30 min. In comparison with 25 cows maintained in the absence of their offspring on Day 2 postpartum, 23 cows maintained immediately adjacent to their calves (suckling not permitted) decreased basal-, milking-, and TRH-induced release of prolactin by 68, 69, and 29%, respectively, on Days 3 to 5 postpartum. We conclude that direct tactile stimulation of teats maintains increased secretion of prolactin, whereas presence of the calf immediately adjacent to their mothers but without suckling, does not stimulate secretion of prolactin. Indeed, the presence of the calf may be inhibitory to the secretion of prolactin in lactating cows.

Our objective was to determine whether melatonin influenced mammary growth in response to mammogenic hormones. Prepubertal female BALB/c mice were injected for 9 days with 1 μg of 17β-estradiol and 1 mg of progesterone or 17β-estradiol/ progesterone plus 50, 100, or 200 μg of melatonin. Area of the parenchyma and total DNA content of the second thoracic gland were similar between controls and melatonin-injected mice. However, μg of DNA/100 mg of mammary tissue were lower in animals treated with 17β-estradiol/progesterone plus 200 μg of melatonin than in controls. Triglyceride content of mammary glands from animals treated with 100 or 200 μg of melatonin/day increased relative to controls. In an in vitroexperiment, thoracic mammary glands of 21-day-old mice were cultured for 6 days in a mammogenic milieu of hormones (17β-estradiol/progesterone, aldosterone, bovine prolactin, growth hormone, and insulin) with 0 (control), 10-6, 10-9, or 10-12Mmelatonin. Relative to controls, 10-12Mmelatonin increased and 10-6Mmelatonin decreased mammary DNA and uptake of [methyl-3H]thymidine. We conclude that high doses of melatonin reduce mammary development in normal mice and that some of this effect may be mediated directly at the mammary tissue.

Mammary tissue cytosols (105,000gsupernatants) were prepared from lactating or from nonlactating, nonpregnant cows, then incubated with [3H]dexamethasone alone (3 × 10-8M) or in combination with various unlabeled hormones (1.9 × 10-7or 1.9 × 10-6M). Cortisol, corticosterone, and progesterone inhibited binding of [3H]dexamethasone, whereas testosterone, cortisone, and estradiol-17 β did not. Binding affinities of mammary cytosols from lactating and nonlactating cows for [3H]dexamethasone (0.5 to 100 nM) were virtually identical averaging 1.4 (± 0.3) and 2.0 (± 1.2) × 10-9M, respectively. However, binding capacities of mammary tissue slices (fmole dexamethasone/μg DNA) were greater in lactating (2.3) than in nonlactating (1.5) cows. Specific cytosolic binding sites sedimented at 8 and 4 S on sucrose gradients. Whole mammary slices were incubated with 3 × 10-9M[3H]dexamethasone alone or in combination with 10-13to 10-5× 3.8 Mprogesterone or 3 × 10-7Mdexamethasone, and cytoplasmic and nuclear fractions (1000gsupernatants and pellets, respectively) were subsequently isolated. Specific cytoplasmic binding of [3H]dexamethasone in these slices was competitively reduced at concentrations in excess of 10-8Mprogesterone in nonlactating tissue and at concentrations in excess of 10-7Min lactating tissue. In contrast, when isolated cytosols were incubated directly, concentrations of progesterone needed to inhibit glucocorticoid binding were identical regardless of the physiological state of the mammary tissue. These data support the hypothesis that progesterone may be sequestered in milk fat and unavailable for competitive binding at the glucocorticoid binding site in lactating tissue but not in nonlactating tissue or in cytosol fractions which are relatively free of milk fat.

Early temporal changes in concentrations of prolactin (PRL) in serum after a sudden change in photoperiod and daily responsiveness to PRL-releasing and inhibiting factors were investigated in prepubertal Holstein bull calves exposed to different photoperiods. In calves switched from 8-hr light:16-hr dark to 16-hr light:8-hr dark, there was no observable change in the daily pattern of serum concentrations of PRL after 1, 2, or 4 days. On the other hand, in animals switched from 16-hr light:8-hr dark to 8-hr light:16-hr dark, there was a consistent increase in serum PRL from 33.4 ng/ml on Day 0 to maximum values of 57.3, 62.7, and 78.9 ng/ml between 14 and 18 hr after onset of light on Days 1, 2, and 4, respectively. Thus, absence of light allowed expression of a daily rhythm in serum concentrations of PRL that persisted for at least 4 days after the photoperiod switch. There were no differences in l-dopa inhibition of PRL release in animals exposed to 16-hr light:8-hr dark at 3 or 15 hr after onset of light. However, thyrotropin-releasing hormone-induced release of PRL was greater 3 hr after onset of light (11 hr after onset of dark) compared with release at 9, 15, and 21 hr after onset of light in animals exposed to 16-hr light:8-hr dark, but not in bulls exposed to 8-hr light:16-hr dark. The results provide evidence that the cue for the putative photosensitive period of PRL secretion in cattle may be more closely associated with onset of dark, not onset of light.

Changing daily exposure of prepubertal bulls from 8 hr of light: 16 hr of dark (8L: 16D) to 16L:8D or 6L:8D:2L:8D increased basal secretion of prolactin 418% 6 weeks later. When daily exposure was changed from 8L:16D (6 weeks) to 6L:14D:2L:2D (6 weeks), basal secretion of prolactin increased only 173%. Among photoperiod exposures, prolactin released into blood after injection of 33 μg/100 kg body weight of thyrotropin-releasing hormone paralleled the changes described for basal conditions. There was no repeatable diurnal secretory pattern for secretion of prolactin. The data support the hypothesis that cattle possess a photosensitive rhythm for secretion of prolactin.

Twelve heifers were exposed to 21° ambient temperature for 10 days, and then subjected to 4.5, 21, or 32° for 9 days in controlled environmental chambers. Serum prolactin (PRL) decreased linearly (P< 0.01; 0.6 ng/ml/°) as the temperature was reduced during the first day from 21 to 4.5°; serum PRL increased linearly (P< 0.05; 1.17 ng/ml/°) as the temperature was increased from 21 to 32°. Between Days 2 and 9 serum PRL averaged 2.6, 13.0, and 27.7 ng/ml (P< 0.05) at 4.5, 21, and 32°, respectively. Injection of thyrotropin-releasing hormone (TRH) caused serum PRL to increase within 5 min from 20.4 to 109.8 ng/ ml at 32°; at 21° serum PRL increased from 15.7 to 62.8 ng/ml, whereas at 4.5° serum PRL did not respond to TRH. Serum growth hormone (GH) averaged 4.0, 6.3, and 9.4 ng/ml at 4.5, 21, and 32°, respectively, but these means were not different (P> 0.10). TRH released GH at all temperatures tested, but the quantity released was unaffected by ambient temperature. Relative humidities of 50 and 90% did not significantly alter (P> 0.05) serum PRL or GH. We conclude that ambient temperature affects basal and TRH-stimulated concentrations of serum PRL but not GH in heifers.The technical assistance of Eileen Bostwick, Larry Brock, and David Kreider is most appreciated.

Compounds native to the pineal were screened using a bovine pituitary cell culture system to investigate their effects on pituitary prolactin secretion. Thyrotropin releasing hormone (10−11-10−7M) and arginine vasotocin (10−10-10−6M) increased prolactin in medium and this increase was linearly related to increasing log concentrations of each peptide. In contrast, dopamine (10−9-10−7M) and norepinephrine (10−10-10−8M) decreased prolactin accumulation in medium and decrease was linearly related to increasing log concentrations of catecholamines. Serotonin (10−10-10−6M) also inhibited prolactin release and decrease was linearly related to increasing log concentrations of serotonin. Melatonin, 5-hydroxytryptophol and 5-methoxytryptophol had no direct effect on pituitary prolactin secretion. We conclude that compounds indigenous to the pineal have the potential to regulate PRL secretion from the pituitary.

The role of the eyes and pineal gland on photoperiod- and season-induced changes in secretion of prolactin were studied in male cattle. Increasing exposure to light from 8 to 16 hr each day increased basal and thyrotropin-releasing-hormone-induced secretion of prolactin 3.7- and 4.1-fold in three sham-pinealectomized steers. In contrast, basal and thyrotropin-releasing- hormone-induced increases in secretion of prolactin did not change in four blind bulls and were markedly suppressed in three pinealectomized steers when exposure to light was increased from 8 to 16 hr/day. There was no diurnal variation in secretion of prolactin regardless of photoperiod or surgical treatment. However, seasonal changes (averaged 46 ng/ml in June-Aug vs 7 ng/ml in Dec-Feb) in secretion of prolactin persisted in blind and pinealectomized steers previously shown to be nonresponsive to changing photoperiods. Ambient temperature and photoperiod account for most, but not all, of the seasonal variation in secretion of prolactin. We hypothesize there is an endogenous annual rhythm in the secretion of prolactin in cattle.

Three experiments were conducted to determine serum growth hormone (GH) response of bull calves (N= 4; 83 kg body wt) to iv injections and infusions of human pancreatic GH-releasing factor 1-40-OH (hpGRF). Peak GH responses to 0, 2.5, 10, and 40 μg hpGRF/100 kg body wt were 7 ± 3, 8 ± 3, 18 ± 7, and 107 ± 55 (mean peak height ± SEM) ng/ml serum, respectively. Only the response to the 40-μg dose was greater (P< 0.05) than the 0-μg dose. Concentrations of prolactin in serum were not affected by hpGRF treatment. In calves injected with hpGRF (20 μg/100 kg body wt) at 6-hr intervals for 48 hr, GH increased from a mean preinjection value of 3.1 ng/ml serum to a mean peak response value of 70 ng/ml serum. Differences in peak GH response between times of injection existed within individual calves (e.g., 10.5 ng/ml vs 184.5 ng/ml serum). Concentrations of GH in calves infused continuously with either 0 or 200 μg hpGRF/hr for 6 hr averaged 7.4 ± 3 and 36.5 ± 11 ng/ml serum, respectively (P< 0.05). Concentrations of GH oscillated markedly in hpGRF-infused calves, but oscillations were asynchronous among calves. We conclude that GH response of bull calves to hpGRF is dose dependent and that repeated injections or continuous infusions of hpGRF elicit GH release, although magnitude of response varies considerably. We hypothesize that differences in GH response to hpGRF within and among calves, and pulsatile secretion in the face of hpGRF infusion may be related to the degree of synchrony among exogenous hpGRF and endogenous GRF and somatostatin.

Interruption of suckling in rats for a single period of up to 12 hr in the immediate postpartum period or on day 10 of lactation had no significant effect on subsequent litter weights or mammary nucleic acid content. Interruption for 24 hr in both early and midlactation reduced litter weights and mammary gland RNA content but had no significant effect on weight of anterior pituitary, adrenal, or mammary gland DNA content. The results suggest that the effect of interrupted nursing is to reduce mammary gland metabolic activity without altering cell number and furthermore, that the effect probably occurs locally within the mammary gland.

Total mammary DNA per 100 g of body weight increased 9.8% from day 20 of pregnancy to day 1 of lactation and 25.9% from day 1 to day 4 of lactation. Ovariectomy on day of parturition had no influence on mammary hyperplasia during early lactation. On the basis of total DNA per 100 g of body weight maximum mammary development occurred on day 8 of lactation (11.51 mg) and 24.8% of mammary growth occurred during lactation. No significant decrease occurred in mammary DNA until after day 24 of lactation. With the onset of lactation total mammary RNA increased much more rapidly than did total mammary DNA. Maximum mammary RNA content was observed on day 21 of lactation (75.06 mg) after which it declined precipitously to day 28 of lactation (39.61 mg). Mammary gland RNA/DNA ratio increased rapidly when lactation was initiated (1.24), attained its maximum on day 21 of lactation (2.95) and then decreased to day 28 of lactation (1.96). It is suggested that RNA/DNA ratios, when plotted against stage of lactation, represent the normal lactation curve of the rat.

Teat ligation of 3 abdominal-inguinal mammary glands of lactating rats markedly decreased DNA and RNA content and RNA/DNA ratio when compared with similar estimates of either contralateral, non-ligated glands or glands from normal rats lactating the same length of time. Suckling stimulus, without milk removal, did not prevent cellular loss and did not maintain level of protein synthesis. DNA and RNA content and RNA/DNA ratio of contralateral, non-ligated glands were greater than those of glands of normal rats lactating comparable periods of time.

Daily injection of 1 or 10 mg of prolactin for 10 days or transplantation of anterior pituitaries (AP) to the kidney capsule for 14 days markedly decreased pituitary weight and prolactin content of mature virgin rats. Ovariectomy reduced prolactin content of pituitaries relative to intact controls, but ovariectomy plus transplantation of AP's reduced prolactin content even further. These results indicated that function of the auto-feedback control of prolactin can occur without the ovary. Prolactin injections and AP transplants increased total mammary DNA and RNA, but these increases were somewhat reduced in ovariectomized rats compared with intact rats. Exogenous prolactin reduced uterine and adrenal weights, stimulated body weight increases but had no significant effects on ovarian weights.

Mammary and uterine development and their relationships to the pituitary content of prolactin and LH were studied during the five estrous cycles after vaginal opening in rats. Cumulative mammary and uterine growth was completed largely by the fourth estrous cycle after puberty. Although there was a lag of 1 day between peak mammary and uterine development, virtually all of this growth occurred during the estrogenic phase of the estrous cycle. Involutionary changes occurred in both organs during the progestational phase of the estrous cycle. Pituitary prolactin concentration was greater at proestrus (0.032 IU/mg) and metestrus (0.041 IU/mg) than at estrus (0.015 IU/mg) or diestrus (0.015 IU/mg). Pituitary LH concentration, which averaged 0.72, 0.31, 0.27, and 0.33 μ/mg at proestrus, estrus, metestrus, and diestrus, respectively, was correlated (r=0.58, p<.01) with changes in uterine RNA/DNA ratio.

Exogenous prolactin (3 or 9 mg NIH–B3/hr) or saline was infused for 2.5 hr into 6 cows. The prolactin or saline infusions were repeated on alternate days for a total of 4 days. Approximately 45 min was required to achieve a stable baseline after the prolactin infusion was started. The exogenous prolactin, which increased serum prolactin approximately 5– and 8–fold above baseline endogenous concentrations, did not prevent the release of endogenous prolactin in response to milking. When the infusion was stopped serum prolactin disappeared with a 1/2 time of 23–25 min. We conclude that the inhibitory effects of exogenous prolactin previously reported on endogenous prolactin secretion must be exerted primarily on synthesis rather than release mechanisms, or the animals may require chronic exposure to high concentrations of prolactin.The able technical assistance of Mrs. Annemieke Ambrosier is gratefully acknowledged.

Within rats lactating 13 to 14 days, non-suckled mammary glands contained significantly more DNA than suckled glands, and this difference increased in a linear manner as the non-suckling interval increased from 0 to 12 hours. It is suggested that increased numbers of leucocytes account for this difference. The presence or absence of milk did not affect RNA content, and it is suggested that the rat mammary gland has the potential to secrete milk proteins at a constant rate up to 12 hours.

The effects of ambient temperature on serum concentrations of prolactin and neurochemical estimates of activity of dopaminergic neurons projecting to the infundib-ulum/pituitary stalk were investigated in Holstein bull calves. Accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the infundibulum/pituitary stalk after intravenous injection of a decarboxylase inhibitor was used to estimate activity of these dopaminergic neurons. Increasing ambient temperature from 21 to 33°C for 22 hr increased serum concentrations of prolactin and decreased activity of the dopaminergic neurons. Conversely, reducing ambient temperature from 22°C to 11°C decreased serum concentrations of prolactin and increased activity of these dopaminergic neruons. It is suggested that alterations in activity of dopaminergic neurons terminating in the infundibulum/pituitary stalk of bull calves may mediate acute temperature-induced changes in secretion of prolactin.

Total DNA of rat mammary glands per 100 g of body weight increased from 2.77 mg to 7.89 mg during pregnancy, a 184.8% increase, and total DNA increased from 4.76 mg to 17.19 mg, a 261.1% increase. Total RNA increased from 3.06 mg to 15.44 mg during pregnancy and these increases paralleled those of total DNA. RNA/DNA ratio increased from 0.64 to 0.90 as gestation advanced.

DNA per 100 g of body weight and total DNA in 6 abdominalinguinal mammary glands one day after weaning did not differ significantly from that of glands on the 21st day of lactation. DNA content decreased significantly 3 days after weaning. On the 21st day of involution mammary DNA per 100 g of body weight was similar to that of glands from sexually mature virgin rats. One day after weaning RNA content decreased 26.2% and on the 21st day of involution it approximated that of glands from sexually mature virgin rats. RNA/DNA ratio was reduced 31.9% one day after weaning and by the 12th day it was lower than that of glands from sexually mature virgin rats.

The influence of varying numbers of suckling young on the number of cells (DNA) and protein synthetic activity (RNA and RNA/DNA) was determined in abdominalinguinal mammary glands of rats lactating 21 days. The data indicated that mammary gland development and secretory activity increased as the number of suckling young per mother rat increased.

Concurrent pregnancy significantly enhanced mammary DNA and RNA in rats lactating 18 days when compared with non-pregnant rats lactating 18 days, but the RNA/DNA ratio was not changed. Concurrent pregnancy in rats lactating 21, 24, and 28 days produced, respectively, no change, significant decrease, and no change in nucleic acid content when compared with lactating, non-pregnant rats. Within days 24 and 28 of lactation, nucleic acid content decreased markedly with advancing pregnancy.Standardization of gestation length showed that glands of rats with 7 or more fetuses contained highly significantly less DNA and RNA than glands of lactating, non-pregnant rats.