Plasticity in Intrinsic Excitability of Hypothalamic Magnocellular Neurosecretory Neurons in Late-Pregnant and Lactating Rats.

Oxytocin and vasopressin secretion from the posterior pituitary gland are required for normal pregnancy and lactation. Oxytocin secretion is relatively low and constant under basal conditions but becomes pulsatile during birth and lactation to stimulate episodic contraction of the uterus for delivery of the fetus and milk ejection during suckling. Vasopressin secretion is maintained in pregnancy and lactation despite reduced osmolality (the principal stimulus for vasopressin secretion) to increase water retention to cope with the cardiovascular demands of pregnancy and lactation. Oxytocin and vasopressin secretion are determined by the action potential (spike) firing of magnocellular neurosecretory neurons of the hypothalamic supraoptic and paraventricular nuclei. In addition to synaptic input activity, spike firing depends on intrinsic excitability conferred by the suite of channels expressed by the neurons. Therefore, we analysed oxytocin and vasopressin neuron activity in anaesthetised non-pregnant, late-pregnant, and lactating rats to test the hypothesis that intrinsic excitability of oxytocin and vasopressin neurons is increased in late pregnancy and lactation to promote oxytocin and vasopressin secretion required for successful pregnancy and lactation. Hazard analysis of spike firing revealed a higher incidence of post-spike hyperexcitability immediately following each spike in oxytocin neurons, but not in vasopressin neurons, in late pregnancy and lactation, which is expected to facilitate high frequency firing during bursts. Despite lower osmolality in late-pregnant and lactating rats, vasopressin neuron activity was not different between non-pregnant, late-pregnant, and lactating rats, and blockade of osmosensitive ΔN-TRPV1 channels inhibited vasopressin neurons to a similar extent in non-pregnant, late-pregnant, and lactating rats. Furthermore, supraoptic nucleus ΔN-TRPV1 mRNA expression was not different between non-pregnant and late-pregnant rats, suggesting that sustained activity of ΔN-TRPV1 channels might maintain vasopressin neuron activity to increase water retention during pregnancy and lactation.

Ocitocina na produção e composição do leite / Oxytocin application in production and milk composition

Objetivou-se analisar a aplicação da ocitocina influenciando na eficiência da ejeção do leite; ao ponto de ocorrer um aumento da produção e mudanças na composição. Foram utilizados 45 animais, durante 105 dias, divididos em três tratamentos, no tratamento 1 (T1) os animais receberam 0,5ml de soro fisiológico, logo após realizava-se a ordenha destes animais, sendo considerado o tratamento controle, no T2 os animais receberam ocitocina antes do início da ordenha, e logo após ordenhava-se estes animais; no T3 os animais receberam ocitocina ao final da ordenha e reordenhava-se estes animais. Para aferir a produção das vacas foram usados medidores de leite, com pesagens de produção realizadas quinzenalmente. Após aferido a produção de todos os animais, as amostras de leite foram coletadas e encaminhadas para as análises posterior. As variáveis físico-químicas do leite analisadas foram: porcentagem de gordura, lactose e proteína. Os resultados foram submetidos à análise de variância e posteriormente, ao teste de comparação de médias, com diferença estatística significativa de 5%. De acordo com os resultados obtidos a ocitocina influenciou na produção leiteira com um aumento na produtividade diária por animal, mas não se observou diferenças significativas nas médias de porcentagem de gordura, lactose e proteína do leite.

This study aimed to analyze the application of oxytocin influence in the milk ejection efficiency; as to occur an increase in production and changes in milk composition. 45 animals were used for 105 days, divided into three treatments, the treatment one received 0.5ml of physiological saline, in the treatment two were given oxytocin to the animals before the milking; and in treatment theree the animals received oxytocin at the end of milking. To measure the cow's milk production, milk meters were used, and were performed fortnightly. After measured the production of all animals, samples were collected and sent for analysis. The physico-chemical variables analyzed were: percentage of fat, lactose and protein. The results were submitted to ANOVA and subsequently the mean comparison test, with statistical significance of 5%. According to obtained results oxytocin influence the milk production with a significant increase in daily productivity per animal, but there was no significant difference in percentage of fat, lactose and milk protein.

Oxytocin Induces Mammary Epithelium Disruption and Could Stimulate Epithelial Cell Exfoliation.

Mammary epithelial cells (MEC) are exfoliated from the epithelium into milk, influencing the number of MEC present in the udder. This process is associated with epithelium integrity. The release of oxytocin (OT) induced by milking causes myoepithelial cell contraction, which, in turn, may stimulate MEC exfoliation through mechanical forces. To investigate the role of OT in MEC exfoliation, we inhibited or induced myoepithelial cell contraction by injecting the OT receptor antagonist atosiban (Ato) or a supraphysiological dose of OT, respectively. Eight cows were assigned to 2 treatments during 2 milkings according to a crossover experimental design: Control+OT (cows were first milked to collect standard milk and then received 5 IU of OT to collect residual milk through a second milking) and Ato + OT (cows were injected with Ato (50 µg/kg of body weight) and milked to collect cisternal milk, then received 5 IU of OT to collect alveolar milk through a second milking). Milk MEC were purified to determine their concentration and number in milk. Mammary epithelium integrity was assessed by measuring the kinetics of plasma lactose concentration. Inhibiting myoepithelial cell contraction by Ato injection decreased the number of exfoliated MEC in milk. In contrast, OT injection increased the concentration of MEC in the residual milk and the number of MEC in the alveolar milk. Ato injection reduced plasma lactose concentration, whereas, in both treatments, OT injections increased it. Our results suggested that myoepithelial cell contraction caused by OT could stimulate MEC exfoliation into milk and was associated with epithelium disruption.

The Peptide Oxytocin Antagonist F-792, When Given Systemically, Does Not Act Centrally in Lactating Rats.

Oxytocin secreted by nerve terminals in the posterior pituitary has important actions for ensuring a successful outcome of pregnancy: it stimulates uterine contractions that lead to birth and it is essential in the milk-ejection reflex, enabling milk to be expelled from the mammary glands into the mouths of suckling young. Oxytocin also has important actions in the brain: released from dendrites of neurones that innervate the posterior pituitary, oxytocin auto-excites the neurones to fire action potentials in co-ordinated bursts, causing secretion of pulses of oxytocin. Central oxytocin actions are blocked by an oxytocin antagonist given into the brain and, consequently, milk transfer stops. Systemic peptide oxytocin antagonist (atosiban) treatment is used clinically in management of pre-term labour, a major obstetric problem. Hence, it is important to know whether an oxytocin antagonist given peripherally can enter the brain and interfere with central oxytocin actions. In the present study, we tested F792, a peptide oxytocin antagonist. In urethane-anaesthetised suckled rats, we show that the mammary gland responsiveness to oxytocin is blocked by i.v. injections of 7 µg/kg of F792, and the milk-ejection reflex is blocked when F792 is given directly into the brain at a dose of 0.2 µg. To critically test whether F792 given systemically can enter the brain, we recorded the suckling- and oxytocin-induced burst-firing of individual antidromically identified oxytocin neurones in the paraventricular nucleus. Given systemically at 100 µg/kg i.v., F792 acted only peripherally, blocking the milk-ejecting actions of oxytocin, but not the burst-firing of oxytocin neurones during suckling (n = 5 neurones in five rats). Hence, this peptide oxytocin antagonist does not enter the brain from the circulation to interfere with an essential oxytocin function in the brain. Furthermore, the functions of oxytocin in the brain evidently cannot be explored with a systemic peptide antagonist.

Do not use drugs to prevent onset of lactation. Relieve the discomfort and wait.

Breast-feeding is not advisable in certain situations and some women do not want to breast-feed. If the woman does not breast-feed, lactation ceases after one or two weeks. Where does the evaluation of methods used to prevent onset of lactation stand in 2012? To answer this question, we reviewed the available evidence, based on the standard Prescrire methodology. Among the physical methods sometimes proposed, breast binding causes greater discomfort than wearing a bra. Dopamine agonists, such as bromocriptine, are effective in inhibiting lactation. But the serious, mainly cardiovascular, adverse effects they provoke are disproportionate to the discomfort they prevent.These drugs are best avoided. High doses of oestrogens inhibit lactation, but the risk of thromboembolism they pose is unreasonable in the postpartum setting. Neither diuretics nor homeopathy have been shown to have any tangible efficacy against the discomfort associated with onset of lactation. In practice, a standard analgesic such as paracetamol generally eases the few days of discomfort or pain associated with the onset of lactation. Wearing a bra is risk-free and sometimes provides relief. Breast discomfort, however intense, does not justify exposing women to the serious adverse effects linked to postpartum administration of dopamine agonists or oestrogens.

The effect of temperament and responsiveness towards humans on the behavior, physiology and milk production of multi-parous dairy cows in a familiar and novel milking environment.

The objectives of this study were to investigate whether; 1) temperament or 2) behavioral responsiveness to humans, can affect the behavior, physiology and productivity of dairy cows being milked in a familiar and novel milking environment. Temperament of multi-parous cows was defined based on exit time from a restraint device, as High Responders (HR; n=10), Medium Responders (MR; n=10) or Low Responders (LR; n=10). The behavioral response of cows to humans was assessed using four tests: restraint, exit speed, avoidance distance test and a voluntary approach test. Cows were milked according to their established routines in a rotary (familiar) milking parlor and behavioral, physiological and production data were collected over five consecutive days, including heart rate, cortisol and oxytocin concentrations and milk yield. The following week, cows were milked in a novel environment (herringbone parlor within the same farm facility) over five consecutive days, and the data and sample collection program was repeated. Cows were then given an exogenous adrenocorticotropic hormone (ACTH) challenge to measure adrenal responsiveness. Exit time was negatively correlated with the behavioral responses of cows to restraint and human avoidance distance (HAD) in the paddock and arena. The behavioral response of cows to the milking process was greater in MR than LR and HR cows in the familiar and novel milking environments. Milk yields were greater in LR than HR cows in the novel but not the familiar milking parlor. Oxytocin concentrations increased during milking in the novel environment, regardless of cow temperament. In the familiar and novel environments, heart rates were higher in HR than LR cows before and during milking and rMSSD was lower in HR cows during milking in a novel environment. There was no difference in cortisol concentrations between LR and HR cows in response to an ACTH challenge, but HR cows had higher baseline cortisol levels than LR cows. The number of leg lifts cows performed when restrained in the crush was associated with several physiological and milk production measures. These results indicate that cows with different temperaments vary in their basal physiology as well as their behavioral and physiological responses to milking and stress associated with being milked in a novel environment.

Treatments for suppression of lactation.

BACKGROUND: Various pharmacologic and non-pharmacologic interventions have been used to suppress lactation after childbirth and relieve associated symptoms. Despite the large volume of literature on the subject, there is currently no universal guideline on the most appropriate approach for suppressing lactation in postpartum women. OBJECTIVES: To evaluate the effectiveness and safety of interventions used for suppression of lactation in postpartum women (who have not breastfed or expressed breastmilk) to determine which approach has the greatest comparative benefits with least risk. SEARCH METHODS: We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (30 June 2012). SELECTION CRITERIA: Randomised trials evaluating the effectiveness of treatments used for suppression of postpartum lactation. DATA COLLECTION AND ANALYSIS: Two review authors independently assessed trial quality and extracted data. MAIN RESULTS: We included 62 trials (6428 women). Twenty-two trials did not contribute data to the meta-analyses. The trials were generally small and of limited quality. Three trials (107 women) indicated that bromocriptine significantly reduced the proportion of women lactating compared with no treatment at or within seven days postpartum (three trials, 107 women; risk ratio (RR) 0.36, 95% confidence interval (CI) 0.24 to 0.54). Seven trials involving oestrogen preparations (diethylstilbestrol, quinestrol, chlorotrianisene, hexestrol) suggested that they significantly reduced the proportion of lactating women compared with no treatment at or within seven days postpartum (RR 0.40, 95% CI 0.29 to 0.56). We found no trials comparing non-pharmacologic methods with no treatment. Trials comparing bromocriptine with other pharmacologic agents such as methergoline, prostaglandins, pyridoxine, carbegoline, diethylstilbestrol and cyclofenil suggested similarity in their effectiveness. Side effects were poorly reported in the trials and no case of thromboembolism was recorded in the four trials that reported it as an outcome. AUTHORS' CONCLUSIONS: There is weak evidence that some pharmacologic treatments (most of which are currently unavailable to the public) are better than no treatment for suppressing lactation symptoms in the first postpartum week. No evidence currently exists to indicate whether non-pharmacologic approaches are more effective than no treatment. Presently, there is insufficient evidence to address the side effects of methods employed for suppressing lactation. When women desire treatment, bromocriptine may be considered where it is registered for lactation suppression in those without predisposition to its major side effects of public concerns. Many trials did not contribute data that could be included in analyses. Large randomised trials are needed to compare the effectiveness of pharmacologic (especially bromocriptine) and non-pharmacologic methods with no treatment. Such trials should consider the acceptability of the intervention and lactation symptoms of concern to women and be large enough to detect clinically important differences in major side effects between comparison groups.

Sudden loss of milk supply following high-dose triamcinolone (Kenacort) injection.

Endogenous corticosteroids are involved in breast development, initiation and maintenance of milk production. Animal studies have shown that exogenous corticosteroids diminish milk production and milk ejection. A high dose depot injection of triamcinolone resulted in dramatic reduction in milk production in an established lactation. Domperidone and frequent expression restored milk production. Lower dose depot injection of betamethasone into the shoulder joint did not noticeably reduce milk production.

Metoclopramide or domperidone for increasing maternal breast milk output: a randomised controlled trial.

OBJECTIVE: To compare the effects of metoclopramide and domperidone on the breast milk output of mothers with infants in neonatal intensive care. DESIGN: Double-blind randomised controlled trial. SETTING: Tertiary level neonatal intensive care unit (NICU). SAMPLE: Eighty mothers expressing breast milk for their infants (mean gestational age 28 weeks) based in NICU and the amounts expressed fell short of the prescribed target. INTERVENTION: Mothers were randomised to receive domperidone or metoclopramide for 10 days (10 mg three times a day). OUTCOME MEASURES: Total milk volume daily for up to 10 days before the medication, 10 days during the trial and up to 10 days after medication. Adverse side effects were also recorded. RESULTS: Mothers produced more milk in the domperidone group and achieved a mean of 96.3% increase in milk volume (mean increase/pretrial volume) compared with a 93.7% increase for metoclopramide. After adjusting for the amount of milk produced prior to medication, the mean amount of milk produced while taking medication for those on domperidone was 31.0 ml/24 h (95% CI -5.67 to 67.6) greater than the mean for those on metoclopramide. Seven mothers taking metoclopramide reported side effects and three taking domperidone; a further eight women (of 29) who had a follow-on prescription for metoclopramide also reported side effects. CONCLUSIONS: Oral domperidone and metoclopramide increased the volume of milk produced by mothers who are expressing to feed their babies in NICU. There were small differences in milk output between the two medications and in the incidence of side effects, but the differences were non-significant.

Effect of 2,4-dichlorophenoxyacetic acid on milk transfer to the litter and prolactin release in lactating rats.

The effects of 2,4-dichlorophenoxyacetic acid (2,4-D) on brain monoamines and the serum level of hormones involved in milk synthesis and on the milk ejection reflex in rats were evaluated. Dams were treated with 2.5, 5, 15, 25, 50 or 70mg 2,4-D/kg bw according to two experimental designs: (a) through food from post partum day 1 (PPD 1) to PPD 16 and the respective control groups or (b) an unique i.p. injection on PPD 11. To measure milk ejection, the litter was separated from the mother at the 11th day of lactation during 8h, returned to their mothers and allowed to suckle for a period of 15min. The procedure was repeated on 3 consecutive days until the end of treatment. The change in litter weight during the suckling period was taken as a measure of the amount of milk ejected during this period. The dams' serum prolactin (PRL), oxytocin (OT) and growth hormone levels were determined by radioimmunoassay. Both treatment regimens produced a dose-dependent decrease in the amount of milk ejected and circulating PRL and OT secreted in response to the suckling stimulus. Administration of OT before returning the pups restored the milk ejection, indicating no impairment in the capacity of the mammary gland to produce and secrete milk. In addition, dopamine levels were increased by the 2,4-D treatments in arcuate nucleus (ArN) and anterior lobe of pituitary gland (AL), while serotonin level was drastically decreased in ArN. 2,4-D treatment increased both calcium-dependent and calcium-independent nitric oxide synthase (NOS) activities in ArN. These results suggest that 2,4-D inhibits the suckling-induced hormone release, milk transfer to the litter at the central level, through a stimulation of hypothalamic NOS and dopamine and by an inhibition of hypothalamic serotonin transmission.

Suitability of low-dosage oxytocin treatment to induce milk ejection in dairy cows.

Chronic use of high oxytocin (OT) dosages can cause a reduced response to endogenous OT. In this study the OT dosages used in the milking practice of 82 dairy cow farms were recorded. The OT dosages per cow used were high, especially when injected i.m. (23+/-2 IU) compared with i.v. (7+/-1 IU). In addition, the minimum OT dosages needed to obtain normal milk removal in cows with disturbed milk ejection were investigated. Seventeen cows routinely treated with OT during milking (group T) and 17 cows without previous OT treatment were used (group C). After cessation of spontaneous milk flow, both T and C groups were injected i.v. with a low dosage of OT (0.2 or 0.5 IU/cow). The time from injection until cessation of the OT-induced milk flow was recorded (response phase). The response phase and the amounts of removed milk by effect of the OT injection increased with increasing OT dosage. Values for 0.2 and 0.5 IU/cow of OT injected i.v. were (response phase and amount of milk removed) 198+/-27 and 302+/-18s and 3.4+/-0.7 kg and 6.5+/-1.3 kg, respectively, for the C group, and 157+/-15 and 221+/-16s and 3.2+/-0.5 and 5.5+/-1.0 kg, respectively, for the T group. Within 20 min of the OT injection, plasma concentrations returned to basal levels. The threshold OT concentration at cessation of milk flow after injection of 0.2 or 0.5 IU/cow of OT was calculated based on the OT plasma half-life. The threshold increased with increasing dosages of OT and was higher in group T (8+/-1 and 14+/-1 pg/mL for 0.2 and 0.5 IU/cow, respectively) than in group C (7+/-1 and 11+/-1 pg/mL for 0.2 and 0.5 IU/cow, respectively). In conclusion, desensitization of the udder toward OT occurs when the udder is exposed to elevated OT plasma concentrations, both short-term during the actual milking and long-term due to chronic high-dosage OT treatment. However, low-dosage OT treatments to induce normal milk removal can minimize the observed side effects.

Milk ejection disorders in Swiss dairy cows: a field study.

A questionnaire was sent to 2099 dairy farms to investigate the occurrence of poor milkability. Based on that, the frequency of poor milkability in Swiss dairy cows was 4% and the percentage of cows treated with oxytocin (OT) was 2%. In addition, 270 dairy farms that had reported cases of animals with poor milkability were contacted for an interview to classify the disorders. Farmers suspected disturbed milk ejection in 52%, anatomical dysfunction of the teat and/or the udder in 16% and milk ejection disorder or impaired milkability caused by discernable environmental factors in 32% of the cases. Forty-eight animals from 18 farms with suspected milk ejection disorders were selected for an experimental field study which included milk flow recording and OT administration to induce milk ejection. After cessation of the spontaneous milk flow, a low dose of OT (0.2, 0.5 or 1 i.u.) was injected i.v. to test the responsiveness of the udder to OT at a physiological level. When milk flow ceased again, 10 i.u. OT was injected i.v. (supraphysiological) to ensure complete udder emptying and to determine the residual milk. Milk ejection disorder could be confirmed in 69% of the cases, i.e. if residual milk was >20% of the total milk. Because in 27% of the animals milk ejection disorder was not confirmed on the basis of elevated residual milk, an anatomical disorder of the teat and/or the udder was suspected. Milk ejection disorder could be confirmed in 69% of the cases whereas in 27% of the suspected cases an anatomical disorder of the teat and/or the udder was suspected. An increased cortisol production in cows with milk ejection disorder was not obvious because faecal concentrations of cortisol metabolites with a 5 beta-androstane-3 alpha,11 oxo-structure were not augmented in animals with disturbed milk ejection.

Effects of intrapartum oxytocin administration and epidural analgesia on the concentration of plasma oxytocin and prolactin, in response to suckling during the second day postpartum.

BACKGROUND: Oxytocin and prolactin stimulate milk ejection and milk production during breastfeeding. The aim of the present study was to make a detailed analysis of maternal release of oxytocin and prolactin in response to breastfeeding during the second day postpartum in mothers who had received oxytocin either intravenously for stimulation of labor or intramuscularly for prevention of postpartum hemorrhage and/or epidural analgesia or those who had received no such treatment in connection with birth. METHODS: In a descriptive comparative study plasma oxytocin and prolactin concentrations were measured in response to suckling during the second day postpartum in women who had received intravenous intrapartum oxytocin (n = 8), intramuscular postpartum oxytocin (n = 13), or epidural analgesia, either with (n = 14) or without (n = 6) intrapartum oxytocin infusion, and women who received none of these interventions (n = 20). Hormone levels were analyzed by enzyme immunoassay. RESULTS: All mothers showed a pulsatile oxytocin pattern during the first 10 minutes of breastfeeding. Women who had received epidural analgesia with oxytocin infusion had the lowest endogenous median oxytocin levels. The more oxytocin infusion the mothers had received during labor, the lower their endogenous oxytocin levels were during a breastfeeding during the second day postpartum. A significant rise of prolactin was observed after 20 minutes in all women, but after 10 minutes in mothers having received oxytocin infusion during labor. In all women, oxytocin variability and the rise of prolactin levels between 0 and 20 minutes correlated significantly with median oxytocin and prolactin levels. CONCLUSION: Oxytocin, released in a pulsatile way, and prolactin were released by breastfeeding during the second day postpartum. Oxytocin infusion decreased endogenous oxytocin levels dose-dependently. Furthermore, oxytocin infusion facilitated the release of prolactin. Epidural analgesia in combination with oxytocin infusion influenced endogenous oxytocin levels negatively.

Maternal lactation characteristics after consumption of an alcoholic soup during the postpartum 'doing-the-month' ritual.

OBJECTIVE: The present study examined whether ethanol exposure influences lactation parameters. Specifically, selected constituents in maternal blood and milk and the lactation performance of Chinese lactating mothers were evaluated after they had consumed chicken soup flavoured with sesame oil and rice wine (CSSR), a diet traditionally prescribed during the postpartum 'doing-the-month' ritual. DESIGN: Twenty-three lactating mothers were examined. Informed consent was obtained from each subject. Each subject was tested on two occasions separated by a week. The target alcohol dosage was 0.3 g/kg body weight. Milk and blood samples were collected prior to consumption of soup and at 120 and 150 min, respectively, after consumption. Levels of various constituents were measured. The time for ejection of the first milk droplet and total milk volume yielded were also measured. RESULTS: Consumption of CSSR influenced TAG, insulin and lactate levels in maternal blood. Likewise, consumption of the soup affected milk composition and its nutritional status, particularly total protein, TAG, fatty acid, beta-hydroxybutyrate and lactate levels. CSSR intake significantly affected TAG and lactate levels in milk. The time for the first milk droplet to be ejected was significantly longer in the CSSR group, indicating that the milk-ejecting reflex is inhibited. However, blood prolactin level increased slightly after ethanol intake. Milk yields were reduced after ingestion of CSSR although the difference was not statistically significant. CONCLUSION: Consumption of CSSR affects not only the composition of maternal blood and milk, but also lactation performance. These findings suggest that an alcoholic diet should be avoided during lactation.

Involvement of neuromedin S in the oxytocin release response to suckling stimulus.

We recently identified neuromedin S (NMS) from the rat hypothalamus as an endogenous ligand for the FM-4/TGR-1 receptor distinct from neuromedin U. In the present study, we examined the role of NMS in the oxytocin release response to suckling stimulation by rat pups. Intracerebroventricular (icv) injection of NMS induced cFos expression in the paraventricular nucleus and supraoptic nucleus. Double immunohistochemical analysis revealed induction of cFos expression in a proportion of oxytocinergic neurons in both nuclei. In addition, icv injection of NMS stimulated oxytocin release dose-dependently in intact rats, and increased milk secretion in lactating rats. On the other hand, icv injection of anti-NMS antiserum into lactating rats significantly suppressed suckling-induced milk ejection. These results suggest that, in the rat, endogenous NMS plays an important role in the oxytocin release response to the suckling stimulus.

The histamine H1 receptor is not involved in local control of mammary blood flow in dairy cows.

Low concentrations of the essential amino acid histidine in circulation have been shown to increase mammary blood flow and it has been suggested that this effect is mediated by histamine. The hypotheses tested in this experiment were that interstitial histamine concentrations in the mammary gland are related to arterial His concentrations and that mammary blood flow is reduced by extracellular histamine via H(1) receptors. The hypotheses were tested by infusing saline or chlorpheniramine, a blocker of the H(1) histamine receptor, into the arterial supply of the mammary glands of lactating cows infused with 44 g/h of amino acid mixtures with or without His for 10 h. Infusates were administered in a 2 x 2 factorial arrangement within a 4 x 4 Latin square to 4 multiparous Holstein cows in mid lactation. Exclusion of His from the infusate decreased protein content in milk from the infused udder half from 3.98 to 3.77%, and increased arterial alpha-aminonitrogen concentration from 3.2 to 3.4 mM. Neither the decreased arterial His concentration nor the H(1) blocker affected plasma flow to the infused udder half. We conclude that histamine is not involved in the regulation of mammary blood flow. The H(1) blocker decreased milk production in the infused udder half from 4.6 to 3.5 kg without affecting protein, fat, and lactose percentages, suggesting an inhibition of milk ejection. Cows on chlorpheniramine ate less feed during the infusion than saline-infused cows, which resulted in lower arterial concentrations and mammary uptakes of acetate. The efficiency of plasma triacylglycerol uptake across the mammary glands was decreased by chlorpheniramine but net uptake of long-chain fatty acids was not affected. The mechanism by which an amino acid deficiency influences mammary blood flow does not involve histamine signaling through the H(1) receptor and remains unidentified.

Effects of twice-daily nursing on milk ejection and milk yield during nursing and milking in dairy cows.

Milk production and hormonal responses to milking in Holstein cows that were milked twice daily, and that either also nursed calves twice daily 2 h after milking for 9 wk after calving (n = 10) or that served as nonnursing controls (n = 8) were examined to assess how nursing affected responses to machine milking. Milk yield at milking during the 9 wk of nursing was lower in nursing cows compared with control cows (26.1 +/- 1.0 vs. 35.5 +/- 1.1 kg) that were only machine milked. During nursing, the amount drunk by calves increased from 6.5 +/- 0.7 kg/d on wk 1 to 12.5 +/- 1.4 kg/d on wk 9. When this was added to the amount of milk obtained at milking, nursing cows did not differ from control cows in total milk produced (35.5 +/- 1.0 vs. 35.5 +/- 1.0 kg). Residual milk yield, after i.v. injection of oxytocin after milking, was higher in nursing cows than in control cows (8.7 +/- 0.8 vs. 3.2 +/- 0.8 kg). During the 6 wk after weaning, milk production was the same for the nursing and control cows (34.0 +/- 1.35 vs. 34.7 +/- 1.42 kg). Plasma oxytocin levels during milking were greater for control cows than for nursing cows (31.7 +/- 5.4 vs. 18.0 +/- 2.8 pg/mL), but were equivalent to concentrations in nursing cows during nursing (35.5 +/- 7.5 pg/mL). Plasma concentrations of prolactin and cortisol increased after both milking (control vs. nursing: prolactin: 40.2 +/- 6.8 vs. 32.9 +/- 6.1 ng/mL; cortisol: 6.4 +/- 1.23 vs. 7.4 +/- 1.10 ng/mL) and nursing (control vs. nursing: prolactin: 18.6 +/- 7.3 vs. 38.9 +/- 6.6 ng/mL; cortisol: 2.34 +/- 1.15 vs. 7.37 +/- 1.04 ng/mL). In contrast to previous studies, there was no obvious advantage for milk production by keeping a calf with the cow. This appears to result from the reduced oxytocin secretion during milking for the nursing cows.

Interaction of extracellular signal-regulated protein kinase 1/2 with actin cytoskeleton in supraoptic oxytocin neurons and astrocytes: role in burst firing.

Neuronal firing patterns determine the manner of neurosecretion, the underlying mechanisms of which are poorly understood. Using supraoptic nuclei in brain slices from lactating rats, we examined the involvement of extracellular signal-regulated protein kinase 1/2 (ERK1/2) and filamentous actin (F-actin) in burst generation by oxytocin (OT) neurons. Blocking phosphorylation of ERK1/2 (pERK1/2) decreased miniature EPSCs and blocked OT-evoked bursts, as did intracellularly loading an antibody against pERK1/2. OT (10 pM) increased cytosolic pERK1/2 close to the cell membrane within the first 5 min, subsiding by 30 min, whereas OT elicited pERK1/2 nuclear translocation in closely associated supraoptic astrocytes. The increased pERK1/2 was tightly correlated with spatiotemporal actin dynamics. In OT neurons, OT initially increased F-actin, particularly at membrane subcortical areas, and then decreased it after 30 min. Both polymerization and depolymerization of actin cytoskeleton were associated with bursts, but only polymerization facilitated OT-evoked bursts. Blocking ERK1/2 activation blocked OT-evoked actin polymerization, whereas depolymerizing F-actin increased pERK1/2 expression. These changes were further identified in vivo. In intact animals, suckling increased ERK1/2 activation in the cytosol and membrane subcortical area F-actin formation in OT neurons, whereas it increased F-actin concentration in astrocytic somata. Coimmunoprecipitation showed that suckling increased molecular interactions between pERK1/2 and actin. Finally, two different blockers of ERK1/2 kinase injected intracerebroventricularly reduced suckling-evoked milk ejections. This is the first demonstration that OT mediation of suckling-evoked bursts/milk ejections is via interactions between pERK1/2 and actin cytoskeleton.