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Central and peripheral inhibition of milk ejection
Livestock Production Science 70 (2001) 135–140 www.elsevier.com / locate / livprodsci
Central and peripheral inhibition of milk ejection a, b Olga Wellnitz *, Rupert M. Bruckmaier a
Institute of Anatomy, Physiology and Hygiene of Domestic Animals, University of Bonn, Katzenburgweg 7 -9, D-53115 Bonn, Germany b Institute of Physiology, Technical University of Munich-Weihenstephan, Weihenstephaner Berg 3, D-85350 Freising, Germany
Abstract Milk ejection can be inhibited as a result of a lack of release of oxytocin (OT) from the pituitary gland, or, when OT is released normally, as a lack of effect on the mammary gland. The former can be overcome by administration of exogenous OT, whereas the latter cannot. Central inhibition of OT release can be caused by several factors, such as relocating cows to unfamiliar surroundings for milking, oestrus, change from suckling to machine milking. In such cases peripheral blood concentrations of b-endorphin and cortisol are elevated. Administration of exogenous cortisol does not cause inhibition of milk ejection. On the other hand, the administration of morphine inhibits OT release from the pituitary, and, therefore, milk ejection. The effect of morphine can be abolished by naloxone (opioid antagonist) administration. However, premilking administration of naloxone does not abolish spontaneous inhibition of milk ejection in cows during milking in unfamiliar surroundings. The role of b-endorphin in central inhibition of milk ejection remains unclear. Peripheral inhibition of milk ejection is caused by exogenous a-adrenergic receptor agonists or OT receptor blocking agents, while OT release from the pituitary is normal. Administered b-adrenergic agonists cause elevated milk flow rates as compared with normal milking. Inhibition of milk ejection by catecholamines cannot be abolished by administration of OT but a-adrenergic receptor blocking agents. Effects of OT receptor blocking agents can be abolished by administration of high dosages of exogenous OT. However, milk flow rates are not reduced as long as milk is present in the cistern. The presence of high concentrations of adrenergic receptors in the teat and lower amounts in more proximal regions of the bovine udder has been demonstrated. Their influence on milkability has been reported and relations between adrenergic receptor concentrations in the udder or on blood platelets and milking characteristics are most likely. Under stress free conditions the sympathetic tone in the mammary gland is reduced during milking and increases between milkings. All these investigations reflect the importance of the sympathic nervous system in the process of milk removal. Milk removal seems to be regulated by the adrenergic system, although the mechanism is not clear. 2001 Elsevier Science B.V. All rights reserved. Keywords: Milk ejection; Opiods; Catecholamines; Cortisol; Peripheral inhibition
1. Introduction In dairy animals, milk removal by either the offspring or by milking machines largely depends upon the active ejection of milk into the cisternal *Corresponding author.
cavities. Milk ejection and, in consequence, milk removal may be impaired when animals are exposed to various stressors. From rodents, decreases in milk yield are reported during restraint or confrontation with novel environments (Cross, 1955; Lau, 1992). Similarly, dairy cows respond with reduced milk ejection towards aversive handling, as indicated by
0301-6226 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0301-6226( 01 )00206-8
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O. Wellnitz, R.M. Bruckmaier / Livestock Production Science 70 (2001) 135 – 140
increased amounts of residual milk (Rushen et al., 1999). Disturbances of milk ejection may lead to reduced milk yields and, possibly, food shortage of the offspring or a loss of dairy production, respectively. In addition, impaired milk ejection is suspected to increase the risk for mammary infections, since withheld milk is an exquisite substrate for microorganisms within the mammary gland. In dairy animals, the vacuum of the milking machine can provoke a collapse of the cavities and thus induce damages of udder tissue. Besides, reduced milk flow may result in climbing of the clusters which in turn may ligature further milk removal. In general, inhibition of milk ejection can emerge from two different levels, i.e. from either a central or a peripheral blockade. Central inhibition of milk ejection means that the release of oxytocin (OT), the principal hormone responsible for milk ejection) from the posterior pituitary is inhibited. In this case OT plasma concentrations do not increase above a threshold level necessary to elicit milk ejection. In contrast to this, peripheral inhibition of milk ejection means that, although OT is physiologically released during milk removal, it is not able to exert its effects on the mammary gland.
2. Central inhibition Central inhibition of milk ejection, i.e. inhibition of OT release from the pituitary gland, is observed during various types of emotional stress. In women, stressors like mental arithmetic or noise lead to a reduction of OT pulse frequency during nursing (Ueda et al., 1994). In cows, OT release was inhibited during suckling by an alien calf (Silveira et al., 1993). Moreover, milking in unfamiliar surroundings has been demonstrated to result in an inhibition of milk ejection, which could nevertheless be abolished by small dosages of exogenous OT (Bruckmaier et al., 1993). The same is true for primiparous cows, milked for the very first time (Bruckmaier et al., 1992) as well as for cows being switched from suckling to machine milking (Tancin et al., 1995). When cows are repeatedly transferred to initially unfamiliar surroundings, OT release reaches its normal when the animals get used to the procedure (Bruckmaier et al., 1996).
To overcome central inhibition of milk ejection it is necessary to elevate OT blood concentrations above the threshold level, either by exogenous OT as mentioned previously or by applying nervous stimuli strong enough to induce endogenous OT release. In contrast to tactile stimulation of the teat, which fails to induce OT secretion in certain stress situations, vaginal stimulation might nevertheless be able to trigger hypophyseal OT release, at least under certain circumstances. One approach to set such stimuli is to inflate the vagina, examples for this are anecdotally known from some primitive rural settings. Under experimental conditions it does hold true that vaginal stimulation is a more intense signal for OT release than is tactile stimulation limited to the udder (Bruckmaier et al., 1992). In cows, milked in unfamiliar surroundings vaginal stimulation induced OT release in some animals (Bruckmaier et al., 1993).
2.1. Opioids The missing OT release in unfamiliar surroundings is accompanied by increased levels of b-endorphin plasma concentrations (Bruckmaier et al., 1993, 1997). When the cows get used to the new surroundings, the b-endorphin concentrations decrease, while OT release is gradually normalised (Bruckmaier et al., 1996). These observations led to the concept that endogenous opioidergic peptides do play a role within the mechanisms causing the central inhibition of milk ejection. This notion is further substantiated by experiments in which milk ejection could be experimentally inhibited in several species by administration of exogenous opioid peptides: In cows, morphine inhibits OT release during milking; this effect could be abolished by administration of naloxone, an opioid antagonist (Tancin et al., 2000). Similar findings are reported for women during breast-feeding (Lindow et al., 1999). In rats, opioidergic peptides were shown to inhibit electrically evoked neurohypophyseal OT release in vitro (Clarke and Patrik, 1983) and to inhibit milk ejection in response to suckling when opioids were injected into the lumbar subarachnoid space (Wright, 1985). Again, these effects could also be abolished by administration of naloxone. In mice, opioidergic peptides inhibit suckling-induced OT release and naloxone abolished this effect (Haldar et al., 1982).
O. Wellnitz, R.M. Bruckmaier / Livestock Production Science 70 (2001) 135 – 140
Naloxone potentiates the OT response to vaginocervical stimulation in goats (Seckl and Lightman, 1987). In periparturient mares, naloxone increases peak OT plasma concentrations caused by vaginocervical stimulation (Aurich et al., 1996). For cows, the data on naloxone effects in OT relaese and milk ejection are somewhat contradictory: in cows, milked in unfamiliar surroundings, naloxone application could not overcome the blockade of OT release (Wellnitz et al., 1997). Moreover, naloxone was not able to overcome disturbed milk ejection in primiparous cows (Kraetzl et al., 2000). Administration of metyrapone, an inhibitor of adrenal corticoid synthesis which stimulates ACTH and b-endorphin through a negative feedback mechanism in rats (Pettibone and Mueller, 1984), had no effect on adrenocorticotropin (ACTH) and b-endorphin concentrations and OT release and milk removal in cows (Wellnitz et al., 1997). Taken together, the demonstration of direct opoid actions on OT release from the pituitary in vitro (Bicknell and Leng, 1982) and the observation that naloxone is able to antagonise the inhibition of OT release by endogenous opioids in several species, cannot be extended to the cow. Suggestions about differences in species concerning the ability of naloxone to reach the pituitary, i.e. to cross the blood–brain barrier, or the inability of naloxone to block the k-opioid receptors (Bruckmaier and Blum, 1998), which are most likely responsible for the modulation of OT release (Summy-Long et al., 1990), are not yet proven.
2.2. Cortisol During milking of cows in unfamiliar surroundings, when milk ejection is inhibited, concentrations of cortisol in blood increase concomitantly with those of b-endorphin (Bruckmaier et al., 1993, 1997). In spite of the common precursor protein pro-opiomelanocortin for both b-endorphin and ACTH (Eipper and Mains, 1980), the ACTH triggered cortisol release is not necessarily simultanoeus to the increase in b-endorphin concentrations (Bishop et al., 1999). Nonetheless, cortisol itself does not seem to have an influence on central inhibition of milk ejection, since intravenous administration of cortisol has no effects on milk ejection in cows (Mayer and Lefcourt, 1987). During routine milking, plasma cortisol concentrations in-
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crease physiologically in cows (Bruckmaier et al., 1993). In sows, plasma cortisol is increased in absence of the piglets, but milk ejection remained undisturbed after return of the piglets (Rushen et al., 1995).
2.3. Catecholamines Catecholamines seem to be involved in OT secretion, at least at the central level: Secretion of OT from magnocellular neurons can be inhibited via noradrenergic neurons (Swanson et al., 1981) by stimulation of b-adrenergic receptors (Tribollet et al., 1978; Crowley and Armstrong, 1992). Propranolol, a b-adrenergic receptor blocking agent which crosses the blood–brain barrier (Hayes and Cooper, 1971), enhances OT release in response to electrical stimulation of the neural stalk in rats (Poulain and Dyer, 1984). In contrast, peripheral catecholamine concentrations are reportedly unaffected during central inhibition of milk ejection in cows (Bruckmaier and Blum, 1998) and propranolol failed to abolish disturbed OT release in unfamiliar surroundings (Wellnitz et al., 1997). Moreover, phentolamine, an a-adrenergic blocking agent, failed to abolish central inhibition of milk ejection in cows (Wellnitz et al., 1997). We therefore conclude that inhibition of milk ejection in cows by emotional stress is not related to an increase in catecholamine secretion. As indicated from studies in sheep in which an inhibiting effect of exogenous adrenaline on OT release during milking could be demonstrated (Barowicz, 1979), it remains open whether the lack of peripheral catecholamine action observed in cows represents a species specificity in either the blood–brain barrier transfer or the principal lack of central catecholamine effects on OT release.
3. Peripheral inhibition During peripheral inhibition of milk ejection, OT release from the pituitary during milking may be normal or even elevated, but OT lacks its effect at the mammary gland. Peripheral inhibition of milk ejection in cows can be experimentally induced by administration of a-adrenergic receptor agonists or an OT receptor blocking agent in presence of normal OT release during milking (Bruckmaier et al., 1997).
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The inhibitory effect of the oxytocin receptor blocking agent could be abolished by administration of high dosages of oxytocin but not the effect of the a-adrenergic agonist. Instead, the inhibition of milk ejection through a-adrenergic receptor stimulation could only be antagonized by administration of the a-adrenergic blocking agent (Bruckmaier et al., 1997). Physiologically, the sympathetic tone in cows is decreased during milking and increased between the milkings to prevent loss of milk by contraction of teat longitudinal and sphincter muscles (Lefcourt, 1982). a- and b-adrenergic receptors in teat smooth muscles of the cows were first described by Peeters et al., 1973. a 1 -, a 2 - and b 2 -adrenergic receptors were detected and quantified in three different regions of the bovine udder, showing that the highest levels of receptor expression were found in the muscular layer of the teat, less in the tissue around the cistern containing great milk ducts and very low concentrations were found in the parenchyma region; this distribution has been established both for the receptor protein (Hammon et al., 1994) as well as for the receptor mRNA (Wellnitz et al., 2000). Administration of an a-adrenergic agonist reduced teat length, but did not reduce milk flow, as long as milk was in the cistern (Bruckmaier et al., 1997). This indicates that the contraction of the smooth muscles of the teats are not responsible for the inhibition of milk flow. High concentrations of a-adrenergic receptors within the region of the large milk ducts around the cistern suggest, that peripheral inhibition of milk ejection may be located in this region, most likely by contraction of smooth muscles around the milk ducts. An influence of the OT receptor action by catecholamines is unlikely, because lowest adrenergic receptor concentrations were found in the parenchyma region were most OT receptors are expected. Some authors believe that the reason for inhibition of milk ejection after catecholamine administration is due to reduced blood flow to the mammary gland (Gorewit and Aromando, 1985) by reducing the amount of OT presented to the receptor sites. However, even pharmacologically high dosages of OT (10 i.u. injected intravenously) could not abolish the inhibition of milk ejection. Assuming that blood flow never stops completely, extremely high OT blood concentrations should then allow for sufficient con-
centrations of OT reaching the myoepithelial cells to elicit milk ejection. The distribution and the ratio of a- and b-adrenergic receptors is important role for the milkability of cows, because milkability and b 2 - and a 2 -adrenergic receptor ratio are (Roets et al., 1986). In addition, correlations between the number of a 2 - and b 2 adrenergic receptors on blood platelets of bulls and the milkability of their daughters were detected (Roets et al., 1995). When a-adrenergic agonists or OT blocking agents are injected after milk ejection had already started, milk flow ceased (Bruckmaier et al., 1997). This emphasises the importance of the sympathetic system for disturbed milk removal, even in situations where pituitary OT release is initiated and milk ejection had started. b-adrenergic receptor stimulation has no effects on intramammary pressure, i.e. it has no effect on milk ejection, instead milk flow rates are enhanced (Bruckmaier et al., 1991). This is likely due to a relaxation of the smooth muscle cells. However, peripheral inhibition of milk ejection was only shown in pharmacological experiments. Its importance for practical dairy farming might therefore be only marginal.
4. Conclusions Within the central inhibition of milk ejection, which occurs during emotional stress situations, bendorphin plays an important role in most species. Central catecholamines seem to be involved in the central inhibition of milk ejection in ewe but likely not in cows. Peripheral inhibition of milk ejection can be provoked by adrenergic receptor stimulation or OT receptor blockade, but is seen exclusively under experimental conditions, although the milk ejection rate seems to be regulated by the adrenergic system. In dairy cows with spontaneously inhibited milk ejection so far only central inhibition of OT release was observed.
References Aurich, J.E., Besognet, B., Daels, P.F., 1996. Evidence for opioidergic inhibition of oxytocin release in periparturient mares. Theriogenology 46, 387–396.
O. Wellnitz, R.M. Bruckmaier / Livestock Production Science 70 (2001) 135 – 140 Barowicz, T., 1979. Inhibitory effect of adrenaline on oxytocin release in the ewe during the milk-ejection reflex. J. Dairy Res. 46 (1), 41–46. Bicknell, R.J., Leng, G., 1982. Endogenous opiates regulate oxytocin but not vasopressin secretion from the neurohypophysis. Nature 298, 161–162. Bishop, J.D., Malven, P.V., Singleton, W.L., Weesner, G.D., 1999. Hormonal and behavioral correlates of emotional states in sexually trained boars. J. Anim. Sci. 77, 3339–3345. Bruckmaier, R.M., Blum, J.W., 1998. Oxytocin release and milk removal in ruminants. J. Dairy Sci. 81, 939–949. Bruckmaier, R.M., Mayer, H., Schams, D., 1991. Effects of a- and b-adrenergic agonists on intramammary pressure and milk flow in dairy cows. J. Dairy Res. 59, 411–419. Bruckmaier, R.M., Pfeilsticker, H.-U., Blum, J.W., 1996. Milk yield, oxytocin an b-endorphin gradually normalize during repeated milking in unfamiliar surroundings. J. Dairy Res. 63, 191–200. Bruckmaier, R.M., Schams, D., Blum, J.W., 1992. Aetiology of disturbed milk ejection in parturient primiparous cows. J. Dairy Res. 59, 479–498. Bruckmaier, R.M., Schams, D., Blum, J.W., 1993. Milk removal in familiar and unfamiliar surroundings: concentrations of oxytocin, prolactin, cortisol and b-endorphin. J. Dairy Res. 60, 449–456. Bruckmaier, R.M., Wellnitz, O., Blum, J.W., 1997. Inhibition of milk ejection in cows by oxytocin receptor blockade, a-adrenergic receptor stimulation and in unfamiliar surroundings. J. Dairy Res. 64, 315–325. Clarke, G., Patrik, G., 1983. Differential inhibitory action by morphine on the release of oxytocin and vasopressin from the isolated neural lobe. Neurosci. Lett. 39, 175–180. Cross, B.A., 1955. Neurohormonal mechanisms in emotional inhibition of milk ejection J. Endocrinology 12, 29–37. Crowley, W.R., Armstrong, W.E., 1992. Neurochemical regulation of oxytocin secretion in lactation. Endocr. Rev. 13, 33–65. Eipper, B.A., Mains, R.E., 1980. Structure and biosynthesis of pro-adrenocorticotropin / endorphin and related peptides. Endocr. Rev. 1, 1–27. Gorewit, R.C., Aromando, M.C., 1985. Mechanisms involved in the adrenaline induced blockade of milk ejection in dairy cattle. Proc. Soc. Exp. Biol. Med. 180, 340–347. Haldar, J., Hoffman, D.L., Zimmerman, E.A., 1982. Morphine, beta-endorphin and [D-Ala2] Met-enkephalin inhibit oxytocin release by acetylcholine and suckling. Peptides 3, 663–668. Hammon, H.M., Bruckmaier, R.M., Honegger, U.E., Blum, J.W., 1994. Distribution and density of a- and b-adrenergic binding sites in the bovine mammary gland. J. Dairy Res. 61, 47–57. Hayes, A., Cooper, R.G., 1971. Studies on the absorption, distribution and excretion of propranolol in rat, dog and monkey. J. Pharmacol. Exp. Ther. 176, 302–311. Kraetzl, W.D., Tancin, V., Schams, D., 2000. Significance of opioids for the inhibition of oxytocin release and milk let down in primiparous fresh lactating dairy cows. 5th Joint EAAP/ ASAS Workshop on Biology of Lactation in Farm Animals Lau, Ch., 1992. Effects of various stressors on milk release in the rat. Physiol. Behav. 51, 1157–1163. Lefcourt, A.M., 1982. Effect of teat stimulation on sympathetic tone in bovine mammary gland. J. Dairy Sci. 65, 2317–2322.
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Lindow, S.W., Hendricks, M.S., Nugent, F.A., Dunne, T.T., van der Spuy, Z.M., 1999. Morphine suppresses the oxytocin response in breast-feeding women. Gynecol. Obstet. Invest. 48 (1), 33–37. Mayer, H., Lefcourt, A.M., 1987. Failure of cortisol injected prior to milking to inhibit milk ejection in dairy cattle. J. Dairy Res. 54, 173–177. Peeters, G., De Bruycker, R., Quintelier, W., Paape, M., 1973. Contractions of the teat muscles of lactating cows. Acta Endocrinol. 177 (Suppl), 50. Pettibone, D.J., Mueller, G.P., 1984. Differential effects of adrenergic agents on plasma levels of immunoreactive beta-endorphin and alpha-melanotropin in rats. Proc. Soc. Exp. Med. 176, 168–174. Poulain, D.A., Dyer, R.G., 1984. Reproductible increases in intramammary pressure after spinal cord stimulation in lactating rats. Exp. Brain Res. 55, 313–316. Roets, E., Burvenich, C., Hamann, J., 1995. Relationship between numbers of a 2 - and b 2 -adrenoceptors on blood cells of bulls and milkability of their daughters. J. Dairy Res. 62, 567–575. Roets, E., Vandeputte-Van Messom, G., Peeters, G., 1986. Relationship between milkability and adrenoceptor concentrations in teat tissue in primiparous cows. J. Dairy Sci. 69, 3120–3130. Rushen, J., Nay, T.S., Wright, L.R., Payne, D.C., Foxcroft, G.R., 1995. Stress and nursing in the pig: role of HPA axis and endogenous opioid peptides. Physiol. Behav. 58 (1), 43–48. ´ A.M.B., Munksgaard, L., 1999. Ferar of Rushen, J., De Passille, people by cows and effects on milk yield, behavior, and heart rate at milking. J. Dairy Sci. 82, 720–727. Seckl, J.R., Lightman, S.L., 1987. Effect of naloxone on oxytocin and vasopressin release during vaginocervical stimulation in the goat. J. Endocrinol. 115, 317–322. Silveira, P.A., Spoon, R.A., Ryan, D.P., Williams, G.L., 1993. Evidence for maternal behavior as a requisite link in sucklingmediated anovulation in cows. Biol. Reprod. 49 (6), 1338– 1346. Summy-Long, J.Y., Rosella-Dumpman, L.M., McLemore, G.L., Kachler, E., 1990. Kappa opiate receptors inhibit release of oxytocin from the magnocellular system during dehydration. Neuroendocrinol. 51, 376–384. ´ Swanson, L.W., Sawchenko, P.E., Berod, A., Hartman, B.K., Helle, K.B., Vanorden, D.E., 1981. An immunohistochemical study of the organisation of catecholaminergic cells and terminal fields in the paraventricular and supraoptic nuclei of the hypothalamus. J. Comp. Neurol. 196, 271–285. Tancin, V., Harcek, L., Broucek, J., Uhrincat, M., Mihina, St., 1995. Effect of suckling during early lactation and changeover to machine milking on plasma oxytocin and cortisol levels and milking characteristics in Holstein cows. J. Dairy Res. 62, 249–256. Tancin, V., Kraetzl, W.-D., Schams, D., 2000. The effect of morphine and naloxone on the release of oxytocin and on milk ejection in dairy cows. J. Dairy Res. 67, 13–20. Tribollet, E., Clarke, G., Dreifuss, J.J., Lincoln, D.W., 1978. The role of central adrenergic receptors in the reflex release of oxytocin. Brain Res. 142, 69–84. Ueda, T., Yokoyama, Y., Irahara, M., Aono, T., 1994. Influence of psychological stress on suckling-induced pulsatile oxytocin release. Obstet. Gynecol. 84 (2), 259–262.
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Wellnitz, O., Bruckmaier, R.M., Blum, J.W., 1997. Naloxone and adrenergic blocking agents fail to abolish central inhibition of milk ejection in cows. J. Dairy Res. 64 (4), 627–631. Wellnitz, O., Zurbriggen, A., Friis, R.R., Blum, J.W., Bruckmaier, R.M., 2000. Measurement of a 1 - and b 2 -adrenergic receptor mRNA distribution in the bovine mammary gland by competi-
tive reverse transcription polymerase chain reaction. J. Dairy Res., submitted Wright, D.M., 1985. Evidence for spinal site at which opioids may act to inhibit the milk ejection reflex. J. Endocrinol. 106, 401–407.
Central and peripheral inhibition of milk ejection a, b Olga Wellnitz *, Rupert M. Bruckmaier a
Institute of Anatomy, Physiology and Hygiene of Domestic Animals, University of Bonn, Katzenburgweg 7 -9, D-53115 Bonn, Germany b Institute of Physiology, Technical University of Munich-Weihenstephan, Weihenstephaner Berg 3, D-85350 Freising, Germany
Abstract Milk ejection can be inhibited as a result of a lack of release of oxytocin (OT) from the pituitary gland, or, when OT is released normally, as a lack of effect on the mammary gland. The former can be overcome by administration of exogenous OT, whereas the latter cannot. Central inhibition of OT release can be caused by several factors, such as relocating cows to unfamiliar surroundings for milking, oestrus, change from suckling to machine milking. In such cases peripheral blood concentrations of b-endorphin and cortisol are elevated. Administration of exogenous cortisol does not cause inhibition of milk ejection. On the other hand, the administration of morphine inhibits OT release from the pituitary, and, therefore, milk ejection. The effect of morphine can be abolished by naloxone (opioid antagonist) administration. However, premilking administration of naloxone does not abolish spontaneous inhibition of milk ejection in cows during milking in unfamiliar surroundings. The role of b-endorphin in central inhibition of milk ejection remains unclear. Peripheral inhibition of milk ejection is caused by exogenous a-adrenergic receptor agonists or OT receptor blocking agents, while OT release from the pituitary is normal. Administered b-adrenergic agonists cause elevated milk flow rates as compared with normal milking. Inhibition of milk ejection by catecholamines cannot be abolished by administration of OT but a-adrenergic receptor blocking agents. Effects of OT receptor blocking agents can be abolished by administration of high dosages of exogenous OT. However, milk flow rates are not reduced as long as milk is present in the cistern. The presence of high concentrations of adrenergic receptors in the teat and lower amounts in more proximal regions of the bovine udder has been demonstrated. Their influence on milkability has been reported and relations between adrenergic receptor concentrations in the udder or on blood platelets and milking characteristics are most likely. Under stress free conditions the sympathetic tone in the mammary gland is reduced during milking and increases between milkings. All these investigations reflect the importance of the sympathic nervous system in the process of milk removal. Milk removal seems to be regulated by the adrenergic system, although the mechanism is not clear. 2001 Elsevier Science B.V. All rights reserved. Keywords: Milk ejection; Opiods; Catecholamines; Cortisol; Peripheral inhibition
1. Introduction In dairy animals, milk removal by either the offspring or by milking machines largely depends upon the active ejection of milk into the cisternal *Corresponding author.
cavities. Milk ejection and, in consequence, milk removal may be impaired when animals are exposed to various stressors. From rodents, decreases in milk yield are reported during restraint or confrontation with novel environments (Cross, 1955; Lau, 1992). Similarly, dairy cows respond with reduced milk ejection towards aversive handling, as indicated by
0301-6226 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0301-6226( 01 )00206-8
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O. Wellnitz, R.M. Bruckmaier / Livestock Production Science 70 (2001) 135 – 140
increased amounts of residual milk (Rushen et al., 1999). Disturbances of milk ejection may lead to reduced milk yields and, possibly, food shortage of the offspring or a loss of dairy production, respectively. In addition, impaired milk ejection is suspected to increase the risk for mammary infections, since withheld milk is an exquisite substrate for microorganisms within the mammary gland. In dairy animals, the vacuum of the milking machine can provoke a collapse of the cavities and thus induce damages of udder tissue. Besides, reduced milk flow may result in climbing of the clusters which in turn may ligature further milk removal. In general, inhibition of milk ejection can emerge from two different levels, i.e. from either a central or a peripheral blockade. Central inhibition of milk ejection means that the release of oxytocin (OT), the principal hormone responsible for milk ejection) from the posterior pituitary is inhibited. In this case OT plasma concentrations do not increase above a threshold level necessary to elicit milk ejection. In contrast to this, peripheral inhibition of milk ejection means that, although OT is physiologically released during milk removal, it is not able to exert its effects on the mammary gland.
2. Central inhibition Central inhibition of milk ejection, i.e. inhibition of OT release from the pituitary gland, is observed during various types of emotional stress. In women, stressors like mental arithmetic or noise lead to a reduction of OT pulse frequency during nursing (Ueda et al., 1994). In cows, OT release was inhibited during suckling by an alien calf (Silveira et al., 1993). Moreover, milking in unfamiliar surroundings has been demonstrated to result in an inhibition of milk ejection, which could nevertheless be abolished by small dosages of exogenous OT (Bruckmaier et al., 1993). The same is true for primiparous cows, milked for the very first time (Bruckmaier et al., 1992) as well as for cows being switched from suckling to machine milking (Tancin et al., 1995). When cows are repeatedly transferred to initially unfamiliar surroundings, OT release reaches its normal when the animals get used to the procedure (Bruckmaier et al., 1996).
To overcome central inhibition of milk ejection it is necessary to elevate OT blood concentrations above the threshold level, either by exogenous OT as mentioned previously or by applying nervous stimuli strong enough to induce endogenous OT release. In contrast to tactile stimulation of the teat, which fails to induce OT secretion in certain stress situations, vaginal stimulation might nevertheless be able to trigger hypophyseal OT release, at least under certain circumstances. One approach to set such stimuli is to inflate the vagina, examples for this are anecdotally known from some primitive rural settings. Under experimental conditions it does hold true that vaginal stimulation is a more intense signal for OT release than is tactile stimulation limited to the udder (Bruckmaier et al., 1992). In cows, milked in unfamiliar surroundings vaginal stimulation induced OT release in some animals (Bruckmaier et al., 1993).
2.1. Opioids The missing OT release in unfamiliar surroundings is accompanied by increased levels of b-endorphin plasma concentrations (Bruckmaier et al., 1993, 1997). When the cows get used to the new surroundings, the b-endorphin concentrations decrease, while OT release is gradually normalised (Bruckmaier et al., 1996). These observations led to the concept that endogenous opioidergic peptides do play a role within the mechanisms causing the central inhibition of milk ejection. This notion is further substantiated by experiments in which milk ejection could be experimentally inhibited in several species by administration of exogenous opioid peptides: In cows, morphine inhibits OT release during milking; this effect could be abolished by administration of naloxone, an opioid antagonist (Tancin et al., 2000). Similar findings are reported for women during breast-feeding (Lindow et al., 1999). In rats, opioidergic peptides were shown to inhibit electrically evoked neurohypophyseal OT release in vitro (Clarke and Patrik, 1983) and to inhibit milk ejection in response to suckling when opioids were injected into the lumbar subarachnoid space (Wright, 1985). Again, these effects could also be abolished by administration of naloxone. In mice, opioidergic peptides inhibit suckling-induced OT release and naloxone abolished this effect (Haldar et al., 1982).
O. Wellnitz, R.M. Bruckmaier / Livestock Production Science 70 (2001) 135 – 140
Naloxone potentiates the OT response to vaginocervical stimulation in goats (Seckl and Lightman, 1987). In periparturient mares, naloxone increases peak OT plasma concentrations caused by vaginocervical stimulation (Aurich et al., 1996). For cows, the data on naloxone effects in OT relaese and milk ejection are somewhat contradictory: in cows, milked in unfamiliar surroundings, naloxone application could not overcome the blockade of OT release (Wellnitz et al., 1997). Moreover, naloxone was not able to overcome disturbed milk ejection in primiparous cows (Kraetzl et al., 2000). Administration of metyrapone, an inhibitor of adrenal corticoid synthesis which stimulates ACTH and b-endorphin through a negative feedback mechanism in rats (Pettibone and Mueller, 1984), had no effect on adrenocorticotropin (ACTH) and b-endorphin concentrations and OT release and milk removal in cows (Wellnitz et al., 1997). Taken together, the demonstration of direct opoid actions on OT release from the pituitary in vitro (Bicknell and Leng, 1982) and the observation that naloxone is able to antagonise the inhibition of OT release by endogenous opioids in several species, cannot be extended to the cow. Suggestions about differences in species concerning the ability of naloxone to reach the pituitary, i.e. to cross the blood–brain barrier, or the inability of naloxone to block the k-opioid receptors (Bruckmaier and Blum, 1998), which are most likely responsible for the modulation of OT release (Summy-Long et al., 1990), are not yet proven.
2.2. Cortisol During milking of cows in unfamiliar surroundings, when milk ejection is inhibited, concentrations of cortisol in blood increase concomitantly with those of b-endorphin (Bruckmaier et al., 1993, 1997). In spite of the common precursor protein pro-opiomelanocortin for both b-endorphin and ACTH (Eipper and Mains, 1980), the ACTH triggered cortisol release is not necessarily simultanoeus to the increase in b-endorphin concentrations (Bishop et al., 1999). Nonetheless, cortisol itself does not seem to have an influence on central inhibition of milk ejection, since intravenous administration of cortisol has no effects on milk ejection in cows (Mayer and Lefcourt, 1987). During routine milking, plasma cortisol concentrations in-
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crease physiologically in cows (Bruckmaier et al., 1993). In sows, plasma cortisol is increased in absence of the piglets, but milk ejection remained undisturbed after return of the piglets (Rushen et al., 1995).
2.3. Catecholamines Catecholamines seem to be involved in OT secretion, at least at the central level: Secretion of OT from magnocellular neurons can be inhibited via noradrenergic neurons (Swanson et al., 1981) by stimulation of b-adrenergic receptors (Tribollet et al., 1978; Crowley and Armstrong, 1992). Propranolol, a b-adrenergic receptor blocking agent which crosses the blood–brain barrier (Hayes and Cooper, 1971), enhances OT release in response to electrical stimulation of the neural stalk in rats (Poulain and Dyer, 1984). In contrast, peripheral catecholamine concentrations are reportedly unaffected during central inhibition of milk ejection in cows (Bruckmaier and Blum, 1998) and propranolol failed to abolish disturbed OT release in unfamiliar surroundings (Wellnitz et al., 1997). Moreover, phentolamine, an a-adrenergic blocking agent, failed to abolish central inhibition of milk ejection in cows (Wellnitz et al., 1997). We therefore conclude that inhibition of milk ejection in cows by emotional stress is not related to an increase in catecholamine secretion. As indicated from studies in sheep in which an inhibiting effect of exogenous adrenaline on OT release during milking could be demonstrated (Barowicz, 1979), it remains open whether the lack of peripheral catecholamine action observed in cows represents a species specificity in either the blood–brain barrier transfer or the principal lack of central catecholamine effects on OT release.
3. Peripheral inhibition During peripheral inhibition of milk ejection, OT release from the pituitary during milking may be normal or even elevated, but OT lacks its effect at the mammary gland. Peripheral inhibition of milk ejection in cows can be experimentally induced by administration of a-adrenergic receptor agonists or an OT receptor blocking agent in presence of normal OT release during milking (Bruckmaier et al., 1997).
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The inhibitory effect of the oxytocin receptor blocking agent could be abolished by administration of high dosages of oxytocin but not the effect of the a-adrenergic agonist. Instead, the inhibition of milk ejection through a-adrenergic receptor stimulation could only be antagonized by administration of the a-adrenergic blocking agent (Bruckmaier et al., 1997). Physiologically, the sympathetic tone in cows is decreased during milking and increased between the milkings to prevent loss of milk by contraction of teat longitudinal and sphincter muscles (Lefcourt, 1982). a- and b-adrenergic receptors in teat smooth muscles of the cows were first described by Peeters et al., 1973. a 1 -, a 2 - and b 2 -adrenergic receptors were detected and quantified in three different regions of the bovine udder, showing that the highest levels of receptor expression were found in the muscular layer of the teat, less in the tissue around the cistern containing great milk ducts and very low concentrations were found in the parenchyma region; this distribution has been established both for the receptor protein (Hammon et al., 1994) as well as for the receptor mRNA (Wellnitz et al., 2000). Administration of an a-adrenergic agonist reduced teat length, but did not reduce milk flow, as long as milk was in the cistern (Bruckmaier et al., 1997). This indicates that the contraction of the smooth muscles of the teats are not responsible for the inhibition of milk flow. High concentrations of a-adrenergic receptors within the region of the large milk ducts around the cistern suggest, that peripheral inhibition of milk ejection may be located in this region, most likely by contraction of smooth muscles around the milk ducts. An influence of the OT receptor action by catecholamines is unlikely, because lowest adrenergic receptor concentrations were found in the parenchyma region were most OT receptors are expected. Some authors believe that the reason for inhibition of milk ejection after catecholamine administration is due to reduced blood flow to the mammary gland (Gorewit and Aromando, 1985) by reducing the amount of OT presented to the receptor sites. However, even pharmacologically high dosages of OT (10 i.u. injected intravenously) could not abolish the inhibition of milk ejection. Assuming that blood flow never stops completely, extremely high OT blood concentrations should then allow for sufficient con-
centrations of OT reaching the myoepithelial cells to elicit milk ejection. The distribution and the ratio of a- and b-adrenergic receptors is important role for the milkability of cows, because milkability and b 2 - and a 2 -adrenergic receptor ratio are (Roets et al., 1986). In addition, correlations between the number of a 2 - and b 2 adrenergic receptors on blood platelets of bulls and the milkability of their daughters were detected (Roets et al., 1995). When a-adrenergic agonists or OT blocking agents are injected after milk ejection had already started, milk flow ceased (Bruckmaier et al., 1997). This emphasises the importance of the sympathetic system for disturbed milk removal, even in situations where pituitary OT release is initiated and milk ejection had started. b-adrenergic receptor stimulation has no effects on intramammary pressure, i.e. it has no effect on milk ejection, instead milk flow rates are enhanced (Bruckmaier et al., 1991). This is likely due to a relaxation of the smooth muscle cells. However, peripheral inhibition of milk ejection was only shown in pharmacological experiments. Its importance for practical dairy farming might therefore be only marginal.
4. Conclusions Within the central inhibition of milk ejection, which occurs during emotional stress situations, bendorphin plays an important role in most species. Central catecholamines seem to be involved in the central inhibition of milk ejection in ewe but likely not in cows. Peripheral inhibition of milk ejection can be provoked by adrenergic receptor stimulation or OT receptor blockade, but is seen exclusively under experimental conditions, although the milk ejection rate seems to be regulated by the adrenergic system. In dairy cows with spontaneously inhibited milk ejection so far only central inhibition of OT release was observed.
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