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Oxytocin and Milk Ejection


Oxytocin

Cows walking to the barn.

Oxytocin is a 9 amino acid long peptide. The amino acid structure of oxytocin is:

Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly

It has a molecular mass of 1007 daltons. Oxytocin has a disulfide bond between the two cysteines. Reduction of the disulfide bond inactivates oxytocin. One IU (international Unit) is approximately 2 micrograms of pure peptide.

Hypothalamic Nuclei and Oxytocin Synthesis

Oxytocin is syntheized in the hypothalamus in specific nuclei, the paraventricular nucleus and the supraoptic nucleus in the hypothalamus. [A cluster of nerve cells in the brain is often called a nucleus. This is different from the nucleus of a single cell.] Neurons in these hypothalamic nuclei synthesize the oxytocin precursor and package it into vesicles. Oxytocin is initially synthesized as a large molecular weight precursor which also consists of the oxytocin-carrier peptide neurophysin. The precursor is proteolytically cleaved in the neuron in the oxytocin-containing vesicle to yield oxytocin bound to neurophysin. The oxytocin-neurophysin complex is the intracellular storage form of oxytocin.

The oxytocin-containing vesicles are transported from the cell body (which is in the hypothalamus), down the axons to the neuron endings in the posterior pituitary. This is called the hypothalamo-neurohypophysial tract. The oxytocin-neurophysin complex is stored in neurosecretory granules called herring bodies in the axon ending.

The synthesis of oxytocin in the cell bodies and its transport to the axon endings occur separately from the milk ejection reflex.


Oxytocin Surge

The oxytocin concentration in the blood normally is increased within 1 to 2 min. after udder stimulation, but the amount released is declining during milking.

Why is pre-stimulation of the cow needed before milking?

  • hygiene - for prevention of mastitis and for maximizing milk quality.
  • milk ejection - [see J. Dairy Sci. 1980 63:800] In this study they compared milking after manual stimulation (washing the teats) vs. nonstimulated (milkers were put straight on the gland with no pre-washing or other stimulation). The manual stimulation resulted in shorter machine-on times (higher milking efficiency) and higher peak and average milk flow rates. Mean peak oxytocin was not different, but the pre-stimulated cows' oxytocin peaked at 2 min. after stimulation, compared with 5 min. after machine-on time for the unstimulated cows.
  • milk flow rate - [see J. Dairy Sci. 1985 68:1813] In this study, average milk flow rate increased with increasing duration of udder stimulation (30, 60, 120 sec.) and milking machine-on time decreased. However, oxytocin concentration was not different, and when premilking stimulation time was added to machine-on time the difference in average milk flow rate was not significant.

*** The timing of oxytocin release relative to milk removal is an important factor affecting milk ejection.

For example, the image below illustrates the milk flow rate of a cow that A) had teats stimulated for 1 min prior to attaching the milking machine, and B) had the milking machine put on immediately without any prior manual stimulation. Note that the machine-on-time is shorter for the pre-stimulated cow and the peak flow rate is higher for the pre-stimulated cow. Also note the initial rise and fall of flow rate during the first min of milking in B. In this case, the milking machine is initially removing the milk present in the cisterns (does not require milk ejection) and is providing the tactile stimulation necessary to elicit the normal release of oxytocin, which causes the second increase in milk flow rate. In both A and B, the final small increase in milk yield probably was caused by machine stripping by the person milking the cow.

The sensitivity of the neuroendocrine reflex seems to decline as lactation progresses. Peak oxytocin seems to come later after mammary stimulation as lactation progresses. Peak oxytocin occurs at 1 min. at 1-2 weeks of lactation , 2 min. at 5-6 weeks, and 15-16 weeks. Maximum oxytocin concentration during milking also declines as lactation progresses. Extra-tactile stimuli often can release oxytocin before milking, but the occurrence of this declines as lactation progresses.

The dry or nonlactating period may serve to restore the sensitivity of the neuroendocrine reflex. Nonlactating cows will release oxytocin in response to udder stimulation. But, virgin heifers do not respond substantially to udder stimulation. Apparently maximum oxytocin release in response to udder stimulation occurs only if the mammary gland is lactating or has lactated.

Maximal prolactin release from the pituitary in response to tactile stimulation of the udder depends on the presence of a fully developed mammary gland.

It is estimated that the bovine pituitary has about 800 micrograms of oxytocin. This is about 40X what is in the blood under resting conditions. Only about 1/3 of pituitary oxytocin is released at a milking.

How much oxytocin is needed to elicit milk ejection? Peak oxytocin is about 11to 65 microunits/ml serum; 40 liters of blood in a cow = about 0.4 to 2.6 IU. Normally you inject 10 IU to cause milk letdown, but as little as 0.02 IU into the jugular can result in milk ejection (see Sagi et al. J. Dairy Sci. 1980 63:2006).

*** Oxytocin receptors on myoepithelial cells can respond to very low levels of oxytocin.

*** Oxytocin has a short half-life in the blood = 0.55 to 3.6 min. This means that the removal of milk by machine or by nursing must be closely timed with stimulation of the teats.


Other Roles of Oxytocin

  • Injection of oxytocin into the ventricles of the cerebellum in rats induces maternal behavior.
  • Oxytocin has insulin-like activity and it may be lipogenic. The mother is rapidly losing a great deal of lipid when milk is removed.
  • Both oxytocin and prolactin are implicated in osmoregulation. The mother is rapidly losing a great deal of water when milk is removed.
  • Oxytocin-containing neurons from the paraventricular and supraoptic nuclei go to other brain regions which are involved in autonomic regulation (such as cardiovascular effects). In these cases, oxytocin is acting as a neurotransmitter.
  • Oxytocin may be directly or indirectly involved in prolactin release during proestrus in the rat, but this may not be the case in prolactin release caused by suckling or milking (Johnston and Negro-Vilar, 1988, Endocrinology 122:341). The interaction between oxytocin (or oxytocenergic neurons in the hypothalamus and brain) and prolactin release from the pituitary remains an area of investigation (see also Mori et al., 1990, Endocrinology 125:1009).

 
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