The Role of Oxytocin in Domestic Animal's Maternal Care: Parturition, Bonding, and Lactation

Abstract

Oxytocin (OXT) is one of the essential hormones in the birth process; however, estradiol, prolactin, cortisol, relaxin, connexin, and prostaglandin are also present. In addition to parturition, the functions in which OXT is also involved in mammals include the induction of maternal behavior, including imprinting and maternal care, social cognition, and affiliative behavior, which can affect allo-parental care. The present article aimed to analyze the role of OXT and the neurophysiologic regulation of this hormone during parturition, how it can promote or impair maternal behavior and bonding, and its importance in lactation in domestic animals.

Keywords: maternal aggression; maternal care; milk ejection; mother–young bonding.

Figure 1

Mechanism of action of oxytocin during myometrial contractions. During parturition, the activation of OXTR at the uterus by OXT starts myometrial contractions. The interaction between OXTR and voltage-regulated Ca²⁺ channels enables the formation of the actomyosin-P complex to produce myocytes’ contraction. After contraction starts, a relation between contraction maintenance and uterine blood flow is responsible for maintaining effective contraction movements and, therefore, a normal course during parturition. In the first instance, myometrial contractions cause compression of uterine blood vessels, reducing blood flow. Hypoxia induces an acidemia state, by lactate increase, which decreases Ca²⁺ influx and, consequently, myometrial contractions stop, restoring blood flow and normoxia, in order to start another effective contraction. The ongoing maternal and fetal stimulation that promotes OXT release maintains rhythmic contractions due to initial hypoxia and acidemia of the myocytes, an effect that decreases contractions, and the cycle begins again when reaching normoxia and a regular uterine blood flow. MLC: myosin-light chain kinase; OXTR: oxytocin receptor; PG: prostaglandins; PKC: protein kinase C; SR: sarcoplasmic reticulum.

Figure 2

The role of oxytocin during parturition. In the final phase of pregnancy, secretion of ACTH due to the maturation of the hypothalamus leads to the release of fetal cortisol. These hormones also respond to the increase in E2 and PGF2α, and the decrease in P4. These hormones promote cervix dilation and the mechanoreceptors’ stimulation in the uterus. The sensory signals are processed in the PVN of the maternal hypothalamus after its transmission through fibers in the spinal cord. The interaction between the hypothalamus and the pituitary causes OXT release by the posterior pituitary into the bloodstream to initiate the physiological process of fetal expulsion. DRG: dorsal root ganglion; OXT: oxytocin; OXTR: oxytocin receptors; P4: progesterone; PGF2α: prostaglandin F2α; PVN: paraventricular nucleus; SON: supraoptic nucleus.

Figure 3

The oxytocinergic positive feedback loop of maternal behavior in sows. The visual, tactile, olfactory, and auditory stimuli that are constantly perceived by the sow participate in maternal responsiveness and attachment due to the activation of vital cerebral regions sensitive to OXT. For example, stimulation and activation of AMY, BNST, PVN, and VTA promote OXT release in the posterior pituitary. After OXT is released, several peripartum maternal behaviors in mammals are influenced by this hormone. In the case of sows, increases in circulating OXT levels before the onset of farrowing motivate nest site seeking and nesting. Particularly, nest building has also been associated with high concentrations of OXT, P4, PRL, and low levels of SOM in aforementioned cerebral regions. After farrowing, the development of maternal attachment and nursing of the piglets requires so-called OXT positive feedback, where piglets stimulate teat mechanoreceptors through suckling, releasing OXT in the maternal brain and promoting maternal behaviors. On the other hand, MPOA axons project to the anterior pituitary to release PRL and participate in milk secretion. AMY: amygdala; AVP: arginine vasopressin; BNST: bed nucleus of the stria terminalis; MPOA: medial preoptic area; OB: olfactory bulb; OXT: oxytocin; P4: progesterone; PFC: prefrontal cortex; PRL: prolactin; PVN: paraventricular nucleus; SOM: somatostatin.

Figure 4

Neurophysiology of milk ejection and oxytocin influence. Milk yield and ejection can be divided into five stages. 1. Environmental stimulus, such as suckling from the calf, activates udder mechanoreceptors in the teat. 2. Through nervous tracts, the sensory signal reaches the hypothalamus. 3. The SON secretory cells are activated to release OXT. 4. When OXT reaches the bloodstream, it acts on the myoepithelial cells of the alveolus, causing the contraction and secretion of milk by the secretory cells (5). This physiological path and the aforementioned stimulus from the offspring are one of the reasons why manual or mechanical stimulation of the udder is important in dairy systems and can depend on OXT administration, regardless of its controversial effects. OXT: oxytocin; SON: supraoptic nucleus.