A great deal of evidence suggests that the oxytocin neuroendocrine system plays a key role in the initiation of maternal behavior following birth. It is now clear that oxytocin, which is synthesized in the neurons of the hypothalamus (specifically in the paraventricular nucleus [PVN] and supraoptic nucleus), is released not only into the general circulation from the posterior pituitary, but centrally during vaginal stimulation and parturition, to act upon oxytocin receptors widely expressed throughout the central nervous system (Forsling, 1986; Barberis and Tribollet, 1996; Gimpl and Fahrenholz, 2001). Moreover, maternal behavior was found to be greatly impaired in post-parturient rats that received PVN lesions during pregnancy (Insel and Harbaugh, 1989), and in female rats administrated an oxytocin antagonist immediately after parturition (van Leengoed et al., 1987). Furthermore, oxytocin receptors proliferate in many forebrain areas including the mPOA, ventromedial hypothalamus, and bed nucleus of the stria terminalis of female rats at the time of partition (Jirikowski et al., 1989; Pedersen et al., 1994). These alterations in oxytocin receptor expression may be partially responsible for the induction of maternal behavior, since intraventricular and mPOA infusion of oxytocin induces a rapid onset of maternal behavior in virgin female rats (Pedersen and Prange Jr, 1979; Pedersen et al., 1994).
Oxytocin and mutual communication in mother-infant bonding
The oxytocin system in the mPOA is associated with the induction of maternal behavior (Pedersen et al., 1994).
Oxytocin receptor levels in the central nucleus of the amygdala were significantly higher in high compared to low LG-ABN females regardless of reproductive status. These findings suggest that individual differences in maternal behaviour may be directly related to variations in oxytocin receptor expression.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4671834/
Indeed, one of the functions of estradiol action within MPOA neurons may be to stimulate the expression of oxytocin receptors (OTRs) so that the critical neurons become responsive to OT,
higher densities of OTR in the mPFC, NA and lateral amygdala than nonmonogamous montane and meadow voles (Fig. 4; Insel & Shapiro, 1992). Indeed, OTR is virtually absent in the NA of nonmonogamous vole species. This robust species difference in OTR density in the mesolimbic reward pathway led to speculation that these receptor populations may be involved in mating-induced partner preference formation. Indeed, infusion of an OT antagonist into the NA or mPFC, but not into the adjacent caudate-putamen, prevents mating-induced partner preferences in female prairie voles (Young et al., 2001).
Insel, T.R. & Shapiro, L.E. Oxytocin receptor distribution reflects social organization
in monogamous and polygamous voles. Proc. Natl. Acad. Sci. USA 89, 5981–5985
(1992).
Liu, Y. & Wang, Z.X. Nucleus accumbens dopamine and oxytocin interact to regu-
late pair bond formation in female prairie voles. Neuroscience 121, 537–544
(2003)
oxytocin receptors in NAcc is essential for pair-bond formation
As already noted, we do not know whether OTRs are localized on neurons expressing D1 or D2 receptors, or both.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4671834/
One possibility is that the hormonal and OT changes associated with the end of pregnancy coupled with maternal interaction with pups in some way boosts the number of OTRs in NAs to levels much higher than those that are typically expressed in nonpregnant females and males. Perhaps epigenetic mechanisms are involved in this effect (Stolzenberg et al., 2014; 2012; cf. Wang et al., 2013). This is certainly and important area for future research.
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