The oxytocin system promotes resilience to the effects of neonatal isolation on adult social attachment in female prairie voles

Abstract

Genes and social experiences interact to create variation in social behavior and vulnerability to develop disorders of the social domain. Socially monogamous prairie voles display remarkable diversity in neuropeptide receptor systems and social behavior. Here, we examine the interaction of early-life adversity and brain oxytocin receptor (OTR) density on adult social attachment in female prairie voles. First, pups were isolated for 3 h per day, or unmanipulated, from postnatal day 1-14. Adult subjects were tested on the partner preference (PP) test to assess social attachment and OTR density in the brain was quantified. Neonatal social isolation impaired female PP formation, without affecting OTR density. Accumbal OTR density was, however, positively correlated with the percent of time spent huddling with the partner in neonatally isolated females. Females with high accumbal OTR binding were resilient to neonatal isolation. These results are consistent with the hypothesis that parental nurturing shapes neural systems underlying social relationships by enhancing striatal OTR signaling. Thus, we next determined whether early touch, mimicking parental licking and grooming, stimulates hypothalamic OT neuron activity. Tactile stimulation induced immediate-early gene activity in OT neurons in neonates. Finally, we investigated whether pharmacologically potentiating OT release using a melanocortin 3/4 agonist, melanotan-II (10 mg kg(-1) subcutaneously), would mitigate the social isolation-induced impairments in attachment behavior. Neonatal melanotan-II administration buffered against the effects of early isolation on partner preference formation. Thus, variation in accumbal OTR density and early OT release induced by parental nurturing may moderate susceptibility to early adverse experiences, including neglect.

Figure 1

Experimental design. In Experiment (Expt) 1, entire litters were isolated from both parents and siblings in a temperature-controlled incubator for 3 h per day from PND1–14, with PND0 as the day of birth. Control litters were left undisturbed, apart from being weighed every 3 days. Anxiety-like behavior was tested in the open field (OF), social attachment in the partner preference test (PPT) after 24 and 48 h of cohabitation, and brains were harvested (BH) to perform autoradiography for OTR. Brains from a subset of females who were naive to behavioral testing also underwent autoradiography. In Expt 2, the impact of a 5min body and anogenital paint-brush tactile stimulation (TS) on hypothalamic oxytocin (OT) and vasopressin (AVP) neuronal activation was tested in PND6–7 pups. For 1 h before and after stimulation, pups were kept in a temperature-controlled incubator. In Expt 3, pups were isolated as in Expt 1, but given melanotan-II (MTII) or saline daily for the first week of life. OTR, oxytocin receptor; PND, postnatal day.

Figure 2

Impact of early isolation on weight, parental care upon reunion and adult behavior. Prairie vole litters were exposed to daily 3h separations from both parents and siblings, or left undisturbed aside from weighing every 3 days. Between PND1–13, weights were averaged across all pups of both sexes within a litter. PND21 weights are averaged across sex. Isolated litters displayed slowed weight gain, and weighed significantly less only on PND13 (a). For 1 h after reunion of the litters, both parents displayed heightened licking and grooming in comparison with controls whose pups were not removed from the nest (b). Adults were tested for their social preference in the partner preference arena (c), reproduced from Barrett and Young. Only the control females and early-isolated males displayed a significant partner preference after 48 h of cohabitation (d and e). Asterisk indicates t-test, P<0.05. PND, postnatal day.

Figure 3

Females with low NAcc OTR are susceptible to early adversity. After 48 h of cohabitation, the percent of time females spent huddling with their partner over total huddling significantly correlated with NAcc OTR binding in the early-isolated (R=0.779, R²=0.607, P=0.008), but not control (R=0.006, R²<0.0001, P>0.05) females (a). Only females with low OTR binding exposed to early isolation did not form a partner preference (b). Representative autoradiographs of low (c) and high (d) OTR NAcc females. Asterisk indicates paired t-test, P<0.05. Scale bar, 1 mm. NAcc, nucleus accumbens. OTR, oxytocin receptor.

Figure 4

Tactile stimulation activates OT neurons in the PVN. PND6–7 neonates were brushed for 5- min to the anogenital and body region or only handled and killed after 1 h. Representative oxytocin (a and b) and vasopressin (c and d) sections in handled (left) and tactile stimulated (right) animals. White arrows indicate cells double labeled for EGR-1 and OT or AVP. Oxytocin cells are green and EGR-1 nuclei are labeled red. Tactile stimulation induced significant EGR-1 activity in oxytocin, but not vasopressin, neurons (e). Asterisk indicates Student's t-test, P<0.05. AVP, vasopressin; EGR-1, early growth response factor-1; OT, oxytocin; PVN, paraventricular nucleus.

Figure 5

MTII buffered against early isolation in females. Voles were treated with the melanocortin agonist MTII for the first week of the two-week isolation. Females displayed slowed weight gain for the first week of life, but accelerated weight gain when the drug was off-board (a). MTII treatment significantly reduced weights on PND2 (P=0.012), 3 (P=0.032) and 5 (P=0.036). Neonatal MTII treatment facilitated female pair bonding after a 6h non-mated cohabitation (b). Asterisk indicates t-test (a) or Wilcoxon test (b), P<0.05. MTII, melanotan-II; PND, postnatal day.