Prenatal stress generates deficits in rat social behavior: Reversal by oxytocin

Abstract

Neurodevelopmental changes induced by environmental stress exposure play a significant but poorly defined role in the etiology of schizophrenia. Exposure of pregnant female rats to a series of unpredictable stresses during the final week of pregnancy generates behavioral deficits and molecular changes in the offspring similar to those observed in schizophrenic individuals. We used this rat prenatal stress preparation to investigate social withdrawal behaviors that may have relevance to the negative symptoms of schizophrenia. The cumulative time adult male offspring of stress-exposed pregnant female rats actively interacted with a weight-matched, same-sex peer was decreased approximately 76% relative to non-stress exposed control rats. Prenatal stress exposure also diminished the quality of the social interaction behavior indicative of reduced social drive. Analysis of the oxytocinergic system in the prenatally stressed male rats revealed significantly less oxytocin mRNA in the paraventricular nucleus and increased oxytocin receptor binding in the central amygdala. Moreover, oxytocin, but not vasopressin, administration into the central amygdala reversed the social incompetence of the prenatally stressed rats without increasing behavior in non-stressed control animals. In addition, cross-fostering pups from prenatally stressed mothers to non-stressed mothers failed to improve the social deficit of the prenatally stressed male offspring. Two behavioral assays designed to measure anxiety did not differentiate the prenatally stressed rats from non-stressed controls. These data indicate that prenatal stress may be an etiologically appropriate animal model for some aspects of schizophrenic social withdrawal. Furthermore, unpredictable prenatal stress exposure selectively degrades social interaction behaviors without increasing anxiety measures.

Figure 1a

Social interaction behaviors in adult male rats (56 days of age) following exposure no stress during pregnancy or following exposure to an unpredictable prenatal stress regimen. Exposure in utero to an unpredictable stress paradigm caused a significant decrease in the cumulative mean time spent by adult males in social interaction (*p < 0.001, n=6) when compared to the mean time of male rats not exposed to stress (no stress control, n=6). Data shown are mean total social interaction times (sec) ± SEM for each group.

Figure 1b

Comparison of contact and non-contact social interaction behaviors in adult male rats exposed to unpredictable prenatal stress. Adult male rats exposed to prenatal stress (n=6/group) spent a significantly larger percentage (*p < 0.001) of their social interaction time engaged in a non-contact behavior (following) with a male target as opposed to behaviors with obligate close physical proximity (anogential exploration or crawling over/under). The distribution for unstressed control rats (n=8) behavioral percentages revealed a significantly greater (*p < 0.001) preference for contact behaviors. Data shown are mean percent of total time spent engaged in social interaction ± SEM.

Figure 1c

Comparison of social interaction behavior in prepubertal (35 days old) and young adult (56 days old) male rats following exposure to an unpredictable prenatal stress paradigm. Data shown are expressed as the mean percent of age-matched, non-stress exposed male rat social interaction time ± SEM. Six rats were tested in each experimental group. Statistical differences are designated by # (p<0.001 compared to control rats) and by ‡ (p<0.05 compared to 35 days old prenatally stressed rats).

Figure 2

Representative autoradiographic images of PVN oxytocin (OT) mRNA expression, and OT and vasopressin (VP) receptor binding in the CeA of a non-stressed control male rats and a prenatally stressed male rats. All images are light-field images captured from film autoradiographs using a CCD camera. A 1 mm scale bar is included in the left-hand image of each image set for comparison purposes.

Figure 3

Effect of cross-fostering on social behavior in prenatally stressed and non-stressed male rats. Rat pups from non-stressed mothers were cross-fostered to other non-stressed mothers or prenatally stressed mothers within 24 hours of birth. Rat pups from prenatally stress mothers were also cross-fostered to other prenatally stressed or non-stressed mother rats. On postnatal day 56, social interaction behavior was determined in randomly selected cross-fostered male offspring. Prenatally stressed male rats, regardless of postnatal rearing, were less inclined to display social interaction behaviors (# - p < 0.001) compared to non-stressed male rats. Data shown are mean social interaction times (sec) ± SEM for each group (n = 6 in all groups).

Figure 4

Cumulative mean time spent engaged in social interaction behavior by prenatally stressed and unstressed control male rats after infusions into the CeA of VP, OT, and saline (vehicle). All post-infusion social interaction mean times for prenatally stressed rats were compared to the mean time of the unstressed, saline-infused control rats (n=8) except for the 1000 ng dose which was compared a 1000 ng dose of OT in unstressed controls. OT infusions (1000 ng) into unstressed control male rats (n=8) had no effect on social interaction. Bilateral infusions of 1000 ng of OT into prenatally stressed rats (n=8) increased social interaction to a level identical to that of unstressed control rats infused with saline or OT. Infusions of two lower doses of OT into prenatally stressed rats, 100 ng (n=8) and 10 ng (n=7), were associated with the same significant (*p < 0.001) reduction in social interaction found in saline-infused, prenatally stressed rats (n=5). Two infused doses of VP, 10 ng (n=7) and 3000 ng (n=8), were also unable to restore normal levels of social interaction in prenatally stressed rats. Data are shown as the mean total social interaction times (sec) ± SEM.

Figure 5

Representative cresyl violet-stained rat brain section used in the oxytocin and vasopressin infusion experiments demonstrating a typical location of the injection cannulae above the CeA. Dye was injected to mark the cannula location and is depicted as the dark spot traversing the internal capsule referred to by the black arrow. The scale on the right is a millimeter scale for size comparison.

Figure 6

Contact and non-contact behaviors after OT or saline infusions into the CeA of prenatally stressed or unstressed control male rats expressed as a percentage of total social interaction time. The mean percents of contact and non-contact behaviors for each group were compared to those of saline-infused unstressed control rats (n=8). Bilateral infusions of 1000 ng of OT into prenatally stressed rats (n=8) increased contact behaviors (sniffing, crawling over/under) to a level identical to that of unstressed control rats infused with saline. Two other infused doses of OT, 100 ng (n=8) and 10 ng (n=8) were associated with the same significant (*p < 0.01) reduction in contact interactions found in saline-infused, prenatally stressed rats (n=8). Data are shown as the mean percent of total social interaction times ± SEM.