Maternal neglect alters reward-anticipatory behavior, social status stability, and reward circuit activation in adult male rats

Introduction

Adverse early life experiences affect neuronal growth and maturation of reward circuits that modify behavior under reward predicting conditions. Previous studies demonstrate that rats undergoing denial of expected reward in the form of maternal contact (DER-animal model of maternal neglect) during early post-natal life developed anhedonia, aggressive play-fight behaviors and aberrant prefrontal cortex structure and neurochemistry. Although many studies revealed social deficiency following early-life stress most reports focus on individual animal tasks. Thus, attention needs to be given on the social effects during group tasks in animals afflicted by early life adversity.

Methods

To investigate the potential impact of the DER experience on the manifestation of behavioral responses induced by natural rewards, we evaluated: 1) naïve adult male sexual preference and performance, and 2) anticipatory behavior during a group 2-phase food anticipation learning task composed of a context-dependent and a cue-dependent learning period.

Results

DER rats efficiently spent time in the vicinity of and initiated sexual intercourse with receptive females suggesting an intact sexual reward motivation and consummation. Interestingly, during the context-dependent phase of food anticipation training DER rats displayed a modified exploratory activity and lower overall reward-context association. Moreover, during the cue-dependent phase DER rats displayed a mild deficit in context-reward association while increased cue-dependent locomotion. Additionally, DER rats displayed unstable food access priority following food presentation. These abnormal behaviours were accompanied by overactivation of the ventral prefrontal cortex and nucleus accumbens, as assessed by pCREB levels.

Conclusions/discussion

Collectively, these data show that the neonatal DER experience resulted in adulthood in altered activation of the reward circuitry, interfered with the normal formation of context-reward associations, and disrupted normal reward access hierarchy formation. These findings provide additional evidence to the deleterious effects of early life adversity on reward system, social hierarchy formation, and brain function.

Manifestations of domination: Assessments of social dominance in rodents

Social hierarchies are ubiquitous features of virtually all animal groups. The varying social ranks of members within these groups have profound effects on both physical and emotional health, with lower-ranked individuals typically being the most adversely affected by their respective ranks. Thus, reliable measures of social dominance in preclinical rodent models are necessary to better understand the effects of an individual's social rank on other behaviors and physiological processes. In this review, we outline the primary methodologies used to assess social dominance in various rodent species: those that are based on analyses of agonistic behaviors, and those that are based on resource competition. In synthesizing this review, we conclude that assays based on resource competition may be better suited to characterize social dominance in a wider variety of rodent species and strains, and in both males and females. Lastly, albeit expectedly, we demonstrate that similarly to many other areas of preclinical research, studies incorporating female subjects are lacking in comparison to those using males. These findings emphasize the need for an increased number of studies assessing social dominance in females to form a more comprehensive understanding of this behavioral phenomenon.

Rats do not eat alone in public: Food-deprived rats socialize rather than competing for baits

Limited resources result in competition among social animals. Nevertheless, social animals also have innate preferences for cooperative behavior. In the present study, 12 dyads of food-deprived rats were tested in four successive trials, and then re-tested as eight triads of food-deprived rats that were unfamiliar to each other. We found that the food-deprived dyads or triads of rats did not compete for the food available to them at regular spatially-marked locations that they had previously learnt. Rather, these rats traveled together to collect the baits. One rat, or two rats in some triads, lead (ran ahead) to collect most of the baits, but "leaders" differed across trials so that, on average, each rat ultimately collected similar amounts of baits. Regardless of which rat collected the baits, the rats traveled together with no substantial difference among them in terms of their total activity. We suggest that rats, which are a social species that has been found to display reciprocity, have evolved to travel and forage together and to share limited resources. Consequently, they displayed a sort of 'peace economy' that on average resulted in equal access to the baits across trials. For social animals, this type of dynamics is more relaxed, tolerant, and effective in the management of conflicts. Rather than competing for the limited available food, the food-deprived rats socialized and coexisted peacefully.

Decreased dominance in a limited access test but normal maternal behavior in micrencephalic rats

Micrencephalic offspring produced by gestational treatment with the antimitotic compound/methylazoxymethanol acetate (MAM) are remarkable for substantial preservation of function concurrent with severe neural stunting. Altered behaviors in these micrencephalics, including light shyness and response perseveration, are similar to those produced by frontal cortex lesions. Consistent with this, the frontal cortex is one of several regions severely stunted by gestational MAM treatment. Because the frontal cortex has been implicated in rodent social behavior, maternal behavior in females and dominance in both sexes were assessed. Dominance was measured via water competition in 24-h water-deprived dyads (1 control and 1 MAM) matched for sex and body weight. Micrencephalic rats exhibited shorter drinking time than controls (males: 101 vs. 219 s, p < 0.001; females: 114 vs. 176 s, p < 0.03), indicating that micrencephalics were more submissive. For maternal behavior tests, micrencephalic and control females were bred to control males and pup retrieval was measured on postnatal days 3-13. Micrencephalic dams were unimpaired in any aspect of pup retrieval. Of 8 standard behavior measures used here and previously, access time in water competition tests produced the clearest differentiation between control and micrencephalic rats. These studies indicate that at least one aspect of social dominance in both sexes is severely reduced by MAM treatment while maternal behavior remains intact.