Elsevier

Behavioural Brain Research

Volume 341, 2 April 2018, Pages 146-153
Behavioural Brain Research

Monoacylglycerol lipase inhibition alters social behavior in male and female rats after post-weaning social isolation

https://doi.org/10.1016/j.bbr.2017.12.038Get rights and content

Abstract

Post-weaning social isolation (PSI) has been shown to increase aggressive behavior and alter medial prefrontal cortex (mPFC) function in rats. The present study sought to determine whether this phenotype would be normalized by increasing levels of the endocannabinoid 2-arachidonoylglycerol (2-AG) using pharmacological inhibition of monoacylglycerol lipase (MAGL). Male and female Sprague-Dawley rats were exposed to either 4 weeks of PSI or social rearing (SR) starting on postnatal day 21, then underwent a 15 min trial of social interaction with a novel, same-sex juvenile rat. Rats were administered an acute injection of the MAGL inhibitor MJN110 or vehicle prior to the social interaction. Rats received either 0 mg/kg (vehicle), 1 mg/kg, or 5 mg/kg of MJN110. Both doses of MJN110 decreased aggressive grooming, a measure of agonistic behavior, in both males and females, largely driven by decreased aggressive grooming in PSI rats. There were no effects of MJN110 on overall social behavior or play behavior, while modest effects were observed on locomotor activity in SR rats only. While social interaction increased c-Fos expression in the mPFC of both males and females, MJN110 reduced c-Fos preferentially in females. These results suggest that 2-AG can modulate specific social behaviors during adolescence, and may affect mPFC function differentially in males and females.

Introduction

Adolescence is a critical period for the development of competent social behavior in gregarious species including rats and humans. Social isolation of rats during this period has long been known to produce persistent behavioral changes known as the isolation syndrome [1]. Outcomes such as increased anxiety-like behavior [2] and overreaction to novelty [3,4] may be manifest as dysfunctional social interaction. Post-weaning social isolation (PSI), also known as isolation rearing, alters social behavior and increases unprovoked aggressive behavior in rats [[5], [6], [7], [8]], [[9], [10], [11], [12], [13]]. Although most investigations of the effects of PSI on aggression have focused exclusively on males, we have shown that 4 weeks of PSI alters social interaction and increases aggressive grooming in both male and female rats [10,11]. Increased aggressive grooming has been observed by other groups after PSI [14,15] or maternal separation [16] of rats and is considered an agonistic behavior [[17], [18], [19],14,16], or an “offensive threat” signal [13].

The ventral aspect of the medial prefrontal cortex (mPFC), which consists of the prelimbic (PL) and infralimbic (IL) subregions, is crucial for the regulation of emotion and undergoes considerable development during adolescence. The mPFC plays an important role in the processing of stressful or threatening stimuli as well as in determining the visceral, cognitive, and emotional responses to stressors [20]. A number of cognitive and behavioral processes that are analogous to PFC-dependent executive functions in humans are mediated by the mPFC in rodents. Executive functions such as the ability to attend to or ignore stimuli, the control of inhibitory responses, and cognitive flexibility involve the PFC [21]. Furthermore, structural and functional abnormalities of the PFC have been linked to aggressive pathologies including antisocial personality disorder [22]. In rodents, PSI produces abnormalities in mPFC structure and function including decreased constitutive expression of immediate early genes [23,24] and synaptic-associated proteins [25], reductions in mPFC volume [23], and changes in dendritic spine morphology [6]. In studies assessing the protein product of the immediate early gene c-Fos, conflicting evidence exists regarding mPFC activation and aggression associated with PSI in rats. We have observed blunted c-Fos expression in the mPFC of male and female Sprague-Dawley rats after 4 weeks of PSI that was associated with increased aggression against a novel conspecific [10]. However, Toth et al. [26] have observed greater c-Fos activation in the anterior cingulate, but not in the IL or PL, of the mPFC of male rats after 8 weeks of PSI and a resident intruder test compared to socially reared (SR) rats. In another study, 8 weeks of PSI enhanced c-Fos activation in male rats after the resident intruder test in both the PL and IL [13]. Thus, additional studies to determine effects of PSI on social interaction-induced c-Fos are warranted.

The endocannabinoids anandamide (AEA) and 2-arachidonylglycerol (2-AG) have been implicated in emotionality [27], anxiety regulation [28], social interaction [29,30] and play behavior [[31], [32], [33]]. Endocannabinoids play a fundamental role in emotional homeostasis, and dysfunctional endocannabinoid signaling has been observed in human and animal models of anxiety disorders. 2-AG is abundant in limbic regions including the mPFC [66]; it is produced on demand in postsynaptic neurons and acts in a retrograde manner on presynaptic cannabinoid type 1 receptors (CB1). The signaling action of 2-AG is terminated primarily via hydrolysis by the serine hydrolase monoacylglycerol lipase (MAGL) [34], and inhibitors of MAGL have been shown to elevate 2-AG levels [35]. MJN110, a selective inhibitor of MAGL, has been found to increase 2-AG levels while having no effect on AEA. CB1 receptor mRNA is dense in the mPFC, and preferentially expressed on GABAergic interneurons, although they are expressed on glutamatergic neurons as well [36]; increased 2-AG in the mPFC may have complex effects in this brain region.

PSI for 8 weeks increased the expression of mRNA transcripts for several genes in the endocannabinoid system throughout the frontal cortex including the PFC [37]. In addition to increased expression of mRNA for CB1 receptors, expression of mRNA for both MAGL and the biosynthetic enzyme diacylglycerol lipase (DAGL) was increased, suggesting enhanced 2-AG turnover after PSI [37]. Thus, alterations in endocannabinoid function may be involved in the social deficits observed after PSI. To test this, male and female rats were subjected to either 4 weeks of PSI or SR beginning at postnatal day (P)21. Rats then received systemic injections of the MAGL inhibitor MJN110 prior to a single trial of social interaction with a novel same sex rat. In addition to assessing social behavior, activation of the immediate early gene protein product c-Fos was measured in the mPFC of the same rats.

Section snippets

Animals

Male and female Sprague-Dawley rats were purchased (Harlan [now Envigo] Laboratories, Indianapolis, IN) and shipped immediately after weaning at P21 (a period corresponding to the early adolescent period) and housed for 4 weeks either individually (post-weaning social isolation [PSI]) or in same-sex groups of 3 (socially reared [SR]) in standard Plexiglas cages before experimentation began. Experimentation took place at P50, a period corresponding to late adolescence [38,39]. Rats were on a

Aggressive grooming

MAGL inhibition via MJN110 decreased aggressive grooming of a novel same-sex rat in both males and females, primarily in PSI rats, as shown in Fig. 2. We observed a significant Rearing x Drug interaction, F (2, 85) = 3.31, p < 0.05. Post-hoc tests indicated that Vehicle (0 mg/kg) treated PSI rats spent more time engaged in aggressive grooming than all other groups, p < .05, and that SR rats at the 5 mg/kg dose spent less time engaged in aggressive grooming than SR vehicle rats and PSI rats at

Discussion

Our primary finding indicates that pharmacological inhibition of MAGL using a systemic injection of MJN110 decreased aggressive grooming of a novel same-sex rat; this was observed in both male and female rats. This decrease was primarily driven by an attenuation of the increase in aggressive grooming that was produced by 4 weeks of post-weaning social isolation. Importantly, MJN110 did not result in decreased non-aggressive social interactions, including play behaviors. These observations

Conflict of interest

The authors have no conflicts of interest to report.

Acknowledgements

This work was supported by NIH Grant R15MH102717. Jazmin Fontenot was supported by NIH BP-ENDURE undergraduate training grant R25GM097633. We also wish to thank the University of Colorado Denver Undergraduate Research Opportunity Program (UROP) for their support, and we thank Raleigh Jonscher and Emma Boxer for their contributions.

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