Intracerebroventricular Oxytocin Self-Administration in Female Rats

Conditioned social preference and reward value of activating oxytocin-receptor-expressing ventral tegmental area neurons following repeated daily binge ethanol intake

BACKGROUND

Individuals with alcohol use disorder (AUD) exhibit a disruption of social behavior and dysregulation of oxytocin signaling in the brain, possibly reflecting decreased activation of oxytocin receptors (OxTRs) in reward pathways in response to social stimuli. We hypothesize that daily binge ethanol intake causes a deficit in social reward and oxytocin signaling in the ventral tegmental area (VTA).

METHODS

After 9 weeks of daily binge ethanol intake (blood ethanol concentration >80 mg%), OxTR-cre mice underwent conditioned place preference for social reward. Separate groups of mice were tested for the effects of binge ethanol on voluntary social interactions, food reward, locomotion, and anxiety-like behaviors. A subset of mice underwent transfection of OxTR-expressing VTA neurons (VTA^Oxtr ) with a light-sensitive opsin, followed by operant training to respond to light delivered to VTA.

RESULTS

Ethanol-naïve male mice increased the time spent on the side previously paired with novel mice while ethanol-treated mice did not. Binge ethanol did not affect conditioned place preference for food reward in males, but this response was weakened in ethanol-treated females. Ethanol treatment also caused a sex-specific impairment of voluntary social interactions with novel mice. There were minimal differences between groups in measures of anxiety and locomotion. Ethanol-naïve mice had significantly greater operant responding for activation of VTA^Oxtr than sham-transfected mice but ethanol-treated mice did not. There was no difference in the number of VTA^Oxtr after binge ethanol.

CONCLUSIONS

Daily binge ethanol causes social reward deficits that cannot be explained by nonspecific effects on other behaviors, at least in males. Only ethanol-naïve mice exhibited positive reinforcement caused by activation of VTA^Oxtr while daily binge ethanol did not alter the number of VTA^Oxtr in either males or females. Thus, subtle dysregulation of VTA^Oxtr function may be related to the social reward deficits caused by daily binge ethanol.

The potential role of oxytocin in addiction: What is the target process?

Oxytocin regulates a variety of centrally-mediated functions, ranging from socio-sexual behavior, maternal care, and affiliation to fear, stress, anxiety. In the past years, both clinical and preclinical studies characterized oxytocin for its modulatory role on reward-related neural substrates mainly involving the interplay with the mesolimbic and mesocortical dopaminergic pathways. This suggests a role of this nonapeptide on the neurobiology of addiction raising the possibility of its therapeutic use. Although far from a precise knowledge of the underlying mechanisms, the putative role of the bed nucleus of the stria terminalis as a key structure where oxytocin may rebalance altered neurochemical processes and neuroplasticity involved in dependence and relapse has been highlighted. This view opens new opportunities to address the health problems related to drug misuse.

Oxytocin treatment for alcoholism: Potential neurocircuitry targets

Oxytocin (OT) has gained considerable interest in recent years as a potential treatment for alcoholism and other substance use disorders. Evidence continues to mount that OT administered either centrally, peripherally or intranasally can decrease ethanol intake in both humans and animal models. The potential mechanisms for the ability of OT to decrease ethanol reward, and importantly, cue- and stress-induced ethanol relapse, are explored by reviewing the specific neuronal circuits involved in mediating these actions and their sensitivity to OT. In addition to dopamine neurons that project from ventral tegmental area (VTA) to nucleus accumbens (NAc) to signal positively reinforcing events, OT receptors (OxTR) are also expressed by dopamine neurons that project from VTA to brain regions that can convey aversive properties of a stimulus. Moreover, OxTR are expressed by non-dopaminergic neurons in the VTA, such as GABA and glutamate neurons, which can both modulate the activity of dopamine VTA neurons locally (in opposite directions) or can project to other brain regions, including the NAc, where it can alter either positive reinforcement or aversion caused by ethanol. The ability of OT to regulate limbic circuitry and the hypothalamic-pituitary-adrenal axis is discussed as a potential mechanism for the ability of OT to inhibit ethanol-induced negative reinforcement. Together, understanding the diversity and complexity of OT regulation of ethanol reward may contribute to more effective use of OT as pharmacotherapy for alcohol use disorder. This article is part of the special issue on Neuropeptides.

Sex-dependent regulation of social reward by oxytocin: an inverted U hypothesis

The rewarding properties of social interactions are essential for the expression of social behavior and the development of adaptive social relationships. Here, we review sex differences in social reward, and more specifically, how oxytocin (OT) acts in the mesolimbic dopamine system (MDS) to mediate the rewarding properties of social interactions in a sex-dependent manner. Evidence from rodents and humans suggests that same-sex social interactions may be more rewarding in females than in males. We propose that there is an inverted U relationship between OT dose, social reward, and neural activity within structures of the MDS in both males and females, and that this dose-response relationship is initiated at lower doses in females than males. As a result, depending on the dose of OT administered, OT could reduce social reward in females, while enhancing it in males. Sex differences in the neural mechanisms regulating social reward may contribute to sex differences in the incidence of a large number of psychiatric and neurodevelopmental disorders. This review addresses the potential significance of a sex-dependent inverted U dose-response function for OT's effects on social reward and in the development of gender-specific therapies for these disorders.

Estrogen-Dependent Upregulation of Adcyap1r1 Expression in Nucleus Accumbens Is Associated With Genetic Predisposition of Sex-Specific QTL for Alcohol Consumption on Rat Chromosome 4

Humans show sex differences related to alcohol use disorders (AUD). Animal model research has the potential to provide important insight into how sex differences affect alcohol consumption, particularly because female animals frequently drink more than males. In previous work, inbred strains of the selectively bred alcohol-preferring (P) and non-preferring (NP) rat lines revealed a highly significant quantitative trait locus (QTL) on rat chromosome 4, with a logarithm of the odds score of 9.2 for alcohol consumption. Recently, interval-specific congenic strains (ISCS) were developed by backcrossing the congenic P.NP line to inbred P (iP) rats to further refine the chromosome 4 QTL region. Two ISCS sub-strains, ISCS-A and ISCS-B, were obtained with a narrowed QTL, where the smallest region of overlap consisted of 8.9 Mb in ISCS-B. Interestingly, we found that females from both ISCS lines consumed significantly less alcohol than female iP controls (p < 0.05), while no differences in alcohol consumption were observed between male ISCS and iP controls. RNA-sequencing was performed on the nucleus accumbens of alcohol-naïve female ISCS-B and iP rats, which revealed differentially expressed genes (DEG) with greater than 2-fold change and that were functionally relevant to behavior. These DEGs included down-regulation of Oxt, Asb4, Gabre, Gabrq, Chat, Slc5a7, Slc18a8, Slc10a4, and Ngfr, and up-regulation of Ttr, Msln, Mpzl2, Wnt6, Slc17a7, Aldh1a2, and Gstm2. Pathway analysis identified significant alterations in gene networks controlling nervous system development and function, as well as cell signaling, GABA and serotonin receptor signaling and G-protein coupled receptor signaling. In addition, β-estradiol was identified as the most significant upstream regulator. The expression levels of estrogen-responsive genes that mapped to the QTL interval and have been previously associated with alcohol consumption were measured using RT-qPCR. We found that expression of the Adcyap1r1 gene, encoding the pituitary adenylate cyclase-activating polypeptide type 1 (PAC₁) receptor, was upregulated in female ISCS-B compared to female iP controls, while no differences were exhibited in males. In addition, sequence variants in the Adcyap1r1 promoter region showed a differential response to estrogen stimulation in vitro. These findings demonstrate that rat chromosome 4 QTL contains genetic variants that respond to estrogen and are associated with female alcohol consumption.

Prosocial effects of prolactin in male rats: Social recognition, social approach and social learning

Prolactin (PRL) and oxytocin (OT) are pituitary hormones essential for lactation, but also promote sexual behavior. OT stimulates social behaviors, such as recognition, approach, and learning, but less is known about PRL in these behaviors. Since PRL and OT have complementary functions in reproduction, we hypothesized that PRL increases social recognition, approach, and learning. Male Long-Evans rats received ovine PRL (oPRL; 0.5, 2.0 or 5.0mg/kg), the PRL antagonist bromocriptine (0.1, 3.0 or 5.0mg/kg) or saline 20 mins before testing for recognition of familiar vs. unfamiliar stimulus males. Saline controls preferred the unfamiliar male (p<0.05), while bromocriptine blocked this preference. oPRL did not increase preference. To measure social approach, we determined if PRL restores approach 2h after defeat by an aggressive male. Defeated rats avoided the aggressive male. 2mg/kg oPRL, before or after defeat, restored approach towards the aggressive male (p<0.05). In non-defeated rats, oPRL or 3mg/kg bromocriptine had no effect. To determine if PRL increases social learning, we tested social transmission of food preference. Rats choose between two unfamiliar flavors, one of which they have previously been exposed to through interaction with a demonstrator rat. Vehicle controls preferred chow with the demonstrated flavor over the novel flavor. oPRL-treated rats were similar. Bromocriptine-treated rats failed to show a preference. When tested one week later, only oPRL-treated rats preferred the demonstrated flavor. The results suggest that PRL is required for social recognition and learning, and that increasing PRL enhances social memory and approach, similar to OT.