The acute intoxicating effects of ethanol are not dependent on the vasopressin 1a or 1b receptors

Neuronal biomarkers as potential therapeutic targets for drug addiction related to sex differences in the brain: Opportunities for personalized treatment approaches

Biological sex disparities manifest at various stages of drug addiction, including craving, substance abuse, abstinence, and relapse. These discrepancies are underpinned by notable distinctions in neurobiological substrates, encompassing brain structures, functions, and neurotransmitter systems implicated in drug addiction. Neuronal biomarkers, such as neurotransmitters, signaling proteins, and genes may be associated with the diagnosis, prognosis, and treatment outcomes in both biological sexes afflicted by drug abuse. Sex differences in the neural reward system, mainly through dopaminergic transmission during drug abuse, can be attributed to modifications in neurotransmitter systems and signaling pathways. This results in distinct patterns of neural activation and responsiveness to addictive substances in males and females. Sex hormones, the estrus/menstrual cycle, and cerebral neurochemistry contribute to the progression of psychological and physiological dependence in both male and female individuals grappling with addiction. Moreover, the alteration of sex hormone balance and neurotransmitter release plays a pivotal role in substance use disorders, subsequently modulating cognitive functions pertinent to reward, including memory formation, decision-making, and locomotor activity. Comparative investigations reveal distinctions in brain region volume, gene expression, neuronal firing, and circuitry in substance use disorders affecting individuals of both biological sexes. This review examines prevalent substance use disorders to elucidate the impact of sex hormones as therapeutic biomarkers on the mesocorticolimbic neurotransmitter systems via diverse mechanisms within the addicted brain. We underscore the imperative necessity of considering these variations to gain a deeper comprehension of addiction mechanisms and potentially discern sex-specific neuronal biomarkers for tailored therapeutic interventions.

Impact of adolescent intermittent ethanol exposure in male and female rats on social drinking and neuropeptide gene expression

BACKGROUND

Alcohol use during adolescence can alter maturational changes that occur in brain regions associated with social and emotional responding. Our previous studies have shown that adult male, but not female rats demonstrate social anxiety-like alterations and enhanced sensitivity to ethanol-induced social facilitation following adolescent intermittent ethanol exposure (AIE). These consequences of AIE may influence adult social drinking in a sex-specific manner.

METHODS

To test the effects of AIE on social drinking, male and female Sprague-Dawley rats exposed to water or ethanol (0 or 4 g/kg, intragastrically, every other day, between postnatal day [P] 25 and 45) were tested as adults (P72-83) in a social drinking paradigm (30-minute access to a 10% ethanol solution in supersac or supersac alone in groups of three same-sex littermates across two 4-day cycles separated by 4 days off). Social behavior was assessed during the last drinking session, along with assessment of oxytocin (OXT), oxytocin receptor (OXTR), vasopressin (AVP), and vasopressin receptors 1a and 1b (AVPR1a, AVPR1b) in the hypothalamus and lateral septum.

RESULTS

Males exposed to AIE consumed more ethanol than water-exposed controls during the second drinking cycle, whereas AIE did not affect supersac intake in males. AIE-exposed females consumed less ethanol and more supersac than water-exposed controls. Water-exposed females drinking ethanol showed more social investigation and significantly higher hypothalamic OXTR, AVP, and AVPR1b gene expression than their counterparts ingesting supersac and AIE females drinking ethanol. In males, hypothalamic AVPR1b gene expression was affected by drinking solution, with significantly higher expression evident in males drinking ethanol than those consuming supersac.

CONCLUSIONS

Collectively, these findings provide new evidence regarding sex-specific effects of AIE on social drinking and suggest that the hypothalamic OXT and AVP systems are implicated in the effects of ingested ethanol on social behavior in a sex- and adolescent-exposure-dependent manner.

Activation of hypothalamic oxytocin neurons reduces binge-like alcohol drinking through signaling at central oxytocin receptors

Preclinical and clinical evidence suggests that exogenous administration of oxytocin (OT) may hold promise as a therapeutic strategy for reducing heavy alcohol drinking. However, it remains unknown whether these effects are mediated by stimulation of endogenous sources of OT and signaling at oxytocin receptors (OTR) in brain or in the periphery. To address this question, we employed a targeted chemogenetic approach to examine whether selective activation of OT-containing neurons in the paraventricular nucleus of the hypothalamus (PVN) alters alcohol consumption in a binge-like drinking ("Drinking-in-the-Dark"; DID) model. Adult male Oxt-IRES-Cre mice received bilateral infusion of a Cre-dependent virus containing an excitatory DREADD (AAV8-hSyn-DIO-hM3Dq-mCherry) or control virus (AAV8-hSyn-DIO-mCherry) into the PVN. Chemogenetic activation of PVN^OT+ neurons following clozapine-N-oxide injection reduced binge-like alcohol drinking in a similar manner as systemic administration of the neuropeptide. Pretreatment with a brain-penetrant OTR antagonist (L-368,899) reversed this effect while systemic administration of a peripherally restricted OTR antagonist (Atosiban) did not alter reduced alcohol drinking following chemogenetic activation of PVN^OT+ neurons. Altogether, these data are the first to demonstrate that targeted activation of hypothalamic (endogenous) OT reduces alcohol consumption, providing further evidence that this neuropeptide plays a role in regulation of alcohol self-administration behavior. Further, results indicate that the ability OT to reduce alcohol drinking is mediated by signaling at OTR in the brain.

Oxytocin and vasopressin: Signalling, behavioural modulation and potential therapeutic effects

Oxytocin (OT) and vasopressin (AVP) are endogenous ligands for OT and AVP receptors in the brain and in the peripheral system. Several studies demonstrate that OT and AVP have opposite roles in modulating stress, anxiety and social behaviours. Interestingly, both peptides and their receptors exhibit high sequence homology which could account for the biased signalling interaction of the peptides with OT and AVP receptors. However, how and under which conditions this crosstalk occurs in vivo remains unclear. In this review we shed light on the complexity of the roles of OT and AVP, by focusing on their signalling and behavioural differences and exploring the crosstalk between the receptor systems. Moreover, we discuss the potential of OT and AVP receptors as therapeutic targets to treat human disorders, such as autism, schizophrenia and drug abuse.

Voluntary alcohol consumption is increased in female, but not male, oxytocin receptor knockout mice

INTRODUCTION

The oxytocin (Oxt) system, while typically associated with the neural regulation of social behaviors, also plays a role in an individual's vulnerability to develop alcohol use disorders (AUD). In humans, changes to the Oxt system, due to early life experience and/or genetic mutations, are associated with increased vulnerability to AUD. While a considerable amount is known about Oxt's role in AUD in males, less is known or understood, about how Oxt may affect AUD in females, likely due to many clinical and preclinical studies of AUD not directly considering sex as a biological variable. This is unfortunate given that females are more vulnerable to the effects of alcohol and have increased alcohol consumption, as compared to males. Therefore, in the current study we wanted to determine whether genetic disruption of the Oxt receptor (Oxtr), that is, Oxtr knockout (-/-) mice, affected stress-induced alcohol consumption in males and females. We hypothesized that genetic disruption of the Oxtr would result in increased stress-induced alcohol consumption in both males and females compared to wild-type (+/+) controls. Though, we predicted that these disruptions might be greater in female Oxtr -/- mice.

METHODS

To test this hypothesis, a two-bottle preference test was utilized along with the forced swim test (FST), and pre- and poststress alcohol consumption and preference measured within each sex (males and females were run separately). As a follow-up experiment, a taste preference test, to control for possible genotypic differences in taste, was also performed.

RESULTS

In males, we found no significant genotypic differences in alcohol consumption or preference. However, in females, we found that genetic disruption of the Oxtr resulted in a greater consumption of alcohol both pre- and poststress compared to controls.

CONCLUSION

These data suggest that in females, disruptions in Oxt signaling may contribute to increased vulnerability to alcohol-associated addiction.

Amygdala Arginine Vasopressin Modulates Chronic Ethanol Withdrawal Anxiety-Like Behavior in the Social Interaction Task

BACKGROUND

Chronic ethanol (EtOH) exposure induces neurobehavioral maladaptations in the brain though the precise changes have not been fully explored. The central nucleus of the amygdala (CEA) regulates anxiety-like behavior induced by withdrawal from chronic intermittent EtOH (CIE) exposure, and the arginine vasopressin (AVP) system within the CEA regulates many anxiety-like behaviors. Thus, adaptations occur in the CEA AVP system due to chronic EtOH exposure, which lead to anxiety-like behaviors in rats.

METHODS

Chronic exposure to a low-dose EtOH (4.5% wt/vol) induces anxiety-like behavior in rats. Wistar or Sprague Dawley rats were exposed to a modified CIE or CIE, while intra-CEA microinjections of AVP or a V1b receptor antagonist were used to elicit or block withdrawal-induced anxiety. Additionally, AVP microinjections into the CEA were given 24 hours following 15 days of continuous high-dose EtOH (7% wt/vol), a time period when rats no longer express anxiety. Chemogenetics was also used to activate the basolateral amygdala (BLA) or deactivate the dorsal periaqueductal gray=(dm/dlPAG) therefore PAG=periaqueductal gray to elicit or block withdrawal-induced anxiety.

RESULTS

AVP microinjected into the CEA in lieu of exposure to the first 2 cycles of CIE was sufficient to induce anxiety-like behavior in these commonly used rat strains. The V1b receptor antagonist, but not an oxytocin receptor agonist, into the CEA during the first 2 withdrawal cycles suppressed anxiety. However, activation of the BLA in lieu of exposure to the first 2 cycles of CIE was insufficient to induce anxiety-like behavior. AVP microinjection into the CEA 24 hours into withdrawal reelicited anxiety-like behavior, and deactivation of the dm/dlPAG reduced this effect of CEA AVP.

CONCLUSIONS

Taken together, this study demonstrates a role of CEA AVP and a CEA-dm/dlPAG circuit in the development of anxiety induced by CIE. Such information is valuable for identifying novel therapeutic targets for alcohol- and anxiety-associated disorders.

Vasopressin and alcohol: a multifaceted relationship

BACKGROUND

Arginine vasopressin (VP) has been implicated in a number of neuropsychiatric disorders with an emphasis on situations where stress increased the severity of the disorder. Based on this hypothesized role for VP in neuropsychiatric disorders, much research is currently being undertaken in humans and animals to test VP as a target for treatment of a number of these disorders including alcohol abuse.

OBJECTIVES

To provide a summary of the literature regarding the role of VP in alcohol- and stress-related behaviors including the use of drugs that target VP in clinical trials.

RESULTS

Changes in various components of the VP system occur with alcohol and stress. Manipulating VP or its receptors can alter alcohol- and stress-related behaviors including tolerance to alcohol, alcohol drinking, and anxiety-like behavior. Finally, the hypothalamic-pituitary-adrenal axis response to alcohol is also altered by manipulating the VP system. However, clinical trials of VP antagonists have had mixed results.

CONCLUSIONS

A review of VP's involvement in alcohol's actions demonstrates that there is much to be learned about brain regions involved in VP-mediated effects on behavior. Thus, future work should focus on elucidating relevant brain regions. By using previous knowledge of the actions of VP and determining the brain regions and/or systems involved in its different behavioral effects, it may be possible to identify a specific receptor subtype target, drug treatment combination, or specific clinical contexts that may point toward a more successful treatment.

Social Context, Stress, Neuropsychiatric Disorders, and the Vasopressin 1b Receptor

The arginine vasopressin 1b receptor (Avpr1b) is involved in the modulation of a variety of behaviors and is an important part of the mammalian hormonal stress axis. The Avpr1b is prominent in hippocampal CA2 pyramidal cells and in the anterior pituitary corticotrophs. Decades of research on this receptor has demonstrated its importance to the modulation of social recognition memory, social forms of aggression, and modulation of the hypothalamic-pituitary-adrenal axis, particularly under conditions of acute stress. Further, work in humans suggests that the Avpr1b may play a role in human neuropsychiatric disorders and its modulation may have therapeutic potential. This paper reviews what is known about the role of the Avpr1b in the context of social behaviors, the stress axis, and human neuropsychiatric disorders. Further, possible mechanisms for how Avpr1b activation within the hippocampus vs. Avpr1b activation within anterior pituitary may interact with one another to affect behavioral output are proposed.

Oxytocin and behavior: Lessons from knockout mice

It is well established that the nonapeptide oxytocin (Oxt) is important for the neural modulation of behaviors in many mammalian species. Since its discovery in 1906 and synthesis in the early 1950s, elegant pharmacological work has helped identify specific neural substrates on which Oxt exerts its effects. More recently, mice with targeted genetic disruptions of the Oxt system-i.e., both the peptide and its receptor (the Oxtr)-have further defined Oxt's actions and laid some important scientific groundwork for studies in other species. In this article, we highlight the scientific contributions that various mouse knockouts of the Oxt system have made to our understanding of Oxt's modulation of behavior. We specifically focus on how the use of these mice has shed light on our understanding of social recognition memory, maternal behavior, aggression, and several nonsocial behaviors. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 190-201, 2017.

Genes and Alcohol Consumption: Studies with Mutant Mice

In this chapter, we review the effects of global null mutant and overexpressing transgenic mouse lines on voluntary self-administration of alcohol. We examine approximately 200 publications pertaining to the effects of 155 mouse genes on alcohol consumption in different drinking models. The targeted genes vary in function and include neurotransmitter, ion channel, neuroimmune, and neuropeptide signaling systems. The alcohol self-administration models include operant conditioning, two- and four-bottle choice continuous and intermittent access, drinking in the dark limited access, chronic intermittent ethanol, and scheduled high alcohol consumption tests. Comparisons of different drinking models using the same mutant mice are potentially the most informative, and we will highlight those examples. More mutants have been tested for continuous two-bottle choice consumption than any other test; of the 137 mouse genes examined using this model, 97 (72%) altered drinking in at least one sex. Overall, the effects of genetic manipulations on alcohol drinking often depend on the sex of the mice, alcohol concentration and time of access, genetic background, as well as the drinking test.

Social housing conditions and oxytocin and vasopressin receptors contribute to ethanol conditioned social preference in female mice

Social behavior modulates response to alcohol. Because oxytocin (OXT) and vasopressin (AVP) contribute to rewarding social behavior, the present study utilized a genetic strategy to determine whether OXT and AVP receptors (OXTR, AVPR1a) are essential for female mice to demonstrate a conditioned social preference for ethanol. The study compared wild-type (WT) and knock-out (KO) females lacking either Oxtr or Avpr1a in a conditioned social preference (CSP) test. KO females and WT females from Het-Het crosses were pair-housed: KO and WT(ko). WT females from Het-WT crosses were pair-housed: WT(wt). Test mice received 2g/kg ethanol or saline ip, and were paired four times each with one stimulus female (CS-) after saline, and with another female (CS+) following ethanol. After pairing, the time spent with CS+ and CS- females was measured. WT(wt) females showed conditioned preference for the CS+ female paired with ethanol, demonstrated by greater interaction time (p<0.05). In both KO lines, ethanol significantly reduced interaction with the CS+ female (p<0.05), and there was no change in interaction for WT(ko) females. Response to odors by habituation-dishabituation was unaffected in both KO lines, and the response to a hypnotic dose of ethanol also was the same as in WT mice. However, anxiety, measured as time on the open arms of the elevated plus maze, was reduced in KO(Oxtr) females compared with WT(wt). The results suggest that Oxtr and Avpr1a are required for conditioned effects of an ethanol-associated social stimulus. The lack of CSP in WT(ko) females suggests that the quality of social interactions during postnatal and postweaning life may modulate development and expression of normal social responses.

Role of the vasopressin 1b receptor in rodent aggressive behavior and synaptic plasticity in hippocampal area CA2

The vasopressin 1b receptor (Avpr1b) is critical for social memory and social aggression in rodents, yet little is known about its specific roles in these behaviors. Some clues to Avpr1b function can be gained from its profile of expression in the brain, which is largely limited to the pyramidal neurons of the CA2 region of the hippocampus, and from experiments showing that inactivation of the gene or antagonism of the receptor leads to a reduction in social aggression. Here we show that partial replacement of the Avpr1b through lentiviral delivery into the dorsal CA2 region restored the probability of socially motivated attack behavior in total Avpr1b knockout mice, without altering anxiety-like behaviors. To further explore the role of the Avpr1b in this hippocampal region, we examined the effects of Avpr1b agonists on pyramidal neurons in mouse and rat hippocampal slices. We found that selective Avpr1b agonists induced significant potentiation of excitatory synaptic responses in CA2, but not in CA1 or in slices from Avpr1b knockout mice. In a way that is mechanistically very similar to synaptic potentiation induced by oxytocin, Avpr1b agonist-induced potentiation of CA2 synapses relies on NMDA (N-methyl-D-aspartic acid) receptor activation, calcium and calcium/calmodulin-dependent protein kinase II activity, but not on cAMP-dependent protein kinase activity or presynaptic mechanisms. Our data indicate that the hippocampal CA2 is important for attacking in response to a male intruder and that the Avpr1b, likely through its role in regulating CA2 synaptic plasticity, is a necessary mediator.

Raphe serotonin neuron-specific oxytocin receptor knockout reduces aggression without affecting anxiety-like behavior in male mice only

Serotonin and oxytocin influence aggressive and anxiety-like behaviors, though it is unclear how the two may interact. That the oxytocin receptor is expressed in the serotonergic raphe nuclei suggests a mechanism by which the two neurotransmitters may cooperatively influence behavior. We hypothesized that oxytocin acts on raphe neurons to influence serotonergically mediated anxiety-like, aggressive and parental care behaviors. We eliminated expression of the oxytocin receptor in raphe neurons by crossing mice expressing Cre recombinase under control of the serotonin transporter promoter (Slc6a4) with our conditional oxytocin receptor knockout line. The knockout mice generated by this cross are normal across a range of behavioral measures: there are no effects for either sex on locomotion in an open-field, olfactory habituation/dishabituation or, surprisingly, anxiety-like behaviors in the elevated O and plus mazes. There was a profound deficit in male aggression: only one of 11 raphe oxytocin receptor knockouts showed any aggressive behavior, compared to 8 of 11 wildtypes. In contrast, female knockouts displayed no deficits in maternal behavior or aggression. Our results show that oxytocin, via its effects on raphe neurons, is a key regulator of resident-intruder aggression in males but not maternal aggression. Furthermore, this reduction in male aggression is quite different from the effects reported previously after forebrain or total elimination of oxytocin receptors. Finally, we conclude that when constitutively eliminated, oxytocin receptors expressed by serotonin cells do not contribute to baseline anxiety-like behaviors or maternal care.

Stimulatory role of the chemokine CCL2 in the migration and peptide expression of embryonic hypothalamic neurons

Neuroinflammation is a feedback mechanism against infection, with recent studies suggesting a neuromodulatory role. The chemokine, (C-C motif) ligand 2 (CCL2), and its receptor, (C-C motif) receptor type 2 (CCR2), affect neuromodulation and migration in response to damage. Although CCL2 co-localizes with neuropeptides in the hypothalamus that control voluntary behavior, the function of CCL2/CCR2 is unknown. This led us to consider the possibility that CCL2 acting through CCR2, under natural conditions, may affect the migration and peptide levels of hypothalamic neurons that control voluntary behavior. This study used primary embryonic hypothalamic neurons to examine the effect of CCL2 on migratory behavior and on levels of the peptides, enkephalin (ENK) and galanin. Treatment with CCL2 led to a significant, dose-dependent increase in the number of migrated neurons and an increase in the velocity and distance traveled. CCL2 also significantly increased the number of ENK-expressing and CCR2/ENK co-expressing neurons and the percentage of neurons that contain higher levels of ENK. Lastly, CCL2 produced a dose-dependent increase in expression of ENK and galanin. These results provide evidence for a stimulatory effect of CCL2 on embryonic hypothalamic neurons involving changes in migratory behavior, expression, and synthesis of neuropeptides that function in controlling behavior. Our results demonstrate that the chemokine, CCL2, functions through its receptor, CCR2, to stimulate the migration and expression of the orexigenic peptides, enkephalin (ENK) and galanin (GAL), in developing embryonic hypothalamic neurons that are important for controlling ingestive behavior. This evidence reveals broad effects of CCL2 in the developing hypothalamus, showing this chemokine system to be tightly linked to the hypothalamic peptide neurons.

Lesions to the CA2 region of the hippocampus impair social memory in mice

The function of the CA2 region of the hippocampus is poorly understood. Although the CA1 and CA3 regions have been extensively studied, for years the CA2 region has primarily been viewed as a linking area between the two. However, the CA2 region is known to have distinct neurochemical and structural features that are different from the other parts of the hippocampus and in recent years it has been suggested that the CA2 region may play a role in the formation and/or recall of olfactory-based memories needed for normal social behavior. Although this hypothesis has been supported by hippocampal lesion studies that have included the CA2 region, no studies have attempted to specifically lesion the CA2 region of the hippocampus in mice to determine the effects on social recognition memory and olfaction. To fill this knowledge gap, we sought to perform excitotoxic N-methyl-D-aspartate lesions of the CA2 region and determine the effects on social recognition memory. We predicted that lesions of the CA2 region would impair social recognition memory. We then went on to test olfaction in CA2-lesioned mice, as social memory requires a functional olfactory system. Consistent with our prediction, we found that CA2-lesioned animals had impaired social recognition. These findings are significant because they confirmed that the CA2 region of the hippocampus is a part of the neural circuitry that regulates social recognition memory, which may have implications for our understanding of the neural regulation of social behavior across species.

Impairments in the initiation of maternal behavior in oxytocin receptor knockout mice

Oxytocin (Oxt) acting through its single receptor subtype, the Oxtr, is important for the coordination of physiology and behavior associated with parturition and maternal care. Knockout mouse models have been helpful in exploring the contributions of Oxt to maternal behavior, including total body Oxt knockout (Oxt −/−) mice, forebrain conditional Oxtr knockout (Oxtr FB/FB) mice, and total body Oxtr knockout (Oxtr −/−) mice. Since Oxtr −/− mice are unable to lactate, maternal behavior has only been examined in virgin females, or in dams within a few hours of parturition, and there have been no studies that have examined their anxiety-like and depression-like behavior following parturition. To improve our understanding of how the absence of Oxt signaling affects maternal behavior, mood and anxiety, we designed a study using Oxtr −/− mice that separated nursing behavior from other aspects of maternal care, such as licking and grooming by thelectomizing (i.e. removing the nipples) of Oxtr +/+ mice and sham-thelectomizing Oxtr −/− mice, and pairing both genotypes with a wet nurse. We then measured pup abandonment, maternal behavior, and postpartum anxiety-like and depression-like behaviors. We hypothesized that genetic disruption of the Oxtr would impact maternal care, mood and anxiety. Specifically, we predicted that Oxtr −/− dams would have impaired maternal care and increased anxiety-like and depression-like behaviors in the postpartum period. We found that Oxtr −/− dams had significantly higher levels of pup abandonment compared to controls, which is consistent with previous work in Oxtr FB/FB mice. Interestingly, Oxtr −/− dams that initiated maternal care did not differ from wildtype controls in measures of maternal behavior. We also did not find any evidence of altered anxiety-like or depressive-like behavior in the postpartum period of Oxtr −/− dams. Thus, our data suggest that Oxt lowers the threshold for the initiation of maternal behavior.

Role of neuropeptides in anxiety, stress, and depression: from animals to humans

Major depression, with its strikingly high prevalence, is the most common cause of disability in communities of Western type, according to data of the World Health Organization. Stress-related mood disorders, besides their deleterious effects on the patient itself, also challenge the healthcare systems with their great social and economic impact. Our knowledge on the neurobiology of these conditions is less than sufficient as exemplified by the high proportion of patients who do not respond to currently available medications targeting monoaminergic systems. The search for new therapeutical strategies became therefore a "hot topic" in neuroscience, and there is a large body of evidence suggesting that brain neuropeptides not only participate is stress physiology, but they may also have clinical relevance. Based on data obtained in animal studies, neuropeptides and their receptors might be targeted by new candidate neuropharmacons with the hope that they will become important and effective tools in the management of stress related mood disorders. In this review, we attempt to summarize the latest evidence obtained using animal models for mood disorders, genetically modified rodent models for anxiety and depression, and we will pay some attention to previously published clinical data on corticotropin releasing factor, urocortin 1, urocortin 2, urocortin 3, arginine-vasopressin, neuropeptide Y, pituitary adenylate-cyclase activating polypeptide, neuropeptide S, oxytocin, substance P and galanin fields of stress research.

Identification of subpopulations of prairie voles differentially susceptible to peer influence to decrease high alcohol intake

Peer influences are critical in the decrease of alcohol (ethanol) abuse and maintenance of abstinence. We previously developed an animal model of inhibitory peer influences on ethanol drinking using prairie voles and here sought to understand whether this influential behavior was due to specific changes in drinking patterns and to variation in a microsatellite sequence in the regulatory region of the vasopressin receptor 1a gene (avpr1a). Adult prairie voles’ drinking patterns were monitored in a lickometer apparatus that recorded each lick a subject exhibited during continuous access to water and 10% ethanol during periods of isolation, pair housing of high and low drinkers, and subsequent isolation. Analysis of fluid consumption confirmed previous results that high drinkers typically decrease ethanol intake when paired with low drinkers, but that a subset of voles do not decrease. Analysis of bout structure revealed differences in the number of ethanol drinking bouts in the subpopulations of high drinkers when paired with low drinkers. Lickometer drinking patterns analyzed by visual and by cross-correlation analyses demonstrated that pair housing did not increase the rate of subjects drinking in bouts occurring at the same time. The length of the avpr1a microsatellite did not predict susceptibility to peer influence or any other drinking behaviors. In summary, subpopulations of high drinkers were identified, by fluid intake and number of drinking bouts, which did or did not lower their ethanol intake when paired with a low drinking peer, and these subpopulations should be explored for testing the efficacy of treatments to decrease ethanol use in groups that are likely to be responsive to different types of therapy.

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

Effect of arginine vasopressin on the behavioral activity in the behavior despair depression rat model

Arginine vasopressin (AVP), a nonapeptide posterior hormone of the pituitary, is mainly synthesized and secreted in the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). Large numbers of studies have reported that AVP plays a role in depression. The present study was to investigate by which level, brain or periphery, AVP affects the behavioral activity in the behavior despair depression rat model. The results showed that (1) either forced swimming or tail suspension significantly increased AVP concentration not only in the brain (PVN, SON, frontal of cortex, hippocampus, amygdala, lumber spinal cord) but also in the periphery (posterior pituitary and serum); (2) intraventricular injection (icv) of AVP decreased the animal immobility time, whereas V₁ receptor antagonist d(CH₂)₅Tyr(Me)AVP (icv) increased the animal immobility time in a dose-dependent manner not only in FST but also in TST, but the V₂ receptor antagonist d(CH₂)₅[D-Ile, Ile, Ala-NH₉]AVP did not change the animal immobility time in FST or TST; (3) V₁, not V₂ receptor antagonist could inhibit the animal immobility time decrease induced by AVP (icv); (4) neither AVP nor its receptor antagonist (including V₁ and V₂ receptor antagonist) influenced the animal immobility time in both FST and TST. The data suggested that AVP in the brain rather than the periphery played a role in the behavior despair depression by V₁, not V₂ receptors, which behavior despair might have a positive feedback effect on central AVP and blood AVP might have a negative feedback on central AVP in the depressive process.

The vasopressin 1b receptor and the neural regulation of social behavior

To date, much of the work in rodents implicating vasopressin (Avp) in the regulation of social behavior has focused on its action via the Avp 1a receptor (Avpr1a). However, there is mounting evidence that the Avp 1b receptor (Avpr1b) also plays a significant role in Avp's modulation of social behavior. The Avpr1b is heavily expressed on the anterior pituitary cortiocotrophs where it acts as an important modulator of the endocrine stress response. In the brain, the Avpr1b is prominent in the CA2 region of the hippocampus, but can also be found in areas such as the paraventricular nucleus of the hypothalamus and the olfactory bulb. Studies that have employed genetic knockouts or pharmacological manipulation of the Avpr1b point to the importance of central Avpr1b in the modulation of social behavior. However, there continues to be a knowledge gap in our understanding of where in the brain this is occurring, as well as how and if the central actions of Avp acting via the Avpr1b interact with the stress axis. In this review we focus on the genetic and pharmacological studies that have implicated the Avpr1b in the neural regulation of social behaviors, including social forms of aggressive behavior, social memory, and social motivation. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Involvement of arginine vasopressin and V1b receptor in alcohol drinking in Sardinian alcohol-preferring rats

BACKGROUND

Recent animal studies have shown that the level of stress-responsive arginine vasopressin (AVP) gene expression in the amygdala is increased during early withdrawal from long-term heroin or cocaine administration. The selective AVP V1b receptor antagonist SSR149415 (capable of exerting antidepressant-like and anxiolytic effects in animal models) also blocked stress-induced reinstatement of drug-seeking behavior. This study was undertaken to investigate the effects of alcohol and to determine whether (i) there are genetically determined differences in basal AVP mRNA levels in the medial/central amygdala (Me/CeA) and medial hypothalamus (MH) between selectively bred Sardinian alcohol-preferring (sP) and alcohol-nonpreferring (sNP) rats; (ii) the AVP mRNA levels are altered by long-term alcohol drinking in sP rats; and (iii) the V1b receptor antagonist SSR149415 alters alcohol drinking in sP rats.

METHODS

In Experiment 1, AVP mRNA levels were measured in the Me/CeA and MH of alcohol-naïve sP and sNP rats, and sP rats exposed to the standard, homecage 2-bottle "alcohol versus water" choice regimen 24 h/d for 17 days. In Experiment 2, SSR149415 (0, 3, 10, or 30 mg/kg; intraperitoneal) was acutely administered 30 minutes before lights off to alcohol-experienced sP rats. Alcohol, water, and food intake were monitored 6 and 24 hours later.

RESULTS

We found higher basal AVP mRNA levels in both Me/CeA and MH of alcohol-naïve sP than sNP rats; alcohol consumption decreased AVP mRNA levels in both brain regions of sP rats, suggesting genetically determined differences between the 2 rat lines and in the effects of alcohol drinking in sP rats. Acute treatment with SSR149415 significantly reduced alcohol intake of sP rats.

CONCLUSION

The stress-responsive AVP/V1b receptor system is 1 component of the neural circuitry underlying high alcohol drinking in sP rats.

Alcohol preference drinking in a mouse line selectively bred for high drinking in the dark

We have selectively bred mice that reach very high blood ethanol concentrations (BECs) after drinking from a single bottle of 20% ethanol. High Drinking in the Dark (HDID-1) mice drink nearly 6g/kg ethanol in 4h and reach average BECs of more than 1.0mg/mL. Previous studies suggest that DID and two-bottle preference for 10% ethanol with continuous access are influenced by many of the same genes. We therefore asked whether HDID-1 mice would differ from the HS/Npt control stock on two-bottle preference drinking. We serially offered mice access to 3-40% ethanol in tap water versus tap water. For ethanol concentrations between 3 and 20%, HDID-1 and HS/Npt controls did not differ in two-bottle preference drinking. At the highest concentrations, the HS/Npt mice drank more than the HDID-1 mice. We also tested the same mice for preference for two concentrations each of quinine, sucrose, and saccharin. Curiously, the mice showed preference ratios (volume of tastant/total fluid drunk) of about 50% for all tastants and concentrations. Thus, neither genotype showed either preference or avoidance for any tastant after high ethanol concentrations. Therefore, we compared naive groups of HDID-1 and HS/Npt mice for tastant preference. Results from this test showed that ethanol-naive mice preferred sweet fluids and avoided quinine but the genotypes did not differ. Finally, we tested HDID-1 and HS mice for an extended period for preference for 15% ethanol versus water during a 2-h access period in the dark. After several weeks, HDID-1 mice consumed significantly more than HS. We conclude that drinking in the dark shows some genetic overlap with other tests of preference drinking, but that the degree of genetic commonality depends on the model used.

The vasopressin Avpr1b receptor: molecular and pharmacological studies

The distribution, pharmacology and function of the arginine vasopressin (Avp) 1b receptor subtype (Avpr1b) has proved more challenging to investigate compared to other members of the Avp receptor family. Avp is increasingly recognised as an important modulator of the hypothalamic-pituitary-adrenal (HPA) axis, an action mediated by the Avpr1b present on anterior pituitary corticotrophs. The Avpr1b is also expressed in some peripheral tissues including pancreas and adrenal, and in the hippocampus (HIP), paraventricular nucleus and olfactory bulb of the rodent brain where its function is unknown. The central distribution of Avpr1bs is far more restricted than that of the Avpr1a, the main Avp receptor subtype found in the brain. Whether Avpr1b expression in rodent tissues is dependent on differences in the length of microsatellite dinucleotide repeats present in the 5' promoter region of the Avpr1b gene remains to be determined. One difficulty of functional studies on the Avpr1b, especially its involvement in the HPA axis response to stress, which prompted the generation of Avpr1b knockout (KO) mouse models, was the shortage of commercially available Avpr1b ligands, particularly antagonists. Research on mice lacking functional Avpr1bs has highlighted behavioural deficits in social memory and aggression. The Avpr1b KO also appears to be an excellent model to study the contribution of the Avpr1b in the HPA axis response to acute and perhaps some chronic (repeated) stressors where corticotrophin-releasing hormone and other genes involved in the HPA axis response to stress do not appear to compensate for the loss of the Avpr1b.

Social dominance in male vasopressin 1b receptor knockout mice

We have previously reported that mice with a targeted disruption of their vasopressin 1b receptor gene, Avpr1b, have mild impairments in social recognition and reduced aggression. The reductions in aggression are limited to social forms of aggression, i.e., maternal and inter-male aggression, while predatory aggression remains unaffected. To further clarify the role of the Avpr1b in the regulation of social behavior we first examined anxiety-like and depression-like behaviors in Avpr1b knockout (Avpr1b -/-) mice. We then went on to test the ability of Avpr1b -/- mice to form dominance hierarchies. No major differences were found between Avpr1b -/- and wildtype mice in anxiety-like behaviors, as measured using an elevated plus maze and an open field test, or depression-like behaviors, as measured using a forced swim test. In the social dominance study we found that Avpr1b -/- mice are able to form dominance hierarchies, though in early hierarchy formation dominant Avpr1b -/- mice display significantly more mounting behavior on Day 1 of testing compared to wildtype controls. Further, non-socially dominant Avpr1b -/- mice spend less time engaged in attack behavior than wildtype controls. These findings suggest that while Avpr1b -/- mice may be able to form dominance hierarchies they appear to employ alternate strategies.

Dopaminergic-neuropeptide interactions in the social brain

A well-mapped set of brain regions is dedicated to social cognition. It is responsive to social cues, engaged in moral decision-making and makes predictions about the likely behaviour of other people. Recent studies of affiliation, using animal models, have revealed that specific neurotransmitters and hormones influence the neural circuits of 'the social brain'. There is converging evidence that the interface between the neuropeptides oxytocin and vasopressin and dopaminergic reward circuits is of particular importance. In the context of recent research, we discuss emerging evidence for the impact of these neuropeptides on the regulation of the social brain. We also examine the putative role of allelic variation in candidate genes on individual differences in social cognitive processing and associated social behaviour.

A conditional knockout mouse line of the oxytocin receptor

Oxytocin plays important roles in reproductive physiology and various behaviors, including maternal behavior and social memory. Its receptor (Oxtr) is present in peripheral tissues and brain, so a conditional knockout (KO, -/-) would be useful to allow elimination of the receptor in specific sites at defined times. We created a line of mice in which loxP sites flank Oxtr coding sequence (floxed) enable Cre recombinase-mediated inactivation of the receptor. We expressed Cre recombinase in these mice either in all tissues (Oxtr(-/-)) or the forebrain (Oxtr(FB/FB)) using the Ca(2+)/calmodulin-dependent protein kinase IIalpha promoter. The latter KO has reduced Oxtr binding beginning 21-28 d postnatally, leading to prominent reductions in the lateral septum, hippocampus, and ventral pallidum. The medial amygdala is spared, and there is significant retention of binding within the olfactory bulb and nucleus and neocortex. We did not observe any deficits in the general health, sensorimotor functions, anxiety-like behaviors, or sucrose intake in either Oxtr(-/-) or Oxtr(FB/FB) mice. Females of both KO types deliver pups, but only the Oxtr(FB/FB) mice are able to eject milk. Oxtr(-/-) males show impaired social memory for familiar females, whereas the Oxtr(FB/FB) males appear to recognize their species but not individuals. Our results confirm the importance of oxytocin in social recognition and demonstrate that spatial and temporal inactivation of the Oxtr will enable finer understanding of the physiological, behavioral, and developmental roles of the receptor.

Alcohol preference in mice lacking the Avpr1a vasopressin receptor

[Arg(8)]-vasopressin (Avp), a nonapeptide hormone, is known to regulate blood pressure, water balance, and a variety of behaviors such as anxiety, aggression, and bonding. Although some evidence that Avp modifies ethanol consumption and some of the effects of ethanol on behavior have been reported, the role of Avp in alcohol consumption and preference is poorly understood. The Avp1a receptor (Avpr1a) is ubiquitously expressed in the central nervous system. To determine the role of Avp signaling on the behavioral effects of alcohol, we examined voluntary ethanol consumption in mice with targeted disruptions of the Avpr1a knockout (Avpr1a KO) gene. Avpr1a KO mice displayed both increased ethanol consumption and preference compared with wild-type (WT) mice. Enhanced ethanol consumption was dramatically and reversibly reduced by treatment with N-methyl-D-aspartic acid antagonists. Basal glutamate release was elevated around the striatum in Avpr1a KO mice. Elevation of extracellular glutamate was also produced in WT mice by local application of an Avpr1a antagonist though a dialysis probe, and this elevation was quickly reversed by stopping the perfusion. These results suggest that Avp can inhibit the release of glutamate from the presynaptic terminal via the Avp1a receptor and that elevation of glutamate levels owing to loss of the inhibitory effect via Avp-Avpr1a signaling may play an important role in the preference for ethanol.

Inactivation of the oxytocin and the vasopressin (Avp) 1b receptor genes, but not the Avp 1a receptor gene, differentially impairs the Bruce effect in laboratory mice (Mus musculus)

The Bruce effect is a pheromonally mediated process whereby exposure to chemosensory cues from an unfamiliar male terminates pregnancy in a recently mated female. Pharmacological and genetic evidence implicates both oxytocin (Oxt) and vasopressin (Avp) in the regulation of social memory in males, but less work has been done in females. We tested the extent to which the Avp receptors (Avprs) 1a and 1b and Oxt are essential for the Bruce effect, a phenomenon that relies on olfactory memory. Adult female mice were paired with stimulus males and monitored for the presence of sperm plugs. Wild-type, heterozygous, and homozygous knockout (KO) females for either the Avpr1a, Avpr1b, or Oxt genes were randomly assigned to one of the following treatment groups: 1) alone (mate removed, no second exposure to another animal); 2) paired continuously (mate kept with female for 10-14 d); 3) familiar male (mate removed, reintroduced 24 h later); or 4) unfamiliar male (mate removed, BalbC male introduced 24 h later). Regardless of genotype, 90-100% of females in the alone or paired continuously groups became pregnant. The Oxt KO females terminated their pregnancies regardless of whether their original mate or an unfamiliar male was reintroduced. The Avpr1b KO mice failed to terminate pregnancy in the presence of an unfamiliar male. The Avpr1a KO mice exhibited a normal Bruce effect. These data demonstrate that both Oxt and the Avpr1b are critical for the normal expression of the Bruce effect but have different effects on the interpretation of social cues.

The role of hypothalamic peptide gene expression in alcohol self-administration behavior

Self-administration of ethanol and food share many common features and Richter hypothesized that an increase in ethanol consumption would decrease feeding to balance the excess calories contained in the ethanol. Previously, we have shown that individual alcohol consumption correlates with neurotransmitter gene expression, especially in the prefrontal cortex. To test the hypothesis of Richter, we measured hypothalamic gene expression of receptors or neuropeptides of known relevance for the regulation of food intake using qPCR and correlated this to individual ethanol consumption in Wistar rats. For validation, gene expression was first correlated with body weight. We found a correlation of dynorphin, somatostatin, melanocortin-4 receptor and serotonin 5-HT(2C) with body weight and trends to correlation for CART, thus confirming the established role of the hypothalamus in the regulation of weight. For ethanol consumption, correlations were found for CRH receptors 1 and 2 and vasopressin while strong trends were observed for galanin receptor 1, orexin receptor 1, MCH and adrenoceptor alpha(1B). Therefore, alcohol consumption does seem to involve several hypothalamic systems which also mediate feeding responses and suggests that the hypothalamus, together with the prefrontal cortex, may determine the 'stopping point' of an individual.

Vasopressin: behavioral roles of an "original" neuropeptide

Vasopressin (Avp) is mainly synthesized in the magnocellular cells of the hypothalamic supraoptic (SON) and paraventricular nuclei (PVN) whose axons project to the posterior pituitary. Avp is then released into the blood stream upon appropriate stimulation (e.g., hemorrhage or dehydration) to act at the kidneys and blood vessels. The brain also contains several populations of smaller, parvocellular neurons whose projections remain within the brain. These populations are located within the PVN, bed nucleus of the stria terminalis (BNST), medial amygdala (MeA) and suprachiasmatic nucleus (SCN). Since the 1950s, research examining the roles of Avp in the brain and periphery has intensified. The development of specific agonists and antagonists for Avp receptors has allowed for a better elucidation of its contributions to physiology and behavior. Anatomical, pharmacological and transgenic, including "knockout," animal studies have implicated Avp in the regulation of various social behaviors across species. Avp plays a prominent role in the regulation of aggression, generally of facilitating or promoting it. Affiliation and certain aspects of pair-bonding are also influenced by Avp. Memory, one of the first brain functions of Avp that was investigated, has been implicated especially strongly in social recognition. The roles of Avp in stress, anxiety, and depressive states are areas of active exploration. In this review, we concentrate on the scientific progress that has been made in understanding the role of Avp in regulating these and other behaviors across species. We also discuss the implications for human behavior.

Attenuated stress response to acute lipopolysaccharide challenge and ethanol administration in vasopressin V1b receptor knockout mice

The arginine vasopressin (Avp) 1b receptor (Avpr1b) present on anterior pituitary corticotrophs is involved in the stimulation of adrenocorticotrophic hormone (ACTH) secretion, especially during times of stress. Corticotrophin-releasing hormone (CRH) is considered the major ACTH secretagogue during acute stress whereas Avp appears to be the more dominant mediator of the hypothalamic-pituitary-adrenal (HPA) axis response during chronic stress situations. To investigate the role of the Avpr1b in the HPA axis response to acute stress, we measured ACTH and corticosterone (CORT) plasma levels in Avpr1b knockout (KO) mice and wild-type controls in response to bacterial lipopolysaccharide (LPS) challenge and ethanol (EtOH) administration. Mice deficient in Avpr1b had markedly compromised plasma ACTH and CORT responses to acute (30 min) LPS, but normal ACTH and CORT response to more extended exposure (4 h) to the immune system activator. The plasma ACTH and CORT levels stimulated by intoxicating, sedative doses of EtOH (3.2 and 4 g/kg) were significantly decreased in the Avpr1b KO mice compared to wild-type littermates. Significantly higher EtOH-induced plasma ACTH and CORT secretion was measured in female than in male Avpr1b wild-type mice. There were no differences in the blood alcohol levels following acute EtOH administration in Avpr1b KO or wild-type mice of either gender. Our results clearly suggest that Avpr1b plays a significant role in the HPA axis response to acute immune stress and EtOH intoxication.