Sexual experience alters D1 receptor-mediated cyclic AMP production in the nucleus accumbens of female Syrian hamsters

The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents

BORLAND, J.M., The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents, NEUROSCI BIOBEH REV 21(1) XXX-XXX, 2024.-Sociality shapes an organisms' life. The nucleus accumbens is a critical brain region for mental health. In the following review, the effects of different types of social interactions on the physiology of neurons in the nucleus accumbens is synthesized. More specifically, the effects of sex behavior, aggression, social defeat, pair-bonding, play behavior, affiliative interactions, parental behaviors, the isolation from social interactions and maternal separation on measures of excitatory synaptic transmission, intracellular signaling and factors of transcription and translation in neurons in the nucleus accumbens in rodent models are reviewed. Similarities and differences in effects depending on the type of social interaction is then discussed. This review improves the understanding of the molecular and synaptic mechanisms of sociality.

Mechanisms, development, and comparative perspectives on experience-dependent plasticity in social behavior

Revealing the mechanisms underlying experience-dependent plasticity is a hallmark of behavioral neuroscience. While the study of social behavior has focused primarily on the neuroendocrine and neural control of social behaviors, the plasticity of these innate behaviors has received relatively less attention. Here, we review studies on mating-dependent changes to social behavior and neural circuitry across mammals, birds, and reptiles. We provide an overview of species similarities and differences in the effects of mating experiences on motivational and performative aspects of sexual behaviors, on sensory processing and preferences, and on the experience-dependent consolidation of sexual behavior. We also discuss recent insights into the neural mechanisms of and developmental influences on mating-dependent changes and outline promising approaches to investigate evolutionary parallels and divergences in experience-dependent plasticity.

Sexual Behavior and Synaptic Plasticity

Although sex drive is present in many animal species, sexual behavior is not static and, like many other behaviors, can be modified by experience. This modification relies on synaptic plasticity, a sophisticated mechanism through which neurons change how they process a given stimulus, and the neurophysiological basis of learning. This review addresses the main plastic effects of steroid sex hormones in the central nervous system (CNS) and the effects of sexual experience on the CNS, including effects on neurogenesis, intracellular signaling, gene expression, and changes in dendritic spines, as well as behavioral changes.

Integrating Neural Circuits Controlling Female Sexual Behavior

The hypothalamus is most often associated with innate behaviors such as is hunger, thirst and sex. While the expression of these behaviors important for survival of the individual or the species is nested within the hypothalamus, the desire (i.e., motivation) for them is centered within the mesolimbic reward circuitry. In this review, we will use female sexual behavior as a model to examine the interaction of these circuits. We will examine the evidence for a hypothalamic circuit that regulates consummatory aspects of reproductive behavior, i.e., lordosis behavior, a measure of sexual receptivity that involves estradiol membrane-initiated signaling in the arcuate nucleus (ARH), activating β-endorphin projections to the medial preoptic nucleus (MPN), which in turn modulate ventromedial hypothalamic nucleus (VMH) activity-the common output from the hypothalamus. Estradiol modulates not only a series of neuropeptides, transmitters and receptors but induces dendritic spines that are for estrogenic induction of lordosis behavior. Simultaneously, in the nucleus accumbens of the mesolimbic system, the mating experience produces long term changes in dopamine signaling and structure. Sexual experience sensitizes the response of nucleus accumbens neurons to dopamine signaling through the induction of a long lasting early immediate gene. While estrogen alone increases spines in the ARH, sexual experience increases dendritic spine density in the nucleus accumbens. These two circuits appear to converge onto the medial preoptic area where there is a reciprocal influence of motivational circuits on consummatory behavior and vice versa. While it has not been formally demonstrated in the human, such circuitry is generally highly conserved and thus, understanding the anatomy, neurochemistry and physiology can provide useful insight into the motivation for sexual behavior and other innate behaviors in humans.

Cell-type specific increases in female hamster nucleus accumbens spine density following female sexual experience

Female sexual behavior is an established model of a naturally motivated behavior which is regulated by activity within the mesolimbic dopamine system. Repeated activation of the mesolimbic circuit by female sexual behavior elevates dopamine release and produces persistent postsynaptic alterations to dopamine D1 receptor signaling within the nucleus accumbens. Here we demonstrate that sexual experience in female Syrian hamsters significantly increases spine density and alters morphology selectively in D1 receptor-expressing medium spiny neurons within the nucleus accumbens core, with no corresponding change in dopamine receptor binding or protein expression. Our findings demonstrate that previous life experience with a naturally motivated behavior has the capacity to induce persistent structural alterations to the mesolimbic circuit that can increase reproductive success and are analogous to the persistent structural changes following repeated exposure to many drugs of abuse.

ΔJunD overexpression in the nucleus accumbens prevents sexual reward in female Syrian hamsters

Motivated behaviors, including sexual experience, activate the mesolimbic dopamine system and produce long-lasting molecular and structural changes in the nucleus accumbens. The transcription factor ΔFosB is hypothesized to partly mediate this experience-dependent plasticity. Previous research in our laboratory has demonstrated that overexpressing ΔFosB in the nucleus accumbens of female Syrian hamsters augments the ability of sexual experience to cause the formation of a conditioned place preference. It is unknown, however, whether ΔFosB-mediated transcription in the nucleus accumbens is required for the behavioral consequences of sexual reward. We therefore used an adeno-associated virus to overexpress ΔJunD, a dominant negative binding partner of ΔFosB that decreases ΔFosB-mediated transcription by competitively heterodimerizing with ΔFosB before binding at promotor regions on target genes, in the nucleus accumbens. We found that overexpression of ΔJunD prevented the formation of a conditioned place preference following repeated sexual experiences. These data, when coupled with our previous findings, suggest that ΔFosB is both necessary and sufficient for behavioral plasticity following sexual experience. Furthermore, these results contribute to an important and growing body of literature demonstrating the necessity of endogenous ΔFosB expression in the nucleus accumbens for adaptive responding to naturally rewarding stimuli.

Vesicular glutamate transporter 2 and tyrosine hydroxylase are not co-localized in Syrian hamster nucleus accumbens afferents

The nucleus accumbens (NAc) is an important brain region for motivation, reinforcement, and reward. Afferents to the NAc can be divided into two anatomically segregated neurochemical phenotypes: dopaminergic inputs, primarily from the midbrain ventral tegmental area (VTA) and glutamatergic inputs from several cortical and sub-cortical structures. A population of glutamatergic neurons exists within the VTA and evidence from rats and mice suggests that these VTA axons may co-release dopamine and glutamate into the NAc. Our laboratory has used sexual experience in Syrian hamsters as a model of experience-dependent plasticity within the NAc. Given that both dopamine and glutamate are involved in this plasticity, it is important to determine whether these neurotransmitters are co-expressed within the mesolimbic pathway of hamsters. We therefore used immunofluorescent staining to investigate the possible co-localization of tyrosine hydroxylase (TH), a dopaminergic marker, and vesicular glutamate transporter 2 (VGLUT2), a glutamatergic marker, within the mesolimbic pathway. PCR analyses identified VGLUT2 gene expression in the VTA. No co-localization of TH and VGLUT2 protein was detected in NAc fibers, nor was there a difference in immunolabeling between males and females. Further studies are needed to resolve this absence of anatomical co-localization of TH and VGLUT2 in hamster striatal afferents with reports of functional co-release in other rodents.

Aggressive experience increases dendritic spine density within the nucleus accumbens core in female Syrian hamsters

Activity within the mesolimbic dopamine system is associated with the performance of naturally motivated behaviors, one of which is aggression. In male rats, aggressive behavior induces neurochemical changes within the nucleus accumbens, a key structure within the mesolimbic dopamine system. Corresponding studies have not been done in females. Female Syrian hamsters live as isolates and when not sexually responsive are aggressive toward either male or female intruders, making them an excellent model for studying aggression in females. We took advantage of this naturally expressed behavior to examine the effects of repeated aggressive experience on the morphology of medium spiny neurons in the nucleus accumbens and caudate nucleus, utilizing a DiOlistic labeling approach. We found that repeated aggressive experience significantly increased spine density within the nucleus accumbens core, with no significant changes in any other brain region examined. At the same time, significant changes in spine morphology were observed in all brain regions following repeated aggressive experience. These data are significant in that they demonstrate that repeated exposure to behaviors that form part of an animal's life history will alter neuronal structure in a way that may shift neurobiological responses to impact future social interactions.

The complex of G protein regulator RGS9-2 and Gβ(5) controls sensitization and signaling kinetics of type 5 adenylyl cyclase in the striatum

Multiple neurotransmitter systems in the striatum converge to regulate the excitability of striatal neurons by activating several heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) that signal to the type 5 adenylyl cyclase (AC5), the key effector enzyme that produces the intracellular second messenger cyclic adenosine monophosphate (cAMP). Plasticity of cAMP signaling in the striatum is thought to play an essential role in the development of drug addiction. We showed that the complex of the ninth regulator of G protein signaling (RGS9-2) with the G protein β subunit (Gβ(5)) critically controlled signaling from dopamine and opioid GPCRs to AC5 in the striatum. RGS9-2/Gβ(5) directly interacted with and suppressed the basal activity of AC5. In addition, the RGS9-2/Gβ(5) complex attenuated the stimulatory action of Gβγ on AC5 by facilitating the GTPase (guanosine triphosphatase) activity of Gα(o), thus promoting the formation of the inactive heterotrimer and inhibiting Gβγ. Furthermore, by increasing the deactivation rate of Gα(i), RGS9-2/Gβ(5) facilitated the recovery of AC5 from inhibition. Mice lacking RGS9 showed increased cAMP production and, upon withdrawal from opioid administration, enhanced sensitization of AC5. Our findings establish RGS9-2/Gβ(5) complexes as regulators of three key aspects of cAMP signaling: basal activity, sensitization, and temporal kinetics of AC5, thus highlighting the role of this complex in regulating both inhibitory and stimulatory GPCRs that shape cAMP signaling in the striatum.

Neural mechanisms of reproduction in females as a predisposing factor for drug addiction

There is an increasing awareness that adolescent females differ from males in their response to drugs of abuse and consequently in their vulnerability to addiction. One possible component of this vulnerability to drug addiction is the neurobiological impact that reproductive physiology and behaviors have on the mesolimbic dopamine system, a key neural pathway mediating drug addiction. In this review, we examine animal models that address the impact of ovarian cyclicity, sexual affiliation, sexual behavior, and maternal care on the long-term plasticity of the mesolimbic dopamine system. The thesis is that this plasticity in synaptic neurotransmission stemming from an individual's normal life history contributes to the pathological impact of drugs of abuse on the neurobiology of this system. Hormones released during reproductive cycles have only transient effects on these dopamine systems, whereas reproductive behaviors produce a persistent sensitization of dopamine release and post-synaptic neuronal responsiveness. Puberty itself may not represent a neurobiological risk factor for drug abuse, but attendant behavioral experiences may have a negative impact on females engaging in drug use.

Delta FosB overexpression in the nucleus accumbens enhances sexual reward in female Syrian hamsters

Repeated activation of the mesolimbic dopamine system results in persistent behavioral alterations accompanied by a pattern of neural plasticity in the nucleus accumbens (NAc). As the accumulation of the transcription factor Delta FosB may be an important component of this plasticity, the question addressed in our research is whether Delta FosB is regulated by sexual experience in females. We have shown that female Syrian hamsters, given sexual experience, exhibit several behavioral alterations including increased sexual efficiency with naïve male hamsters, sexual reward and enhanced responsiveness to psychomotor stimulants (e.g. amphetamine). We recently demonstrated that sexual experience increased the levels of Delta FosB in the NAc of female Syrian hamsters. The focus of this study was to explore the functional consequences of this induction by determining if the constitutive overexpression of Delta FosB by adeno-associated virus (AAV) vectors in the NAc could mimic the behavioral effects of sexual experience. Animals with AAV-mediated overexpression of Delta FosB in the NAc showed evidence of sexual reward in a conditioned place preference paradigm under conditions in which control animals receiving an injection of AAV-green fluorescent protein (GFP) into the NAc did not. Sexual behavior tests further showed that males paired with the AAV-Delta FosB females had increased copulatory efficiency as measured by the proportion of mounts that included intromission compared to males mated with the AAV-GFP females. These results support a role for Delta FosB in mediating natural motivated behaviors, in this case female sexual behavior, and provide new insight into the possible endogenous actions of Delta FosB.

Dopamine and monogamy

Social attachments play a central role in human society. In fact, such attachments are so important that deficits in the ability to form meaningful social bonds are associated with a variety of psychological disorders. Although mother-infant bonding has been studied for many years, we only recently have begun to examine the processes that underlie social bonds between adults. Over the past decade, central dopamine has become a focus of such research, especially its role in pair bonding between mates in species that display monogamous life strategies. Neuroanatomical and pharmacological studies in rodents have firmly established central dopamine systems, especially the mesocorticolimbic dopamine circuitry, in the formation, expression, and maintenance of monogamous pair bonds. As this research has progressed, it has become apparent that there is considerable overlap between the processes that underlie pair bonding and those that mediate responses to abused substances. This suggests that social bonding and substance abuse each may affect the other. Herein we review the current state of knowledge of central dopamine involvement in pair bond formation, expression, and maintenance. We first describe the neuroanatomical substrate within which dopamine exerts its effects on social bonding. We then describe dopamine receptor subtype-specific influences on pair bonding and how dopamine receptor activation may interact with activation of other neurochemical systems. Finally, we describe possible interactions between social bonding and substance abuse.

Sexual experience in female rodents: cellular mechanisms and functional consequences

The neurobiology of female sexual behavior has largely focused on mechanisms of hormone action on nerve cells and how these effects translate into the display of copulatory motor patterns. Of equal importance, though less studied, are some of the consequences of engaging in sexual behavior, including the rewarding properties of sexual interactions and how sexual experience alters copulatory efficiency. This review summarizes the effects of sexual experience on reward processes and copulation in female Syrian hamsters. Neural correlates of these sexual interactions include long-term cellular changes in dopamine transmission and postsynaptic signaling pathways related to neuronal plasticity (e.g., dendritic spine formation). Taken together, these studies suggest that sexual experience enhances the reinforcing properties of sexual behavior, which has the coincident outcome of increasing copulatory efficiency in a way that can increase reproductive success.

Repeated quinpirole treatments produce neurochemical sensitization and associated behavioral changes in female hamsters

RATIONALE

Repeated stimulation of dopaminergic pathways with dopamine receptor agonists can produce both neurochemical and behavioral sensitization.

OBJECTIVES

The present study was designed to examine whether repeated treatment with the D2-like dopamine receptor agonist, quinpirole, would produce neurochemical sensitization of D1 dopamine receptor-mediated processes and associated behavioral changes in female hamsters in a manner analogous to that previously used to sensitize heterologous dopamine signaling pathways in derived cell lines.

MATERIALS AND METHODS

Female hamsters received two injections of quinpirole (1.5 mg/kg) or saline each week for 7 weeks, during which time pouching behavior and body weight were monitored. Over the next 2 weeks, hamsters were tested for differences in prepulse inhibition of the acoustic startle response (PPI) and sexual behavior. Adenylate cyclase activation assays were then performed on dissected tissue from the nucleus accumbens and caudate-putamen.

RESULTS

Repeated treatment with quinpirole increased pouching behavior and body weight and disrupted PPI. No changes in sexual activity in response to repeated quinpirole were found. Prior quinpirole treatment enhanced D1 dopamine receptor-stimulated adenylate cyclase activity in the caudate-putamen that was blocked by co-incubation with the D1 dopamine antagonist, SCH23390.

CONCLUSIONS

These results show that repeated activation of D2-like receptors in vivo can produce changes in feeding behavior and sensory processing that is associated with sensitization of D1 dopamine receptor-mediated signaling in the caudate-putamen.

Sensitization of adenylate cyclase by Galpha i/o-coupled receptors

Activation of receptors coupled to inhibitory G proteins (Galpha i/o) has opposing consequences for cyclic AMP accumulation and the activity of cyclic AMP-dependent protein kinase, depending on the duration of stimulation. Acute activation inhibits the activity of adenylate cyclase, thereby attenuating cyclic AMP accumulation; in contrast, persistent activation of Galpha i/o-coupled receptors produces a paradoxical enhancement of adenylate cyclase activity, thus increasing cyclic AMP accumulation when the action of the inhibitory receptor is terminated. This heterologous sensitization of cyclic AMP signaling, also called superactivation or supersensitization, likely represents a cellular adaptive response, a mechanism by which the cell compensates for chronic inhibitory input. Recent advances in our knowledge of G protein-mediated signaling, regulation of adenylate cyclase, and other cellular signaling mechanisms have extensively increased our insight into the mechanisms and significance of this phenomenon. In particular, recent evidence points to the Galpha(s)-adenylate cyclase interface as a locus for the expression of the sensitized adenylate cyclase response, and to isoform-specific phosphorylation of adenylate cyclase as one mechanism that can produce sensitization. Galpha i/o-coupled receptor-induced heterologous sensitization may contribute to enhanced Galpha(s)-coupled receptor signaling following neurotransmitter elevations induced by the administration of drugs of abuse and during other types of neuronal function or dysfunction. This review will focus on recent advances in our understanding of signaling pathways that are involved in sensitization and describe the potential role of sensitization in neuronal function.