Ventral Tegmental Area Dopamine Cell Activation during Male Rat Sexual Behavior Regulates Neuroplasticity and d-Amphetamine Cross-Sensitization following Sex Abstinence

Reciprocal effects of single or repeated exposure to methylphenidate or sex in adult male rats

RATIONALE

Exposure to rewards can alter behavioral reactivity to them. For example, stimulants sensitize locomotor activation, whereas sexual experience sensitizes copulatory behaviors. Moreover, rewards can cross-sensitize one another. Although stimulants are known to cross-sensitize locomotor effects, the evidence for cross-sensitization between stimulants and sex is less clear.

OBJECTIVES

This study determined the effects of single and repeated pre-exposure to methylphenidate (MPH) or sex on one another in adult male rats.

METHODS

Cross-sensitization between MPH (5 mg/kg) and sex (30 min with sexually experienced female) was examined. Adult male rats were pre-exposed to 0, 1, or 10 trials of either sex or MPH before being exposed to the other reward. Locomotor chambers were used in MPH trials. Bilevel chambers were used in sexual trials, and sexual behaviors were video scored.

RESULTS

The amount of prior sexual experience differentially influenced the ceiling of MPH-dependent sensitization; in the last drug trial, locomotion was highest in males given 1 previous sexual trial compared with 0 or 10. Compared with MPH-naive males, pre-exposure to MPH (1 and 10 trials) reduced the number of ejaculations without impacting sexual performance (intromission/mount latency and frequency).

CONCLUSIONS

These findings indicate that the degree of pre-exposure to a reward can differentially affect reactivity to novel rewards. The results showed that previous findings of cross-sensitization between amphetamine and sex do not extend to MPH. However, exposure to MPH prior to sexual experience can increase the amount of sexual stimulation needed to achieve ejaculation.

The Protective Effect of Social Reward on Opioid and Psychostimulant Reward and Relapse: Behavior, Pharmacology, and Brain Regions

Until recently, most modern neuroscience research on addiction using animal models did not incorporate manipulations of social factors. Social factors play a critical role in human addiction: social isolation and exclusion can promote drug use and relapse, while social connections and inclusion tend to be protective. Here, we discuss the state of the literature on social factors in animal models of opioid and psychostimulant preference, self-administration, and relapse. We first summarize results from rodent studies on behavioral, pharmacological, and circuit mechanisms of the protective effect of traditional experimenter-controlled social interaction procedures on opioid and psychostimulant conditioned place preference, self-administration, and relapse. Next, we summarize behavioral and brain-mechanism results from studies using newer operant social-interaction procedures that inhibit opioid and psychostimulant self-administration and relapse. We conclude by discussing how the reviewed studies point to future directions for the addiction field and other neuroscience and psychiatric fields, and their implications for mechanistic understanding of addiction and development of new treatments.SIGNIFICANCE STATEMENT In this review, we propose that incorporating social factors into modern neuroscience research on addiction could improve mechanistic accounts of addiction and help close gaps in translating discovery to treatment. We first summarize rodent studies on behavioral, pharmacological, and circuit mechanisms of the protective effect of both traditional experimenter-controlled and newer operant social-interaction procedures. We then discuss potential future directions and clinical implications.

Hearing, touching, and multisensory integration during mate choice

Mate choice is a potent generator of diversity and a fundamental pillar for sexual selection and evolution. Mate choice is a multistage affair, where complex sensory information and elaborate actions are used to identify, scrutinize, and evaluate potential mating partners. While widely accepted that communication during mate assessment relies on multimodal cues, most studies investigating the mechanisms controlling this fundamental behavior have restricted their focus to the dominant sensory modality used by the species under examination, such as vision in humans and smell in rodents. However, despite their undeniable importance for the initial recognition, attraction, and approach towards a potential mate, other modalities gain relevance as the interaction progresses, amongst which are touch and audition. In this review, we will: (1) focus on recent findings of how touch and audition can contribute to the evaluation and choice of mating partners, and (2) outline our current knowledge regarding the neuronal circuits processing touch and audition (amongst others) in the context of mate choice and ask (3) how these neural circuits are connected to areas that have been studied in the light of multisensory integration.

Neuroplastic changes in c-Fos, ΔFosB, BDNF, trkB, and Arc expression in the hippocampus of male Roman rats: differential effects of sexual activity

Sexual activity causes differential changes in the expression of markers of neural activation (c‐Fos and ΔFosB) and neural plasticity (Arc and BDNF/trkB), as determined either by Western Blot (BDNF, trkB, Arc, and ΔFosB) or immunohistochemistry (BDNF, trkB, Arc, and c‐Fos), in the hippocampus of male Roman high (RHA) and low avoidance (RLA) rats, two psychogenetically selected rat lines that display marked differences in sexual behavior (RHA rats exhibit higher sexual motivation and better copulatory performance than RLA rats). Both methods showed (with some differences) that sexual activity modifies the expression levels of these markers in the hippocampus of Roman rats depending on: (i) the level of sexual experience, that is, changes were usually more evident in sexually naïve than in experienced rats; (ii) the hippocampal partition, that is, BDNF and Arc increased in the dorsal but tended to decrease in the ventral hippocampus; (iii) the marker considered, that is, in sexually experienced animals BDNF, c‐Fos, and Arc levels were similar to those of controls, while ΔFosB levels increased; and (iv) the rat line, that is, changes were usually larger in RHA than RLA rats. These findings resemble those of early studies in RHA and RLA rats showing that sexual activity influences the expression of these markers in the nucleus accumbens, medial prefrontal cortex, and ventral tegmental area, and show for the first time that also in the hippocampus sexual activity induces neural activation and plasticity, events that occur mainly during the first phase of the acquisition of sexual experience and depend on the genotypic/phenotypic characteristics of the animals.

Calcium-binding proteins typify the dopaminergic neuronal subtypes in the ventral tegmental area of zebra finch, Taeniopygia guttata

Calcium-binding proteins (CBPs) regulate neuronal function in midbrain dopamine (DA)-ergic neurons in mammals by buffering and sensing the intracellular Ca²⁺ , and vesicular release. In birds, the equivalent set of neurons are important in song learning, directed singing, courtship, and energy balance, yet the status of CBPs in these neurons is unknown. Herein, for the first time, we probe the nature of CBPs, namely, Calbindin-, Calretinin-, Parvalbumin-, and Secretagogin-expressing DA neurons in the ventral tegmental area (VTA) and substantia nigra (SN) in the midbrain of zebra finch, Taeniopygia guttata. qRT-PCR analysis of ventral midbrain tissue fragment revealed higher Calbindin- and Calretinin-mRNA levels compared to Parvalbumin and Secretagogin. Application of immunofluorescence showed CBP-immunoreactive (-i) neurons in VTA (anterior [VTAa], mid [VTAm], caudal [VTAc]), SN (compacta [SNc], and reticulata [SNr]). Compared to VTAa, higher Calbindin- and Parvalbumin-immunoreactivity (-ir), and lower Calretinin-ir were observed in VTAm and VTAc. Secretagogin-ir was highly localized to VTAa. In SN, Calbindin- and Calretinin-ir were higher in SNc, SNr was Parvalbumin enriched, and Secretagogin-ir was not detected. Weak, moderate, and intense tyrosine hydroxylase (TH)-i VTA neurons were demarcated as subtypes 1, 2, and 3, respectively. While subtype 1 TH-i neurons were neither Calbindin- nor Calretinin-i, ∼80 and ∼65% subtype 2 and ∼30 and ∼45% subtype 3 TH-i neurons co-expressed Calbindin and Calretinin, respectively. All TH-i neuronal subtypes co-expressed Parvalbumin with reciprocal relationship with TH-ir. We suggest that the CBPs may determine VTA DA neuronal heterogeneity and differentially regulate their activity in T. guttata.

Phasic firing of dopaminergic neurons in the ventral tegmental area triggers peripheral immune responses

Dopaminergic neurons in the ventral tegmental area (VTA) play a crucial role in the processing of reward-related information. Recent studies with pharmacological manipulations of VTA neuronal activity demonstrated a VTA-induced immunoenhancement in peripheral organs. Here, to examine the detailed physiological dynamics, we took an optogenetic approach in which VTA dopaminergic neurons were selectively activated with millisecond precision. Optogenetic phasic, rather than tonic, stimulation of VTA dopaminergic neurons increased serum cytokine levels, such as IL-2, IL-4 and TNF-α. These results provide direct evidence to link dopaminergic neuronal phasic firing to peripheral immunity. Next, we tested whether cytokine induction in male mice was boosted by female encounters, a natural condition that induces increased active VTA neurons and gamma power. Female encounters increased serum IL-2 levels, which were abolished by pharmacological inhibition of VTA neuronal activity. Taken together, our results highlight the importance of the brain reward system in the treatment and management of immune-related disorders.

Functional Connectivity of Nucleus Accumbens Is Associated with Lifelong Premature Ejaculation in Male Adults : A Resting-state fMRI Study

PURPOSE

Ejaculation represents a crucial component of sexual behavior in men, which is involved in reward functions of certain brain areas including the nucleus accumbens (NAcc). Lifelong premature ejaculation (PE) is one of the most prevalent sexual dysfunctions in men. It is suggested to be related to abnormal brain function. This study aimed to explore changes of the functional connectivity patterns of NAcc and possible correlations of the neuroimaging abnormalities with clinical features in lifelong PE patients.

METHODS

The sample consisted with 42 lifelong PE patients and 30 healthy controls. All participants underwent functional magnetic resonance imaging scans and clinical symptoms. The functional connectivity (FC) approach was applied to investigate differences of NAcc-seed intrinsic connectivity between two groups and correlation analysis was used to access possible relationships between the imaging findings and clinical features, such as premature ejaculation diagnostic tool (PEDT) or intravaginal ejaculatory latency time (IELT).

RESULTS

Results showed that lifelong PE patients had decreased FC between the NAcc and thalamus, superior temporal pole, superior temporal cortex (STC), inferior frontal gyrus (IFG), orbitofrontal cortex, caudate and putamen. A significantly negative correlation between the PEDT score and NAcc-STC connectivity (r = -0.46) was found in lifelong PE patients, while IELT score positively correlated with the NAcc-IFG connectivity (r = 0.48) and NAcc-thalamus connectivity (r = 0.46).

CONCLUSION

The findings may facilitate a more sophisticated understanding of neural mechanisms of lifelong PE, particularly associated with the NAcc-related intrinsic connectivity during the resting state.

NMDA Receptors in Accumbal D1 Neurons Influence Chronic Sugar Consumption and Relapse

Glutamatergic input via NMDA and AMPA receptors within the mesolimbic dopamine (DA) pathway plays a critical role in the development of addictive behavior and relapse toward drugs of abuse. Although well-established for drugs of abuse, it is not clear whether glutamate receptors within the mesolimbic system are involved in mediating chronic consumption and relapse following abstinence from a non-drug reward. Here, we evaluated the contribution of mesolimbic glutamate receptors in mediating chronic sugar consumption and the sugar-deprivation effect (SDE), which is used as a measure of relapse-like behavior following abstinence. We studied four inducible mutant mouse lines lacking the GluA1 or GluN1 subunit in either DA transporter (DAT) or D1R-expressing neurons in an automated monitoring system for free-choice sugar drinking in the home cage. Mice lacking either GluA1 or GluN1 in D1R-expressing neurons (GluA1^D1CreERT2 or GluN1^D1CreERT2mice) have altered sugar consumption in both sexes, whereas GluA1^DATCreERT2 and GluN1^DATCreERT2do not differ from their respective littermate controls. In terms of relapse-like behavior, female GluN1^D1CreERT2mice show a more pronounced SDE. Given that glutamate receptors within the mesolimbic system play a critical role in mediating relapse behavior of alcohol and other drugs of abuse, it is surprising that these receptors do not mediate the SDE, or in the case of female GluN1^D1CreERT2 mice, show an opposing effect. We conclude that a relapse-like phenotype of sugar consumption differs from that of drugs of abuse on the molecular level, at least with respect to the contribution of mesolimbic glutamate receptors.

Different baseline physical activity predicts susceptibility and resilience to chronic social defeat stress in mice: Involvement of dopamine neurons

Physical inactivity, the fourth leading mortality risk factor worldwide, is associated with chronic mental illness. Identifying the mechanisms underlying different levels of baseline physical activity and the effects of these levels on the susceptibility to stress is very important. However, whether different levels of baseline physical activity influence the susceptibility and resilience to chronic social defeat stress (CSDS), and the underlying mechanisms in the brain remain unclear. The present study segregated wild-type mice into low baseline physical activity (LBPA) and high baseline physical activity (HBPA) groups based on short term voluntary wheel running (VWR). LBPA mice showed obvious susceptibility to CSDS, while HBPA mice were resilient to CSDS. In addition, the expression of tyrosine hydroxylase (TH) in the ventral tegmental area (VTA) was lower in LBPA mice than in HBPA mice. Furthermore, activation of TH neurons in the VTA of LBPA mice by chemogenetic methods increased the levels of VWR and resilience to CSDS. In contrast, inhibiting TH neurons in the VTA of HBPA mice lowered the levels of VWR and increased their susceptibility to CSDS. Thus, this study suggests that different baseline physical activities might be mediated by the dopamine system. This system also affects the susceptibility and resilience to CSDS, possibly via alteration of the baseline physical activity. This perspective on the neural control and impacts on VWR may aid the development of strategies to motivate and sustain voluntary physical activity. Furthermore, this can maximize the impacts of regular physical activity toward stress-reduction and health promotion.

Compound 21, a two-edged sword with both DREADD-selective and off-target outcomes in rats

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) represent a technical revolution in integrative neuroscience. However, the first used ligands exhibited dose-dependent selectivity for their molecular target, leading to potential unspecific effects. Compound 21 (C21) was recently proposed as an alternative, but in vivo characterization of its properties is not sufficient yet. Here, we evaluated its potency to selectively modulate the activity of nigral dopaminergic (DA) neurons through the canonical DREADD receptor hM4Di using TH-Cre rats. In males, 1 mg.kg-1 of C21 strongly increased nigral neurons activity in control animals, indicative of a significant off-target effect. Reducing the dose to 0.5 mg.kg-1 circumvented this unspecific effect, while activated the inhibitory DREADDs and selectively reduced nigral neurons firing. In females, 0.5 mg.kg-1 of C21 induced a transient and residual off-target effect that may mitigated the inhibitory DREADDs-mediated effect. This study raises up the necessity to test selectivity and efficacy of chosen ligands for each new experimental condition.

Enhancement of Drug Seeking Following Drug Taking in a Sexual Context Requires Anterior Cingulate Cortex Activity in Male Rats

Individual variance in vulnerability to develop addictions is influenced by social factors. Specifically, drug-taking in a sexual context appears to enhance further drug-seeking behavior in human users, as these users identify the effects of drugs to enhance sexual pleasure as a primary reason for continued drug use. Methamphetamine (Meth) is commonly used in this context. Similarly, male rats that self-administered Meth immediately followed by sexual behavior display enhanced drug-seeking behavior, including attenuation of extinction and increased reinstatement to seeking of Meth-associated cues. Hence, drug-taking in a sexual context enhances vulnerability for addiction. However, the neural mechanisms by which this occurs are unknown. Here the hypothesis was tested that medial prefrontal cortex is essential for this effect of Meth and sex when experienced concurrently. First it was shown that CaMKII neurons in the anterior cingulate area (ACA) were co-activated by both Meth and sex. Next, chemogenetic inactivation of ACA CaMKII cells using AAV5-CaMKIIa-hM4Di-mCherry was shown not to affect Meth-induced locomotor activity or sexual behavior. Subsequently, chemogenetic inactivation of ACA CaMKII neurons during Meth self-administration followed by sexual behavior was shown to prevent the effects of Meth and sex on enhanced reinstatement of Meth-seeking but did not affect enhanced drug-seeking during extinction tests. These results indicate that ACA CaMKII cell activation during exposure to Meth in a sexual context plays an essential role in the subsequent enhancement of drug-seeking during reinstatement tests.

TRPM8 as the rapid testosterone signaling receptor: Implications in the regulation of dimorphic sexual and social behaviors

Testosterone regulates dimorphic sexual behaviors in all vertebrates. However, the molecular mechanism underlying these behaviors remains unclear. Here, we report that a newly identified rapid testosterone signaling receptor, Transient Receptor Potential Melastatin 8 (TRPM8), regulates dimorphic sexual and social behaviors in mice. We found that, along with higher steroid levels in the circulation, TRPM8^-/- male mice exhibit increased mounting frequency indiscriminate of sex, delayed sexual satiety, and increased aggression compared to wild-type controls, while TRPM8^-/- females display an increased olfaction-exploratory behavior. Furthermore, neuronal responses to acute testosterone application onto the amygdala were attenuated in TRPM8^-/- males but remained unchanged in females. Moreover, activation of dopaminergic neurons in the ventral tegmental area following mating was impaired in TRPM8^-/- males. Together, these results demonstrate that TRPM8 regulates dimorphic sexual and social behaviors, and potentially constitutes a signalosome for mediation of sex-reward mechanism in males. Thus, deficiency of TRPM8 might lead to a delayed sexual satiety phenomenon.

Effects of inactivation of the ventral tegmental area on prefronto-accumbens activity and sexual motivation in male rats

Sexual motivation requires the processing of sexual stimuli. The prefrontal cortex (PFC) and nucleus accumbens (NAcc) receive dopaminergic innervation from the ventral tegmental area (VTA). Both structures participate in processing stimuli, and their adequate functioning is modulated by dopamine and other neurotransmitters. This study was designed to determine the effect of inactivation of the VTA on sexual motivation, relative power (RP) and electroencephalographic (EEG) correlation of the PFC and NAcc in male rats. A total of 20 rats implanted with electrodes in the left medial prefrontal cortex (mPFC) and NAcc, and with bilateral cannulae in the VTA, were divided into two groups of 10 rats each, one injected with tetrodotoxin (TTX), the other with a vehicle solution (VEH). EEGs from the mPFC and NAcc were recorded during the awake-quiet state in the presence of either a receptive or non-receptive female. The TTX group showed a lower preference for the receptive female accompanied by a lower RP of the 8-13 and 14-30 Hz bands in the mPFC. Also, in the presence of the receptive female, the TTX group had a lower RP of the 8-13 Hz band in the NAcc, but a higher prefronto-accumbens correlation in the same band. These results provide evidence that VTA activity is necessary for the adequate functioning of the mPFC and NAcc and, therefore, also for the adequate processing of sexually-relevant stimuli that allows the induction and maintenance of sexual motivation in male rats.

Effects of non-contingent cocaine on 3alpha-androstanediol. I. Disruption of male sexual behavior

One of the hallmarks of drug abuse is a reduction in the salience of, and motivation for, natural rewards, such as mating. The effects of psychostimulants on male sexual interest and performance are conflicting; use of psychostimulants can produce increases in risky sexual behaviors but have detrimental effects on sexual ability. We hypothesize that these conflicting effects on sexual behavior are due to interactions between cocaine and androgens, such as testosterone and its neuroactive metabolite, 3α-androstanediol (3α-diol). Male rats were administered saline or cocaine (5, 10, or 20mg/kg, i.p.). Motor behavior was observed in the first 30min following drug-administration, and then sexual responding was assessed for 15min. Levels of androgens (testosterone, 3ɑ-diol, and testosterone's aromatized metabolite, estradiol) were measured in circulation and brain regions (frontal cortex, hippocampus, hypothalamus/striatum (hypo/str), and midbrain). Cocaine had no effect on measures of sexual interest (i.e. anogenital investigation). However, cocaine had substantial effects on consummatory sexual behaviors, such as the latency to mount/intromit and the number of sexual contacts. Frontal cortex and hypo/str 3α-diol levels were strongly correlated with consummatory behaviors in saline administered rats; however, this relationship was disrupted by cocaine at all dosages, concomitant with impaired sexual behaviors. Additionally, there was a shift in metabolism at low dosages of cocaine to push testosterone metabolism in the midbrain towards 3α-diol. On the contrary, moderate and high dosages of cocaine shifted testosterone metabolism towards estradiol. These data demonstrate that the association between cortical and hypo/str 3α-diol levels and sexual behavior of male rats is disrupted by non-contingent cocaine and that there may be dose-dependent effects of acute cocaine on androgen metabolism.

Modulating Dopamine Signaling and Behavior with Chemogenetics: Concepts, Progress, and Challenges

For more than 60 years, dopamine (DA) has been known as a critical modulatory neurotransmitter regulating locomotion, reward-based motivation, and endocrine functions. Disturbances in DA signaling have been linked to an array of different neurologic and psychiatric disorders, including Parkinson's disease, schizophrenia, and addiction, but the underlying pathologic mechanisms have never been fully elucidated. One major obstacle limiting interpretation of standard pharmacological and transgenic interventions is the complexity of the DA system, which only appears to widen as research progresses. Nonetheless, development of new genetic tools, such as chemogenetics, has led to an entirely new era for functional studies of neuronal signaling. By exploiting receptors that are engineered to respond selectively to an otherwise inert ligand, so-called Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), chemogenetics enables pharmacological remote control of neuronal activity. Here we review the recent, extensive application of this technique to the DA field and how its use has advanced the study of the DA system and contributed to our general understanding of DA signaling and related behaviors. Moreover, we discuss the challenges and pitfalls associated with the chemogenetic technology, such as the metabolism of the DREADD ligand clozapine N-oxide (CNO) to the D2 receptor antagonist clozapine. We conclude that despite the recent concerns regarding CNO, the chemogenetic toolbox provides an exceptional approach to study neuronal function. The huge potential should promote continued investigations and additional refinements to further expound key mechanisms of DA signaling and circuitries in normal as well as maladaptive behaviors.

Neural Circuits Underlying Rodent Sociality: A Comparative Approach

All mammals begin life in social groups, but for some species, social relationships persist and develop throughout the course of an individual's life. Research in multiple rodent species provides evidence of relatively conserved circuitry underlying social behaviors and processes such as social recognition and memory, social reward, and social approach/avoidance. Species exhibiting different complex social behaviors and social systems (such as social monogamy or familiarity preferences) can be characterized in part by when and how they display specific social behaviors. Prairie and meadow voles are closely related species that exhibit similarly selective peer preferences but different mating systems, aiding direct comparison of the mechanisms underlying affiliative behavior. This chapter draws on research in voles as well as other rodents to explore the mechanisms involved in individual social behavior processes, as well as specific complex social patterns. Contrasts between vole species exemplify how the laboratory study of diverse species improves our understanding of the mechanisms underlying social behavior. We identify several additional rodent species whose interesting social structures and available ecological and behavioral field data make them good candidates for study. New techniques and integration across laboratory and field settings will provide exciting opportunities for future mechanistic work in non-model species.

Pharmacological and proteomic analyses of neonatal polyI:C-treated adult mice

Perinatal virus infection is an environmental risk factor for neurodevelopmental disorders such as schizophrenia. We previously demonstrated that neonatal treatment with a viral mimetic, polyriboinosinic-polyribocytidilic acid (polyI:C), in mice leads to emotional and cognitive deficits in adolescence. Here, we investigated the effects of antipsychotics on polyI:C-induced behavioral abnormalities. We also performed a proteomic analysis in the hippocampus of polyI:C-treated adult mice using two-dimensional electrophoresis to understand the changes in protein expression following neonatal immune activation. Neonatal mice were subcutaneously injected with polyI:C for 5 days (postnatal day 2-6). At 10 weeks, sensorimotor gating, emotional and cognitive function were analyzed in behavioral tests. Clozapine improved PPI deficit and emotional and cognitive dysfunction in polyI:C-treated mice. However, haloperidol improved only PPI deficit. Proteomic analysis revealed that two candidate proteins were obtained in the hippocampus of polyI:C-treated mice, including aldehyde dehydrogenase family 1 member L1 (ALDH1L1) and collapsin response mediator protein 5 (CRMP5). These data suggest that the neonatal polyI:C-treated mouse model may be useful for evaluating antipsychotic activity of compounds. Moreover, changes in the protein expression of ALDH1L1 and CRMP5 support our previous findings that astrocyte-neuron interaction plays a role in the pathophysiology of neurodevelopmental disorders induced by neonatal immune activation.

Oxytocin induces penile erection and yawning when injected into the bed nucleus of the stria terminalis: Involvement of glutamic acid, dopamine, and nitric oxide

Oxytocin (5-100ng), but not Arg⁸-vasopressin (100ng), injected unilaterally into the bed nucleus of the stria terminalis (BNST) induces penile erection and yawning in a dose-dependent manner in male rats. The minimal effective dose was 20ng for penile erection and 5ng for yawning. Oxytocin responses were abolished not only by the oxytocin receptor antagonist d(CH₂)₅Tyr(Me)²-Orn⁸-vasotocin (1μg), but also by (+) MK-801 (1μg), an excitatory amino acid receptor antagonist of the N-methyl-d-aspartic acid (NMDA) subtype, SCH 23390 (1μg), a D1 receptor antagonist, but not haloperidol (1μg), a D2 receptor antagonist, and SMTC (40μg), an inhibitor of neuronal nitric oxide synthase, injected into the BNST 15min before oxytocin. Oxytocin-induced penile erection, but not yawning, was also abolished by CNQX (1μg), an excitatory amino acid receptor antagonist of the AMPA subtype. In contrast, oxytocin responses were not reduced by bicuculline (20ng), a GABA_A receptor antagonist, phaclofen (5μg), a GABA_B receptor antagonist, CP 376395, a CRF receptor-1 antagonist (5μg), or astressin 2B, a CRF receptor-2 antagonist (150ng). Considering the ability of NMDA (100ng) to induce penile erection and yawning when injected into the BNST and the available evidence showing possible interaction among oxytocin, glutamic acid, and dopamine in the BNST, oxytocin possibly activates glutamatergic neurotransmission in the BNST. This in turn leads to the activation of neural pathways projecting back to the paraventricular nucleus, medial preoptic area, ventral tegmental area, and/or ventral subiculum/amygdala, thereby inducing penile erection and yawning.

Dopamine, Noradrenaline and Differences in Sexual Behavior between Roman High and Low Avoidance Male Rats: A Microdialysis Study in the Medial Prefrontal Cortex

Roman High- (RHA) and Low-Avoidance (RLA) outbred rats, which differ for a respectively rapid vs. poor acquisition of the active avoidance response in the shuttle-box, display differences in sexual activity when put in the presence of a sexually receptive female rat. Indeed RHA rats show higher levels of sexual motivation and copulatory performance than RLA rats, which persist also after repeated sexual activity. These differences have been correlated to a higher tone of the mesolimbic dopaminergic system of RHA rats vs. RLA rats, revealed by the higher increase of dopamine found in the dialysate obtained from the nucleus accumbens of RHA than RLA rats during sexual activity. This work shows that extracellular dopamine and noradrenaline (NA) also, increase in the dialysate from the medial prefrontal cortex (mPFC) of male RHA and RLA rats put in the presence of an inaccessible female rat and more markedly during direct sexual interaction. Such increases in dopamine (and its main metabolite 3,4-dihydroxyphenylacetic acid, DOPAC) and NA were found in both sexually naïve and experienced animals, but they were higher: (i) in RHA than in RLA rats; and (ii) in sexually experienced RHA and RLA rats than in their naïve counterparts. Finally, the differences in dopamine and NA in the mPFC occurred concomitantly to those in sexual activity, as RHA rats displayed higher levels of sexual motivation and copulatory performance than RLA rats in both the sexually naïve and experienced conditions. These results suggest that a higher dopaminergic tone also occurs in the mPFC, together with an increased noradrenergic tone, which may be involved in the different copulatory patterns found in RHA and RLA rats, as suggested for the mesolimbic dopaminergic system.