Involvement of mesolimbic dopamine neurons in sexual behaviors: implications for the neurobiology of motivation

Aging, testosterone, and neuroplasticity: friend or foe?

Neuroplasticity or neural plasticity implicates the adaptive potential of the brain in response to extrinsic and intrinsic stimuli. The concept has been utilized in different contexts such as injury and neurological disease. Neuroplasticity mechanisms have been classified into neuroregenerative and function-restoring processes. In the context of injury, neuroplasticity has been defined in three post-injury epochs. Testosterone plays a key yet double-edged role in the regulation of several neuroplasticity alterations. Research has shown that testosterone levels are affected by numerous factors such as age, stress, surgical procedures on gonads, and pharmacological treatments. There is an ongoing debate for testosterone replacement therapy (TRT) in aging men; however, TRT is more useful in young individuals with testosterone deficit and more specific subgroups with cognitive dysfunction. Therefore, it is important to pay early attention to testosterone profile and precisely uncover its harms and benefits. In the present review, we discuss the influence of environmental factors, aging, and gender on testosterone-associated alterations in neuroplasticity, as well as the two-sided actions of testosterone in the nervous system. Finally, we provide practical insights for further study of pharmacological treatments for hormonal disorders focusing on restoring neuroplasticity.

Endocannabinoids Released in the Ventral Tegmental Area During Copulation to Satiety Modulate Changes in Glutamate Receptors Associated With Synaptic Plasticity Processes

Endocannabinoids modulate mesolimbic (MSL) dopamine (DA) neurons firing at the ventral tegmental area (VTA). These neurons are activated by copulation, increasing DA release in nucleus accumbens (NAcc). Copulation to satiety in male rats implies repeated ejaculation within a short period (around 2.5 h), during which NAcc dopamine concentrations remain elevated, suggesting continuous neuronal activation. During the 72 h that follow copulation to satiety, males exhibit long-lasting changes suggestive of brain plasticity processes. Enhanced DA neuron activity triggers the synthesis and release of endocannabinoids (eCBs) in the VTA, which participate in several long-term synaptic plasticity processes. Blockade of cannabinoid type 1 receptors (CB1Rs) during copulation to satiety interferes with the appearance of the plastic changes. Glutamatergic inputs to the VTA express CB1Rs and contribute to DA neuron burst firing and synaptic plasticity. We hypothesized that eCBs, released during copulation to satiety, would activate VTA CB1Rs and modulate synaptic plasticity processes involving glutamatergic transmission. To test this hypothesis, we determined changes in VTA CB1R density, phosphorylation, and internalization in rats that copulated to satiety 24 h earlier as compared both to animals that ejaculated only once and to sexually experienced unmated males. Changes in glutamate AMPAR and NMDAR densities and subunit composition and in ERK1/2 activation were determined in the VTA of males that copulated to satiety in the presence or absence of AM251, a CB1R antagonist. The CB1R density decreased and the proportion of phosphorylated CB1Rs increased in the animals that copulated compared to control rats. The CB1R internalization was detected only in sexually satiated males. A decrease in α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR) density, blocked by AM251 pretreatment, and an increase in the proportion of GluA2-AMPARs occurred in sexually satiated rats. GluN2A- N-methyl-D-aspartate receptor (NMDAR) expression decreased, and GluN2B-NMDARs increased in these animals, both of which were prevented by AM251 pre-treatment. An increase in phosphorylated ERK1/2 emerged in males copulating to satiety in the presence of AM251. Results demonstrate that during copulation to satiety, eCBs activate CB1Rs in the VTA, producing changes in glutamate receptors compatible with a reduced neuronal activation. These changes could play a role in the induction of the long-lasting physiological changes that characterize sexually satiated rats.

Endocannabinoid and dopaminergic system: the pas de deux underlying human motivation and behaviors

Endocannabinoid system (ECS) has been identified ever since cannabinoid, an active substance of Cannabis, was known to interact with endogenous cannabinoid (endocannabinoid/eCB) receptors. It later turned out that eCB was more intricate than previously thought. It has a pervasive role and exerts a multitude of cellular signaling mechanisms, regulating various physiological neurotransmission pathways in the human brain, including the dopaminergic (DA) system. eCB roles toward DA system were robust, clearly delineated, and reproducible with respect to physiological as well as pathological neurochemical and neurobehavioral manifestations of DA system, particularly those involving the nigrostriatal and mesocorticolimbic pathways. The eCB–DA system regulates the basics in the Maslow’s pyramid of hierarchy of needs required for individual survival such as food and sexual activity for reproductive purpose to those of higher needs in the pyramid, including self-actualization behaviors leading to achievement and reward (e.g., academic- and/or work-related performance and achievements). It is, thus, interesting to specifically discuss the eCB–DA system, not only on the molecular level, but also its tremendous potential to be developed as a future therapeutic strategy for various neuropsychiatric problems, including obesity, drug addiction and withdrawal, pathological hypersexuality, or low motivation behaviors.

Microbiota-gut-brain axis as a regulator of reward processes

Our gut harbours trillions of microorganisms essential for the maintenance of homeostasis and host physiology in health and disease. In the last decade, there has been a growing interest in understanding the bidirectional pathway of communication between our microbiota and the central nervous system. With regard to reward processes there is accumulating evidence from both animal and human studies that this axis may be a key factor in gating reward valence. Focusing on the mesocorticolimbic pathway, we will discuss how the intestinal microbiota is involved in regulating brain reward functions, both in natural (i.e. eating, social or sexual behaviours) and non-natural reinforcers (drug addiction behaviours including those relevant to alcohol, psychostimulants, opioids and cannabinoids). We will integrate preclinical and clinical evidence suggesting that the microbiota-gut-brain axis could be implicated in the development of disorders associated with alterations in the reward system and how it may be targeted as a promising therapeutic strategy. Cover Image for this issue: https://doi.org/10.1111/jnc.15065.

Endocannabinoids mediate long-lasting behavioural and physiological changes in male rats induced by the repeated activation of the mesolimbic system by copulation to satiety

Sexually satiated male rats exhibit long-lasting physiological changes, suggestive of brain plasticity, the most conspicuous of which are a sexual behaviour inhibition and a generalised drug hypersensitivity. Copulation activates the mesolimbic circuit increasing dopamine (DA) release in the nucleus accumbens (NAcc) and, enhanced midbrain DA neuron activity promotes endocannabinoid (eCB) release in the ventral tegmental area (VTA). The objective of this work was to explore the possible participation of DA and/or eCB transmission in the induction of these two long-lasting phenomena. To this aim we analysed the effect of blocking DA or CB1 receptors during the process of copulation to exhaustion, on the expression 24 h later, of the sexual inhibitory state and the hypersensitivity to two different drugs: 8-OH-DPAT, a 5-HT_1A receptor agonist, and yohimbine, an α₂-adrenoceptor antagonist. Blockade of DA receptors failed to prevent these phenomena, while blockade of CB1 receptors interfered with the appearance of the sexual inhibition and the hypersensitivity to both drugs in the sexually satiated animals. Specific blockade of CB1 receptors in the VTA during copulation to satiety mimicked these results, suggesting that both eCB-mediated effects were exerted in this brain region. It is concluded that eCBs play a role in the induction of behavioural and physiological changes, triggered by copulation to satiety, by acting at the VTA, while increased NAcc DA levels appear not to contribute to the changes induced by intense copulation. Results pose sexual satiety as a useful model for the study of brain plasticity phenomena induced by natural rewards.

Glutamate Afferents From the Medial Prefrontal Cortex Mediate Nucleus Accumbens Activation by Female Sexual Behavior

Low levels of desire and arousal are the primary sexual dysfunctions in women, necessitating neurobiological studies of sexual motivation in female animal models. As the mesocorticolimbic system is a primary neural circuit underlying sexual motivation, the goal of this study was to test the hypothesis that medial prefrontal cortex (mPFC) glutamate mediates sexual behavior activation of the nucleus accumbens. Glutamatergic neurons in the mPFC were activated by sex behavior, and these sex-activated cells shown to project to the nucleus accumbens. During sexual interactions with the male, glutamate transients recorded in the nucleus accumbens of female hamsters were specifically associated with the receipt of intromissions from the male. Further, inhibition of the mPFC during sex significantly decreased nucleus accumbens activation. Glutamatergic medial prefrontal cortical input to the nucleus accumbens mediates the activity in the nucleus accumbens during female sexual behavior. These results offer novel insights into the neurobiology of the motivational control of female sexual behavior and provide attractive avenues for pursuing target-specific and clinically-relevant therapies for sexual dysfunction in women.

Nucleus accumbens dopamine increases sexual motivation in sexually satiated male rats

RATIONALE

The influence of the main dopaminergic brain regions controlling copulation, the medial preoptic area (mPOA) and the nucleus accumbens (NAcc), on male rat sexual behavior expression has not been fully established.

OBJECTIVE

This work analyzes the sexual effects of dopamine (DA) receptor activation in the mPOA or the NAcc of sexually active male rats, with an intact (sexually experienced) or a reduced (sexually exhausted) sexual motivation.

METHODS

The non-specific DA receptor agonist apomorphine and the D2-like receptor agonist quinpirole were infused into the mPOA or the NAcc of sexually experienced or sexually exhausted male rats and their sexual behavior recorded.

RESULTS

DA receptor activation neither in the mPOA nor in the NAcc modified the copulatory behavior of sexually experienced male rats. DA receptor stimulation in the NAcc, but not in the mPOA, reversed the characteristic sexual inhibition of sexually satiated rats, and D2-like receptors were found to participate in this effect.

CONCLUSION

The optimal sexual performance of sexually experienced male rats cannot be further improved by DA receptor activation at either brain region. In sexually satiated rats, which are sexually inhibited and have a diminished sexual motivation, NAcc DA receptor stimulation appears to play a key role in their capacity to respond to a motivational significant stimulus, the receptive female, with the participation of D2-like receptors. Activation of DA receptors with the same drug, at the same dose and in the same brain region, produces different effects on copulatory behavior that depend on the animal's sexual motivational state.

The lateral preoptic area and ventral pallidum embolden behavior

While recently completing a study of the effects of stimulating the lateral preoptic area (LPO) and ventral pallidum (VP) on locomotion and other movements, we also noticed LPO and VP effects on motivational drive and threat tolerance. Here, we have investigated these latter effects by testing conditioned place preference (CPP), behavior on the elevated plus maze (EPM) and the willingness of sated rats to occupy a harshly lit open field center to acquire sweet pellets, a measure of threat tolerance, following infusions of vehicle or bicuculline (bic) into the LPO and VP. LPO-bic infusions robustly increased total locomotion, and, in direct proportion, occupancy of both the harshly lit field center and open arms of the EPM. LPO bic also generated CPP, but did not increase sweet pellet ingestion. These effects were attenuated by dopamine D1 and D2 receptor antagonists, whether given individually or as a cocktail and systemically or infused bilaterally into the nucleus accumbens. VP-bic infusions did not increase total locomotion, but preferentially increased field center occupancy. VP-bic-infused rats compulsively ingested sweet pellets and did so even under the spotlight, whereas harsh illumination suppressed pellet ingestion in the control groups. VP bic produced CPP and increased open arm occupancy on the EPM. These effects were attenuated by pretreatment with dopamine receptor antagonists given systemically or as bilateral infusions into the VP, except for % distance in the field center (by D1 or D2 antagonists) and pellet ingestion (by D1 antagonist). Thus, boldness generated in association with LPO activation is tightly tied to locomotor activation and, as is locomotion itself, strongly DA dependent, whereas that accompanying stimulation of the VP is independent of locomotor activation and, at least in part, DA signaling. Furthermore, respective emboldened behaviors elicited from neither LPO nor VP could clearly be attributed to goal pursuit. Rather, emboldening of behavior seems more to be a fixed action response not fundamentally different than previously for reported locomotion, pivoting, backing, gnawing, and eating elicited by basal forebrain stimulation.

Testosterone and Corticosterone in the Mesocorticolimbic System of Male Rats: Effects of Gonadectomy and Caloric Restriction

Steroid hormones can modulate motivated behaviors through the mesocorticolimbic system. Gonadectomy (GDX) is a common method to determine how steroids influence the mesocorticolimbic system, and caloric restriction (CR) is often used to invigorate motivated behaviors. A common assumption is that the effects of these manipulations on brain steroid levels reflects circulating steroid levels. We now know that the brain regulates local steroid levels in a region-specific manner; however, previous studies have low spatial resolution. Using ultrasensitive liquid chromatography tandem mass spectrometry, we examined steroids in microdissected regions of the mesocorticolimbic system (ventral tegmental area, nucleus accumbens, medial prefrontal cortex). We examined whether GDX or CR influences systemic and local steroids, particularly testosterone (T) and steroidogenic enzyme transcripts. Adult male rats underwent a GDX surgery and/or CR for either 2 or 6 weeks. Levels of T, the primary steroid of interest, were higher in all brain regions than in the blood, whereas corticosterone (CORT) was lower in the brain than in the blood. Importantly, GDX completely eliminated T in the blood and lowered T in the brain. Yet, T remained present in the brain, even 6 weeks after GDX. CR decreased both T and CORT in the blood and brain. Steroidogenic enzyme (Cyp17a1, 3β-hydroxysteroid dehydrogenase, aromatase) transcripts and androgen receptor transcripts were expressed in the mesocorticolimbic system and differentially affected by GDX and CR. Together, these results suggest that T is synthesized within the mesocorticolimbic system. These results provide a foundation for future studies examining how neurosteroids influence behaviors mediated by the mesocorticolimbic system.

Individual prolactin reactivity modulates response of nucleus accumbens to erotic stimuli during acute cannabis intoxication: an fMRI pilot study

RATIONALE

Self-report studies indicate that cannabis could increase sexual desire in some users. We hypothesized that intoxication increases activation of brain areas responsive to visual erotica, which could be useful in the treatment of hypoactive sexual desire disorder, a condition marked by a lack of sexual desire.

OBJECTIVES

The aim of this study is to assess the aphrodisiacal properties of cannabis.

METHODS

We conducted an open-randomized study with 21 heterosexual casual cannabis users. A 3T MRI was used to measure brain activation in response to erotic pictures. Blood samples were collected to determine the serum levels of cannabinoids, cortisol and prolactin. Participants were grouped according to whether they had ever experienced any aphrodisiacal effects during intoxication (Group A) or not (Group non-A).

RESULTS

Intoxication was found to significantly increase activation in the right nucleus accumbens in the Group A while significantly decreasing activation in the Group non-A. There was also a significant interaction between the group and intoxication, with elevated prolactin in the Group non-A during intoxication. No intoxication-related differences in subjective picture evaluations were found.

CONCLUSION

Cannabis intoxication increases activation of the right nucleus accumbens to erotic stimuli. This effect is limited to users whose prolactin is not elevated in response to intoxication. This effect may be useful in the treatment of low sexual desire.

Prosexual Effect of Chrysactinia mexicana A. Gray (Asteraceae), False Damiana, in a Model of Male Sexual Behavior

Chrysactinia mexicana A. Gray (Asteraceae) and Turnera diffusa Willd (Turneraceae) are employed in traditional medicine as aphrodisiacs; however, there is no scientific evidence supporting the prosexual properties of C. mexicana. The aim of this study was to determine whether an aqueous extract of C. mexicana (Cm) stimulates rat male sexual behavior in the sexual exhaustion paradigm. Sexually exhausted (SExh) male rats were treated with Cm (80, 160, and 320 mg/kg), an aqueous extract of T. diffusa (Td), or yohimbine. The sexual exhaustion state in the control group was characterized by a low percentage of males exhibiting mounts, intromissions, and ejaculations and no males demonstrating mating behavior after ejaculation. Cm (320 mg/kg), Td, or yohimbine significantly increased the proportion of SExh rats that ejaculated and resumed copulation after ejaculation. In males that exhibited reversal of sexual exhaustion, Cm (320 mg/kg) improved sexual performance by reducing the number of intromissions and shrinking ejaculation latency. The effects of treatments on sexual behavior were not related with alterations in general locomotion. In conclusion, the prosexual effects of Cm, as well as those of Td, are established at a central level, which supports the traditional use of C. mexicana for stimulating sexual activity.

Mood, Food, and Fertility: Adaptations of the Maternal Brain

Successfully rearing young places multiple demands on the mammalian female. These are met by a wide array of alterations in maternal physiology and behavior that are coordinated with the needs of the developing young, and include adaptations in neuroendocrine systems not directly involved in maternal behavior or lactation. In this article, attenuations in the behavioral and neuroendocrine responses to stressors, the alterations in metabolic pathways facilitating both increased food intake and conservation of energy, and the changes in fertility that occur postpartum are described. The mechanisms underlying these processes as well as the factors that contribute to them and the relative contributions of these stimuli at different times postpartum are also reviewed. The induction and maintenance of the adaptations observed in the postpartum maternal brain are dependent on mother-young interaction and, in most cases, on suckling stimulation and its consequences for the hormonal profile of the mother. The peptide hormone prolactin acting on receptors within the brain makes a major contribution to changes in metabolic pathways, suppression of fertility and the attenuation of the neuroendocrine response to stress during lactation. Oxytocin is also released, both into the circulation and in some hypothalamic nuclei, in response to suckling stimulation and this hormone has been implicated in the decrease in anxiety behavior seen in the early postpartum period. The relative importance of these hormones changes across lactation and it is becoming increasingly clear that many of the adaptations to motherhood reviewed here reflect the outcome of multiple influences. © 2016 American Physiological Society. Compr Physiol 6:1493-1518, 2016.

Catecholaminergic based therapies for functional recovery after TBI

Among the many pathophysiologic consequences of traumatic brain injury are changes in catecholamines, including dopamine, epinephrine, and norepinephrine. In the context of TBI, dopamine is the one most extensively studied, though some research exploring epinephrine and norepinephrine have also been published. The purpose of this review is to summarize the evidence surrounding use of drugs that target the catecholaminergic system on pathophysiological and functional outcomes of TBI using published evidence from pre-clinical and clinical brain injury studies. Evidence of the effects of specific drugs that target catecholamines as agonists or antagonists will be discussed. Taken together, available evidence suggests that therapies targeting the catecholaminergic system may attenuate functional deficits after TBI. Notably, it is fairly common for TBI patients to be treated with catecholamine agonists for either physiological symptoms of TBI (e.g. altered cerebral perfusion pressures) or a co-occuring condition (e.g. shock), or cognitive symptoms (e.g. attentional and arousal deficits). Previous clinical trials are limited by methodological limitations, failure to replicate findings, challenges translating therapies to clinical practice, the complexity or lack of specificity of catecholamine receptors, as well as potentially counfounding effects of personal and genetic factors. Overall, there is a need for additional research evidence, along with a need for systematic dissemination of important study details and results as outlined in the common data elements published by the National Institute of Neurological Diseases and Stroke. Ultimately, a better understanding of catecholamines in the context of TBI may lead to therapeutic advancements. This article is part of a Special Issue entitled SI:Brain injury and recovery.

Food restriction dissociates sexual motivation, sexual performance, and the rewarding consequences of copulation in female Syrian hamsters

Animals can switch their behavioral priorities from ingestive to sex behaviors to optimize reproductive success in environments where energy fluctuates. We hypothesized that energy availability differentially affects the appetitive (motivation), consummatory (performance), and learned (rewarding) components of behavior. In Experiment 1, appetitive and consummatory aspects of sex behavior were dissociated in the majority of female Syrian hamsters restricted to 75% of their ad libitum food intake for between 8 and 11 days. Food restriction significantly inhibited vaginal scent marking, decreased the preference for spending time with male hamsters vs. spending time with food, and increased food hoarding with no significant effect on consummatory behaviors such as the incidence of lordosis or food intake. In Experiments 2 and 3, we attempted to use a similar level of food restriction to dissociate sexual appetite from sexual reward. In hamsters, formation of a conditioned place preference (CPP) for copulatory reward is reflected in increased nucleus accumbens (NAc) neural activation, measured as immunocytochemical staining for c-Fos, the protein product of the immediate-early gene, c-fos. In Experiment 2, neural activation increased 1h after copulation in the NAc, and did not differ significantly between 10-day food-restricted and ad libitum-fed females in any brain area examined. In Experiment 3, females were either food-restricted or fed ad libitum over 8-30 days of conditioning with copulatory stimuli. Food-restricted females showed significantly fewer appetitive behaviors, but no difference in formation of a CPP compared to females fed ad libitum. Together these data are consistent with the idea that mild levels of food restriction that inhibit appetitive behaviors fail to attenuate consummatory behaviors and the rewarding consequences of copulation. Thus, appetitive sex behaviors are, at least partially, neuroanatomically and behaviorally distinct from both consummatory behaviors and copulatory reward.

Estradiol and song affect female zebra finch behavior independent of dopamine in the striatum

Female songbirds display preferences for certain song characteristics, but the neural and hormonal mechanisms mediating these preferences are not fully clear. The present study sought to further explore the role of estradiol, as well as assess potential roles of dopaminergic systems, on behavioral responses to song. Adult female zebra finches were treated with estradiol and exposed to tutored or untutored song or silence. Behavior was quantified and neurochemistry of the nucleus accumbens and striatum was examined with high performance liquid chromatography. As a control, the responses of these two systems to treatment with raclopride, a specific D2 receptor antagonist, were also evaluated. This manipulation did not affect dopamine (DA), but did increase DOPAC and the DOPAC/DA ratio. Estradiol reduced the display of two behaviors, distance calls and visual scanning, but had no effect on dopaminergic responses. Auditory stimulus exposure affected other vocalizations, but song presentation did not modulate the levels of DA or its metabolite, DOPAC in the nucleus accumbens or striatum. Collectively, the results suggest that both estradiol and auditory stimuli can modify the behavioral responses of adult zebra finches, but they may not change DA concentration or turnover in striatal dopamine neurons.

Persistent cognitive dysfunction after traumatic brain injury: A dopamine hypothesis

Traumatic brain injury (TBI) represents a significant cause of death and disability in industrialized countries. Of particular importance to patients the chronic effect that TBI has on cognitive function. Therapeutic strategies have been difficult to evaluate because of the complexity of injuries and variety of patient presentations within a TBI population. However, pharmacotherapies targeting dopamine (DA) have consistently shown benefits in attention, behavioral outcome, executive function, and memory. Still it remains unclear what aspect of TBI pathology is targeted by DA therapies and what time-course of treatment is most beneficial for patient outcomes. Fortunately, ongoing research in animal models has begun to elucidate the pathophysiology of DA alterations after TBI. The purpose of this review is to discuss clinical and experimental research examining DAergic therapies after TBI, which will in turn elucidate the importance of DA for cognitive function/dysfunction after TBI as well as highlight the areas that require further study.

Estradiol induces region-specific inhibition of ZENK but does not affect the behavioral preference for tutored song in adult female zebra finches

Female zebra finches display a preference for songs of males raised with tutors compared to those from males without tutors. To determine how this behavioral preference may be mediated by auditory perception sites, the social behavior network, and the dopamine reward system, and whether responses of these regions are affected by estradiol, females were treated with hormone or blank implants. An auditory choice test was conducted followed by exposure to tutored or untutored song or silence to examine induction of the immediate early gene, ZENK. Birds spent significantly more time near tutored than untutored song, regardless of estrogen treatment, and estradiol significantly decreased the density of ZENK immunoreactive neurons within the ventromedial hypothalamus. These results suggest that selective neural and behavioral responses can be induced by both high quality vocalizations and estradiol, although they are not necessarily correlated.

Brain circuits regulating energy homeostasis

Recent years have seen an impetus in the study for central mechanisms regulating energy balance, and caloric intake possibly as a response to the obesity pandemic. This renewed interest as well as drastic improvements in the tools that are now currently available to neuroscientists, has yielded a great deal of insight into the mechanisms by which the brain regulates metabolic function, and volitional aspects of feeding in response to metabolic signals like leptin, insulin and ghrelin. Among these mechanisms are the complex intracellular signals elicited by these hormones in neurons. Moreover, these signals produce and modulate the metabolism of the cell at the level of the mitochondria. Finally, these signals promote plastic changes that alter the synaptic circuitry in a number of circuits and ultimately affect cellular, physiological and behavioral responses in defense of energy homeostasis. These mechanisms are surveyed in this review.

Twenty years of dopamine research: individual differences in the response of accumbal dopamine to environmental and pharmacological challenges

Individual differences in the dopaminergic system of the nucleus accumbens of rats have extensively been reported. These individual differences have frequently been used to explain individual differences in response to environmental and pharmacological challenges. Remarkably, only little attention is paid to the factors that underlie these individual differences. This review gives an overview of the studies that have been performed in our institute during the last 20 years to investigate individual differences in accumbal dopamine release. Data are summarised demonstrating that individual differences in accumbal dopamine release are due to individual differences in: the functional reactivity of the noradrenergic system, the accumbal concentration of vesicular monoamine transporters and tyrosine hydroxylase as well as in the quantal size of the presynaptic pools of dopamine. Our data are embedded in the available literature to create a model that illustrates the putative hardware giving rise to the individual-specific release of accumbal dopamine. An important role is contributed to individual differences in the reactivity of the: hypothalamic-pituitary-adrenal axes, the reactivity of second messenger systems as well in the aminergic reactivity of the accumbens shell and core. The consequences of the individual-specific make-up and reactivity of the nucleus accumbens on the regulation of behaviour and the response to drugs of abuse will also be discussed. Apart from agents that interact with dopaminergic receptors, re-uptake or breakdown, noradrenergic agents as well as agents that interact with vesicular monoamine transporters or tyrosine hydroxylase are suggested to have therapeutic effects in subjects that are suffering from diseases in which the dopaminergic system is disturbed.

Cocaine during adolescence enhances dopamine in response to a natural reinforcer

The use of cocaine during adolescent development could alter the normal growth of brain regions affected by cocaine, specifically the reward system, and impact the adult mesolimbic system. However, there is scant literature aimed at determining whether animals are more vulnerable to the adverse effects of drugs during adolescence. The present study investigated whether cocaine pretreatment in either adolescence or adulthood altered the dopaminergic response to a naturally reinforcing substance in adulthood. To evaluate the responsivity of the mesolimbic system after repeated cocaine, sucrose was offered during the dialysis procedure and dialysates were collected. Regardless of age all saline pretreated rats had significant increases in sucrose-induced extracellular dopamine (DA) levels in the nucleus accumbens septi (NAcc) as compared to baseline levels. Rats pretreated with cocaine as adults also had significant increases in DA levels after sucrose. Interestingly, sucrose intake significantly enhanced DA levels in cocaine pretreated adolescent rats as compared to all other conditions. The results from the present study show that in rats pretreated with cocaine during adolescence there is an enhanced response of the dopaminergic system in animals exposed to a naturally reinforcing substance. Therefore, cocaine exposure during adolescence results in long-term functional changes in the mesolimbic pathway. Future studies need to ascertain the underlying mechanisms and their potential role in cocaine addiction.

Effects of sexual experience on conspecific odor preference and estrous odor-induced activation of the vomeronasal projection pathway and the nucleus accumbens in male rats

Male rats prefer odors from estrous females to those from sexually active males. Several studies, however, have demonstrated that prior sexual experience was required to develop the preference for estrous odor. Immunohistological methods for visualizing Fos protein have been shown that in sexually experienced male rats, estrous odors activate brain areas throughout the vomeronasal projection pathway (VN pathway) and the nucleus accumbens (NAcc). In the present study, we examined the contribution of prior sexual experience to the estrous odor-induced neuronal activation of these brain areas in relation to the development of the preference for estrous odor. Sexually experienced testosterone-implanted castrates showed the preference for the odor from an estrous female as opposed to the odor from a sexually active male. In these subjects, significant increment of Fos-like immunoreactivity (Fos-Li) after exposure to estrous female soiled bedding was observed in all brain regions examined, confirming the results of previous studies. Sexually naïve subjects, on the other hand, did not show the preference for estrous odor and the significant increment of Fos-Li was observed only in the accessory olfactory bulb (AOB) and the posterior-dorsal medial amygdala (MePD) of the VN pathway. These results suggested that sexual experience is required for the estrous odor-induced activation of more central portions of the VN pathway, such as the medial preoptic area (mPOA) and the bed nucleus of the stria terminalis (BNST), and the NAcc. The activation of some of these brain regions, therefore, is probably involved in the development of the preference for estrous odor.

Neurophysiological correlates of affiliative behaviour between humans and dogs

Few physiological parameters for positive human-companion animal contact have been identified and those that are established have all been in humans. The implication is that if the physiological reactions are mutual, dogs would experience the same psychological benefits from these neurophysiological changes as humans. Therefore, we have determined the role of certain neurochemicals during affiliation behaviour on an interspecies basis. Our results indicate that concentrations of beta-endorphin, oxytocin, prolactin, beta-phenylethylamine, and dopamine increased in both species after positive interspecies interaction, while that of cortisol decreased in the humans only. Indicators of mutual physiological changes during positive interaction between dog lovers and dogs may contribute to a better understanding of the human-animal bond in veterinary practice.

Sexual behavior reduces hypothalamic androgen receptor immunoreactivity

Male sexual behavior is regulated by limbic areas like the medial preoptic nucleus (MPN), the bed nucleus of the stria terminalis (BST), the nucleus accumbens (nAcc) and the ventromedial hypothalamic nucleus (VMN). Neurons in these brain areas are rich in androgen receptors (AR) and express FOS-immunoreactivity in response to mating. In many species sexual satiation, a state of sexual behavior inhibition, is attained after multiple ejaculations. The mechanisms underlying sexual satiation are largely unknown. In this study we show that sexual activity reduces androgen receptor immunoreactivity (AR-ir) in some of the brain areas associated with the control of male sexual behavior, but not in others. Thus, one ejaculation reduced the AR-ir in the MPN and nAcc, but not in the BST and VMN. Copulation to satiation, on the other hand, reduced AR-ir in the MPN, nAcc and VMN, and not in the BST. The AR-ir reduction observed in the MPN of sexually satiated rats was drastic when compared to that of animals ejaculating once. Serum androgen levels did not vary after one ejaculation or copulation to exhaustion. These data reveal that sexual activity reduces AR in specific brain areas and suggest the possibility that such a reduction underlies the sexual inhibition that characterizes sexual satiety.

Exposure to female rats produces differences in c-fos induction between sexually-naïve and experienced male rats

Sexual incentive-induced Fos-like immunoreactivity (Fos-Li) within six neural regions implicated in male sexual behavior was investigated in both sexually-naïve and experienced male rats. Sexual experience was limited to one copulation culminating in ejaculation 24 h prior to testing. On test-day, subjects were placed within a cylindrical arena for 15 min on the opposite side of a perforated, Plexiglas partition from one of three targets: an uninhabited area, a non-estrous female, or an estrous female. Then 1 h later, each subject was sacrificed and its brain prepared for subsequent immunocytochemical staining. Analyses revealed a main effect of target stimulus on c-fos expression within the nucleus accumbens shell and core of male subjects. In addition, sexually-experienced subjects demonstrated significantly more Fos-Li within the nucleus accumbens shell in response to an estrous female versus a non-estrous female. There was also greater estrous cue-induced Fos-Li in the nucleus accumbens shell of experienced subjects when compared to naïve subjects. These data support previous suggestions implicating the nucleus accumbens in the generation of male sexual motivation. In addition, copulatory experience, even when limited to one ejaculation, seems to mediate long-term changes in the response properties of nucleus accumbens neurons that may reflect the value enhancement of primary female incentives.

CREB activity in the nucleus accumbens shell controls gating of behavioral responses to emotional stimuli

The transcription factor cAMP response element (CRE)-binding protein (CREB) has been shown to regulate neural plasticity. Drugs of abuse activate CREB in the nucleus accumbens, an important part of the brain's reward pathways, and local manipulations of CREB activity have been shown to affect cocaine reward, suggesting an active role of CREB in adaptive processes that follow exposure to drugs of abuse. Using CRE-LacZ reporter mice, we show that not only rewarding stimuli such as morphine, but also aversive stimuli such as stress, activate CRE-mediated transcription in the nucleus accumbens shell. Using viral-mediated gene transfer to locally alter the activity of CREB, we show that this manipulation affects morphine reward, as well as the preference for sucrose, a more natural reward. We then show that local changes in CREB activity induce a more general syndrome, by altering reactions to anxiogenic, aversive, and nociceptive stimuli as well. Increased CREB activity in the nucleus accumbens shell decreases an animal's responses to each of these stimuli, whereas decreased CREB activity induces an opposite phenotype. These results show that environmental stimuli regulate CRE-mediated transcription within the nucleus accumbens shell, and that changes in CREB activity within this brain area subsequently alter gating between emotional stimuli and their behavioral responses. This control appears to be independent of the intrinsic appetitive or aversive value of the stimulus. The potential relevance of these data to addiction and mood disorders is discussed.

Sexually conditioned incentives: attenuation of motivational impact during dopamine receptor antagonism

The motivational impact of sexually conditioned incentives was examined in two experiments. In Experiment 1, male Long-Evans rats copulated to ejaculation in the presence of one of two scents (orange or almond extract) on five separate occasions. On alternating days, subjects spent an equal amount of time in social isolation with the opposing scent. Following the 10-day conditioning regimen, subjects ran more rapidly down an operant runway toward a goalbox containing the sex-paired scent (CS+) compared to trials on which the isolation-paired scent (CS-) or no scent was provided. In Experiment 2, comparably conditioned male rats were first given a baseline runway trial with an unscented goalbox. The following day, subjects were pretreated with one of four doses of haloperidol (0.0, 0.075, 0.15, or 0.30 mg/kg i.p.) 45 min prior to being tested in the runway for their motivation to approach either the CS+ or CS- scents. Control subjects given vehicle injections performed comparably to subjects from Experiment 1, taking significantly less time to approach the CS+ than an unscented goalbox. This decrease in run latency was not observed in subjects within the 0.075 and 0.15 mg/kg haloperidol groups. Subjects in the 0.30 mg/kg haloperidol groups took significantly more time to approach both the CS+ and CS- compared to their baseline run times. These data reveal that an olfactory cue associated with sexual reward becomes a conditioned incentive capable of eliciting approach behavior, and that dopamine receptor antagonism (at moderate but not high doses) selectively attenuates this cue-induced motivation.

Sexual behavior induction of c-Fos in the nucleus accumbens and amphetamine-stimulated locomotor activity are sensitized by previous sexual experience in female Syrian hamsters

Dopamine transmission in the nucleus accumbens can be activated by drugs, stress, or motivated behaviors, and repeated exposure to these stimuli can sensitize this dopamine response. The objectives of this study were to determine whether female sexual behavior activates nucleus accumbens neurons and whether past sexual experience cross-sensitizes neuronal responses in the nucleus accumbens to amphetamine. Using immunocytochemical labeling, c-Fos expression in different subregions (shell vs core at the rostral, middle, and caudal levels) of the nucleus accumbens was examined in female hamsters that had varying amounts of sexual experience. Female hamsters, given either 6 weeks of sexual experience or remaining sexually naive, were tested for sexual behavior by exposure to adult male hamsters. Previous sexual experience increased c-Fos labeling in the rostral and caudal levels but not in the middle levels of the nucleus accumbens. Testing for sexual behavior increased labeling in the core, but not the shell, of the nucleus accumbens. To validate that female sexual behavior can sensitize neurons in the mesolimbic dopamine pathway, the locomotor responses of sexually experienced and sexually naive females to an amphetamine injection were then compared. Amphetamine increased general locomotor activity in all females. However, sexually experienced animals responded sooner to amphetamine than did sexually naive animals. These data indicate that female sexual behavior can activate neurons in the nucleus accumbens and that sexual experience can cross-sensitize neuronal responses to amphetamine. In addition, these results provide additional evidence for functional differences between the shell and core of the nucleus accumbens and across its anteroposterior axis.

Dopamine antagonism attenuates the unconditioned incentive value of estrous female cues

The role of dopaminergic transmission in the incentive-motivational processes involved in the generation of male sexual behavior was examined. Three groups of sexually naïve Long-Evans male rats traversed a straight alley for one of three goalbox targets: an empty goalbox, a nonestrous female, or an estrous female. A Plexiglas partition within the goalbox allowed for the perception of visual, auditory, and olfactory cues, but prevented physical contact. Baseline run times revealed that subjects returned to the goalbox significantly faster for an estrous female than for a nonestrous female, replicating our earlier work on the inherent incentive value of primary female cues. When subjects were then pretreated with the dopamine receptor antagonist, haloperidol (0.0, 0.075, or 0.15 mg/kg), they expressed decreased sexual motivation as reflected by increased run times for estrous female targets. Subjects' run times for the empty goalbox condition were unaffected by haloperidol, suggesting that the drug did not reliably impair motoric capacity. Results support the contention that central dopaminergic systems are involved in the regulation of the positive, unconditioned incentive value of estrous female cues.

NMDA, but not dopamine D(2), receptors in the rat nucleus accumbens areinvolved in guidance of instrumental behavior by stimuli predicting reward magnitude

Expectancy of future reward is an important factor guiding the speed of instrumental behavior. The present study sought to explore whether signals transmitted via the NMDA subtype of glutamate receptors and via dopamine D(2) receptors in the nucleus accumbens (NAc) are critical for the determination of reaction times (RTs) of instrumental responses by the expectancy of future reward. A simple RT task for rats demanding conditioned lever release was used in which the upcoming reward magnitude (5 or 1 pellet) was signaled in advance by discriminative stimuli. In trained rats, RTs of conditioned responses with expectancy of a high reward magnitude were found to be significantly shorter. The shortening of RTs by stimuli predictive of high reward to be obtained was dose-dependently impaired by bilateral intra-NAc infusion of the competitive NMDA antagonist dl-2-amino-5-phosphonovaleric acid (APV) (1, 2, or 10 microg in 0.5 microl/side), but not by infusion of the preferential dopamine D(2) antagonist haloperidol (5 and 12.5 microg in 0.5 microl/side) or by infusion of vehicle (0.5 microl/side). In conclusion, the data reveal that in well trained animals stimulation of intra-NAc NMDA, but not of dopamine D(2), receptors, is critically involved in guiding the speed of instrumental responses according to stimuli predictive of the upcoming reward magnitude.

An integrative neuroanatomical perspective on some subcortical substrates of adaptive responding with emphasis on the nucleus accumbens

Neuroanatomical substrates associated in the literature with adaptive responding are discussed, with a focus on the nucleus accumbens. While it is emphasized that the accumbens exhibits multiple levels of complex organization, a fairly complete list of brief descriptions of recent studies devoted specifically to the accumbens shell and core subterritories is presented in tabular format. The distinct patterns of connectivity of the accumbens core and shell and structures related to them by connections are described. Multiple inputs, outputs and abundant reciprocity of connections within the ventral parts of the basal ganglia are emphasized and the implications for "through-put" of impulses is considered. It is noted, at least on neuroanatomical grounds, that there is ample reason to expect feed forward processing from shell and structures with which it is associated to core and structures with which it is associated. Furthermore, the potential for additional feed forward processing involving several forebrain functional anatomical systems, inlcuding the ventral striatopallidum, extended amygdala and magnocellular basal forebrain complex is considered. It is intended that from the considerations recorded here a conceptual framework will begin to emerge that is amenable to further experimental substantiation as regards how multiple basal forebrain systems and the cortices to which they are related by connections work together to fashion a unitary object--the adaptive response.

Neonatal ventral hippocampal lesions attenuate the nucleus accumbens dopamine response to stress: an electrochemical study in the adult rat

Neonatal damage to the ventral hippocampus (VH) can lead, during adulthood, to behaviours that are believed to reflect enhanced mesocorticolimbic dopamine (DA) transmission. In the present study, the effects of neonatal excitotoxic lesions to the VH on spontaneous locomotor activity and stress-elicited increases in extracellular nucleus accumbens (NAcc) DA levels were examined in adult rats. Male pups received, on postnatal day 7, bilateral injections of either an ibotenic acid solution (lesioned) or vehicle (sham-lesioned) into the VH. At 3-4 months of age, animals were assessed during five daily sessions for changes in spontaneous locomotor activity associated with habituation to a novel environment. Voltammetry was used in separate groups of sham- and VH-lesioned animals to monitor the NAcc DA response to each of five once-daily exposures to tail-pinch stress. The results indicate that while VH-lesioned animals seem to habituate to novelty, they remain hyperactive relative to sham-lesioned controls. In contrast, however, stress consistently elicited in VH-lesioned animals smaller and shorter-lasting increases in NAcc DA than in sham-lesioned controls. These data suggest that neonatal excitotoxic damage to VH leads to changes in DA function that persist into adulthood. The blunted response to stress seen in VH-lesioned animals indicates that one consequence of such damage is a functional hyporeactivity in meso-NAcc DA neurons. The fact that these animals are spontaneously more active suggests compensatory changes in DA function that are efferent to DA terminals in NAcc.

Yohimbine interacts with the dopaminergic system to reverse sexual satiation: further evidence for a role of sexual motivation in sexual exhaustion

The possible interaction of yohimbine with the dopaminergic system in the mediation of sexual behaviour expression in sexually exhausted male rats was investigated. The behavioural effects of the simultaneous injection of yohimbine (500 microg/kg) plus apomorphine (50 microg/kg) and those of the combined treatment of haloperidol (125 microg), a nonspecific dopamine receptor antagonist, with an effective dose of yohimbine (2000 microg/kg) on sexually satiated rats were evaluated. Data show that yohimbine and apomorphine, per se, dose-dependently reverse sexual exhaustion by increasing the percentage of sexually satiated rats copulating and resuming copulation after ejaculation. Injection of haloperidol simultaneous to an effective dose of yohimbine, blocked the ability of the latter to reverse sexual satiation. The combined treatment with subthreshold doses of apomorphine and yohimbine synergised to reverse the sexual inhibition characteristic of sexual exhaustion. Data suggest that the dopaminergic system might be the final pathway for the yohimbine-induced sexual behaviour expression in satiated rats. The possible role of sexual motivation in the sexual exhaustion phenomenon is discussed.

Influence of perinatal factors on the nucleus accumbens dopamine response to repeated stress during adulthood: an electrochemical study in the rat

Evidence from animal studies suggests that a period of anoxia to the fetus, a consequence common to many birth complications, results in long-term alterations in ventral mesencephalic dopamine function. Long-term functional changes in these dopamine neurons, in particular those that innervate the nucleus accumbens, also occur when animals are repeatedly stressed. In the present study, we examined the possibility that a period of anoxia during a Cesarean section birth can later alter the development of stress-induced sensitization of dopamine transmission in the nucleus accumbens. Dams were decapitated on the last day of gestation and the entire uterus was removed by Cesarean section. Pups were then delivered either immediately (Cesarean section group) or were immersed in a 37 degrees C saline bath for 3.5 or 13.5 min (Cesarean section+anoxia groups) before delivery of the pups. A fourth group of pups that were born vaginally served as controls (Vaginal group). Three to four months postnatally, animals from each group were implanted with monoamine-selective carbon-fiber electrodes into the nucleus accumbens. Voltammetry was used to monitor the dopamine response to each of five consecutive, once daily, 15-min exposures to tail-pinch stress. The results show that the first exposure to stress elicited dopamine signal increases of comparable amplitudes and durations in all animals. However, when compared to the initial stress response, the fourth and fifth exposures to tail-pinch elicited significantly longer-lasting dopamine responses in animals born by Cesarean section, either with or without added anoxia. In contrast, there was no significant day-to-day enhancement of the stress response in control, vaginally born animals. The findings reported here provide experimental support for the idea that birth complications may contribute to the pathophysiology of psychiatric disorders, in particular those that involve central dopamine dysfunction, such as schizophrenia. Specifically, our results suggest that subtle alterations in birth procedure may be sufficient to increase the sensitivity of mesolimbic dopamine neurons to the effects of repeated stress in the adult animal.

Comparative sexual side effects of bupropion, fluoxetine, paroxetine, and sertraline

OBJECTIVE

To investigate patient reported prosexual side effects of the aminoketone antidepressant bupropion (INN, amfebutamone) and to compare directly the sexual side effects of bupropion and the selective serotonin reuptake inhibitor (SSRI) antidepressants fluoxetine, paroxetine, and sertraline.

METHODS

One hundred seven psychiatric outpatient respondents receiving current treatment with one of the above antidepressants anonymously completed questionnaires that allowed reporting of both decreases and increases in sexual function. The main outcome measures were antidepressant-associated changes in libido, arousal, duration of time from arousal to orgasm, intensity of orgasm, and duration of orgasm relative to that experienced before the onset of the patients' psychiatric illnesses.

RESULTS

Bupropion-treated patients reported significant increases in libido, level of arousal, intensity of orgasm, and duration of orgasm beyond levels experienced premorbidly. The three SSRIs to an equal degree significantly decreased libido, arousal, duration of orgasm, and intensity of orgasm below levels experienced premorbidly. Overall, 27% of the SSRI-treated patients had no adverse sexual side effects; in contrast, 86% of patients treated with bupropion had no adverse sexual effects, and 77% of bupropion-treated patients reported at least one aspect of heightened sexual functioning.

CONCLUSIONS

SSRI-induced adverse sexual effects appear to be the rule rather than the exception and may be substantially underreported unless patients are specifically asked about the effects of these medications on various aspects of sexual function. In contrast, prosexual effects were reported by the majority of patients treated with bupropion. The findings are reviewed in light of the neurochemistry of these agents and the sexual response.

Sexually arousing events and relapse to heroin-seeking in sexually experienced male rats

We have shown previously, using a reinstatement procedure, that both priming injections of heroin and exposure to footshock stress reinstate heroin-taking behavior following prolonged drug-free periods. In the present study, we examined the effect of another highly arousing event, exposure to a sexually active female, on reinstatement of heroin-seeking. Male rats were first given sexual experience, being allowed to copulate on 4 occasions with sexually active females and were then trained to self-administer heroin (100 micrograms/kg per infusion, IV) for 4 3-h sessions/day for 5-6 days and 1 6-h session/day for an additional 6 days. Extinction sessions were then given for 4 days, 6-h/day, during which saline was substituted for heroin. On tests for reinstatement, males were presented with: 1. The wire-mesh side of an empty cage (baseline condition), 2. the sight, odor, and smell of a sexually inactive female, 3. the sight, odor, and smell of a female in heat, 4. a female in heat, and allowed to copulate, 5. intermittent footshock (15 min, 0.5 mA, 0.5 s on, mean off period of 40 s), or 6. a priming injection of heroin (0.25 mg/kg, SC). Reinstatement of heroin-taking behavior was observed after exposure to the priming injection of heroin and to footshock stress. Reinstatement of heroin-taking behavior was not induced by exposure to the females in any of the conditions. Thus, motivational arousal, as such, does not appear to be a sufficient stimulus for relapse to heroin-taking.

Medial prefrontal cortical D1 receptor modulation of the meso-accumbens dopamine response to stress: an electrochemical study in freely-behaving rats

Voltammetry was used to study the role of prefrontal cortex (PFC) dopamine (DA) in modulating the nucleus accumbens (NAcc) DA response to stress. Signal increases elicited in NAcc by 15 min of restraint were monitored in freely-behaving rats following intra-PFC microinjections of D1 and D2 receptor-selective drugs. The exact site of injection was first determined by assessing the electrochemical response to stress at two dorsal-ventral levels of PFC. Consistent with previous reports, a pronounced stress response was observed ventrally at sites within the infralimbic PFC but not dorsally within the superficial layers of PFC. When microinjected into the infralimbic PFC, the D1 receptor antagonist SCH 23390 significantly enhanced the NAcc stress response. While the D1 receptor agonist SKF 38393 tended to decrease the NAcc stress response, it failed to do so reliably. Neither sulpiride (D2 receptor antagonist) nor quinpirole (D2 receptor agonist) had a significant effect. Finally, systemic administration of the selective DA uptake inhibitor GBR 12909 dose-dependently potentiated stress-induced signal increases in NAcc and in PFC, indicating that the electrochemical responses to stress in both regions were due primarily to increases in extracellular DA levels. Together, these data add to other evidence indicating that the PFC exerts an inhibitory influence on subcortical DA transmission. Specifically, the present results suggest that the NAcc DA response to stress is dampened by the concurrent activation of meso-PFC DA neurons and that this action is mediated, at least in part, by D1 receptors in PFC.

Neurobehavioural mechanisms of reward and motivation

The analysis of the behavioural and neural mechanisms of reinforcement and motivation has benefited from the recent application of learning theory and better anatomical knowledge of the connectivity of certain key neural structures, such as the nucleus accumbens. This progress has enabled the dissection of motivational processes into components that can begin to be related to the functioning of specific limbic cortical structures that project to different compartments of the ventral striatum.