Subcortical multiple unit activity changes during rat male sexual behavior

The endogenous cannabinoid system modulates male sexual behavior expression

The endocannabinoid system (ECS) plays a key neuromodulatory role in the brain. Main features of endocannabinoids (eCBs) are that they are produced on demand, in response to enhanced neuronal activity, act as retrograde messengers, and participate in the induction of brain plasticity processes. Sexual activity is a motivated behavior and therefore, the mesolimbic dopaminergic system (MSL) plays a central role in the control of its appetitive component (drive to engage in copulation). In turn, copulation activates mesolimbic dopamine neurons and repeated copulation produces the continuous activation of the MSL system. Sustained sexual activity leads to the achievement of sexual satiety, which main outcome is the transient transformation of sexually active male rats into sexually inhibited animals. Thus, 24 h after copulation to satiety, the sexually satiated males exhibit a decreased sexual motivation and do not respond to the presence of a sexually receptive female with sexual activity. Interestingly, blockade of cannabinoid receptor 1 (CB1R) during the copulation to satiety process, interferes with both the appearance of the long-lasting sexual inhibition and the decrease in sexual motivation in the sexually satiated males. This effect is reproduced when blocking CB1R at the ventral tegmental area evidencing the involvement of MSL eCBs in the induction of this sexual inhibitory state. Here we review the available evidence regarding the effects of cannabinoids, including exogenously administered eCBs, on male rodent sexual behavior of both sexually competent animals and rat sub populations spontaneously showing copulatory deficits, considered useful to model some human male sexual dysfunctions. We also include the effects of cannabis preparations on human male sexual activity. Finally, we review the role played by the ECS in the control of male sexual behavior expression with the aid of the sexual satiety phenomenon. Sexual satiety appears as a suitable model for the study of the relationship between eCB signaling, MSL synaptic plasticity and the modulation of male sexual motivation under physiological conditions that might be useful for the understanding of MSL functioning, eCB-mediated plasticity and their relationship with motivational processes.

Crosstalk between the endocannabinoid and mid-brain dopaminergic systems: Implication in dopamine dysregulation

Endocannabinoids (eCBs) and the expanded endocannabinoid system (ECS)-"endocannabinoidome", consists of the endogenous ligands, eCBs, their canonical and non-canonical receptor subtypes, and their synthesizing and metabolizing enzymes. This system modulates a wide range of body functions and acts as a retrograde signaling system within the central nervous system (CNS) by inhibition of classical transmitters, and plays a vital modulatory function on dopamine, a major neurotransmitter in the CNS. Dopamine is involved in different behavioral processes and contributes to different brain disorders-including Parkinson's disease, schizophrenia, and drug addiction. After synthesis in the neuronal cytosol, dopamine is packaged into synaptic vesicles until released by extracellular signals. Calcium dependent neuronal activation results in the vesicular release of dopamine and interacts with different neurotransmitter systems. The ECS, among others, is involved in the regulation of dopamine release and the interaction occurs either through direct or indirect mechanisms. The cross-talk between the ECS and the dopaminergic system has important influence in various dopamine-related neurobiological and pathologic conditions and investigating this interaction might help identify therapeutic targets and options in disorders of the CNS associated with dopamine dysregulation.

Pentylenetetrazol-induced seizures are followed by a reduction in the multiunitary activity of hippocampal CA1 pyramidal neurons in adult rats

Pentylenetetrazol (PTZ) blocks the inhibitory action of GABA, triggering a Glu-mediated hyperexcitation of the dendritic spines in hippocampal CA1 pyramidal neurons that leads to the generation of epileptiform seizures. The aim of this work was to determine the effect of PTZ on the electrical activity of the hippocampal pyramidal neurons in male rats. Bipolar electrodes were implanted stereotaxically in the right and left hippocampal CA1 fields of adults, and PTZ (65 mg/kg) was administered i.p. Simultaneous recordings of the field activity and the firing rate (multiunitary activity, MUA) were analyzed at 10, 20, and 30 min post-administration of PTZ. Only rats that presented tonic-clonic seizures during the first 1-5 min after PTZ treatment were included in the study. The recordings of the field activity were analyzed in 4 frequency bands. In both the right and left hippocampal CA1 fields, the relative power corresponding to the slow waves (4-7 Hz) increased, while in the bands 13-30 Hz and 31-50 Hz, it decreased at 10, 20, and 30 min post-PTZ. MUA recordings were analyzed at four levels. The highest levels corresponded to larger amplitudes of the action potentials in the pyramidal neurons. The firing rates of the PTZ-treated rats did not differ from baseline but presented a significant decrement at 10, 20, and 30 min post-PTZ. The decreased firing rate of the hippocampal CA1 pyramidal neurons after PTZ treatment could be associated with plastic changes of dendritic spines along with some microenvironmental adaptations at synaptic level, after neuronal PTZ-mediated hyperexcitation.

The nucleus accumbens dopamine increase, typically triggered by sexual stimuli in male rats, is no longer produced when animals are sexually inhibited due to sexual satiety

RATIONALE

Exposure of male rats to an inaccessible receptive female and copulation increases dopamine (DA) levels in the nucleus accumbens (NAcc). Males copulating to satiety become sexually inhibited and most of them do not display sexual activity when presented with a sexually receptive female 24 h later. This inhibitory state can be pharmacologically reversed. There are no studies exploring NAcc DA levels during this sexual inhibitory state.

OBJECTIVES

To characterize changes in NAcc DA and its metabolites' levels during sexual satiety development, during the well-established sexual inhibitory state 24 h later, and during its pharmacological reversal.

METHODS

Changes in NAcc DA and its metabolites were measured in sexually experienced male rats, using in vivo microdialysis, during copulation to satiety, when presented to a new sexually receptive female 24 h later, and during the pharmacological reversal of the sexual inhibition by anandamide.

RESULTS

NAcc DA levels remained increased during copulation to satiety. DA basal levels were significantly reduced 24 h after copulation to satiety, as compared to the initial basal levels. Presenting a receptive female behind a barrier 24 h after satiety did not induce the typical NAcc DA elevation in the sexually satiated males but there was a decrease that persisted when they got access to the female, with which they did not copulate. Anandamide injection slightly increased NAcc DA levels coinciding with sexual satiety reversal.

CONCLUSIONS

Reduced NAcc DA concentrations coincide with the inhibition of an instinctive, natural rewarding behavior suggesting that there might be a DA concentration threshold needed to be responsive to a rewarding stimulus.

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.

The glucagon-like peptide-1 receptor agonist, exendin-4, reduces sexual interaction behaviors in a brain site-specific manner in sexually naïve male mice

Besides reducing food intake and controlling energy balance, glucagon-like peptide-1 (GLP-1) suppresses the reinforcing properties of palatable foods and addictive drugs. This reduction in reward involves activation of GLP-1 receptors (GLP-1R) within areas processing natural and artificial rewards, including the laterodorsal tegmental area (LDTg), ventral tegmental area (VTA) and nucleus accumbens (NAc) shell. These areas are part of a neurocircuitry mediating reward from addictive drugs and natural rewards including sexual behaviors. The male sexual encounter with a female includes three different stages: a pre-sexual interaction phase with social behaviors, which is followed by a sexual interaction phase with mounting and intromission of the female, and ends with a post-sexual interaction phase characterized by self-grooming behaviors. Albeit GLP-1 modulates reward, the influence of GLP-1R activation on sexual interaction is unknown. Thus, we infused the GLP-1R agonist, exendin-4 (Ex4), into sub-regions of the reward neurocircuitry in sexually naïve male mice and recorded their novel interaction with an estrus female. We found that Ex4 into the LDTg, posterior VTA or NAc shell reduces pre-sexual interaction behaviors and activation of GLP-1R in the LDTg or posterior VTA decreases sexual interaction behaviors. Contrarily, Ex4 infusion into anterior VTA does not influence these behaviors. Furthermore, self-grooming behaviors are not influenced by activation of GLP-1R in the aforementioned areas. These data highlight that activation of GLP-1R in reward-related areas reduces different aspects of the sexual interaction chain and further supports a role of the GLP-1R in social behaviors.

The role of ghrelin signalling for sexual behaviour in male mice

Ghrelin, a gut-brain signal, is well known to regulate energy homeostasis, food intake and appetite foremost via hypothalamic ghrelin receptors (GHS-R1A). In addition, ghrelin activates the reward systems in the brain, namely the mesolimbic dopamine system, and regulates thereby the rewarding properties of addictive drugs as well as of palatable foods. Given that the mesolimbic dopamine system mandates the reinforcing properties of addictive drugs and natural rewards, such as sexual behaviour, we hypothesize that ghrelin plays an important role for male sexual behaviour, a subject for the present studies. Herein we show that ghrelin treatment increases, whereas pharmacological suppression (using the GHSR-1A antagonist JMV2959) or genetic deletion of the GHS-R1A in male mice decreases the sexual motivation for as well as sexual behaviour with female mice in oestrus. Pre-treatment with L-dopa (a dopamine precursor) prior to treatment with JMV2959 significantly increased the preference for female mouse compared with vehicle treatment. On the contrary, treatment with 5-hydroxythyptohan (a precursor for serotonin) prior to treatment with JMV2959 decreased the sexual motivation compared to vehicle. In separate experiments, we show that ghrelin and GHS-R1A antagonism do not affect the time spent over female bedding as measured in the androgen-dependent bedding test. Collectively, these data show that the hunger hormone ghrelin and its receptor are required for normal sexual behaviour in male mice and that the effects of the ghrelin signalling system on sexual behaviour involve dopamine neurotransmission.

Ghrelin and GHS-R1A signaling within the ventral and laterodorsal tegmental area regulate sexual behavior in sexually naïve male mice

In addition to food intake and energy balance regulation, ghrelin mediate the rewarding and motivational properties of palatable food as well as addictive drugs. The ability of ghrelin to regulate reinforcement involves the cholinergic-dopaminergic reward link, which encompasses a cholinergic projection from the laterodorsal tegmental area (LDTg) to the ventral tegmental area (VTA) together with mesolimbic dopaminergic projections from the VTA to the nucleus accumbens (NAc). Recently, systemic ghrelin was shown to regulate sexual behavior and motivation in male mice via dopamine neurotransmission. The present study therefore elucidates the role of ghrelin and ghrelin receptor (GHS-R1A) antagonist treatment within NAc, VTA or LDTg for sexual behavior in sexually naïve male mice. Local administration of the GHSR-1A antagonist, JMV2959, into the VTA or LDTg was found to reduce the preference for female mice, the number of mounts and the duration of mounting as well as to prolong the latency to mount. This was further substantiated by the findings that ghrelin administration into the VTA or LDTg increased the number of mounts and the duration of mounting and decreased the latency to mount. Moreover, ghrelin administered into the LDTg increased the preference for female mice. Accumbal administration of ghrelin increased whereas GHS-R1A antagonist decreased the intake of palatable food, but did not alter sexual behavior. In males exposed to sexual interaction, systemic administration of ghrelin increases whereas JMV2959 decreases the turnover of dopamine in the VTA. These data suggest that ghrelin signaling within the tegmental areas is required for sexual behavior in sexually naïve male mice.

Low anandamide doses facilitate male rat sexual behaviour through the activation of CB1 receptors

RATIONALE

Endocannabinoids (eCN) exert biphasic effects on several behaviours; however, they have only been reported to inhibit male sexual behaviour. eCN, the endogenous ligands for CB1 receptors, are released in response to neuronal stimulation and regulate the functioning of the mesocorticolimbic system (MCL), which is activated by male sexual behaviour. We hypothesised that eCN might exert biphasic effects on male rat copulatory behaviour and be released during copulation to satiation as a result of the repeated activation of the MCL system.

OBJECTIVES

The study was conducted to determine if the eCN anandamide exerts biphasic effects on sexual behaviour expression of sexually experienced and sexually satiated male rats and to establish the possible participation of eCN in the sexual satiation phenomenon.

METHODS

The eCN anandamide and the CB1 receptor antagonist AM251 were systemically administered to sexually experienced or sexually satiated rats and their effects on copulatory behaviour analysed.

RESULTS

Low anandamide doses facilitated sexual behaviour expression in sexually experienced and in sexually satiated rats by acting at CB1 receptors. AM251 blocked the establishment of the sexual inhibition that characterises sexual satiation, but did not reverse it once established.

CONCLUSIONS

Anandamide exerts dose-based biphasic effects on copulatory behaviour of sexually experienced male rats and facilitates sexual behaviour expression of sexually satiated animals at low doses. eCN participate in the establishment, but not in the maintenance of the sexual inhibitory state that characterises the sexual satiation phenomenon.

A forebrain-retrorubral pathway involved in male sex behavior is GABAergic and activated with mating in gerbils

The ventral bed nuclei of the stria terminalis (BST) and medial preoptic nucleus (MPN) of gerbils contain cells that regulate male sex behavior via a largely uncrossed pathway to the retrorubral field (RRF). Our goal was to learn more about cells at the pathway source and target. To determine if the pathway uses GABA as its transmitter, we used immunocytochemistry (ICC) to study glutamic acid decarboxlyase(67) (GAD(67)) colocalization with fluoro-gold (FG) in the ventral BST and MPN after applying FG to the RRF. To determine if the pathway is activated with mating, we studied FG-Fos colocalization in the ventral BST of recently mated males. The ventral BST expresses Fos with mating and is the major pathway source. To determine to what extent other GABAergic cells in the ventral BST are activated with mating, we studied Fos colocalization with GAD(67) mRNA visualized by in situ hybridization (ISH). We also looked for GAD(67) mRNA in RRF cells. Almost all ventral BST and MPNm cells projecting to the RRF (95-97%) and most ventral BST cells activated with mating (89%), were GABAergic. GABAergic cells were also seen in the RRF. RRF-projecting cells represented 37% of ventral BST cells activated with mating. Their activation may reflect arousal and anticipation of sexual reward. Among ventral BST cells that project to the RRF, 14% were activated with mating, consistent with how much of this pathway is needed for mating. The activated GABAergic cells that do not project to the RRF may release GABA locally and inhibit ejaculation.

Multiunit recording of the cerebellar cortex, inferior olive, and fastigial nucleus during copulation in naive and sexually experienced male rats

The sexual behavior of male rats constitutes a natural model to study learning of motor skills at the level of the central nervous system. We previously showed that sexual behavior increases Fos expression in granule cells at lobules 6 to 9 of the vermis cerebellum. Herein, we obtained multiunit recordings of lobules 6a and 7 during the training period of naive subjects, and during consecutive ejaculations of expert males. Recordings from both lobules and the inferior olive showed that the maximum amplitude of mount, intromission, and ejaculation signals were similar, but sexual behavior during training tests produced a decrease in the amplitude for mount and intromission signals. The fastigial nucleus showed an inverse mirror-like response. Thus, the cerebellum is involved in the neural basis of sexual behavior and the learning of appropriate behavioral displays during copulation, with a wiring that involves the cerebellar cortex, inferior olive, and fastigial nucleus.

Lesions of the medial prefrontal cortex cause maladaptive sexual behavior in male rats

BACKGROUND

An inability to inhibit behaviors once they become maladaptive is a component of several psychiatric illnesses, and the medial prefrontal cortex (mPFC) was identified as a potential mediator of behavioral inhibition. The current study tested if the mPFC is involved in inhibition of sexual behavior when associated with aversive outcomes.

METHODS

Using male rats, effects of lesions of the infralimbic and prelimbic areas of the mPFC on expression of sexual behavior and ability to inhibit mating were tested using a paradigm of copulation-contingent aversion.

RESULTS

Medial prefrontal cortex lesions did not alter expression of sexual behavior. In contrast, mPFC lesions completely blocked the acquisition of sex-aversion conditioning and lesioned animals continued to mate, in contrast to the robust behavioral inhibition toward copulation in mPFC intact male animals, resulting in only 22% of intact male animals continuing to mate. However, rats with mPFC lesions were capable of forming a conditioned place preference to sexual reward and conditioned place aversion for lithium chloride, suggesting that these lesions did not alter associative learning or sensitivity for lithium chloride.

CONCLUSIONS

The current study indicates that animals with mPFC lesions are likely capable of forming the associations with aversive outcomes of their behavior but lack the ability to suppress seeking of sexual reward in the face of aversive consequences. These data may contribute to a better understanding of a common pathology underlying impulse control disorders, as compulsive sexual behavior has a high prevalence of comorbidity with psychiatric disorders and Parkinson's disease.

Anatomy and neurochemistry of the pair bond

Studies in monogamous rodents have begun to elucidate the neural circuitry underlying the formation and maintenance of selective pair bonds between mates. This research suggests that at least three distinct, yet interconnected, neural pathways interact in the establishment of the pair bond. These include circuits involved in conveying somatosensory information from the genitalia to the brain during sexual activity, the mesolimbic dopamine circuits of reward and reinforcement, and neuropeptidergic circuits involved specifically in the processing of socially salient cues. Here we present an integrated description of the interaction of these circuits in a model of pair bond formation in rodents with a discussion of the implications of these findings for evolution, individual variation, and human bonding.

Has dopamine a physiological role in the control of sexual behavior? A critical review of the evidence

The role of dopaminergic systems in the control of sexual behavior has been a subject of study for at least 40 years. Not surprisingly, reviews of the area have been published at variable intervals. However, the earlier reviews have been summaries of published research rather than a critical analysis of it. They have focused upon the conclusions presented in the original research papers rather than on evaluating the reliability and functional significance of the data reported to support these conclusions. During the last few years, important new knowledge concerning dopaminergic systems and their behavioral functions as well as the possible role of these systems in sexual behavior has been obtained. For the first time, it is now possible to integrate the data obtained in studies of sexual behavior into the wider context of general dopaminergic functions. To make this possible, we first present an analysis of the nature and organization of sexual behavior followed by a summary of current knowledge about the brain structures of crucial importance for this behavior. We then proceed with a description of the dopaminergic systems within or projecting to these structures. Whenever possible, we also try to include data on the electrophysiological actions of dopamine. Thereafter, we proceed with analyses of pharmacological data and release studies, both in males and in females. Consistently throughout this discussion, we make an effort to distinguish pharmacological effects on sexual behavior from a possible physiological role of dopamine. By pharmacological effects, we mean here drug-induced alterations in behavior that are not the result of the normal actions of synaptically released dopamine in the untreated animal. The conclusion of this endeavor is that pharmacological effects of dopaminergic drugs are variable in both males and females, independently of whether the drugs are administered systemically or intracerebrally. We conclude that the pharmacological data basically reinforce the notion that dopamine is important for motor functions and general arousal. These actions could, in fact, explain most of the effects seen on sexual behavior. Studies of dopamine release, in both males and females, have focused on the nucleus accumbens, a structure with at most a marginal importance for sexual behavior. Since accumbens dopamine release is associated with all kinds of events, aversive as well as appetitive, it can have no specific effect on sexual behavior but promotes arousal and activation of non-specific motor patterns. Preoptic and paraventricular nucleus release of dopamine may have some relationship to mechanisms of ejaculation or to the neuroendocrine consequences of sexual activity or they can be related to other autonomic processes associated with copulation. There is no compelling indication in existing experimental data that dopamine is of any particular importance for sexual motivation. There is experimental evidence showing that it is of no importance for sexual reward.

Sexual motivation--an inquiry into events determining the occurrence of sexual behavior

For the individual engaged in it, sexual behavior has no finality or purpose other than its own execution. Data are presented showing that the execution of sexual reflexes can promote learning, i.e. it functions as reinforcement. Furthermore, positive affect is generated. Based on these principles, a model of sexual motivation has been elaborated. The conceptual framework is the incentive motivation theory previously proposed by Bindra D, A motivational view of learning, performance, and behavior modification, Psychol Rev 1974: 81:199-213; A Theory of Intelligent Behavior, New York: Wiley, 1976; How adaptive behavior is produced: a perceptual-motivational alternative to response reinforcement, Behav Brain Sci 1978; 1:41-52. Although the model is intended for application to most mammals, the rat is used as example. Essentially, sexual approach behaviors are activated by appropriate incentives (conditioned in the male, unconditioned in the female). Approach is, in the inexperienced male, followed by the execution of copulatory reflexes as a consequence of accidentally obtained tactile stimulation of the perineal region. In the female, copulatory acts are activated by tactile stimulation of the flanks and hinds provided by the mounting male. The role of conditioning for the execution of copulatory reflexes and for the acquisition of incentive value of neutral stimuli is analyzed. It is also shown that the incentive properties of sexual acts are not substantially different from those of other incentives. Sexual exhaustion is suggested to be either a case of negative alliesthesia or of stimulus habituation and the Coolidge effect is, in consequence, an example of dishabituation. Studies in women and men support this proposal. It is emphasized that sexual behavior is best understood as being entirely mechanistic albeit not deterministic.