Conditioned place preference produced by infusion of Met-enkephalin into the medial preoptic area

Mu opioid receptor stimulation in the medial preoptic area or nucleus accumbens facilitates song and reward in flocking European starlings

It has been proposed that social cohesion in gregarious animals is reinforced both by a positive affective state induced by social interactions and by the prevention of a negative state that would be caused by social separation. Opioids that bind to mu opioid receptors (MORs) act in numerous brain regions to induce positive and to reduce negative affective states. Here we explored a potential role for MORs in affective states that may impact flocking behavior in mixed-sex flocks of nonbreeding European starlings, Sturnus vulgaris. Singing behavior, which is considered central to flock cohesion, and other social behaviors were quantified after infusions of the MOR agonist D-Ala2, N-Me-Phe4, glycinol5-ENK (DAMGO) into either the medial preoptic area (POM) or the nucleus accumbens (NAC), regions previously implicated in affective state and flock cohesion. We focused on beak wiping, a potential sign of stress or redirected aggression in this species, to provide insight into a presumed negative state. We also used conditioned place preference (CPP) tests to provide insight into the extent to which infusions of DAMGO into POM or NAC that stimulated song might be rewarding. We found that MOR stimulation in either POM or NAC dose-dependently promoted singing behavior, reduced beak wiping, and induced a CPP. Subtle differences in responses to MOR stimulation between NAC and POM also suggest potential functional differences in the roles of these two regions. Finally, because the location of NAC has only recently been identified in songbirds, we additionally performed a tract tracing study that confirmed the presence of dopaminergic projections from the ventral tegmental area to NAC, suggesting homology with mammalian NAC. These findings support the possibility that MORs in POM and NAC play a dual role in reinforcing social cohesion in flocks by facilitating positive and reducing negative affective states.

Behavioral, Neural, and Molecular Mechanisms of Conditioned Mate Preference: The Role of Opioids and First Experiences of Sexual Reward

Although mechanisms of mate preference are thought to be relatively hard-wired, experience with appetitive and consummatory sexual reward has been shown to condition preferences for partner related cues and even objects that predict sexual reward. Here, we reviewed evidence from laboratory species and humans on sexually conditioned place, partner, and ejaculatory preferences in males and females, as well as the neurochemical, molecular, and epigenetic mechanisms putatively responsible. From a comprehensive review of the available data, we concluded that opioid transmission at μ opioid receptors forms the basis of sexual pleasure and reward, which then sensitizes dopamine, oxytocin, and vasopressin systems responsible for attention, arousal, and bonding, leading to cortical activation that creates awareness of attraction and desire. First experiences with sexual reward states follow a pattern of sexual imprinting, during which partner- and/or object-related cues become crystallized by conditioning into idiosyncratic “types” that are found sexually attractive and arousing. These mechanisms tie reward and reproduction together, blending proximate and ultimate causality in the maintenance of variability within a species.

Using seasonality and birdsong to understand mechanisms underlying context-appropriate shifts in social motivation and reward

Social motivation and reward are dynamic and flexible, shifting adaptively across contexts to meet changing social demands. This is exceptionally apparent when seasonal contexts are considered in seasonally breeding songbirds as they cycle from periods of sexual motivation and reward during the breeding season to periods of extreme gregariousness outside the breeding season when non-sexual social interactions gain reward value, motivating birds to form flocks. Here we review evidence demonstrating a key integrative role for the medial preoptic area (mPOA) in the seasonally-appropriate adjustment of behaviors, with seasonal changes in dopamine activity in mPOA adjusting social motivation and changes in opioid activity modifying social reward. Experiments demonstrate that dramatic seasonal fluctuations in steroid hormone concentrations alter patterns of opioid- and dopamine-related protein and gene expression in mPOA to modify social motivation and reward to meet seasonal changes in social demands. These studies of birdsong and seasonality provide new insights into neural and endocrine mechanisms underlying adaptive changes in social motivation and reward and highlight an underappreciated, evolutionarily conserved role for the mPOA in important social behaviors in non-reproductive contexts.

Endogenous opioids facilitate intrinsically-rewarded birdsong

Many songbirds sing in non-reproductive contexts while in flocks. Singing in such gregarious contexts is critical for maintaining and learning songs; however, song is not directed towards other individuals and has no obvious, immediate social consequences. Studies using conditioned place preference (CPP) tests of reward indicate that song production in gregarious contexts correlates positively with a bird’s intrinsic reward state and with opioid markers in the medial preoptic nucleus (mPOA). However, the causal involvement of opioids in gregarious song is unknown. Here we report that the selective mu opioid receptor (MOR) agonist fentanyl dose-dependently facilitates gregarious song and reduces stress/anxiety-related behavior in male and female European starlings. Furthermore, infusion of siRNA targeting MORs specifically in mPOA both suppresses gregarious song and disrupts the positive association between affective state and singing behavior, as revealed using CPP tests of song-associated reward. Results strongly implicate opioids in gregarious song and suggest that endogenous opioids in the mPOA may facilitate song by influencing an individual’s intrinsic reward state.

Mu opioid receptors in the medial preoptic area govern social play behavior in adolescent male rats

Neural systems underlying important behaviors are usually highly conserved across species. The medial preoptic area (MPOA) has been demonstrated to play a crucial role in reward associated with affiliative, nonsexual, social communication in songbirds. However, the role of MPOA in affiliative, rewarding social behaviors (eg, social play behavior) in mammals remains largely unknown. Here we applied our insights from songbirds to rats to determine whether opioids in the MPOA govern social play behavior in rats. Using an immediate early gene (ie, Egr1, early growth response 1) expression approach, we identified increased numbers of Egr1-labeled cells in the MPOA after social play in adolescent male rats. We also demonstrated that cells expressing mu opioid receptors (MORs, gene name Oprm1) in the MPOA displayed increased Egr1 expression when adolescent rats were engaged in social play using double immunofluorescence labeling of MOR and Egr1. Furthermore, using short hairpin RNA-mediated gene silencing we revealed that knockdown of Oprm1 in the MPOA reduced the number of total play bouts and the frequency of pouncing. Last, RNA sequencing differential gene expression analysis identified genes involved in neuronal signaling with altered expression after Oprm1 knockdown, and identified Egr1 as potentially a key modulator for Oprm1 in the regulation of social play behavior. Altogether, these results show that the MPOA is involved in social play behavior in adolescent male rats and support the hypothesis that the MPOA is part of a conserved neural circuit across vertebrates in which opioids act to govern affiliative, intrinsically rewarded social behaviors.

Why Do Birds Flock? A Role for Opioids in the Reinforcement of Gregarious Social Interactions

The formation of social groups provides safety and opportunities for individuals to develop and practice important social skills. However, joining a social group does not result in any form of obvious, immediate reinforcement (e.g., it does not result in immediate copulation or a food reward), and individuals often remain in social groups despite agonistic responses from conspecifics. Much is known about neural and endocrine mechanisms underlying the motivation to perform mate- or offspring-directed behaviors. In contrast, relatively little is known about mechanisms underlying affiliative behaviors outside of these primary reproductive contexts. Studies on flocking behavior in songbirds are beginning to fill this knowledge gap. Here we review behavioral evidence that supports the hypothesis that non-sexual affiliative, flocking behaviors are both (1) rewarded by positive social interactions with conspecifics, and (2) reinforced because affiliative contact reduces a negative affective state caused by social isolation. We provide evidence from studies in European starlings, Sturnus vulgaris, that mu opioid receptors in the medial preoptic nucleus (mPOA) play a central role in both reward and the reduction of a negative affective state induced by social interactions in flocks, and discuss potential roles for nonapeptide/opioid interactions and steroid hormones. Finally, we develop the case that non-sexual affiliative social behaviors may be modified by two complementary output pathways from mPOA, with a projection from mPOA to the periaqueductal gray integrating information during social interactions that reduces negative affect and a projection from mPOA to the ventral tegmental area integrating information leading to social approach and reward.

Oprm1 A112G, a single nucleotide polymorphism, alters expression of stress-responsive genes in multiple brain regions in male and female mice

BACKGROUND

OPRM1 A118G, a functional human mu-opioid receptor (MOR) polymorphism, is associated with drug dependence and altered stress responsivity in humans as well as altered MOR signaling. MOR signaling can regulate many cellular processes, including gene expression, and many of the long-term, stable effects of drugs and stress may stem from changes in gene expression in diverse brain regions. A mouse model bearing an equivalent polymorphism (Oprm1 A112G) was previously generated and studied. Mice homozygous for the G112 allele show differences in opioid- and stress-related phenotypes.

APPROACH

The current study examines the expression of 24 genes related to drug and stress responsivity in the caudoputamen, nucleus accumbens, hypothalamus, hippocampus, and amygdala of drug-naïve, stress-minimized, male and female mice homozygous for either the G112 variant allele or the wild-type A112 allele.

RESULTS

We detected nominal genotype-dependent changes in gene expression of multiple genes. We also detected nominal sex-dependent as well as sex-by-genotype interaction effects on gene expression. Of these, four genotype-dependent differences survived correction for multiple testing: Avp and Gal in the hypothalamus and Oprl1 and Cnr1 in the hippocampus.

CONCLUSIONS

Changes in the regulation of these genes by mu-opioid receptors encoded by the G112 allele may be involved in some of the behavioral and molecular consequences of this polymorphism observed in mice.

Song practice as a rewarding form of play in songbirds

In adult songbirds, the primary functions of song are mate attraction and territory defense; yet, many songbirds sing at high rates as juveniles and outside these primary contexts as adults. Singing outside primary contexts is critical for song learning and maintenance, and ultimately necessary for breeding success. However, this type of singing (i.e., song "practice") occurs even in the absence of immediate or obvious extrinsic reinforcement; that is, it does not attract mates or repel competitors. Here we review studies that support the hypothesis that song practice is stimulated and maintained by intrinsic reward mechanisms (i.e., that it is associated with a positive affective state). Additionally, we propose that song practice can be considered a rewarding form of play behavior similar to forms of play observed in multiple young animals as they practice sequences of motor events that are used later in primary adult reproductive contexts. This review highlights research suggesting at least partially overlapping roles for neural reward systems in birdsong and mammalian play and evidence that steroid hormones modify these systems to shift animals from periods of intrinsically rewarded motor exploration (i.e., singing in birds and play in mammals) to the use of similar motor patterns in primary reproductive contexts.

Associations Between Environmental Resources and the "Wanting" and "Liking" of Male Song in Female Songbirds

Reproductive success requires animals to adjust social and sexual behaviors in response to changes in environmental resources. In many species, males produce courtship signals to attract females; however, not all females are attracted by these signals. One possible explanation for this is that environmental resources alter neural mechanisms underlying motivation and reward in females so that male courtship is attractive when conditions are most favorable for an individual to breed. Here, we first introduce resource-dependent breeding behaviors of female songbirds. We then review studies that show associations between neural systems underlying motivation and reward, female responses to male courtship stimuli, and environmental resources necessary for breeding success (e.g., in female starlings, a nest cavity). Overall, we review evidence supporting the working hypotheses that (1) dopamine underlies sexually-motivated female responses to male courtship stimuli (i.e., song), (2) opioids underlie reward induced in females by hearing male courtship song, and (3) these systems are possibly modified by resources such that male courtship song is only attractive and rewarding to females with access to limited environmental resources essential for breeding success.

Song-associated reward correlates with endocannabinoid-related gene expression in male European starlings (Sturnus vulgaris)

Vocal communication is required for successful social interactions in numerous species. During the breeding season, songbirds produce songs that are reinforced by behavioral consequences (e.g., copulation). However, some songbirds also produce songs not obviously directed at other individuals. The consequences maintaining or reinforcing these songs are less obvious and the neural mechanisms associated with undirected communication are not well-understood. Previous studies indicate that undirected singing is intrinsically rewarding and mediated by opioid or dopaminergic systems; however, endocannabinoids are also involved in regulating reward and singing behavior. We used a conditioned place preference paradigm to examine song-associated reward in European starlings and quantitative real-time PCR to measure expression of endocannabinoid-related neural markers (CB₁, FABP7, FABP5, FAAH, DAGLα), in brain regions involved in social behavior, reward and motivation (ventral tegmental area [VTA], periaqueductal gray [PAG], and medial preoptic nucleus [POM]), and a song control region (Area X). Our results indicate that starlings producing high rates of song developed a conditioned place preference, suggesting that undirected song is associated with a positive affective state. We found a significant positive relationship between song-associated reward and CB₁ receptors in VTA and a significant negative relationship between song-associated reward and CB₁ in PAG. There was a significant positive relationship between reward and the cannabinoid transporter FABP7 in POM and a significant negative relationship between reward and FABP7 in PAG. In Area X, FABP5 and DAGLα correlated positively with singing. These results suggest a role for endocannabinoid signaling in vocal production and reward associated with undirected communication.

Reward associated with singing behavior correlates with opioid-related gene expression in the medial preoptic nucleus in male European starlings

Birdsong consists of species-specific learned vocal sequences that are used primarily to attract mates and to repel competitors during the breeding season. However, many birds continue to sing at times when vocal production has no immediate or obvious impact on conspecific behavior. The mechanisms that ensure that animals produce important behaviors in contexts in which the function of these behaviors is not immediate or obvious are not known. One possibility is that animals engage in such behaviors because they are associated with pleasure. Here we examined the hypothesis that male European starlings sing outside of the breeding season in part because the act of singing in this context is facilitated and/or maintained by opioid-mediated reward. We measured song-associated reward using a conditioned place preference (CPP) test in male starlings producing fall, non-breeding season-typical song. We used quantitative real time PCR to measure expression of the enkephalin opioid precursor preproenkephalin (PENK) and mu opioid receptors (MOR) in the medial preoptic nucleus (POM; a region in which opioids are implicated in both reward and starling fall song) and additionally the song control region HVC as a control. Starlings developed a strong preference for a place that had been paired previously with the act of producing fall-typical song, indicating that fall song production was associated with a positive affective state. Both PENK and MOR mRNA expression in the POM, but not HVC, correlated positively with both individual reward state (as reflected in CPP) and undirected singing behavior. These results suggest that singing induces opioid receptor and enkephalin expression in the POM and consequent reward, and/or that opioid release in the POM induced by individual or environmental factors (e.g., the presence of food, safety of a flock or the absence of predators) induces a positive affective state which then facilitates singing behavior.

Sexual performance and precontact 50-kHz ultrasonic vocalizations in WAG/Rij rats: effects of opioid receptor treatment

WAG/Rij rats are genetically selected animals that model absence epilepsy in rats. Ultrasonic vocalizations and sexual behavior - both ethologically relevant markers of reward system functioning - are poorly described in this strain. The aim of our experiment was to investigate reward-dependent precontact 50-kHz vocalizations (PVs) and copulatory behavior as well as the effects of opioid receptor treatment on such behaviors in sexually experienced WAG/Rij males and rats from two control strains: Sprague-Dawley and Crl: Han Wistar. We analyzed the effects of the opioid receptor antagonist naltrexone (3 mg/kg) and the agonist morphine (1 mg/kg) administration. Additionally, we analyzed the initiation of copulation in sexually naïve males before drug treatment. A significantly lower number of sexually naïve WAG/Rij rats initiated copulation. Sexually experienced WAG/Rij males differed at the control session (after physiological saline treatment) compared with Sprague-Dawley rats: WAG/Rij rats displayed more 50-kHz precontact vocalizations and had longer mount and intromission latencies, longer ejaculation latency, longer postejaculatory latency to exploration, longer 22-kHz vocalization duration after ejaculation, and longer postejaculatory intromission latency. Compared with Crl: Han Wistar rats, WAG/Rij males displayed longer mount latency and shorter 22-kHz vocalization duration. Neither naltrexone nor morphine affected PVs in all groups. On the other hand, opioid receptor treatment differently influenced the number of intromissions required to achieve ejaculation and 22-kHz postejaculatory vocalization duration in WAG/Rij rats than in both control groups. This suggests functional differences in the opioid system in this strain. As a result of the number of males that initiated copulation as well as the number of intromissions to ejaculation and 22-kHz postejaculatory vocalizations which all depend on D1 receptor activation, we suggest that the proportion of opioid receptor to D1 receptors in WAG/Rij rats is different when compared with the control strains. The reward system of Wag/Rij rats with absence epilepsy is sensitive to social rewards (high level of precontact 50-kHz ultrasounds) although this strain displays a lower level of sexual motivation (longer mount latency) compared with other control strains. A lower number of sexually naïve rats initiating copulation and longer mount latency in sexually experienced males could suggest a moderate depressive-like syndrome in this strain of rats.

Curvilinear relationships between mu-opioid receptor labeling and undirected song in male European starlings (Sturnus vulgaris)

Female-directed communication in male songbirds has been reasonably well studied; yet, relatively little is known about communication in other social contexts. Songbirds also produce song that is not clearly directed towards another individual (undirected song) when alone or in flocks. Although the precise functions of undirected song may differ across species, this type of song is considered important for flock maintenance, song learning or practice. Past studies show that undirected song is tightly coupled to analgesia and positive affective state, which are both mediated by opioid activity. Furthermore, labeling for the opioid met-enkephalin in the medial preoptic nucleus (POM) correlates positively with undirected song production. We propose that undirected song is facilitated and maintained by opioid receptor activity in the POM and other brain regions involved in affective state, analgesia, and social behavior. To provide insight into this hypothesis, we used immunohistochemistry to examine relationships between undirected song and mu-opioid receptors in male starlings. Polynomial regression analyses revealed significant inverted-U shaped relationships between measures of undirected song and mu-opioid receptor labeling in the POM, medial bed nucleus of the stria terminalis (BSTm), and periaqueductal gray (PAG). These results suggest that low rates of undirected song may stimulate and/or be maintained by mu-opioid receptor activity; however, it may be that sustained levels of mu-opioid receptor activity associated with high rates of undirected song cause mu-opioid receptor down-regulation. The results indicate that mu-opioid receptor activity in POM, BSTm, and PAG may underlie previous links identified between undirected song, analgesia, and affective state.

Links between breeding readiness, opioid immunolabeling, and the affective state induced by hearing male courtship song in female European starlings (Sturnus vulgaris)

Male courtship vocalizations represent a potent signal designed to attract females; however, not all females find male signals equally attractive. We explored the possibility that the affective state induced by hearing courtship vocalizations depends on the motivational state of a receiver. We used a conditioned place preference test of reward to determine the extent to which the rewarding properties of hearing male courtship song differed in female European starlings categorized as nest box owners (a sign of breeding readiness) or non-owners. Nest box owners developed a preference for a chamber in which they previously heard male courtship song. Non-owners displayed no preference for a chamber in which they previously heard song. Positive correlations were identified between the preference a female developed for the song-paired chamber and female nesting and dominance behaviors observed prior to conditioning (indices of the motivation to breed). Immunolabeling for met-enkephalin (an opioid neuropeptide involved in reward) in the medial preoptic nucleus, ventromedial nucleus of the hypothalamus, nucleus accumbens, and periaqueductal gray was higher in females with compared to those without nest boxes. Both nest box entries and song-induced place preference also correlated positively with met-enkephalin labeling in the ventromedial nucleus of the hypothalamus. These findings indicate that the reward value of vocal signals is linked to individual differences in motivational state; and that differences in enkephalin activity may play a role in modifying an individual's motivational state and/or the reward value of song.

Context-dependent links between song production and opioid-mediated analgesia in male European starlings (Sturnus vulgaris)

Little is known about the neural mechanisms that ensure appropriate vocal behaviors within specific social contexts. Male songbirds produce spontaneous (undirected) songs as well as female-directed courtship songs. Opioid neuropeptide activity in specific brain regions is rewarding, at least in mammals, and past studies suggest that the opioid met-enkephalin in such areas is more tightly linked to undirected than female-directed song. Recent data using a song-associated place preference paradigm further suggest that production of undirected but not directed song is tightly linked to intrinsic reward. Opioids have analgesic properties. Therefore, if production of undirected song is closely linked to opioid-mediated reward, the production of undirected but not directed song should be associated with analgesia. Consistent with this prediction, in male starlings we identified a positive correlation between analgesia (decreased reactivity to a hot water bath) and undirected song (in non-breeding season condition males in affiliative flocks) but not female-directed song (in breeding season condition males presented with females). When breeding condition males were divided according to social status, a negative correlation was found in subordinate males (i.e. males that failed to acquire a nest box). These data are consistent with the hypotheses 1) that the production of undirected song is facilitated or maintained by opioids (and/or other neuromodulators that also induce analgesia) and 2) that production of female-directed song is not linked in the same way to release of the same neuromodulators. Results also demonstrate a link between analgesia and song in subordinate individuals lacking a nesting territory within the breeding season. Overall, the findings indicate that distinct neural mechanisms regulate communication in different social contexts and support the working hypothesis that undirected but not directed song is tightly linked to opioid release.

The role of motivation and reward neural systems in vocal communication in songbirds

Many vertebrates are highly motivated to communicate, suggesting that the consequences of communication may be rewarding. Past studies show that dopamine and opioids in the medial preoptic nucleus (mPOA) and ventral tegmental area (VTA) play distinct roles in motivation and reward. In songbirds, multiple lines of recent evidence indicate that the roles of dopamine and opioid activity in mPOA and VTA in male birdsong differ depending upon whether song is used to attract females (sexually-motivated) or is produced spontaneously (undirected). Evidence is reviewed supporting the hypotheses that (1) mPOA and VTA interact to influence the context in which a male sings, (2) distinct patterns of dopamine activity underlie the motivation to produce sexually-motivated and undirected song, (3) sexually-motivated communication is externally reinforced by opioids released as part of social interactions, and (4) undirected communication is facilitated and rewarded by immediate opioid release linked to the act of singing.

Pleasure seeking and birdsong

Songbirds sing at high rates within multiple contexts, suggesting that they are highly motivated to communicate and that the act of singing itself may be rewarding. Little is known about the neural regulation of the motivation to communicate. Dopamine and opioid neuropeptides play a primary role in reward seeking and sensory pleasure. In songbirds, these neurochemicals are found within brain regions implicated in both motivation and reward, including the medial preoptic nucleus (mPOA) and ventral tegmental area (VTA). Several lines of research indicate that dopamine and opioids in these regions play a role in birdsong that differs depending upon whether song is used to attract females (female-directed song) or is not directed towards other individuals (undirected song). Evidence is reviewed supporting the hypotheses: (1) that distinct patterns of dopamine activity influence the motivation to produce undirected and female-directed song, (2) that undirected communication is intrinsically reinforced by immediate release of opioids induced by the act of singing, and (3) that directed communication is socially reinforced by opioids released as part of social interactions.

Administration of the GABAA receptor antagonist picrotoxin into rat supramammillary nucleus induces c-Fos in reward-related brain structures. Supramammillary picrotoxin and c-Fos expression

BACKGROUND

Picrotoxin blocks GABAA receptors, whose activation typically inhibits neuronal firing activity. We recently found that rats learn to selectively self-administer picrotoxin or bicuculline, another GABAA receptor antagonist, into the supramammillary nucleus (SuM), a posterior hypothalamic structure localized anterior to the ventral tegmental area. Other drugs such as nicotine or the excitatory amino acid AMPA are also self-administered into the SuM. The SuM appears to be functionally linked with the mesolimbic dopamine system and is closely connected with other brain structures that are implicated in motivational processes, including the prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus. Here, we hypothesized that these brain structures are activated by picrotoxin injections into the SuM.

RESULTS

Picrotoxin administration into the SuM markedly facilitated locomotion and rearing. Further, it increased c-Fos expression in this region, suggesting blockade of tonic inhibition and thus the disinhibition of local neurons. This manipulation also increased c-Fos expression in structures including the ventral tegmental area, medial shell of the nucleus accumbens, medial prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus.

CONCLUSIONS

Picrotoxin administration into the SuM appears to disinhibit local neurons and recruits activation of brain structures associated with motivational processes, including the mesolimbic dopamine system, prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus. These regions may be involved in mediating positive motivational effects triggered by intra-SuM picrotoxin.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Evidence for opioid involvement in the motivation to sing

Songbirds produce high rates of song within multiple social contexts, suggesting that they are highly motivated to sing and that song production itself may be rewarding. Progress has been made in understanding the neural basis of song learning and sensorimotor processing, however little is known about neurobiological mechanisms regulating the motivation to sing. Neural systems involved in motivation and reward have been conserved across species and in songbirds are neuroanatomically well-positioned to influence the song control system. Opioid neuropeptides within these systems play a primary role in hedonic reward, at least in mammals. In songbirds, opioid neuropeptides and receptors are found throughout the song control system and within several brain regions implicated in both motivation and reward, including the medial preoptic nucleus (POM) and ventral tegmental area (VTA). Growing research shows these regions to play a role in birdsong that differs depending upon whether song is sexually motivated in response to a female, used for territorial defense or sung as part of a flock but not directed towards an individual (undirected song). Opioid pharmacological manipulations and immunocytochemical data demonstrate a role for opioid activity possibly within VTA and POM in the regulation of song production. Although future research is needed, data suggest that opioids may be most critically involved in reinforcing song that does not result in any obvious form of immediate externally mediated reinforcement, such as undirected song produced in large flocks or during song learning. Data are reviewed supporting the idea that dopamine activity underlies the motivation or drive to sing, but that opioid release is what makes song production rewarding.

Pharmacological evidence for a motivational role of kappa-opioid systems in ethanol dependence

The purpose of this study was to test the hypothesis that activation of the dynorphin/kappa (kappa)-opioid system has a role in the increased consumption of ethanol in dependent animals. The effects of three opioid receptor antagonists with different effects on opioid receptors, naltrexone, nalmefene, and nor-binaltorphimine (nor-BNI), were compared in their ability to decrease ethanol self-administration in nondependent and ethanol-dependent male Wistar rats. Nalmefene and naltrexone are both opioid receptor ligands with comparable molecular weights and pharmacokinetic profiles, but differing specificity for the three opioid receptor subtypes at low doses, while nor-BNI is a selective kappa-opioid receptor antagonist. Dependence was induced in half the animals by subjecting them to a 4-week intermittent vapor exposure period in which animals were exposed to ethanol vapor for 14 h per day. Subsequent to dependence induction, nalmefene, naltrexone, and nor-BNI were tested for their ability to modulate self-administration of ethanol in vapor-exposed and control rats. The results indicated that both nalmefene and naltrexone induced a significant dose-dependent decrease in the number of lever presses for ethanol in both groups of animals. However, in ethanol-dependent animals, nalmefene was significantly more effective in suppressing ethanol intake than naltrexone. Nor-BNI selectively attenuated ethanol-dependent self-administration while leaving nondependent ethanol self-administration intact. Because naltrexone is primarily selective for the mu-opioid receptor, and nalmefene is primarily selective for the mu- and kappa-opioid receptor subtypes, the fact that nalmefene demonstrates more suppression in dependent animals suggests that opioid systems distinct from the mu-regulated portion may be involved in the increased drinking seen during withdrawal in dependent animals. The results with nor-BNI confirm that kappa-opioid receptor antagonism selectively decreases dependence-induced ethanol self-administration, which supports the hypothesis that dynorphin/kappa-opioid systems are dysregulated in dependence and contribute to the increased drinking seen during acute withdrawal in dependent rats.

Evidence for opioid involvement in the regulation of song production in male European starlings (Sturnus vulgaris)

Many social animals vocalize at high rates, suggesting that vocal communication is highly motivated and rewarding. In songbirds, much is known about the neural control of vocal behavior; however, little is known about neurobiological mechanisms regulating the motivation to communicate. This study examined a possible role for opioid neuropeptides in motivation and reward associated with song production in male European starlings (Sturnus vulgaris). Peripheral opioid blockade facilitated male song production. Furthermore, methionine-enkephalin immunolabeled fiber densities within brain regions in which opioids are known to regulate motivation and reward (i.e., the medial preoptic nucleus and ventral tegmental area) related positively to male song production. These data suggest that song production might be regulated by opioid activity within motivation and reward neural systems.

Activation of μ opioid receptors in the medial preoptic area following copulation in male rats

The current study tested the hypothesis that sexual behavior is a biological stimulus for release of endogenous opioid peptides. In particular, activation of mu opioid receptors (MOR) in the medial preoptic area (MPOA), a key area for regulation of male sexual behavior, was studied in male rats. MOR endocytosis or internalization was used as a marker for ligand-induced receptor activation, utilizing confocal, electron, and bright microscopic analysis. Indeed, mating including one ejaculation induced receptor activation in the MPOA, demonstrated by increased immunoreactivity for MOR, increased numbers of endosome-like particles immunoreactive for MOR inside the cytoplasm of neurons, and increased percentage of neurons with three or more endosome-like particles inside the cytosol. Moreover, it was demonstrated that MOR activation occurred within 30 min following mating and was still evident after 6 h. Mating-induced internalization was prevented by treatment with the opioid receptor antagonist naloxone before mating, suggesting that mating-induced receptor activation is a result of action of endogenous MOR ligands. i.c.v. injections of MOR ligand [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin resulted in internalization of the MOR in a similar manner observed following mating. Finally, mating induced Fos expression in MOR containing neurons in the MPOA. However, naloxone pretreatment did not prevent Fos activation of MOR neurons, suggesting that Fos induction was not the result of MOR activation. In summary, these results provide further evidence that endogenous opioid peptides are released in the MPOA during male sexual behavior.

Emotional behaviour as the result of stochastic interactions. A process crucial for cognition

This paper is aimed at exhibiting two striking features of the usual approach of emotional expression in science and philosophy, suggesting a different perspective. One is the generally shared belief that emotions are a state of utter disarray, which hampers objective knowledge; the other is the search for causal explanation, along a wide range of categorized approaches (psychology, neurosciences, developmental biology) each proposing its own theoretical framework. In both cases the result is to play down emotional expression. Alternatively, we propose to view emotions as something crucial in the choice of our conceptual tools, ideas and involvements, in the genesis of which various explanations interact in a complex stochastic way. Rather than being a harmful disruption of the mind calling for identification of a definite causality, emotional behaviour appears as a necessary process in cognition, which is irreducible to a unique origin.

Potentiation of opioid-induced conditioned place preference by the selective serotonin reuptake inhibitor fluoxetine

The ability of the selective serotonin reuptake inhibitor, fluoxetine, to modify the effects of morphine, N-((S)-2-benzyl-3[(S) 2-amino-4-methylthio)butyldithio-]-1-oxopropyl)-L-alanine benzylester (RB 120; mixed inhibitor of enkephalin metabolism), and 4-¿[2-[[3-(1H-indol-3-yl))-2-methyl-1-oxo-2-[[(tricyclo[3,3,1,1] dec-2-yloxy) carbonyl] amino¿ propyl] amino]-1-phenylethyl] amino¿-4-oxo-[R-(R*,R*)]-butanoate N-methyl-D-glucamine (CI 988; cholecystokinin receptor subtype [CCK(2)] antagonist), was assessed using conditioned place preference. RB 120 and morphine both induced significant, dose-dependent conditioned place preference, whilst CI 988 failed to elicit conditioned place preference. A subthreshold dose of fluoxetine (2.5 mg/kg) potentiated the morphine submaximal response. Notably, the combination of a subthreshold dose of fluoxetine (2.5 mg/kg) with RB 120 (5 mg/kg) or CI 988 (3 mg/kg) was devoid of any significant conditioned place preference properties. Fluoxetine may act via enhanced serotonergic activity to modulate enkephalinergic tone. Agents that increase enkephalinergic tone more directly such as RB 120 and CI 988, at submaximal doses, did not induce conditioned place preference when co-administered with fluoxetine. These data suggest that fluoxetine, in combination with CI 988 or RB 120, might prove to be a beneficial treatment strategy for opioid drug addiction, though further studies are necessary.

Affective properties of intra-medial preoptic area injections of testosterone in male rats

On alternating days, adult male Long-Evans rats implanted with unilateral cannulae in the medial preoptic area (MPOA) received intracerebral injections of testosterone (0.05, 0.1 or 0.2 microg/0.5 microl), or saline immediately prior to confinement for 30 min to one of two compartments of a place preference apparatus. All rats received 8 days of pairings (4 hormone and 4 saline). On day 9, the rats were given a hormone-free 20-min test session during which they had access to all compartments of the apparatus. Intra-MPOA injections of testosterone (0.1 microg) produced a conditioned place preference, while injections of a higher dose (0.2 microg) produced a conditioned place aversion. The rewarding effects of intra-MPOA testosterone may in part mediate the facilatory effects of testosterone on motivational aspects of sexual behavior.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

The neurosteroids, progesterone and 3alpha,5alpha-THP, enhance sexual motivation, receptivity, and proceptivity in female rats

The effects of progesterone (P) and the neurosteroid and P metabolite, 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP) on ovariectomized (ovx), estradiol-3-benzoate (EB)-primed rats on sexual motivation, receptivity, and proceptivity were examined. Changes in central P and 3alpha,5alpha-THP were measured following administration of EB, EB+P, EB+3alpha,5alpha-THP, or EB+inhibitor of 5alpha-reductase or P metabolism (epostane and finasteride)+P (Expt. 1). Partner preference was measured as the duration of time females in these different hormonal treatments spent in proximity to a male vs. female conspecific (Expt. 2). Receptivity (lordosis quotients and ratings) and proceptivity (darting, hopping, ear wiggling, and pacing), for different hormone treatments were assessed (Expt. 3 and Expt. 4, respectively). Conditioned place preference following hormone treatments and paced mating enabled assessment of sexual motivation (Expt. 5). Central P and 3alpha,5alpha-THP were measured in various combinations of hormone/mating conditions (Expt. 6). Studies revealed that 3alpha,5alpha-THP has a significant role in these reproductive measures. Brain concentrations of 3alpha, 5alpha-THP were significantly higher in animals receiving EB+P or EB+3alpha,5alpha-THP compared to animals receiving EB alone, or EB+P in conjunction with an inhibitor of P metabolism. EB+P and EB+3alpha, 5alpha-THP significantly increased time spent in proximity to the male, receptivity and proceptivity. When administered to ovx, EB-primed rats, the progestin metabolite, 3alpha,5alpha-THP, had effects on these behaviors similar to P. Epostane, an inhibitor of P and 3alpha,5alpha-THP biosynthesis, and finasteride, an inhibitor of P metabolism to 3alpha,5alpha-THP, administered to EB+P animals reduced male partner preference, proceptive, and receptive behaviors to levels seen in EB+vehicle animals. Notably, whole brain 3alpha, 5alpha-THP levels were significantly increased and whole brain P levels were significantly reduced in paced mated rats compared to standard mated, and receptive non-mated animals. These studies suggest that P and 3alpha,5alpha-THP may have some common effects on reproductive behavior, e.g., sexual motivation, receptivity, and proceptivity.

Enhanced sensitivity of the nucleus accumbens proenkephalin system to alcohol in rats selectively bred for alcohol preference

Evidence suggests that alcohol-induced activation of the endogenous opioid system is part of a neurobiological mechanism that may be functionally involved in alcohol reinforcement and high alcohol drinking behavior. We postulate that a genetic predisposition toward alcohol drinking is accompanied by increased responsiveness of the opioid system to alcohol. To test this hypothesis, the present study compared the effect of an acute alcohol challenge on enkephalin gene expression in discrete brain regions which are high in preproenkephalin (PPENK) mRNA content and/or are important in mediating alcohol reward in rats selectively bred for alcohol preference (P) or nonpreference (NP). PPENK mRNA content was measured by in situ hybridization performed with a 36 base oligonucleotide probe for PPENK mRNA and was quantified using a computerized image-analysis system. Blood alcohol concentration (BAC) and rate of alcohol elimination following alcohol infusion were similar in P and NP rats. P and NP rats did not differ in basal content of PPENK mRNA in any of the brain areas examined prior to onset of infusion. An intragastric (I.G.) infusion of alcohol (2.5 g/kg b.wt) produced a significant increase in PPENK mRNA in the nucleus accumbens (both shell and core) of P but not NP rats at 1 h after the onset of infusion which coincided with the time at which peak BAC was attained. In contrast, at 8 h after the onset of the alcohol infusion, when BAC was falling toward baseline, PPENK mRNA was decreased in the nucleus accumbens of both P and NP rats and in the anterior striatum and amygdala of NP rats. The results suggest that enhanced responsiveness of the enkephalinergic system to alcohol is associated with, and may be functionally involved in, mediating high alcohol drinking behavior.

Cholecystokinin modulates the aversive component of morphine withdrawal syndrome in rats

The conditioned place aversion paradigm was used to investigate the role of cholecystokinin in the aversive/dysphoric component of morphine abstinence. Several cholecystokinin ligands were chronically administered during the development of morphine dependence: the CCKA antagonist devazepide, the CCKB antagonists PD-134,308 and L-365,260, and the CCKB agonist BC 264. The CCK-B antagonists L-365,260 and PD-134,308 decreased and completely blocked (respectively) the place aversion induced by naloxone in morphine dependent animals whereas BC 264 and devazepide were inactive in this model. No effect was observed in non-dependent animals after chronic administration of these CCK-ligands. These results show a distinct role for CCK receptors in the regulation of the motivational component of morphine abstinence, probably related to their differential effects in the regulation of limbic dopaminergic neurons.

Enkephalinase inhibition facilitates sexual behavior in the male rat but does not produce conditioned place preference

The effects of two enkephalinase inhibitors, SCH 34826 and phospho-leu-phe, on male rat sexual behavior and conditioned place preference were evaluated. SCH 34826, administered intraperitoneally, reduced the ejaculation latency to both the first and second ejaculation at a dose of 30 mg/kg. This dose also reduced the first postejaculatory interval. No other effect was obtained with this drug. Phospho-leu-phe, administered intracerebroventricularly, increased mount and intromission latency at doses of 50 and 100 micrograms. A dose of 25 micrograms reduced the latency to the first ejaculation as well as the number of preejaculatory intromissions. The postejaculatory interval was also reduced at this dose. SCH 34826, 100 and 30 mg/kg, and phospho-leu-phe, 25 micrograms, had no effect in the conditioned place preference procedure. These observations seem to suggest that there is no functionally relevant tonic release of enkephalins. Therefore, the effects obtained on sexual behavior may indicate that enkephalins are released before and during the course of sexual activity. The function of such a release could be to facilitate ejaculatory mechanisms in the way found in the present studies. Previous work has shown that ejaculation-induced reward is opioid dependent, further supporting the hypothesis of opioid release during sexual activity. Taken together, these data suggest an important role for opioids, probably enkephalins, in the physiological control of sexual behavior.

Unlike morphine the endogenous enkephalins protected by RB101 are unable to establish a conditioned place preference in mice

The mixed inhibitor prodrug, RB101, was used to study the motivational properties of the endogenous opioid peptides, the enkephalins. In the conditioned place preference test, which is commonly used to investigate the reinforcing properties of drugs, mice alternately treated with morphine (3 mg/kg i.p.) on the initially non-preferred compartment and with saline on the preferred one, for four place pairings, spent more time in the drug-associated compartment. This shift in place preference after the conditioning procedure was not found after treatment with RB101 (80 mg/kg i.p.). Administration of naloxone (1 mg/kg s.c.) after the conditioning phase increased the preference for the drug-associated compartment of mice treated with 6 mg/kg (i.p.) of morphine. This illustrates the negative motivational properties of morphine withdrawal or the establishment of psychic dependence on the drug. In contrast, no modification of preference was observed after injection of naloxone in animals treated with a high dose of RB101 (160 mg/kg i.p.). The failure to establish conditioned place preference by inhibiting endogenous enkephalin metabolism, and the lack of development of psychic dependence after RB101 administration demonstrate for the first time the interest of mixed inhibitors of enkephalin-degrading enzymes as potent new non-addictive analgesics.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Inhibitory effect of opiates on male rat sexual behavior may be mediated by opiate receptors outside the central nervous system

The importance of opiate receptors outside the central nervous system for the inhibitory actions of morphine on male rat sexual behavior was evaluated. Morphine (10 mg/kg) produced an almost complete inhibition of sexual behavior. This inhibition was antagonized by naloxone at a dose of 1 mg/kg but not at a dose of 0.25 mg/kg. The quaternary opioid antagonist methylnaloxone effectively blocked the inhibitory actions of morphine at a dose of 20 mg/kg but not at a dose of 5 mg/kg. Since the affinity of methylnaloxone for opiate receptors is about 5% of that of naloxone, it may be concluded that both antagonists were about equally effective in inhibiting the effects of morphine. Furthermore, the opiate-like drug loperamide was found to inhibit sexual behavior. This drug acts mainly outside the central nervous system. Its effect was blocked by both naloxone and methylnaloxone, suggesting that opiate receptors are involved. It was also shown that methylnaloxone is unable to block the reinforcing effects of morphine in the conditioned place preference procedure. Because the reinforcing effects of opiates seem to be localized to the central nervous system, it may be proposed that methylnaloxone does not antagonize morphine's central effects. Moreover, loperamide had no effect in the place preference procedure, suggesting that this drug does not act at central opioid receptors. Taken together, these data show that peripheral opioid receptors are responsible for at least some of the inhibitory actions of morphine on male sexual behavior. After treatment with morphine + methylnaloxone, ejaculatory mechanisms were facilitated, reflected in a reduced number of preejaculatory intromissions and a shortened ejaculation latency.(ABSTRACT TRUNCATED AT 250 WORDS)