Dopamine antagonism attenuates the unconditioned incentive value of estrous female cues

Blockade of serotonin 5-HT2A receptors potentiates dopamine D2 activation-induced disruption of pup retrieval on an elevated plus maze, but has no effect on D2 blockade-induced one

Appetitive aspect of rat maternal behavior, such as pup retrieval, is motivationally driven and sensitive to dopamine disturbances. Activation or blockade of dopamine D₂ receptors causes a similar disruption of pup retrieval, which may also reflect an increase in maternal anxiety and/or a disruption of executive function. Recent work indicates that serotonin 5-HT_2A receptors also play an important role in rat maternal behavior. Given the well-known modulation of 5-HT_2A on the mesolimbic and mesocortical dopamine functions, the present study examined the extent to which blockade of 5-HT_2A receptors on dopamine D₂-mediated maternal effects using a pup retrieval on the elevated plus maze (EPM) test. Sprague-Dawley postpartum female rats were acutely injected with quinpirole (a D₂ agonist, 0.10 and 0.25 mg/kg, sc), or haloperidol (a D₂ antagonist, 0.1 or 0.2 mg/kg, sc), in combination of MDL100907 (a 5-HT_2A receptor antagonist, 1.0 mg/kg, sc, 30 min before quinpirole or haloperidol injection) or saline and tested at 30, 90 and 240 min after quinpirole or haloperidol injection on postpartum days 3 and 7. Quinpirole and haloperidol decreased the number of pup retrieved (an index of maternal motivation) and sequential retrieval score (an index of executive function), prolonged the pup retrieval latencies, reduced the percentage of time spent on the open arms (an index of maternal anxiety), and decreased the distance travelled on the maze in a dose-dependent and time-dependent fashion. MDL100907 treatment by itself had no effect on pup retrieval, but it exacerbated the quinpirole-induced disruption of pup retrieval, but had no effect on the haloperidol-induced one. These findings suggest a complex interactive effect between 5-HT_2A and D₂ receptors on one or several maternal processes (maternal motivation, anxiety and executive function), and support the idea that one molecular mechanism by which 5-HT_2A receptors mediate maternal behavior is through its modulation of D₂ receptors.

Dopamine mediates testosterone-induced social reward in male Syrian hamsters

Adolescent maturation of responses to social stimuli is essential for adult-typical sociosexual behavior. Naturally occurring developmental changes in male Syrian hamster responses to a salient social cue, female hamster vaginal secretions (VS), provide a good model system for investigating neuroendocrine mechanisms of adolescent change in social reward. Sexually naïve adult, but not juvenile, males show a conditioned place preference (CPP) to VS, indicating that VS is not rewarding before puberty. In this series of experiments, the authors examined the roles of testosterone and dopamine receptor activation in mediating the adolescent gain in positive valence of VS. Experiment 1 showed that testosterone replacement is necessary for gonadectomized adult hamsters to form a CPP to VS. Experiment 2 showed that testosterone treatment is sufficient for juvenile hamsters to form a CPP to VS, and that the dopamine receptor antagonist haloperidol blocks formation of a CPP to VS in these animals. Experiments 3 and 4 demonstrated that the disruption of VS CPP with low doses of haloperidol is the result of a reduction in the attractive properties of VS and not attributable to aversive properties of haloperidol. Together, these studies demonstrate that the unconditioned rewarding properties of a social cue necessary for successful adult sociosexual interactions come about as the result of the pubertal increase in circulating testosterone in male hamsters. Furthermore, this social reward can be prevented by dopamine receptor antagonism, indicating that hypothalamic and/or mesocorticolimbic dopaminergic circuits are targets for hormonal activation of social reward.

Prefrontal electroencephalographic activity during the working memory processes involved in a sexually motivated task in male rats

The prefrontal cortex is involved in working memory functions, and several studies using food or drink as rewards have demonstrated that the rat is capable of performing tasks that involve working memory. Sexual activity is another highly-rewarding, motivated behaviour that has proven to be an efficient incentive in classical operant tasks. The objective of this study was to determine whether the functional activity of the medial prefrontal cortex (mPFC) changes in relation to the working memory processes involved in a sexually motivated task performed in male rats. Thus, male Wistar rats implanted in the mPFC were subjected to a nonmatching-to-sample task in a T-maze using sexual interaction as a reinforcer during a 4-day training period. On the basis of their performance during training, the rats were classified as 'good-learners' or 'bad-learners'. Only the good-learner rats showed an increase in the absolute power of the 8-13 Hz band during both the sample and test runs; a finding that could be related to learning of the working memory elements entailed in the task. During the maintenance phase only (i.e., once the rule had been learned well), the good-learner rats also showed an increased correlation of the 8-13 Hz band during the sample run, indicating that a high degree of coupling between the prefrontal cortices is necessary for the processing required to allow the rats to make correct decisions in the maintenance phase. Taken together, these data show that mPFC activity changes in relation to the working memory processes involved in a sexually motivated task in male rats.

Adolescent cannabinoid exposure attenuates adult female sexual motivation but does not alter adulthood CB1R expression or estrous cyclicity

Adolescence is a developmental period characterized by neuronal remodeling and the maturation of adult emotionality, reproductive behavior and social behavior. We examined whether chronic cannabinoid exposure in adolescent rats alters female sexual motivation, estrous cyclicity, sucrose preference, and CB(1)R expression in adulthood. Female rats were administered with the synthetic cannabinoid agonist, CP-55,940 (0.4 mg/kg, intraperitoneal), daily during adolescent development (PND 35-45). In a subset of subjects, socio-sexual motivation was investigated in adulthood (PND 75-86) using a runway apparatus. Estrous cyclicity was tracked in adulthood via vaginal cytology and a single-mount test. A two-bottle sucrose preference test was also conducted to determine whether predicted changes in socio-sexual motivation might be linked to alterations in hedonic processing. CB(1)R expression was examined in two separate subsets of subjects, one sacrificed following drug treatment (PND 46) and one before behavioral testing (PND 74). Drug treatment significantly decreased adult preference for a male conspecific (sexual motivation), as assessed by both Run Time and Proximity Time, but did not affect estrous cyclicity or sucrose preference. CP-55,940 treatment also induced immediate, but transient, decreases in CB(1)R expression in the ventromedial nucleus of the hypothalamus and amygdala. Drug treatment did not affect CB(1)R expression in the nucleus accumbens (core or shell) or globus pallidus at either time point. We suggest that the endocannabinoid system may play a role in the maturation of neuroendocrine axes and adult female reproductive behavior, and that chronic exposure to cannabinoids during adolescence disrupts these neurodevelopmental processes.

The rewarding nature of social interactions

The objective of this short review is to highlight rewarding aspects of social interactions for humans and discuss their neural basis. Thereby we report recent research findings to illustrate how social stimuli in general are processed in the reward system and highlight the role of Theory of Mind as one mediating process for experiencing social reward during social interactions. In conclusion we discuss clinical implications for psychiatry and psychotherapy.

The rewarding nature of social interactions

The objective of this short review is to highlight rewarding aspects of social interactions for humans and discuss their neural basis. Thereby we report recent research findings to illustrate how social stimuli in general are processed in the reward system and highlight the role of Theory of Mind as one mediating process for experiencing social reward during social interactions. In conclusion we discuss clinical implications for psychiatry and psychotherapy.

Acute cannabinoid administration attenuates female socio-sexual motivation

Endocannabinoids may normally inhibit the generation and expression of female estrous behaviors. Previous work in our laboratory demonstrated that acute administration of a CB(1) receptor antagonist (AM251) increased sexual incentive motivation in estrous female rats. The current experiment examined the effect of CP55,940, a synthetic cannabinoid agonist, on sexual motivation. Seventy-two ovariectomized female Long-Evans rats were tested for their socio-sexual motivation via a runway methodology. Baseline motivation to approach and maintain close proximity to an empty goalbox, a female conspecific, and a male conspecific was assessed over six trials. Subjects were then grouped into nine experimental conditions and re-tested for their socio-sexual motivation after one of three possible hormonal treatments and three drug doses. Hormone treatments were: oil (nonestrous), 10 microg estradiol benzoate (partially estrous), and 10 microg estradiol+500 microg progesterone (fully estrous). Drug doses were: 0, 20, or 40 microg/kg CP55,940 (IP, 30 min prior to testing). As expected, hormonal priming with both estradiol and progesterone significantly increased sexual motivation in females that did not receive drug treatment. This occurred even though females were kept sexually-naïve throughout the experiment. CP55,940 dose-dependently attenuated sexual motivation for a male target in estrous females; the 40 microg/kg dose completely blocked sexual motivation. However, this same dose also significantly reduced social motivation for another female. Cannabinoid agonists reduce female sexual motivation, either directly by inhibiting estrus or indirectly by increasing social anxiety.

Orexin A attenuates unconditioned sexual motivation in male rats

Orexins are neuropeptides involved in multiple neurophysiological functions such as reward and motivation. However, it is not clear whether orexins are implicated in sexual motivation. This study aims to evaluate the effects of orexin A and the OX(1)R antagonist SB334867 on unconditioned sexual motivation. Forty-five male Wistar rats are divided into four groups. The four groups are respectively administered intracerebroventricularly with saline, orexin A (1, 10 microg), 10% DMSO (cyclodextrin) and SB334867 (5, 15 microg) 10-15 min before sexual motivation tests. The preference for a receptive female to a male in an open arena with two tethered animals is designated as unconditioned sexual motivation. The results show that orexin A reduces the female preference (reducing time in the female zone and/or increasing time in the male zone), the number of visits for the female zone and the total distance traveled in sexually high-motivated males. SB334867 has no effect on the female preference, the number of visits and the distance traveled in either sexually high-motivated or low-motivated males. Our experiments reveal that centrally administered orexin A attenuates sexual motivation in high-motivated males although endogenous orexin A might not play an important role in the expression of unconditioned sexual motivation.

The effects of sexual experience and estrus on male-seeking motivated behavior in the female rat

Ovariectomized (OVX) female rats were trained to traverse a straight alley and return to a goal box where they had previously encountered a male rat, a female rat or an empty goal box. The time required to run the alley was used as an index of the subjects' motivation to re-engage the goal box target. Subjects were tested in both estrus and non-estrus, first sexually naïve and then again after sexual experience. Female rats ran most quickly for a male target, most slowly for an empty goal box, and at intermediate speeds for a female target. Sexual experience tended to slow run times for all but male targets. Estrus enhanced approach behavior for males and an empty goal box, but tended to slow the approach toward females, both before and after sexual experience. This latter finding was further investigated in a second experiment in which sexually naïve OVX females were tested during estrus and non-estrus in a locomotor activity apparatus, a runway with an empty goal box, and an open field. Estrus produced no changes in spontaneous locomotion either in the activity box or the open field, but decreased run times in the alley and increased the number of center-square entries in the open-field. Thus, estrus produces increases in sexual motivation that selectively enhance exploratory, presumably male-seeking behavior, but not simple spontaneous locomotion.

Sexual behavior in male rodents

The hormonal factors and neural circuitry that control copulation are similar across rodent species, although there are differences in specific behavior patterns. Both estradiol (E) and dihydrotestosterone (DHT) contribute to the activation of mating, although E is more important for copulation and DHT for genital reflexes. Hormonal activation of the medial preoptic area (MPOA) is most effective, although implants in the medial amygdala (MeA) can also stimulate mounting in castrates. Chemosensory inputs from the main and accessory olfactory systems are the most important stimuli for mating in rodents, especially in hamsters, although genitosensory input also contributes. Dopamine agonists facilitate sexual behavior, and serotonin (5-HT) is generally inhibitory, though certain 5-HT receptor subtypes facilitate erection or ejaculation. Norepinephrine agonists and opiates have dose-dependent effects, with low doses facilitating and high doses inhibiting behavior.

The effect of acute bupropion on sexual motivation and behavior in the female rat

Recent clinical studies have suggested that the atypical antidepressant, bupropion (Wellbutrin), may stimulate sexual desire in women. Two experiments were conducted, testing the effect of acute bupropion administration on the sexual motivation and copulatory behavior of female rats. In the first experiment, 63 sexually-experienced, female Long-Evans rats were tested in a runway for their motivation to approach an empty goalbox, a nonestrous female, and an adult male. Both latency to approach and time spent in close proximity to the targets were used as dependent variables. Subjects were tested in both a nonestrous (OVX) and estrous (OVX+15 microg estradiol+500 microg progesterone) state, and following administration of 0.0, 7.5, or 15 mg/kg bupropion hydrochloride (subcutaneous, 45 min prior to testing). Results indicated that pre-treatment with ovarian hormones significantly increased the sexual motivation of the subjects. Bupropion treatment had no significant effect, either stimulatory or inhibitory, on subjects' socio-sexual motivation. In the second experiment, 60 female subjects were paired with an adult male for a thirty-minute copulatory test. Subjects were tested under one of three hormonal conditions: nonestrous (no hormones), 15 mug estradiol, or 15 microg estradiol+500 microg progesterone. Subjects were also pre-treated with either physiological saline or 15 mg/kg bupropion. Results indicated that while hormonal administration had a strong effect on female sexual behavior, bupropion treatment did not significantly affect either lordosis or the emission of hop-darts. Males paired with bupropion-treated females successfully achieved a greater number of ejaculations and demonstrated significantly shortened post-ejaculatory intervals. It is possible that bupropion treatment enhanced female attractiveness.

Different functionality of the medial and orbital prefrontal cortex during a sexually motivated task in rats

This study was designed to analyze whether the electroencephalographic (EEG) activity of the medial (mPFC) and orbital prefrontal cortex (oPFC) was modified during the performance of male rats in a T maze under two different conditions, sexually motivated (with previous intromission and females in the goal boxes of the lateral arms) or sexually non-motivated (without previous intromission and with empty goal boxes). Relative power (RP) of three EEG band frequencies, and inter-hemispheric correlation (r) were calculated and a comparison was made between rats under motivated and non-motivated conditions. In the mPFC of sexually motivated males, an increase of the RP in the 6-7 Hz band as well as a decrease in the 8-11 Hz band was observed in relation to an awake-quiet state and during the walk in the maze stem. Similarly, an increase in the r of the 6-7 Hz band was observed during the walk in the maze stem and when remaining near to a receptive female, when compared to non-motivated males. In the oPFC, only the RP of the 6-7 Hz band was increased during the walk in the maze stem of the motivated males. These data suggest that, among sexually motivated males, the mPFC is involved both in anticipatory and motor execution during the performance of the T maze task, whereas the oPFC is only involved in the motor execution of the T maze. These results are in line with other studies suggesting that the mPFC and oPFC are functionally distinct, regions which may work together during certain behaviors and physiological conditions.

Dopamine and serotonin: influences on male sexual behavior

Steroid hormones regulate sexual behavior primarily by slow, genomically mediated effects. These effects are realized, in part, by enhancing the processing of relevant sensory stimuli, altering the synthesis, release, and/or receptors for neurotransmitters in integrative areas, and increasing the responsiveness of appropriate motor outputs. Dopamine has facilitative effects on sexual motivation, copulatory proficiency, and genital reflexes. Dopamine in the nigrostriatal tract influences motor activity; in the mesolimbic tract it activates numerous motivated behaviors, including copulation; in the medial preoptic area (MPOA) it controls genital reflexes, copulatory patterns, and specifically sexual motivation. Testosterone increases nitric oxide synthase in the MPOA; nitric oxide increases basal and female-stimulated dopamine release, which in turn facilitates copulation and genital reflexes. Serotonin (5-HT) is primarily inhibitory, although stimulation of 5-HT(2C) receptors increases erections and inhibits ejaculation, whereas stimulation of 5-HT(1A) receptors has the opposite effects: facilitation of ejaculation and, in some circumstances, inhibition of erection. 5-HT is released in the anterior lateral hypothalamus at the time of ejaculation. Microinjections of selective serotonin reuptake inhibitors there delay the onset of copulation and delay ejaculation after copulation begins. One means for this inhibition is a decrease in dopamine release in the mesolimbic tract.

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.

Locomotion induced by non-contingent intracranial stimulation: comparison to psychomotor stimulant

Non-contingent experimenter-applied stimulation (nEAS) to the ventral mesencephalon, unlike contingent intracranial self-stimulation (ICSS), elicits high rates of general locomotion. This locomotion may be due to the nature of the presentation of stimulation, in that nEAS is non-contingent, while ICSS depends on a specific and focused response (e.g., bar pressing). Psychomotor stimulants also elicit high amounts of general locomotion, with the locomotion attributed to increased dopamine release. Interestingly, dopamine release decreases or is absent with repeated ICSS, but not nEAS. This suggests that the locomotion elicited by nEAS may be the result of DA release similar to that observed with psychomotor stimulants. To determine the relationship between locomotion induced by nEAS and psychomotor stimulants, locomotion elicited by nEAS was directly compared to that produced by cocaine, a psychomotor stimulant and indirect DA agonist. Six groups of rats were examined: (1) DA+ group: rats were implanted with a stimulating electrode in the ventral mesencephalon and activation of DA neurons was verified during surgery by monitoring DA release in the striatum; (2) DA- group: rats were also implanted with stimulating electrodes, but the location in the ventral mesencephalon did not elicit DA release; (3) 10-mg/kg cocaine group: rats were exposed to a low dose (10 mg/kg) of cocaine; (4) 40-mg/kg cocaine group: rats were exposed to a high dose (40 mg/kg) of cocaine; (5) saline group: rats were injected with saline; and (6) naive group: rats received no treatment. The topography of behavior was assessed in all rats during four periods: a pre-treatment baseline, treatment, early post-treatment, and a late post-treatment end point. The results suggest that locomotion elicited by nEAS was stereotypic, dependent upon DA release and similar, but not identical, to psychomotor stimulant-induced locomotion.

Opponent process properties of self-administered cocaine

Over the past decade, data collected in our laboratory have demonstrated that self-administered cocaine produces Opponent-Process-like behavioral effects. Animals running a straight alley once each day for IV cocaine develop over trials an approach-avoidance conflict about re-entering the goal box. This conflict behavior is characterized by a stop in forward locomotion (usually at the very mouth of the goal box) followed by a turn and 'retreat' back toward the goal box. The results of a series of studies conducted over the past decade collectively suggest that the behavioral ambivalence exemplified by rats running the alley for IV cocaine stems from concurrent and opponent positive (rewarding) and negative (anxiogenic) properties of the drug--both of which are associated with the goal box. These opponent properties of cocaine have been shown to result from temporally distinct affective states. Using a conditioned place preference test, we have been able to demonstrate that while the initial immediate effects of IV cocaine are reinforcing, the state present 15 min post-injection is aversive. In our most recent work, the co-administration of IV cocaine with either oral ethanol or IV heroin was found to greatly diminish the development and occurrence of retreat behaviors in the runway. It may therefore be that the high incidence of co-abuse of cocaine with either ethanol or heroin, stems from the users' motivation to alleviate some of the negative side effects of cocaine. It would seem then that the Opponent Process Theory has provided a useful conceptual framework for the study of the behavioral consequences of self-administered cocaine including the notion that both positive and negative reinforcement mechanisms are involved in the development and maintenance of cocaine abuse.

Opioids, cocaine, and food change runtime distribution in a rat runway procedure

RATIONALE

The nature of the relationship between the dose of a drug of abuse and its reinforcing effect has come under close scrutiny. It is currently debated if the steep ascending part of the biphasic dose-response curve typically obtained in multiple-dosing lever-press-based operant conditioning procedures represents a satiety-driven, all-or-none response or if the response is gradual and tightly adjusted to the various doses of the reinforcer.

OBJECTIVES

Dose-response relationships of drug reinforcers (remifentanil, alfentanil, morphine, cocaine) as well as a physiological reinforcer, i.e. food (sweetened condensed milk) were investigated in a different operant conditioning paradigm, i.e. a runway procedure.

METHODS

Administration of the mu opioid receptor agonists remifentanil (0.0032-0.1 mg/kg IV), alfentanil (0.032 mg/kg IV), morphine (0.032-1 mg/kg IV), the psychostimulant cocaine (0.001-0.1 mg/kg IV) or sweetened condensed milk (diluted 1:100-1:3 in water) was made contingent upon alley running for male Sprague-Dawley rats.

RESULTS

All drug reinforcers dose-dependently decreased runtimes and, more importantly, significantly increased the percentage of runs at a certain speed (> or =10 cm/s) from an average 29% to an average 71% (water versus milk, 22% versus 83%).

CONCLUSION

Both drug- and food reinforcers produced a discontinuous, qualitative change in the rats' operant behavior rather than a simple gradual increase along a continuum, an effect that could be seen clearly only after a histogram analysis of runtime distribution.

Effects of haloperidol on cue-induced autonomic and behavioral indices of heroin reward and motivation

RATIONALE

Most theoretical conceptions of motivation presume an internal state of sympathetic nervous system activation that precedes and accompanies goal-seeking behavior.

OBJECTIVES

The present study investigates the animals' physiological and behavioral response to presentation of environmental cues predictive of availability (S+) or non-availability (S-) of heroin in the goal box of a straight-arm alley.

METHODS

Animals were trained to discriminate between two olfactory cues, one predictive of the delivery of i.v. heroin (0.1 mg/kg) upon the rat's entry into the goal box of the runway (the S+) and another cue (the S-) predictive of i.v. saline upon goal-box entry. Once discriminative performance had stabilized, animals were challenged with each of four haloperidol treatments in a counterbalanced manner (0.0, 0.075, 0.15, 0.3 mg/kg i.p.). Run times and heart rates (measured via radiotelemetry) served as the dependent variables on every trial.

RESULTS

Both behavioral and physiological measures of motivation responded differentially to S+ and S- cues. Haloperidol had no effect during or immediately following S- trials, nor prior to reinforcer delivery on S+ trials. However, the behavioral and physiological consequences of heroin delivery during dopamine receptor antagonism were reliable - animals ran more slowly and showed less activation (lower heart rates) on the first S+ trial following a heroin + haloperidol experience.

CONCLUSIONS

The current data demonstrate that physiological and behavioral indices of cue-induced motivation remained intact during haloperidol challenge, while the reinforcing consequences of heroin appear to have been attenuated by dopamine receptor antagonism.

Lack of opioid or dopaminergic effects on unconditioned sexual incentive motivation in male rats

The effects of dopaminergic and opioidergic drugs on sexual incentive motivation were evaluated in sexually inexperienced male rats subjected to a choice procedure. Various parameters of ambulatory activity were recorded as well. Two drugs stimulating dopaminergic neurotransmission, amphetamine and apomorphine, failed to affect sexual incentive motivation, although ambulatory activity was enhanced by amphetamine. The dopamine antagonist cis(Z)-flupenthixol reduced sexual incentive motivation, but only at a dose that severely disrupted motor function. Morphine had marginal effects on sexual motivation but reduced ambulatory activity. These effects were not reduced by a peripheral opioid antagonist, methylnaloxone. Loperamide, a peripheral opioid agonist, reduced sexual motivation through an opioid-independent action. Naloxone was ineffective. Neither dopamine nor opioids seem to be important for sexual incentive motivation.

Nucleus accumbens shell and core dopamine: differential role in behavior and addiction

Drug addiction can be conceptualized as a disturbance of behavior motivated by drug-conditioned incentives. This abnormality has been explained by Incentive-Sensitization and Allostatic-Counteradaptive theories as the result of non-associative mechanisms acting at the stage of the expression of incentive motivation and responding for drug reinforcement. Each one of these theories, however, does not account per se for two basic properties of the motivational disturbance of drug addiction: (1). focussing on drug- at the expenses of non-drug-incentives; (2). virtual irreversibility. To account for the above aspects we have proposed an associative learning hypothesis. According to this hypothesis the basic disturbance of drug addiction takes place at the stage of acquisition of motivation and in particular of Pavlovian incentive learning. Drugs share with non-drug rewards the property of stimulating dopamine (DA) transmission in the nucleus accumbens shell but this effect does not undergo habituation upon repeated drug exposure, as instead is the case of non-drug rewards. Repetitive, non-decremental stimulation of DA transmission by drugs in the nucleus accumbens septi (NAc) shell abnormally strengthens stimulus-drug associations. Thus, stimuli contingent upon drug reward acquire powerful incentive properties after a relatively limited number of predictive associations with the drug and become particularly resistant to extinction. Non-contingent occurrence of drug-conditioned incentive cues or contexts strongly facilitates and eventually reinstates drug self-administration. Repeated drug exposure also induces a process of sensitization of drug-induced stimulation of DA transmission in the NAc core. The precise significance of this adaptive change for the mechanism of drug addiction is unclear given the complexity and uncertainties surrounding the role of NAc core DA in responding but might be more directly related to instrumental performance.

Effects of dopamine receptor antagonism with haloperidol on nurturing behavior in the biparental prairie vole

Dopamine (DA) receptor activity in lactating rats is critical for retrieval and licking of pups, whereas its inactivity facilitates quiescent nursing. The role of DA in the maternal behavior of other species and its role in paternal behavior are unknown. This experiment examined the effects of the DA antagonist haloperidol (HAL) on parental behavior in the biparental prairie vole (Microtus ochrogaster). Three days after birth of pups, parental behavior of male and female voles was observed for 30 min beginning 1 h after intraperitoneal injection of 0.1, 0.5, or 2.5 mg/kg of HAL. Controls received the propylene glycol vehicle. Control males were slower to contact pups, licked them more, and quiescently huddled/nursed less than control females. Even at the lowest dose of HAL that had no effect on general activity, pup licking was decreased in both sexes and the latency to contact pups increased in males. The latency to contact pups was most increased in females by the highest HAL dose. Retrieval of pups was not often displayed by any group. HAL dose-dependently decreased the latency and increased the duration of huddling/nursing in both sexes, but did not affect litter weight gains. These data indicate some subtle species differences in the dopaminergic regulation of parenting, as well as sex differences in the sensitivity of some vole parental behaviors to HAL.

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.

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.