Neuroplasticitat en el sistema mesolímbic induït per recompensa natural i posterior abstenció de recompenses. (2010)

INTRODUCCIÓ

The mesolimbic dopamine (DA) system, consisting of dopaminergic neurons in the ventral tegmental area (VTA) with projections to the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC), plays a critical role in the motivating and rewarding aspects of behavior including aggression (1), feeding (2-7), drinking (8), mating (9-11) and social bonding (12-13). Drugs of abuse converge upon the mesolimbic DA system (14-15). Moreover, repeated drug administration can induce neuronal alterations in these pathways, that in turn play a putative role in increasing the susceptibility to drug relapse, or in the transition from drug use to drug addiction (16-18). The behavioral effects of repeated drug administration include a sensitized locomotor response to psychostimulants and opiates (19-21), an enhanced conditioned drug reward (22-24), Ind increased operant responses for cues associated with prior drug intake (25). Furthermore, repeated drug administration results in long-lasting changes in dendritic morphology and spine density throughout the mesolimbic circuit (16, 26-31), and induces gene expression changes (32-35). Finally, repeated drug administration alters synaptic strength at excitatory and inhibitory synapses on midbrain dopamine neurons (36-41), and neurons in the NAc (42-44). It is currently unclear if similar alterations in the mesolimbic system occur with repeated exposure to natural rewards. Determining whether such alterations overlap with or are unique to drugs of abuse may lead to a better understanding of the cellular mechanisms underlying the differences between normal reward reinforcement versus compulsive seeking of a particular reward.

Supporting the hypothesis that stimuli other than drugs can cause neuronal alterations in the mesolimbic system are findings that stressful stimuli activate dopamine systems (45-47), and cause psychomotor stimulant sensitization (21, 48-50) and relapse in self-administration models (51-54). However, few studies have investigated whether natural rewarding behaviors can also produce functional changes in the DA system (6, 55-56). Therefore, the hypothesis was tested that male sexual experience causes neuronal alterations within the mesolimbic DA system via analysis of effects of sexual experience on locomotor sensitization, conditioned place preference, and dendrite morphology of NAc neurons. Furthermore, we hypothesized that an abstinence period from sexual behavior (sexual reward) is critical for the onset of these changes, based on recent observations that abstinence from drugs play a key role in the development of neural plasticity associated with repeated drug exposure (40, 57-59).

MÈTODES

Locomotor Activity Testing

Locomotor activity was measured using custom-designed locomotor activity chambers (LACs), modeled on chambers designed by Segal and Kuczenski (60). Locomotor activity was measured using a 16×16 photobeam array (San Diego Instruments, San Diego, CA) and expressed as crossovers per minute(s). A crossover was recorded each time the animal entered any of the “active zones” of the chamber, depicted as shaded areas in Figura 1A (61).

     

figura 1

Locomotor response of sexually experienced and naïve animals to saline or amphetamine administration. A is a schematic diagram of the zone map used to measure locomotor activity. A crossover is recorded each time the animal enters one of the black ...

RESULTATS

Experimenta 2

The goal of experiment 2 was to test if sexual experience results in locomotor sensitization in animals that mated during consecutive days, and in the same environment as in which they are exposed to AMPH. Exposure to the sex-paired environment caused increased locomotor activity during the 15 minutes prior to each mating session (Figure S1 in Supplement 1), illustrating the learned association between sexual behavior and environment. In addition, experiment 2 investigated the temporal pattern of locomotor sensitization to AMPH in sexually experienced male rats. The locomotor response to AMPH or SAL was measured one day (Day 8), one week (Day 14) and one month (Day 35) following the last mating session. As in experiment 1, sexually experienced rats displayed a greater locomotor response to AMPH compared to naïve animals. Moreover, this effect was evident on all three testing days. figura 2 illustrates locomotor activity during the last 60 minutes of the tests during which the most robust differences were observed, and data for first 30 minutes are shown in Figure S2 (Supplement 1). Naïve and experienced animals did not differ in their response to SAL on any of the testing days, and rats that received AMPH displayed increased locomotor activity when compared to their SAL controls (figura 2; p-values are listed in figure legend).

     

figura 2

Locomotor response of sexually experienced and naïve animals to saline or amphetamine administered one day (Day 8; A,B), one week (Day 14; C,D) or one month (Day 35; E,F) following the last mating session. Mean +/− SEM of total number ...

Experimenta 3

Experiment 3 investigated the effect sexual experience on conditioned AMPH reward. AMPH CPP was tested in sexually naïve and experienced males 10 days following the final mating session. Sexually experienced animals show an enhanced conditioned AMPH reward. Specifically, sexually experienced males formed a strong preference for the AMPH-paired chamber with the lower doses of 0.5 and 1.0 mg/kg but not with the higher doses 2.5 or 5.0 mg/kg. In contrast, sexually naïve males only formed a strong preference for the AMPH-paired chamber with the higher doses, 2.5 and 5.0 mg/kg, and not the lower doses (Figura 3A; p-values are listed in figure legend).

     

figura 3

Conditioned place preference of sexually experienced and naive animals in response to amphetamine either 10 days following (A) or during (B) mating sessions. Mean +/− SEM of CPP score, defined as the time spent in the AMPH-paired chamber in the ...

Experimenta 5

The purpose of Experiment 5 was to examine morphological alterations in the mesolimbic system, specifically the NAc, following sexual experience. Morphological alterations were evident one week (Figure 4H, J and L; p-values are listed in the figure legend), but not one day (Figure 4G, I and K), following the last mating session. In particular, significant increases in numbers of dendrites (indicative of increased dendritic branching) were detected in NAc core and shell (Figure 4H and J). In addition, numbers of dendritic spines were significantly increased in both the shell and core regions, one week, but not one day, after sex experience (Figure 4L).

     

figura 4

Dendritic morphology in the NAc of sexually experienced and naïve animals. Sexual experience caused an increase in numbers of dendrites and dendritic spines, illustrated by images (A,B) and camera lucida drawings (C,D) of representative NAc shell ...

DISCUSSIÓ

This study demonstrates that sexual experience and the post-experience abstinence from sexual behavior induce functional and morphological alterations in the mesolimbic system of male rats. Functional changes were evident in the form of a sensitized locomotor response and an enhanced conditioned reward with AMPH following sexual experience.

The sensitized locomotor response was observed as early as 1 day and maintained up to 28 days after last mating session. By contrast, the enhanced conditioned AMPH reward was only evident following an abstinence period from sexual behavior. Morphological alterations in both core and shell subregions of NAc were observed 7 days, but not 1 day, following the last mating session in sexually experienced animals. Together these data demonstrate that sexual experience induces plasticity in the mesolimbic system and that an abstinence period from mating is critical for the development of some, but not all mesolimbic system changes.

It is well recognized that natural rewarding behaviors and drugs of abuse act within the same neural pathways (64). Indeed, drugs of abuse have been demonstrated to affect the expression of rewarding behaviors (65-67), including male rat sexual behavior (67-70). The alterations in sexual behavior and motivation caused by repeated drug administration are dependent on a withdrawal or abstinence period from drug, as well as the environment in which the drug was presented. The current study showed that exposure to sexual behavior alters responsiveness to drugs of abuse. It was determined that sexually experienced male rats are sensitized to the locomotor effects of AMPH, and that this phenomenon is long-lasting and independent of an abstinence period from mating. Moreover, the sensitized locomotor response was independent of mating schedule or mating environment and was observed following either consecutive or intermittent mating sessions that occurred in the same or different environment as drug exposure. Studies conducted in female hamsters showed that sexually experienced female hamsters display a more rapid onset of AMPH-induced locomotor response compared to sexually naïve controls (71). However, rodents display sexual dimorphic responses to psychostimulants (72-73). Thus, the current studies expand the findings in female hamsters and demonstrate in male rats, the fast onset and the long duration of the enhanced locomotor responses to psychostimulants following sexual behavior.

It is unclear from the current studies which elements of sexual behavior contribute to the AMPH locomotor sensitization and if social interactions are sufficient. Animals in experiment 2 that failed to reach the criteria for sexual experience (displayed mounts and intromissions, but did not copulate to 5 ejaculations during the mating sessions) did not show a sensitized response (Figure S3 in Supplement 1). Therefore, an additional experiment was performed during which males were exposed to a receptive female without physical interaction, or displayed mounts and intromissions, neither of which resulted in sensitized locomotor responses to AMPH (Figure S4 in Supplement 1). Thus, social interactions do not appear to contribute to the effects of sexual experience on AMPH sensitization, but rather copulation including ejaculation appears essential for this form of plasticity.

In addition to a sensitized behavioral response, sexual experience enhances the conditioned reward value of AMPH, but only following abstinence from sexual reward. Previous work using CPP has shown that repeated exposures to psychostimulants or opiates augment drug-induced rewarding effects in line with the drug-induced locomotor sensitization (22-24). Repeated administration for 5 days of either cocaine (10 mg/kg), d- amphetamine (0.5 mg/kg) or morphine (5 mg/kg) sensitizes the rewarding effects of cocaine when tested 3 days following the cessation of drug pre-treatment. The sensitized effect was displayed by observing a conditioned preference with fewer conditioning trials (from 3 to 2) and with lower drug doses compared to SAL pre-treated control animals. The sensitized conditioned reward caused by repeated cocaine was found 7 days, but not 14 days, after final pretreatment of cocaine (23). A similar study utilizing 5 days of morphine (5.0 mg/kg) shows an augmented conditioned reward response to morphine when conditioning started 3, 10, or 21 days after drug pre-treatment. This augmented response was absent 1 day following morphine pre-treatment (24). Such findings suggest that a period of drug withdrawal of at least 3 days is required for the sensitized or cross-sensitized conditioned reward for both psychostimulants and opiates. Sexual experience, like repeated drug administration, may be instilling similar neuroadaptations in the mesolimbic system responsible for this sensitized drug responsiveness once the reward has been removed. It is currently unclear if reward abstinence is associated with stress and thus acts as a psychological stressor contributing to the observed alterations.

Clearly, there is an interplay between the effects of natural and drug reward. Reward cross-sensitization suggests that the long-lasting effects of both sexual behavior and drugs are mediated by common cellular or molecular mechanisms. Therefore, it is hypothesized that the sex behavior-induced alterations regulate the reinforcing components of sexual behavior and thus may be critical for positive reinforcement of rewarding behaviors in general. However, a subsequent abstinence from sexual reward may induce a state of increased reward seeking, or vulnerability to the effects of addictive substances similar to the effects of abstinence an ‘incubation of drug craving’ (25, 33, 74). In general, sexual behavior in male rodents does not causes compulsive seeking for sex, shown using copulation-malaise associative conditioning experiments (75), although the influence of abstinence has not been tested.

Dendritic morphology has been examined in depth in the fields of learning and memory (76-77) i l'addicció (59, 78-79), and is known to be influenced by environmental (80) and hormonal factors (81-82). Since the synaptic inputs are predominantly on dendrites or dendritic spines, they are the most likely target of experience-induced neuroplasticity (26, 83). Natural fluctuations or administration of gonadal hormones have been found to cause dendritic changes within several hours (84-87). Lalso, perturbations to the system, such stress (88) or chronic cocaine (79), cause dendritic alterations detectable within 24 hours.

Here, changes to dendritic morphology of medium spiny neurons in both the NAc core and shell were not observed within 24 hours, and instead required a period of abstinence following sexual experience. The structural alterations induced by sexual experience and subsequent abstinence resemble those seen following repeated exposure to psychostimulants (16-17, 26, 30). By contrast, DA depletion in the NAc results in a decreased number of dendrites and complexity in the shell (18, 89). Hence sexual experience-induced changes may be dependent on endogenous DA action in the NAc. However, mating-induced morphological alterations were only evident 7 days following the last mating session and coincide with the enhanced conditioned AMPH reward in sexually experienced animals. These data suggest that these increases in dendritic arborization and spines are not required for the expression of short-term locomotor sensitization to AMPH, yet may play a role in the maintenance and long-term expression of the sensitization. Previous studies of repeated drug administration have also noted a disconnect between long-term sensitization and morphological alterations in the NAc (89-94). It remains unclear what the functional relevance is of the morphological alterations, but it may play a role in the long term changes in function and gene expression.

In summary, the data presented here demonstrate that sexual behavior – a natural rewarding stimulus – can induce long-lasting neuroadaptations in the mesolimbic system. Our findings suggest that behavioral plasticity, particularly a sensitized locomotor response, is an immediate and long-term outcome of sexual experience. Moreover, an abstinence period may allow for neuroadaptations critical for observed morphological changes in the NAc and subsequent enhanced conditioned drug reward. This behavioral and neural plasticity follows a similar, but not identical, profile as seen in drug-sensitized animals. These data are of particular interest since we show that an abstinence from the natural reward induces a vulnerable state to drug administration. Understanding how both natural behaviors and drugs of abuse activate these systems causing neuroadaptations may provide us with a better understanding of reinforcement and reward in general, and provide further insight into the mechanisms of drug addiction.

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