Novel-object place conditioning in adolescent and adult male and female rats: effects of social isolation

Elevated levels of novelty seeking are often seen during adolescence. Recent studies using a conditioned place preference (CPP) paradigm have shown that novelty may be rewarding for adult rats. The present study explored the impact of age, sex, and isolation stress on novelty seeking and novelty reward by assessing novel object-induced CPP in adolescent and adult male and female Sprague-Dawley rats housed either socially or in isolation. Responding to the novel objects during conditioning was higher in adolescent animals than adults, and was suppressed by social isolation only in adulthood, particularly among males. Novel object CPP was strong among adolescent males, whereas only socially isolated adult males demonstrated preference for the compartment paired with the novel objects. This age difference was not evident in females, with both adolescent and adult group-housed females, but not their isolated counterparts, showing novel-object place conditioning. These dissociations between novelty-directed behaviors during conditioning and novelty reward in the CPP paradigm support the suggestion that mechanisms underlying novelty seeking are separable from those involved in the rewarding effects of novelty. High levels of novelty seeking demonstrated by adolescents do not necessarily predict high rewarding properties of novelty, with the latter also being influenced by environmental and gender-related factors.     

Social behavior and social motivation in adolescent rats: role of housing conditions and partner's activity.

The present study investigated 1) the effects of individual and grouped housing on social investigation, social contact behavior, and play behavior in adolescent rats tested with low socially active (grouped) and high socially active (isolated) play partners; and 2) the effects of long-term (8 days) and short-term (24 h) isolation on social behavioral manifestations and social motivation in terms of preference or avoidance of play partners. Social isolation of adolescent rats activated play behavior and social behaviors different from play, but play was predominantly affected under the conditions of this study. Long-term isolation was more effective than short-term, and resulted in greater manifestation of play and social preference. Adolescent rats were able to modify their social behaviors in response to social activity of the play partner: in isolated animals exposed to low socially active group-housed partners, play behavior was transformed into social activities unrelated to play; exposure of group-housed adolescents to high socially active previously isolated partners resulted in an increase of play behavior. Testing that allowed avoidance of social contacts revealed a dissociation between manifestations of play behavior and social motivation: group-housed play partners of isolated animals showed elevated levels of play behavior but a tendency to avoid their isolated pairmates.     

Nicotine-induced conditioned place preference in adolescent and adult rats

About 1 million American adolescents start smoking every year. Adolescents may be unusually sensitive to certain consequences of nicotine, demonstrating, for instance, significantly higher rates of dependence than adults at the same level of nicotine use. To explore whether adolescents may be more sensitive to rewarding properties of nicotine than adults, the present study used an animal model to assess the rewarding effects of a low nicotine dose (0.6 mg/kg) in a conditioned place preference (CPP) paradigm. Locomotor activity during conditioning and testing was also evaluated. Nicotine was observed to induce place preference conditioning in adolescent Sprague-Dawley rats, whereas the training dose of 0.6 mg/kg failed to produce convincing place preference in their adult counterparts. Age differences were also apparent in terms of nicotine influences on motor activity, with adults being more sensitive to nicotine-suppressant effects and only adolescents showing an emergence of nicotine-stimulatory effects upon repeated exposures. An increased predisposition to stimulatory nicotine effects during adolescence may contribute to age-specific rewarding properties of the drug as revealed using the CPP paradigm in this experiment. Increased sensitivity to stimulatory and rewarding effects during adolescence could potentially contribute to the high rate of nicotine use and dependence among human adolescents.     

Ethanol‐induced locomotor activity in adolescent rats and the relationship with ethanol‐induced conditioned place preference and conditioned taste aversion

Adolescent rats exhibit ethanol‐induced locomotor activity (LMA), which is considered an index of ethanol's motivational properties likely to predict ethanol self‐administration, but few studies have reported or correlated ethanol‐induced LMA with conditioned place preference (CPP) by ethanol at this age. The present study assessed age‐related differences in ethanol's motor stimulating effects and analyzed the association between ethanol‐induced LMA and conventional measures of ethanol‐induced reinforcement. Experiment 1 compared ethanol‐induced LMA in adolescent and adult rats. Subsequent experiments analyzed ethanol‐induced CPP and conditioned taste aversion (CTA) in adolescent rats evaluated for ethanol‐induced LMA. Adolescent rats exhibit a robust LMA after high‐dose ethanol. Ethanol‐induced LMA was fairly similar across adolescents and adults. As expected, adolescents were sensitive to ethanol's aversive reinforcement, but they also exhibited CPP. These measures of ethanol reinforcement, however, were not related to ethanol‐induced LMA. Spontaneous LMA in an open field was, however, negatively associated with ethanol‐induced CTA. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 55: 429–442, 2013     

Tickling induces reward in adolescent rats

In adolescent rats, 50-kHz vocalizations are most evident during tickling and rough-and-tumble play. The following experiments evaluated whether 50-kHz vocalizations reflect positive social affect by determining (1) if tickling is a rewarding event, (2) if social or isolate housing conditions differentially influence the response (since housing condition has been found to effect the reward magnitude of social encounters), and (3) if drugs that work on mu-opiate receptors, which has been hypothesized to control positive social affect, modulate tickling. Tickling was positively reinforcing as demonstrated by elevated operant behavior, conditioned place preference, and approach measures. A significant negative correlation between vocalization rate and approach latency measures was found. Social housing reduced tickle-induced vocalizations and approach speeds compared to isolate housing. Naloxone (1 mg/kg) increased vocalization in the socially housed rats and decreased it in isolated Subjects (Ss). These findings suggest that tickling can be used to induce positive social affect in rodents, and that it is modulated by endogenous opioids.     

Social isolation in adolescence alters behaviors in the forced swim and sucrose preference tests in female but not in male rats

Social interactions in rodents are rewarding and motivating and social isolation is aversive. Accumulating evidence suggests that disruption of the social environment in adolescence has long-term effects on social interactions, on anxiety-like behavior and on stress reactivity. In previous work we showed that adolescent isolation produced increased reactivity to acute and to repeated stress in female rats, whereas lower corticosterone responses to acute stress and decreased anxiety-related behavior were noted in isolated males. These results indicate a sex specific impact on the effects of social stress in adolescence. However, little is known about whether social isolation impacts behaviors related to affect and whether it does so differently in male and female rats. The present study investigated the impact of adolescent social isolation from day 30-50 of age in male and female Sprague Dawley rats on behavior in the forced swim test at the end of adolescence and in adulthood and on behavior in the sucrose preference test in adulthood. Adult female rats that were isolated in adolescence exhibited increased climbing on the first and second day of the forced swim test and showed an increased preference for sucrose compared to adult females that were group-housed in adolescence. There were no effects in male rats. The results indicate that social isolation in adolescence produces a stable and active behavioral phenotype in adult female rats.     

Adolescent social stress and social context influence the intake of ethanol and sucrose in male rats soon and long after the stress exposures

Social instability stress in adolescent rats (SS; postnatal day 30–45, daily 1 hr isolation +new cage partner) alters behavioural responses to psychostimulants, but differences in voluntary consumption of natural and drug rewards are unknown. SS also is associated with an atypical behavioural repertoire, for example reduced social interactions. Here, we investigated whether SS rats differ from control (CTL) rats in ethanol (EtOH) or sucrose intake in experiments involving different social contexts: alone, in the presence of an unfamiliar peer, in the presence of its cage partner, or in competition against its cage partner. SS rats drank more EtOH than CTL rats irrespective of social context, although the effects were driven primarily by those tested soon after the test procedure rather than weeks later in adulthood. SS and CTL rats did not differ in sucrose intake, except in adulthood under conditions of competition for limited access (SS>CTL). Adolescent rats drank more sucrose than adults, in keeping with evidence that adolescents are more sensitive to natural rewards than adult animals. Overall, adolescent SS modified the reward value of EtOH and sucrose, perhaps through stress/glucocorticoids modifying the development of the mesocorticolimbic system.     

Social and physical environment alter cocaine conditioned place preference and dopaminergic markers in adolescent male rats

This study was done to determine whether social and environmental factors alter cocaine reward and proteins implicated in mediating drug reward in rats during early adolescence. On postnatal day (PND) 23, rats were housed under conditions where both social (number of rats per cage) and environmental (availability of toys) factors were manipulated. Socially isolated rats were housed alone impoverished with no toys (II) or enriched with toys (IE). Social rats were housed two rats/cage with no toys (SI2) or with toys (SE2), or three/cage with (SE3) or without (SI3) toys. On PND 43, cocaine conditioned place preference (CPP) sessions began with the post-test done on PND 47. Cocaine CPP was established in response to 5 or 10 mg/kg cocaine in II rats, and CPP was decreased with the addition of cage mates or toys. No CPP was seen to any dose in SI3 or SE3 rats. Enriched housing (SE3) increased dopamine transporter (DAT) protein in the nucleus accumbens compared to II. There also were differential effects of cocaine on tyrosine hydroxylase and DAT depending on housing, with both increased by cocaine in II but not SE3 rats. DARPP-32 was unchanged by housing or cocaine, while phospho-Thr³⁴-DARPP-32 was increased by cocaine treatment across conditions. Thus, both social and environmental enrichment decrease cocaine CPP during adolescence and different housing alters proteins that regulate dopaminergic neurotransmission in a manner that may account for the observed differences in cocaine-induced reward.     

Differences in the social consequences of ethanol emerge during the course of adolescence in rats: social facilitation, social inhibition, and anxiolysis

The present experiments explored social consequences of ethanol during adolescence by examining dose-dependent ethanol-induced social facilitation and inhibition in a non-anxiogenic (familiar) environment, and ethanol-related anxiolysis in an anxiogenic (unfamiliar) environment in early (P28) and late (P42) adolescent rats. Pronounced age-related differences in the social consequences of ethanol emerged during the course of adolescence, with early adolescents being uniquely sensitive to activating effects of low doses of ethanol when tested in the familiar context in terms of play fighting-an adolescent-characteristic form of social interactions, but conversely less sensitive than late adolescents to ethanol-associated social suppression when tested at higher ethanol doses in this context. Early adolescents were also less sensitive than late adolescents to the anxiolytic effects of ethanol revealed in the unfamiliar test situation, when indexed in terms of increases in social investigation and the ethanol-induced transformation of social avoidance into social preference. Anti-anxiety properties of ethanol were found to be sex-dependent in older animals, with late adolescent females being more sensitive to ethanol anxiolysis than their male counterparts. Considerable ontogenetic differences in the social consequences of ethanol are evident even within the adolescent period, with early adolescence being a time of particularly pronounced adolescent-typical sensitivities to ethanol.     

Limited physical contact through a mesh barrier is sufficient for social reward-conditioned place preference in adolescent male rats

Adolescence is a period of enhanced sensitivity to social influences and vulnerability to drug abuse. Social reward in adolescent rats has been demonstrated with the conditioned place preference (CPP) model, but it is not clear whether limited contact with another rat without play is sufficient to produce reward. We investigated this issue using an apparatus containing two main compartment, each with a wire mesh barrier that allowed rats placed on either side of the barrier to have limited physical contact. Adolescent male rats were given two conditioning sessions/day for 2 or 8 days following baseline preference tests. Rats were placed into their preferred side alone for one daily 10-min session and into their initially non-preferred side (i.e., CS) for the other session during which they either had restricted or unrestricted physical access to another rat (Rat/Mesh or Rat/Phys, respectively) or to a tennis ball (Ball/Mesh or Ball/Phys, respectively) unconditioned stimulus (US). Only the Rat/Phys group exhibited CPP after 2 CS-US pairings; however, after 8 CS-US pairings, the Rat/Mesh and Ball/Phys groups also exhibited CPP. During conditioning, the rat US elicited more robust approach and contact behavior compared to the ball, regardless of physical or restricted access. The incidence of contact and/or approach increased as the number of exposures increased. The results suggest that the rank order of US reward efficacy was physical contact with a rat>limited contact with a rat>physical contact with a ball, and that rough-and-tumble play is not necessary to establish social reward-CPP. The findings have important implications for emerging drug self-administration models in which two rats self-administering drug intravenously have limited physical contact via a mesh barrier shared between their respective operant conditioning chambers.     

Preoperative differential housing and dorsal hippocampal lesions in rats

Rats housed in impoverished environments often show greater behavioral deficits after receiving brain lesions than to rats housed in standard or enriched environments. However, the resemblance between the effects of social isolation and those of hippocampal lesions in rats prompted the suggestion that rats socially isolated at weaning rather than grouped counterparts may show less behavioral change after sustaining dorsal hippocampal lesions when adult. In socially reared rats, hippocampal lesions produced increased ambulation and object contact in an open field, reduced passive avoidance in a runway task, and produced faster acquisition of active avoidance in a shuttle box, but there were no such differences in isolation-reared rats. Ambulation and object contact in isolates were intermediate to those of rats with lesions and intact group-housed rats, and the behavior of isolates during passive and active avoidance training was generally similar to that of grouped rats with lesions. The introduction of a distractor during approach training in an alley reduced running speeds more in rats with lesions than in controls. The several significant interactions between housing state and lesion state suggest that neural pathways associated with the hippocampal formation may mediate some behavioral effects of differential housing.     

Developmental social isolation affects adult behavior,social interaction,and dopamine metabolite levels in zebrafish

The zebrafish is a social vertebrate and an excellent translational model for a variety of human disorders. Abnormal social behavior is a hallmark of several human brain disorders. Social behavioral problems can arise as a result of adverse early social environment. Little is known about the effects of early social isolation in adult zebrafish. We compared zebrafish that were isolated for either short (7 days) or long duration (180 days) to socially housed zebrafish, testing their behavior across ontogenesis (ages 10, 30, 60, 90, 120, 180 days), and shoal cohesion and whole‐brain monoamines and their metabolites in adulthood. Long social isolation increased locomotion and decreased shoal cohesion and anxiety in the open‐field in adult. Additionally, both short and long social isolation reduced dopamine metabolite levels in response to social stimuli. Thus, early social isolation has lasting effects in zebrafish, and may be employed to generate zebrafish models of human neuropsychiatric conditions.     

Effects of prenatal alcohol exposure on social competence: Asymmetry in play partner preference among heterogeneous triads of male and female rats

Social behavior deficits associated with prenatal alcohol exposure (PAE) are frequently described in terms of impaired social competence, which can be defined as the effectiveness in social interaction and the ability to employ social skills successfully within different interpersonal contexts. Play behavior—which peaks during adolescence—is critical for developing social competence, as well as for motor, cognitive, and emotional development. Studies of play behavior typically utilize protocols where animals interact in dyads. However, less is understood about how the social environment may shape PAE-related social behavior deficits, particularly in more complex social contexts. Here, we assess play partner preference utilizing a novel approach in which adolescent male and female animals interact within same-sex triads comprised of animals from mixed prenatal treatments to determine how play partner identity and social group composition interact to shape behavior. When triads included one PAE animal and two control animals (i.e., control animals had the option to play either with a fellow control or a PAE playmate), we observed play target asymmetry whereby controls preferentially played with fellow controls. Notably, these results were consistent for triads of both males and females, with subtle differences in frequency of initiations versus reciprocations. We found no play target asymmetry, however, when triads included two PAE animals and one control animal or different configurations of control and pair-fed animals. Taken together, play target asymmetry resulting from ineffective social interactions, including a failure to engage with, respond to, and/or solicit play from control play partners appropriately, suggests that PAE negatively impacts the development of social competence.     

A novel social fear conditioning procedure alters social behavior and mTOR signaling in differentially housed adolescent rats

Vulnerabilities to fear‐related disorders can be enhanced following early life adversity. This study sought to determine whether post‐weaning social isolation (PSI), an animal model of early life adversity, alters the development of social fear in an innovative model of conditioned social fear. Male and female Sprague‐Dawley rats underwent either social rearing (SR) or PSI for 4 weeks following weaning. Rats were then assigned to groups consisting of either Footshock only, Social conditioned stimulus (CS) only, or Paired footshock with a social CS. Social behavior was assessed the next day. We observed a novel behavioral response in PSI rats, running in circles, that was rarely observed in SR rats; moreover, this behavior was augmented after Paired treatment in PSI rats. Other social behaviors were altered by both PSI and Paired footshock and social CS. The mammalian target of rapamycin (mTOR) pathway was assessed using immunohistochemistry for phosphorylated ribosomal protein S6 (pS6) in subregions of the prefrontal cortex (PFC) and amygdala. Paired treatment produced opposite effects in the PFC and amygdala in males, but no differences were observed in females. Conditioned social fear produced alterations in social behavior and the mTOR pathway that are dependent upon rearing condition and sex.     

Sequential analysis of juvenile isolation-induced decreased social behavior in the adult rat.

The consequences of juvenile isolation on adult social behavior were studied in detail using two different analysis methods: frequency, duration, and latency of behavioral elements, and sequential analysis. Rats were either isolated or socially housed during weeks 4 and 5 of age, and after the isolation period housed in pairs with a rat of identical housing condition until the time of testing at 12 weeks of age. Juvenile isolation caused marked effects on the frequency, duration, and latency of various social behavioral elements, whereas the non-social activities such as ambulation, rearing, and self-grooming were hardly affected. Juvenile isolation reduced social exploration, anogenital sniffing, and approach/following and increased the latency to the first occurrence of these social behavioral elements. In contrast, the sequential analysis revealed that the structure of social behavior was barely affected by juvenile isolation. Some transitions were less pronounced in juvenile isolated rats compared to non-isolated rats, but no significant differences were observed in transitions between social elements. Thus, juvenile isolation bisected the time spent on adult social interactions, whereas it did not disrupt the sequential structure of social behavior. The present data suggest that juvenile isolation reduced the motivation for adult social behavior, but when social contact is initiated, a relatively normal social behavioral pattern is displayed.     

Adolescent pre-exposure to ethanol or MDMA prolongs the conditioned rewarding effects of MDMA

Adolescents often take ethanol (EtOH) in combination with MDMA (3,4-methylenedioxymethylamphetamine). In the present work we studied the effect of repeated intermittent adolescent pre-exposure to both drugs on the behavioral and neurochemical effects of MDMA in mice. Sixteen days after pre-treatment, the rewarding and reinstating effects of MDMA in the conditioned place preference (CPP) paradigm were evaluated, along with the levels of biogenic amines, basal motor activity and corticosterone response to different challenges. Pre-exposure to EtOH, MDMA or EtOH + MDMA did not affect the CPP induced by 10 mg/kg of MDMA. However, adolescent exposure to EtOH or MDMA increased the duration of the conditioned rewarding effects of MDMA. Following extinction of the CPP, a priming dose of 5 mg/kg of MDMA elicited reinstatement in all the groups, with the duration of this reinstated CPP being longer in mice pre-treated with MDMA. After reinstatement, an increase in monoamine levels was observed in mice pre-exposed to EtOH (DA, DOPAC and 5-HT in the striatum and 5-HIAA in the cortex and hippocampus) or MDMA (5-HT in the hippocampus). Basal motor activity and basal levels of corticosterone were not affected by any of these pre-treatments, but the group pre-exposed to MDMA showed higher levels of corticosterone in response to the administration of 10 mg/kg of MDMA. Behavioral and hormonal effects of adolescent exposure to MDMA were reversed by co-administration of EtOH. Our results suggest that exposure to EtOH or MDMA during adolescence prolongs the rewarding properties of MDMA.     

Cocaine and morphine-induced place conditioning in adolescent and adult rats

The periadolescent period in the rat is characterized by alterations in novelty seeking and exploratory behavior, as well as changes in the behavioral responsiveness to many drugs of abuse. These alterations may be predictive of alterations in the reward value of drugs of abuse. The present experiments examined whether adolescent rats (34-37 days old) differ from their adult counterparts in the expression of drug-induced place conditioning for morphine (0, 2.5, or 5 mg/kg; Experiment I) and cocaine (0, 5, or 10 mg/kg; Experiments II and III). Animals received multiple conditioning days, followed 24 h later by a drug-free CPP test. All drugs were given intraperitoneally immediately prior to confinement in the CS+ compartment, while vehicle injections were given prior to exposure to the CS- chamber. For both drugs, there were no significant differences between adolescents and adults in amount of place conditioning seen when drug exposure was paired with the nonpreferred chamber. When cocaine was paired with either the preferred or nonpreferred compartment (Experiment III), again, the magnitude of the place conditioning observed did not differ between adolescents and adults. The lack of age differences in expression of drug-induced place conditioning in the present experiments is not likely a result of ceiling effects, because the data suggest that the doses used included near-threshold doses. Although these findings need to be confirmed using other approaches for assessing drug reward before concluding that adolescent and adult rats exhibit similar sensitivity to the rewarding effects of morphine and cocaine, the current data revealed no differences between adolescents and adults in the magnitude of place conditioning expressed for morphine or cocaine.     

Naloxone attenuates the conditioned place preference induced by wheel running in rats

Pairings, during which an episode of wheel running is followed by confinement in a distinctive place, produce conditioned place preference (CPP) in rats. This finding indicates that wheel running has a rewarding effect that outlasts the activity itself. In two similar experiments, we tested the hypothesis that this rewarding effect of wheel running is mediated by endogenous opioids. During a paired trial, the rats in the naloxone group were first allowed to wheel run for 2 h, then injected with naloxone (0.5 or 0.1 mg/kg in Experiments 1 and 2, respectively), and 10 min later placed in a distinctive chamber. During an unpaired trial, these rats were confined in an adjoining chamber without wheel running. Naloxone was injected before placement in both chambers, so that if naloxone-induced conditioned place aversion occurred, it would have counteracting effects on performance during the preference test. The rats in the saline group were similarly treated, except that saline was injected instead of naloxone. CPP occurred in the saline group, but not in the naloxone group. Thus, naloxone attenuated the CPP induced by wheel running. This finding supports the hypothesis that the rewarding effect of wheel running is mediated by endogenous opioids.     

Differential behavioral responses of the spontaneously hypertensive rat to methylphenidate and methamphetamine: lack of a rewarding effect of repeated methylphenidate treatment

Several questions remain unanswered concerning the effects of long-term methylphenidate treatment in individuals with attention-deficit/hyperactivity disorder (ADHD). It has been speculated that repeated methylphenidate treatment may facilitate abuse of the drug or psychological dependence. In the present study, we conducted conditioned place preference (CPP) tests to investigate whether the repeated treatment of methylphenidate results to greater "liking" of the drug. We compared the effect of methylphenidate with that of methamphetamine, a drug with high abuse and dependence liability; also used as a treatment of ADHD. Prior to CPP tests, adolescent spontaneously hypertensive rats (SHR) (putative rodent model of ADHD) and Wistar rats (strain used to represent the "normal" heterogeneous population) were administered intraperitoneally with methylphenidate (1.25, 5 and 20 mg/kg) or methamphetamine (1.25 and 5 mg/kg) for 14 days in their home cages. CPP tests were commenced and rats were conditioned with the two stimulants at the doses stated. We found that (1) repeated administration of methylphenidate and methamphetamine was rewarding in Wistar rats (2) stimulant-treated SHR showed CPP only to methamphetamine but not to methylphenidate. The observation that Wistar rats, but not SHR showed CPP to methylphenidate indicates vulnerability of "normal" individuals to methylphenidate abuse and dependence following repeated exposure or administration of the drug. The findings in SHR suggest the safety of methylphenidate as an ADHD intervention insofar as its behavioral effects are compared with those of methamphetamine, and to the extent that the SHR appropriately models ADHD in humans.     

Aging-related changes in the effects of social isolation on social behavior in rats

Aging is generally associated with cognitive dysfunction and alterations in emotional response. Moreover, in social situations, aging decreases social interaction with unfamiliar individuals, suggesting the decline of social cognition/motivation and a high level of anxiety. Although it is known that isolation housing has various effects on subsequent behavior, including social interaction depending on the age at isolation, the effects of isolation on aged subjects have not been examined. In the present study, we investigated the effects of aging and different periods of isolation housing on social interaction in male F344/N rats. Young (3-4 months old) and aged (24-25 months old) rats were either group-housed or socially isolated for 2 or 4 weeks. The rats were tested with age-matched and group-housed unfamiliar males in a social interaction test, and social (e.g. approach/following and sniffing) and non-social behaviors (e.g. self-grooming and ambulation) were recorded. The results indicated that group-housed aged rats showed less approach/following, sniffing, and ambulation than group-housed young rats. Moreover, in young rats, isolation housing gradually increased approach/following and sniffing depending on the isolation period. In contrast, in aged rats, more prolonged isolation (4 weeks) attenuated the 2-week isolation-induced increase of sniffing behavior and had no effect on approach/following. The present study suggests that aging decreases social investigation and induces high emotional response to a novel social environment, and that the behaviors can be differentially affected by social isolation depending on the age at isolation and the period of isolation.