Shoshana Eitan

Attenuation of cocaine-induced locomotor sensitization in rats sustaining genetic or pharmacologic antagonism of ghrelin receptors.

Systemic infusions of the orexigenic peptide ghrelin (GHR) increase dopamine levels within the nucleus accumbens and augment cocaine‐stimulated locomotion and conditioned place preference in rats; observations that suggest an important role for GHR and GHR receptors (GHR‐Rs) in drug reinforcement. In the present studies, we examined the development of cocaine locomotor sensitization in rats, sustaining either pharmacologic antagonism or genetic ablation of GHR‐Rs. In a pharmacologic study, adult male rats were injected (i.p.) with either 0, 3 or 6 mg/kg JMV 2959 (a GHR‐R1 receptor antagonist), and 20 minutes later, with either vehicle or 10 mg/kg cocaine HCl on each of 7 consecutive days. Rats pretreated with JMV 2959 showed significantly attenuated cocaine‐induced hyperlocomotion. In a second study, adult wild‐type (WT) or mutant rats sustaining ENU‐induced knockout of GHR‐R [GHR‐R (−/−)] received daily injections (i.p.) of vehicle (0.9% saline) or 10.0 mg/kg cocaine HCl for 14 successive days. GHR‐R null rats treated repeatedly with cocaine showed diminished development of cocaine locomotor sensitization relative to WT rats treated with cocaine. To verify the lack of GHR‐R function in the GHR‐R (−/−) rats, a separate feeding experiment was conducted in which WT rats, but not GHR‐R (−/−) rats, were noted to eat more after a systemic injection of 15 nmol GHR than after vehicle. These results suggest that GHR‐R activity is required for the induction of locomotor sensitization to cocaine and complement an emerging literature implicating central GHR systems in drug reward. GHR is an orexigenic gut peptide that is transported across the blood–brain barrier and interacts with GHR‐Rs located on ventral tegmental dopamine neurons.

Different affective response to opioid withdrawal in adolescent and adult mice

AIMS Drug withdrawal is suggested to play a role in precipitating mood disorders in individuals with familial predisposition. Age-related differences in affective responses to withdrawal might explain the increased risk of mental illnesses when drug use begins during adolescence. Since there is a lack of animal research examining the effects of opioid withdrawal during adolescence, the present study examined whether there are age-related differences in affective responses to opioid withdrawal. MAIN METHODS Adolescent and adult mice were injected with two different morphine regimens, namely low and high, which differed in the dosage. Three and nine days following discontinuation of morphine administration, immobility time in the forced swim test (FST) and locomotion (total distance traveled) were evaluated. KEY FINDINGS On withdrawal day 3 (WD3), adolescent mice exhibited a decrease in immobility as compared to controls. No significant differences in immobility were observed on withdrawal day 9 (WD9). This effect on FST behaviors was not due to changes in overall motor activity, since no differences in locomotion were observed on either WD3 or WD9 in adolescent mice. In adults, no differences in either FST or locomotor behaviors were observed on WD3. As expected, on WD9, adult mice exhibited an increase in immobility and a decrease in locomotion. SIGNIFICANCE This study demonstrates age-dependent differences in both FST scores and locomotor behaviors during opioid withdrawal. FST behaviors are classically used to evaluate mood in rodents, thus this study suggests that opioid withdrawal might affect mood differentially across age.

Pharmacologic antagonism of ghrelin receptors attenuates development of nicotine induced locomotor sensitization in rats.

AIMS Ghrelin (GHR) is an orexigenic gut peptide that interacts with ghrelin receptors (GHR-Rs) to modulate brain reinforcement circuits. Systemic GHR infusions augment cocaine stimulated locomotion and conditioned place preference (CPP) in rats, whereas genetic or pharmacological ablation of GHR-Rs has been shown to attenuate the acute locomotor-enhancing effects of nicotine, cocaine, amphetamine and alcohol and to blunt the CPP induced by food, alcohol, amphetamine and cocaine in mice. The stimulant nicotine can induce CPP and like amphetamine and cocaine, repeated administration of nicotine induces locomotor sensitization in rats. A key issue is whether pharmacological antagonism of GHR-Rs would similarly attenuate nicotine-induced locomotor sensitization. METHOD To examine the role of GHR-Rs in the behavioral sensitizing effects of nicotine, adult male rats were injected with either 0, 3 or 6 mg/kg of the GHR-R receptor antagonist JMV 2959 (i.p.) and 20 min later with either vehicle or 0.4 mg/kg nicotine hydrogen tartrate (s.c.) on each of 7 consecutive days. RESULTS Rats treated with nicotine alone showed robust locomotor sensitization, whereas rats pretreated with JMV 2959 showed significantly attenuated nicotine-induced hyperlocomotion. CONCLUSIONS These results suggest that GHR-R activity is required for the induction of locomotor sensitization to nicotine and complement an emerging literature implicating central GHR systems in drug reward/reinforcement.

Increased elevated plus maze open-arm time in mice during naloxone-precipitated morphine withdrawal.

Opioid withdrawal is known to be anxiogenic in humans and, using the elevated plus maze (EPM), was demonstrated to also be anxiogenic in rats. Thus, this study characterizes EPM behaviors of mice during naloxone-precipitated morphine withdrawal. Naloxone did not significantly change EPM behaviors of drug-naïve mice. Additionally, morphine-dependent mice in which withdrawal was not precipitated (i.e. morphine-dependent mice receiving saline) spent less time in the open-arms compared to the controls. Surprisingly, increased open-arm time was observed in morphine-dependent mice undergoing naloxone-precipitated withdrawal. This increase was not because of total motor activity, as no significant differences in total activity were observed. Moreover, morphine dependency was necessary, given that there was not a significant increase in open-arm time for mice undergoing withdrawal from acute morphine. Increased open-arm time during withdrawal is unexpected, given that opioid withdrawal is usually associated with anxiety. Additionally, even in mice, naloxone-precipitated morphine withdrawal is known be aversive and increases plasma corticosterone levels. In conclusion, this study demonstrates somewhat unexpected EPM behavior in mice undergoing naloxone-precipitated morphine withdrawal. Possible interpretations of these EPM results, though somewhat speculative, raise the possibility that EPM behaviors might not be driven exclusively by anxiety levels but rather by other withdrawal-induced behaviors.

Increased elevated plus maze open-arm time in mice during spontaneous morphine withdrawal

Rats undergoing both naloxone-precipitated and spontaneous opioid withdrawal exhibit anxiogenic behaviors in the elevated plus maze (EPM). Recently, we observed an unexpected result, namely mice exhibited increased EPM open-arm time during naloxone-precipitated morphine withdrawal. This was surprising since this behavioral outcome is usually associated with an anxiolytic profile. This study demonstrates that mice exhibit an increase in both EPM open-arm time and % open-arm entries also during spontaneous opioid withdrawal.

Social influences on morphine conditioned place preference in adolescent mice

Social/peer influences are among the strongest predictors of adolescent drug use. However, this important subject does not get much attention in pre‐clinical studies. We recently observed that exposure to different social partners modulates morphine locomotor sensitization. Sensitivity to the hyper‐locomotor response of drugs of abuse is a predictor of sensitivity to other drug‐induced behaviors. Thus, this study examined how exposure to different social partners affected the rewarding properties of morphine. All animals were group‐housed four per cage in one of two conditions referred to as ‘only’ and ‘cage‐mates’. In the mixed treatment condition, morphine‐ and saline‐treated mice were housed together. These groups are referred to as ‘morphine cage‐mates’ and ‘saline cage‐mates’, respectively. In the separated treatment conditions, all mice in the cage received morphine (i.e. ‘morphine only’) or saline (i.e. ‘saline only’), and cages were visually separated from each other. All animals were subsequently individually tested for the acquisition of morphine conditioned place preference (CPP) following one conditioning session with 10, 20 or 40 mg/kg morphine or saline. As expected, one conditioning session established morphine CPP in the morphine only animals, but not in the saline only animals. Notably, morphine CPP was not acquired by the morphine cage‐mate animals. Additionally, 40 mg/kg morphine was sufficient to establish morphine CPP in the saline cage‐mate animals. These results indicate that social environment has an effect on the rewarding properties of morphine. It suggests that exposure to different peers can alter the abuse potential of opioids and potentially other illicit drugs.

Differential effects of oxycodone, hydrocodone, and morphine on the responses of D2/D3 dopamine receptors.

Oxycodone and hydrocodone are opioids which are widely used for pain management and are also commonly misused and abused. The exposure to opioid analgesics has been associated with altered responses of D2-like dopamine receptors (D2DRs). Our recent results suggest that various opioids will differentially modulate the responses of D2DRs. The D2DRs are known to be involved in the pathology of addiction and other mental illnesses, indicating the need to improve our understanding of the effects of opioid analgesics on the responses of the D2DRs. Thus, in this study, we first established equianalgesic oral doses of oxycodone, hydrocodone, and morphine using the tail withdrawal assay. Then, mice were orally administered (gavage) with the various opioids or saline once daily for 6 days. Twenty-four hours later, the mice were tested for their locomotor response to quinpirole, a D2/D3 dopamine receptor agonist. Mice pretreated with oxycodone showed significantly greater locomotor supersensitivity to quinpirole than did morphine-pretreated mice, while hydrocodone-pretreated mice showed sensitivity in between that of mice treated with morphine and oxycodone. This finding suggests that various opioids differentially modulate the responses of D2DRs. It provides further evidence supporting of the notion that various opioids carry differential risks to the dopamine reward system.

Extracellular signal-regulated kinase activation in the amygdala mediates elevated plus maze behavior during opioid withdrawal.

This study examined whether activation of extracellular signal-regulated kinase (ERK) contributes to the increased open-arm time observed in the elevated plus maze (EPM) during opioid withdrawal. We applied SL327, a selective ERK kinase (MEK) inhibitor, to specific limbic areas and examined the effect on EPM behaviors of controls and during naloxone-precipitated morphine withdrawal. We next confirmed that ERK activation increased in limbic areas of mice undergoing naloxone-precipitated morphine withdrawal. Direct injection of SL327 into the amygdala blocked the withdrawal-induced increase in open-arm time; however, injecting SL327 into the septum had no effect. Consistent with these results, both 0.2 and 2 mg/kg naloxone increased ERK activation in the central amygdala of morphine-dependent mice. In drug-naive mice, 2 mg/kg naloxone, but not 0.2 mg/kg, increased ERK activation in the central amygdala. During withdrawal, increased ERK activation was also observed in the lateral septum. In the locus coeruleus, a significant increase was observed only in morphine-dependent mice receiving 2 mg/kg, but not 0.2 mg/kg naloxone. In conclusion, ERK activation in limbic areas is likely involved in both the aversive properties of naloxone and in the affective/emotional symptoms of opioid withdrawal, including mediating EPM behaviors.

Morphine alters the locomotor responses to a D2/D3 dopamine receptor agonist differentially in adolescent and adult mice

The D2-like dopamine receptors mediate the emotional/aversive state during morphine withdrawal. Given age-dependent differences in the affective responses to withdrawal, this study examined whether the response to dopamine receptor agonists is altered differentially across ages following morphine administration. Adolescent and adult mice were injected with morphine (twice daily, 10–40 mg/kg, s.c.) or saline for 6 days. Subsequently, they were examined for their locomotor response to quinpirole, a D2/D3 receptor agonist, and SKF 38393, a D1 receptor agonist. Quinpirole dose-dependently reduced locomotion in drug-naïve animals. Initial suppression was also observed in morphine-treated animals, but was followed by enhanced locomotion. Notably, this enhanced locomotion was markedly greater in adolescents than adults. Quinpirole-induced hypo-locomotion is thought to be mediated by the presynaptic D2Short receptors, whereas its activating effect is mediated by postsynaptic D2Long/D3 receptors. This suggests that following morphine administration, the postsynaptic, but not the presynaptic, dopaminergic signaling is differentially modulated across ages. This locomotor supersensitivity was not observed for SKF 38393, a D1 dopamine receptor agonist. The D2/D3 receptors are involved in the pathophysiology of many mental illnesses. Thus, this study offers a potential explanation for the increased psychiatric disorder co-morbidities when drug use begins during adolescence.

Social environment alters opioid-induced hyperalgesia and antinociceptive tolerance in adolescent mice.

Chronic opioid treatment is complicated by the development of tolerance and hyperalgesia. Social environment alters both opioid‐induced behaviours and nociceptive mechanisms. Our previous studies demonstrated that, in adolescent rodents, the susceptibility to acquire opioid dependence and reward is dependent on the nature of social housing conditions. Specifically, our previous studies demonstrate that housing morphine‐treated mice with drug‐naïve animals mitigates the abuse liability of opioids. Thus, this study tested the effect of social housing conditions on the development of adaptive processes to morphine antinociception.