Social environment modulates morphine sensitivity: A partial role of vasopressin V1b receptor

Social factors influence drug abuse in adolescents; this is partially attributed to peer pressure in humans. Similarly, using rodent models, some research suggests that social housing condition can influence rodents' drug taking behavior. Despite this, few studies have examined the role that intoxicated peers have on drug-naive cage-mates. This dissertation examined how social environment affects opioid sensitivity and hormone production. This was accomplished by comparing the opioid sensitivity of mice housed in mixed cages (some animals received opioids and some were drug-naive) to cages where all the mice were treated with the same drug (all saline or all morphine). These studies identified an adolescent-specific vulnerability to social environment-induced alteration of morphine sensitivity. Interaction with drug-intoxicated cage-mates enhanced locomotor sensitivity in previously drug-naive males and altered their production of testosterone. Conversely, interaction of morphine experienced mice with drug-naive cage-mates afforded protection from the rewarding properties of morphine. In other words, morphine-treated mice housed with drug-naive cage-mates demonstrated attenuated reward compared to morphine-treated mice housed with other morphine-treated mice. In addition, part of the neurobiological basis of the social-environment effect was identified. Antagonism of V1b receptors decreased morphine reward in morphine-treated mice housed only with other morphine-treated mice. These results suggest a role of vasopressin in the peer influence on drug sensitivity observed in adolescents. This body of work further elucidates the role of peer influence on opioid sensitivity. Future studies should further reveal the role of healthy peer relationships and should aid in combating drug abuse in this at-risk demographic

Social environment modulates morphine sensitivity: A partial role of vasopressin V1b receptor

Social factors influence drug abuse in adolescents; this is partially attributed to peer pressure in humans. Similarly, using rodent models, some research suggests that social housing condition can influence rodents' drug taking behavior. Despite this, few studies have examined the role that intoxicated peers have on drug-naive cage-mates. This dissertation examined how social environment affects opioid sensitivity and hormone production. This was accomplished by comparing the opioid sensitivity of mice housed in mixed cages (some animals received opioids and some were drug-naive) to cages where all the mice were treated with the same drug (all saline or all morphine). These studies identified an adolescent-specific vulnerability to social environment-induced alteration of morphine sensitivity. Interaction with drug-intoxicated cage-mates enhanced locomotor sensitivity in previously drug-naive males and altered their production of testosterone. Conversely, interaction of morphine experienced mice with drug-naive cage-mates afforded protection from the rewarding properties of morphine. In other words, morphine-treated mice housed with drug-naive cage-mates demonstrated attenuated reward compared to morphine-treated mice housed with other morphine-treated mice. In addition, part of the neurobiological basis of the social-environment effect was identified. Antagonism of V1b receptors decreased morphine reward in morphine-treated mice housed only with other morphine-treated mice. These results suggest a role of vasopressin in the peer influence on drug sensitivity observed in adolescents. This body of work further elucidates the role of peer influence on opioid sensitivity. Future studies should further reveal the role of healthy peer relationships and should aid in combating drug abuse in this at-risk demographic

Regulation by endogenous dopamine of the expression of the clock protein, PERIOD2, in the forebrain of the male Wistar rat

Daily exposure to light is the most powerful means of entraining circadian rhythms of clock gene expression in the suprachiasmatic nucleus (SCN), the body's master timekeeper, and in other brain regions and peripheral tissues; however, a variety of non-photic stimuli have also been found to entrain rhythmic clock gene expression in the brain, including motivationally significant events such as stressors and rewards. The present experiments investigated the role of the neurotransmitter, dopamine (DA), in entraining the daily rhythm of the clock protein, PERIOD2 (PER2), in the SCN of the male Wistar rat, as well as in several regions of the limbic forebrain that regulate motivational states: namely, the dorsal striatum, the oval nucleus of the bed nucleus of the stria terminalis (BNSTov), the central nucleus of the amygdala (CEA), the basolateral amygdala (BLA), and the dentate gyrus (DG). In the dorsal striatum, the normal daily peak of PER2 expression was blunted following selective destruction of midbrain DA neurons with 6-hydroxydopamine (6-OHDA) and after blockade of D2 DA receptors with the antagonist, raclopride, whereas daily injections of a D2 agonist but not a D1 agonist restored and entrained the PER2 rhythm in the 6-OHDA-lesioned striatum. Disruption of catecholamine signaling in general using systemic injections of the tyrosine hydroxylase inhibitor, alpha-methyl-para-tyrosine (AMPT), or daily morphine injections and withdrawal of morphine also blunted the normal PER2 peak in the dorsal striatum. Together, these results suggest that daily stimulation of D2 receptors is necessary for the striatal PER2 rhythm. In the BNSTov and CEA, daily injections of a D2 agonist or morphine increased PER2 expression near the time of injection, but other manipulations of DA signaling or catecholamines in general had no effect on PER2. In the BLA and DG, disruption of catecholamine signaling using AMPT or morphine injections blunted the normal PER2 peak in these regions but DA-selective manipulations had no effect on PER2. None of these manipulations affected the normal PER2 rhythm in the SCN. Taken together, these findings indicate that daily stimulation of 02 receptors regulates the PER2 rhythm in the dorsal striatum, and that DA does not directly contribute to the normal PER2 rhythm in the SCN or in any other forebrain region examined

Emotional Impairment and Persistent Upregulation of mGlu5 Receptor following Morphine Abstinence: Implications of an mGlu5-MOPr Interaction.

BACKGROUND: A difficult problem in treating opioid addicts is the maintenance of a drug-free state because of the negative emotional symptoms associated with withdrawal, which may trigger relapse. Several lines of evidence suggest a role for the metabotropic glutamate receptor 5 in opioid addiction; however, its involvement during opioid withdrawal is not clear. METHODS: Mice were treated with a 7-day escalating-dose morphine administration paradigm. Following withdrawal, the development of affective behaviors was assessed using the 3-chambered box, open-field, elevated plus-maze and forced-swim tests. Metabotropic glutamate receptor 5 autoradiographic binding was performed in mouse brains undergoing chronic morphine treatment and 7 days withdrawal. Moreover, since there is evidence showing direct effects of opioid drugs on the metabotropic glutamate receptor 5 system, the presence of an metabotropic glutamate receptor 5/μ-opioid receptor interaction was assessed by performing metabotropic glutamate receptor 5 autoradiographic binding in brains of mice lacking the μ-opioid receptor gene. RESULTS: Withdrawal from chronic morphine administration induced anxiety-like, depressive-like, and impaired sociability behaviors concomitant with a marked upregulation of metabotropic glutamate receptor 5 binding. Administration of the metabotropic glutamate receptor 5 antagonist, 3-((2-Methyl-4-thiazolyl)ethynyl)pyridine, reversed morphine abstinence-induced depressive-like behaviors. A brain region-specific increase in metabotropic glutamate receptor 5 binding was observed in the nucleus accumbens shell, thalamus, hypothalamus, and amygdala of μ-opioid receptor knockout mice compared with controls. CONCLUSIONS: These results suggest an association between metabotropic glutamate receptor 5 alterations and the emergence of opioid withdrawal-related affective behaviors. This study supports metabotropic glutamate receptor 5 system as a target for the development of pharmacotherapies for the treatment of opioid addiction. Moreover, our data show direct effects of μ-opioid receptor system manipulation on metabotropic glutamate receptor 5 binding in the brain

Cingulate NMDA NR2B receptors contribute to morphine-induced analgesic tolerance

Morphine is widely used to treat chronic pain, however its utility is hindered by the development of tolerance to its analgesic effects. While N-methyl-D-aspartate (NMDA) receptors are known to play roles in morphine tolerance and dependence, less is known about the roles of individual NMDA receptor subtypes. In this study, Ro 256981, an antagonist of the NMDA receptor subunit NR2B, was used to reduce the expression of analgesic tolerance to morphine. The mechanisms altered with chronic drug use share similarities with those underlying the establishment of long-tem potentiation (LTP) and behavioral memory. Since NMDA NR2B receptors in the anterior cingulate cortex (ACC) play roles in the establishment of LTP and fear memory, we explored their role in changes that occur in this region after chronic morphine. Both systemic and intra-ACC inhibition of NR2B in morphine-tolerant animals inhibited the expression of analgesic tolerance. Electrophysiological recordings revealed a significant increase in the NR2B component of NMDA receptor mediated excitatory postsynaptic currents (EPSCs), at both synaptic and extra-synaptic sites. However, there was no change in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor mediated EPSCs. This study suggests that selective inhibition of NMDA NR2B receptors may prove useful in combating the development of analgesic tolerance to morphine and proposes a novel role for the ACC in opioid tolerance and morphine induced changes in synaptic plasticity

The Role of Social Environment on Morphine Response in Adolescent Mice

Drug abuse is strongly influenced by socio-environmental factors. Also, social environment is one of the most important predictors for adolescent drug use. In this dissertation, I examined how social housing conditions affect morphine reward, dependence, and antinociception. I also explored possible mechanisms that may underlie these responses. Mice were group-housed in one of two conditions referred to as ‘only’ and ‘mixed’. In the only condition, all mice in the cage receive the same treatment and are physically and visually separated from mice that receive different treatments (i.e., saline only and morphine only). Mice in the mixed condition were housed together with mice that received a different treatment- (i.e., morphine cage-mate mice are housed with drug naïve mice and saline cage-mate mice are housed with morphine-treated mice). Being housed with drug-naive mice conferred a protective effect for the rewarding properties of morphine, as well as the occurrence of withdrawal symptoms. Similarly, while it did not prevent the development of tolerance to morphine analgesia, it did reduce the persistence of this tolerance. Moreover, it provided protection from morphine-induced hyperalgesia. Additionally, some of the neurobiological mechanisms underlying this protection were identified. Inhibiting the sensation of social grooming in morphine cage-mates blocked the protective effect of being housed with drug-naïve animals. Additionally, increased expression of vasopressin mRNA was observed in the striatum of morphine only animals, while being housed with drug-naïve mice protected against this effect. In line with this finding, antagonizing V1b receptors blocked the development of morphine reward in morphine only animals. Lastly, I identified various genes with increased expression levels in striatum of morphine only animals, but not morphine cage-mates. This makes them potential targets for future studies aiming to reveal the underlying molecular mechanisms involved in the protective effects of being housed with drug-naïve animals. These studies further support the notion that social conditions alter the propensity for developing opioid addiction, and can be used in the development of more efficacious behavioral and pharmacological treatments for adolescent opioid addicts

mGluR5 Involvement in Methamphetamine Reward and the Co-Morbidity of Schizophrenia and Stimulant Use Disorders

Methamphetamine (Meth) is a potent, widely-used stimulant. Stimulant abuse occurs more frequently in the schizophrenia patient population than the general population. The co-morbidity of stimulant use disorders and schizophrenia presents an understudied phenomenon and suggests overlapping brain states of these two pathologies. There are currently no FDA-approved pharmacotherapies for Meth addiction and relapse to Meth use remains a substantial challenge for abstinent Meth addicts. One factor that contributes to craving and relapse is exposure to environmental or contextual cues associated with Meth use. This effect is the consequence of associative learning that occurs between the rewarding properties of drugs and drug-related, contextual cues that become extremely salient to an individual. This drug-induced associative learning can be studied in the laboratory with humans and rodents using conditioned place preference (CPP). In the laboratory, repeated Meth administration also produces a progressive enhancement in motor activity termed motor sensitization. It is hypothesized that the neuronal adaptations associated with motor sensitization may model aspects of the brain changes that are associated with Meth abuse in humans. Schizophrenia patients and rodent models of the disorder demonstrate deficits in sensorimotor gating, or the inability to properly filter sensory information properly. Sensorimotor gating deficits are measured in the laboratory through the use of pre-pulse inhibition (PPI) of the acoustic startle response. PPI is defined as a normal suppression of the startle response when a weaker acoustic stimulus is presented before a louder startling acoustic stimulus. The current dissertation project utilized Meth-induced CPP, motor sensitization, and PPI to elucidate neuronal underpinnings of Meth addiction and schizophrenia / Meth addiction co-morbidity. The glutamate transmitter system is important for memory processing, addiction, and the neuropathology of schizophrenia. Glutamate activates both ionotropic and metabotropic receptors; the group I subtype 5 metabotropic glutamate receptor (mGluR5) is highly expressed in brain regions known to be important for stimulant reward. Thus, mGluR5 functions to fine tune neuronal excitability and modulate drug-induced behaviors. It is important, and clinically relevant, to study the neuronal adaptations that occur following repeated Meth administration in order to evaluate the ability of potential pharmacotherapy targets to influence Meth-induced behaviors in the general and schizophrenia patient population. The goal of this dissertation project was to elucidate the role of mGluR5 in different phases of Meth-induced associative learning in normal and schizophrenia-like rodents and to determine the correlation between deficits associated with schizophrenia and responding to the rewarding effects of Meth

Daily morphine injection and withdrawal disrupt 24-h wheel running and PERIOD2 expression patterns in the rat limbic forebrain

Symptoms of opiate withdrawal include disturbances in circadian rhythms. Here, we examined in male Wistar rats the effects of a daily, mid-morning morphine injection (5-40mg/kg, i.p.) and the subsequent withdrawal of morphine on 24-h patterns of wheel running and expression of the clock protein, PERIOD2 (PER2), in the master circadian clock, the suprachiasmatic nucleus (SCN), and regions of the limbic forebrain. Rats were killed either within 24 h of the last morphine injection or 2 days later. Nighttime wheel running was suppressed during daily morphine injections and following the withdrawal of morphine. Daily morphine injections and their subsequent withdrawal did not affect PER2 expression in the SCN, but blunted the normal daily peak of PER2 in the dorsal striatum, oval nucleus of the bed nucleus of the stria terminalis (BNSTov), central nucleus of the amygdala (CEA), basolateral amygdala (BLA), and dentate gyrus of the hippocampus (DG). We then examined the effect of injecting the D2/3 dopamine agonist, quinpirole (1 mg/kg, i.p.), or the alpha 2 adrenergic agonist, clonidine (0.1 mg/kg, i.p.), two drugs that alleviate opiate withdrawal symptoms, following withdrawal of the daily morphine injection. Quinpirole restored the daily PER2 pattern in the BNSTov and CEA, whereas clonidine restored and entrained a new PER2 pattern in the striatum, BLA, and DG. Together, these findings suggest that disruption of daily PER2 patterns in the forebrain might contribute to the circadian symptoms observed in opiate withdrawal. Furthermore, pharmacological treatments for withdrawal can restore PER2 patterns in regions of the limbic forebrain

Behavioral and neurochemical assessment of the role of ERK pathway in the psychopharmacological effects of ethanol, caffeine and of their interaction

The intracellular signaling cascades constitute the means by which addictive substances induce the remodeling of the circuits involved in motivated behaviors which underlie the learning processes and the development of memories at the basis of the progression of addiction. A molecule protagonist in such signaling cascades is the Extracellular signal-Regulated Kinase (ERK), a member of the mitogen-activated protein kinase, that constitutes an important biochemical factor common to many cellular functions. The abundant expression of ERK in brain areas of the addiction circuits emphasizes the relevance of these kinases in modulating behavioral functions mediated by these circuits. Thus, the general aim of the present doctoral thesis was to study the role of ERK in terms of protein kinase expression and behavioral responses induced by ethanol, caffeine and their association. The first and second chapters examined the involvement of protein kinases ERK in different aspects of ethanol-induced place conditioning. Specifically, in the first chapter we used the MEK inhibitor SL327 to study how the blockade of this cascade could affect the acquisition and expression of Conditioned Place Preference (CPP) and Conditioned Place Aversion (CPA) elicited by ethanol. The second chapter explored the results of the pharmacological relationship between caffeine and ethanol in ethanol-elicited CPP and CPA. In this chapter we also investigated the expression of pERK as a result of 1) the acute administration of both substances and 2) the presentation of stimuli positively (CPP) or negatively (CPA) conditioned to ethanol. The last chapter examines the pharmacological relationship between caffeine and ethanol through the analysis of horizontal and vertical locomotion and evaluated whether there was reciprocal cross-sensitization with respect to these effects. We also examined the phosphorylation of DARPP-32(Thr75), another factor of intracellular signaling cascade, and of ERK in the nucleus accumbens. The findings of these studies revealed that MEK/ERK pathway is differentially involved in distinct phases of associative learning behavior expressed in the CPP and CPA elicited by ethanol. Moreover, our data disclose that ERK activation takes place differentially in distinct brain regions depending on the motivational significance of the conditioned stimulus. Furthermore, we demonstrated that caffeine significantly impaires ethanol-elicited place conditioning (both CPP and CPA) and prevents ethanol-induced pERK expression in several brain areas with different activation patterns depending on the brain area examined. Our observations also revealed that caffeine and ethanol affect horizontal and vertical locomotion in different manner and without undergoing cross-sensitization. Finally, caffeine prevents ethanol-elicited pERK expression in the nucleus accumbens whereas there were no effects on pDARPP-32(Thr75). Taken together the results of the present thesis offer new insights into the complexity of the involvement of ERK cascade in the acquisition and expression of associative learning and provide new information about the antagonistic interaction between caffeine and ethanol expressed in place conditioning and locomotor activation

Acupuncture Stimulation Attenuates Impaired Emotional-Like Behaviors and Activation of the Noradrenergic System during Protracted Abstinence following Chronic Morphine Exposure in Rats

The purpose of this study was to evaluate whether acupuncture stimulation attenuates withdrawal-induced behaviors in the rats during protracted abstinence following chronic morphine exposure. To do this, male rats were first exposed to morphine gradually from 20 to 100 mg/kg for 5 days, and subsequently naloxone was injected once to extend despair-related withdrawal behaviors for 4 weeks. Acupuncture stimulation was performed once at the SP6 (Sanyinjiao) acupoint on rat’s; hind leg for 5 min during protracted abstinence from morphine. The acupuncture stimulation significantly decreased despair-like behavior deficits in the forced swimming test and low sociability in the open-field test as well as increased open-arm exploration in the elevated plus maze test in the last week of 4-week withdrawal period. Also the acupuncture stimulation significantly suppressed the increase in the hypothalamic corticotropin-releasing factor (CRF) expression, the decrease in the tyrosine hydroxylase expression in the locus coeruleus, and the decrease in the hippocampal brain-derived neurotrophic factor mRNA expression, induced by repeated injection of morphine. Taken together, these findings demonstrate that the acupuncture stimulation of SP6 significantly reduces withdrawal-induced behaviors, induced by repeated administration of morphine in rats, possibly through the modulation of hypothalamic CRF and the central noradrenergic system