Intravenous Morphine Self-administration Research Articles

Negative allosteric modulation of CB1 cannabinoid receptor signalling decreases intravenous morphine self-administration and relapse in mice.

The endocannabinoid system interacts with the reward system to modulate responsiveness to natural reinforcers, as well as drugs of abuse. Previous preclinical studies suggested that direct blockade of CB1 cannabinoid receptors (CB1R) could be leveraged as a potential pharmacological approach to treat substance use disorder, but this strategy failed during clinical trials due to severe psychiatric side effects. Alternative strategies have emerged to circumvent the side effects of direct CB1 binding through the development of allosteric modulators. We hypothesized that negative allosteric modulation of CB1R signalling would reduce the reinforcing properties of morphine and decrease behaviours associated with opioid misuse. By employing intravenous self-administration in mice, we studied the effects of GAT358, a functionally-biased CB1R negative allosteric modulator (NAM), on morphine intake, relapse-like behaviour and motivation to work for morphine infusions. GAT358 reduced morphine infusion intake during the maintenance phase of morphine self-administration under a fixed ratio 1 schedule of reinforcement. GAT358 also decreased morphine-seeking behaviour after forced abstinence. Moreover, GAT358 dose dependently decreased the motivation to obtain morphine infusions under a progressive ratio schedule of reinforcement. Strikingly, GAT358 did not affect the motivation to work for food rewards in an identical progressive ratio task, suggesting that the effect of GAT358 in decreasing opioid self-administration was reward specific. Furthermore, GAT58 did not produce motor ataxia in the rotarod test. Our results suggest that CB1R NAMs reduced the reinforcing properties of morphine and could represent a viable therapeutic route to safely decrease misuse of opioids.

Treatment with dopamine \u03b2-hydroxylase (DBH) inhibitors prevents morphine use and relapse-like behavior in rats

BackgroundOpioid use disorders are serious contributors to the harms associated with the drug use. Unfortunately, therapeutic interventions for opioid addicts after detoxification have been limited and not sufficiently effective. Recently, several studies have led to promising results with disulfiram (DSF), a dopamine β-hydroxylase (DBH) inhibitor, showing that it is a potent agent against not only alcohol but also addiction to various drugs.Materials and methodsThis study was designed to examine whether DSF and nepicastat (NEP; another DBH inhibitor) modify morphine intake and reinstatement of seeking-behavior using the rat model of intravenous morphine self-administration. Additionally, we intended to estimate the effects of both inhibitors on the locomotor activity as well as on extracellular dopamine and its metabolite levels in the nucleus accumbens using microdialysis in naive rats.ResultsWe demonstrated that both DBH inhibitors reduced responding to morphine self-administration. Moreover, DSF and NEP administered acutely before reinstatement test sessions consistently attenuated the reinforcing effects of morphine and a morphine-associated conditioned cue. The observed effects for lower doses (6.25–25 mg/kg; ip) of both DBH inhibitors seem to be independent of locomotor activity reduction and dopamine level in the nucleus accumbens. Neither DSF nor NEP administered daily during morphine abstinence with extinction training sessions had any effect on active lever-responding and changed the reinstatement induced by morphine priming doses. Reinstatement of drug-seeking behavior induced by a conditioned cue previously associated with morphine delivery was attenuated following repeated administration of DSF or NEP during the abstinence period.ConclusionThese results seem to point to the significance of DBH inhibition as a potential pharmacotherapy against morphine use disorders.Graphic abstract

Potentiation of glutamatergic synaptic transmission onto lateral habenula neurons following early life stress and intravenous morphine self-administration in rats.

Early life stress presents an important risk factor for drug addiction and comorbid depression and anxiety through persistent effects on the mesolimbic dopamine pathways. Using an early life stress model for child neglect (a single 24 h episode of maternal deprivation, MD) in rats, recent published works from our lab show that MD induces dysfunction in the ventral tegmental area and its negative controller, the lateral habenula (LHb). MD-induced potentiation of glutamatergic synaptic transmission onto LHb neurons shifts the coordination of excitation/inhibition (E/I) balance towards excitation, resulting in an increase in the overall spontaneous neuronal activity with elevation in bursting and tonic firing, and in the intrinsic excitability of LHb neurons in early adolescent male rats. Here, we explored how MD affects intravenous morphine self-administration (MSA) acquisition and sucrose preference as well as glutamatergic synaptic function in LHb neurons of adult male rats self-administering morphine. We found that MD-induced increases in LHb neuronal and glutamatergic synaptic activity and E/I ratio persisted into adulthood. Moreover, MD significantly reduced morphine intake, triggered anhedonia-like behaviour in the sucrose preference test and was associated with persistent glutamatergic potentiation 24 h after the last MSA session. MSA also altered the decay time kinetics of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR) currents in LHb neurons of control rats during this time period. Our data highlight that early life stress-induced glutamatergic plasticity in LHb may dampen the positive reinforcing and motivational properties of both natural rewards and opioids, and may contribute to the development of anhedonia and dysphoric states associated with opioids.

Indices of dentate gyrus neurogenesis are unaffected immediately after or following withdrawal from morphine self-administration compared to saline self-administering control male rats

Opiates – including morphine – are powerful analgesics with high abuse potential. In rodents, chronic opiate exposure or self-administration negatively impacts hippocampal-dependent function, an effect perhaps due in part to the well-documented opiate-induced inhibition of dentate gyrus (DG) precursor proliferation and neurogenesis. Recently, however, intravenous (i.v.) morphine self-administration (MSA) was reported to enhance the survival of new rat DG neurons. To reconcile these disparate results, we used rat i.v. MSA to assess 1) whether a slightly-higher dose MSA paradigm also increases new DG neuron survival; 2) how MSA influences cells in different stages of DG neurogenesis, particularly maturation and survival; and 3) if MSA-induced changes in DG neurogenesis persist through a period of abstinence. To label basal levels of proliferation, rats received the S-phase marker bromodeoxyuridine (BrdU, i.p.) 24 -h prior to 21 days (D) of i.v. MSA or saline self-administration (SSA). Either immediately after SA (0-D) or after 4 weeks in the home cage (28-D withdrawal), stereology was used to quantify DG proliferating precursors (or cells in cell cycle; Ki67+ cells), neuroblast/immature neurons (DCX+ cells), and surviving DG granule cells (BrdU+ cells). Analysis revealed the number of DG cells immunopositive for these neurogenesis-relevant markers was similar between MSA and SSA rats at the 0-D or 28-D timepoints. These negative data highlight the impact experimental parameters, timepoint selection, and quantification approach have on neurogenesis results, and are discussed in the context of the large literature showing the negative impact of opiates on DG neurogenesis.

Intravenous morphine self-administration alters accumbal microRNA profiles in the mouse brain.

A significant amount of evidence indicates that microRNAs (miRNAs) play an important role in drug addiction. The nucleus accumbens (NAc) is a critical part of the brain's reward circuit and is involved in a variety of psychiatric disorders, including depression, anxiety, and drug addiction. However, few studies have examined the expression of miRNAs and their functional roles in the NAc under conditions of morphine addiction. In this study, mice were intravenously infused with morphine (0.01, 0.03, 0.3, 1 and 3 mg/kg/infusion) and showed inverted U-shaped response. After morphine self-administration, NAc was used to analyze the functional networks of altered miRNAs and their putative target mRNAs in the NAc following intravenous self-administration of morphine. We utilized several bioinformatics tools, including Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway mapping and CyTargetLinker. We found that 62 miRNAs were altered and exhibited differential expression patterns. The putative targets were related to diverse regulatory functions, such as neurogenesis, neurodegeneration, and synaptic plasticity, as well as the pharmacological effects of morphine (receptor internalization/endocytosis). The present findings provide novel insights into the regulatory mechanisms of accumbal molecules under conditions of morphine addiction and identify several novel biomarkers associated with morphine addiction.

Effects of isoflurane anesthesia and intravenous morphine self-administration on regional glucose metabolism ([18 F]FDG-PET) of male Sprague-Dawley rats.

Although certain drugs of abuse are known to disrupt brain glucose metabolism (BGluM), the effects of opiates on BGluM are not well characterized. Moreover, preclinical positron emission tomography (PET) studies anesthetize animals during the scan, which limits clinical applications. We investigated the effects of (i) isoflurane anesthesia and (ii) intravenous morphine self-administration (MSA) on BGluM in rats. Jugular vein cannulated adult male Sprague-Dawley rats self-administered either saline (SSA) or morphine (0.5mg/kg/infusion, 4h/day for 12days). All animals were scanned twice with [18 F]-fluoro-deoxy-glucose (FDG)-PET/CT at a baseline and at 2-day withdrawal from self-administration. After the IV injection of FDG, one batch of animals (n=14) was anesthetized with isoflurane and the other batch (n=16) was kept awake during the FDG uptake (45min). After FDG uptake, all animals were anesthetized in order to perform a PET/CT scan (30min). Isoflurane anesthesia, as compared to the awake condition, reduced BGluM in the olfactory, cortex, thalamus, and basal ganglia, while increasing BGluM in the midbrain, hypothalamus, hippocampus, and cerebellum. Morphine self-administered animals exhibited withdrawal signs (piloerection and increased defecation), drug seeking, and locomotor stimulation to morphine (0.5mg/kg) during the 2day withdrawal. The BGluM in the striatum was increased in the MSA group as compared to the SSA group; this effect was observed only in the isoflurane anesthesia, not the awake condition. These findings suggest that the choice of the FDG uptake condition may be important in preclinical PET studies and increased BGluM in the striatum may be associated with opiate seeking in withdrawal.

Individual differences in initial morphine sensitivity as a predictor for the development of opiate addiction in rats

Individuals report a wide range of analgesia to similar doses of opiates, and not all opiate users become addicted. This suggests that there may be certain predispositions that influence one to develop opiate addiction. We investigated the relationship between the individual differences in initial morphine sensitivity and the subsequent development of opiate addiction-like behavior using a hot plate test and an intravenous morphine self-administration (MSA) paradigm in rats. Using a median split of initial morphine antinociception, animals were defined as low antinociception (LA) and high antinociception (HA) groups. Thus, the LA group represents the animals that were less sensitive to initial morphine antinociception as compared to those of the HA group. The animals were allowed to self-administer either saline or morphine (0.5mg/kg/infusion, 4hr/day) 5days per week for 3 weeks. Spontaneous locomotor activity was measured on self-administration days 10 and 15. Individual differences in initial morphine sensitivity were not correlated with the amount of morphine self-administered by the animals on day 1. In the second-week of MSA, the LA group exhibited increased morphine intake and locomotor hyperactivity as compared to those of the HA group. Therefore, certain animals that are less sensitive to initial morphine antinociception may be susceptible to developing opiate addiction. The current findings may have clinical implications for future research on the biological mechanisms of opiate addiction and preclinical medication development.

Reducing effect of saikosaponin A, an active ingredient of Bupleurum falcatum, on alcohol self-administration in rats: Possible involvement of the GABAB receptor

Recent studies demonstrated that treatment with saikosaponin A (SSA) – an active ingredient of the medicinal herb, Bupleurum falcatum L. – selectively suppressed, likely via a GABAB receptor-mediated mechanism, intravenous self-administration of morphine and cocaine in rats [Yoon et al., 2012; 2013]. The present study was designed to investigate whether the capacity of SSA to suppress morphine and cocaine self-administration extends to oral alcohol self-administration. To this end, selectively bred Sardinian alcohol-preferring (sP) rats were trained to lever-respond on a Fixed Ratio (FR) 4 (FR4) schedule of reinforcement for alcohol (15%, v/v) in daily 30-min sessions. Once responding had stabilized, rats were tested under the FR4 (measure of alcohol reinforcing properties) and Progressive Ratio (PR; measure of alcohol motivational properties) schedules of reinforcement. The possible involvement of the GABAB receptor system was investigated testing the effect of (a) pretreatment with the GABAB receptor antagonist, SCH50911, and (b) combined treatment with the positive allosteric modulator of the GABAB receptor, GS39783. Treatment with SSA (0, 0.25, 0.5, and 1mg/kg, i.p.) markedly reduced lever-responding for alcohol, amount of self-administered alcohol, and breakpoint for alcohol (defined as the lowest response requirement not achieved in the PR experiment). Pretreatment with 2mg/kg SCH50911 (i.p.) resulted in a partial blockade of the reducing effect of 0.5mg/kg SSA on lever-responding for alcohol and amount of self-administered alcohol. Combination of per se ineffective doses of GS39783 (5mg/kg, i.g.) and SSA (0.1mg/kg, i.p.) reduced lever-responding for alcohol and amount of self-administered alcohol. These results (a) extend to alcohol self-administration the capacity of SSA to suppress morphine and cocaine self-administration in rats and (b) suggest that the GABAB receptor system is likely part of the neural substrate underlying the reducing effect of SSA on alcohol self-administration.

Increased intravenous morphine self-administration following Roux-en-Y gastric bypass in dietary obese rats.

Roux-en-Y gastric bypass (RYGB) surgery is a commonly performed and very effective method to achieve significant, long-term weight loss. Opioid analgesics are primarily used to manage postoperative pain as fewer alternative medication options are available for bariatric surgery patients than for the general population. Recent clinical studies support a greater risk for substance use following bariatric surgery, including an increased use of opioid medications. The present study is the first to study morphine self-administration in a rat model of RYGB. High fat diet-induced obese (HFD-DIO) rats underwent RYGB (n=14) or sham-surgery with ad libitum HFD (SHAM, n=14) or a restricted amount that resulted in weight matched to the RYGB cohort (SHAM-WM, n=8). An additional normal-diet (ND, n=7), intact (no surgery) group of rats was included. Two months after the surgeries, rats were fitted with jugular catheters and trained on a fixed ratio-2 lick task to obtain morphine intravenously. Both morphine-seeking (number of licks on an empty spout to obtain morphine infusion) and consumption (number of infusion) were significantly greater in RYGB than any control group beginning on day 3 and reached a two-fold increase over a period of two weeks. These findings demonstrate that RYGB increases motivation for taking morphine and that this effect is independent of weight loss. Further research is warranted to reveal the underlying mechanisms and to determine whether increased morphine use represents a risk for opioid addiction following RYGB. Identifying risk factors preoperatively could help with personalized postoperative care to prevent opioid abuse and addiction.

GABAB receptor as therapeutic target for drug addiction: from baclofen to positive allosteric modulators

The present paper summarizes experimental and clinical data indicating the therapeutic potential of the GABAB receptor agonist, baclofen, in the treatment of alcohol use disorder (AUD) and substance use disorder (SUD). Multiple preclinical studies have demonstrated the ability of baclofen to suppress alcohol drinking (including binge- and relapse-like drinking), oral alcohol self-administration, and intravenous self-administration of cocaine, nicotine, amphetamine, methamphetamine, morphine, and heroin in rodents. Some randomized, controlled trials (RCTs) and case reports support the efficacy of baclofen in suppressing alcohol consumption, craving for alcohol, and alcohol withdrawal symptomatology in alcohol-dependent patients. Data from RCTs and open studies investigating baclofen efficacy on SUD are currently less conclusive. Interest in testing high doses of baclofen in AUD and SUD treatment has recently emerged. Preclinical research has extended the anti-addictive properties of baclofen to positive allosteric modulators of the GABAB receptor (GABAB PAMs). In light of their more favourable side effect profile (compared to baclofen), GABAB PAMs may represent a major step forward in a GABAB receptor-based pharmacotherapy of AUD and SUD.

Varied behavioral responses induced by morphine in the tree shrew: a possible model for human opiate addiction.

Tree shrews represent a suitable animal model to study the pathogenesis of human diseases as they are phylogenetically close to primates and have a well-developed central nervous system that possesses many homologies with primates. Therefore, in our study, we investigated whether tree shrews can be used to explore the addictive behaviors induced by morphine. Firstly, to investigate the psychoactive effect of morphine on tree shrews’ behavior, the number of jumping and shuttling, which represent the vertical and horizontal locomotor activity respectively, was examined following the injection of different dosage of morphine. Our results showed intramuscular (IM) injection of morphine (5 or 10 mg/kg) significantly increased the locomotor activity of tree shrews 30–60 min post-injection. Then, using the conditioned place preference/aversion (CPP/CPA) paradigm, we found morphine-conditioned tree shrews exhibited place preference in the morphine-paired chamber on the test day. In addition, naloxone-precipitated withdrawal induced place aversion in the chronic morphine-dependent tree shrews. We evaluated the craving for morphine drinking by assessing the break point that reflects the maximum effort animals will expend to get the drug. Our data showed the break point was significantly increased when compared to the baseline on the 1st, 7th and 14th day after the abstinence. Moreover, in the intravenous morphine self-administration experiment, tree shrews conditioned with morphine responded on the active lever significantly more frequently than on the inactive lever after training. These results suggest that tree shrew may be a potential candidate for study the addictive behaviors and the underling neurological mechanisms.

Association between initial morphine intake and body weight change, acoustic startle reflex and drug seeking in rats.

Although chronic use of opiates can induce physical dependence and addiction, individual differences contributing to these symptoms are largely unknown. Using intravenous morphine self-administration (MSA), we investigated whether individual differences in drug intake are associated with weight change, acoustic startle reflex (ASR), pre-pulse inhibition (PPI), and drug seeking during spontaneous withdrawal. Male Sprague-Dawley rats self-administered morphine (0.5 mg/kg/infusion) or saline for 3 weeks (4-6 h/day, 5 days/week) and drug intake and body weight were monitored daily. The ASR and the PPI (baseline, 1 day and 1 week) and drug seeking (1 week) were measured during spontaneous withdrawal. Morphine animals did not gain weight (101 % ± 0.69), while the control animals did (115 % ± 1.06) after 3 weeks of self-administration. The ASR and the PPI were not significantly different between morphine and saline animals in 1-day or 1-week withdrawal. However, individual differences in initial (first 10 min), but not total (4-6 h), morphine intake of the daily sessions were positively correlated with weight change (r = 0.437, p = 0.037) and drug seeking (r = 0.424, p = 0.035) while inversely correlated with the ASR (r = -0.544, p = 0.005) in 1-week withdrawal from chronic morphine. A subgroup of animals that self-administered a larger amount of morphine at the beginning of the daily sessions exhibited subsequent weight gain, reduced ASR, and enhanced drug seeking in morphine withdrawal. Thus, individual differences in initial morphine intake may reveal a novel behavioral phenotype in opioid addiction.

Region-specific alterations in glutamate receptor 1 phosphorylation during context-induced drug seeking after withdrawal from morphine self-administration

Re-exposure to morphine-associated environments elicits morphine-seeking behavior after a long period of withdrawal in rats with a history of morphine dependence. Adaptations in glutamate receptor 1 (GluR1) phosphorylation in limbic brain regions have been shown to occur during withdrawal from addictive drugs, such as cocaine, methamphetamine, and heroin. However, whether similar adaptations exist after spontaneous withdrawal from repeated morphine intake has not been studied. In our present study, alterations in GluR1 expression and GluR1 phosphorylation at serine 845 (Ser845) and serine 831 (Ser831) in multiple limbic brain regions of rats were measured following context-induced drug craving after 1 or 10 days of withdrawal from intravenous morphine self-administration. Phosphorylation of GluR1 at Ser845, but not Ser831, increases in the nucleus accumbens and central amygdala from 1 to 10 days of withdrawal, and there were no changes in GluR1 phosphorylation at Ser845 or Ser831 in the hippocampal CA1 subregion from 1 to 10 days of withdrawal. Significant positive correlations between numbers of drug-seeking responses and GluR1 phosphorylation at Ser845 in the nucleus accumbens were found in individual animals. These results suggest that time-dependent and region-specific changes in phosphorylation of GluR1 at Ser845, but not Ser831, are involved in the drug-seeking behavior elicited by re-exposure to the morphine-associated context.

Effect of saikosaponin A on maintenance of intravenous morphine self-administration

In this study, we investigated the effects of saikosaponin A (SSA), a major compound of Bupleurum falcatum L., on morphine self-administration behavior. Male Sprague–Dawley rats were trained to self-administer intravenous morphine (0.1mg/kg per injection over 5s) during daily 1-h sessions under a fixed-ratio 1 schedule. Rats were pretreated with SSA (0.25, 0.5, 1.0mg/kg) by intraperitoneal injection 30min prior to the start of the test session. Results demonstrated that pretreatment with SSA reduced morphine-maintained responding dose-dependently. Additionally, SSA inhibition of morphine-reinforced behavior was blocked by the selective GABAB receptor antagonist (2S)(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911), but not the selective GABAA receptor antagonist bicuculline. Together, these results suggest that SSA may effectively suppress morphine-reinforced behavior by activating GABAB receptors.

Dorsal as well as ventral striatal lesions affect levels of intravenous cocaine and morphine self-administration in rats

While the ventral striatum has long been implicated in the rewarding properties of psychomotor stimulants and opiates, little attention has been paid to the possible contribution of more dorsal regions of the striatum. We have thus examined the effects of lesions in three different striatal subregions on cocaine and morphine self-administration. Different groups of rats were trained to self-administer intravenous cocaine (1.0 mg/kg/infusion) or morphine (0.5 mg/kg/infusion) first under fixed ratio (FR) and then under progressive ratio (PR) schedules of reinforcement. Upon completion of the training, independent groups received bilateral electrolytic or sham lesions of the dorsal portion of the caudate-putamen (dCPu), the ventral portion of the caudate-putamen (vCPu) or the more ventral nucleus accumbens (NAS). Following recovery, they were tested for self-administration of cocaine (0.25, 0.5, 1.0 and 1.5 mg/kg/infusion) or morphine (0.125, 0.25, 0.5 and 0.75 mg/kg/infusion) under the PR schedule. The PR responding for each drug was significantly reduced in a dose-dependent manner following lesions of dCPu, vCPu and NAS. While the relative effectiveness of these lesions is likely to be specific to the conditions of this experiment, NAS lesions reduced self-administration of each drug to a greater extent than did dCPu or vCPu lesions.

Qualitative differences between C57BL/6J and DBA/2J mice in morphine potentiation of brain stimulation reward and intravenous self-administration

The C57BL/6J (C57) and DBA/2J (DBA) mice are the most common genotypes used to identify chromosomal regions and neurochemical mechanisms of interest in opioid addiction. Unfortunately, outside of the oral two-bottle choice procedure, limited and sometimes controversial evidence is available for determining their relative sensitivity to the rewarding effects of morphine. The purpose of this study was to utilize classically accepted models of drug abuse liability to determine relative susceptibility to the rewarding effects of morphine. The ability of morphine or amphetamine to potentiate lateral hypothalamic brain stimulation and intravenous morphine self-administration (across three doses in a fixed ratio schedule and at the highest dose in progressive ratio schedules) was investigated in both genotypes. In both measures, C57 and DBA mice differed dramatically in their response to morphine. Morphine potentiated rewarding stimulation in the C57 mice but antagonized it in the DBA mice. Consistent with these findings, intravenous morphine did not serve as a positive reinforcer in DBA mice under conditions that were effective in the C57 mice using a fixed ratio schedule and failed to sustain levels of responding sufficient to maintain a constant rate of drug intake under a progressive ratio schedule. In contrast, amphetamine potentiated the rewarding effects of brain stimulation similarly in the two genotypes. These findings provide strong evidence that morphine is rewarding in the C57 genotype and not in the DBA genotype. Understanding their relative susceptibility is important given the prominence of these genotypes in candidate gene identification and gene mapping.

Agmatine blocks acquisition and re-acquisition of intravenous morphine self-administration in rats

Our previous studies showed that agmatine inhibits morphine-induced conditioned place preference, locomotor sensitization and drug discrimination in rats. In the present study, we investigated the effects of agmatine on intravenous morphine self-administration in rats. At a dose of 80 mg/kg/infusion, agmatine did not substitute for intravenous morphine (0.5 mg/kg/infusion) self-administration, suggesting that agmatine itself has no reinforcing effect. However, pretreatment with agmatine (40 or 80 mg/kg, i.g.) significantly inhibited the acquisition of intravenous morphine self-administration as assessed by the nose-poke response and morphine intake. The mean number of days required to meet the acquisition criteria for intravenous morphine self-administration was significantly prolonged. After acquisition of intravenous morphine self-administration, chronic administration of agmatine (40 or 80 mg/kg × 30 days, bid, i.g.) during the extinction period significantly prevented the re-acquisition of intravenous morphine self-administration. The ability of agmatine to inhibit the acquisition and re-acquisition of intravenous morphine self-administration suggests a possible use of agmatine in the treatment of opioid dependence.

Age-dependent morphine intake and cue-induced reinstatement, but not escalation in intake, by adolescent and adult male rats

Despite increasing rates of opioid abuse by human adolescents, few laboratory experiments address adolescent vulnerability to opiates. We examined intravenous morphine self-administration after adolescent- vs. adult-onset, followed by extinction and cue-induced reinstatement. Adolescent male Sprague–Dawley rats [postnatal day (P) 35 at start] and adults (P91) acquired lever pressing maintained by 0.375 mg/kg/infusion morphine on a fixed ratio one schedule of reinforcement. Subjects were subsequently divided into short or long daily access conditions (ShAcc, 1-h vs. LgAcc, 8-h; 18 sessions). After extinction, cue-induced reinstatement was recorded over 1 h. During the first six 1-h acquisition sessions and continuing throughout ShAcc conditions, adolescent-onset rats self-administered less morphine than adults, an effect commonly interpreted as higher drug sensitivity. In contrast under LgAcc conditions, escalation of morphine intake was similar across ages. Extinction of drug-seeking was similar across ages, although rats from LgAcc conditions pressed more than ShAcc conditions. Notably, cue-induced reinstatement was less robust in rats that began morphine self-administration during adolescence vs. adulthood. Although increased sensitivity of younger rats to morphine reinforcement under ShAcc conditions might help explain opioid abuse by human adolescents, lower rates of reinstatement in younger rats might suggest that adolescent development includes some protective factors that dampen the long-term impact of early drug intake.

Selective breeding for intravenous drug self-administration in rats: a pilot study

Although the role of genetic factors in the response to drugs of abuse has been emphasized, no earlier studies have applied selective breeding to intravenous drug self-administration. Here we report the effects of six generations of selective breeding for rat lines with low or high levels of intravenous drug self-administration (LS and HS lines, respectively). Rats from the outbred founder population and the first selected generation were evaluated for intravenous self-administration of either morphine or cocaine. All subsequent generations were assessed for self-administration of cocaine, using a multifactorial score based on how rapidly self-administration behavior was acquired, levels of self-administration during acquisition, and the response to different doses of cocaine. All changes in cocaine self-administration that occurred in generations three through six were consistent with effects of selection, with most measures differing in sixth-generation LS and HS animals. Sixth-generation HS rats self-administered approximately five times more injections of low-dose cocaine than LS animals under fixed-ratio-5 (a schedule in which an injection is delivered after five lever presses). These findings support a role of genetic factors in influencing cocaine-reinforced behavior. Establishment of the LS and HS lines will allow future studies to evaluate the role of specific genetic factors that underlie these differences and may contribute to substance abuse disorders in humans.

Mid- to late gestational morphine exposure does not alter the rewarding properties of morphine in adult male rats

Prenatal exposure to drugs of abuse often leads to physiological and neurobiological abnormalities including decreased brain and body weight, cognitive deficits and behavioral alterations. A handful of studies showed increased vulnerability to drug abuse in prenatally drug-exposed offspring. Our work also demonstrated that prenatal exposure to analgesic doses of morphine during gestation days 11–18 increases μ-opioid receptor density in the nucleus accumbens and central amygdala of adult male rats. Both the nucleus accumbens and central amygdala play important roles in modulating drug-induced reward via the mesolimbic dopaminergic system. Therefore, two types of behavioral paradigms were used to test the hypothesis that the same prenatal morphine exposure would enhance the rewarding effects of morphine, making drug-exposed offspring more vulnerable to abuse this drug in adulthood. All experiments were performed with adult male offspring of saline-injected, morphine-injected or non-injected (control) dams. (1) The unbiased conditioned place preference (CPP) paradigm was used to investigate whether prenatal morphine exposure sensitizes adult male rats to non-contingent morphine reward. These adult animals were conditioned with 0.1, 0.3, 1, 3 or 5 mg/kg morphine. All control, prenatally saline- and morphine-exposed male rats preferred the morphine-paired compartment relative to the saline-paired compartment. However, the magnitude of morphine CPP in adult male rats was not dependent on the conditioning dose of morphine or prenatal morphine exposure. (2) Intravenous morphine self-administration was used to assess the behavioral response to contingent morphine reward. Each rat self-administered one of four doses of morphine (0.3, 1, 2 or 3 mg/kg/infusion). Morphine self-administration was not altered in prenatally morphine-exposed adult male offspring. Control males self-administered significantly less morphine at the lowest dose of morphine than both prenatally saline- and morphine-exposed males. Although our data show that prenatal exposure to an analgesic dose of morphine during the time of opioid receptor appearance does not enhance morphine CPP or self-administration, they do not exclude the possibility that this prenatal morphine exposure enhances the rewarding properties of other drugs of abuse.