Acute Delta 9-tetrahydrocannabinol administration differentially alters the hippocampal opioid system in adult female and male rats.

Our prior studies demonstrated that the rat hippocampal opioid system can undergo sex-specific adaptations to external stimuli that can influence opioid-associated learning processes. This opioid system extensively overlaps with the cannabinoid system. Moreover, acute administration of Δ9 Tetrahydrocannabinoid (THC), the primary psychoactive constituent of cannabis, can alter cognitive behaviors that involve the hippocampus. Here, we use light and electron microscopic immunocytochemical methods to examine the effects of acute THC (5 mg/kg, i.p., 1 h) on mossy fiber Leu-Enkephalin (LEnk) levels and the distribution and phosphorylation levels of delta and mu opioid receptors (DORs and MORs, respectively) in CA3 pyramidal cells and parvalbumin dentate hilar interneurons of adult female and male Sprague-Dawley rats. In females with elevated estrogen states (proestrus/estrus stage), acute THC altered the opioid system so that it resembled that seen in vehicle-injected females with low estrogen states (diestrus) and males: (1) mossy fiber LEnk levels in CA2/3a decreased; (2) phosphorylated-DOR levels in CA2/3a pyramidal cells increased; and (3) phosphorylated-MOR levels increased in most CA3b laminae. In males, acute THC resulted in the internalization of MORs in parvalbumin-containing interneuron dendrites which would decrease disinhibition of granule cells. In both sexes, acute THC redistributed DORs to the near plasma membrane of CA3 pyramidal cell dendrites, however, the dendritic region varied with sex. Additionally, acute THC also resulted in a sex-specific redistribution of DORs within CA3 pyramidal cell dendrites which could differentially promote synaptic plasticity and/or opioid-associated learning processes in both females and males.

Nalmefene, a mu opioid receptor antagonist/kappa opioid receptor partial agonist, potentiates cocaine motivation but not intake with extended access self-administration in adult male mice.

The mu opioid receptor antagonist/kappa opioid receptor (KOR) partial agonist nalmefene (NMF), a close structural analog of naltrexone (NTX), has been shown to reduce cocaine reward in preclinical models. Given the greater KOR potency and improved bioavailability compared to NTX, NMF may be a promising pharmacotherapeutic for cocaine use disorder (CUD). Here we examine the effects of NMF pretreatment on chronic daily extended access (4h) cocaine intravenous self-administration (IVSA) in adult male C57Bl/6J mice. METHODS: separate groups of mice had daily 4h cocaine IVSA sessions (0.25 or 0.5 mg/kg/inf, FR1) for 14 days. Starting on day 8, mice were pretreated with NMF (0, 1, or 10 mg/kg) 30m before each session. A separate group of mice acquired cocaine IVSA [seven days FR1 then four FR3 of 4h daily sessions (0.5 mg/kg/inf)] prior to a single progressive ratio 3 session to examine the effect of 1 mg/kg NMF on cocaine motivation. RESULTS: No significant effect of NMF pretreatment on cocaine intake was observed. Acute pretreatment of 1 mg/kg NMF significantly potentiated cocaine motivation as measured by progressive ratio breakpoint. CONCLUSIONS: NMF did not significantly attenuate cocaine intake and increased motivation for cocaine suggesting that NMF may not be suitable for non-abstinent CUD patients. Further research is needed with KOR selective partial or full agonists to determine their effect on cocaine reinforcement.

Review of addiction risk potential associated with adolescent opioid use.

Adolescence is a critical period of development with robust behavioral, morphological, hormonal, and neurochemical changes including changes in brain regions implicated in the reinforcing effects of drugs such as opioids. Here we examine the preclinical and, where appropriate complementary clinical literature, for the behavioral and neurological changes induced by adolescent opioid exposure/use and their long-term consequences during adulthood. Adolescent opioid exposure results in a widened biphasic shift in reinforcement with increased impact of positive rewarding aspects during initial use and profound negative reinforcement during adulthood. Females may have enhanced vulnerability due to fast onset of antinociceptive tolerance and reduced severity of somatic withdrawal symptoms during adolescence. Overall, adolescent opioid exposure, be it legally prescribed protracted intake or illicit consumption, results in significant and prolonged consequences of increased opioid reward concomitant with reduced analgesic efficacy and exacerbated somatic withdrawal severity during opioid use/exposure in adulthood. These findings are highly relevant to physicians, parents, law makers, and the general public as adolescent opioid exposure/misuse results in heightened risk for substance use disorders.

Effects of Kappa opioid receptor blockade by LY2444296 HCl, a selective short-acting antagonist, during chronic extended access cocaine self-administration and re-exposure in rat.

RATIONALE: Cocaine addiction is a chronic brain disease characterized by compulsive drug intake and dysregulation of brain reward systems. Few preclinical studies have modeled the natural longitudinal course of cocaine addiction. Extended access self-administration protocols are powerful tools for modeling the advanced stages of addiction; however, few studies have duration of drug access longer than 12 h/session, potentially limiting their construct validity. Identification of changes in cocaine intake patterns during the development of addictive-like states may allow better treatments for vulnerable subjects. The kappa opioid receptor (KOPr) system has been implicated in the neurobiological regulation of addictive states as well as mood and stress disorders, with selective KOPr antagonists proposed as possible pharmacotherapeutic agents. Chronic cocaine exposure increases the expression of KOPr and its endogenous agonists, the dynorphins, in several brain areas in rodents. OBJECTIVES: To examine the behavioral pattern of intake during chronic (14 days) 18 h intravenous cocaine self-administration (0.5 mg/kg/infusion) and the effect of a novel short-acting KOPr antagonist LY2444296 HCl (3 mg/kg) administered during sessions 8 to 14 of chronic 18 h/day cocaine self-administration and prior to a single re-exposure session after 2 cocaine-free withdrawal days. RESULTS: Both daily and hourly cocaine intake patterns changed over 14 days of 18 h self-administration. LY pretreatment affected the pattern of self-administration across the second week of extended access cocaine self-administration and prevented the increase in cocaine intake during re-exposure. CONCLUSIONS: Overall, the KOPr antagonist attenuated escalated cocaine consumption in a rat model of extended access cocaine self-administration.

Naltrexone and nalmefene attenuate cocaine place preference in male mice.

Cocaine addiction treatment is difficult due to the current lack of approved pharmacotherapuetics. Several preclinical and clinical studies have demonstrated that the mu opioid receptor (MOPr) antagonist/kappa opioid receptor (KOPr) partial agonist naltrexone (NTX) reduces the subjective effects and self-administration of cocaine. However, very limited research has examined the ability of the structurally similar MOPr antagonist/KOPr partial agonist nalmefene (NMF) to reduce cocaine reward. Here we examine the effect of low (1 mg/kg) and high (10 mg/kg) doses of NTX or NMF on cocaine place preference. In vivo characterization of these NTX and NMF doses were performed to examine their effectiveness at MOPr and KOPr. RESULTS: Both NTX doses and high dose NMF significantly reduced cocaine place preference. Conversely, a significant place avoidance was observed for high dose NTX and both NMF doses. Interestingly, neither NTX nor NMF blocked cocaine-induced hyperlocomotion. High dose NTX and both NMF doses fully blocked MOPr agonist morphine-induced thermal analgesia as well as KOPr agonist U50,488H-induced locomotor discoordination. However, low dose NTX fully blocked morphine analgesia but not U50,488H locomotor discoordination suggesting that low dose NTX is effective at MOPr but not KOPr. CONCLUSION: Both NTX and NMF block the place preference, but not locomotor activating, effects of cocaine. These results suggest that both NTX and NMF may be viable pharmacotheraputics for some aspects of cocaine addiction. This is an important step to understanding the potential mechanism(s) of action of NTX and NMF for the development of more efficacious pharmacological treatments for substance use disorders.

Alcohol affects the P3 component of an adaptive stop signal task ERP.

BACKGROUND: The P3 component of the event-related potential (ERP) has been particularly useful in alcohol research for identifying endophenotypes of alcohol-use disorder (AUD) risk in sober subjects. However, practice and/or fatigue reduce P3 amplitude, limiting the ability to ascertain acute and adaptive effects of alcohol exposure. Here, we report acute alcohol effects on P3 amplitude and latency using an adaptive stop signal task (aSST). METHODS: One hundred forty-eight non-dependent moderate to heavy social drinkers, ages 21 to 27, participated in two single-blind, alcohol or placebo, counterbalanced sessions approximately 1 week apart. During each session, subjects performed an adaptive stop signal task (aSST) at 1) baseline, 2) upon reaching the target 60 mg/dL breath alcohol concentration or at the equivalent time during the placebo session, and 3) approximately 135 min later while the breath alcohol concentration was clamped. Here, we report on differences between baseline and first subsequent measurements across the experimental sessions. During each aSST run, the stop signal delay (SSD, the time between stop and go signals) adjusted trial-by-trial, based on the subject's performance. RESULTS: The aSST reliably generated a STOP P3 component that did not change significantly with repeated task performance. The pre-infusion SSD distribution was bimodal, with mean values several hundred msec apart (FAST: 153 msec and SLOW: 390 msec). This suggested different response strategies: FAST SSD favoring "going" over "stopping", and SLOW SSD favoring "stopping" over "going". Exposure to alcohol at 60 mg/dL differentially affected the amplitude and latency of the STOP P3 according to SSD group. Alcohol significantly reduced P3 amplitude in the SLOW SSD compared to the FAST SSD group, but significantly increased P3 latency in the FAST SSD compared to the SLOW SSD group. CONCLUSIONS: The aSST is a robust and sensitive task for detecting alcohol-induced changes in inhibition behavior as measured by the P3 component in a within-subject design. Alcohol was associated with P3 component changes, which varied by SSD group, suggesting a differential effect as a function of task strategy. Overall, the data support the potential utility of the aSST in the detection of alcohol response-related AUD risk.

Effects of group II metabotropic glutamate receptor modulation on ethanol- and sucrose-seeking and consumption in the rat.

Previous studies suggest that group II metabotropic glutamate receptors (mGluR2/3) are involved in regulating ethanol-seeking and consumption. The mGluR2/3 agonist LY379268 (LY37) and selective mGluR2 positive allosteric modulator biphenyl­indanone A (BINA) were used to investigate the relative contribution of mGlu2 and mGlu3 receptors on ethanol- and sucrose-seeking and consumption. A microinjection study was then performed to examine the role of nucleus accumbens (NAc) core mGluR2/3 on ethanol-seeking. For the systemic experiments, separate groups of male Wistar rats [LY37 (0-2.0 mg/kg); BINA (0-20 mg/kg)] were trained to complete a response requirement (RR) resulting in access to 10% ethanol or 2% sucrose (in separate groups) for a 20­min drinking period. Animals then underwent consummatory testing (weekly drug injections with RR1) followed by appetitive testing (weekly drug injections followed by extinction session). A separate group of male Wistar rats was surgically implanted with bilateral guide cannulae directed toward the NAc core and had weekly microinjections followed by an extinction session. Systemic administration of the mGluR2/3 agonist LY37 significantly reduced ethanol- and sucrose-seeking. The same treatment also reduced sucrose consumption and body weight (24­h post injection). Systemic administration of the selective mGluR2 PAM BINA, however, had no effect on either seeking or consumption of ethanol or sucrose. Intra-accumbens core LY37 significantly reduced ethanol-seeking. These findings suggest that systemic mGluR2/3 agonism, but not allosteric modulation of mGluR2, reduces reinforcer-seeking. In particular, NAc core group II mGluR may be involved in regulating ethanol-seeking.

Intravenous alcohol self-administration in the P rat.

Alcohol consumption produces a complex array of effects that can be divided into two types: the explicit pharmacological effects of ethanol (which can be temporally separate from time of intake) and the more temporally "relevant" effects (primarily olfactory and taste) that bridge the time from intake to onset of the pharmacological effects. Intravenous (IV) self-administration of ethanol limits the confounding "non-pharmacological" effects associated with oral consumption, allows for controlled and precise dosing, and bypasses first order absorption kinetics, allowing for more direct and better-controlled assessment of alcohol's effect on the brain. IV ethanol self-administration has been reliably demonstrated in mouse and human experimental models; however, models of IV self-administration have been historically problematic in the rat. An operant multiple-schedule study design was used to elucidate the role of each component of a compound IV-ethanol plus oral-sucrose reinforcer. Male alcohol-preferring P rats had free access to both food and water during all IV self-administration sessions. Animals were trained to press a lever for orally delivered 1% sucrose (1S) on a fixed ratio 4 schedule, and then surgically implanted with an indwelling jugular catheter. Animals were then trained to respond on a multiple FR4-FR4 schedule composed of alternating 2.5-min components across 30-min sessions. For the multiple schedule, two components were used: an oral 1S only and an oral 1S plus IV 20% ethanol (25 mg/kg/injection). Average total ethanol intake was 0.47 ± 0.04 g/kg. We found significantly higher earning of sucrose-only reinforcers and greater sucrose-lever error responding relative to the compound oral-sucrose plus IV-ethanol reinforcer. These response patterns suggest that sucrose, not ethanol, was responsible for driving overall responding. The work with a compound IV ethanol-oral sucrose reinforcer presented here suggests that the existing intravenous ethanol self-administration methodology cannot overcome the aversive properties of ethanol via this route in the rat.