Spontaneous withdrawal in opiate-dependent Fischer 344, Lewis and Sprague-Dawley rats

Diurnal sex differences in morphine withdrawal revealed by continuous assessment of voluntary home cage wheel running in the rat

Animal studies modeling recreational opioid use show more severe withdrawal symptoms in male compared to female rats, whereas our study modeling opioid use for pain showed a greater withdrawal-induced decrease in wheel running in female rats. The objective of this experiment was to determine whether sex differences in spontaneous morphine withdrawal are caused by differences in assessment method (i.e., wheel running vs. somatic symptoms). Twice daily injections of morphine (5 - 20 mg/kg, s.c.) for 5 days produced a dose and time dependent decrease in wheel running that was greater in male compared to female rats. Termination of morphine administration resulted in an overall decrease in running and a decrease in the amount of running during the dark phase of the light cycle from 95 % to approximately 75 %. In male rats, this decrease in the percent of dark running was caused by a large decrease in dark phase running, whereas female rats had a slightly higher increase in light phase running. Withdrawal also reduced maximal running speed and caused a decrease in body weight that was larger in male than female rats. Withdrawal symptoms were greatest on the day following the last morphine injection, but persisted for all 3 days of assessment. Morphine withdrawal produced a greater decrease in dark phase wheel running and body weight in male rats and a greater increase in light phase running in female rats. Voluntary home cage wheel running provides a continuous measure of opioid withdrawal that is consistent with other measures of opioid withdrawal.

Effects of pharmacological and environmental manipulations on choice between fentanyl and shock avoidance/escape in male and female rats under mutually exclusive and non-exclusive choice conditions

Substance use disorders are defined by persistent drug consumption despite adverse consequences. Accordingly, we developed two fentanyl-vs-shock avoidance/escape choice procedures in which male and female rats responded under a fixed-ratio (FR)1:FR1 concurrent schedule of shock avoidance/escape and IV fentanyl under either mutually exclusive or non-exclusive choice conditions. Initial experiments using a discrete-trial procedure determined behavioral allocation between mutually exclusive shock avoidance/escape and different fentanyl doses (0.32-18 μg/kg/infusion; Experiment 1). Shock intensity (0.1-0.7 mA) and shock avoidance/escape response requirement (FR1-16) were also manipulated (Experiment 2). Next, we used a free-operant procedure in which shock avoidance/escape and fentanyl were continuously available under non-exclusive conditions, and response-shock (R-S) interval (30-1000 s) was manipulated (Experiment 3). Finally, we tested the hypothesis that extended-access fentanyl self-administration would produce fentanyl dependence, establish fentanyl withdrawal as an endogenous negative reinforcer, and increase fentanyl choice in both procedures (Experiments 4 and 5). The shock avoidance/escape contingency decreased fentanyl self-administration, and rats consistently chose shock avoidance/escape over fentanyl in both choice conditions. Decreasing shock intensity or increasing shock avoidance/escape response requirement failed to increase fentanyl choice, suggesting that fentanyl and shock avoidance/escape are independent economic commodities. Increasing the R-S interval increased fentanyl choice but failed to increase shock delivery. Extended fentanyl access engendered high fentanyl intake and opioid withdrawal signs but failed to increase fentanyl choice under either choice condition. These results suggest that neither positive fentanyl reinforcement nor negative reinforcement by fentanyl withdrawal is sufficient to reduce shock avoidance/escape-maintained responding and increase foot shock as an adverse consequence.

Strain and sex-related behavioral variability of oxycodone dependence in rats

Over the past two decades, the escalating prescription of opioid medications for pain management has culminated in a widespread opioid epidemic, significantly impacting public health, social dynamics, and economic stability. The urgent need for improved treatments for opioid addiction necessitates a deeper understanding of its biological underpinnings, with genetic variations playing a crucial role in individual susceptibility to opioid use disorder (OUD) and influencing clinical practices. In this study, we leverage the genetic diversity of four rat strains (ACI/N, BN/NHsd, WKY/N, and F344/N) to examine the contribution of genetic factors to oxycodone metabolism and addiction-like behaviors. We used the extended access to intravenous oxycodone self-administration procedure (12 h/day, 0.15 mg/kg/injection) to comprehensively characterize oxycodone-related behaviors and pharmacokinetics. We measured escalation of oxycodone self-administration, motivation for drug consumption, tolerance to the analgesic effects of oxycodone, withdrawal-induced hyperalgesia, and oxycodone-induced respiratory depression. Additionally, we examined oxycodone-seeking behavior after four weeks of withdrawal by reintroducing the animals to environmental and cue stimuli previously associated with oxycodone self-administration. The findings revealed notable strain differences in several behavioral measures, including oxycodone metabolism. Intriguingly, BN/NHsd and WKY/N strains exhibited similar drug intake and escalation patterns but displayed significant disparities in oxycodone and oxymorphone metabolism. Minimal sex differences were observed within strains, primarily relating to oxycodone metabolism. In conclusion, this study identifies strain differences in the behavioral responses and pharmacokinetics associated with oxycodone self-administration in rats, providing a robust foundation for identifying genetic and molecular variants associated with various facets of the opioid addiction process.

Advances in the characterization of negative affect caused by acute and protracted opioid withdrawal using animal models

Opioid use disorder (OUD) is a chronic brain disease which originates from long-term neuroadaptations that develop after repeated opioid consumption and withdrawal episodes. These neuroadaptations lead among other things to the development of a negative affect, which includes loss of motivation for natural rewards, higher anxiety, social deficits, heightened stress reactivity, an inability to identify and describe emotions, physical and/or emotional pain, malaise, dysphoria, sleep disorders and chronic irritability. The urge for relief from this negative affect is one of major causes of relapse, and thus represents a critical challenge for treatment and relapse prevention. Animal models of negative affect induced by opioid withdrawal have recapitulated the development of a negative emotional state with signs such as anhedonia, increased anxiety responses, increased despair-like behaviour and deficits in social interaction. This research has been critical to determine neurocircuitry adaptations during chronic opioid administration or upon withdrawal. In this review, we summarize the recent literature of rodent models of (i) acute withdrawal, (ii) protracted abstinence from passive administration of opioids, (iii) withdrawal or protracted abstinence from opioid self-administration. Finally, we describe neurocircuitry involved in acute withdrawal and protracted abstinence. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Establishing preclinical withdrawal syndrome symptomatology following heroin self-administration in male and female rats

Opioid use disorder (OUD) is a significant health problem, and understanding mechanisms of various aspects of OUD including drug use and withdrawal is important. Preclinical models provide an ideal opportunity to evaluate mechanisms underlying opioid withdrawal. Current models are limited by their reliance upon forced opioid administration, focus on the acute (and not protracted) syndrome, and exclusion of females. In this study, male and female rats self-administered heroin (maintenance dose of 12.5 μg/kg/infusion) and opioid withdrawal after abrupt discontinuation was measured. In Phase 1, acute withdrawal symptoms were rated in male and female rats at 0, 16, 48, and 72 hr after the last self-administration session. Total somatic signs increased until 48 hr (predominantly in females), and heroin intake positively correlated with total somatic signs at the 48 and 72 hr timepoints. Measures of hyperactivity and anxiety-like behavior increased by 16 and 48 hr, respectively. In Phase 2, symptoms were assessed at baseline, acute, and protracted (168 and 312 hr after self-administration) timepoints in a subset of male and female rats from Phase 1. The total number of somatic signs did not differ across timepoints, though females displayed significantly higher body temperature at all timepoints compared with males, indicating sex-specific protracted withdrawal symptomatology. These data provide a thorough characterization of rodent opioid withdrawal symptomatology after self-administration and abrupt discontinuation that serve as a foundation for future studies designed to mimic the human experience, and demonstrate the importance of characterizing acute and protracted withdrawal with sex-specificity in preclinical models of opioid self-administration. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

High-fat diet attenuates morphine withdrawal effects on sensory-evoked locus coeruleus norepinephrine neural activity in male obese rats

Objective: These experiments sought to characterize the effects of obesity propensity and obesogenic diet on locus coeruleus (LC) norepinephrine (NE) activity and determine the effects of obesity on LC neural responses to morphine withdrawal.Methods: In vivo single-unit LC electrophysiological activity was measured in obese prone (OP) and obese resistant (OR) male SD rats following high-fat (HFD: 45% fat) or low-fat (LFD; 10% fat) feeding. A separate cohort of LFD and HFD rats underwent in vivo LC recording on day 3 of spontaneous morphine withdrawal following an escalation dose paradigm (5-15 mg/kg; SQ twice daily).Results: OP (LFD: 34 cells/7 rats; HFD: 32 cells/6 rats) had higher spontaneous and tonic activity, and lower sensory-evoked activity compared with OR (LFD: 31 cells/6 rats; HFD: 41 cells/7 rats). Interacting effect of diet x strain status was observed on signal-to-noise ratio with OR-LFD having higher ratio than OP-LFD and OP-HFD. Morphine treatment decreased body weights. Withdrawal increased sensory-evoked rate in LFD (morphine; 20 cells/10 rats; saline 24 cells/6 rats) but not HFD (saline: 22 cells/7 rats; morphine: 21 cells/5 rats) rats. In a separate group of age-matched SD rats, a similar weight loss (5-7%) in response to the morphine did not alter sensory-evoked rate but decreased signal-to-noise ratio (Control: 22 cells/8 rats; Weight-matched: 23 cells/8 rats).Discussion: Taken together, our findings suggest that obesity and diet alter the sensory-evoked LC-NE neural responses, which could have implication for emotional stress and opioid-withdrawal behaviors.

A drug-vs-food "choice" self-administration procedure in rats to investigate pharmacological and environmental mechanisms of substance use disorders

BACKGROUND

Preclinical drug self-administration procedures are commonly used to investigate expression, mechanisms, and treatment of substance use disorders.

NEW METHOD

The aims were to back-translate an intravenous drug-vs-food choice procedure primarily utilized in monkeys to male and female rats and to develop a surgical method for sustained intravenous catheter patency suitable for long-term drug-choice studies.

RESULTS

The surgical protocol resulted in a median intravenous jugular catheter patency in male and female rats of 126 days (range: 25-365 days). Drug-vs-food choice was established with opioids (fentanyl and heroin), psychostimulants (cocaine, methamphetamine, and amphetamine), and an opioid/psychostimulant mixture (fentanyl + methamphetamine). The average time from catheter implantation to stable choice behavior across all drugs was 27 sessions (range: 16-44 sessions). Choice behavior stabilized more quickly for cocaine and fentanyl than for other drugs. Manipulations of both environmental variables (e.g., response requirement or food reinforcer magnitude) and pharmacological variables (e.g., extended access drug self-administration or continuous buprenorphine treatment via osmotic pump) significantly shifted opioid-vs-food choice consistent with previous monkey studies.

COMPARISON WITH EXISTING METHODS

Duration of intravenous catheter patency in rats was suitable for long-term, within-subject drug choice studies. Effects of environmental and pharmacological manipulations in rats confirmed and extended previous results from monkeys.

CONCLUSIONS

The concordance of behavioral results between rats and monkeys using the present drug-vs-food choice procedure supports its utility to improve our basic understanding of the expression and mechanisms of substance use disorders towards to development of more effective therapeutics.

Limited potential of cebranopadol to produce opioid-type physical dependence in rodents

Cebranopadol is a novel potent analgesic agonist at the nociceptin/orphanin FQ peptide (NOP) and classical opioid receptors. As NOP receptor activation has been shown to reduce side effects related to the activation of μ-opioid peptide (MOP) receptors, the present study evaluated opioid-type physical dependence produced by cebranopadol in mice and rats. In a naloxone-precipitated withdrawal assay in mice, a regimen of seven escalating doses of cebranopadol over 2 days produced only very limited physical dependence as evidenced by very little withdrawal symptoms (jumping) even at cebranopadol doses clearly exceeding the analgesic dose range. In contrast, mice showed clear withdrawal symptoms when treated with morphine within the analgesic dose range. In the rat, spontaneous withdrawal (by cessation of drug treatment; in terms of weight loss and behavioral score) was studied after 4-week subacute administration. Naloxone-precipitated withdrawal (in terms of weight loss and behavioral score) was studied in the same groups of rats after 1-week re-administration following the spontaneous withdrawal period. In both tests, cebranopadol-treated rats showed only few signs of withdrawal, while withdrawal effects in rats treated with morphine were clearly evident. These findings demonstrate a low potential of cebranopadol to produce opioid-type physical dependence in rodents. The prospect of this promising finding into the clinical setting remains to be established.

Depression of home cage wheel running is an objective measure of spontaneous morphine withdrawal in rats with and without persistent pain

Opioid withdrawal in humans is often subtle and almost always spontaneous. In contrast, most preclinical studies precipitate withdrawal by administration of an opioid receptor antagonist such as naloxone. These animal studies rely on measurement of physiological symptoms (e.g., wet dog shakes) in the period immediately following naloxone administration. To more closely model the human condition, we tested the hypothesis that depression of home cage wheel running will provide an objective method to measure the magnitude and duration of spontaneous morphine withdrawal. Rats were allowed access to a running wheel in their home cage for 8days prior to implantation of two 75mg morphine or placebo pellets. The pellets were removed 3 or 5days later to induce spontaneous withdrawal. In normal pain-free rats, removal of the morphine pellets depressed wheel running for 48h compared to rats that had placebo pellets removed. Morphine withdrawal-induced depression of wheel running was greatly enhanced in rats with persistent inflammatory pain induced by injection of Complete Freund's Adjuvant (CFA) into the hindpaw. Removal of the morphine pellets following 3days of treatment depressed wheel running in these rats for over 6days. These data demonstrate that home cage wheel running provides an objective and more clinically relevant method to assess spontaneous morphine withdrawal compared to precipitated withdrawal in laboratory rats. Moreover, the enhanced withdrawal in rats with persistent inflammatory pain suggests that pain patients may be especially susceptible to opioid withdrawal.

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 (0.5 mg/kg/infusion, 4 h/day for 12 days). 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 (45 min). After FDG uptake, all animals were anesthetized in order to perform a PET/CT scan (30 min). 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 (0.5 mg/kg) during the 2 day 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.

Multi-system state shifts and cognitive deficits induced by chronic morphine during abstinence

Chronic morphine administration induces neural plasticity followed by withdraw. And clinic observation indicates that obvious cognitive deficits are found during withdrawal. However, current neural substrates that regulate dysfunction in withdrawal are unknown. In our studies, chronic morphine administration was used to induce the spontaneous withdrawal model in rats. A series of cognitive abilities was tested to explore brain function. To further evaluate the neural substrates of dysfunction, Manganese-enhanced MRI(MEMRI) was used to map the dysfunctional regions in vivo.We observed that chronic morphine administration could induce obvious withdrawal behaviors in abstinence followed by cognitive impairments, such as impairments in working memory, reward, interaction and enhancement of anxiety. Our in-vivo MEMRI data using the voxel-wise comparisons showed that the manganese-enhanced signal intensity (VMI) within morphine withdrawal groups was increased in cingulate cortex (Cg), secondary motor cortex (M2), CA3 subfield of hippocampus, dorsal striatum (D-striatum), retrosplenial cortex (RS), shell subregion of NAc (AcbSh), core subregion of NAc (AcbC), central nucleus of amygdala (CeC), basolateral amygdaloid nucleus (BLA), central amygdaloid nucleus (CeM), anterior hypothalamic area, central (AHC), ventral tegmental area (VTA) and scaphoid thalamic nucleus (SC).However, decreasing of VMI was found in the ventrolateral striatum (V-striatum) and lateral posterior thalamic nucleus (LP) compared to the control group. These brain regions were beleived to be components of the memory, executive, limbic and regulatory systems. Therefore, our present studies indicate that withdrawal induced by chronic morphine adiministration could disturb brain function leading to multi-systems state shifts and cognitive deficits in abstinence.

Corticotropin-releasing factor (CRF) receptor-1 is involved in cardiac noradrenergic activity observed during naloxone-precipitated morphine withdrawal

BACKGROUND AND PURPOSE

The negative affective states of withdrawal involve the recruitment of brain and peripheral stress circuitry [noradrenergic activity, induction of the hypothalamic-pituitary-adrenocortical (HPA) axis and activation of heat shock proteins (Hsps)]. Corticotropin-releasing factor (CRF) pathways are important mediators in the negative symptoms of opioid withdrawal. We performed a series of experiments to characterize the role of the CRF₁ receptor in the response of stress systems to morphine withdrawal and its effect in the heart using genetically engineered mice lacking functional CRF₁ receptors.

EXPERIMENTAL APPROACH

Wild-type and CRF₁ receptor-knockout mice were treated with increasing doses of morphine. Precipitated withdrawal was induced by naloxone. Plasma adrenocorticotropic hormone (ACTH) and corticosterone levels, the expression of myocardial Hsp27, Hsp27 phosphorylated at Ser⁸², membrane (MB)- COMT, soluble (S)-COMT protein and NA turnover were evaluated by RIA, immunoblotting and HPLC.

KEY RESULTS

During morphine withdrawal we observed an enhancement of NA turnover in parallel with an increase in mean arterial blood pressure (MAP) and heart rate (HR) in wild-type mice. In addition, naloxone-precipitated morphine withdrawal induced an activation of HPA axis and Hsp27. The principal finding of the present study was that plasma ACTH and corticosterone levels, MB-COMT, S-COMT, NA turnover, and Hsp27 expression and activation observed during morphine withdrawal were significantly inhibited in the CRF₁ receptor-knockout mice.

CONCLUSION AND IMPLICATIONS

Our results demonstrate that CRF/CRF₁ receptor activation may contribute to stress-induced cardiovascular dysfunction after naloxone-precipitated morphine withdrawal and suggest that CRF/CRF₁ receptor pathways could contribute to cardiovascular disease associated with opioid addiction.

3,4-Methylenedioxypyrovalerone (MDPV)-induced conditioned taste avoidance in the F344/N and LEW rat strains

The inbred Fischer (F344) and Lewis (LEW) rats, while originally developed as animal models for cancer and tissue transplantation research, have since been used to study genetic differences in a variety of physiological and behavioral endpoints. In this context, LEW rats show greater sensitivity to the aversive effects of cocaine as compared to F344 rats in a conditioned taste avoidance procedure. Like cocaine, 3,4-methylenedioxypyrovalerone (MDPV; "bath salts") acts as a dopamine transport blocker and possesses aversive properties, making it a good candidate for assessing whether the aforementioned strain differences with cocaine would generalize to drugs with similar biochemical action. Accordingly, male F344 and LEW rats were exposed to a novel saccharin solution followed by injections of one of four doses of MDPV in a taste avoidance procedure. Over the four saccharin/MDPV pairings during conditioning, core body temperatures were also assessed. Similar to previous research, MDPV induced robust dose-dependent taste avoidance, although no effect of strain was observed. MDPV also produced hyperthermia that was independent of strain and unrelated to the conditioned taste avoidance. These findings argue for a complex influence of multiple (and likely interacting) monoaminergic systems mediating MDPV-induced taste avoidance in the two strains and suggest different mechanisms of avoidance learning for cocaine and MDPV.

Conditioned taste avoidance induced by Δ(9)-tetrahydrocannabinol in the Fischer (F344) and Lewis (LEW) rat strains

Although Fischer (F344) and Lewis (LEW) rats differ in their sensitivity to the rewarding effects of ∆(9)-tetrahydrocannabinol (THC), no data have been reported on differences in their sensitivity to the drug's aversive effects, a limiting factor in drug use and abuse. Examining the degree of differences (if any) in such effects in these strains may help further characterize possible genetic factors important to abuse vulnerability. Accordingly, the aversive effects of THC (1-5.6 mg/kg; intraperitoneal) were examined in 32F344 and 32 LEW subjects using the conditioned taste avoidance (CTA) procedure. Thermoregulation was assessed following an acute injection of THC (same as CTA groups) after a week washout period following the last trial. Subjects in both strains displayed dose-dependent THC-induced taste avoidance, with no significant strain difference. THC induced dose-dependent decreases in core body temperature in both strains. LEW subjects displayed lower core body temperatures than F344 rats, although this effect was independent of THC and was likely stress related. These results were discussed in terms of the nature of THC-induced taste avoidance and the basis of strain differences in the aversive effects of drugs of abuse.

Affective and neuroendocrine effects of withdrawal from chronic, long-acting opiate administration

Although the long-acting opiate methadone is commonly used to treat drug addiction, relatively little is known about the effects of withdrawal from this drug in preclinical models. The current study examined affective, neuroendocrine, and somatic signs of withdrawal from the longer-acting methadone derivative l-alpha-acetylmethydol (LAAM) in rats. Anxiety-like behavior during both spontaneous and antagonist-precipitated withdrawal was measured by potentiation of the startle reflex. Withdrawal elevated corticosterone and somatic signs and blunted circadian variations in baseline startle responding. In addition, fear to an explicit, Pavlovian conditioned stimulus (fear-potentiated startle) was enhanced. These data suggest that anxiety-like behavior as measured using potentiated startle responding does not emerge spontaneously during withdrawal from chronic opiate exposure - in contrast to withdrawal from acute drug exposure - but rather is manifested as exaggerated fear in response to explicit threat cues.

Fischer rats are more sensitive than Lewis rats to the suppressive effects of morphine and the aversive kappa-opioid agonist spiradoline

Data have suggested that rats avoid intake of an otherwise palatable saccharin cue when paired with a drug of abuse, at least, in part, because the value of the taste cue pales in anticipation of the availability of the highly rewarding drug. Earlier support for this hypothesis was provided by the finding that, relative to the less sensitive Fischer rats, Lewis rats exhibit greater avoidance of a saccharin cue when paired with a rewarding sucrose or cocaine unconditioned stimulus (US), but not when paired with the aversive agent, lithium chloride. More recent data, however, have shown that Fischer rats actually exhibit greater, not less, avoidance of the same saccharin cue when morphine serves as the US. Therefore, Experiment 1 evaluated morphine-induced suppression of intake of the taste cue in Lewis and Fischer rats when the morphine US was administered subcutaneously, rather than intraperitoneally. Experiment 2 examined the effect of strain on the suppression of intake of the saccharin cue when paired with spiradoline, a selective kappa-opioid receptor agonist. The results confirmed that Fischer rats are more responsive to the suppressive effects of morphine than Lewis rats, and that Fischer rats also exhibit greater avoidance of the saccharin cue when paired with spiradoline, despite the fact that spiradoline is devoid of reinforcing properties. Taken together, the data suggest that the facilitated morphine-induced suppression observed in Fischer rats, compared with Lewis rats, may reflect an increased sensitivity to the aversive, kappa-mediated properties of opiates.

Differential effects on natural reward processing in rats after repeated heroin

RATIONALE

Heroin users report reward deficits as well as reward enhancements (to drug stimuli). To better understand the causal relation between chronic heroin and alterations in natural reward processing, we used experimental techniques in animal models.

METHODS

Separate groups of rats were trained in several food reward paradigms: conditioned place preference (CPP), food-reinforced lever pressing under a progressive ratio schedule of reinforcement, free feeding, and lever pressing with conditioned reinforcement. After training, the rats were subjected to 10 daily heroin (2 mg/kg) or saline vehicle injections and tested at 3, 15, and 30 days post-treatment.

RESULTS

Repeated heroin treatment abolished the CPP and significantly reduced break points for food reward at 3, 15, and 30 days post-treatment. Repeated heroin did not affect free feeding. Finally, repeated heroin significantly enhanced responding for a food-based conditioned reinforcer.

CONCLUSIONS

Repeated heroin decreases the attractiveness of food-associated cues and reduces motivation to work for natural reward. However, it appears to enhance natural conditioned reward processes that involve the acquisition of novel responding. Thus, repeated heroin appears to produce differential effects on natural reward processing depending on the nature of the reward-directed behavior.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).