Brain neurotransmitter turnover correlated with morphine-seeking behavior of rats

Role of dopamine D1 receptor in the modulation of memory consolidation by passive and self-administered heroin and associated conditioned stimuli

It has been proposed that opiates modulate memory consolidation, but recent work has indicated that this effect may be mediated by how the drug is experienced (i.e., passive injections vs. self-administration). Because the dopamine (DA) D1 receptor is involved in processing of learning signals and attribution of salience to events experienced by an organism, two studies in male Sprague-Dawley rats tested the effect of blocking this receptor on modulation of memory consolidation by passive and self-administered heroin, in addition to conditioned memory modulation by heroin-paired cues. Using the object location memory task, Study 1 employed SCH23390 (0, 0.05, 0.10 mg/kg, SC) to modulate enhancement of memory consolidation induced by post-training injections of heroin (1 mg/kg, SC) as well as by exposure to the environment paired with heroin injections (6 pairings, 1 h each, 1 mg/kg). Study 2 was conducted in rats that could self-administer heroin (0.05 mg/kg/infusion, IV) and tested whether SCH23390 (0 and 0.1 mg/kg, SC) could prevent memory modulation induced by a change in schedule of self-administration (from fixed to variable ratio). It was found that while repeated passive injections of heroin retained their enhancing effect on memory, when self-administered, heroin enhanced consolidation of object location memory only at the beginning of self-administration and after a change in schedule. Importantly, SCH23390 blocked memory modulation by heroin when passively administered and when the drug was self-administered on a novel schedule. SCH23390 also blocked conditioned memory modulation induced by post-training exposure to heroin-paired cues. Taken together, these results suggest that modulation of memory consolidation by unconditioned and conditioned opiate reinforcers involve a D1-dependent mechanism of salience attribution linked to the anticipation of drug effects.

The effects of passive and active administration of heroin, and associated conditioned stimuli, on consolidation of object memory

Mode of administration (i.e., active vs passive) could influence the modulatory action that drugs of abuse exert on memory consolidation. Similarly, drug conditioned stimuli modulate memory consolidation and, therefore, acquisition and extinction of this conditioned response could also be influenced by mode of drug administration. Exploring these questions in male Sprague-Dawley rats, Study 1 assessed memory modulation by post-training 0, 0.3 and 1 mg/kg heroin injected subcutaneously in operant chambers (i.e., drug conditioned context). Study 2 asked a similar question but in rats trained to self-administer 0.05 mg/kg/infusion heroin intravenously, as well as in rats that received identical amounts of intravenous heroin but passively, using a yoked design. The period of heroin exposure was followed by repeated drug-free confinement in the conditioned context, and by sessions during which responses on the active lever had no scheduled consequences. Study 2 also included a cue-induced reinstatement session during which lever responses reactivated a light cue previously paired with intravenous heroin infusions. The post-training effects of injected/self-administered/yoked heroin, extinction and reinstatement sessions on memory consolidation were tested using the object location memory task. It was found that post-sample heroin enhanced memory in injected and yoked, but not self-administering, rats. However, post-sample exposure to the heroin cues (i.e., context or/and light cue) modulated memory equally in all groups. Taken together, these data support the conclusion that mode of administration impacts the cognitive consequences of exposure to drugs but not of environmental stimuli linked to their reinforcing effects.

Opioid and neuroHIV Comorbidity - Current and Future Perspectives

With the current national opioid crisis, it is critical to examine the mechanisms underlying pathophysiologic interactions between human immunodeficiency virus (HIV) and opioids in the central nervous system (CNS). Recent advances in experimental models, methodology, and our understanding of disease processes at the molecular and cellular levels reveal opioid-HIV interactions with increasing clarity. However, despite the substantial new insight, the unique impact of opioids on the severity, progression, and prognosis of neuroHIV and HIV-associated neurocognitive disorders (HAND) are not fully understood. In this review, we explore, in detail, what is currently known about mechanisms underlying opioid interactions with HIV, with emphasis on individual HIV-1-expressed gene products at the molecular, cellular and systems levels. Furthermore, we review preclinical and clinical studies with a focus on key considerations when addressing questions of whether opioid-HIV interactive pathogenesis results in unique structural or functional deficits not seen with either disease alone. These considerations include, understanding the combined consequences of HIV-1 genetic variants, host variants, and μ-opioid receptor (MOR) and HIV chemokine co-receptor interactions on the comorbidity. Lastly, we present topics that need to be considered in the future to better understand the unique contributions of opioids to the pathophysiology of neuroHIV. Graphical Abstract Blood-brain barrier and the neurovascular unit. With HIV and opiate co-exposure (represented below the dotted line), there is breakdown of tight junction proteins and increased leakage of paracellular compounds into the brain. Despite this, opiate exposure selectively increases the expression of some efflux transporters, thereby restricting brain penetration of specific drugs.

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

RATIONALE

Although chronic use of opiates can induce physical dependence and addiction, individual differences contributing to these symptoms are largely unknown.

OBJECTIVES

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

The volitional nature of nicotine exposure alters anandamide and oleoylethanolamide levels in the ventral tegmental area

Cannabinoid-1 receptors (CB(1)) have an important role in nicotine reward and their function is disrupted by chronic nicotine exposure, suggesting nicotine-induced alterations in endocannabinoid (eCB) signaling. However, the effects of nicotine on brain eCB levels have not been rigorously evaluated. Volitional intake of nicotine produces physiological and behavioral effects distinct from forced drug administration, although the mechanisms underlying these effects are not known. This study compared the effects of volitional nicotine self-administration (SA) and forced nicotine exposure (yoked administration (YA)) on levels of eCBs and related neuroactive lipids in the ventral tegmental area (VTA) and other brain regions. Brain lipid levels were indexed both by in vivo microdialysis in the VTA and lipid extractions from brain tissues. Nicotine SA, but not YA, reduced baseline VTA dialysate oleoylethanolamide (OEA) levels relative to nicotine-naïve controls, and increased anandamide (AEA) release during nicotine intake. In contrast, all nicotine exposure paradigms increased VTA dialysate 2-arachidonoyl glycerol (2-AG) levels. Thus, nicotine differentially modulates brain lipid (2-AG, AEA, and OEA) signaling, and these modulations are influenced by the volitional nature of the drug exposure. Corresponding bulk tissue analysis failed to identify these lipid changes. Nicotine exposure had no effect on fatty acid amide hydrolase activity in the VTA, suggesting that changes in AEA and OEA signaling result from alterations in their nicotine-induced biosynthesis. Both CB(1) (by AEA and 2-AG) and non-CB(1) (by OEA) targets can alter the excitability and activity of the dopaminergic neurons in the VTA. Collectively, these findings implicate disrupted lipid signaling in the motivational effects of nicotine.

Generational association studies of dopaminergic genes in reward deficiency syndrome (RDS) subjects: selecting appropriate phenotypes for reward dependence behaviors

Abnormal behaviors involving dopaminergic gene polymorphisms often reflect an insufficiency of usual feelings of satisfaction, or Reward Deficiency Syndrome (RDS). RDS results from a dysfunction in the "brain reward cascade," a complex interaction among neurotransmitters (primarily dopaminergic and opioidergic). Individuals with a family history of alcoholism or other addictions may be born with a deficiency in the ability to produce or use these neurotransmitters. Exposure to prolonged periods of stress and alcohol or other substances also can lead to a corruption of the brain reward cascade function. We evaluated the potential association of four variants of dopaminergic candidate genes in RDS (dopamine D1 receptor gene [DRD1]; dopamine D2 receptor gene [DRD2]; dopamine transporter gene [DAT1]; dopamine beta-hydroxylase gene [DBH]).

METHODOLOGY

We genotyped an experimental group of 55 subjects derived from up to five generations of two independent multiple-affected families compared to rigorously screened control subjects (e.g., N = 30 super controls for DRD2 gene polymorphisms). Data related to RDS behaviors were collected on these subjects plus 13 deceased family members.

RESULTS

Among the genotyped family members, the DRD2 Taq1 and the DAT1 10/10 alleles were significantly (at least p < 0.015) more often found in the RDS families vs. controls. The TaqA1 allele occurred in 100% of Family A individuals (N = 32) and 47.8% of Family B subjects (11 of 23). No significant differences were found between the experimental and control positive rates for the other variants.

CONCLUSIONS

Although our sample size was limited, and linkage analysis is necessary, the results support the putative role of dopaminergic polymorphisms in RDS behaviors. This study shows the importance of a nonspecific RDS phenotype and informs an understanding of how evaluating single subset behaviors of RDS may lead to spurious results. Utilization of a nonspecific "reward" phenotype may be a paradigm shift in future association and linkage studies involving dopaminergic polymorphisms and other neurotransmitter gene candidates.

Neural Changes Developed during the Extinction of Cocaine Self-Administration Behavior

The high rate of recidivism in cocaine addiction after prolonged periods of abstinence poses a significant problem for the effective treatment of this condition. Moreover, the neurobiological basis of this relapse phenomenon remains poorly understood. In this review, we will discuss the evidence currently available regarding the neurobiological changes during the extinction of cocaine self-administration. Specifically, we will focus on alterations in the dopaminergic, opioidergic, glutamatergic, cholinergic, serotoninergic and CRF systems described in self-administration experiments and extinction studies after chronic cocaine administration. We will also discuss the differences related to contingent versus non-contingent cocaine administration, which highlights the importance of environmental cues on drug effects and extinction. The findings discussed in this review may aid the development of more effective therapeutic approaches to treat cocaine relapse.

Self-administered and yoked nicotine produce robust increases in blood pressure and changes in heart rate with modest effects of behavioral contingency in rats

Experimenter-administered nicotine produces reliable increases in blood pressure and changes in heart rate. However, an extensive literature demonstrates that the effects of psychoactive drugs are dependent on whether administration is contingent on behavior. The present study assessed the cardiovascular effects of nicotine and whether those effects vary as a function of whether nicotine was self-administered or response-independent. Rats were divided into three groups according to a yoked design. The pattern of infusions for each triad was determined by the animal self-administering nicotine; the other two animals received either yoked nicotine or saline. Heart rate and blood pressure were measured during eighteen daily, 1h drug sessions by radiotelemetry. Each session was preceded and followed by a 20 minute period during which cardiovascular function was monitored in the operant chambers, but drug was not available. Acute exposure to yoked nicotine produced a rapid rise in blood pressure that was larger than the increase observed with self-administered nicotine. Additional infusions during the first session resulted in a similar sustained elevation in blood pressure in the nicotine groups. Over subsequent sessions, self-administered nicotine produced a larger effect on systolic blood pressure particularly early in each session, although for both self-administered and yoked nicotine the hypertensive effects waned partially with repeated test sessions. This decrease was fully accounted for by a pre-session decrease in pressure; relative to pre-session levels the strong hypertensive effects of nicotine persisted. Initial exposure to nicotine produced a short-lived bradycardia that in subsequent sessions was replaced with a longer-lasting nicotine-induced tachycardia; neither effect was related to the behavioral contingency of nicotine delivery. Together, these data provide a rich picture of the cardiovascular effects of nicotine. Effects of behavioral contingency were observed, but differences were limited. Other non-pharmacological factors such as baseline shifts potentially related to nicotine-associated cues deserve further attention.

Chronic binge-like moderate ethanol drinking in rats results in widespread decreases in brain serotonin, dopamine, and norepinephrine turnover rates reversed by ethanol intake

This research was initiated to assess the turnover rates (TORs) of dopamine (DA), norepinephrine (NA), serotonin (5-HT), aspartate, glutamate, and GABA in brain regions during rodent ethanol/sucrose (EtOH) and sucrose (SUC) drinking and in animals with a history of EtOH or SUC drinking to further characterize the neuronal systems that underlie compulsive consumption. Groups of five male rats were used, with two trained to drink EtOH solutions, two to drink SUC and one to serve as a non-drinking control. When stable drinking patterns were obtained, rats were pulse labeled intravenously and killed 60 or 90 min later and the TORs of DA, norepinephrine, 5-HT, aspartate, glutamate, and GABA determined in brain regions. Changes in the TOR of 5-HT, DA, and NA were detected specific to EtOH drinking, SUC drinking or a history of EtOH or SUC drinking. An acute EtOH deprivation effect was detected that was mostly reversed with EtOH drinking. These results suggest that binge-like drinking of moderate amounts of EtOH produces a deficit in neuronal function that could set the stage for the alleviation of anhedonic stimuli with further EtOH intake that strengthen EtOH seeking behaviors which may contribute to increased EtOH use in at risk individuals.

Self-administered and passive cocaine infusions produce different effects on corticosterone concentrations in the medial prefrontal cortex (MPC) of rats

Although our lab, as well as several others, has demonstrated a role for corticosterone in cocaine self-administration, there are no studies of the central dynamics of this hormone over the course of a behavioral session when rats are self-administering cocaine or receiving passive injections. The assay of corticosterone in microdialysates collected during such sessions allows for determinations of changes in brain corticosterone during drug-taking behavior. By using the combination of microdialysis in terminal fields for the mesocorticolimbic dopaminergic system and the yoked-triad model, one can distinguish between the direct cocaine-induced activation of the hypothalamo-pituitary-adrenal (HPA) axis from the activation of the HPA axis related to drug-taking. In these experiments, we measured corticosterone in microdialysis samples collected from probes aimed at the medial prefrontal cortex, nucleus accumbens and basolateral amygdala in rats self-administering cocaine and receiving identical, passive infusions of cocaine or saline. While corticosterone was increased in all three brain regions in rats receiving cocaine, medial prefrontal cortex corticosterone was increased significantly more in rats receiving non-contingent infusions of the drug compared to rats self-administering cocaine. The results of these experiments demonstrate that control over drug delivery can affect the influence of a hormonal input on the functional characteristics of specific anatomical projections of the central nervous system. These results also provide evidence of the role steroid hormones play in shaping the functional activity of the brain.

Neuronal adaptations, neuroendocrine and immune correlates of heroin self-administration

Opioid receptor-mediated action in the central nervous system (CNS) has been consistently shown to trigger changes in the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS) and suppress a variety of parameters of immune function in investigator-delivered paradigms. Overwhelming evidence supports the concept that the CNS undergoes numerous and complex neuronal adaptive changes in addicts, and in animal models of heroin addiction as a result of the training of drug stimuli to serve as reinforcers, altering the function of individual neurons and the larger neural circuits within which the neurons operate. Taken together, these advances suggest that since plastic neuronal changes occur in drug addiction and related animal model paradigms, profiles of neuroendocrine and immune function would differ in a rat model of heroin self-administration compared to passive infusion of drug. Self-administration of heroin induces neuronal circuitry adaptations in specific brain regions that may be related to alterations in neuroendocrine and T lymphocyte function also observed. Animals self-administering (SA) heroin exhibit increased mu-opioid receptor agonist ([D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO))-stimulated guanosine-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) binding in the anterior hypothalamus (50% and 33%) and rostral medial thalamus (33% and 36%) compared with control animals receiving identical non-contingent injections of yoked-heroin (YH) or yoked-saline (YS), respectively. No changes in agonist-stimulated G-protein sensitization were observed in 14 other brain regions studied. No changes in mu-opioid receptor density, ((3)H-DAMGO binding) were seen in all brain regions examined. The neuronal changes in SA animals were correlated with elevated adrenocorticotrophic hormone (ACTH) (64% and 104%) and glucocorticoid production (198% and 79%) compared with YH and YS groups, respectively. Neuroendocrine adaptive changes in SA animals were associated with thymic hypoplasia. Splenic T lymphocytes from animals that had self-administered heroin showed a profoundly reduced ability to proliferate in response to concanavalin A (50% and 48% compared with YH and YS controls, respectively; Fig. 1A), or a monoclonal antibody (R73) to the CD3/T-cell receptor complex (anti-TCR) plus IL-2 (55% and 59% compared with YH and YS controls, respectively; Fig. 1B). Self-administration of heroin selectively alters T lymphocyte function, as no effects on natural killer cell activity or macrophage functions were observed. These findings may have relevance to the acquisition and documented increased incidence of infectious diseases, including HIV, in heroin addicts, due to a pre-existing T-cell immunodeficient state.

Chronic pain alters drug self-administration: implications for addiction and pain mechanisms

This review article focuses on the impact that the presence of pain has on drug self-administration in rodents, and the potential for using self-administration to study both addiction and pain, as well as their interaction. The literature on the effects of noxious input to the brain on both spinal and supraspinal neuronal activity is reviewed as well as the evidence that human and rodent neurobiology is affected similarly by noxious stimulation. The convergence of peripheral input to somatosensory systems with limbic forebrain structures is briefly discussed in the context of how the activity of one system may influence activity within the other system. Finally, the literature on how pain influences drug-seeking behaviors in rodents is reviewed, with a final discussion of how these techniques might be able to contribute to the development of novel analgesic treatments that minimize addiction and tolerance.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Conditioned and sensitized responses to stimulant drugs in humans

In animal models considerable evidence suggests that increased motivation to seek and ingest drugs of abuse are related to conditioned and sensitized activations of the mesolimbic dopamine (DA) system. Direct evidence for these phenomena in humans, though, is sparse. However, recent studies support the following. First, the acute administration of drugs of abuse across pharmacological classes increases extracellular DA levels within the human ventral striatum. Second, individual differences in the magnitude of this response correlate with rewarding effects of the drugs and the personality trait of novelty seeking. Third, transiently diminishing DA transmission in humans decreases drug craving, the propensity to preferentially respond to reward-paired stimuli, and the ability to sustain responding for future drug reward. Finally, very recent studies suggest that repeated exposure to stimulant drugs, either on the street or in the laboratory, can lead to conditioned and sensitized behavioral responses and DA release. In contrast to these findings, though, in individuals with a long history of substance abuse, drug-induced DA release is decreased. This diminished DA release could reflect two different phenomena. First, it is possible that drug withdrawal related decrements in DA cell function persist longer than previously suspected. Second, drug-paired stimuli may gain marked conditioned control over the release of DA and the expression of sensitization leading to reduced DA release when drug-related cues are absent. Based on these observations a two-factor hypothesis of the role of DA in drug abuse is proposed. In the presence of drug cues, conditioned and sensitized DA release would occur leading to focused drug-seeking behavior. In comparison, in the absence of drug-related stimuli DA function would be reduced, diminishing the ability of individuals to sustain goal-directed behavior and long-term objectives. This conditioned control of the expression of sensitized DA release could aggravate susceptibility to relapse, narrow the range of interests and perturb decision-making, accounting for a wide range of addiction related phenomena.

Comparing the physiological and subjective effects of self-administered vs yoked cocaine in humans

RATIONALE

Studies with laboratory animals demonstrating different effects of self- vs experimenter-administered drug suggest that the ability to control or predict drug delivery may be an important determinant of drug action.

OBJECTIVE

This study assessed whether self-administered and yoked cocaine injections produce different effects in humans.

METHODS

Ten inpatient volunteers with experience in using cocaine participated in a double-blind, yoked design during which four experimental test sessions were conducted. During two sessions, participants controlled if and when up to six 40 mg/70 kg i.v. cocaine injections were given. During two sessions, participants received noncontingent exposure to the same pattern of injections given during the preceding session (i.e., yoked) under blind conditions. Sessions followed a fixed-order, ABAB design. Measures of subjective and physiological response to cocaine were taken throughout each session.

RESULTS

Cardiovascular safety parameters were exceeded in some individuals after yoked, but not self-administered, cocaine resulting in some scheduled injections being delayed or withheld. Mean systolic and diastolic blood pressures were higher following yoked compared to self-administered cocaine. In contrast, analysis of the subjective effects revealed only small and generally nonsignificant differences in the effects of self-administered vs yoked cocaine.

CONCLUSIONS

These results suggest that under the laboratory methods employed, control over the schedule of drug delivery may not alter the subjective effects of cocaine in humans. In contrast, the cardiovascular effects of cocaine appear to be greater when the drug is administered noncontingently.

Clonidine maintains intrathecal self-administration in rats following spinal nerve ligation

Clonidine is approved for spinal administration against neuropathic pain, and reverses both spontaneous and elicited pain in humans following spinal administration. Rodent studies that seek to model pharmacology in pain states have historically relied on reflexive withdrawal from noxious stimuli as the primary endpoint. Drug self-administration studies have face validity in the drug abuse field for modeling drug abuse in humans, however, this methodology has not been applied to address issues related to drug seeking behaviors that may be relevant for other human populations, such as patients with neuropathic pain. Rats without spinal nerve ligation (SNL) failed to acquire intrathecal clonidine self-administration over 10 days of access. Rats were found to self-administer intrathecal infusions of clonidine following SNL in a stable and dose-responsive manner, however, and clonidine was self-administered throughout the day with 66% of total drug intake occurring during the dark cycle. Substitution of clonidine with saline or with clonidine and the alpha2-adrenoceptor antagonist idazoxan resulted in extinction of responding in SNL animals. Food reinforcement was initially decreased in SNL rats self-administering clonidine compared to normal animals, however, tolerance developed to this effect of clonidine in SNL rats after 5 days. These data demonstrate that drug self-administration can be applied to questions other than drug abuse, and provides an additional measure for development of novel therapeutic strategies for chronic pain treatment.

Mu-opioid self-administration vs passive administration in heroin abusers produces differential EEG activation

Psychoactive drug self-administration (SA) produces different neurobiological effects than passive administration (PA) in non-human animals; however, such consequences have never been examined in human drug abusers. The present study compared electroencephalographic (EEG) activation produced by intravenous PA and SA of the mu-opioid fentanyl in eight heroin-dependent, methadone-stabilized male participants. In phase 1, participants received cumulative PA of fentanyl (up to 1.5 mg/70 kg; session 1), then bolus PA of placebo and fentanyl 1.5 mg/70 kg (session 2). High-dose fentanyl significantly increased the amplitude of slow-frequency (delta- and theta-band) EEG activity. In phase 2, bolus fentanyl 1.5 mg/70 kg was available for SA, requiring the participant to complete 1500 responses, in each of two sessions after saline or naloxone pretreatment. Delta EEG peak amplitude increases were greater following fentanyl SA than fentanyl PA, primarily over the central midline region, and were attenuated by naloxone pretreatment. The EEG increase and its attenuation by naloxone agree with preclinical evidence and suggest that SA-related EEG responses were mediated by opioid receptors.

Neuroadaptive effects of active versus passive drug administration in addiction research

Increasing knowledge of the genome sequences of several organisms and the development of genome-wide, high-throughput screening techniques for gene expression are likely to generate a vast amount of data aimed at elucidating the molecular mechanisms of addiction. These findings are likely to have potential for future addiction pharmacotherapies. However, it is important to employ animal models that dissociate the molecular and cellular consequences of the direct pharmacological effects of addictive drugs from those that result from the cognitive processes associated with self-administration of these drugs. In this article, we suggest that the short-term and long-term neuroadaptive effects of addictive drugs in the brain depend crucially on the drug-exposure paradigm used [i.e. passive (non-contingent) drug exposure and active (contingent) self-administration]. This has important ramifications for future molecular and cellular studies of drug addiction.

Brain neurotransmitter turnover rates during rat intravenous cocaine self-administration

The turnover rates of dopamine, norepinephrine, serotonin, aspartate, glutamate and GABA were measured in 27 brain regions of rats self-administering cocaine and in yoked cocaine- and yoked vehicle-infused controls using radioactive pulse-labeling procedures to identify brain neuronal systems underlying self-administration. Changes in the activity of heretofore unrecognized dopamine, norepinephrine, serotonin, glutamate and GABA innervations of the forebrain specific to cocaine self-administration were found. This included innervations of the nucleus accumbens, ventral pallidum, lateral hypothalamus and the anterior and posterior cingulate, entorhinal-subicular and visual cortices. Turnover rates also were calculated using metabolite/neurotransmitter ratios which were inconsistent with the pulse-label technologies indicating that ratio procedures are not accurate measures of neurotransmitter utilization. Results with the pulse-label technique provide evidence of the involvement of neuronal systems in cocaine self-administration not previously known, some of which may have a broader role in brain reinforcement processes for natural reinforcers (i.e. food, water, etc.) since drugs of abuse are thought to produce reinforcing effects by modulating activity in these endogenous systems.

Self-administration of heroin produces alterations in the expression of inducible nitric oxide synthase

Nitric oxide plays a critical role in the immune response, and our studies have shown that heroin induces a reduction in the expression of iNOS, the enzyme responsible for nitric oxide production. The present study evaluated the effect of heroin self-administration on iNOS expression using a three-group design. Group one (self-administration) was trained to press a lever for i.v. administration of heroin. Group two (yoked heroin) received a simultaneous equivalent infusion of heroin determined by the responses of a 'partner' animal in the first group. A third group (yoked saline) also was yoked to the first group, but received i.v. injections of saline. Immediately following the last session, all rats received an injection of lipopolysaccharide (LPS) to induce iNOS expression. About 6 h after the injection of LPS, iNOS mRNA and protein expression were determined in spleen, lung, and liver. Additionally, the accumulation of plasma nitrite/nitrate, the more stable end products of nitric oxide degradation were measured. Although there was not a consistent difference between the self-administering and yoked-heroin animals, the results show that rats will self-administer a sufficient amount of heroin to induce a pronounced, widespread reduction in the expression of iNOS.

Cue dependency of nicotine self-administration and smoking

A paradox exists regarding the reinforcing properties of nicotine. The abuse liability associated with smoking equals or exceeds that of other addictive drugs, yet the euphoric, reinforcing and other psychological effects of nicotine, compared to these other drugs, are more subtle, are manifest under more restricted conditions, and do not readily predict the difficulty most smokers experience in achieving abstinence. One possible resolution to this apparent inconsistency is that environmental cues associated with drug delivery become conditioned reinforcers and take on powerful incentive properties that are critically important for sustaining smoking in humans and nicotine self-administration in animals. We tested this hypothesis by using a widely employed self-administration paradigm in which rats press a lever at high rates for 1 h/day to obtain intravenous infusions of nicotine that are paired with two types of visual stimuli: a chamber light that when turned on signals drug availability and a 1-s cue light that signals drug delivery. We show that these visual cues are at least as important as nicotine in sustaining a high rate of responding once self-administration has been established, in the degree to which withdrawing nicotine extinguishes the behavior, and in the reinstatement of lever pressing after extinction. Additional studies demonstrated that the importance of these cues was manifest under both fixed ratio and progressive ratio (PR) schedules of reinforcement. The possibility that nicotine-paired cues are as important as nicotine in smoking behavior should refocus our attention on the psychology and neurobiology of conditioned reinforcers in order to stimulate the development of more effective treatment programs for smoking cessation.

Differential effects of response-contingent and response-independent nicotine in rats

Passive administration of nicotine activates the hypothalamic-pituitary-adrenocortical axis and sympathetic nervous system. However, little is known about the effects of self-administered nicotine. Drug-naive rats were trained to respond for food reinforcement and then tested in one, 1-h session in which they received response-contingent i.v. nicotine or response-independent i.v. nicotine or saline. Blood draws were taken immediately prior to the session, 15 min after the first infusion and immediately after the session. Both response-contingent and response-independent nicotine (RI/N) increased corticosterone within 15 min, however, corticosterone levels returned to baseline in animals receiving response-contingent nicotine (RC/N) by the end of the session while remaining elevated in those receiving RI/N. Furthermore, only RI/N increased plasma epinephrine and norepinephrine levels; RC/N produced no effect. These differences indicate that nicotine's acute effects are powerfully modified by the presence of a contingency relationship between drug administration and the animal's behavior and that this relationship develops very rapidly.

Nucleus accumbens cell firing during maintenance, extinction, and reinstatement of cocaine self-administration behavior in rats

Electrophysiological recording procedures were used to examine nucleus accumbens (Acb) cell firing in rats (N=13) during cocaine self-administration sessions consisting of three phases. In phase one (maintenance), each lever press resulted in an intravenous cocaine infusion (0.33 mg, 6 s) paired with a tone-houselight stimulus (20 s). Of 144 Acb cells recorded during maintenance, 39 neurons (27%) exhibited phasic firing relative to the cocaine-reinforced response [4-8]. Briefly, Acb neurons showed increases in firing rate within seconds preceding the reinforced response and/or changes (increases or decreases) in activity within seconds following response completion. In phase two (extinction), saline was substituted for cocaine in the task. Results indicated that cells displaying exclusively anticipatory discharges during maintenance exhibited similar phasic activity during extinction. However, neurons that displayed post-response activity during the maintenance phase typically showed significant attenuation of phasic firing rates during extinction. After 30 min of no responding, animals were 'primed' with an intravenous infusion of cocaine, and self-administration was reestablished during phase three (reinstatement). Results showed that pre-response discharge patterns remained relatively intact while post-response cells typically exhibited a partial recovery of phasic activity. Similar findings were observed during other extinction experiments in which the stimulus only was removed (CS extinction). These findings support the notion that specific factors operating within the self-administration context differentially control pre- versus post-response discharge patterns of Acb neurons.

In vivo CRF release in rat amygdala is increased during cocaine withdrawal in self-administering rats

Previous studies have suggested a role for corticotropin-releasing factor (CRF) in the central nucleus of the amygdala (CeA) in the aversive and anxiogenic effects of withdrawal from opiates and ethanol. To test whether this role of CRF extends to cocaine withdrawal as well, the release of CRF in rat amygdala was monitored by intracranial microdialysis during a 12-hour session of intravenous cocaine self-administration and subsequent 12-hour cocaine withdrawal period. Cocaine self-administration tended to lower dialysate CRF concentrations to approximately 75% of CRF levels in controls. In contrast, subsequent cocaine withdrawal produced a profound increase in CRF release, which reached peak levels of approximately 400% of baseline between 11 and 12 hours post-cocaine. These results provide evidence that cocaine withdrawal activates CRF neurons in the amygdala, a site that has been implicated in emotional and anxiogenic effects of stress and drug withdrawal syndromes.

Modifications of dopamine D1 receptor complex in rats self-administering cocaine

Cocaine is spontaneously and experimentally self-administered and, when given repeatedly, it induces a stable form of sensitization to a previously assessed minimum active dose. In the present study, triads of rats chronically implanted with a jugular catheter were treated as follows: one animal was trained to self-inject cocaine, while the other two passively received either cocaine or saline whenever the self-administering rat completed the response requirement. After 30 days of stable responding, the animals were sacrificed and dopamine D1 receptor density and adenylyl cyclase activity were measured in different brain areas. Animals receiving cocaine (both self-administering and yoked) showed a down-regulation of dopamine D1 receptor number and of dopamine stimulated adenylyl cyclase activity in the nucleus accumbens, as compared to saline rats. In the olfactory tubercle, dopamine stimulated adenylyl cyclase activity appeared selectively and significantly down-regulated in self-administering animals.

The role of corticosteroids in nicotine's physiological and behavioral effects

This paper reviews evidence indicating that adrenal corticosteroids modulate the responsiveness of mice and rats to nicotine. Adrenalectomy increases, and both acute and chronic corticosteroid administration decrease, some of the physiological and behavioral effects of nicotine. One function of adrenal steroids may be to regulate stress-induced changes in nicotine sensitivity. Another is to mediate the development of chronic tolerance when nicotine is given intermittently, and when the resulting tolerance has a learned component. A role of glucocorticoids in the development of tolerance to nicotine is suggested by the findings that a conditioned elevation of plasma corticosterone, which anticipates nicotine delivery, accompanies the development of chronic tolerance and that environmental cues evoke a conditioned corticosterone response, but only after they have become associated with nicotine delivery. The mechanisms by which adrenal steroids modulate nicotine sensitivity are not known, although recent in vitro evidence suggests that steroids can rapidly and reversibly reduce nicotinic receptor function. While most of the data are consistent with the hypothesis that corticosteroids reduce nicotine responsiveness, and thus promote a learned form of tolerance, there are new findings that corticosteroids increase the development of sensitization to the locomotor-activating effects of nicotine. These data suggest that formulations postulating a unidirectional effect of corticosteroids on nicotine's actions (e.g. decreased sensitivity) must be revised to take into account interacting variables such as the specific nicotine effect being studied and whether that effect normally exhibits tolerance or sensitization. Finally, research is presented which indicates that the corticosterone-elevating effects of nicotine, previously reported for experimenter-administered drug, are also produced when nicotine administration is contingent on an operant response, and at a dose which sustains the development of nicotine self-administration in rats. These findings highlight the feasibility of using self-administration models in future explorations of the relationship between adrenal steroids and nicotine function.

Cardiovascular effects of cocaine during operant cocaine self-administration

The aim of this study was to investigate the acute and chronic effects of cocaine self-administration behavior on cardiovascular function. Mean blood pressure and heart rate were measured by radio-telemetry during several experimental conditions. Initial control studies eliminated possible confounds related to the effects of saline injections and operant responding on heart rate and blood pressure. When rats were first allowed to self-administer 0.5-mg/kg injections of cocaine (FR(fixed ratio)10:TO 30 s), there was a significant increase in blood pressure. Tolerance developed to this effect within 3 daily sessions. A significant decrease in blood pressure and heart rate was observed during saline-substitution sessions. Increasing the injection dose of cocaine (1.0, 2.0 and 4.0 mg/kg per injection) did not produce a dramatic increase in blood pressure or heart rate despite significant cumulative cocaine intake (20-27 mg/kg). The cardiovascular effects of cocaine administration did not approach magnitudes previously reported. The results of the current study suggest that operant-conditioned behaviour and/or the direct reinforcing effects of cocaine modulates the cardiovascular effects of cocaine.

Fluctuations in nucleus accumbens dopamine concentration during intravenous cocaine self-administration in rats

Fluctuations in extracellular dopamine and DOPAC levels in nucleus accumbens septi (NAS) were monitored in 1-min microdialysis samples taken from rats engaged in intravenous cocaine self-administration. For four rats the dose per injection was fixed at 2.0 mg/kg; for four others the dose per injection was varied irregularly, from one response to the next, between three levels (0.5, 1.0 and 2.0 mg/kg). Regardless of the dosing regimen, extracellular dopamine levels were tonically elevated by 200-800% within the cocaine self-administration periods, fluctuating phasically within this range between responses. In the fixed dose condition, the phasic increases following each injection (and the phasic decreases preceding them) averaged approximately 50% of the mean tonic elevation. Phasic fluctuations in dopamine levels remained time-locked to lever-presses even when response rate was irregular, because of the variable dose condition. In the variable dose condition greater increases in dopamine and longer inter-response times followed injections of the higher doses; dopamine fluctuations were consistent with the multiple-infusion pharmacokinetics of cocaine. DOPAC levels showed a slow tonic depression during cocaine self-administration, but individual injections were not associated with discernible phasic fluctuations of DOPAC. These data are consistent with the hypothesis that falling dopamine levels trigger successive responses in the intravenous cocaine self-administration paradigm, but inconsistent with the notion that extracellular dopamine levels are depleted at the times within sessions when the animal initiates drug-seeking responses.

Firing patterns of nucleus accumbens neurons during cocaine self-administration in rats

The firing patterns of neurons in the nucleus accumbens (NA) were recorded in rats trained to self-administer cocaine via response contingent intravenous drug infusions. Recordings were obtained from permanently implanted multiple electrode arrays (8 microwires) inserted bilaterally into the NA and/or ventral striatum (NA-VS) in animals exhibiting stable responding (inter-infusion intervals, INT) during test sessions consisting of 16-30 drug delivery episodes. Electronically isolated and identified NA-VS neurons showed distinct patterns of phasic increases in firing relative to the occurrence of the reinforced lever press. Two particular firing patterns, however, were repeatedly encountered in different animals. In one type, a marked increase was observed in discharge following response contingent drug delivery. A second firing pattern showed two distinct temporally separated brief firing peaks (bursts), one immediately prior to the initiation of responding, and the other a brief discharge commencing within 200 ms after the initiation of drug delivery. The time between firing peaks was found to be modifiable by changing the response/reward (FR) ratio for drug delivery. A third finding was that the correlates of the self-administration response were not solely the result of drug infusion since, (1) phasic firing increases were not observed when the drug was delivered non-contingently during the same session and, (2) the emergence of patterns was frequently delayed within the session until after drug self-administration behavior stabilized at regular INTs. The findings are discussed in terms of the significance of NA-VS neuron firing correlates for the initiation and maintenance of cocaine self-administration.

Neurochemical changes associated with the action of acute administration of diazepam in reversing the behavioral paradigm conditioned emotional response (CER)

Neurotransmitter turnover of biogenic monoamines (dopamine, norepinephrine, and serotonin) and amino acids (glutamate, aspartate, and gamma-aminobutyric acid) was evaluated in rats exposed to the conditioned emotional response (CER) paradigm in the absence (total suppression) or presence of acute 5 mg/kg i.p. diazepam (which reversed suppression and restored normal responding). Based on previous studies of CER, with controls for shock and stimulus histories, the results with respect to the anxiolytic could be divided into several categories: changes in turnover which are associated only with the CER behavior; changes associated only with the drug, diazepam; changes which augmented the effects of the behavior; or changes which were the reverse of those associated with the behavior. Due to the multitude and complexity of the results, not all observations have clear explanations at this time. However, for the CER behavior per se, it is apparent that a combination of neurotransmitters, including some implications about acetylcholine, act in concert to bring about the behavioral suppression. The action of diazepam is more complex, involving the full spectrum of neurotransmitters to bring about its direct and indirect effects.

Acute and multiple injection effects of magnesium on responding maintained by cocaine, extinction from cocaine, glucose + saccharin, and food

In a variety of behavioral experiments, magnesium has effects that are similar to cocaine and other psychomotor stimulants. Of particular relevance to the present experiments is the recent finding that magnesium maintains responding in cocaine-trained rats. It would be expected, therefore, that injections of magnesium would alter the rate of responding maintained by self-administered cocaine in rats. Five experiments examined the specificity and selectivity of this interaction. Acute and multiple injections of MgCl2 (15-250 mg/kg) produced dose-dependent reductions in responding maintained by cocaine (0.1-2 mg/kg/infusion). Testing for acute injection effects occurred following injections, while testing for multiple injection effects occurred prior to daily injections. Doses of 30 and 125 mg/kg MgCl2 reduced responding maintained by doses of cocaine that were below the training dose of 0.75 mg/kg/infusion. MgCl2 in a dose of 250 mg/kg markedly suppressed responding maintained by each dose of cocaine. A magnesium-deficient diet produced a dose-dependent increase in responding maintained by 0.1 mg/kg/infusion cocaine. In order to determine the specificity and selectively of these effects, acute and multiple injections of MgCl2 were examined on glucose + saccharin- and food-maintained responding. The acute effects of MgCl2 injections were specific because food-maintained responding was not affected, except by the highest dose of 250 mg/kg. This demonstrates that lever pressing was not nonspecifically reduced by 30 and 125 mg/kg MgCl2 during cocaine availability. However, the effects on cocaine-maintained responding were not selective for cocaine because glucose + saccharin-maintained responding and responding during extinction from cocaine were affected by MgCl2 in a manner similar to cocaine-maintained responding.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnesium-maintained self-administration responding in cocaine-trained rats

Magnesium chloride (MgCl2) produces behavioral effects similar to those of psychomotor stimulants in a variety of behavioral situations. Because MgCl2 appears to have stimulant properties, the ability of MgCl2 to maintain responding in a rat self-administration paradigm was examined in seven experiments under different access and schedule conditions in cocaine-trained rats. These varied from the availability of MgCl2 for a single day's test session subsequent to 1 h availability of cocaine, to the availability of MgCl2 for 10 or 20 days after cocaine availability was totally discontinued. Fixed ratio 1, fixed ratio 5, and progressive ratio 1, 2 and 3 schedules of drug delivery were used. The results demonstrate that MgCl2 may substitute for self-administered cocaine because it maintained responding; it did so dose dependently to maintain a constant level of MgCl2 intake; and it did so over a 10-day period of time both with and without access to cocaine on test days. Responding maintained by MgCl2 when cocaine was no longer available was similar under fixed ratio 1 and 5 schedule conditions. The progressive ratio breakpoints for MgCl2 were significantly higher than those for saline, but significantly lower than those for cocaine. These data indicate that MgCl2 has some reinforcing efficacy in cocaine-trained rats, particularly under fixed ratio 1 and 5 schedules, but has a low abuse potential compared to cocaine.

Oral self-administration of ethanol and not experimenter-administered ethanol facilitates rewarding electrical brain stimulation

The effects of ethanol on brain-stimulation reward (BSR) were investigated in rats orally self-administering ethanol. Electrodes were stereotaxically implanted in the medial forebrain bundle (MFB) of male F-344 rats. A rate free threshold procedure was used. Animals demonstrated significant threshold-lowering effects after considerable ethanol self-administration experience. To elucidate the significance of the contingent nature of the route of administration in the threshold-lowering effects of ethanol on BSR, a comparison of animals self-administering ethanol to yoked animals receiving it passively through a gastric cannula was made. Significant threshold-lowering effects were only found in the animals self-administering ethanol and not those receiving it noncontingently. Thus, to the extent that brain-stimulation reward is a model of drug-induced euphoria, these results suggest that the reinforcing effects of ethanol are dependent to a greater degree on an interaction between experimental, environmental and pharmacological factors, than other abused drugs.

Failure of magnesium to maintain self-administration in cocaine-naive rats

Previous research has shown that magnesium interacts with cocaine in such a way that it potentiates its action in a variety of behavioral situations. More recently, it has been demonstrated that magnesium will dose dependently substitute for cocaine self-administration and reduce the intake of cocaine. It is of considerable interest to determine if magnesium would be self-administered in cocaine-naive animals. The results of two experiments demonstrate that magnesium is not self-administered by cocaine-naive rats since although responding for magnesium chloride is above hypertonic saline control levels on day 1 of access, this responding is not maintained on subsequent days, does not occur in a regularly spaced pattern over time, and is not inversely related to dose. Taken together these data indicate that magnesium is a substitute for cocaine that has low abuse potential.

Ethanol oral self-administration and rewarding brain stimulation

The effects of orally self-administered ethanol (ETOH) on responding for rewarding brain stimulation were studied. Bipolar electrodes were implanted in either the lateral hypothalamic region of the medial forebrain bundle (MFB-LH) or the ventral tegmental area (VTA) of 6 male F-344 rats. After surgery subjects were trained in a continuous reinforcement procedure (CRF) for constant current rewarding brain stimulation. On alternate days subjects were allowed to drink an ethanol and sucrose solution (12% and 5%, respectively) for 30 min and subsequently tested on the brain stimulation procedure. All subjects showed facilitation of responding (increase in rate) after ingesting low to moderate doses of ETOH (0.4-1.7 g/kg). Depression of responding (decrease in rate) or return to baseline levels (control solution rate) was observed only in those subjects which ingested 2 g/kg or greater during the drinking period. These results indicate that low to moderate doses of self-administered ethanol will increase responding for rewarding brain stimulation. Further, the results suggest that this facilitation of responding is, at least in part, a function of the method of administration and/or the contingent nature of the ethanol delivery (self-administration).

Opponent process theory of motivation: neurobiological evidence from studies of opiate dependence

One hypothetical model for a mechanism of drug dependence involves the development of an adaptive process that is initiated to counter the acute effects of the drug. This adaptive process persists after the drug has been cleared from the brain, leaving an opposing reaction unopposed (abstinence signs). From a motivational perspective a particularly attractive hypothesis has been that of opponent process theory (32). Here many reinforcers elicit positive affective and hedonic processes that are opposed by negative affective and hedonic processes. Thus the intense pleasure of the opiate drug "rush" or "high" would be opposed by aversive withdrawal symptoms. The present paper presents neurobiological evidence to support the opponent process concept and suggests neural circuitry that may be involved. The region of the nucleus accumbens in the forebrain of the rat has been shown to be a particularly sensitive substrate not only for the acute reinforcing properties of opiate drugs, but also for the response disruptive effects of opiate antagonists in opiate dependent rats. This region also appears to be particularly sensitive to the aversive stimulus effects of opiate antagonists using a place aversion measure in dependent rats. These results suggest that the region of the nucleus accumbens and its neural circuitry may be an important neural substrate for both the positive and negative motivational aspects of drug dependence.

Specific effects of punishment on amino acids turnover in discrete rat brain regions

Specific effects of punishment on the turnover rates of aspartate (Asp), glutamate (Glu) and gamma-aminobutyric acid (GABA) in 14 brain regions were investigated in rats exposed to punishment. Two yoked controls were also used in an attempt to separate the nonspecific effects of response rate, reinforcement density and direct effects of punisher (foot shock). Punished and unpunished littermate rats had similar response rates, and the reinforcement density was almost identical for both groups. A third group (yoked-shock rats) received food and shock independent of responding whenever these were given to the punished rats. When compared to the unpunished rats, the punishment increased the turnover rates of the three amino acids in all brain regions examined except GABA turnover in the caudate-putamen and preoptic-diagonal band. The majority of these changes by the punishment were similar to the effects of the yoked-shock (yoked-shock versus unpunished), although the magnitude of increase by the punishment was mostly larger than that by the yoked-shock. Six changes by the punishment (increase in the turnover rates of Asp in the thalamus, Glu in the hypothalamus and GABA in the cingulate cortex, entorhinal-subicular cortex, dentate gyrus and hypothalamus) appeared to be the specific effects of punishment since the yoked-shock did not affect these parameters. These results suggest that the punishment caused a hyperexcitation of the amino acidergic neurons in the limbic systems, particularly those in Papez's circuit.

Brain stimulation reward: effects of ethanol

This paper briefly describes and compares the effects of ethanol with those of other abuse substances on brain stimulation reward. The most frequently observed effects of abuse substances on this phenomenon is an increase in sensitivity of the animal to the stimulation. This increased sensitivity to rewarding brain stimulation has been studied as a model of drug-induced euphoria. Although many studies have reported that ethanol does increase the sensitivity of animals to this stimulation, there is much less consistency in results between laboratories than observed with the abused opiates or psychomotor stimulants. Data is presented that suggests that associative factors, e.g., self- versus experimenter-administered ethanol, as well as route of administration and time of brain stimulation testing may all contribute to the variability in results obtained between laboratories. Further, the effects of ethanol on brain stimulation reward are more like those of other sedative-hypnotics than the opiates or psychomotor stimulants.

Effect of heroin-conditioned auditory stimuli on cerebral functional activity in rats

Cerebral functional activity was measured as changes in distribution of the free fatty acid [1-14C]octanoate in autoradiograms obtained from rats during brief presentation of a tone previously paired to infusions of heroin or saline. Rats were trained in groups of three consisting of one heroin self-administering animal and two animals receiving yoked infusions of heroin or saline. Behavioral experiments in separate groups of rats demonstrated that these training parameters imparts secondary reinforcing properties to the tone for animals self-administering heroin while the tone remains behaviorally neutral in yoked-infusion animals. The optical densities of thirty-seven brain regions were normalized to a relative index for comparisons between groups. Previous pairing of the tone to heroin infusions irrespective of behavior (yoked-heroin vs. yoked-saline groups) produced functional activity changes in fifteen brain areas. In addition, nineteen regional differences in octanoate labeling density were evident when comparison was made between animals previously trained to self-administer heroin to those receiving yoked-heroin infusions, while twelve differences were noted when comparisons were made between the yoked vehicle and self administration group. These functional activity changes are presumed related to the secondary reinforcing capacity of the tone acquired by association with heroin, and may identify neural substrates involved in auditory signalled conditioning of positive reinforcement to opiates.

Specific effects of punishment on biogenic monoamine turnover in discrete rat brain regions

Specific effects of punishment on the turnover rates of norepinephrine, dopamine and serotonin (5-HT) in brain regions were investigated in rats exposed to punishment. Two yoked controls were also used in an attempt to separate the non-specific effects of response rate, reinforcement density and direct effects of punisher (foot shock). Punished and unpunished littermate rats had similar response rates, and the reinforcement density was almost identical for both groups. A third group (yoked-shock rats) received food and shock independent of responding whenever these were given to the punished rats. When compared to the unpunished rats, changes in the monoamine turnover rates resulting from the punishment were similar to the effects of yoked-shock with respect to the direction of action in most cases (13 out of 17 changes). These changes may be related to non-specific effects of the shock. Four changes by the punishment were determined to be specific effects of the punishment since the yoked-shock had no effect or changed the turnover to the opposite direction. Among these, increase in 5-HT turnover rate in the frontal cortex (greater than 7-fold) was the largest change. These results and reported effects of drugs which act on serotonergic systems on the punished behavior suggest that the increase in 5-HT neuronal activity in the frontal cortex is involved in the behavioral suppression induced by the punishment.

Effects of heroin and cocaine on brain activity in rats using [1-14C]octanoate as a fast functional tracer

Brain activity was measured autoradiographically using [1-14C]octanoate (OCTO) as a fast functional tracer in rats receiving either saline, heroin or cocaine. Regional optical densities were normalized to a relative optical density index for comparisons of OCTO labeling between treatment groups. Heroin significantly increased labeling in the dentate gyrus and cocaine increased density in the anterior cingulate cortex, globus pallidus, hippocampus CA3-4, lateral septum, hypothalamus and ventral tegmentum. Heroin and cocaine induced significant, but opposing effects in medial cortex and bed nucleus of the stria terminalis. Both drugs decreased labeling density in the nucleus accumbens and piriform cortex, and increased density in the substantia nigra, subthalamus, medial septum, claustrum, lateral hypothalamus and hippocampus CA2. These results demonstrate the ability of the OCTO method to discriminate the brief metabolic effects of different drug classes, and suggest that heroin and cocaine may activate a common functional system in the brain.

Kainic acid lesions of the nucleus accumbens selectively attenuate morphine self-administration

The influence of kainic acid lesions of intrinsic and efferent neurons of the central medial nucleus accumbens on responding simultaneously maintained by food, water and morphine self-administration was assessed. Rats were trained on a multioperant baseline to respond on three different levers that resulted in either a food pellet, the presentation of a water dipper or an infusion of morphine. While responding on the morphine lever was related to dose (0.83-13.2 mg/infusion), increasing concentrations of the drug had little or no effect on responding maintained by food and water before the lesion. Bilateral infusions of the neurotoxin into the nucleus accumbens decreased morphine self-administration but did not appreciably alter food or water intake. Food extinction probes before the lesion produced significant increases in drug intake and decreases in responding on the water lever, but the neurotoxin lesion attenuated the food extinction induced decrease in water intake. These data suggested that kainic acid lesions of the nucleus accumbens decrease the reinforcing efficacy of morphine but do not alter the reinforcing properties of food and water. The neuronal systems potentially involved in mediating the reinforcing effects of environmental events are discussed.