Factors affecting voluntary morphine intake in self-maintained addicted rats

Differential Effect of Fixed Ratio Magnitude on the Rate of Lever-Pressing and Interinjection Intervals of Cocaine Self-Administration in Rats

Background

Many features of self-administration behavior may be explained by reference to the properties of schedules of reinforcement. Schedules alter the probability of a behavior being reinforced and thereby increase, or decrease, the frequency of the behavior and fixed ratio (FR) magnitude reportedly alters the rate of responding to cocaine. A pharmacokinetic/pharmacodynamic interaction theory states that lever-pressing behavior is induced only when cocaine levels in the body are above the priming/remission threshold and below the satiety threshold-a range termed the compulsion zone. This theory successfully explains cocaine self-administration in rats on a progressive ratio and the FR1 schedule.

Objectives

To determine the effects of high FR magnitude on the rate of self-administration of cocaine and the rate of lever-pressing behavior when cocaine levels are within the compulsion zone.

Methods

Rats acquired cocaine self-administration on an FR1 schedule and then were switched to sessions that started with FR1 and then FR 5, 10, 20, or 50. An only FR1 session was run each week between FR1/FR50 sessions and then only FR1 sessions were conducted for several weeks.

Results

Interinjection intervals at a unit dose of 3 µmol/kg were regular at both FR1 and FR50 but were longer by the time required to complete the 50 presses. When responding by rats was maintained under an FR50 schedule of cocaine presentations, compared to baseline FR1 sessions, dramatic increases in the number of lever-presses were observed after access to cocaine was terminated, a previously unreported finding. However, lever-pressing occurred only when cocaine levels were in the compulsion zone, and this duration was unchanged. The increase in lever-pressing persisted for weeks. Interinjection intervals at FR1 were not altered after exposure to FR50.

Conclusions

Although previously considered key to understanding the regulation of cocaine self-administration behavior, FR magnitude simply increased interinjection intervals by the time required to complete 50 lever-presses. The dramatic increase in the rate of lever-pressing was caused by the high FR schedule rather than cocaine. The utility of the schedule-induced increase in the rate of lever-pressing is unclear. The compulsion zone theory provides a rational pharmacological basis for understanding cocaine self-administration behavior.

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.

Establishment of multi-stage intravenous self-administration paradigms in mice

Genetically tractable animal models provide needed strategies to resolve the biological basis of drug addiction. Intravenous self-administration (IVSA) is the gold standard for modeling psychostimulant and opioid addiction in animals, but technical limitations have precluded the widespread use of IVSA in mice. Here, we describe IVSA paradigms for mice that capture the multi-stage nature of the disorder and permit predictive modeling. In these paradigms, C57BL/6J mice with long-standing indwelling jugular catheters engaged in cocaine- or remifentanil-associated lever responding that was fixed ratio-dependent, dose-dependent, extinguished by withholding the drug, and reinstated by the presentation of drug-paired cues. The application of multivariate analysis suggested that drug taking in both paradigms was a function of two latent variables we termed incentive motivation and discriminative control. Machine learning revealed that vulnerability to drug seeking and relapse were predicted by a mouse's a priori response to novelty, sensitivity to drug-induced locomotion, and drug-taking behavior. The application of these behavioral and statistical-analysis approaches to genetically-engineered mice will facilitate the identification of neural circuits driving addiction susceptibility and relapse and focused therapeutic development.

Fos-expressing neuronal ensembles in rat infralimbic cortex encode initial and maintained oxycodone seeking in rats

Neuronal ensembles within the infralimbic cortex (IL) and their projections to the nucleus accumbens (NAc) mediate opiate seeking in well-trained rats. However, it is unclear how early this circuitry is recruited during oxycodone self-administration. Here, we used retrograde labelling (CTb) and immunohistochemistry to identify NAc-projecting neurons in the IL that were activated during initial oxycodone seeking. Next, we sought to determine the role of IL neuronal ensembles in initial oxycodone self-administration. We used the Daun02 procedure in male and female Fos-LacZ rats to chemogenetically inactivate IL Fos-expressing neurons at different time points in oxycodone self-administration training: immediately after meeting criteria for acquisition of behaviour and following nine daily sessions with increasing schedules of reinforcement (FR1, FR2 and FR3) in which rats demonstrated stable oxycodone intake under increasing effort to self-administer. We found that Daun02 infusions attenuated oxycodone seeking at both the initial learning and well-trained time points. These results suggest that IL neuronal ensembles are formed during initial learning of oxycodone self-administration and required for the maintenance and expression of oxycodone seeking.

What Have We Learned (or Expect to) From Analysis of Murine Genetic Models Related to Substance Use Disorders?

The tremendous public health problem created by substance use disorders (SUDs) presents a major opportunity for mouse genetics. Inbred mouse strains exhibit substantial and heritable differences in their responses to drugs of abuse (DOA) and in many of the behaviors associated with susceptibility to SUD. Therefore, genetic discoveries emerging from analysis of murine genetic models can provide critically needed insight into the neurobiological effects of DOA, and they can reveal how genetic factors affect susceptibility drug addiction. There are already indications, emerging from our prior analyses of murine genetic models of responses related to SUDs that mouse genetic models of SUD can provide actionable information, which can lead to new approaches for alleviating SUDs. Lastly, we consider the features of murine genetic models that enable causative genetic factors to be successfully identified; and the methodologies that facilitate genetic discovery.

Therapeutic Potential of Histamine H3 Receptors in Substance Use Disorders

Substance use disorders are a leading cause of morbidity and mortality, and available pharmacological treatments are of modest efficacy. Histamine is a biogenic amine with four types of receptors. The histamine H₃ receptor (H₃R) is an autoreceptor and also an heteroreceptor. H₃Rs are highly expressed in the basal ganglia, hippocampus and cortex, and regulate a number of neurotransmitters including acetylcholine, norepinephrine, GABA and dopamine. Its function and localization suggest that the H₃R may be relevant to a number of psychiatric disorders and could represent a potential therapeutic target for substance use disorders. The purpose of the present review is to summarize preclinical studies investigating the effects of H₃R agonists and antagonists on animal models of alcohol, nicotine and psychostimulant use. At present, the effects of H₃R antagonists such as thioperamide, pitolisant or ciproxifan have been investigated in drug-induced locomotion, conditioned place preference, drug self-administration, reinstatement, sensitization and drug discrimination. For alcohol and nicotine, the effects of H₃R ligands on two-bottle choice and memory tasks, respectively, have also been investigated. The results of these studies are inconsistent. For alcohol, H₃R antagonists generally decreased the reward-related properties of ethanol, which suggests that H₃R antagonists may be effective as a treatment option for alcohol use disorder. However, the effects of H₃R antagonists on nicotine and psychostimulant motivation and reward are less clear. H₃R antagonists potentiated the abuse-related properties of nicotine, but only a handful of studies have been conducted. For psychostimulants, evidence is mixed and suggests that more research is needed to establish whether H₃R antagonists are a viable therapeutic option. The fact that different drugs of abuse have different brain targets may explain the differential effects of H₃R ligands.

The Role of Dopamine D3 Receptors in Tobacco Use Disorder: A Synthesis of the Preclinical and Clinical Literature

Tobacco smoking is a significant cause of preventable morbidity and mortality globally. Current pharmacological approaches to treat tobacco use disorder (TUD) are only partly effective and novel approaches are needed. Dopamine has a well-established role in substance use disorders, including TUD, and there has been a long-standing interest in developing agents that target the dopaminergic system to treat substance use disorders. Dopamine has 5 receptor subtypes (DRD1 to DRD5). Given the localization and safety profile of the dopamine receptor D3 (DRD3), it is of therapeutic potential for TUD. In this chapter, the preclinical and clinical literature investigating the role of DRD3 in processes relevant to TUD will be reviewed, including in nicotine reinforcement, drug reinstatement, conditioned stimuli and cue-reactivity, executive function, and withdrawal. Similarities and differences in findings from the animal and human work will be synthesized and findings will be discussed in relation to the therapeutic potential of targeting DRD3 in TUD.

Dramatic increase in lever-pressing activity in rats after training on the progressive ratio schedule of cocaine self-administration

Transition from the highest rate of lever-pressing activity during the unloading (extinction) phase of a cocaine self-administration session to an extremely low activity rate during the remission phase is in many cases gradual. This makes it difficult to assess the duration of the unloading phase after a fixed ratio 1 (FR1) or breakpoint after a progressive-ratio (PR) self-administration session. In addition, 3–5 days of training under the PR schedule results in a dramatic and persistent increase in the rate of presses during PR sessions and in the unloading phase following FR1 self-administration sessions. The goals of this study were to find the definition of the last press demarcating the border between the unloading and remission phases of the session and to determine if this border was also affected by PR training. Rats were trained to self-administer cocaine under the FR1 schedule and then under the PR schedule of drug delivery. Distributions of inter-press intervals (IPIs) during the unloading phase in sessions before and after PR training were compared. It was found that the distribution of cocaine-induced IPIs during the unloading phase was lognormal, bimodal, and independent of previously injected cocaine unit doses. The first mode represented intervals within the short bouts of stereotypic presses and the second mode represented intervals between bouts. The two modes were approximately 0.7 s and 21 s during unloading prior to and 0.6 s and 1.5 s after PR self-administration training. The total number of presses per unloading phase increased eightfold. When the FR1 schedule was restored, the intervals between bouts remained very short for at least 7–10 days and only then started a gradual increase towards baseline levels. The last unloading press was defined as the press followed by the IPI longer than the defined criterion. PR training resulted in a substantial and long-lasting increase in lever-pressing activity during unloading. The duration of the unloading phase did not depend on the rate of lever-pressing activity.

The gold-standard in preclinical abuse liability testing: It's all relative

All new molecular entities (NMEs) with targeted or indirect effects on the central nervous system (CNS) must be evaluated for their abuse liability as a part of their nonclinical development plan. Inherently key in the drug control review is the term "relative abuse liability". The basis for determination of drug control is critically dependent on the nonclinical assessment of the reinforcing attributes of the NME in animals (rat is the regulatory preferred species) in a standard operant conditioning paradigm. Pharmaceutical representatives without a background in behavioral analysis or operant conditioning models must weigh through conceptually-intriguing language and constructs that accurately convey and communicate the relative potential for abuse to drug regulatory experts in the field. Effective statutory language in the preclinical assessment of relative abuse liabilities for schedule control status reviews must be 1) specific; 2) concise; 3) familiar to the regulators; 4) unambiguous; 5) constructive; and 6) formalized with respect to both international and national drug control policies. In this review we attempt to define and highlight the importance of the statutory language used to report self-administration study results to both parties engaged in NDA approval process.

Preclinical Determinants of Drug Choice under Concurrent Schedules of Drug Self-Administration

Drug self-administration procedures have played a critical role in the experimental analysis of psychoactive compounds, such as cocaine, for over 50 years. While there are numerous permutations of this procedure, this paper will specifically focus on choice procedures using concurrent schedules of intravenous drug self-administration. The aims of this paper are to first highlight the evolution of drug choice procedures and then review the subsequent preclinical body of literature utilizing these choice procedures to understand the environmental, pharmacological, and biological determinants of the reinforcing stimulus effects of drugs. A main rationale for this paper is our proposition that choice schedules are underutilized in investigating the reinforcing effects of drugs in assays of drug self-administration. Moreover, we will conclude with potential future directions and unexplored scientific space for the use of drug choice procedures.

Heroin self-administration and reinstatement of heroin-seeking in adolescent vs. adult male rats

RATIONALE

Heroin abuse is prevalent among teenagers, and early onset drug use might predict long-term drug dependence. However, adolescent sensitivity to drug reinforcement has not been explored thoroughly in animal models.

OBJECTIVES

This study aimed to compare intravenous (i.v.) self-administration of heroin, as well as extinction and reinstatement of heroin-seeking, in adolescent vs. adult male rats.

METHODS

Adolescent (35 days old at start) and adult (86 days old at start) male Sprague-Dawley rats spontaneously acquired lever pressing maintained by i.v. heroin infusions. In experiment 1, self-administration was tested on a fixed ratio 1 schedule of reinforcement (0.05 and 0.025 mg/kg per infusion), followed by within-session extinction and reinstatement tests after 1 or 12 days of abstinence. In experiment 2, self-administration was tested on a progressive ratio schedule (0.0125-0.1 mg/kg per infusion), followed 12 days later by a single test of extinction responding in the presence of cues.

RESULTS

In experiment 1, adolescent rats self-administered more heroin than adults. After 1 or 12 days of abstinence, adolescents exhibited less heroin-seeking than adults, although levels of heroin-seeking increased over abstinence period for both age groups. In experiment 2, adolescents and adults reached the same maximal response ratio (breakpoint), although adolescents earned more infusions when response requirements were low. For extinction responding in the presence of cues, heroin-seeking was similar across ages.

CONCLUSIONS

Lower levels of heroin-seeking suggest that younger rats are less sensitive than adults to some residual effects of heroin intake.

Cocaine self-administration in rats: discrete trials procedures

A discrete trials procedure involves splitting up a self-administration session so that there are multiple distinct trials and inter-trial-intervals. This schedule is well suited to be used over 24 h periods which allows insight into diurnal variability in self-administration behavior. DT is also well suited for investigations using pretreatments for increasing or decreasing both high and low probability behavior.

Morphine deprivation increases self-administration of the fast- and short-acting mu-opioid receptor agonist remifentanil in the rat

Opiate dependence and withdrawal have long been hypothesized to enhance the reinforcing effects of opiates; however, opiate agonist self-administration in these states has yet to be systematically assessed. To address this issue, the reinforcing property of the short-acting mu-opioid agonist, remifentanil, was assessed in morphine-dependent (MD), morphine-dependent and -withdrawn (MW), and nondependent, control (C) rats. Dependence was established by twice daily administration of increasing doses of morphine for 4 days (10, 20, 30, and 40 mg/kg s.c.) and then maintained with a daily injection of the large dose. Morphine deprivation-induced withdrawal (defined by weight loss and hyperalgesia) was apparent 24, but not 12, h after morphine treatment. Remifentanil self-administration (0.4, 0.8, 1.6, 3.2, or 6.4 mug/kg/infusion) was assessed over 20 successive, daily, 1-h sessions, either 12 or 24 h after the maintenance dose of morphine. Compared with the control group, the MD group demonstrated suppressed remifentanil self-administration, whereas the MW group exhibited enhanced responding for every dose of remifentanil. The increased responding observed in the MW group compared with the control and MD groups resulted in an upward shift in the remifentanil dose-response curve, an effect that was expressed only after repeated exposure to the contingency, demonstrating that morphine withdrawal ultimately enhances the reinforcing effects of remifentanil.

Self-administration of drugs in animals and humans as a model and an investigative tool

AIM

To review briefly the methods, assumptions, models, accomplishments, drawbacks and future directions of research using drug self-administration in animals and humans.

BACKGROUND

The use of drug self-administration to study addiction is based on the assumption that drugs reinforce the behavior that results in their delivery. A wide range of drug self-administration techniques have been developed to model specific aspects of addiction. These techniques are highly amenable to being combined with a wide variety of neuroscience techniques.

CONCLUSIONS

The identification of drug use as behavior that is reinforced by drugs has contributed greatly to the understanding and treatment of addiction. As part of a program of pre-clinical research that also involves screening with a variety of simpler behavioral techniques, drug self-administration procedures can provide an important last step in testing potential treatments for addiction. There is currently a concerted effort to develop self-administration procedures that model the extreme nature of the behavior engendered by addiction. As advances continue to be made in neuroscience techniques, self-administration should continue to provide a means of applying these techniques within a sophisticated and valid model of human drug addiction.

Critical assessment of how to study addiction and its treatment: human and non-human animal models

Laboratory models, both animal and human, have made enormous contributions to our understanding of addiction. For addictive disorders, animal models have the great advantage of possessing both face validity and a significant degree of predictive validity, already demonstrated. Another important advantage to this field is the ability of reciprocal interplay between preclinical and clinical experiments. These models have made important contributions to the development of medications to treat addictive disorders and will likely result in even more advances in the future. Human laboratory models have gone beyond data obtained from patient histories and enabled investigators to make direct observations of human drug self-administration and test the effects of putative medications on this behavior. This review examines in detail some animal and human models that have led not only to important theories of addiction mechanisms but also to medications shown to be effective in the clinic.

An historical note on Darwin and nonhuman drug self-administration

This note brings to the attention of readers a quote from Charles Darwin on the scientific implications of nonhuman drug self-administration. The quote is from The Descent of Man; and Selection in Relation to Sex (2nd ed.; C. Darwin, 1874/1998). Consistent with Darwin's prescience in many areas of science, he discerned potential scientific importance in voluntary nonhuman drug self-administration almost a century before that potential was realized in any substantive or systematic manner.

Antagonism of heroin and morphine self-administration in rats by the morphine-6beta-glucuronide antagonist 3-O-methylnaltrexone

In mice, 3-O-methylnaltrexone blocks the analgesic actions of morphine-6beta-glucuronide and heroin at doses which are inactive against morphine. We found a similar selectivity in rats. 3-O-Methylnaltrexone antagonized the analgesic actions of 6-acetylmorphine in Sprague-Dawley rats and heroin in Wistar rats at doses that were inactive against morphine. Inclusion of a fixed dose of 3-O-methylnaltrexone significantly shifted the analgesic dose-response curves for 6-acetylmorphine and heroin without altering the morphine dose-response curves. In a self-administration model, 3-O-methylnaltrexone treatment significantly increased both heroin and morphine intake during the first hour, suggestive of an antagonist effect. This effect at doses of 3-O-methylnaltrexone which were inactive against morphine analgesia implied a role for the morphine-6beta-glucuronide opioid receptor in the reinforcing properties of heroin and morphine.

Dopaminergic involvement in the process of reinforcement from diethyl ether vapor in rats

1. Male and female Wistar rats were placed in boxes for 30 min daily and allowed to nose-poke in two holes on opposite walls, which opened into evaporation chambers through valves. One chamber contained diethyl ether vapor another-air, the contents being alternated randomly. 2. Rats inhaled the contents of evaporation chambers while nose-poking differed by the level of intake of diethyl ether vapor. Rats with the intake time of more than 3 sec formed about 15% of population. Their preference for diethyl ether was above 0.55. 3. There was significant negative correlation between the time of vapor inhalation and the time of immobility in forced swimming test in females but not in males. 4. Withdrawal of diethyl ether vapor decreased the inhalation time. On the first day after ether deprivation inhalation time rose above average level. Relationship between concentration of ether vapor and the time of its inhalation was inverted U-shaped function. Substitution of acetone vapor elevated the time of the vapor inhalation. 5. D-amphetamine in dose 0.05 mg/kg elevated the time of inhalation of and preference for ether vapor in some rats. In doses 0.05-1.0 mg/kg amphetamine selectively suppressed the time of vapor inhalation. 6. Haloperidol in doses 0.05 and 0.1 mg/kg elevated the time of vapor inhalation on the first day in females and suppressed it in doses 0.05-0.3 mg/kg dose-dependently on the second day. Atypical neuroleptic cis-carbidine elevated the time of vapor inhalation in doses 2.5 and 5.0 mg/kg and suppressed it at 10 mg/kg. 7. Diethyl ether vapor can be established as reinforcer in rats. Female rats are more liable to reinforcement from ether vapor than males and show more pronounced response to haloperidol. This may be related to its more active behavior in the forced swimming situation. 8. The results point to potential involvement of dopamine system in the process of reinforcement from ether vapor.

Inhibitory effects of velvet antler water extract on morphine-induced conditioned place preference and DA receptor supersensitivity in mice

The repeated administration of morphine in mice produced conditioned place preference (CPP) and postsynaptic dopamine (DA) receptor supersensitivity. CPP effects were evaluated assessing the increased time spent by the mice to morphine and the inhibition of CPP by the decreased time spent by the mice in the nonpreferred compartment. Postsynaptic DA receptor supersensitivity in mice displaying a morphine-induced CPP was evidenced by the enhanced response in ambulatory activity to the DA agonist. The oral administration of velvet antler water extract (VAWE) from Cerrus elaphus, prior to the morphine treatment in mice inhibited the development of morphine-induced CPP and postsynaptic DA receptor supersensitivity. These results suggest that the inhibitory effects of VAWE on morphine-induced CPP may be closely related to the recovery of dysfunction in the dopaminergic system produced by morphine because both phenomena were inhibited by VAWE. But a single administration of VAWE did not inhibit apomorphine-induced climbing behavior, indicating that antidopaminergic activity was not produced by a single administration of VAWE at the postsynaptic DA receptors. From the above results, it is presumed that VAWE may be useful for prevention and therapy of the adverse actions of morphine.

Reinforcement processes in opiate addiction: a homeostatic model

The development of tolerance and dependence has traditionally been considered an integral aspect of the drug addiction process, and opiate dependence has been studied extensively as a model system in this regard. However, recent emphasis on the positive reinforcing properties of drugs has led to the suggestion that tolerance, dependence, and withdrawal may be of secondary or even negligible importance in motivating compulsive drug use. The current article argues for an integrated view of addiction in the form of a homeostatic neuroadaptation model which emphasizes the motivational significance of both the positive affective state produced by opiates and the negative affective state characteristic of drug withdrawal. The model is supported by evidence at both the behavioral and neural systems levels of analysis. Understanding the important distinction between somatic and affective components of opiate withdrawal is key to recognizing the factors which contribute to the motivational significance of opiate dependence and withdrawal. In addition, the critical role of conditioning processes in the maintenance of compulsive drug use and relapse after periods of abstention is discussed. Finally, it is argued that both the positive reinforcement produced by acute administration of a drug and the negative affective state produced by withdrawal are common to multiple classes of abused drugs, suggesting that an understanding of homeostatic neuroadaptation within motivational systems provides a key to the etiology, treatment and prevention of drug addiction.

Inhibition by MK-801 of morphine-induced conditioned place preference and postsynaptic dopamine receptor supersensitivity in mice

Intraperitoncel injection of morphine (5 mg/kg) in mice every other day for 8 days produced conditioned place preference (CPP). CPP effects were evaluated by assessing the difference in time spent in the drug-paired compartment and the saline-paired compartment of the place conditioning apparatus. The injection of a noncompetitive NMDA antagonist, MK-801 (0.05 and 0.1 mg/kg. IP), prior to and during morphine treatment in mice inhibited morphine-induced CPP. The development of postsynaptic dopamine (DA) receptor supersensitivity in mice displaying a morphine-induced CPP was evidenced by the enhanced response in ambulatory activity to the DA agonist, apomorphine (2 mg/kg). MK-801 inhibited that development of postsynaptic DA receptor supersensitivity. MK-801 also inhibited apomorphine-induced climbing behavior, suggesting that MK-801 inhibits dopaminergic activation mediated via the NMDA receptor. These results suggest that the development of morphine-induced CPP may be associated with the development of postsynaptic DA receptor supersensitivity. The development of morphine-induced CPP and DA receptor supersensitivity may be closely related to NMDA receptor-mediated dopaminergic activity, because morphine-induced changes in sensitivity to apomorphine, as well as apomorphine-induced climbing behavior in morphine treated mice, were both blocked by MK-801.

Modelling drug kinetics with brain stimulation: dopamine antagonists increase self-stimulation

The rewarding effects of brain stimulation and drugs are believed to depend on a common neural system. However, the pattern of responding produced by drug reinforcers is different from the pattern produced by conventional brain stimulation. Furthermore, pharmacological antagonists of reinforcement increase the rate of drug self-administration but depress self-stimulation. To test the hypothesis that the differences in the characteristics of brain stimulation and drugs as reinforcers are due to differences in the kinetics of drugs and brain stimulation, we modelled drug kinetics with frequency-modulated trains of brain stimulation. We report that animals will self-administer such brain stimulation in a manner that resembles drug self-administration and that, under these conditions, dopamine antagonists can increase the rate of self-stimulation.

Effect of dose on cocaine self-administration behavior and dopamine levels in the nucleus accumbens

The reinforcing properties of cocaine are thought to be primarily mediated by the release of dopamine (DA) in the nucleus accumbens (N ACC). The extracellular concentration of DA in the N ACC was monitored with in vivo microdialysis procedures during ongoing cocaine self-administration to achieve a more detailed understanding of how DA mediates the reinforcing effects of cocaine. A dose-dependent decrease in lever pressing behavior occurred as the dose of cocaine was increased. The mean number of lever presses (in 20 min intervals) for 0.25, 0.50 and 0.75 mg/infusion doses was 5.6 +/- 0.7, 3.3 +/- 0.3 and 2.4 +/- 0.3, respectively. However, a simple inverse relationship did not occur between lever pressing behavior and the total amount of cocaine injected. Lever pressing behavior significantly increased cocaine intake as the dose of cocaine was increased. The total amount of cocaine intake that occurred during the 3 h self-administration period of the 0.25, 0.50 and 0.75 mg/infusion doses was 12.0 +/- 1.8 mg, 14.6 +/- 0.37 mg and 16.6 +/- 1.2 mg. Correspondingly, the extracellular concentration of DA in the N ACC was increased and maintained at significantly higher levels as the dose of cocaine was increased. The average concentration of DA that occurred during the self-administration of 0.25, 0.50 and 0.75 mg/infusion doses of cocaine was 269 +/- 26%, 381 +/- 21% and 464 +/- 49% of the basal DA concentration. As dose is increased, a corresponding increase occurs in both cocaine intake and in the extracellular concentration of DA in the N ACC.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological mechanisms in posttraumatic stress disorder. Relevance for substance abuse

Recent studies suggest a significant biological contribution to posttraumatic stress disorder (PTSD). In particular, central catecholamine and endogenous opioid systems have been implicated both in this syndrome and in substance abuse. We review relevant animal and human studies that support these hypotheses and suggest that this overlap may contribute to the incidence of substance abuse in PTSD. The animal studies have primarily employed the learned helpless and conditioned emotional response models and have included assessments of brain catecholamines, locus ceruleus activity, and behavioral correlates in rodents and nonhuman primates. Human studies have used only indirect measures to assess these variables. However, both therapeutic approaches and attempts at self-medication for PTSD have supported this hypothesis.

Differential involvement of central cholinergic mechanisms in the aversive stimulus properties of morphine and amphetamine

Previously, it was reported that pretreatment with the centrally-acting cholinergic antagonist atropine, but not the peripherally-acting antagonist, methyl-atropine, may serve to attenuate the positive reinforcing properties of morphine and conversely, to enhance those of amphetamine as evidenced within a drug self-administration paradigm in rats. In parallel, evidence from several sources would suggest that there may be a functional relationship between the neurochemical mechanisms mediating these drugs' positive reinforcing properties and their seemingly paradoxical capacity to act as aversive stimuli, as evidenced within a conditioned taste aversion (CTA) paradigm. Accordingly, the present study undertook to examine whether a similar differential involvement of central cholinergic mechanisms established for these drugs' positive reinforcing effects may be obtained for morphine and amphetamine-induced CTA. Using a conventional CTA paradigm, animals were pretreated with either intraperitoneal (IP) atropine or methyl-atropine (0.6 mg/kg) 40 minutes prior to consuming a novel 0.1% saccharin solution. This taste stimulus was paired with IP injection of 15 mg/kg morphine or vehicle. Results showed that atropine (but not methyl-atropine) pretreatment served to attenuate the morphine CTA. In a second experiment, atropine-pretreatment failed to attenuate, and may have slightly potentiated, a CTA induced by 1 mg/kg amphetamine. Atropine pretreatment did not affect a CTA induced by the emetic agent, lithium chloride. Pretreatment with the peripherally-acting methyl-atropine had no effect on the amphetamine CTA and served, if anything, to slightly attenuate the lithium chloride CTA.(ABSTRACT TRUNCATED AT 250 WORDS)

Conditioned taste aversion induced by self-administered drugs: paradox revisited

In this paper we have reviewed the literature on Conditioned Taste Aversion (CTA) with specific attention to the "apparent paradox" in this literature. This paradox refers to the fact that drugs which are self-administered (SA) by animals and are therefore presumed to possess positive reinforcing properties are also endowed with the capacity to induce a CTA. We have argued that the CTA literature contains evidence of the existence of two qualitatively distinct types of CTA, one which is mediated by emetic agents and the other induced by SA drugs. We first provided evidence to support the notion that the traditional explanation of CTA as a function of "drug toxicity" and its resultant gastrointestinal distress does not fit the data on the nature of CTA induced by SA drugs. We proposed instead that "drug shyness" or the novelty of the drug state of these psychoactive SA drugs constitutes a better explanation of the CTA of SA drugs. We provided further evidence suggesting a functional relationship between the positive reinforcing and aversive properties of SA drugs. We have based this contention on a review of the behavioral, physiological and neurochemical data concerning the nature of CTA of SA drugs. The examination of these data reveals that the neural mechanisms underlying both the positive and aversive properties of SA drugs are the same and at the same time different from the neural mechanisms underlying the induction of CTA by emetic agents. Finally, we discussed the relevance of this interaction between the positive and aversive properties of SA drugs in the context of their abuse liability and the control they exert on drug-oriented behavior.

Further evaluation of morphine aversion: maintenance of a taste aversion using a low, nonaversive morphine dose

Previously, in an investigation of morphine-conditioned taste aversion (CTA), we found that limited preexposure to a low, nonaversive (non-CTA-inducing) dose of morphine (2.5 mg/kg) was as effective as preexposure to a higher, CTA-inducing dose (15 mg/kg) in blocking the formation of a subsequent morphine CTA. In the present study, we examined the capacity of this low, 2.5-mg/kg morphine dose to maintain a CTA initially induced by the 15-mg/kg dose. A standard CTA procedure was used. Results indicated that rats given three initial taste-drug pairings with 15 mg/kg morphine followed on subsequent pairing days by treatment with the low, non-CTA-inducing, 2.5-mg/kg dose continued to exhibit a strong CTA over 8 pairing days. A similar pattern was observed for animals continuing to receive taste-drug pairings with the 15-mg/kg dose. Animals receiving only one taste-drug pairing with the 15-mg/kg dose, followed on subsequent pairing days by 2.5-mg/kg conditioning, failed to show such a pattern of CTA. An intermediate CTA pattern was seen with animals conditioned with 15, 10, 5, and repeated 2.5-mg/kg doses over consecutive pairing days. These data suggest that exposure to a low dose of morphine, with no apparent CTA-inducing properties, is sufficient to maintain a previously established morphine taste aversion. Potential implications for understanding the apparent discriminative complexity of morphine's motivational properties are discussed.

Aversive stimulus properties of morphine: evaluation using the drug preexposure conditioned taste aversion paradigm

Interpretation of the finding that positive-reinforcing drugs such as morphine also possess possible aversive properties, as revealed by their ability to induce a conditioned taste aversion (CTA), remains problematic. This issue was addressed in the present study using the drug preexposure CTA paradigm. Water-deprived rats were given noncontingent preexposure to one of three doses of morphine (2.5, 5.0, or 15.0 mg/kg) or drug vehicle. Subsequently, animals in each of these preexposure groups were presented with a novel 0.1% saccharin-flavored solution followed immediately by injection with one of the same three morphine doses or drug vehicle. This procedure was repeated at 5-day intervals until six saccharin presentations had been performed. Results indicated that while the three morphine doses were differentially potent as taste aversion-conditioning agents, they were equipotent as preexposure agents serving to disrupt CTA. These data suggest that preexposure to morphine's predominantly positive-reinforcing (and non-CTA-inducing) properties is sufficient for preexposure disruption of subsequent morphine CTA. A second experiment indicated that the minimal effective preexposure dose is between 0.3 and 1.25 mg/kg of morphine. It is suggested that an important commonality may exist between the discriminative stimulus properties of morphine as a CTA-inducing agent and as a positive reinforcer.

Involvement of dopamine in the aversive stimulus properties of cocaine in rats

Previous studies of cocaine self-administration have demonstrated central dopaminergic involvement in cocaine's positive reinforcing properties. The present study reports the ability of pimozide, a dopamine receptor antagonist, to attenuate a conditioned taste aversion induced by repeated injections of cocaine. Rats placed on a daily water deprivation schedule were subsequently presented with a novel saccharin taste in their drinking fluid immediately followed by administration of four 9 mg/kg injections of cocaine spaced at 20 min intervals. These animals exhibited a reduction in saccharin intake on subsequent presentations. Animals pretreated with pimozide 90 min prior to the saccharin-cocaine pairings failed to show this reduction. In a second experiment using an identical procedure, repeated injections of lithium chloride were shown to induce a CTA both in pimozide-pretreated and control animals. The results of these two experiments are consistent with the notion that a functional relationship may exist between neurochemical mechanisms underlying both the aversive (CTA-inducing) and positive reinforcing properties of self-administered drugs such as cocaine.

The potencies and selectivities of four calcium antagonists as inhibitors of uterine contractions in the rat in vivo

The potencies of four calcium antagonists (nifedipine, gallopamil, verapamil and diltiazem) at inhibiting uterine contractions in vivo have been assessed in the conscious ovariectomized, post-partum rat. Their selectivities for this action, relative to their effects on blood pressure and heart rate, have been compared with salbutamol. All compounds produced a dose-dependent inhibition of intra-uterine pressure cycles. The rank order of potency was salbutamol greater than nifedipine greater than diltiazem = gallopamil greater than verapamil. All compounds produced a dose-dependent fall of mean blood pressure. The rank order of potency was salbutamol greater than nifedipine greater than gallopamil greater than verapamil greater than diltiazem. Salbutamol and nifedipine produced a tachycardia, which was very marked with salbutamol. Gallopamil, verapamil and diltiazem induced a moderate tachycardia at low doses but temporary cessation of heart beat occurred at high doses. Nifedipine and diltiazem, like salbutamol, exhibited some selectivity for inhibition of uterine contractions relative to their cardiovascular actions. Gallopamil and verapamil showed no selectivity for the uterus.

Failure to establish a conditioned place preference with ethanol in rats

Previous studies have demonstrated that many drugs of abuse are able to produce a conditioned place preference in rats. We sought to determine if ethanol, injected in a wide range of doses, could also produce a conditioned place preference. Statistical analysis of our results indicated that the IP administration of the drug (50, 100, 150, 300, 600, 800, or 1000 mg/kg) failed to produce either a conditioned place preference or aversion compared to vehicle injected control rats. Under similar testing conditions a conditioned place preference was obtained with amphetamine (2 mg/kg) and this preference was not secondary to conditioned hyperactivity. In another experiment, rats were injected with ethanol through indwelling jugular cannulae at doses similar to those reported [24,26] to support (1, 2 mg/kg) or not to support (8 mg/kg) self-administration by rats. We also failed to obtain a conditioned place preference using these doses. Blood and brain ethanol levels, determined 1, 2 or 5 minutes after the administration of 2 mg/kg (IV) indicated very low ethanol levels. These results may suggest that rats do not self-administer ethanol for its intoxicating properties, and that the affective state produced by ethanol administration per se is not readily conditionable to environmental cues.

Limbic acetylcholine turnover rates correlated with rat morphine-seeking behaviors

Acetylcholine (ACh) turnover rates were measured in fourteen brain regions of rats intravenously self-administering morphine and in yoked-morphine and yoked-vehicle infused littermates to identify cholinergic neuronal pathways potentially involved in opiate reinforcement processes. Rats receiving chronic passive administration of morphine had increased ACh turnover rates in the frontal cortex and diagonal band and decreased rates in the medial septum. The significant changes in animals self-administering the drug were prominent in limbic regions with increases in the frontal cortex and decreases in the pyriform cortex, nucleus accumbens, amygdala and ventral tegmental area. Some components of opiate reinforcement may be mediated by increases in the activity of cholinergic ventral pallidal and diagonal band fibers innervating the frontal cortex and by decreases in activity of cholinergic fibers innervating the ventral tegmental area. These data and turnover rates for dopamine, norepinephrine, serotonin, aspartate, glutamate and gamma-aminobutyric acid previously determined in similarly treated animals are consistent with two neuronal circuits that may be involved in opiate seeking behaviors and opiate reinforcement processes.

Comparison of two intravenous infusion schedules for inducing physical dependence upon ethanol in rats

Male hooded rats were implanted with intravenous (IV) cannulas and housed in operant chambers. The effectiveness of two infusion schedules for producing physical dependence upon ethanol was assessed. Twenty-three rats (12 ethanol, 11 control) were tested under a 4 hr interinfusion schedule (one infusion every 4 hr, around the clock), and 29 rats (15 ethanol, 14 control) were tested under a 2 hr interinfusion schedule. During the dependence induction phase, which lasted for 6 days, the experimental rats received ethanol (30% v/v) at an average daily dose which ranged from 8.4-11.8 g/kg, and the dose administered per infusion was adjusted according to the degree of intoxication of each animal. The pair-fed control subjects received infusions of isocaloric control solutions (either 29% v/v propylene glycol or 31% v/v glycerol). Following the dependence induction phase, ethanol was withdrawn, and withdrawal symptoms were assessed. Blood ethanol levels (BEL) and signs of intoxication were determined through all phases of the experiment. During the dependence induction phase, mortality was close to zero, and weight loss was held to about 10%. Tolerance to ethanol developed in all experimental rats. During the withdrawal period, all ethanol rats developed clear withdrawal symptoms, while control subjects did not. Ethanol elimination rates ranged between 45-50 mg/dl/hr and withdrawal symptoms began when BEL fell below 200 mg/dl, and were severe when BEL approached zero. The 2 hr schedule proved superior to the 4 hr schedule in that it led to greater stability of BEL during dependence induction, and a tendency for the withdrawal reaction to be more severe.(ABSTRACT TRUNCATED AT 250 WORDS)

Conditioned place preference: an evaluation of morphine's positive reinforcing properties

The place-preference paradigm was evaluated as a measure of morphine's positive reinforcing properties. Previous place-preference studies obtained a morphine place preference of 26%-63%. In order to examine whether differences in procedure may account for this scatter, the present experiment investigated whether there is any difference in the absolute magnitude of preference when animals are conditioned on their non-preferred side of the box or when animals are randomly assigned to the side of conditioning. Furthermore, the number of conditioning days was extended with 3 intervening test days, and drug doses were doubled following each test day. The results showed no significant difference between conditioning animals on their non-preferred side or randomly assigning them to the side of conditioning. However, by extending the number of conditioning days, as well as by following the drug regimen used, the animals showed a greater magnitude of preference than that observed in previous studies. The implications of these findings for the usage of this paradigm as a measure of morphine's positive reinforcing properties are discussed.

Reinforcer interactions under concurrent schedules of food, water, and intravenous morphine

Responding by six rats was maintained under a concurrent chained fixed-ratio 1, fixed-ratio 9 schedule (conc chain FR1 FR9 ) of food, water, and morphine presentations. The subjects had continuous access to the schedule contingencies on a reversed 12-h light-dark cycle. Local rates and temporal patterns were very similar for responding maintained by the three reinforcers with food and water intake occurring predominantly during the dark cycle, while morphine infusions were evenly distributed. Food and water extinction (24-h duration) decreased the number of ratios completed on both the food and water levers. Moreover, food extinction resulted in a large increase in I.V. morphine self-administration. Morphine extinction increased responding on the morphine lever while almost eliminating responding on the water lever. Changes in the dose of morphine (2.5-40 mg/kg/injection) did not significantly affect food and water intake, but were inversely related to responding on the morphine lever. Saline substitutions resulted in effects similar to those observed during morphine extinction. The schedule used in this study provides a method for examining the specificity of a number of pharmacological and neurochemical manipulations.

Cortical evoked potential changes during classical conditioning of morphine dependence in rats

A classical conditioning experiment was designed to determine if a conditioned neural response would develop and persist in cortical evoked potentials elicited by a foreleg stimulus (CS+) that was paired with morphine administration during the development of dependence and subsequent withdrawal. A stimulus to the other foreleg (CS-) was presented explicitly unpaired with morphine delivery. After dependence was established, the rats were taken from the experimental chamber and withdrawn from morphine for 6 days in their home cages. Finally, during the testing phase, the animals were returned to the experimental chamber and the foreleg stimuli were presented. The CS+ was paired with either morphine or saline injections. Changes due to both morphine effects only and conditioning were observed. The conditioned response, however, was present only in the cortical evoked potentials recorded from those animals receiving contralateral foreleg stimulation as the CS+. The conditioned neural response persisted after withdrawal and was present in both the drug-free and morphine-intoxicated animals. These results provide support for the relapse theory that a nonextinguished conditioned response is retained after withdrawal. However, further experiments are necessary to determine if these conditioned responses can elicit drug-seeking behavior.

Acute responses during blood substitution in the conscious rat

Changes in the cellular components of blood in response to exchange transfusion with the fluorocarbon emulsion blood substitute Fluosol-DA 20% have been examined in conscious chronically catheterized rats. Continuous isovolemic exchange transfusion at 1 ml X min-1 for 40 min reduced the mean haematocrit from 36% to less than 2%. The animals behaved normally during blood replacement and no significant changes in mean arterial blood pressure, heart rate or respiration rate were observed. An exponential decline in circulating red and total white cell count was observed during exchange transfusion. However, the rates of decline were significantly different for red and white cells as was reflected in the half-times (t1/2) for the decrease in cell numbers (red blood cells: 7.5 min; white blood cells: 27.5 min) and the fractional turnover rates (k). An exponential decline in total plasma protein and plasma albumin concentrations occurred in response to blood substitution with highly significant inverse log-linear relationships with time and with closely similar half-times (t1/2) for the decrease in concentration (total protein: 10.5 min; albumin: 11.3 min) and the fractional turnover rates (k). If a single well-mixed compartment is assumed for each type of blood cell and plasma proteins, the differences in turnover rates can be attributed to the volumes of these compartments (red blood cells: 10.9 ml.; white blood cells: 40.0 ml.; total protein: 15.2 ml.; albumin: 16.4 ml.). Possible anatomical boundaries for these compartments are considered. Plasma sodium concentration showed a significant decrease in response to blood substitution; no corresponding changes in either plasma potassium or total plasma calcium concentrations were observed. A progressive rise in plasma glucose concentration occurred during blood substitution, with levels at the end of the experiment approximately 2-fold higher than the mean initial value. There was also a small but not statistically significant decrease in plasma urea concentration during the procedure.

Effect of early and later colony housing on oral ingestion of morphine in rats

Male and female rats were raised from weaning either in isolation or in a large colony. At 65 days of age, half the rats in each environment were moved to the other. At 80 days, the animals were given continuous access to water and to a sequence of 7 solutions: 3 sweet or bitter-sweet control solutions and 4 different concentrations of morphine hydrochloride (MHCl) in 10% sucrose solution. Rats housed in the colony at the time of testing drank less MHCl solution than isolated rats, but no less of the control solutions. Colony-dwelling rats previously housed in isolation tended to drink more MHCl solution than those housed in the colony since weaning, but this effect reached statistical significance only at the lowest concentration of MHCl. These data were related to the hypothesis that colony rats avoid morphine because it interferes with complex, species-specific behavior.

An analysis of drug-seeking behavior in animals

This article reviews the literature on the behavioral aspects of opiate dependence. Available data on the variables affecting drug-maintained behavior are presented. These variables are: (a) reinforcement variables, including parameters such as delay, magnitude, rate and duration of the reinforcing stimulus; (b) antecedent conditions, such as deprivation and satiation; (c) organismic variables, such as genotype, species, sex and age; (d) current environmental contingencies, such as the schedule of reinforcement in effect, or extinction; and (e) such experiential variables as pharmacological and behavioral history of the organism. The review ends with a discussion of the implications of these variables for the treatment of drug abuse.