Nalorphine's ability to substitute for morphine in a drug discrimination procedure is a function of training dose

Drug discrimination learning: Interoceptive stimulus control of behavior and its implications for regulated and dysregulated drug intake

Drug discrimination research has generated rich evidence for the capacity of interoceptive drug stimuli to control behavior by serving as discriminative cues. Owing to its neuropharmacological specificity, drug discrimination learning has been widely used to characterize the stimulus effects and neuropharmacological underpinning of drugs. Apart from such utility, discriminative drug stimuli may help regulate drug use by disambiguating conditioned associations and post-intake outcomes. First, this review summarizes the evidence supporting interoceptive regulation of drug intake from the literature of exteroceptive discriminative control of drug-related behavior, effects of drug priming, and self-titration of drug intake. Second, an overview of interoceptive control of reward-seeking and the animal model of discriminated goal-tracking is provided to illustrate interoceptive stimulus control of the initiation and patterning of drug intake. Third, we highlight the importance of interoceptive control of aversion-avoidance in the termination of drug-use episodes and describe the animal model of discriminated taste avoidance that supports such a position. In bridging these discriminative functions of drug stimuli, we propose that interoceptive drug stimuli help regulate intake by disambiguating whether intake will be rewarding, nonrewarding, or aversive. The reflection and discussion on current theoretical formulations of interoceptive control of drug intake may further scientific advances to improve animal models to study the mechanisms by which interoceptive stimuli regulate drug intake, as well as how alterations of interoceptive processes may contribute to the transition to dysregulated drug use.

Serial feature positive and feature negative discrimination learning in a taste avoidance preparation: implications for interoceptive control of behavior

BACKGROUND

Psychoactive drugs produce interoceptive stimuli that can guide appropriate behaviors by initiating or inhibiting responding.

OBJECTIVE

The current study investigated whether an interoceptive morphine state produces similar patterns of serial feature positive (FP) and feature negative (FN) discrimination learning under comparable conditions in a taste avoidance design.

METHODS

Male Sprague-Dawley rats were trained under 10 cycles of FP or FN discrimination. In the FP task, morphine (10 mg/kg, IP) signaled that a saccharin solution was followed by LiCl (1.2 mEq, IP), while the vehicle (saline) signaled that the LiCl was withheld. In the FN task, the contingency was reversed.

RESULTS

The FP-trained rats acquired the discrimination after three training cycles, consuming significantly less saccharin on morphine, than on vehicle, sessions ( P  < 0.05). The FN-trained rats acquired the discrimination after six training cycles, consuming more on morphine than on vehicle sessions ( P < 0.05). However, FN-trained rats never recovered saccharin consumption to baseline levels and 40% of the rats continued to avoid saccharin (consuming 0 ml) on morphine sessions. Control rats that never received LiCl consumed high levels of saccharin on morphine and vehicle sessions, indicating that morphine did not produce unconditioned suppression of saccharin consumption.

CONCLUSION

The difficulty to acquire FN discrimination might reflect the limitations of learning about safety contingencies in the taste avoidance design. The rapidity of FP learning when a drug state signals an aversive contingency may have implications for the general role of interoceptive stimuli in the control of behavior.

Modulation of morphine physical dependence and discriminative stimulus effects by ovarian hormones: Role of estradiol

Ovarian hormones influence the activity of endogenous opioids, and exogenous administration of estradiol reduces opioid intake and opioid seeking in animal models of opioid reward and reinforcement. The purpose of this study was to examine the effects of ovarian hormones on the discriminative stimulus effects of morphine and naloxone-precipitated opioid withdrawal. To this end, separate groups of ovariectomized female rats were trained to discriminate the stimulus effects of either 3.0 or 10 mg/kg morphine, and substitution tests were conducted with estradiol or progesterone alone and in combination with morphine. At the conclusion of discrimination testing, rats were treated chronically with estradiol, progesterone, or their combination, and challenged with naloxone to measure opioid-like withdrawal symptoms. Finally, the effects of estradiol, progesterone, and their combination were examined on naloxone-precipitated withdrawal in morphine-dependent rats. Neither estradiol nor progesterone substituted for the morphine discriminative stimulus, but estradiol significantly increased the potency of morphine in rats trained to discriminate 10 mg/kg but not 3 mg/kg morphine. When administered chronically, neither hormone nor their combination produced an opioid-like withdrawal syndrome following a naloxone challenge. Acute administration of estradiol, but not progesterone or a combination of estradiol and progesterone, significantly reduced naloxone-precipitated weight loss in morphine-dependent rats. These data indicate that estradiol influences the behavioral effects of morphine, possibly by increasing endogenous tone at mu opioid receptors.

Impact of the Aversive Effects of Drugs on Their Use and Abuse

Drug use and abuse are complex issues in that the basis of each may involve different determinants and consequences, and the transition from one to the other may be equally multifaceted. A recent model of the addiction cycle (as proposed by Koob and his colleagues) illustrates how drug-taking patterns transition from impulsive (acute use) to compulsive (chronic use) as a function of various neuroadaptations leading to the downregulation of DA systems, upregulation of stress systems, and the dysregulation of the prefrontal/orbitofrontal cortex. Although the nature of reinforcement in the initiation and mediation of these effects may differ (positive vs. negative), the role of reinforcement in drug intake (acute and chronic) is well characterized. However, drugs of abuse have other stimulus properties that may be important in their use and abuse. One such property is their aversive effects that limit drug intake instead of initiating and maintaining it. Evidence of such effects comes from both clinical and preclinical populations. In support of this position, the present review describes the aversive effects of drugs (assessed primarily in conditioned taste aversion learning), the fact that they occur concurrently with reward as assessed in combined taste aversion/place preference designs, the role of aversive effects in drug-taking (in balance with their rewarding effects), the dissociation of these affective properties in that they can be affected in different ways by the same manipulations, and the impact of various parametric, experiential, and subject factors on the aversive effects of drugs and the consequent impact of these factors on their use and abuse potential.

Conditioned Taste Avoidance Drug Discrimination Procedure: Assessments and Applications

In the present chapter, we summarize much of the work on the taste avoidance drug discrimination procedure, presenting the logic for its initial introduction and the extension of the procedure in the investigation of the discriminative properties of various drugs. Results from these assessments parallel those from more traditional operant and maze designs in classifying and characterizing the discriminative properties of drug. At the same time, this design reveals a procedure that is sensitive in such assessments by indexing these stimulus properties more rapidly and at lower doses than in the more traditional procedures (in some cases for drugs heretofore resistant in their detection). Importantly, much remains to be learned about the taste avoidance procedure in that the nature of such learning remains unknown and the specific parameters under which it can be established and generalized and its neurochemical and neuroanatomical bases are largely unexplored. The application of drug discrimination learning to human drug abuse continues to be an important consideration for this specific design (as well as that of drug discrimination procedures in general), and recent parallels between drug use and food intake in terms of its regulation by interoceptive stimuli suggests a possible role of the loss of stimulus control in drug escalation and addiction (with possible therapeutic implications via the modulation of these interoceptive cues).

Role of training dose in drug discrimination: a review

Drug discrimination has been an important technique in behavioural pharmacology for at least 40 years. The characteristics of drug-produced discriminative stimuli are influenced by behavioural and pharmacological variables, including the doses used to establish discriminations. This review covers studies on the effects of varying the training dose of a drug in a search for general principles that are applicable across different drug classes and methodological approaches. With respect to quantitative changes, relationships between training dose and the rate of acquisition or magnitude of stimulus control were found for most drug classes. Acquisition accelerated with dose up to a point beyond which drug-induced impairments of performance had a deleterious impact. Sensitivity to the training drug as measured by ED(50) values typically increased when the training dose was reduced. Qualitative changes were more complex and appeared to fall into three categories: (a) changes in profiles of generalization between partial and full agonists; (b) reduced specificity of some discriminations at small training doses; and (c) changes in the relative salience of actions mediated through different neurotransmitter systems or from central and peripheral sites. Three-lever discrimination procedures incorporating 'drug versus drug' or 'dose versus dose' contingencies enabled detection of more subtle differences than the simple 'drug versus no drug' approach when applied to the opioid, hallucinogen and barbiturate classes of drugs. These conclusions have implications for the interpretation of data from studies that use either within-subject or between-subject designs for studying the discriminative stimulus effects of drugs.

Patterns of nicotinic receptor antagonism: nicotine discrimination studies

Evaluation of the discriminative stimulus effects of drugs is a useful procedure for identification of receptor mediation of in vivo drug effects. This assay can be enhanced when the stimulus effects of different doses of agonist are evaluated. In the present study, rats were trained to discriminate small or large doses of nicotine from saline, and interactions of these effects with nicotinic receptor antagonists and partial agonists were determined. The insurmountable nicotine antagonist mecamylamine blocked both the discriminative stimulus and response rate-reducing effects of nicotine but was less effective against the large dose of nicotine. The α4β2*-selective, competitive antagonist dihydro-β-erythrodine (DHβE) antagonized the discriminative stimulus effects of both doses but was less effective against the larger training dose of nicotine. Schild analyses of DHβE suggested that different nicotinic receptor populations may be mediating the stimulus effects of large and small doses of nicotine. This suggestion was supported by observations that the discriminative stimulus effects of the partial agonist cytisine were more like those of the large dose than of the small dose of nicotine and that cytisine antagonized the effects of only the small nicotine dose. Varenicline produced nicotine-like effects in both training dose groups but reduced the discriminative stimulus effects of intermediate doses of nicotine in the group trained to the small dose of nicotine. Overall, these results suggest that small doses of nicotine produce their stimulus effects via α4β2* nicotine receptors, whereas larger doses of nicotine recruit additional nicotine receptor subtypes, as revealed by drug discrimination assays in rats.

Discriminative stimulus properties of naloxone in Long-Evans rats: assessment with the conditioned taste aversion baseline of drug discrimination learning

RATIONALE

The characterization of the discriminative stimulus properties of naloxone has focused primarily on its actions at the mu opioid receptor, although naloxone also displays an affinity for delta and kappa receptor subtypes.

OBJECTIVES

The present study extends this characterization of the naloxone cue by investigating if relatively specific antagonists for the mu (naltrexone: 0.10-0.56 mg/kg), delta (naltrindole: 1-18 mg/kg), and kappa (MR2266: 1.8-10 mg/kg) opioid receptor subtypes will substitute for naloxone in animals trained to discriminate naloxone from its vehicle. The temporal nature of the naloxone cue was examined by varying pretreatment time points (15, 30, 45, 60 min). Finally, various doses of naltrexone methobromide (1-18 mg/kg) were assessed to determine peripheral mediation of the cue.

MATERIALS AND METHODS

Female Long-Evans rats (N = 30) received an injection of naloxone (1 mg/kg; i.p.) 15 min prior to a pairing of saccharin (20-min access) and the emetic LiCl (1.8 mEq; i.p.; n = 16, group NL) or vehicle (n = 14, group NW); on other days, they were injected with saline prior to saccharin alone. Substitution tests with compounds with various receptor affinities and selective CNS and PNS actions were then assessed.

RESULTS

Only naloxone and naltrexone produced dose-dependent decreases in saccharin consumption. Naloxone administered at 15 and 30 min before saccharin produced decreases in consumption similar to that displayed on training days. Naltrexone methobromide substituted only at the highest dose tested (18 mg/kg).

CONCLUSIONS

Naloxone's stimulus effects appear to be mediated centrally via activity at the mu opioid receptor.

Intentional overdose of Large Animal Immobilon

We describe a case of voluntary self-injection with Large Animal Immobilon, a veterinary anaesthesia product containing etorphine, a very strong opioid, and acepromazine, a phenothiazine. This resulted in cardiorespiratory arrest and the need for sustained haemodynamic support after resuscitation. Large Animal Immobilon is used under specific conditions only, mainly in zoo and wildlife medicine. Primary toxicological analysis, although guided by the presumed toxin, could only detect a metabolite of acepromazine in the urine. Further analysis was able to show some traces of etorphine. A number of topics are treated, including the apparent potency of the etorphine and the selection of a suitable antidote, taking into account the different properties of the respective agents.

Interactions between opioids and cocaine on locomotor activity in rats: influence of an opioid's relative efficacy at the mu receptor

RATIONALE

Cocaine and mu opioid agonists increase central dopamine concentrations and produce robust interactions at both neurochemical and behavioral levels. Although the interactions between cocaine and high-efficacy mu opioids have been well characterized, the interactions between cocaine and lower efficacy opioids have not been as extensively examined.

OBJECTIVE

The purpose of this study was to examine the interactions between cocaine and opioids possessing a range of relative efficacy at the mu receptor.

METHODS

Male, Long-Evans rats were habituated to an open-field, locomotor activity chamber, and the effects of cocaine and various opioids were tested under a cumulative dosing procedure. In this procedure, a selected dose of an opioid was administered during the first component of a session, with increasing doses of cocaine administered during subsequent components.

RESULTS

When administered alone, cocaine produced dose-dependent increases in locomotor activity that was stable across 5 weeks of behavioral testing. The high-efficacy mu opioid levorphanol, and the low-efficacy opioids buprenorphine, butorphanol, nalbuphine and (-)-pentazocine, dose-dependently enhanced the effects of cocaine at doses that did not alter locomotor activity when administered alone. In contrast, the opioid antagonist naloxone, and to a lesser extent, the kappa opioid spiradoline attenuated the effects of cocaine at doses that did not alter locomotor activity when administered alone. Across an extensive dose range, the low-efficacy opioid nalorphine failed to alter cocaine's locomotor-activating effects.

CONCLUSIONS

These data suggest that low-efficacy opioids possessing significant mu-agonist activity (e.g. buprenorphine, butorphanol, nalbuphine, (-)-pentazocine) may potentiate the effects of cocaine in a manner similar to that typically observed with high-efficacy mu opioids.

Delta opioid discrimination learning in the rat: assessment with the selective delta agonist SNC80

The majority of reports assessing opioid drug discrimination learning (DDL) have concentrated on characterizing the stimulus properties of compounds selective for mu and kappa opioid receptors. Assessments of delta opioid DDL have been limited and, to date, these assessments have been restricted to the monkey and pigeon. No assessment of delta stimulus control has been examined in rodents. To that end, the present experiment examined discriminative control by the selective delta agonist SNC80 in rats and its generalization to and antagonism by compounds relatively selective to the delta and mu receptor subtypes using the conditioned taste aversion baseline of DDL. Animals injected with 5.6 mg/kg of SNC80 prior to a saccharin-LiCl pairing and with the SNC80 vehicle prior to saccharin alone acquired the discrimination within seven conditioning cycles. The discriminative effects of SNC80 were maximal at 20 min, partial at 120 min, and lost at 240 min. The discrimination was dose dependent in that as the dose of SNC80 increased, the amount of saccharin consumed decreased. In subsequent generalization tests, the delta agonist SNC162 produced SNC80-appropriate responding at a dose of 18 mg/kg. Conversely, the mu agonist morphine produced vehicle-appropriate responding at all doses tested. These selective generalization patterns with SNC162 and morphine suggest that the discriminative effects of SNC80 are mediated at the delta, but not the mu, receptor, a conclusion supported by the fact that SNC80's discriminative control was completely blocked by the delta-selective antagonist NTI, but not by the mu-selective antagonist naltrexone. The present findings indicate that not only do rats readily discriminate both mu- and kappa-selective agonists from their respective vehicles, but they also discriminate compounds that are selective for the delta receptor subtype, thus extending the class of compounds that can serve such discriminative functions for the rat.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.