Receptor mechanisms of opioid drug discrimination

Receptor theory of opioid action has provided an extremely useful interpretive framework for the discriminative stimulus effects of opioids. By and large, receptor theory has been applied to opioid actions as they are measured in in vitro and reflex systems. It is clear, however, that it can also assist in interpreting data from experiments addressing operantly conditioned behavior, and provide a link between these data and those obtained using other procedures. The current paper describes the criteria that can be used to determine whether a drug effect is receptor mediated and applies these criteria to the effects of mu and kappa opioids in drug-discrimination studies. Criteria for distinguishing between drug effects occurring through one, as opposed to two, receptor systems are described and again applied to the discriminative stimulus effects of mu and kappa opioids. The potential difficulties that can be caused by postreceptor variability and the presence of multiple receptor systems are noted, since they can modify the effects predicted from simple receptor theory, and are likely to play an important role when studies of opioid action are made in the whole animal. In discrimination studies, complicating variables include dose of the training drug, subject species, nature of the training drug, and context of the discrimination. Finally, the ability of receptor theory to guide future investigation of the phenomenon of partial generalization is explored.

Continuous glutamate leakage from brain cells is balanced by compensatory high-affinity reuptake transport

The glutamate (and aspartate) uptake blocker threo-3-hydroxyaspartate (20 microM) was added to superfusion fluids employed for in vivo microdialysis of corpus striatum, and to incubation medium for striatal slices (5 microM). In vivo it caused an increase in glutamate and aspartate concentrations in the superfusion fluid. In vitro it caused increases in the levels of glutamate, aspartate, GABA, taurine and glutamine in the incubation fluid. Tetrodotoxin (1 microM) did not influence the rises in glutamate or aspartate. It is concluded from these results that there is a continuous outward leakage of glutamate, and aspartate, from neural cells which is normally balanced by an inward flux due to reuptake processes. This leakage is distinct from synaptic release of these substances due to spike activity, since tetrodotoxin added to striatal slices did not diminish the action of threo-3-hydroxyaspartate. The significance of the findings for mechanisms leading to ischaemic or hypoxic brain damage, and basic mechanisms in epilepsy is discussed.

Serum colony stimulating activity and colony forming cells in murine brucellosis: relationship to immunopathology

Intravenous injection of mice with Brucella abortus vaccine strain 19, results in a chronic infection, immunity to which is dependent on T cell activation of the macrophages. A major feature of the infection is splenomegaly characterized by massive numbers of macrophages. We report here investigations of the haemopoietic precursors of macrophages, the colony forming cells (CFC), and the growth factors, colony stimulating factors (CSF), controlling their production. Comparison was made amongst three mouse strains, CBA, BALB/c and C57B1/10, as well as the F1 (CBA x BALB/c), which differ in the degree of splenomegaly developed and their ability to rid themselves of infection. The proportion of colony forming cells in the spleen peaked 2 to 3 weeks after infection and was higher in those strains which developed stronger splenomegaly. On the other hand there was no relation between colony forming cells and ability to control infection. Serum CSF also peaked 2-3 weeks post infection, with similar titres in all mouse strains studied. Bone marrow exhibited an early loss of total cellularity after infection followed by recovery. There was a sharp peak in the proportion of colony forming cells in the bone marrow 2 weeks post infection. Spleen and bone marrow CFC and serum CSF all returned to normal levels before infection was resolved.

Environmental modification of tolerance to morphine discriminative stimulus properties in rats

The development of tolerance to the discriminative stimulus properties of morphine was examined in rats trained to discriminate saline and 3.2 mg/kg morphine under a multiple timeout 15 min, 5 min fixed-ratio 30 schedule of food delivery. Generalization gradients were generated by administering increasing doses of morphine before successive timeout periods within the experimental session. Over the course of the study, the minimal discriminable dose (MDD) of morphine under control conditions fluctuated but did not systematically increase or decrease. Acute pretreatments of 3.2-17.8 mg/kg morphine 4-24 h before a generalization test resulted in minor changes in the MDD. To examine development of tolerance, supplemental doses of morphine (17.8 mg/kg) or saline were administered twice daily while discrimination training was either suspended or continued. Tolerance was assessed by weekly generalization tests. Greater tolerance developed to the morphine stimulus when training was suspended than when training was continued. For both training conditions, response rates during generalization tests were markedly suppressed during supplemental morphine administration, and original generalization gradients were recaptured within 2 weeks after termination of supplemental morphine administration. Supplemental saline administration did not alter the discriminative or rate-altering effects of morphine under either training condition. Thus, the magnitude of tolerance to a morphine discriminative stimulus reflected an interaction of supplemental drug treatment with the training conditions imposed during that treatment.

Excitatory amino acid neurotransmitters in the corticostriate pathway: studies using intracerebral microdialysis in vivo

The concentration of extracellular excitatory amino acids in the striatum of conscious, unrestrained rats was measured using intracerebral microdialysis, during chemical stimulation of the striatum in intact and hemidecorticate animals. Chemical stimulation of the striatum with tityustoxin (0.1 microM) evoked a rise in dialysate concentration of glutamate (to 383% of basal) and aspartate (to 156% of basal), accompanied by a drop in glutamine (to 55% of basal). These changes showed significant attenuation after treatment with L-proline (1 mM) or 2-chloroadenosine (15 microM). Unilateral degeneration of the corticostriate pathway, produced by frontal hemidecortication, caused a reduction in both basal and stimulated levels of glutamate in the lesioned side, whereas no effect was observed in the intact side. Similarly, basal and stimulated levels of glutamine were unchanged in the intact side, but were increased in the lesioned side. These results provide in vivo evidence for glutamate and possibly aspartate being neurotransmitters in the corticostriate pathway. In addition they lend support to previous studies in vitro, which implicated glutamine as the principal precursor for neurotransmitter glutamate.

Differential modification of pentobarbital stimulus control by d-amphetamine and ethanol

The ability of d-amphetamine and ethanol to alter discriminative stimulus control by pentobarbital was examined in pigeons. Saline and pentobarbital (5.6 mg/kg) were established as discriminative stimuli for food-maintained responding in six birds. Dose-response functions for stimulus control and response rate were determined for pentobarbital alone and in combination with selected doses of d-amphetamine or ethanol. In tests of stimulus generalization, d-amphetamine alone did not exert pentobarbital-like stimulus control, while ethanol alone evoked variable degrees of pentobarbital-like stimulus control. d-Amphetamine attenuated pentobarbital stimulus control. Doses of 1.0 or 3.2 mg/kg d-amphetamine increased the dose of pentobarbital required for stimulus control in five of six birds. Combinations of high d-amphetamine and pentobarbital doses yielded less than additive rate suppression. Ethanol produced variable effects on pentobarbital stimulus control, with moderate doses generally decreasing, and high doses increasing, the dose of pentobarbital required for stimulus control. A high ethanol dose decreased the pentobarbital dose required for rate suppression. Taken together, these data demonstrate that pentobarbital stimulus control can be altered by drugs within or without the sedative hypnotic class.

Modification of morphine tolerance by behavioral variables

These experiments assessed whether the opportunity to perform a target operant in the presence of morphine would alter the development of behavioral tolerance. Morphine tolerance was assessed in rats responding under a fixed-ratio 30 schedule of food delivery. Separate groups of rats were administered 10 mg/kg of morphine either pre- or postsession for 9 weeks. The degree of drug tolerance was assessed by determining cumulative dose-response functions for morphine before, during and after chronic administration. Three to 4-fold tolerance to the rate-decreasing effects of morphine developed in rats receiving morphine presession, whereas no tolerance developed in rats receiving an equal dose of morphine postsession. Morphine sensitivity returned to initial values 4 weeks after termination of chronic administration. Eight weeks after termination of chronic administration, the drug-daily session relationship was reversed and the rats were re-exposed to 10 mg/kg of morphine for 9 additional weeks. There were fewer differences between groups receiving morphine pre- or postsession during this second chronic administration phase. During chronic administration of morphine, the dose of naloxone required to suppress response rates decreased 100-fold in rats receiving morphine presession, but only 10-fold in rats receiving morphine postsession. In contrast, chronic administration of morphine did not alter the rate-decreasing effects of the nonopioids d-amphetamine, ketamine or pentobarbital. These experiments suggest that reinforcement of an operant response in the presence of morphine promoted the development of pharmacologically specific behavioral tolerance to morphine.

Discriminative stimulus properties of amphetamine and other stimulants in lead-exposed and normal rats

The present study examined the discriminative stimulus properties of amphetamine (AMP) at progressively lower doses in lead-exposed and normal rats. In addition, generalization gradients of AMP, apomorphine, methylphenidate, and caffeine to both high and low training doses of AMP were determined in these rats. Under the high AMP training dose condition (1.0 mg/kg, IP) generalization gradients of AMP were similar for lead-exposed and control rats. When the training doses were progressively lowered, the lead-exposed rats tended to require a higher range of AMP doses (0.24-0.49 mg/kg) than did control rats (0.18-0.32 mg/kg) to maintain discriminative control. In parallel with this, the minimal discriminable doses tended to be higher for lead-exposed rats than for control rats. Methylphenidate generalization gradients were different for lead-exposed and control rats under the high AMP training condition but became similar under the low AMP training condition. No differences attributable to training dose or lead exposure were evident for apomorphine or caffeine.

Colony-forming cells and colony-stimulating activity during listeriosis in genetically resistant or susceptible mice

Serum colony-forming activity (CSA) and colony-forming cells (CFC) of resistant (C57BL/10 ScSn) and susceptible (BALB/cJ) mice were studied during Listeria monocytogenes infection. Key findings were also checked in susceptible CBA/H mice. Prompt, bacterial dose-dependent increases in serum CSA were observed in all mice following infection. In response to the same challenge dose, serum CSA increased more in susceptible mice, possibly because rapid bacterial proliferation lead to high bacterial numbers. Thus CSA is not a limiting factor which accounts for the differences in Listeria resistance, but is produced in response to bacterial load. In uninfected mice, there were higher numbers of colony-forming cells in the bone marrow and spleen of resistant mice than in susceptible mice. By 24 hr postinfection there was a sharp drop in total cell numbers including CFC, in the bone marrow of resistant C57BL/10 ScSn mice. This coincides with the time when monocytes have been first observed in the blood of infected mice and when differences in bacterial growth between the mouse strains were first observable. Since the superior resistance of C57BL/10 mice has been shown to be radiosensitive, it is probable that this larger, readily mobilized reserve of monocyte/granulocyte precursors in the resistant mice plays an important role in early control of infection. The significance of this is discussed.

Effects of acute morphine pretreatment on the rate-decreasing and antagonist activity of naloxone

Rats responded under a schedule in which every 30th lever press (fixed ratio 30) produced a food pellet during sessions divided into six 5-min ratio components separated by 10-min timeout (TO) periods. Cumulative doses of morphine or naloxone were administered at the start of consecutive TO periods. When given alone, morphine decreased response rates in a dose-dependent manner, abolishing responding at 10 or 17.8 mg/kg. Naloxone doses of 0.1 and 1.0 mg/kg restored rates and patterns of behavior suppressed by a dose of 17.8 mg/kg morphine; doses of 0.32 to 10 mg/kg prevented the rate-decreasing effects of cumulative morphine doses. When administered alone, naloxone initially decreased response rates at a cumulative dose of 32 to 100 mg/kg; with repeated testing and intervening morphine exposure, the required cumulative dose was decreased to 10 or 32 mg/kg. An acute 10 mg/kg morphine pretreatment, given 4 h before the session, decreased the cumulative naloxone dose required to suppress rates an additional 10- to 30-fold. This effect was time-dependent and dose-dependent, and the usual naloxone dose-response function could be recaptured 1 week after the pretreatment effect was obtained. In contrast, acute morphine pretreatment did not alter either the cumulative dose of morphine itself required to suppress rates or the naloxone dose required to reverse or prevent morphine's rate-decreasing effects.

Effect of splenectomy on production of interferon and colony-stimulating factor in Listeria monocytogenes-infected mice

Splenectomy of mice genetically susceptible to Listeria monocytogenes genetically susceptible mouse strains increased their resistance to L. monocytogenes infection but had no effect on the course of infection in L. monocytogenes-resistant mice. However, splenectomy led to a dramatic decrease in the production of colony-stimulating factor and interferon after L. monocytogenes infection in all mouse strains examined.

Antagonism of the effect of delta sleep-inducing peptide by naloxone in the rat

The somnogenic properties of delta sleep-inducing peptide (DSIP) were investigated using electroencephalographic criteria. The peptide caused a significant increase in both REM and non-REM sleep at the expense of waking when injected into the lateral ventricle of the rat brain in four doses of 5 micrograms during the dark (waking) phase of the light/dark cycle. Furthermore this sleep promoting effect was blocked by pre-treatment with the opiate antagonist naloxone at a dose level (0.1 mg/Kg s.c.) considered selective for mu-receptors.

Reinforcing and discriminative stimulus properties of mixed agonist-antagonist opioids

Nine mixed agonist-antagonist opioids were evaluated in macaque monkeys for their ability to serve as positive reinforcers and for their discriminative stimulus similarity to etorphine and ethylketazocine. For tests of reinforcing properties, various doses of each drug were substituted for codeine under a fixed-ratio 30 time-out 600 sec schedule of i.v. delivery. Discriminative properties were assessed in separate groups of monkeys for which etorphine and saline, or ethylketazocine and saline, were established as discriminative stimuli for responses maintained under a fixed-ratio 20 schedule of food delivery. Two patterns of reinforcing and discriminative stimulus properties were observed. Buprenorphine, butorphanol, GPA 1657, nalbuphine, propiram and WY 16225 (dezocine) functioned as positive reinforcers and occasioned etorphine-appropriate but not ethylketazocine-appropriate responses. dl-Profadol also functioned as a positive reinforcer; its stereoisomers occasioned etorphine-appropriate but not, in general, ethylketazocine-appropriate responses. In contrast, levallorphan and oxilorphan did not function as positive reinforcers and occasioned ethylketazocine-appropriate but no more than 30% etorphine-appropriate responses. Under these experimental conditions, the reinforcing and discriminative stimulus profiles of the mixed agonist-antagonist opioids paralleled those of etorphine-like (mu) or ethylketazocine-like (kappa) opioid agonists.

Antagonism of the discriminative effects of ethylketazocine, cyclazocine, and nalorphine in macaques

dl-Ethylketazocine (EKC, 0.01 mg/kg) and saline were established as discriminative stimuli for food-maintained responding in macaque monkeys. Thirty consecutive presses on a right or left lever were reinforced with food, contingent on whether EKC or saline were administered before the session. For tests of antagonism, naltrexone, or UM 979 [(l)-5,9-alpha-dimethyl-2-(3-furylmethyl)-2'-hydroxy-6,7-benzomorphan] was administered concomitantly with EKC, dl-cyclazocine, or nalorphine. Both antagonists blocked completely the EKC discriminative stimulus. The antagonism of the stimulus and rate-altering effects of EKC was surmountable, with considerable intersubject variability in the magnitude of the EKC dose increase required to overcome the blockade. Cyclazocine and nalorphine, mixed agonist-antagonist opioids that share stimulus properties with EKC, were also susceptible to antagonism. Naltrexone antagonized completely the EKC stimulus effects of nalorphine; naltrexone and UM 979 antagonized completely the EKC stimulus effects of cyclazocine. Naltrexone antagonism of the cyclazocine stimulus was not surmountable, due to a lack of antagonism of the rate-decreasing effects of high cyclazocine doses.

Limitations on the antagonistic actions of opioid antagonists

This paper reviews factors that may limit or modify the behavioral actions of opioid antagonists under schedules of operant behavior in laboratory animals. The surmountable antagonism of opioid agonists by the antagonists is limited by the characteristics of both. Decreases in schedule-controlled responding produced by the prototypic agonist morphine are reversed by appropriate doses of antagonists such as naloxone, naltrexone, nalorphine, and cyclazocine. The ability of each antagonist to reverse the behavioral effects of morphine is limited, however, by the doses of agonist and antagonist used. The efficacies of high doses of antagonists are limited by the drugs' direct rate-decreasing actions. There also are differences among agonists in their sensitivity to antagonism. Naloxone and naltrexone are more potent as antagonists of morphine than of cyclazocine under certain conditions. Effective antagonism of the behavioural effects of opioid agonists appears to require entry into the central nervous system, indicated by the ineffectiveness of quaternary naltrexone, a peripherally acting opioid antagonist. The direct behavioral actions of the antagonists also can be modified by pharmacological conditions. The potency of antagonists in decreasing response rates is increased during chronic morphine administration. In addition, supersensitivity to the direct rate-decreasing actions of naloxone and naltrexone may develop after chronic antagonists administration. Such changes in the potency of antagonists also may modify the interactions of agonists and their antagonists.

Classification of narcotics on the basis of their reinforcing, discriminative, and antagonist effects in rhesus monkeys

Drug discrimination and drug reinforcement procedures were used to classify a variety of narcotic agonists, mixed agonist-antagonists, and antagonists in the rhesus monkey. A five-way classification was formed. Morphine-like agonists were compounds that shared the capacities to reinforce responding, to produce discriminative stimulus effects similar to those of morphine, and to suppress withdrawal in morphine-dependent rhesus monkeys. Morphine-like mixed agonists-antagonists were differentiated from pure agonists primarily by their capacity to elicit abstinence signs in morphine-dependent monkeys. A third class of narcotic agonists had ethylketazocine as a prototype. These compounds shared a distinctive set of interoceptive stimuli, failed to maintain significant responding relative to morphine-like agonists, and neither suppressed nor elicited withdrawal in morphine-dependent rhesus monkeys. The ethylketazocine-like mixed agonist-antagonists were similar to the ethylketazocine-like agonists, except that they shared the capacity to precipitate a morphine withdrawal syndrome. Narcotic antagonists (e.g., naltrexone) did not share interoceptive effects with either morphine or ethylketazocine, but were able to block the discriminative effects of both types of agonists. Furthermore, narcotic antagonists induced narcotic abstinence in morphine-dependent rhesus monkeys. The present classification scheme may be useful in identifying the properties of new narcotics, for clarifying relationships between narcotics and other pharmacological classes, and in comparing classifications based on other effects of narcotics.

Maintenance of behavior by ketamine and related compounds in rhesus monkeys with different self-administration histories

Rhesus monkeys lever-pressed under a fixed-ratio 30 time-out 600 sec schedule of i.v. injection of codeine (0.32 mg/kg/injection) or, in a second group of monkeys, ketamine (1.0 mg/kg/injection). During single session substitutions, the maintenance drug was replaced with saline or doses of various other drugs. At appropriate doses, ketamine maintained responding when substituted for ketamine. Phencyclidine, dexoxadrol and dextrorphan maintained responding when substituted for ketamine but did not maintain responding when substituted for codeine. Cyclazocine and SKF-10,047 (N-allyl-normetazocine) did not maintain responding when substituted for either ketamine or codeine; ethylketazocine did not maintain responding when substituted for ketamine. For those drugs maintained under behavior, fixed-ratio response rate and the number of injection dose and then decreased at higher injection doses. Substituted drugs maintained maximum response rates at the following injection doses: codeine, 0.32 mg/kg; ketamine, 1.0 mg/kg; phencyclidine, 0.03 mg/kg, dexoxadrol, 0.32 mg/kg; and dextrorphan, 1.0 mg/kg. Under only the ketamine maintenance schedule, the rate of responding during the timeout component varied as a function of the substitution dose of ketamine, codeine, phencyclidine and dexoxadrol, with the dose that maintained maximal fixed-ratio rates also engendering the highest rates of timeout responding.

Behavioral effects of levonantradol and nantradol in the rhesus monkey

In rhesus monkeys, acute administration of levonantradol and nantradol produced signs of CNS depression, including ataxia with body sag, pupil dilation, ptosis, dozing, and reduced responsivity to external stimuli. Neither compound suppressed the morphine withdrawal syndrome; however, both alleviated the chronic abdominal contraction associated with withdrawal. The directly observable effects of these compounds were not antagonized by naloxone. When levonantradol was administered every 6 hours, marked tolerance developed to both the effects of levonantradol and nabilone and THC. No signs of withdrawal were observed when levonantradol injections were abruptly discontinued. When substituted in lieu of codeine under an intravenous drug self-administration procedure, neither levonantradol nor nantradol maintained responding at rates higher than those maintained by their vehicle. Finally, the discriminative effects of levonantradol were not equivalent to those of the narcotics ethylketazocine or etorphine.

History of drug exposure as a determinant of drug self-administration

Drug self-administration is controlled, in part, by the subject's history of drug exposure. Although a history of drug administration is not necessary for many drugs to function as reinforcers, prior exposure can increase the likelihood that certain drugs, such as ethanol, will maintain behavior. While it has been demonstrated that physiological dependence is not necessary for a drug to function as a reinforcer, the conditions under which such dependence is maintained can control the later self-administration of the drug. Once drug-maintained behaviors are established, the particular drug that maintains behavior can influence the initial pattern of intake of a new drug and thus the dose of that drug that will maintain behavior. Additionally, under certain conditions, similarity between the discriminative stimulus effects of the drug that previously maintained behavior and those of a new drug can increase the likelihood that the new drug will function as a reinforcer. Finally, stimuli that have been paired with drug administration can powerfully control later drug-maintained behavior, the direction of such control being determined by the conditions under which such pairing occurred. In summary, both the type of drug with which a subject has experience as well as the contingencies governing that experience contribute to subsequent drug self-administration.