Naloxone as a stimulus for drug discrimination learning

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).

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.

Discriminative stimulus effects of the cannabinoid CB1 receptor antagonist rimonabant in rats

OBJECTIVE

To examine the discriminative stimulus effects of the cannabinoid CB(1) receptor (CB(1)R) antagonist/inverse agonist rimonabant (SR141716A) using a discriminated taste aversion (DTA) procedure.

MATERIALS AND METHODS

Groups of rats were trained to discriminate between drug (5.6 or 3 mg/kg) and vehicle in DTA (t' = 20 min). The 30-min drinking opportunity after rimonabant pretreatment was followed by injection of lithium chloride (120 mg/kg) in the experimental (EXP) animals. When offered fluid after vehicle pretreatment, EXP animals subsequently were given intraperitoneal saline (NaCl, 10 ml/kg). Post-drinking treatment for controls (CONT) was NaCl irrespective of the pretreatment condition (rimonabant or vehicle). Tests examined other doses and drugs (t' = 20 min).

RESULTS

The rimonabant analog AM251 (1 to 5.6 mg/kg) substituted for rimonabant. AM281 also appeared to substitute, but interpretation is complicated by unconditioned effects (drinking suppressed also in the CONT group). The CB(2)R antagonists SR144528 (18 and 30 mg/kg), AM630 (1 to 10 mg/kg), and the CB(1)R agonist methanandamide (mAEA, 3 and 10 mg/kg) did not substitute. There was a dose-related attenuation of the rimonabant-induced suppression of saccharin drinking when Delta9-tetrahydrocannabinol (Delta9-THC; 0.3 to 5.6 mg/kg), but not mAEA (1 to 10 mg/kg), was given together with rimonabant (3 mg/kg). Unconditioned effects occurred with the mAEA-rimonabant combination, not evident for combinations of rimonabant and Delta9-THC. mAEA (10 mg/kg) plus AM251 (5.6 mg/kg) resulted in strong unconditioned effects.

CONCLUSION

Rimonabant induces a discriminative stimulus in DTA that continues to show potential for further examination of cannabinoid receptor antagonism.

Stimulus properties of nicotine, amphetamine, and chlordiazepoxide as positive features in a pavlovian appetitive discrimination task in rats

Recent experiments from our laboratory have demonstrated that drug states can signal when environmental cues will be followed by rewarding outcomes (ie Pavlovian conditioning). However, little is known about the generality of this approach and whether it can be used for studying the pharmacological properties of drug states. Accordingly, the present experiments tested the pharmacological specificity of nicotine (0.4 mg/kg), amphetamine (1 mg/kg), and chlordiazepoxide (CDP, 5 mg/kg) in this Pavlovian drug discrimination procedure. Following drug administration, presentation of a conditional stimulus (CS) was followed by brief access to sucrose. When saline was administered, the same CS was presented but sucrose was withheld. In substitution tests, rats in each condition received varying doses of all training drugs and caffeine. Anticipatory food seeking developed during the CS on drug sessions but not on saline sessions for all drug features (ie drug state-specific conditional response (CR)). In generalization tests, this CR decreased as a function of decreases in the training dose. Median effective doses (ED50s) were calculated for nicotine (0.054 mg/kg), amphetamine (0.26 mg/kg), and CDP (2.48 mg/kg). No compound tested substituted for the CDP training drug. Partial substitution was evident between nicotine and amphetamine; CDP did not substitute for either of these drug features. Caffeine fully substituted for nicotine (ED50 = 15.45 mg/kg) and amphetamine (ED50 = 3.70 mg/kg), but not for CDP. These results are consistent with the hypothesis that drug states can occasion appetitive Pavlovian CRs in a pharmacologically specific manner.

Discriminative stimulus effects in rats of SR-141716 (rimonabant), a cannabinoid CB1 receptor antagonist

OBJECTIVE

To examine the discriminative stimulus effects of (i) the cannabinoid CB(1) receptor antagonist SR-141716 (SR, 5.6 mg/kg) and vehicle, and (ii) the cannabinoid receptor agonist Delta(9)-THC (THC, 1.8 mg/kg) and vehicle using a discriminated taste aversion (DTA) procedure.

METHODS

Two groups of rats ( n=6) were trained to discriminate between these drugs and vehicle in DTA ( t'=20 min). The 30-min drinking bout of tap water following drug (SR or THC) treatment was followed by an injection of lithium chloride (LiCl, 120 mg/kg) in the experimental animals. When offered water after vehicle pretreatment, experimental animals subsequently were given IP saline (NaCl, 10 ml/kg). Post-drinking treatment for controls ( n=6) was NaCl, irrespective of the pretreatment condition (SR, THC or vehicle). Additional water was provided during the afternoon (30 min) with no other manipulations. Food was available ad lib at all times. When the discriminations were established other doses and drugs were examined ( t'=20 min). In testing there were no post-drinking treatments.

RESULTS

The SR-related analog AM-251 (dose range: 1-5.6 mg/kg) substituted for SR, whereas other drugs such as the cannabinoid CB(2) receptor antagonist SR-144528 (3 and 10 mg/kg), THC (1-10 mg/kg), flumazenil (1-10 mg/kg), naloxone (1-10 mg/kg), morphine (10 and 18 mg/kg) and d-amphetamine (1 and 3 mg/kg) did not. There was a dose-related attenuation of SR-induced suppression of drinking when THC (1.8-10 mg/kg) was given together with SR (5.6 mg/kg). In the THC trained rats, SR (1-10 mg/kg), morphine (10 and 18 mg/kg) and d-amphetamine (1 and 3 mg/kg) did not substitute for THC. SR (1 mg/kg) attenuated the THC (1.8 mg/kg) induced suppression of drinking. Together with 3 mg/kg SR and 1.8 mg/kg THC, drinking was roughly equally suppressed in both the experimental group and the controls.

CONCLUSION

SR-141716 induces a discriminative stimulus complex in DTA that shows potential for further examination of cannabinoid receptor antagonism.

5-HT3 receptor ligands lack discriminative stimulus properties

The putative discriminative stimulus of the 5-HT3 receptor antagonists ondansetron and (DL)-11-[(2-methyl-1H-imidazol-1-yl)methyl]-4,5,6,7,10,11,12-octahydroazepinol[3,2,1-jk]-carbazol-12-one hydrochloride (DU122932), and of the 5-HT3 receptor agonists 2-methyl-5-HT and 3,4-dichlorophenylbiguanide (3,4DCPB) were investigated in a standard two-lever, food-reinforced drug-saline discrimination procedure with groups of rats (N= 10 per group). In three groups of rats after 80 sessions with training doses ranging from 0.1 to 4.0 mg/kg po, stimulus control by ondansetron, DU122932 and 2-methyl-5-HT was still absent. The same 30 animals thereafter rapidly learned to discriminate chlordiazepoxide (CDP) from vehicle. In three other groups of rats, stimulus control by CDP was first established. Then, the vehicle was gradually (from 0.1 to 2.0 mg/kg po) replaced by either ondansetron, DU122932 or 2-methyl-5-HT. Finally, the dose of CDP was gradually decreased. In all three groups, stimulus control disappeared. A seventh group was trained to discriminate 3,4DCPB (5.0 mg/kg po) from saline. When training was not successful, dose and route were changed but discrimination was not attained. It is concluded that in the rat, using the classical two lever discrimination procedure, the 5-HT3 receptor ligands ondansetron, DU122932, 2-methyl-5-HT and 3,4DCPB are incapable of producing an internal state that can act as a stimulus to control responding.

The effects of chronic morphine on the generalization of buprenorphine stimulus control: an assessment of kappa antagonist activity

Rats trained to discriminate the mixed mu agonist/kappa antagonist buprenorphine from its vehicle generalize buprenorphine control to morphine. Buprenorphine, however, does not generalize to MR2266. The generalization to morphine suggests that buprenorphine's mu agonist properties mediated in part its discriminative control. The failure to generalize to MR2266, a compound reported to block kappa-mediated effects, however, suggests that its kappa antagonist activity was not involved in its discriminative effects. The ability of buprenorphine's mu (but not kappa) activity to establish stimulus control may be a function of the overshadowing of the kappa properties of buprenorphine by its concurrent mu activity. To test this possibility, in the present experiment rats were chronically exposed to morphine prior to buprenorphine discrimination training. This procedure has been reported to result in tolerance to buprenorphine's mu agonist effects and a more pronounced display of its kappa antagonist properties. The rats were then tested for the generalization of buprenorphine control to morphine, MR2266, and pentobarbital. As expected, buprenorphine failed to generalize to the nonopioid pentobarbital. Although subjects were tolerant to morphine (as evidenced by reductions in morphine-induced behavioral effects and a rightward shift in the doses of morphine substituting for buprenorphine), buprenorphine still failed to generalize to MR2266. The failure of buprenorphine to generalize to MR2266 under conditions that should have allowed for the development of stimulus control by buprenorphine's kappa antagonist activity was discussed in terms of the general inability of kappa antagonist activity to support discrimination learning.

Electrical stimulation of the dorsal raphe nucleus as a discriminative stimulus: generalization to (+/-)-DOI

Electrical stimulation of the dorsal raphe nucleus of Sprague-Dawley rats was used as the cue for discrimination using a taste aversion paradigm. Rats were trained to associate saccharin drinking during electrical stimulation of the dorsal raphe nucleus with LiCl injection after the session as the aversive unconditioned stimulus. In sessions without stimulation, rats were allowed to consume saccharin and received a saline injection after the session. Suppression of saccharin consumption during electrical stimulation was learned within 12 trials. Rats trained in the reverse discrimination, i.e., sessions with no electrical stimulation paired with LiCl injection, showed a similar learning curve. Animals injected prior to the session with the hallucinogenic 5-HT2 agonist (+/-)-DOI associated DOI with electrical stimulation of the dorsal raphe nucleus. Thus, animals may be trained to discriminate electrical stimulation of the dorsal raphe nucleus. Furthermore, animals generalize from activation of 5-HT2 receptors to electrical stimulation of the dorsal raphe nucleus.

Nalorphine as a stimulus in drug discrimination learning: assessment of the role of mu- and kappa-receptor subtypes

Using the conditioned taste aversion baseline of drug discrimination learning, animals were trained to discriminate nalorphine from distilled water. In subsequent generalization tests, the mu-opiate agonist morphine substituted for the nalorphine stimulus in a dose-dependent manner, while the kappa-opiate agonist U50,488H and the mu-opiate antagonists naloxone and naltrexone failed to do so. That the mu-agonist morphine substituted for the nalorphine stimulus while a kappa-agonist and mu-antagonists failed to substitute indicate that the discriminative control that was established with nalorphine in the present study was mu-agonist receptor-mediated. The basis for this selective control by the mu-receptor subtype may be related to the relative salience of receptor activity in opiate-naive animals. The present results suggest that discriminative control by compounds with activity at multiple receptor sites is not uniformly mediated by specific activity at all of those sites. The specific site mediating discriminative control appears to be a function of the specific training drug.

Discriminative stimulus properties of the benzodiazepine receptor inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM)

The purpose of this study was to determine whether rats could be trained to discriminate the stimulus properties of the benzodiazepine (BZ) receptor inverse agonist DMCM from saline in a conditioned taste aversion paradigm. On a drug trial, water-deprived rats were injected with DMCM (0.55-0.6 mg/kg IP), allowed access to a 0.25% saccharin solution for 30 min, and then injected with LiCl. On non-drug trials, saline injections bracketed the drinking period. Conditioned controls were treated similarly with DMCM and saline on drug and non-drug trials, but received injections of saline instead of LiCl. At the completion of training, CMCM produced a 69% reduction of saccharin consumption on drug trials, compared with 23% for conditioned controls. The stimulus properties of DMCM were then measured by its ability to reduce the preference for saccharin over water in a two-bottle choice test. DMCM reduced saccharin preference in rats that received discrimination training from 68% to 19%, but did not alter saccharin preference in conditioned controls. Other compounds with varying activity at BZ receptors were evaluated for their ability to substitute for the discriminative stimulus effects of DMCM. Two BZ receptor inverse agonists, beta-CCE (10-18 mg/kg) and FG 7142 (3.2-18 mg/kg), substituted completely for DMCM. Partial substitution for DMCM was shown by the BZ receptor antagonist CGS 8216 (3.2-10 mg/kg) and the non-BZ convulsant pentylenetetrazol (10-20 mg/kg). The BZ receptor agonists chlordiazepoxide (0.32-5.0 mg/kg), diazepam (0.32-10 mg/kg), and alprazolam (0.1-3.2 mg/kg) and the BZ receptor antagonist flumazenil (1.0-32 mg/kg) failed to substitute for the DMCM stimulus. Pretreatment with flumazenil (1.0 mg/kg) blocked the stimulus effects of the training dose of DMCM and produced a shift to the right of the DMCM generalization curve. The pattern of compounds that substituted for the DMCM stimulus and the blockade of that stimulus by flumazenil indicate that the stimulus properties of DMCM are associated with its effects as a BZ receptor inverse agonist.

Buprenorphine as a stimulus in drug discrimination learning: an assessment of mu and kappa receptor activity

Using the conditioned taste aversion baseline of drug discrimination learning, different groups of animals were trained to discriminate either buprenorphine or morphine from distilled water. Specifically, animals were injected with buprenorphine or morphine prior to a saccharin-LiCl pairing and the drug vehicle prior to saccharin alone. By the fifth conditioning trial, animals differentially consumed saccharin on the basis of administration of the drug or its vehicle. In subsequent generalization tests, buprenorphine stimulus control generalized completely to the mu agonist morphine in four of the five subjects tested, while morphine stimulus control completely generalized to buprenorphine in two of five subjects and partially generalized in the remaining three. Buprenorphine failed to generalize to the relatively selective kappa antagonist MR2266 and the broad-based antagonist diprenorphine. Morphine also failed to generalize to MR2266, but did generalize to diprenorphine. That morphine and buprenorphine displayed some degree of cross-generalization suggests that these compounds share some stimulus property, presumably their agonist activity at the mu receptor, and that the mu activity of these compounds was used in the establishment of the discrimination, a conclusion supported by the fact that compounds with mu antagonist activity (e.g., naloxone, MR2266) blocked both buprenorphine and morphine stimulus control. That buprenorphine failed to generalize to compounds with kappa antagonist activity suggests that animals trained to discriminate buprenorphine from its vehicle do not use the kappa antagonist activity of the drug in the establishment of the discrimination. The basis for the differential ability of various receptor subtypes to mediate the discriminative properties of compounds with mixed receptor activity was discussed.

Evidence for state-dependent retrieval in conditioned place aversion

State-dependent retrieval (SDR) in conditioned place aversion (CPA) was observed using Long-Evans male rats, with three different aversive drugs injected ip: lithium chloride (31.8 mg/kg), FG 7142 (10 mg/kg), and naloxone (5 mg/kg). Experiment 1 showed that state-dependent dissociation was complete with lithium chloride following two conditioning trials, but disappeared when the number of learning sessions was increased. Experiment 2 showed that following two conditioning trials, such complete state-dependent dissociations were also observed when FG 7142 or naloxone were used during conditioning. The results of this study showed that the expression of SDR is not monotonic in CPA paradigm but depends on the stage of learning. In addition, SDR can be observed following administration of a variety of drugs known to produce CPA, but differing in their neurochemical targets and the nature of the aversive effects they induce. Finally, these results are discussed with regard to what is known about SDR in other paradigms.

Cholecystokinin as a stimulus in drug discrimination learning

Animals were trained to discriminate a relatively low dose of the octapeptide cholecystokinin (CCK) from distilled water within the conditioned taste aversion baseline of drug discrimination learning. Specifically, rats were injected with CCK (5.6 micrograms/kg) prior to the presentation of saccharin-LiCl pairings and with the CCK vehicle prior to the presentation of saccharin alone. After 10 conditioning trials (40 days), subjects acquired the discrimination, avoiding saccharin consumption following administration of CCK and consuming the same saccharin solution following the drug vehicle. Once the discrimination was acquired, a generalization function was determined for doses above and below that of the training stimulus. At doses below the training dose of CCK (i.e., 0, 3.2, and 4.2 micrograms/kg), subjects drank at control levels, whereas at the training dose and above (10 micrograms/kg) subjects significantly reduced consumption. That a relatively low dose of CCK can be used as a discriminative stimulus within a drug discrimination design may be important in that the procedure can now be used in the assessment of the pharmacological characteristics of CCK at a dose similar to that used in other behavioral assessments of the compound.

Diprenorphine as a stimulus in drug discrimination learning

Using the conditioned taste aversion baseline of drug discrimination learning, animals were trained to discriminate diprenorphine from distilled water. In subsequent generalization tests, the opiate antagonists naltrexone and naloxone and the mixed opiate agonist/antagonist nalorphine substituted for the diprenorphine stimulus in a dose-dependent manner, while the opiate agonist morphine and the nonopiate pentobarbital failed to substitute even at the highest doses tested. That a range of opiate antagonists substituted for the diprenorphine stimulus (and an opiate agonist and a nonopiate failed to substitute) suggest that diprenorphine's antagonist properties may mediate the discrimination, presumably by blocking endogenous opiate activity. The ability of these drugs to substitute for the diprenorphine stimulus may also be a function of this receptor activity. The differences in the specific generalization patterns reported in the present assessment and those of earlier reports were discussed.

Conditioned taste aversions support drug discrimination learning at low dosages of morphine

The present experiment shows that a conditioned taste aversion procedure can support discrimination learning at dosages of morphine comparable to those required to produce motivational effects. Sprague-Dawley rats were injected with 4.0 mg/kg morphine sulfate prior to a saccharin-lithium chloride pairing, and physiological saline prior to a saccharin-saline pairing. The rats avoided the saccharin solution following the administration of morphine and consumed significantly more saccharin following saline administration after four discrimination cycles. After this initial discrimination the subjects were trained with progressively lower doses of morphine. Discrimination learning was apparent at doses of 2.0, 1.5, 1.0, 0.75 and 0.5 mg/kg. Animals initially trained with 1.0 mg/kg morphine also learned the discrimination but required 10 training cycles. After this initial discrimination the subjects were trained with progressively lower dosages of morphine and showed a discrimination at a dosage of 0.5 mg/kg.

Assessment of the stimulus properties of anxiolytic drugs by means of the conditioned taste aversion procedure

The conditioned taste aversion (CTA) procedure has recently been described as a more rapid alternative to two-lever operant procedures in drug discrimination research. We trained different groups of rats to discriminate the benzodiazepine chlordiazepoxide (CDP, 20 mg/kg) or the 5-hydroxytryptamine1A (5-HT1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.4 mg/kg) from saline by means of the CTA procedure. The results were in agreement with findings from two-lever operant drug discrimination procedures. However, discrimination training took 40 sessions in the case of CDP and 72 sessions for 8-OH-DPAT, which is comparable to results obtained with two-lever operant procedures. Dose-response curves were determined and generalization tests were performed for different benzodiazepine and nonbenzodiazepine anxiolytics. Baseline behavior deteriorated in the course of generalization and substitution testing, thus preventing further generalization testing. Our experience is that the use of the CTA procedure in drug discrimination research does not have sufficient advantages over traditionally used procedures to replace the latter.

Discriminative stimulus properties of the benzodiazepine receptor antagonist flumazenil

Rats were trained to discriminate the stimulus properties of the benzodiazepine (BZ) receptor antagonist flumazenil using a conditioned taste aversion procedure. On drug trials, fluid-restricted rats were injected with flumazenil (32 mg/kg), given access to a 0.25% saccharin solution for 30 min, and injected with LiCl (1.8 mEq/kg IP). On saline trials, injections of saline bracketed the period of saccharin consumption. Acquisition of the discriminated taste aversion, as measured by differential effects on drinking between saline and drug trials, developed after only five pairings of flumazenil with the LiCl injections. Flumazenil did not alter saccharin consumption in unconditioned controls (N = 9) that never received LiCl. The discrimination was also measured by flumazenil's ability to reduce the preference for saccharin over tap water using two-bottle choice tests. Flumazenil demonstrated dose-dependent generalization upon decreasing the training dose as low as 1 mg/kg. Two other BZ receptor antagonists of different chemical structure, CGS 8216 and ZK 93426, substituted completely for the flumazenil stimulus. Partial generalization was exhibited to the partial inverse agonists FG 7142 and beta-CCE, while the full inverse agonists DMCM and PTZ failed to substitute for the flumazenil stimulus. The BZ receptor agonists diazepam and alprazolam failed to substitute for the flumazenil stimulus, although partial generalization was shown with CDP. The results suggest that the BZ receptor antagonist flumazenil may produce intrinsic discriminative stimulus effects that are independent from those of BZ receptor agonists or inverse agonists.

Antagonism of drug discrimination learning within the conditioned taste aversion procedure

Animals injected with morphine prior to the presentation of a saccharin-LiCl pairing and the morphine vehicle prior to saccharin alone rapidly acquired the drug discrimination, avoiding saccharin following the administration of morphine and consuming saccharin following its vehicle after only four conditioning trials. Once stimulus control was established, the opiate antagonist naloxone (1 mg/kg) was administered prior to morphine in a test of its ability to antagonize the morphine stimulus. Pretreatment times ranged from 10 to 180 min. Naloxone antagonized the stimulus properties of morphine for all subjects, although there were individual differences in the onset, duration (time course) and degree of antagonism. Together with the rapid acquisition typically reported in this design, the fact that antagonism was demonstrated in the present study suggests that the conditioned taste aversion procedure may be useful in the general assessment of drug discriminations.

Naloxone as a stimulus in drug discrimination learning: generalization to other opiate antagonists

Nonopiate dependent animals were trained to discriminate the opiate antagonist naloxone (1 mg/kg) from distilled water within the conditioned taste aversion baseline of drug discrimination learning. Specifically, rats injected with naloxone prior to a saccharin-LiCl pairing, and with its vehicle prior to saccharin alone, rapidly acquired the drug discrimination, avoiding saccharin following the administration of naloxone and consuming saccharin following its vehicle after only three conditioning trials. Once the discrimination was acquired, generalization tests revealed that the opiate antagonists diprenorphine and naltrexone and the mixed opiate agonist/antagonist nalorphine completely generalized to the naloxone cue at doses of 1.8, 5.6 and 18 mg/kg, respectively. That discriminative control was established with a low dose of naloxone (i.e., 1 mg/kg) and other compounds with opiate antagonist activity generalized to the naloxone cue suggest that the stimulus effects of naloxone were likely mediated through the opiate receptor. Because each of these compounds are reported to bind to the mu receptor (with varying affinities and varying degrees of selectivity), the stimulus properties of naloxone are likely mediated at this specific receptor subtype.

Drug discrimination using a conditioned taste-aversion paradigm in rhesus monkeys

The development of drug discrimination was assessed in rhesus monkeys using the conditioned taste-aversion paradigm. Monkeys were initially trained to respond under a fixed-ratio 30-response schedule of food-pellet delivery to assess the rate-decreasing effects of alprazolam (0.03 to 3 mg/kg, i.m., 60 min presession). Alprazolam decreased responding at doses greater than 0.1 mg/kg. Discriminative stimulus effects of alprazolam were then assessed by giving 0.03 mg/kg before sessions in which 1.8 mEq/kg lithium chloride was given immediately after the session (alprazolam/lithium session). On intervening days, saline was given before and after the session (saline/saline session). Rates of responding decreased over successive alprazolam/lithium sessions and also during the saline/saline session that immediately followed an alprazolam/lithium session. During subsequent saline/saline sessions, rates of responding returned to levels near baseline rates within two to four sessions. The discriminative stimulus effects of alprazolam were then assessed by giving 0.1 mg/kg before sessions in which 1 mg/kg d-amphetamine was given immediately after the session (alprazolam/d-amphetamine session). Rates of responding decreased during subsequent alprazolam/d-amphetamine sessions in drug-experienced monkeys, but did not decrease during intervening saline/saline sessions. These findings demonstrate that drug stimuli associated with postsession drug injections can rapidly develop control over behavior and suggest that similar methods be explored in the assessment of drug discrimination.

Discriminated taste aversion with chlordiazepoxide

Discriminative stimulus effects have been studied extensively with the two-response, food-reinforced operant procedure and more recently also with discriminated taste aversion (DTA) procedures. DTA procedures have the advantage of a more rapid discrimination training. However, the test phase, i.e., drug substitution, of the DTA procedure is more time consuming (1 test per 4 days) than the test phase of the two-response procedure (2 tests per 5 days). The present study investigated whether a DTA procedure with 2 tests per 5 days could be implemented. In addition, the specificity of the DTA procedure was investigated. Rats were trained to discriminate chlordiazepoxide (CDP, 20 mg/kg, IP) from vehicle using a discriminated taste aversion procedure. Selective suppression of saccharin consumption after CDP injections was maximal after seven CDP-LiCl pairings. In subsequent substitution tests, with 2 tests per 5 days, CDP-mimicking effects were found only for another benzodiazepine, diazepam, and for a barbiturate, pentobarbital. The results indicate that rats can be rapidly trained to discriminate CDP from vehicle in the discriminated taste aversion procedure and that the CDP-cue so produced has the same specificity as in a two-response, food-reinforced operant procedure. However, the DTA procedure has a number of drawbacks that make its advantage over the two-response procedure questionable.

Quantal vs. graded generalization in drug discrimination: measuring a graded response

In drug discrimination research, detection of drug stimuli by animals is used for investigating various properties of psychoactive drugs. The major issue addressed by this paper is whether detection of drug stimuli by animals is a quantal or graded event. Some data suggest that detection of a drug stimulus by animals is quantal in nature. Thus, variations in drug stimulus substitution may only reflect variations in threshold for detecting the training stimulus rather than the current concept of these data reflecting graded responding to stimulus intensity. Therefore, drug discrimination procedures may have limited utility for detecting quantitative differences in the subjective effects of varying drug doses. In order to examine this problem, a method for measuring continuous response gradients in individual animals is needed. Tests for quantal responding generally use the distribution of responses on two manipulanda as the dependent measure. However, this variable may be inadequate for detecting a graded response, and may actually reflect loss of stimulus control or a deterioration in performance, rather than changes in response magnitude. Most alternative measures utilize response rate. Unfortunately, these measures are influenced by the direct rate-altering properties of some drugs. One possible alternative method is conditioned taste aversion as the discriminative task. This paradigm provides a means for not only ascertaining if graded discriminative responses occur in individual animals, but also more rapidly training a drug discrimination. Thus, using conditioned taste aversion techniques for measuring a drug discrimination may provide better indices for detecting response gradations.