Discriminative stimulus effects of pentobarbital in pigeons

The Discriminative Stimulus Properties of Drugs Used to Treat Depression and Anxiety

Drug discrimination is a powerful tool for evaluating the stimulus effects of psychoactive drugs and for linking these effects to pharmacological mechanisms. This chapter reviews the primary findings from drug discrimination studies of antidepressant and anxiolytic drugs, including novel pharmacological mechanisms. The stimulus properties revealed from these animal studies largely correspond to the receptor affinities of antidepressant and anxiolytic drugs, indicating that subjective effects may correspond to either therapeutic or side effects of these medications. We discuss drug discrimination findings concerning adjunctive medications and novel pharmacologic strategies in antidepressant and anxiolytic research. Future directions for drug discrimination work include an urgent need to explore the subjective effects of medications in animal models, to better understand shifts in stimulus sensitivity during prolonged treatments, and to further characterize stimulus effects in female subjects. We conclude that drug discrimination is an informative preclinical procedure that reveals the interoceptive effects of pharmacological mechanisms as they relate to behaviors that are not captured in other preclinical models.

Effects of drugs and drug combinations in pigeons trained to discriminate among pentobarbital, dizocilpine, a combination of these drugs, and saline

Drugs with multiple actions can have complex discriminative-stimulus properties. An approach to studying such drugs is to train subjects to discriminate among drug combinations and individual drugs in the combination so that all of the complex discriminative stimuli are present during training. In the current experiments, a four-choice procedure was used to train pigeons to discriminate among dizocilpine (noncompetitive NMDA receptor blocker), pentobarbital (GABA(A) receptor agonist), a fixed-dose combination of these two drugs, and saline. Following extended training, low doses of pentobarbital or dizocilpine administered alone produced saline-appropriate responding. Higher doses of pentobarbital produced responding on the pentobarbital-appropriate key and higher doses of dizocilpine produced responding on the dizocilpine key. Administering the lowest doses of pentobarbital and dizocilpine together resulted in responding on the saline-appropriate key. Increasing the dose of pentobarbital in the presence of low doses of dizocilpine produced responding primarily on the pentobarbital-appropriate key; increasing the dose of dizocilpine in the presence of the lowest dose of pentobarbital produced responding primarily on the dizocilpine-appropriate key. Combining the higher doses of pentobarbital and dizocilpine resulted in responding primarily on the drug-combination key. Low doses of phencyclidine or ethanol produced responding on the saline-appropriate key, but intermediate doses resulted in individual subjects responding predominately on either the pentobarbital key, the dizocilpine key, or the drug-combination key depending on the subject. After the highest dose of phencyclidine or ethanol, most subjects responded predominantly on the drug-combination key. Low doses of other drugs tested produced responding on the saline-appropriate key. With the highest diazepam doses responding was largely confined to the pentobarbital-appropriate key. The highest doses of dextromethorphan or dextrorphan resulted in responding on the dizocilpine key more frequently than on other keys. Across a range of doses, morphine produced responding predominantly on the saline key. The results using the four-key procedure emphasized the role of both GABA(A) and NMDA receptors in the complex discriminative stimulus properties of phencyclidine and of ethanol.

Inhaled toluene vapor as a discriminative stimulus

Few studies exist exploring the discriminative stimulus effects of inhalants and none that have trained an interoceptive discrimination using the inhaled route. This study was designed to assess if it was possible to train an inhaled toluene discrimination. The second objective was to determine whether the discrimination was based on interoceptive or exteroceptive stimulus effects. Eight B6SJLF1/J mice were trained to discriminate 10 min of exposure to 6000 ppm inhaled toluene vapor from air, using a standard food-reinforced operant procedure. Toluene vapor produced robust, concentration-dependent, discriminative stimulus effects, with concentrations of 4000 ppm and higher producing full substitution. Substitution of inhaled toluene vapor for the training condition was exposure-time dependent. A minimum of 7 min of exposure to 6000 ppm was required to produce complete substitution. Injected intraperitoneal toluene produced dose-dependent full substitution for inhaled toluene vapor. Both inhaled and intraperitoneal ethylbenzene produced similar levels of partial substitution for 6000 ppm toluene vapor. Inhaled isoflurane vapor produced no substitution for toluene vapor. These results show that a toluene vapor discrimination can be successfully trained in mice and the discrimination is selective for toluene compared to ethylbenzene and isoflurane. The results also suggest that the discrimination was likely to have been based primarily on interoceptive rather than exteroceptive stimulus effects.

Systemic and intracerebroventricular administration of sodium barbital induced a place preference in rats

We have shown previously that 15 mg/kg pentobarbital induces a conditioned place preference (CPP), but it is unsuitable for intracranial administration. Since the long-acting barbiturate, sodium barbital, is soluble at a neutral pH, we tested whether it would induce a CPP when administered centrally. Furthermore, because barbital has a long duration of action, and because we obtained a significant CPP to systemically administered barbital using 30-minute conditioning trials, we tested whether longer conditioning trials would produce a more robust CPP. Using a three-compartment apparatus and an unbiased procedure, we found that systemic administration of barbital induced a significant CPP at 8 and 24 mg/kg, but not 2.7 or 72 mg/kg (i.p.). When rats were conditioned to 24 mg/kg barbital for conditioning trials of (1/2), 1, 3, or 6 hours, only the 30-min conditioning trial produced a CPP. Finally, 240 and 480 microg intracerebroventricular (ICV) barbital induced a significant CPP, but 60 or 120 microg did not. These findings suggest that: (1) like pentobarbital, barbital has reinforcing properties measured in the CPP test; (2) the CPP is impaired, rather than enhanced, by increasing the duration of drug-context pairing; and (3) the reinforcing effects of barbiturates are centrally mediated.

Principles of drug abuse liability assessment in laboratory animals

This paper describes the rationale for use of preclinical assessments of abuse liability in laboratory animals, and then discusses "cross-cutting" methodological issues that apply to behavioral evaluations intended to contribute to an abuse liability evaluation package. Issues include use of: (1) positive and negative control conditions; (2) full dose-effect evaluations, (3) multiple dependent measures, (4) pharmacokinetic evaluations to guide choice of dose ranges, (5) a species for which good methodological and comparative data are available to aid interpretation of results, and (6) appropriate methods for the group or single-subject experimental design selected. The remainder of the paper describes basic methodology by which three core pieces of behavioral data required by the Food and Drug Administration for its use in the overall abuse liability analysis can be obtained preclinically. Reinforcing effects are assessed in study of drug self-administration; drug discrimination assesses degree of overlap of interoceptive stimulus effects with relevant comparison drugs; physical dependence potential is determined by assessing whether a withdrawal syndrome occurs after chronic drug administration. Background and methodological issues specific to each procedure are discussed. A key consideration for cross-cutting and specific methodological issues is that choices made enable confident interpretation of both positive and negative results.

Drug discrimination under concurrent variable-ratio variable-ratio schedules

Pigeons were trained to discriminate 5 mg/kg pentobarbital from saline under concurrent variable-ratio (VR) VR schedules, in which responses on the pentobarbital-biased lever were reinforced under the VR schedule with the smaller response requirements when pentobarbital was given before the session, and responses on the saline-biased key were reinforced under the VR schedule with the larger response requirements. When saline was administered before the session, the reinforcement contingencies associated with the two response keys were reversed. When responding stabilized under concurrent VR 20 VR 30, concurrent VR 10 VR 40, or concurrent VR 5 VR 50 schedules, pigeons responded almost exclusively on the key on which fewer responses were required to produce the reinforcer. When other doses of pentobarbital and other drugs were substituted for the training dose, low doses of all drugs produced responding on the saline-biased key. Higher doses of pentobarbital and chlordiazepoxide produced responding only on the pentobarbital-biased key, whereas higher doses of ethanol and phencyclidine produced responding only on this key less often. d-Amphetamine produced responding primarily on the saline-biased key. When drugs generalized to pentobarbital, the shape of the generalization curve under concurrent VR VR schedules was more often graded than quantal in shape. Thus, drug discrimination can be established under concurrent VR VR schedules, but the shapes of drug-discrimination dose-response curves under concurrent VR VR schedules more closely resemble those seen under interval schedules than those seen under fixed-ratio schedules. Graded dose-response curves under concurrent VR VR schedules may relate to probability matching and difficulty in discriminating differences in reinforcement frequency.

Discrimination of pentobarbital doses and drug mixtures under fixed-ratio and fixed-interval reinforcement schedules

Pigeons were trained to discriminate among 5 mg/kg pentobarbital, 10mg/kg pentobarbital, and saline, under either fixed-interval (FI) or fixed-ratio (FR) reinforcement schedules. When baseline responding stabilized, a higher percentage of responses occurred on the key that produced the reinforcer under the FR schedule than under the FI schedule. After low doses of pentobarbital, responding shifted from the saline key to the 5 mg/kg pentobarbital key; at higher doses of pentobarbital responding shifted to the 10mg/kg pentobarbital key under both schedules. After low doses of ethanol and chlordiazepoxide, responding shifted from the saline key to the 5 mg/kg pentobarbital key, but after high doses of these drugs, responding continued to occur on the 5 mg/kg pentobarbital key under both reinforcement schedules. A 5 mg/kg dose of pentobarbital increased responding on the 10 mg/kg pentobarbital key when it was combined with pentobarbital, ethanol or chlordiazepoxide. Phencyclidine and D-amphetamine produced responding largely on the saline key under both reinforcement schedules. Under the FR schedule, pentobarbital dose-response curves were usually quantal, whereas under the FI schedule the pentobarbital dose-response curves usually were graded.

Secobarbital in humans discriminating triazolam under two-response and novel-response procedures

Humans were trained to discriminate the benzodiazepine triazolam (0.32 mg/70 kg) from placebo under a two-response (drug vs. placebo) drug discrimination procedure. Dose-effect curves for several drugs were then determined in a crossover design using the two-response procedure and a 'novel-response procedure' that provided a novel-appropriate response for drugs unlike triazolam or placebo. Three subjects were tested with triazolam (0.1-0.32 mg/70 kg), the barbiturate secobarbital (56-177 mg/70 kg), and caffeine (320 and 560 mg/70 kg). Triazolam dose dependently increased triazolam-appropriate responding under both procedures and generally did not occasion novel-appropriate responding under the novel-response procedure. Secobarbital substituted for triazolam in the two-response procedure and dose-dependently increased novel-appropriate responding as well as occasioned some triazolam-appropriate responding in the novel-response procedure. Caffeine generally occasioned placebo-appropriate responding under the two-response procedure and a mix of novel- and placebo-appropriate responding under the novel-response procedure. Triazolam and secobarbital produced qualitatively similar self-reported drug effects. These results suggest that the novel-response procedure for human drug discrimination may enhance the pharmacological selectivity of triazolam- and placebo-appropriate responding.

Behavior of rats under fixed consecutive number schedules: effects of drugs of abuse

Four rats responded under a simple fixed consecutive number schedule in which eight or more consecutive responses on the run lever, followed by a single response on the reinforcement lever, produced the food reinforcer. Under this simple schedule, dose-response curves were determined for diazepam, morphine, pentobarbital, and phencyclidine. The rats were then trained to respond under a multiple fixed consecutive number schedule in which a discriminative stimulus signaled when the response requirement on the run lever had been completed in one of the two fixed consecutive number component schedules. Under control conditions, the percentage of reinforced runs under the multiple-schedule component with the discriminative stimulus added was much higher than the percentage of reinforced runs under the multiple-schedule component without the discriminative stimulus. All of the drugs decreased the percentage of reinforced runs under each of the fixed consecutive number schedules by increasing the conditional probability of short run lengths. This effect was most consistently produced by morphine. The drugs produced few differences in responding between the multiple fixed consecutive number components. Responding under the simple fixed consecutive number schedule, however, was affected at lower doses of the drugs than was responding under the same fixed consecutive number schedule when it was a component of the multiple schedule. This result may be due to the difference in schedule context or, perhaps, to the order of the experiments.

Drug discrimination under a concurrent schedule

Three pigeons were trained to discriminate a 5.0 mg/kg dose of pentobarbital from saline under a two-key concurrent schedule with responding on the key associated with the presession injection, under both stimulus conditions, producing four times as many reinforcers as responding on the other key. This concurrent schedule resulted in approximately 70% responding to the higher reinforcement key under the pentobarbital stimulus and approximately 30% responding to that key under the saline stimulus. During testing, then, the pigeons were able to dose-dependently emit higher (>70%) or lower (<30%) values than were established under the control conditions. Dose-response curves were determined for pentobarbital (twice), methamphetamine, phencyclidine, chlordiazepoxide, and the combination of pentobarbital and the barbiturate antagonist bemegride. The results obtained with pentobarbital and chlordiazepoxide showed that, as the dose increased, pentobarbital-appropriate responding also increased. Methamphetamine produced relatively flat dose--response curves, whereas phencyclidine administration produced inconsistent effects on responding. The combination of the training dose of pentobarbital with increasing doses of bemegride produced a decrease in pentobarbital-appropriate responding. The results also showed that the dose-response curves for pentobarbital and chlordiazepoxide, instead of being all or none, were graded functions of the drug dose.

Effects of several benzodiazepines, alone and in combination with flumazenil, in rhesus monkeys trained to discriminate pentobarbital from saline

The purpose of the present study was to further investigate the relationship between the DS effects of PB and those of benzodiazepines (BZs) and to begin to collect pharmacological information concerning receptor mechanisms involved in this behavioral effect of BZs. Rhesus monkeys (n = 3), trained to discriminate pentobarbital (PB; 10 mg/kg, IG) from saline under a discrete-trials shock avoidance procedure, were given IG diazepam (0.3-10 mg/kg), chlordiazepoxide (1.0-30 mg/kg), or etizolam (0.3-10 mg/kg) alone and in combination with flumazenil (0.01-1.7 mg/kg, IM). Flumazenil was administered 10 min prior to the administration of saline, PB or the BZs. All three BZs fully substituted for PB in all monkeys. Diazepam was the most potent with a mean ED50 of 0.81 mg/kg (SEM = 0.04) while chlordiazepoxide was the least potent (mean ED50 = 5.78 mg/kg, SEM = 1.22 mg/kg). The ED50 for etizolam was 1.22 mg/kg (SEM = 0.37 mg/kg). Pretreatment with flumazenil (0.01-1.0 mg/kg) resulted in a dose-related parallel shift to the right in the dose-response function for PB-appropriate responding in all monkeys for all three BZs. The mean (n = 3) pKB value with 0.1 mg/kg flumazenil was 6.51 (SEM = 0.42) for diazepam and 6.57 (SEM = 0.17) for chlordiazepoxide. This value could not be calculated for etizolam because only one monkey was tested with 0.1 mg/kg flumazenil. However, the mean pKB for etizolam considering all monkeys and all doses of flumazenil was 6.58 (SEM = 0.47). Apparent pA2 values for flumazenil with diazepam were 6.02 for one monkey and 7.11 for another. All three BZs tended to increase average latency to respond. Apparent pKB and pA2 analysis may prove useful for elucidating receptor mechanisms involved in the behavioral effects of BZs.

Pentobarbital-like discriminative stimulus effects of direct GABA agonists in rats

The discriminative stimulus effects of direct and indirect-acting GABAergic drugs were investigated in rats trained to discriminate 5 mg/kg pentobarbital (PB) from saline under a two-lever fixed ratio (FR) 32 schedule of food reinforcement. PB and diazepam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Muscimol, thiomuscimol and 4,5,6,7-tetrahydroisoxazolo [5,4-c]-pyridin-3-ol (THIP) produced intermediate levels of pentobarbital-lever responding (40-60%), accompanied by dose-dependent decreases in rates of responding following THIP and muscimol administration. The GABAA agonist progabide and its metabolite 4-([(4-chlorophenyl) (5-fluoro-2-hydroxyphenyl)methylene]amino)] butyric acid (SL 75102) also partially substituted for PB, producing means of 39-73% PB-lever responding. The GABAB agonist, baclofen, completely failed to substitute for PB even at doses that decreased rates of responding. These results show that the discriminative stimulus effects of indirect GABAA agonists, PB and diazepam, although similar to one another, differ from those of direct GABAA receptor agonists, which produced only partial substitution for PB. The GABAB agonist, baclofen, can be distinguished by lacking any ability to substitute for PB. These results contribute to a further understanding of the similarities and differences in the behavioral effects of different types of GABA agonists.

Discriminative and reinforcing effects of brotizolam in rhesus monkeys

The reinforcing and discriminative stimulus effects of brotizolam, a benzodiazepine-hypnotic, were evaluated in rhesus monkeys. In one experiment, separate groups of monkeys (N = 3 group) were trained to discriminate pentobarbital (10 mg/kg, IG) or d-amphetamine. (0.56-1.0 mg/kg, IG) from saline, in a discrete-trials avoidance/escape paradigm. Pentobarbital (5.6-10 mg/kg), diazepam (1.0-1.7 mg/kg), and brotizolam (0.3-1.7 mg/kg) resulted in 100% drug-lever responding in all three pentobarbital-trained monkeys. In d-amphetamine-trained monkeys brotizolam administration resulted only in saline-lever responding. In another experiment, monkeys were surgically prepared with indwelling intravenous catheters and lever pressing resulted in an injection of 0.1 mg/kg/injection sodium methohexital under a fixed-ratio 10 (FR 10) schedule. Pentobarbital (0.01-0.3 mg/kg/injection) and diazepam (0.003-0.10 mg/kg/injection) maintained responding above saline control levels when substituted for methohexital. Brotizolam (0.001-0.01 mg/kg/injection) resulted in more injections received compared to saline, but fewer injections compared to pentobarbital or diazepam. Thus, results from the present experiment suggest that brotizolam would have pentobarbital-like subjective effects. However, the abuse liability of brotizolam may be lower than that for diazepam.

Comparative study of the behavioral, neurophysiological, and motor effects of psychotropic drugs in the dog

The purpose of this study was to present a comparative and detailed picture of behavioral, physiological, and motor effects of some barbiturates, benzodiazepines, and neuroleptics, using an operant procedure of differential reinforcement of response duration (DRRD) in the dog. An increase in response rate with low doses of barbiturates and benzodiazepines is interpreted in terms of behavioral disinhibition, anxiolytic effect, and enhancement of motivation. The depressive effects noted with higher doses of benzodiazepines and barbiturates, as well as with all doses of neuroleptics, can be explained in different ways. For benzodiazepines and barbiturates, ataxic effects more than sedative or motivational effects are implicated. However, for neuroleptics, loss of motivation or "anhedonia" better explains the decrease in operant responding. Differences between the main classes of psychotropic drugs but also within a class between 1,4 benzodiazepines or 1,5 benzodiazepines, hypnotic or nonhypnotic benzodiazepines, and typical or atypical neuroleptics are highlighted and correlated with human observations.

Drug discrimination in rats successively trained to discriminate diazepam and pentobarbital

In Phase 1, rats were trained to discriminate either diazepam or pentobarbital from the no-drug condition. Diazepam, pentobarbital, triazolam, meprobamate, and zopiclone occasioned 100% drug-lever responding in tests under both training conditions; but the generalization gradients determined under the pentobarbital training condition were shifted to the right of those determined under the diazepam training condition. In Phase 2, the training drugs were reversed for the two groups, as well as which lever was paired with drug or no drug, in an effort to produce greater specificity of the Phase 2 discrimination. In Phase 2 tests, the Phase 1 training drug occasioned responding on the Phase 2 drug lever in all rats, suggesting that retraining overrode the Phase 1 discrimination. There were indications, however, that Phase 1 training influenced Phase 2 responding: 1) Rats ceased responding partway through no-drug training sessions using the former drug lever, and criterion performance was somewhat more difficult to maintain in Phase 2. 2) In Phase 2, dose-effect curves determined under pentobarbital training were shifted even further to the right of those determined under diazepam training than in Phase 1.

Differential generalization to pentobarbital in rats trained to discriminate lorazepam, chlordiazepoxide, diazepam, or triazolam

In drug discrimination studies benzodiazepine-trained animals have typically responded on the drug lever when tested with barbiturates. In a recent study, greater specificity appeared to be shown when lorazepam was used as a training drug. The generality and limits of this finding were explored in the present set of experiments. The asymmetrical cross-generalization found in lorazepam- and pentobarbital-trained baboons was replicated in rats and was shown not to be a function of either lorazepam (0.1., 0.32, or 1.0 mg/kg) or pentobarbital (10 or 25 mg/kg) training dose (i.e., pentobarbital-trained rats responded on the drug lever in tests with lorazepam, but lorazepam-trained rats did not show comparable pentobarbital generalization). In the next experiment, groups of rats were trained to discriminate chlordiazepoxide (10 mg/kg), triazolam (0.1 mg/kg), or diazepam (1.0 mg/kg). Generalization to both lorazepam and pentobarbital was shown by these rats. Finally after daily pentobarbital administration, lorazepam-trained rats made a sufficient number of responses after high pentobarbital doses to permit extension of the range of pentobarbital doses tested. Pentobarbital generalization increased, but still did not occur in all rats and was unreliable in successive tests in the same rats. These results suggest less homogeneity in the discriminative stimulus effects of "depressant drugs" than generally has been recognized.

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.

Effects of training dose on discrimination and cross-generalization of chlordiazepoxide, pentobarbital and ethanol in the rat

Six groups of rats (N = 8), trained to discriminate chlordiazepoxide (5 or 20 mg/kg), pentobarbital (5 or 15 mg/kg) or ethanol (750 or 1500 mg/kg) from saline in a two-lever food-reinforced procedure, were tested for stimulus generalization with the three drugs. Training drug, but not training dose, affected the extent of generalization to a test drug; symmetrical generalization between chlordiazepoxide and pentobarbital and asymmetrical generalization between chlordiazepoxide and ethanol and between pentobarbital and ethanol was observed. Training dose level affected slope and ED50 of the generalization gradients of training drugs and substitution drugs, discriminative performance, response bias and threshold dose for response suppression. Indices of lever selection and percentage drug-appropriate lever responses yielded similar generalization maxima, slopes and ED50S. The potency of chlordiazepoxide relative to the potency of pentobarbital to induce drug stimulus generalization varied across the experimental groups. The results indicate differences between the discriminative effects of chlordiazepoxide, pentobarbital and ethanol. It is suggested that the discriminative effects of chlordiazepoxide, pentobarbital and ethanol are not based on their response rate modulating effects and that training dose is not a determinant for the extent of cross-generalization between these compounds.

Discriminative stimulus properties of valproic acid in the pigeon

Pigeons were successfully trained to discriminate 60 mg/kg valproic acid from saline using a two-key drug discrimination procedure. When 5-80 mg/kg doses of valproic acid were administered during generalization tests the percentage of responses directed to the valproic acid-appropriate key varied directly with dose. The effects of administering the training dose of valproic acid at presession injection intervals ranging from 15 to 120 min were described by an inverted U-shaped function; the 30-min interval used during discrimination training engendered the largest percentage of valproic acid-appropriate responses. The discriminative stimulus properties of valproic acid failed to generalize to the anticonvulsant compounds phenobarbital (10, 20 mg/kg), phenytoin (2.5, 5 mg/kg), and ethosuximide (40, 80 mg/kg), indicating that not all anticonvulsant compounds share similar discriminative properties. Clonazepam (0.25, 0.50 mg/kg) and diazepam (1, 2 mg/kg), two benzodiazepines with anticonvulsant properties, produced quite different effects. The stimulus properties of valproic acid generalized to all doses of clonazepam, whereas intermediate generalization was evident with diazepam. Pentylenetetrazol (10, 20 mg/kg), chlorpromazine (5, 10 mg/kg), tripelennamine (2.5, 5.0 mg/kg), d-amphetamine (0.5, 1.0 mg/kg), morphine (1.25, 2.50 mg/kg), and imipramine (2.5, 5.0 mg/kg) induced only saline-like patterns of responding. The concomitant administration of pentylenetetrazol failed to antagonize the discriminative stimulus properties exerted by the training dose of valproic acid.

The reinforcing properties of diazepam under several conditions in the rhesus monkey

Diazepam self-administration was studied in rhesus monkeys under several conditions of availability. Lever-press responding was maintained in twelve monkeys under a fixed-ratio 10 (FR 10) schedule of IV cocaine or pentobarbital delivery in daily sessions of 1-3 h duration. Each of several doses of diazepam (0.012-0.4 mg/kg/infusion) or vehicle was periodically substituted for 5-14 consecutive sessions. Between each substitution, responding was maintained by the baseline drug (cocaine or pentobarbital). Another procedure was to decrease the response requirement for drug delivery to a fixed-ratio one (FR 1). In three of eleven monkeys studied under conditions of a cocaine baseline and the FR 10 schedule, responding was maintained by diazepam and was inversely related to dose. In each of five monkeys tested in a similar manner but with a pentobarbital baseline, at least one dose of diazepam maintained responding above vehicle levels. Three of these monkeys had previously failed to self-administer diazepam under the cocaine baseline condition. Subsequently when two of these monkeys were returned to the cocaine baseline, diazepam was not self-administered above vehicle levels. Under FR 1 conditions of substitution, vehicle and pentobarbital intake increased in each monkey tested and cocaine intake increased in two of four monkeys. Diazepam self-administration also increased but did not exceed vehicle levels under the FR 1 schedule. However, in two monkeys the number of diazepam infusions was increased compared to the FR 10 substitution condition. These results emphasize the importance of testing drugs under several conditions to determine their relative dependence potential.

Ethanol and pentobarbital have different behavioral effects in the rat

Stimulus control was established in rats with 600 mg/kg ethanol and saline by employing a two-lever response choice task and an FR10 schedule of food reinforcement. Once trained, rats were tested with lower doses of ethanol (300 and 450 mg/kg) and with pentobarbital (0.75 - 4.0 mg/kg). The 3.0 and 4.0 mg/kg doses of pentobarbital were observed to produce ethanol-like responding and decreasing doses of both ethanol and pentobarbital produced dose-related effects upon discriminative performance. The ED50 for ethanol was 372 mg/kg and for pentobarbital was 1.09 mg/kg. Graphic representation of the dose-response relationships suggested that these drugs possess different sites and/or mechanisms of action.

Effects of barbiturates and other sedative hypnotics in pigeons trained to discriminate phencyclidine from saline

Pigeons were trained to peck the center key (lighted white) of three response keys to turn off the center keylight and to light one of the side keys with a red keylight and the other side key with a green keylight. Five responses (fixed-ratio component) on either side key relighted the center key. Food was delivered following 10 fixed-ratio components on the red key if 1.5 mg/kg phencyclidine had been given before the session. The position of the red and green keylights on the side keys varied randomly each time they were lighted by a peck on the center key. Subsequently, increasing doses of phencyclidine, barbital, amobarbital, phenobarbital, methaqualone, methyprylon, diazepam, oxazepam, and d-amphetamine were substituted for the training dose of phencyclidine, using a cumulative dosing procedure. At low doses of the sedative hypnotics, birds pecked the keylight color associated with saline. At higher doses, birds pecked both key colors. At the highest doses of pentobarbital and amobarbital, some birds responded almost exclusively On he color associated with phencyclidine. When responding on keys of both colors occurred following administration of phencyclidine or other sedative hypnotics, this responding was controlled by key position rather than by key color.

Ro 15-1788 antagonizes the discriminative stimulus effects of diazepam in rats but not similar effects of pentobarbital

The benzodiazepine antagonist properties of Ro 15-1788 were evaluated in rats trained to discriminate between saline and either 1.0 mg/kg of diazepam or 10 mg/kg of pentobarbital in a two-choice discrete-trial shock avoidance procedure. When administered alone, 1.0 mg/kg of diazepam and 10 mg/kg of pentobarbital produced comparable amounts of drug-appropriate responding (less than 84%), whether rats were trained to discriminate between diazepam or pentobarbital and saline. Ro 15-1788 (3-32 mg/kg, p.o.), administered 10 min before diazepam or pentobarbital, produced a dose-related blockade of the discriminative effects of diazepam in both groups of rats, but was completely ineffective in blocking the discriminative effects of pentobarbital. The dose-effect curve for the discriminative effects of diazepam was shifted to the right in a parallel fashion 3- and 13-fold by 10 and 32 mg/kg of Ro 15-1788, respectively, indicating that Ro 15-1788 acts as a surmountable, competitive antagonist of diazepam. When administered alone, Ro 15-1788 (32-100 mg/kg, p.o.) produced primarily saline-appropriate responding, although 100 mg/kg of Ro 15-1788 produced drug-appropriate responding in one out of eight rats. When administered orally 30 min after diazepam, Ro 15-1788 (32 mg/kg) completely reversed within 10 min the discriminative effects of diazepam. The blockade of diazepam's discriminative effects by 32 mg/kg of Ro 15-1788 appeared to last at least as long (approximately 2 hr) as the effects of diazepam alone.

Discriminative, disinhibitory, and depressant effects of several anticonvulsants

The discriminative attributes of drugs were used to assess the degree to which several anticonvulsants have behavioral effects resembling those of pentobarbital. Rats were trained to make alternative responses to obtain water, depending on whether they had been injected IP with pentobarbital (10 mg/kg) or saline 10 min before the session. The pentobarbital response was chosen in tests with phenobarbital, dimethylphenobarbital, or methsuximide in the anticonvulsant dosage range. Increasing doses increased the percentage pentobarbital choice. Response rate was generally increased by doses that increased percentage of pentobarbital choice. The rats predominantly chose the saline response when administered phenytoin, primidone, or phenylethylmalondiamide, even at doses that were sufficiently high to reduce the response rate. The results suggest that different types of depressant effects are associated with the anticonvulsants tested.

Discriminative stimulus effects of pentobarbital in rhesus monkeys: tests of stimulus generalization and duration of action

Rhesus monkeys were trained to emit 20 or 30 consecutive responses on one lever following an IM injection of pentobarbital (10 or 18 mg/kg) and the same number of consecutive responses on another lever following an injection of saline. The required number of correct consecutive responses in both cases resulted in food delivery. When responding was reliably under the control of the presession injection, the ability of a variety of other compounds to produce pentobarbital-appropriate responding was examined. Diazepam, clobazam, methohexital, pentobarbital, and phenobarbital, given 10 or 20 min before the session, produced dose-related pentobarbital-appropriate responding in each monkey. Ethylketazocine and dextromethorphan produced responding primarily on the saline-appropriate lever, whereas codeine, cyclazocine, dextrorphan, and ketamine resulted in responding that was, on the average, intermediate between that appropriate for pentobarbital and that appropriate for saline. When tested at various times after their injection, methohexital (3.2 mg/kg) and pentobarbital (10 mg/kg) produced pentobarbital-appropriate responding within 10 min. Barbital (56 mg/kg) resulted in pentobarbital-appropriate responding only if at least 1 h intervened between the injection and the experimental session. The discriminative effects of methohexital, pentobarbital, and barbital lasted approximately 20-60, 120-240, and 480-720 min, respectively. The time-course of the discriminative stimulus effects of barbiturates in the rhesus monkey appears to parallel closely other pharmacological actions of these compounds.

Generalization of the discriminative stimulus properties of phencyclidine to other drugs in the pigeon using color tracking under second order schedules

Pigeons were trained to track the location of a red or green key color under a second order schedule, with reinforcement of responses to each color contingent upon whether 1.5 mg/kg phencyclidine (PCP) or saline had been administered before the session. When cumulative dose-effect curves were determined for other drugs substituted for PCP, pigeons responded predominantly on the PCP key after other doses of PCP, ketamine, pentobarbital, d-cyclazocine and l-cyclazocine, but not after saline, lactic acid solution, d-amphetamine, chlordiazepoxide, scopolamine, morphine, naltrexone, or d-, or l-methadone. l-Cyclazocine was slightly more potent than d-cyclazocine in producing PCP key responding. PCP key responding after the optical isomers of cyclazocine was not blocked by naltrexone.

Selective blockade of the discriminative stimulus effects of pentobarbital in pigeons

The ability of CNS stimulants to block the discriminative effects of pentobarbital was studied in pigeons trained to discriminate IM pentobarbital (5 mg/kg) from saline. Pentobarbital, when administered alone, consistently produced greater than 90% pentobarbital-appropriate responding. The concomitant administration of pentobarbital and increasing doses of bemegride or pentylenetetrazol resulted in a dose-related decrease in pentobarbital-appropriate responses. In contrast, picrotoxin, another CNS stimulant, had little or no effect on pentobarbital-appropriate responding produced by pentobarbital.