Effects of anti-phencyclidine and anti-(+)-methamphetamine monoclonal antibodies alone and in combination on the discrimination of phencyclidine and (+)-methamphetamine by pigeons

RATIONALE

Drug-specific monoclonal antibodies against phencyclidine (PCP) and (+)-methamphetamine [(+)-METH] should bind to these drugs to block their discriminative stimulus effects.

OBJECTIVES

To determine if mouse monoclonal antibodies against PCP and (+)-METH can block the discriminative stimulus effects of the drugs in pigeons.

MATERIALS AND METHODS

Pigeons were trained to discriminate among intramuscular injections of saline, 1 mg/kg PCP, and 2 mg/kg (+)-METH. After responding stabilized, cumulative dose-response curves were obtained for PCP and (+)-METH. Doses of an anti-PCP antibody at 620 mg/kg (anti-PCP mAb6B5) with a K (D) of 1.3 nM for PCP and no measurable affinity for (+)-METH and 1,000 mg/kg doses of anti-(+)-METH antibody (anti-METH mAb6H7) with a K (D) of 41 nM for (+)-METH and no measurable affinity for PCP were subsequently administered, first alone and later in combination after which the dose-response curves were redetermined.

RESULTS

When the antibodies were given alone, the anti-PCP antibody blocked the discriminative stimulus effects of PCP, but not those of (+)-METH, and the anti-(+)-METH antibody blocked the discriminative stimulus effects of (+)-METH, but not those of PCP. The anti-PCP antibody shifted the PCP dose-response curve further to the right and for a longer time than the anti-(+)-METH antibody shifted the dose response curve for (+)-METH. When the anti-PCP and anti-(+)-METH antibodies were administered on the same day, the discriminative stimulus effects of both drugs were completely blocked 1 day after antibody administration.

CONCLUSIONS

These experiments demonstrate the high specificity of the antibodies for the drugs to which they bind and show that monoclonal antibodies can be combined to antagonize the effects of more than one drug.

Pharmacodynamics of a monoclonal antiphencyclidine Fab with broad selectivity for phencyclidine-like drugs

The development of treatment strategies for drug intoxication has been hindered in part by the lack of clinically useful antagonists. Consequently, the major goal of these studies was to determine whether a monoclonal antibody Fab fragment (of IgG) could be used as an effective drug class-selective antagonist and to understand better the dose-response relationships for reversing CNS drug toxicity. Changes in drug-induced locomotor effects in a rat model were used to assess the ability of the antiphencyclidine (anti-PCP) Fab to reverse the behavioral effects of PCP and other potent arylcyclohexylamines. In experiments to determine the pharmacodynamics of Fabinduced antagonism of behavioral effects, the Fab completely reversed all PCP-induced locomotor effects in a Fab dose-dependent manner with a minimal effective dose of 0.18 mole-equivalents of Fab and an ED50 value of about one-third mole-equivalent. The anti-PCP Fab also completely reversed the locomotor effects induced by two other structurally related potent analogs of PCP: 1-[1-(2-thienyl)cyclohexyl]piperidine and N-ethyl-1-phenylcyclohexylamine. In addition, pharmacological and immunological selectivity was further tested by treatment of the behavioral effects induced by the structurally unrelated locomotor stimulant (+)methamphetamine. The antibody did not effectively reverse the effects of methamphetamine-induced locomotor activity. These results indicate that antibody-based medications can be developed to treat toxicity caused by classes of drugs as well as by individual drugs.

Butorphanol-mediated antinociception in mice: partial agonist effects and mu receptor involvement

In the present experiments, we characterized the agonist and antagonist effects of butorphanol in mice. In the mouse radiant-heat tail-flick test, the mu agonists morphine and fentanyl and the kappa agonist U50,488H were fully effective as analgesics, whereas butorphanol was partially effective (producing 82% of maximal possible analgesic effect). Naltrexone was approximately equipotent in antagonizing the effects of morphine, fentanyl and butorphanol; in vivo apparent pA2 values for these naltrexone/agonist interactions were 7.5 (unconstrained). Naltrexone was approximately 10 times less potent in antagonizing the effect of U50,488H (average apparent pK(B) = 6.7). The selective mu antagonist beta-funaltrexamine (0.1-1.0 mg/kg) antagonized the effects of butorphanol in a dose-dependent insurmountable manner. Pretreatment with nor-binaltorphimine (32 mg/kg), a kappa-selective antagonist, did not reliably antagonize butorphanol, and naltrindole (20 and 32 mg/kg), a delta-selective antagonist, failed to antagonize the effects of butorphanol. Low doses of butorphanol (1.0, 1.8 or 3.2 mg/kg) caused parallel, rightward shifts in the dose-effect curve for morphine and parallel leftward shifts in the dose-effect curve for U50,488H. Taken together, the results of the present study suggest that butorphanol is a partial agonist in the mouse radiant-heat tail-flick test and that activity at mu receptors accounts for the majority of its antinociceptive effects.

Antiphencyclidine monoclonal Fab fragments reverse phencyclidine-induced behavioral effects and ataxia in rats

Antiphencyclidine monoclonal antibody binding fragments (anti-PCP Fab) were studied in rats as a possible treatment for phencyclidine (PCP) overdose. Each male Sprague-Dawley rat (n = 4 per group) received an i.v. dose of 1 mg/kg of PCP followed 5 min later (as toxicity maximized) by one of three treatments in a random cross-over design. The treatments were 1 ml of saline, a nonspecific polyclonal human Fab, or a high affinity (Kd = 1.8 nM) anti-PCP monoclonal Fab. The doses of the nonspecific and anti-PCP Fab were 0.3, 1.0 and 3.0 times the mole equivalent (mol-eq) dose of PCP. Changes in locomotor activity and ataxia were the best indicators of PCP-induced behaviors among several time-dependent behavioral changes that were evaluated. PCP administration followed by saline treatment resulted in increases in locomotor activity and ataxia that declined to base line after 35 to 40 min. Anti-PCP Fab at 1.0 and 3.0 times the mol-eq dose of PCP significantly (P < .05) and rapidly reversed PCP-induced behaviors to base-line values. Although the 0.3 mol-eq dose of Fab appeared to slightly decrease the behavioral toxicity, the effects were not statistically different from controls in most cases. No significant effects on PCP-induced behaviors were observed after any dose of the nonspecific Fab. In addition, pharmacological and immunological specificity were tested further by treatment of MK-801 {(+)-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine-}-induced behavioral effects. MK-801 is a PCP-like, noncompetitive N-methyl-D-aspartate receptor antagonist which is structurally unrelated to PCP. The anti-PCP Fab treatment had no effect on MK-801-induced locomotor activity. These data clearly show that anti-PCP Fab is a specific PCP antagonist that can rapidly reverse PCP-induced behavioral toxicity in the rat.

[3H]MK-801 binding to well-washed rat brain membranes following cessation of chronic phencyclidine treatment

Rats were implanted with osmotic minipumps SC that infused either saline or 10 mg/kg/day phencyclidine (PCP) for 10 days, a regimen that produces dependence to PCP. At the end of this 10-day infusion period, the pumps were removed and the rats were sacrificed either immediately or at various time points (12 h, 1, 2, and 7 days) after pump removal. The saturation binding parameters of [3H]MK-801 were then determined in well-washed cortical/hippocampal membranes prepared from these rats. Neither the Bmax nor the Kd of [3H]MK-801 binding in membranes of PCP-treated rats differed from that determined using membranes from saline-treated rats at any time point studied. These results suggest that alterations in PCP receptors do not play a major role in the production of PCP dependence.

Sexual dimorphic effects of chronic phencyclidine in rats

The behavioral effects of phencyclidine (PCP) were studied in male and female Sprague-Dawley rats to determine if chronic infusions would result in sexually dimorphic effects. Rats were trained to make operant responses for food during 30-min response periods that occurred 4 times each day. After attaining stable baseline behaviors, 10 mg of PCP/kg/day was infused s.c. for 10 days. Females were more profoundly affected than males. In the females, response rates were suppressed to 30-71% of control rates during the first 7 days of infusion. In contrast, response rate in male rats never fell below 77% of control during the infusion period. By the eighth infusion day both sexes had become tolerant to these behavioral effects. After stopping infusions there was clear evidence that behavioral dependence had developed; however, the abstinence effects in males and females were similar. Saturation studies of [3H]dizocilpine (MK-801; (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine) binding to brain membranes were conducted to determine if there were sex-dependent receptor differences. There were no significant differences in Kd +/- S.D. (7.6 +/- 1.5 and 7.1 +/- 0.9 nM for males and females, respectively) or Bmax +/- S.D. (4.1 +/- 0.2 and 4.0 +/- 0.5 pmol/mg protein for males and females, respectively).

Tolerance to and dependence on MK-801 (dizocilpine) in rats

Rats were trained to respond under a fixed-ratio 30 schedule for food presentation during four daily 0.5-h sessions occurring every 6 h. After stable baseline response was established, osmotic minipumps were implanted that infused vehicle or (+)-5 methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine hydrogen maleate (dizocilpine; MK-801), SC. Behavioral sessions continued to be conducted daily. After 10 days the infusion pumps were removed. Vehicle and 0.10 mg/kg per day MK-801 did not affect behavior during infusions or after cessation of dosing. Dosing with 0.32 and 0.56 mg/kg per day initially suppressed responding, but tolerance developed to these effects. After the infusions were stopped, a dose-dependent disruption of operant behavior occurred. Response rates for the 0.32 and 0.56 mg/kg per day infusion groups were suppressed to 41 and 27% of preinfusion control response rates, respectively, the day after dosing stopped; however, no physical signs of abstinence were observed. Response rates recovered toward control over the next 2-4 days. In a separate experiment, the suppression of response produced by abstinence from 0.32 mg/kg per day of MK-801 (SC) for 10.5 days was reversed by readministration of MK-801 (IP). These results demonstrate that MK-801 produces dependence, as evidenced by the emergence of a behavioral abstinence syndrome after cessation of dosing.

Effects of cocaine self-administration on ethanol, food and water intake in the rat

Because cocaine and ethanol are frequently abused in combination, the present study was performed to assess the behavioral consequences of concurrent access to both of these drugs. Rats trained to respond for food under a fixed-ratio 40 (FR 40) schedule of reinforcement during four, 30-min periods each day (every 6 h) were given free access to a 5% (w/v) ethanol solution and to water. Once a stable baseline of food, ethanol and water intake was established, intravenous cocaine was made available under a fixed-ratio 1 (FR1) schedule. After cocaine self-administration had been established for 10 days, the ethanol was removed from the chambers for 3 days. After the ethanol was returned to the chambers, saline was substituted for cocaine for 3 days. Following saline substitution, animals were given increased cocaine availability. Before cocaine was made available, rats consumed primarily the ethanol solution, averaging 31.7 ml of the 5% solution (4.2 g/kg ethanol), 10.2 ml of water and 148 food pellets/day. When cocaine (0.2 mg/kg per injection, i.v.) was made available, rats self-administered an average of 40-85 injections per day. Self-administration of cocaine resulted in small decreases in ethanol and food intake, as well as some changes in the pattern of ethanol intake. However, removing the ethanol from the chambers had no effect upon food and cocaine intake. Substitution of saline for the cocaine altered the pattern, but not the amount of ethanol intake. There was a trend toward increased ethanol intake during the study, which may have been related to repeated cycles of cocaine availability.

Phencyclidine dependence: the relationship of dose and serum concentrations to operant behavioral effects

The dependence-producing properties of 10 days of chronic i.v. infusions of phencyclidine (PCP) and the relationship between PCP serum concentrations and behavioral effects were studied in Sprague-Dawley rats. For dependence studies, rats were trained to respond for food under a fixed-ratio 30 schedule during half-hour response periods every 6 hr. After training, implantation of jugular catheters, and restabilization of behavior, the rats were infused with PCP.HCl at 3.2, 5.6, 10.0 or 17.8 mg/kg/day (n = 5 or 6 per dose). The two higher doses initially decreased response rates, but tolerance developed within 4 to 5 days. When PCP infusions were terminated, dose-dependent decreases in session response rate occurred in the three highest dose groups (P less than .05). Mild, overt signs of abstinence were observed only in the highest dose group. Response rates returned to base line within 2 to 3 days after stopping PCP infusions. PCP serum concentrations in rats infused with 10 mg of PCP.HCl/kg/day for 10 days were stable from hour 24 to day 10 (mean steady-state concentration (+/- S.D.) = 97 (+/- 20) ng of PCP/ml; n = 4). The average terminal elimination half-life after stopping infusions on day 10 was 4.6 hr. Comparison of the average response rates with the average serum concentrations showed that during the first 24 hr of infusions, the rate of responding for food decreased as PCP concentrations increased; however, once the animals became tolerant to PCP there was no relationship. In contrast, during the first 24 hr after stopping infusions, response rates decreased as serum concentrations decreased.

Effects of terminating chronic phencyclidine on schedule-controlled behavior in rats

Six rats were trained to respond under a multiple fixed-ratio 30, fixed-interval 3-min schedule for food presentation. Acute administration of phencyclidine (0.1-3.2 mg/kg, IP) produced decreases in fixed-ratio response rates at doses above 0.3 mg/kg, but fixed-interval response rates were only decreased at the highest dose. However, the pattern of fixed-interval responding (as evidenced by quarter-life values) was affected at doses above 0.3 mg/kg. Osmotic minipumps were implanted, SC, which infused saline (2 rats) or phencyclidine (4 rats, 10.0 mg/kg/day) for 10 days, and then removed. Daily behavioral sessions were conducted during infusions and for 10 days afterwards. The effects of phencyclidine infusions on fixed-ratio responding were variable. Fixed-interval response rate and quarter-life values were only modestly affected during drug infusion. All three parameters were markedly affected upon cessation of chronic phencyclidine dosing, but there did not appear to be differential effects between the schedule components. No effects on responding were observed during or after saline infusions.

Phencyclidine pharmacokinetics and concentration-response relationships in the pigeon

Phencyclidine (PCP) pharmacokinetics and drug discrimination were examined in pigeons (n = 6 in both groups) after intramuscular doses of 1.48 mg/kg. PCP absorption was rapid with maximum measured plasma concentrations ranging from 559 to 1450 ng/ml at 10-30 min after dosing, which corresponded to the time of maximum PCP stimulus effects in the drug discrimination studies. The terminal elimination half-life was 0.88 hr (harmonic mean). Average values for the volume of distribution and total body clearance were 1.6 l/kg and 18.2 ml/min/kg, respectively. In the behavioral studies, pigeons discriminated PCP-like effects from about 2 min to 2 hr after dosing. An average value for response on the PCP-appropriate key and for PCP concentration at each time point from 2 min to 2 hr was calculated from the individual subject data. Least-squares linear regression analysis of these data showed a highly significant relationship between the ability to discriminate PCP and log PCP concentration (y = 103x - 219, r2 = .810, p less than 0.005). This analysis suggests PCP concentration is a good predictor of behavioral efficacy.

Alterations in rat brain [3H]-TCP binding following chronic phencyclidine administration

Rats were chronically infused with phencyclidine (PCP, 13.3 mg PCP.HCl/kg/day) or saline, s.c., for 10 days using osmotic minipumps (n = 5 for each group). Twenty-four hours after the cessation of dosing, the rats were sacrificed, and brains were removed for analysis of PCP receptor binding. Saturation studies of the binding of [3H]-TCP to brain homogenates revealed statistically significant increases in the maximum binding capacity (Bmax) and decreases in the affinity for [3H]-TCP in the PCP-treated group.

A comparison of interoceptive and exteroceptive discrimination in the pigeon

In pigeons performing a conditional discrimination under a second-order, color-tracking procedure, stimulus control of responding was established using a blinking versus a nonblinking light as exteroceptive stimuli (light-discrimination group). Another group performing under the same second-order schedule of reinforcement was trained to discriminate the interoceptive stimuli produced by an IM injection of 1.5 mg/kg phencyclidine (PCP) versus saline (drug-discrimination group). In the drug-discrimination group, administration of PCP or pentobarbital resulted in dose-dependent increases in PCP-appropriate responding, while, in general, d-amphetamine did not result in appreciable drug-appropriate responding. In the light-discrimination group, all three drugs over the same dose ranges resulted in decreased discriminative control over responding. In both groups, doses of PCP and pentobarbital which resulted in intermediate (30 to 70%) levels of stimulus-appropriate responding were associated with responding at a single key position rather than tracking a key color. In contrast, intermediate responding after d-amphetamine administration was not associated with position responding in either group. These results emphasize the similarity between discriminative control maintained by interoceptive drug stimuli and exteroceptive visual stimuli.

Interaction of the discriminative stimulus effects of phencyclidine with those of (+)-N-allylnormetazocine, pentobarbital and d-amphetamine

Pigeons trained to discriminate 1.0 mg/kg phencyclidine from saline were used to study the interaction between the stimulus effects of phencyclidine and those of (+)-N-allylnormetazocine [(+) NANM], pentobarbital and d-amphetamine using a cumulative-dosing procedure. Both (+) NANM and pentobarbital enhanced the discriminative stimulus effects of phencyclidine. The enhancement of the phencyclidine stimulus by pentobarbital was predicted by adding the effects of the individual drugs, but the enhancement of the phencyclidine stimulus by (+) NANM was sometimes more than would have been expected from adding the effects of the individual drugs. d-Amphetamine did not enhance the discriminative stimulus effects of phencyclidine, but neither did it interfere with these effects. Combinations of (+) NANM or pentobarbital with phencyclidine also enhanced the rate-decreasing effects of phencyclidine, but to a lesser extent than they enhanced the discriminative stimulus effects of phencyclidine. d-Amphetamine only slightly enhanced the rate-decreasing effects of phencyclidine.

Structure-activity relationships of arylcyclohexylamines as discriminative stimuli in pigeons

The effects of a variety of arylcyclohexylamines, opioid drugs and other drugs were studied for phencyclidine-like effects in pigeons trained to discriminate phencyclidine (PCP) from saline. There was a good correlation between the relative potency of arylcyclohexylamines as PCP-like discriminative stimuli in pigeons and these drugs as discriminative stimuli in rats. Substitution of methyl groups on the piperidine or cyclohexyl rings of PCP reduced potency, but not efficacy, whereas substitution of hydroxyl groups decreased both potency and efficacy. Replacement of the phenyl ring with a thienyl ring increased PCP-like activity, but replacement of the piperidine ring with a pyrrolidine ring or a morpholine ring decreased potency. Compounds with methyl or ethyl groups on the nitrogen atom replacing the piperidine ring also were active. These data suggest that N-alkyl substitutions decrease potency but not efficacy, whereas hydroxylation of the cyclohexyl ring decreases efficacy as well. The data also support the suggestion that size of the aromatic ring is also a determinant of PCP-like activity. Both optical isomers of cyclazocine and N-allylnormetazocine were active as PCP-like discriminative stimuli, although the (-)-isomer was more potent than the (+)-isomer for cyclazocine and the reverse was true for N-allylnormetazocine. The pigeon shows less stereospecific activity with these drugs than the rat and especially the squirrel monkey. A variety of other opioid-like chemical structures, as well as other drugs such as d-amphetamine and pentobarbital, were inactive as PCP-like discriminative stimuli.

Centrally mediated opioid induced depression of hepatic glutathione: effects of intracerebroventricular administration of mu, kappa, sigma and delta agonists

It has recently been demonstrated that morphine produces a loss of hepatocellular glutathione in mice by virtue of its action within the central nervous system. The ability of opioid receptor antagonists to abolish morphine's effect on hepatic glutathione suggests that this action is opioid-receptor mediated. The involvement of opioid receptors in this phenomenon is confirmed in the present study in mice by the ability of naltrexone, 100 micrograms administered intracerebroventricularly (i.c.v.), to completely block the decrease in hepatic glutathione induced by an i.c.v. injection of 100 micrograms of morphine. Intracerebroventricular administration of the selective mu (mu) opioid receptor agonist, (D-Ala2,N-MePhe4,Gly-ol5)enkephalin (DAGO; 25-50 micrograms), or the selective delta (delta) opioid agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE; 3-50 micrograms), like morphine, produced significant decreases in hepatic glutathione 3 h after administration. The selective kappa (kappa) opioid receptor agonists, ethylketocyclazocine (1-30 micrograms) and trans-(+/-)3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide-methane sulfonate (U50 488; 10-300 micrograms), as well as the selective sigma (sigma) opioid agonists, phencyclidine (PCP; 50-300 micrograms) and N-allylnormetazocine (SKF 10,047; 1-30 micrograms), had no effect on the concentrations of glutathione in the liver. It appears from these data that stimulation of mu- or delta-, but not kappa- or sigma-opioid receptors within the central nervous system results in a loss of hepatocellular glutathione.

Behavioral dependence on phencyclidine in rats

The abuse of PCP continues to be an important medical problem in many urban areas. The probability that dependence on PCP may contribute to its compulsive use and relapse is supported by animal studies demonstrating its dependence liability. In the present study, five rats were housed in operant chambers and trained to respond on a lever under a fixed-ratio 30 schedule of food presentation. They obtained all their daily food during four 30-min response periods occurring every 6 hr. After stable baselines of behavior were established the rats were injected with PCP (3.0-7.5 mg/kg/injection), i.p., 1 hr before each response session for 7-10 days. Following chronic dosing, the drug injections were replaced with saline injections for 10 days. Disruptions in behavior were observed upon cessation of relatively brief chronic exposure to PCP (as little as 7 days) and at relatively low doses (5.6 mg/kg/6 hr = 22.4 mg/kg/day). The behavioral disruption was not accompanied by overt signs of abstinence and persisted for up to 48 hr.

Interactions between phencyclidine and central nervous system depressants evaluated in mice and rats

The effects of phencyclidine (PCP) alone and in combination with the CNS depressants, pentobarbital (PB) or ethanol (ETOH), were determined in mice using the inverted screen test and in rats using disruption of milk drinking behavior. The effects of PB and ETOH alone, and in combination, were also determined so that the PCP combinations could be compared to this clinically relevant interaction. These homergic drug interactions were analyzed using the dose-addition model by isobolographic analyses. Most drug combinations resulted in shifts to the left of the dose-effect curves relative to the dose-effect curves for the drugs alone; in no cases were shifts to the right (antagonism) observed. In general, the interactions between PCP and ETOH or PB were quantitatively less (infra-additive) than the interaction between the CNS depressants (dose-additive) when studied in mice. In the rat studies, the interactions between PCP and ETOH or PB were, overall, quantitatively greater (dose-additive or supra-additive) than the ETOH-PB interactions (infra-additive). Since even infra-additive interactions may result in substantially enhanced effects, these results suggest that coabuse of PCP with CNS depressant drugs could produce marked behavioral toxicity.

The discriminative stimulus properties of barbiturate stereoisomers

Pigeons were trained to discriminate an IM injection of racemic pentobarbital sodium (5.0 mg/kg) from saline under a second-order color-tracking schedule of mixed grain presentation. In a time-course study, maximal pentobarbital-appropriate responding occurred 15 min after administration of 5.0 mg/kg racemic pentobarbital sodium, the pretreatment time used for subsequent experiments. Racemic pentobarbital sodium, the R(+) and S(-) isomers of pentobarbital, racemic secobarbital sodium, and the R(+) and S(-) isomers of secobarbital all produced dose-dependent increases in pentobarbital-appropriate responding. Racemic secobarbital sodium, the secobarbital isomers, and the R(+) isomer of pentobarbital were equipotent to each other and slightly less potent than racemic pentobarbital and the S(-)-pentobarbital isomer in this regard. Except for R(+)-pentobarbital, all barbiturates caused dose-dependent decreases in response rate over the dose range tested.

Quantitative analysis of naloxone antagonism of the discriminative stimulus properties of morphine in the pigeon

Pigeons were trained to discriminate morphine (5.0 mg/kg) from saline under a second-order fixed ratio 10 (fixed-ratio 5) color-tracking schedule for food reinforcement. After reliable stimulus control was established, cumulative graded doses of morphine (0.3-30.0 mg/kg) were tested and resulted in dose-dependent increases in morphine-appropriate key pecking and decreases in response rate. Cumulative doses of naloxone (0.1-10.0 mg/kg) or consecutive injections of saline did not elicit morphine-appropriate responding or affect response rate. Pre-treatment with naloxone (0.1-1.0 mg/kg) before determination of cumulative dose-effect curves for morphine caused the morphine generalization curves to be shifted, in a parallel manner, rightward. Dose-ratio analysis of naloxone antagonism of morphine generalization, using a Schild plot with the slope constrained to -1, gave an apparent pA2 value (95% confidence limits) of 6.53 (6.18-6.89).

Approaches to the study of drug interactions in behavioral pharmacology

This paper discusses some of the approaches which have been used for analyzing drug interactions with an emphasis on applications for behavioral pharmacology. Two broad categories of drug interactions are defined: heterergic, when only one of the drugs is active in the behavioral measure employed, and homergic, when both of the interacting drugs have similar action. Two distinct models are presented for homergic drug interactions. The effect-addition model predicts that the combined action of two drugs is equal to the arithmetic sum of the individual effects. This is referred to as effect-additive and deviations from the predicted effects are described accordingly. The dose-addition model takes both dose and effect into account, and thus has a sounder theoretical basis. Leftward shifts in the dose-effect curves are described as equal to (dose-additive), or greater than (supra-additive) or less than (infra-additive) predicted on the basis of the relative potencies of the interacting drugs. Isobolographic methods facilitate data reduction and allow a graphic depiction of dose-addition analysis. A survey of the literature utilizing isobolographic techniques is presented.

The effects of acute and repeated clonidine administration on fixed-interval performance in rhesus monkeys

The effects of clonidine before, during and after 95 days of chronic administration of 100 micrograms/kg/day clonidine were investigated in five rhesus monkeys which had been trained to lever press under a fixed interval 5-min schedule of food presentation. The effects of clonidine (3-100 micrograms/kg) were generally to cause dose-related decreases in rates of responding and quarter-life, and increases in the line slopes of rate-dependency plots. Before chronic treatment, 200 micrograms/kg of clonidine caused somewhat less response rate suppression than 100 micrograms/kg. During chronic treatment, one monkey showed some development of tolerance to the rate-suppressant effects and another showed tolerance to the quarter-life effects, but the group as a whole did not show evidence of tolerance development to these effects of clonidine. Following cessation of chronic treatment, no overt signs of dependence were observed, however, some subjects did show disruption of schedule-controlled performance during the first week. In general, however, no consistent evidence for the development of tolerance or dependence to clonidine were observed under these dosing conditions.

Discriminative stimulus properties and brain distribution of phencyclidine in rats following administration by injection and smoke inhalation

Four male Sprague-Dawley rats were trained to discriminate IP injections of 3.0 mg/kg phencyclidine (PCP) from saline under a 2-lever fixed-ratio 32 schedule of food presentation. After reliable discriminative control of lever choice was established, other doses of injected PCP were tested resulting in dose-dependent increases in PCP-lever selection and dose-dependent decreases in rates of responding. When doses of PCP were administered by exposure to smoke from cigarettes containing PCP, a dose-dependent increase in PCP-lever responding was also observed. delta 9-Tetrahydrocannabinol administered via smoke exposure, up to doses which markedly suppressed response rates, did not result in PCP-appropriate responding, demonstrating the specificity of the PCP stimulus by the inhalation route. Brain levels and distribution of 3H-PCP were determined in rats administered doses calculated to result in 50% generalization by the IP injection or smoke inhalation routes. By both routes of administration roughly equivalent brain levels were attained and the distribution was relatively even across the seven brain areas analyzed. These results demonstrate the validity of using the injection route of administration when studying PCP experimentally, in spite of the fact that PCP is abused primarily by smoking.

Reinforcing properties of clonidine in rhesus monkeys

Intravenous clonidine self-administration was studied in rhesus monkeys under conditions of limited and unlimited access. Limited access consisted of daily 2-h experimental sessions with drug available on a fixed ratio 10 schedule. For unlimited access, drug was available 23 h/day with each response resulting in an injection. In all animals under both conditions, responding was maintained at levels that were above those maintained by saline injections at doses between 0.3 and 10 microgram/kg/inj, and the number of injections taken per session depended upon the dose. Under conditions of limited access, peak self-administration rates varied between animals but averaged approximately 30 inj/session. Total session intake was occasionally in excess of 1.0 mg/kg. Under conditions of unlimited access animals frequently self-administered more than 300 inj/day and intakes averaging 3.6 mg/kg/day occurred at the highest dose tested (10 microgram/kg/inj). When saline was substituted for clonidine after periods of clonidine access that ranged from 10-40 days, withdrawal signs included facial flushing, refusal of preferred food, restlessness, salivation, and emesis. These signs could be reversed with IV clonidine but could not be reliably precipitated with IV naloxone.