The effect of zolpidem on operant behavior and its relation to pharmacokinetics after intravenous and subcutaneous administration: concentration-effect relations

We characterized the effects of i.v. and s.c. zolpidem (1-8 mg/kg) under a differential reinforcement of low-rate schedule (i.e. DRL 45 s) in 3-hour sessions. Both behavioral and pharmacokinetic-pharmacodynamic analyses were used with the intent to compare the effects of zolpidem with those of benzodiazepines reported previously under the same behavioral paradigm. Zolpidem increased the shorter-response [inter-response times (IRTs)<45 s] rate and decreased the reinforcement rate in a dose- and time-related fashion. The behavioral profiles of zolpidem were mainly similar to those of benzodiazepines, except zolpidem produced far fewer shorter IRT responses. Pharmacokinetically, zolpidem decays biexponentially with distributional and terminal elimination half-lives of 5.2 and 42 min, respectively. The absorption rate constant and absolute bioavailability for s.c. zolpidem were 0.083/min and of 84.1%, respectively. The pharmacodynamic parameters for the reinforcement rate, an index of timing performance, were determined by integration of behavioral and pharmacokinetic profiles in a between-subject design using the effect-linked inhibitory sigmoidal E(max) model. The pharmacokinetic-pharmacodynamic analysis revealed that the potency of zolpidem (concentration required to produce 50% maximal effects, IC(50)) in disrupting the timing performance was 0.129 microg/ml. The pharmacodynamic estimates of zolpidem were compared to our previous results for benzodiazepines.

Arteriovenous serum cocaine concentration difference after intravenous bolus injection and constant-rate infusions: relation to pharmacodynamic estimates in rats

We characterized the pharmacokinetics of cocaine using both arterial and venous serum data after a bolus dose (2 mg/kg) and two constant-rate infusions (12.24 and 24.48 microg/min) for 2 h in rats. A published behavioral effect was used to investigate the effects of arteriovenous serum concentration differences on pharmacodynamic estimates for the 2 mg/kg dose. Significant temporal arteriovenous serum cocaine and benzoylecgonine (the major metabolite) concentration differences existed after cocaine administrations. The AUCs for arterial serum data were greater than the AUCs for venous data, indicating that cocaine was metabolized more extensively in the venous sampling site. Cocaine's behavioral effect could be directly related to serum concentrations with no hysteresis observed between the effects and arterial or venous serum concentrations. The pharmacodynamic estimates derived from arterial serum data approximated those from the venous data due to the most decline of cocaine's effect occurred in the elimination phase during which serum cocaine concentrations were not significantly different between the two sampling sites.

Simultaneous pharmacokinetic modeling of cocaine and its metabolites, norcocaine and benzoylecgonine, after intravenous and oral administration in rats

To accurately assess the mechanism of involvement of the active metabolite norcocaine in the effects of oral cocaine, it is essential to determine the rate and extent of the formation of norcocaine. Although this study was designed specifically for this aim, it was also of interest to characterize the metabolite kinetics of benzoylecgonine for comparative purpose. We first characterized the pharmacokinetics of cocaine, norcocaine, and benzoylecgonine by the i.v. route of administration; all three drugs decayed biexponentially. These pharmacokinetic estimates were then used for determination of the formation of norcocaine and benzoylecgonine after i.v. and p.o. (20-40 mg/kg) cocaine administration. Although t(1/2alpha), and t(1/2beta) were similar across the three compounds, the values of volume of distribution in the central compartment and clearance for benzoylecgonine were much smaller than those of cocaine and norcocaine. Norcocaine was not detected following i.v. cocaine; however, serum norcocaine concentrations were as high as those of oral cocaine. Both routes of cocaine administration produced benzoylecgonine. A pharmacokinetic model for the metabolite kinetics was proposed by sequentially adding the models that most adequately described the formation of each metabolite to the model of cocaine. For oral cocaine, the absolute bioavailability was 3.48%, whereas 6.04 and 2.26% of cocaine were converted to benzoylecgonine and norcocaine, respectively, during first-pass absorption regardless of dose. Furthermore, the majority of norcocaine and 92% of benzoylecgonine were formed during the first-pass absorption, leaving 8% of benzoylecgonine produced in systemic circulation. The profile of norcocaine as a metabolite confirmed the involvement of norcocaine in cocaine's behavioral effects.

Spontaneous activity as a contingency-controlled behavior within an operant context: alprazolam concentration-effect relations after subcutaneous administration in rats

RATIONALE

Environmental factors affect serum drug concentration-effect relations. For example, after midazolam administration, longer pre-session delays imposed in experimental chambers produced differential concentration-effect relations compared to those of shorter delays.

OBJECTIVES

To evaluate the extent to which serum concentrations determine alprazolam's effects on spontaneous activity in the presence and absence of a differential reinforcement of low rate (DRL 45-s) contingency using pharmacokinetic-pharmacodynamic analysis. Serum concentrations reported here were simulated from our published pharmacokinetic parameters for alprazolam.

METHODS

One group (n=8) was used to investigate alprazolam's effects on spontaneous activity within the DRL contingency by placing an activity platform beneath each operant chamber to monitor concurrently both spontaneous activity (large and small movements) and DRL performance (shorter-response and reinforcement rates) in 3-h sessions; a parallel group (n=7) was used without the operant context. The concentration-effect relation of the reinforcement rate was compared and contrasted with those of spontaneous activity.

RESULTS

Alprazolam decreased large and small movements within the DRL contingency, which corresponded to that of reinforcement rates under the DRL 45-s schedule. In contrast, without the DRL contingency, alprazolam's effects on small movements were short-lived (i.e., 30 min) and no effects on large movements were detected. Hence, the predicted concentration-effect relations for the reinforcement rate function described those of spontaneous activity well within the operant context, but not those without the operant context. Furthermore, the latter showed no correlation between serum alprazolam concentration and large movements; a significant, but low negative correlation for small movements was observed.

CONCLUSIONS

The duration of alprazolam's action was dependent on not only dose size but also the behavioral measure examined. By imposing the DRL contingency, spontaneous activity behaves as an ideal pharmacodynamic measure (i.e., continuous, sensitive, and objective).

Two procedures establishing preference for oral cocaine and lidocaine solutions which do not use an associative history with a reinforcer

Groups of rats were exposed to daily, 3-h, fixed-time 1-min food-pellet delivery sessions, a procedure that produces overdrinking (schedule-induced polydipsia, SIP). Previous research demonstrated that rats drinking a drug or non-drug solution come to prefer that solution to water if the solution had (a) a past association with either a highly acceptable vehicle (e.g. glucose/saccharin), or (b) allowed rats to eschew drinking an unacceptable solution under SIP conditions. The present experiments show that under the solution-avoidance procedure, preference for a concurrent, alternative solution (0.19 mg/ml lidocaine or 0.24 mg/ml cocaine) occurred when a concentrated quinine solution alternative was either abruptly removed or faded. A concentrated cocaine solution, however, was minimally effective in producing a preference for 0.19 mg/ml lidocaine to water when cocaine concentration was faded. Flavor/nutrient-conditioning (conditioned reinforcement) and solution-eschewing (avoidance) procedures may throw light on the kinds of historical situations that determine the genesis of stable preferences for drugs and other substances.

Negligible tolerance produced by chronic intravenous alprazolam administration: a low serum drug concentration effect

In 3-hr sessions, the authors investigated the onset, peak, and disappearance of the effects of alprazolam on performance under a differential reinforcement of low rate 45-s schedule in rats. Alprazolam was administered chronically as a daily bolus dose (2 mg/kg) via the intravenous route. Alprazolam decreased the reinforcement rate and increased the shorter response (nonreinforced) rate in a dose- and time-related fashion. Tolerance did not develop to the decreases in reinforcement rate; tolerance to increases in shorter response rate was negligible, occurring only at the low-concentration range. Clinically, an optimal dose regimen should be designed to avoid the tolerance development that occurs in the low serum benzodiazepine concentration range.

Contribution of the active metabolite, norcocaine, to cocaine's effects after intravenous and oral administration in rats: pharmacodynamics

RATIONALE

Oral cocaine is more effective than IV cocaine by pharmacokinetic and pharmacodynamic analysis. One explanation is involvement of the active metabolite, norcocaine, in cocaine's effects.

OBJECTIVES

To evaluate norcocaine's contribution to oral cocaine's effects, norcocaine's effects as a parent compound were determined and compared to those of cocaine using a differential reinforcement of low rate (DRL 45-s) schedule and spontaneous activity (large and small movements) after IV and PO routes of administration.

METHODS

The effects of cocaine and norcocaine on DRL performance (shorter-response and reinforcement rates) and spontaneous activity were investigated in 3-h sessions. The changes in effects across time (effect-time profiles) and dose-response curves (DRCs) were constructed to evaluate the duration of action and potency (ED50) of both drugs.

RESULTS

Under the DRL 45-s schedule, effect-time profiles showed both drugs via the two routes of administration significantly increasing and decreasing shorter-response rates and reinforcement rates, respectively. However, cocaine produced greater effects on shorterresponse rates than norcocaine, while both drugs produced comparable effects on reinforcement rates. For spontaneous activity, although IV cocaine, PO cocaine, and PO norcocaine dose- and time-dependently increased spontaneous activity, cocaine's effects were more profound than those of norcocaine. Effect-time profiles revealed that the duration of drug action was a function of dose, route, and behavioral paradigm used. According to ED50 values, IV cocaine was more effective than PO cocaine; however, PO cocaine was more effective than IV cocaine as judged by ED50 values corrected for absolute oral bioavailability.

CONCLUSIONS

Norcocaine's contribution to oral cocaine's effects on DRL performance is evident. Other mechanism(s), such as a greater acute tolerance to IV cocaine's effects than to PO cocaine's effects, can be excluded.

Oral cocaine pharmacokinetics and pharmacodynamics in a cumulative-dose regimen: pharmacokinetic-pharmacodynamic modeling of concurrent operant and spontaneous behavior within an operant context

Despite wide use of cumulative-dosing procedures to evaluate dose-response relations, limited attention has been paid to investigating drug concentration-effect relations. We first characterized the pharmacokinetic (PK) parameters for i.v. (2 mg/kg) and oral cocaine (20 and 40 mg/kg) in rats. Cocaine's concentration-time profile for the escalating cumulative-dose regimen was simulated from PK parameters, dose size (1, 2, 7, 20, and 45 mg/kg by the oral route), and dosing interval (tau, 35 min) as well as validated from blood sampling at various time points. This concentration-time profile was integrated with pharmacodynamic (PD) profiles of differential reinforcement of low rate performance and spontaneous activity (large and small movements) under a differential reinforcement of low rate 45-s schedule. Effects on three behavioral measures were characterized by integrated PK-PD models using the sigmoid E(max) (for increases in shorter response rate or large movements) and inhibitory E(max) (for decreases in density of reinforcement) models. But for the intrinsic differences in baseline and efficacy values among the behavioral endpoints, one set of PD parameters (i.e., potency and Hill factors) predicted concentration-effect relations for the three behavioral indices across all five doses. Concurrent monitoring of operant and spontaneous activity behavior within an operant context provides a novel behavioral paradigm to investigate drug effects on spontaneous activity under conditions where a behavioral contingency exists. Additionally, a cumulative-dosing procedure is efficient for determining the entire dose-response relation and provides an ideal mode to study phenomena such as sensitization or tolerance by varying dose size and/or tau.

High-performance liquid chromatographic determination of cocaine and its metabolites in serum microsamples with fluorimetric detection and its application to pharmacokinetics in rats

A sensitive, selective and simple HPLC method with fluorimetric detection is described for quantitating cocaine and its three metabolites in rat serum microsamples (50 microl). Chromatographic separation is achieved on a Hypersil BDS C18 column (100X2.1 mm, 5 microm) with an isocratic mobile phase consisting of methanol-acetonitrile-25.8 mM sodium acetate buffer, pH 2.6, containing 1.0 x 10(-4) M tetrabutylammonium phosphate (14:10:76, v/v/v). The detection limit (0.5 ng/ml) for all the compounds, using direct fluorometric detection operated at excitation and emission wavelengths of 230 and 315 nm, respectively, was approximately five-times lower than that of using a UV detector operated at 235 nm. The effects of ratio of 2-propanol to chloroform in extraction solvents on the recovery and precision for cocaine and its metabolites were systematically examined. The method was used to study the pharmacokinetics of cocaine after administration of intravenous 2 mg/kg and oral 20 mg/kg doses.

Intravenous and oral clozapine pharmacokinetics, pharmacodynamics, and concentration-effect relations: acute tolerance

We examined the pharmacokinetics and pharmacodynamics of intravenous (1-5 mg/kg) and oral clozapine (2.5-10 mg/kg) in rats (terminal half-life=81.8 min; oral bioavailability=5.32%). Both dose- and concentration-effect relations of clozapine were characterized. Clozapine's effects were similar to those of benzodiazepines because of the similarity in effect-time profiles between the two classes of drugs. The IC(50) value increased as a function of dose; consequently, clozapine's relative potency decreased linearly with the logarithm of AUC((0-infinity)), or bioavailable dose regardless of route of administration. The IC(50) is an index for the sensitivity of behavioral performance to clozapine; relative potency provides an index for estimating the extent of acute tolerance. As IC(50) increases, relative potency decreases, and consequently, acute tolerance increases. Our results demonstrated that greater acute tolerance was observed for i.v. clozapine than for p.o. clozapine; however, clozapine exhibited a single concentration-effect relation across dose and route of administration after correcting for relative potencies.

Post-injection delays in experimental chambers, but not in home cages, produce both sensitization and tolerance of operant behaviour to midazolam: relation to pharmacokinetics

The effects of post-injection delay time and environmental context on behaviour after subcutaneous administration of 3 mg/kg midazolam were investigated under a differential reinforcement of low rate schedule (i.e. DRL 45 s) in 3 h sessions. Post-injection delays were varied (0-120 min) for two groups of rats placed in either the experimental chamber (group 1) or home cage (group 2) during the pre-session delay times. Midazolam increased shorter-response (inter-response times < 45 s) rates and decreased reinforcement rates in a time-related manner. Reinforcement rate-time profiles were also integrated with parallel pharmacokinetics. Post-injection delays in either environment yielded performances that mirrored the pharmacokinetic profile operative at the corresponding time-delay points. At higher concentrations (> 0.12 microg/ml) the pharmacokinetics of midazolam largely determined the behavioural effects in both groups, regardless of post-injection delays. However, at lower drug concentrations, longer post-injection delays (> 60 min) in the experimental chamber produced both sensitization and tolerance, as measured by greater increases in shorter-response rates and a more rapid return of the reinforcement rate, respectively. Interaction of the discriminative stimulus effects of midazolam with the context probably alters the magnitude of behavioural effects when the delay occurs in the experimental chambers, whereas no such interaction is present in group 2. The DRL schedule with post-injection delays in experimental chambers provides a useful behavioural paradigm for studying both sensitization and tolerance.

Dose independent pharmacokinetics of caffeine after intravenous administration under a chronic food-limited regimen

Several studies have shown that caffeine follows non-linear pharmacokinetics in both rats and humans. Recent data have demonstrated that caffeine may following linear pharmacokinetics when administered orally and intraperitoneally to food-limited rats. In this study the pharmacokinetics of caffeine was analyzed following intravenous (i.v.) administration to rats under a food-limited regimen. Four rats were administered four doses of caffeine and a standard dose of the caffeine metabolites, paraxanthine, theobromine, and theophylline. Caffeine pharmacokinetic parameters were dose independent following intravenous doses ranging from 1 to 20 mg/kg. Furthermore, the caffeine area under the curve (AUC) increased linearly as a function of dose. The mean fraction of caffeine converted to paraxanthine, theobromine, and theophylline was 16%, 16%, and 7%, respectively. The linear pharmacokinetics demonstrated in the present study may be attributed to the induction of hepatic metabolism under a chronic food-limited regimen.

Establishing preference for lidocaine solution to water: comparison between a fading and an abrupt-removal procedure for withdrawing a compound vehicle

Rats were exposed to daily, 3-h, fixed-time 1-min food-pellet delivery sessions, which is a procedure that produces overdrinking (schedule-induced polydipsia). In previous polydipsia studies, rats came to prefer solutions of drug or non-drug agents to concurrently presented water if the agents had first been offered in a glucose-saccharin vehicle that was slowly eliminated (faded), leaving a choice between a substance in water vehicle vs water. In the first experiment, a more rapid vehicle-fading procedure was used to produce a preference for 0.19mg/ml lidocaine to water. In the second experiment, the vehicle was abruptly changed to water, which also resulted in a strong preference for lidocaine solution, although the lidocaine solution volumes ingested for the final sessions were significantly less than in the first experiment. The results are consistent with a conditioned flavor/nutrient preference interpretation for the institution of the lidocaine preference in both experiments. Although flavor/nutrient conditioning can be a sufficient condition for generating a substance preference, a previous experiment showed that it was not a necessary condition.

Determination of zolpidem in serum microsamples by high-performance liquid chromatography and its application to pharmacokinetics in rats

A single-solvent extraction step high-performance liquid chromatographic method is described for quantitating zolpidem in rat serum microsamples (50 microl). The separation used a 2.1 mm I.D. reversed-phase OD-5-100 C18 column, 5 microm particle size with an isocratic mobile phase consisting of methanol-acetonitrile-26 mM sodium acetate buffer (adjusted to pH 2.0 with 40% phosphoric acid) containing 0.26 mM tetrabutylammonium phosphate (13:10:77, v/v/v). The detection limit was 3 ng/ml for zolpidem using an ultraviolet detector operated at 240 nm. The recovery was greater than 87% with analysis performed in 12 min. The method is simple, rapid, and applicable to pharmacokinetic studies of zolpidem after administering two intravenous bolus doses (1 and 4 mg/kg) in rats.

Pharmacokinetic determinants of cocaine's differential effects on locomotor and operant behavior

Dose-response, effect-time and concentration-effect relations of intravenous cocaine (1-4 mg/kg) were investigated on contingency-controlled [fixed-ratio (FR) 70 performance] and unconditioned (locomotor activity) behaviors. Cocaine dose-response curves exhibited decreasing rates of response under the FR 70 schedule but increasing locomotor activity in a dose-related fashion. Effect-time profiles confirmed that these changes were time-dependent and provided additional clarity by mirroring the biexponential decay of cocaine concentrations with time. The duration of action of cocaine was comparatively shorter on locomotor activity than on FR performance. We integrated effect-time profiles of the two behaviors with concentration-time profiles simulated from our previously published pharmacokinetic parameters to derive cocaine's pharmacodynamic parameters. Classical inhibitory Emax and sigmoidal Emax models were used to describe cocaine's effects on FR performance and locomotor activity, respectively. Simultaneous pharmacokinetic-pharmacodynamic modeling reveals evidence of acute tolerance to cocaine in locomotor activity, as indicated by decreasing potency with dose, but not in contingency-controlled behavior.

A double-peak phenomenon in the pharmacokinetics of alprazolam after oral administration

The pharmacokinetics of alprazolam (ALP) after i.v. and p.o. administration in rats were characterized. ALP decayed biexponentially after the i.v. dose (1.25 mg/kg), but the concentration-time profiles after the p.o. doses (7 and 12.5 mg/kg) exhibited a double-peak phenomenon. The presence of two peaks was confirmed by statistical analysis of the serum concentration data of ALP, as well as by observed double peaks in the serum concentration-time profiles of the two active metabolites (alpha-hydroxyalprazolam and 4-hydroxyalprazolam). An absorption model incorporating a delay site is proposed to describe the data, and the absolute oral bioavailability is estimated to be about 30%. The two peaks were approximately 80 to 115 min apart, and there was a delay in the absorption of close to 80% of oral ALP, regardless of dose. We hypothesize that the mechanism underlying the double-peak phenomenon is due to reduction in gastric motility caused by the muscle relaxant effect of ALP. This hypothesis is supported by the observed longer delay in the appearance of the second peak at the higher p.o. dose. Enterohepatic recycling is precluded from being the underlying mechanism, because of the presence of double peaks after the p.o. doses but not after the i.v. dose. This is the first reported case of double peaks for oral ALP, and this phenomenon has not been reported for other benzodiazepines. The double-peak phenomenon caused by the hypothesized mechanism may have important therapeutic and drug interaction implications, especially because benzodiazepines are commonly coadministered with other drugs.

Cocaine pharmacodynamics after intravenous and oral administration in rats: relation to pharmacokinetics

RATIONALE

To design optimal dose regimes for oral cocaine, it is essential to characterize pharmacokinetics (PK) of cocaine after IV and PO administration.

OBJECTIVES

To investigate the absolute bioavailability of oral cocaine, its effectiveness and the relation between PK and PD in a within-subject design.

METHODS

We used the effects of IV and PO cocaine on a contingency-controlled timing behavior, the differential reinforcement of low rate schedule (DRL 45-s) in 3-h sessions, as the PD measures [i.e., the shorter-response rate (srr) and the reinforcement rate (rr)]. Cocaine PK parameters were determined by simultaneous modeling of the concentration-time profiles (CTPs) after IV 2 mg/kg and PO 20 mg/kg cocaine administration. The absolute oral cocaine bioavailability was determined pharmacokinetically (F) and pharmacodynamically (Fsrr and Frr).

RESULTS

IV and PO cocaine increased the shorter response rate and decreased the reinforcement rate in a dose- and time-related fashion, which mirrored the respective prototypical serum cocaine CTPs. After the absorption phase, the serum cocaine CTP of PO cocaine paralleled that of IV cocaine. The duration of action for PO cocaine was longer than that for IV cocaine owing to its larger mean residence time. The active metabolite, norcocaine, was not detected after IV but after PO cocaine administration. The value of F was 4.66% which was significantly lower than the values of Fsrr (13.67%) and Frr (32.63%). Furthermore, the concentration-effect relations for the reinforcement rate revealed that PO cocaine was more potent than IV cocaine.

CONCLUSIONS

Oral cocaine is more effective behaviorally than from predictions made in terms of its PK. The differences in active metabolite profiles as well as the rate and extent of acute tolerance for IV versus PO cocaine may account for the greater potency observed for oral cocaine.

Within-subject variability in cocaine pharmacokinetics and pharmacodynamics after intraperitoneal compared with intravenous cocaine administration

Performance in rats (Rattus norvegicus) was measured on a differential reinforcement of low-rate schedule (DRL 45-s) in 1.5-hr sessions after 2 mg/kg intravenous (i.v.) or 10-20 mg/kg intraperitoneal (i.p.) cocaine administration, with each dose given twice and separated by 3-5 days. For successive i.v. doses, cocaine effects were similar, with minimal within-subject variability. For i.p. cocaine, the effects were not always similar; performance was variable and sometimes remained at baseline level. These diminished effects occurred following either the 1st or 2nd i.p. injection. A parallel pharmacokinetic study of cocaine confirmed that within-subject variability existed in cocaine concentration-time profiles after i.p. cocaine, and that a low serum cocaine concentration-time profile could account for the diminished effects. The i.p. route for cocaine administration should be used with caution.

Establishing oral preference for quinine, phencyclidine and caffeine solutions in rats

Rats were given fixed-time, 1-min food-pellet delivery sessions, for 3 h every day, which resulted in over drinking (schedule-induced polydipsia). In previous research, groups of animals came to prefer solutions of cocaine or lidocaine to concurrently presented water, if the drugs were first offered in a glucose/saccharin vehicle, which was then gradually eliminated, so that the choice became a drug solution in water vehicle versus water. In the present experiment, the same procedure produced a preference for 0.025 mg/ml quinine solution to water, indicating that a bitter solution that was not a topical anesthetic agent could come to be preferred. After establishing preference for quinine solution, it was possible to gradually remove quinine while increasing phencyclidine (PCP) concentration to produce preference for 0.075 mg/ml PCP solution, and similarly to produce subsequent preference for 0.1 mg/ml caffeine solution, although not for all animals. For additional groups, drinking either 0.15 mg/ml PCP or 0.1 mg/ml caffeine, while gradually reducing the glucose/saccharin vehicle to water vehicle was a less successful procedure for establishing the respective drug preferences. The latter result suggests that first instituting a preference for quinine solution to water facilitated the later establishment of preferences for PCP and caffeine solutions.

Sensitization of operant behavior to oral cocaine with increasing- and repetitive-dose regimens

This study evaluated whether operant behavior was subject to sensitization upon repeated exposure to cocaine. The behavior of eight rats was reinforced by food-pellet delivery under a differential reinforcement of low-rate (DRL) 45-s schedule, in daily sessions of 190 min. Each session was composed of five 35-min subsessions, with each subsession preceded by a 3-min time out period (TO). For selected sessions, a cumulative dose-effect relation for cocaine, using increasing doses, was first determined by oral gavage administration of a dose during each subsession. Three such cycles were given, separated by between seven and ten sessions. Later, four further cycles were given, using a repetitive 10 mg/kg cocaine per os (p.o.) dose, rather than an increasing dose. Under both dosing procedures, within a cocaine cycle (session), shorter (<45 s) inter-response times (IRT) progressively increased with doses, and reinforced responses decreased. As cycles were repeated, the shorter IRT increases became progressively more marked, indicating the development of sensitization to the effect of cocaine. With the second procedure (repetitive 10 mg/kg doses), reinforcement rate decreases became more marked as cycles were repeated, but this change did not occur using the increasing-dose procedure. Upon later exposure to repetitive 5 mg/kg dosing cycles, none of these rate changes occurred, and performances were equivalent to those occurring under saline cycles. Exposure to cumulative dose regimens of oral cocaine can result in the sensitization of operant behavior to the effects of cocaine.

Pharmacokinetic-pharmacodynamic modeling of the psychomotor stimulant effect of cocaine after intravenous administration: timing performance deficits

We investigated dose-response cocaine pharmacokinetic and metabolite profiles in a within-subject design after intravenous bolus cocaine administration (1-4 mg/kg) in rats under a food-limited regimen. Cocaine was rapidly distributed (T1/2beta = 1.09 min) and eliminated (T1/2alpha = 14.93 min). Norcocaine was not detected. The free fraction of cocaine was 31.3-33.1% for serum cocaine concentrations of 0.5 to 1 microg/ml. Parallel pharmacodynamics was studied using performance on a contingency-controlled timing behavior, a differential reinforcement of low rate schedule (45 s) in 3-h sessions. Cocaine increased the shorter-response rate and decreased the density of reinforcement in a dose- and time-related fashion. The increased shorter-response rate is the stimulatory effect herein reported. The changes in shorter-response rate and the density of reinforcement were directly interpretable as functions of cocaine concentrations in the respective hypothetical effect compartments by using sigmoidal Emax and inhibitory Emax models, respectively. Because the concentration at half of Emax for the shorter-response rate (EC50 = 0.467 microg/ml) was greater than that for density of reinforcement (IC50 = 0.070 microg/ml), the former began to return toward baseline sooner than the latter. Only as cocaine concentration decreased to values smaller than the EC50 did the density of reinforcement begin to return toward baseline. Thus, the density of reinforcement is an index for evaluating the deficit in timing performance. The concentration-effect plot confirmed that the intensity of the effects of cocaine depends solely on concentration regardless of the dose. These results demonstrated that the pharmacokinetic-pharmacodynamic analysis allows the identification of the stimulant action of cocaine, which in turn delineates its consequence on timing performance.

Determination of clozapine and its metabolite, N-desmethylclozapine, in serum microsamples by high-performance liquid chromatography and its application to pharmacokinetics in rats

A single solvent extraction step high-performance liquid chromatographic method is described for quantitating clozapine and its metabolite, N-desmethylclozapine, in rat serum microsamples (50 microl). The separation used a 2.1-mm I.D. reversed-phase Symmetry C18 column with an isocratic mobile phase consisting of methanol-acetonitrile-28.6 mM sodium acetate buffer, pH 2.6 (10:20:70, v/v/v). The detection limit was 2.5 ng/ml for all the compounds using an ultraviolet detector operated at 230 nm. The method was used to study the pharmacokinetics of clozapine after an intravenous bolus dose (2.5 mg/kg).

Caffeine has similar pharmacokinetics and behavioral effects via the i.p. and p.o. routes of administration

Caffeine administered intraperitoneally (i.p.) or orally (p.o.) decreased the reinforcement rate and increased the nonreinforced response rate in a dose-related fashion under a differential reinforcement of low rate schedule (DRL 45-s) in 3-h sessions. These effects were similar following both routes of caffeine administration. The parallel pharmacokinetics for i.p. and p.o. caffeine were each determined and related to the respective effects of caffeine on reinforcement rate. Serum caffeine concentrations were similar across the session after the absorption phase for a given dose. Consequently, the effect remained in approximately the same range within a dose, and no single dose possessed a full concentration-effect relation for the two routes. The effects of i.p. and p.o. caffeine on reinforcement rate plateaued at doses higher than 40 mg/kg, which produced a serum caffeine concentration of approximately 25 microg/ml regardless of the route of administration. The EC50 values were 7.34 and 9.93 microg/ml for i.p. and p.o. caffeine, respectively. This study as well as our previous studies demonstrated that the i.p. route is dependable for studying caffeine dose response relations but not for studying other drugs (e.g., midazolam). The possible mechanism accounting for this difference is discussed.

Pharmacokinetic-pharmacodynamic modeling of the coexistence of stimulatory and sedative components for midazolam

Midazolam increased the shorter-response rate and decreased the reinforcement rate of a contingency-controlled timing behavior--a differential-reinforcement-of-low-rate 45-s schedule. The responding rate changes observed were immediately interpretable as functions of midazolam concentration during a 3-h session--a period for investigating the onset, peak, and disappearance of midazolam effect--in rats. That the midazolam pharmacokinetic-pharmacodynamic model was a direct application of our alprazolam pharmacokinetic-pharmacodynamic model implies that both drugs exhibit similar pharmacological effects. The two peaks of the shorter-response rate increases produced by midazolam were modeled as a stimulation-sedation model that consisted of two opposing effect-link sigmoidal Emax functions. The stimulation-sedation model suggested that midazolam possesses both stimulatory and sedative effects in a continuous but sequential fashion, and hypothesizes the coexistence of stimulation and sedation components for midazolam; this model may help delineate possible mechanisms for rebound side effects and of tolerance in humans. The reinforcement rate was, then, an index for evaluating the deficit in timing performance.

Independent interaction of alprazolam and caffeine under chronic dose regimens on differential reinforcement of low-rate (DRL 45-s) performance

In previous research, we found an independent interaction of alprazolam and caffeine in rats under acute dose regimens using two measures (reinforcement rate and shorter-response rate) of a differential reinforcement of low rate performance (DRL 45-s) in 3-h sessions. Applying the same behavioral endpoints, the present study investigated the alprazolam-caffeine interaction under chronic dose regimens. Both drugs were administered by the oral route. Acute alprazolam and caffeine dose-response curves (DRCs) were characterized and were then used to determine the maintenance dose for the respective chronic dose regimens. Both drugs decreased the reinforcement rate and increased the shorter-response rate in a dose-related fashion. An alprazolam DRC also was determined during chronic-caffeine, chronic-alprazolam, and concurrent chronic-caffeine-alprazolam dose regimens. Complete tolerance to caffeine-induced rate changes was observed on the second day. Incomplete tolerance occurred only at higher alprazolam doses (7-12.5 mg/kg). Cross tolerance was not found between alprazolam and caffeine. Upon discontinuation of both drugs, performance progressively returned to baseline. The four alprazolam DRCs as well as the effect-time profiles demonstrated that caffeine altered neither the magnitudes nor the patterns of alprazolam effects on the two rates under chronic dose regimens. The Pöch DRC method further confirmed the independent interaction of alprazolam and caffeine. Thus, the independence of the interaction held for both the acute and chronic dose regimens despite the development of tolerance in the latter regimens.

Pharmacokinetic-pharmacodynamic modeling of stimulatory and sedative effects of alprazolam: timing performance deficits

Alprazolam decreased the reinforcement rate and increased the shorter-response rate of contingency-controlled timing behavior under a differential reinforcement of low-rate schedule (DRL 45-s) in rats. An integrated pharmacokinetic-pharmacodynamic (PK-PD) model was developed to describe and characterize the effects of i.v. and s. c. administration of alprazolam. The onset, peak and disappearance of alprazolam effects were evaluated during a 3-hr session. After s. c. alprazolam administration, two peak increases in shorter-response rate occurred at moderate alprazolam serum levels, first in the ascending and then in the descending limb of the concentration-time profile. We used a stimulation-sedation PD model incorporating two opposing effect-link sigmoidal Emax functions to model the two peaks after s.c. alprazolam administration. The model suggested that alprazolam possesses both stimulatory and sedative effects in a continuous but sequential fashion, which corresponded to low- and high-concentration effects as indicated by the EC50 values of 0.09 and 0.18 microg/ml, respectively. Owing to the rapid onset of i.v. administration, the first peak (a transition phase before the onset of the sedative effect) was absent, with the presence of the second peak again coinciding with the offset of the sedative effect. The reinforcement rate (IC50 = 0.02 microg/ml) characterized by the indirect response model to account for the initial hysteresis is an index for evaluating the deficit in timing performance. Although the effects of alprazolam can be described in behavioral terms, simultaneous PK-PD optimization numerically defines the performance and hypothesizes the coexistence of stimulation and sedation components for alprazolam. The stimulation-sedation model may help in delineating the possible mechanisms for adverse rebound side effects and of tolerance in humans.

Schedule induction conditions not only exaggerate intake but also enhance drug solution choice

In previous research, rats exposed to daily, 3 h sessions of schedule-induced polydipsia (SIP) self-administered high doses of cocaine orally. However, a strong and durable preference for cocaine solution to water requires training in addition to mere oral self-administration exposure. If cocaine is dissolved in a preferred vehicle solution, and the vehicle is subsequently faded to water, then a strong preference for cocaine remains. A similar preference can be instituted for lidocaine solution. Such preferences may develop because the gustatory property of a drug becomes associated with the preferred vehicle and remains to function as a durable conditioned reinforcer after vehicle fading. To determine if drug preference is solely a function of this posited conditioning mechanism, or whether it also depends upon the SIP condition, rats were exposed to daily, 3 h sessions of single-ration feeding, rather than the SIP condition. A preferred vehicle (glucose/saccharin solution) was slowly faded from a 0.19 mg/ml lidocaine solution, which was presented concurrently with a choice for water. Although a preference for lidocaine solution to water could be generated, it occurred for only 5 out of 9 rats, and the preference was relatively unstable. By contrast, in two previous studies using SIP, 26 out of 27 rats maintained a preference for lidocaine solution. Thus, SIP not only exaggerates the amount of drug solution ingested but also contributes to the fixation of the associative drug solution choice.

Establishing preference for oral cocaine without an associative history with a reinforcer

Using schedule-induced polydipsia (SIP), a stable, oral, preference for cocaine solution to water presented concurrently can be established by first presenting cocaine in a preferred vehicle, and subsequently fading the vehicle to water. The present study showed that rats exposed to a history of choosing 0.48 mg/ml cocaine in preference to concentrated lidocaine (2 mg/ml) under SIP conditions subsequently maintained this preference when the lidocaine concentration was progressively decreased to zero (water). Preference for lidocaine was tested by fixing it at 0.38 mg/ml (equimolar with 0.48 mg/ml cocaine) while subsequently increasing cocaine from 0.48 mg/ml to 2.5 mg/ml. Lidocaine preference did not occur, rather, preference for concentrated cocaine persisted. The results suggest that the avoidance of lidocaine solution may force the discrimination of the relation between high oral cocaine intake and its reinforcing effects, thereby instituting a stable preference for cocaine.

Determination of cocaine and its metabolites in serum microsamples by high-performance liquid chromatography and its application to pharmacokinetics in rats

A single-solvent extraction step high-performance liquid chromatographic method is described for quantitating cocaine and its three metabolites in rat serum microsamples (50 microl). The separation used a 2.1-mm I.D. reversed-phase Brownlee C18 column with an isocratic mobile phase consisting of methanol-acetonitrile-25.8 mM sodium acetate buffer, pH 2.2, containing 1.29-10(-4) M tetrabutylammonium phosphate (12.5:10:77.5, v/v/v). The detection limit was 2.5 ng/ml for all the compounds using an ultraviolet detector operated at 235 nm. The method was used to study the pharmacokinetics of cocaine after an intravenous (i.v.) bolus dose (4 mg/kg).

Differential reinforcement of low rate performance, pharmacokinetics and pharmacokinetic-pharmacodynamic modeling: independent interaction of alprazolam and caffeine

To investigate the interaction between alprazolam and caffeine, performance on a differential reinforcement of low-rate behavior schedule and the respective pharmacokinetics (PK) were explored in concurrent studies. Alprazolam PK was not altered by caffeine, but alprazolam retarded caffeine absorption indirectly, as inferred by the lack of i.v. drug administration PK interaction, thereby decreasing serum methylxanthine concentrations. Inasmuch as alprazolam was more potent and short-lived than caffeine in decreasing the reinforcement rate (consonant with their respective t(1/2) values, 0.44 and 3.1 hr), the alprazolam/caffeine potency ratio decreased across the session time, which determined the expression of the combined effects. Thus, the decreased methylxanthine level yielded slightly less disruption in performance for the observed combined effect, compared to the expected calculated effect, only near the end of a session. The interaction was PK linked and mainly not distinguishable from independence as indicated by the Poch dose-response curve method and the integration of PK and pharmacodynamics. The sigmoid maximal effect-link pharmacodynamic model indicated that caffeine did not alter the concentration at half of the maximal effect value of alprazolam and suggested that the interaction is not competitive, but independent. Although the nature of the benzodiazepine-methylxanthine interaction has been controversial in other behavioral studies, as is the role of PK in determining behavior, this and our previous study make it evident that the interaction is independent not only across doses and routes of administration, but also with respect to two indices of differential reinforcement of low rate performance.

Transparent and gauze dressings and their effect on infection rates of central venous catheters: a review of past and current literature

Infection has been identified as a potentially life-threatening complication of central venous catheterization. The type of dressing applied to the catheter site has been suggested to influence the incidence of catheter-related infection. Although numerous research studies were conducted before 1990 and in the past 5 years to compare the effect of gauze and transparent dressing on the infection rate of central venous catheters, results are conflicting. This literature review analyzes some of those efforts.

Tolerance to the anxiolytic effects of the triazolobenzodiazepines triazolam and alprazolam, as measured by the ingestion of a hypertonic sodium chloride solution in rats

Rats were adapted to a water-deprivation regimen that allowed daily 1-hr drinking sessions of a single fluid, either 1.5% NaCl solution or water. Oral, presession, acute doses of triazolam (0.05-1.6 mg/kg) or alprazolam (0.4-6.4 mg/kg) produced dose-related increases in session NaCl solution ingestion. The relative potencies found for these two drugs approximated their relative Ki values reported in the literature. The increased ingestion produced in this and other experiments by anxiolytic agents is presented as an alternative way of evaluating punishment attenuation, and hence anxiolytic activity. Chronic oral dosing (every 2nd day) with triazolam (0.2 mg/kg) or alprazolam (1.2 mg/kg) led, after several weeks, to partial, but surmountable, tolerance to the increased NaCl solution ingestion produced by these drugs, which was confirmed by a rightward shift in the dose-effect relations. During chronic dosing, no corresponding declines in the independently evaluated increases in water intake produced by these drugs occurred. Abrupt drug discontinuation produced a precipitous decrease in NaCl solution intake, with subsequent recovery. For triazolam, the initial discontinuation decrease in intake to below the original base line suggested the possible accrual of a mild physiological dependence under this moderate chronic dose regimen.

Pharmacokinetics and bioavailability of midazolam after intravenous, subcutaneous, intraperitoneal and oral administration under a chronic food-limited regimen: relating DRL performance to pharmacokinetics

The effects of midazolam on animal behavior often are evaluated under a chronically food-limited regimen, which is used to implement food-reinforced performance, but the corresponding pharmacokinetics are lacking. The present study investigated the pharmacokinetics of midazolam after i.v., s.c., i.p., and p.o. administration in food-limited rates. A two-compartment model best described the concentration-time profiles for the four routes of administration. The rate of midazolam absorption was rapid, and peak concentrations were attained in less than 7 min for the three extravascular routes. The mean volume of distribution of the central compartment and clearance were 0.77 l/kg and 2.03 l/h per keg, respectively. Midazolam elimination half-lives for the four routes of administration ranged from 23.1 to 49.5 min, and metabolites could not be detected. The mean absolute bioavailability was route-dependent: 39.3% (s.c.) 19.2% (i.p.) and 4.6% (p.o.). The markedly low oral bioavailability found in food-limited rats contrasted to the value reported for free-feeding rates (45%). Although the i.p. route yielded the highest maximum concentration on occasion, serum midazolam concentration-time profiles were variable, but did correspond to respective sedative responses. DRL 45-s performance after s.c., i.p., and p.o. administration further supported the advisability of using the s.c. route of administration, as opposed to the i.p. route, for studying midazolam dose-response relations. The bioavailability values assessed from DRL performance also agree with the measured pharmacokinetic values.

Alprazolam, caffeine and their interaction: relating DRL performance to pharmacokinetics

Three-hour sessions of differential reinforcement of low-rate behavior (DRL 45-s) in rats were used to investigate alprazolam, caffeine, and their interactions at the onset, peak and disappearance of serum alprazolam, while caffeine level remained constant. The dose-response curve (DRC) method of Pöch permitted an extensive evaluation of the kind (additivity or independence) of interactions occurring in combined drug effects. The alprazolam and caffeine DRCs were used to derive theoretical additive and independent relations, and the observed combined effects compared to these functions. Behavior-time profiles of the combined effects were similarly compared. Serum alprazolam and caffeine concentrations correlated with their respective behavior-time profiles. No acute tolerance was observed either for the individual drugs or their combinations. Alprazolam was more potent than caffeine in disrupting DRL behavior. Because alprazolam is much shorter-lived (t1/2 = 32 min) than caffeine (t1/2 = 3 h) in rats, potency ratios between alprazolam and caffeine changed across session time (from 123 to 4), which determined the expression of the combined effects. Although the combined effects were not distinguishable in terms of additivity or independence in both the DRCs and in the behavior-time profiles, they showed neither synergism nor antagonism. The pharmacokinetics of alprazolam were not altered by caffeine, but those of caffeine were affected by alprazolam.

Determination of midazolam and its metabolites in serum microsamples by high-performance liquid chromatography and its application to pharmacokinetics in rats

A single-solvent extraction step high-performance liquid chromatographic method is described for quantitating midazolam and its two hydroxy metabolites in rat serum microsamples (50 microliters). The separation used a 2 mm I.D. reversed-phase Symmetry C18 column with an isocratic mobile phase consisting of methanol-acetonitrile-14.9 mM sodium acetate in water at pH 3.0 (10:23:67, v/v). The detection limit was 10 ng/ml for all the compounds using an ultraviolet detector operated at 230 nm. The method was used to study the pharmacokinetics of midazolam after an intravenous bolus dose (0.75 mg/kg).

Conditions sufficient for the production of oral cocaine or lidocaine self-administration in preference to water

Groups of rats were given a chronic history of drinking cocaine solutions of different concentrations in daily, 3-h schedule induced polydipsia sessions. Animals failed to develop a preference for cocaine solution to concurrently presented water. Schedule-induction conditions were maintained, and the animals were divided into separate groups, drinking either cocaine or lidocaine placed in a highly acceptable vehicle (glucose-saccharin solution). Animals preferred their respective drug solutions to concurrently presented water, and these preferences remained stable after the glucose-saccharin vehicle was gradually faded to water, leaving only cocaine or lidocaine, respectively, in the solution. Thus a stable preference for drug solution to water could be instituted in rats for either cocaine or lidocaine solution (putative reinforcing and nonreinforcing agents, respectively) given an appropriate associative history, with high intakes maintained by schedule-induction. Conditions sufficient for the initiation of an oral preference and high intake for a putatively reinforcing drug cannot be assumed to occur owing to the drug's reinforcing property in the absence of demonstrating the ineffectiveness of an appropriate negative control substance.

Dose-dependent surmountability of locomotor activity in caffeine tolerance

For two chronic intraperitoneal caffeine dose regimens (10 and 80 mg/kg per day), tolerance developed rapidly (2-3 days) to the stimulatory effects of caffeine on locomotor activity. However, surmountability of the tolerant activity rate levels by caffeine administration was dose dependent: Activity rate was restored fully by acute caffeine administration for the 10 mg/kg per day series, but not for the 80 mg/kg per day series. The extent of tolerance was also dose-dependent: Tolerance was incomplete for the low-dose daily caffeine series but complete for the high-dose series. Upon discontinuation of daily caffeine dosing, activity rate decreased to the original baseline levels for both chronic series. Caffeine tolerance and the quantification of its surmountability may be explained by the pharmacokinetics of caffeine and the upregulation of adenosine receptors.

Oral and IP caffeine pharmacokinetics under a chronic food-limitation condition

For food-limited rats, serum caffeine was proportional to IP caffeine doses (10-40 mg/kg) for Cmax and area under the curve [AUC(0-24 h)], whereas the three dimethylxanthine (DMX) metabolites of caffeine were disproportional over the dose range. Steady-state concentrations of caffeine and the three metabolites were evident at the 11th day of chronic, daily caffeine IP 20 mg/kg doses. Both caffeine and the three metabolites were dose proportional for Cmax and AUC(0-24 h) by schedule-induced oral caffeine self-administration within the dose range taken (9-38 mg/kg). These results contrast with the nonlinear kinetics of caffeine reported for rats under ad lib conditions. Elimination rate constants (Kel) remained the same for the two routes, but apparent volume of distribution (AVd) and clearance (Cl) were different. The order of the Kel values was caffeine > paraxanthine > theophylline > theobromine. The effects of linear vs. nonlinear caffeine pharmacokinetics may have distinct implications for the resulting pharmacodynamics.

Stimulus control of addictive behavior: persistence in the presence and absence of a drug

Rats were exposed daily to a food schedule-induced polydipsia condition, in which water and 0.16 mg/ml cocaine solution were available concurrently, with the cocaine solution indicated by a discriminative stimulus (SD) light. The cocaine solution was preferred, and the preference was maintained when the SD was gradually eliminated by fading its intensity. For a second group, if cocaine concentration was the stimulus gradually eliminated, preference for the resulting solution (water) indicated by the SD was stably maintained. For two additional groups, if either the light SD or the cocaine stimulus was removed abruptly rather than gradually, few animals retained preferences. These studies reveal the importance of establishing strong stimulus control for the initiation and persistence of drug abuse behavior.

Tolerance to oral and IP caffeine: locomotor activity and pharmacokinetics

Locomotor activity increase was a bitonic function of acute caffeine IP doses (2.5-40 mg/kg) in rats. When the schedule-induced polydipsic, orally self-administered dose of caffeine was increased over blocks of daily 3 h sessions from 9.3 to 36.5 mg/kg, postsession activity increased monotonically as a function of dose. The rate of tolerance development to the increase in locomotor activity produced by caffeine depended on the route of administration. Tolerance onset occurred on the fourth day of chronic IP doses, but remained incomplete after 21 doses. With the highest dose level of oral caffeine self-administration, tolerance developed on day 13, but remained incomplete even after 17 doses. Acute tolerance occurred for each of the IP doses, whereas a linear relation between locomotor activity and serum caffeine concentration was obtained after oral self-administration. There were two- to threefold higher locomotor activity AUCs(4 h) with oral caffeine at three dose levels compared to the activity AUCs(4 h) for IP doses.

Acute and chronic nicotine effects on multiple-schedule behavior: oral and SC routes

For rats responding on a 3 h FI 4 min FR 20 schedule of food reinforcement, presession SC nicotine doses (0.1-0.8 mg/kg) produced depression in all responding followed by stimulation of FI responding that was dependent upon both time and dose. With daily presession 0.8 mg/kg SC nicotine injections for 9 days, no tolerance to the depressive or stimulatory effects of nicotine occurred. When nicotine solutions were orally self-administered by presession exposure to 3 h of schedule-induced polydipsia, the subsequent FR responding was unaffected, but the degree of FI response stimulation and its duration occurred in a dose-related fashion (1.18-4.10 mg/kg). Prolonged daily sessions of oral nicotine self-administration provide a technique for investigating the effects of chronic exposure to nicotine. The postingestive effects of nicotine reveal stimulatory effects that last for at least 3 h.

Determination of alprazolam and its major metabolites in serum microsamples by high-performance liquid chromatography and its application to pharmacokinetics in rats

A high-performance liquid chromatographic method using a single-solvent extraction step is described for quantitating alprazolam and its metabolites in rat serum microsamples (50 microliters). The separation used a 2-mm I.D. reversed-phase Ultrasphere C18 column with a mobile phase of methanol-acetonitrile-sodium acetate buffer. By decreasing the methanol content in the mobile phase, triazolam and its metabolites can be extracted and separated using the same method. The detection limit was 5 ng/ml for all the compounds using an ultraviolet detector at 230 nm. The method showed the effect of aging on alprazolam pharmacokinetics following 0.8 mg/kg intravenous bolus alprazolam administration.

Determination of cocaethylene, cocaine and their metabolites in rat serum microsamples by high-performance liquid chromatography, and its application to pharmacokinetic studies in rodents

A single-solvent extraction step high-performance liquid chromatographic method is described for quantitating cocaethylene in rat serum microsamples (50 microliters), a substance formed in vivo when cocaine and ethanol are present concurrently. The separation used a 2 mm I.D. reversed-phase Nova-Pak C18 column with a mobile phase of acetonitrile-phosphate buffer containing an ion-pairing reagent. With an ultraviolet detector operated at 230 nm, a linear response was observed from 0.05 to 2.0 micrograms/ml with a detection limit of 5 ng/ml for cocaethylene, cocaine and norcocaine. The method showed a longer half-life for cocaethylene than for cocaine in rat.

Oral cocaine self-administration: relation of locomotor activity to pharmacokinetics

Rats were exposed to daily schedule-induced polydipsia sessions in which solutions of cocaine HCl were available. Both cocaine solution concentration (0.08-0.32 mg/ml) and session duration (0.25-3 h) were varied to determine their effects on locomotor activity rate. Additional animals were used to determine the effect of session length on serum cocaine and metabolite levels when drinking 0.32 mg/ml cocaine solution. Changes in locomotor activity rate were related to serum cocaine concentration by a linear concentration-effect model. By estimation from the linear model, the serum cocaine concentration threshold for increasing locomotor activity was about 0.01 microgram/ml. Under these schedule-induction conditions, there was no evidence for the development of acute tolerance to the locomotor-stimulating activity of cocaine.

Orally self-administered cocaine: reinforcing efficacy by the place preference method

In three separate place preference conditioning (PPC) experiments, groups of rats were exposed to different modes of receiving cocaine: IP cocaine doses (7.5 mg/kg), PO cocaine self-administered bolus doses (15 mg/kg), and 1-h schedule-induced cocaine-solution drinking sessions (19.1 mg/kg). Oral cocaine self-administration of PO bolus and schedule induction took place in situations that preceded transfer into an apparatus for PPC sessions. Thus, the reinforcing efficacies of the pharmacological consequences of both oral cocaine self-administration methods were evaluated by a procedure separate from the self-administration behavior itself. The IP cocaine dose imposition and the two oral cocaine self-administration arrangements all resulted in dose-exposure conditions sufficient for the production of PPC. The serum and brain cocaine pharmacokinetics sufficient for the production of reinforcing efficacy were measured and related to previous data.

Sustained synergism by chronic caffeine of the motor control deficit produced by midazolam

To evaluate the effects of chronic caffeine on the impairment of discriminative fine motor control produced by midazolam, rats were trained to hold a force transducer steady to deliver food pellets. Chronic, daily doses of midazolam (3 mg/kg SC) led to a stable level of motor impairment. Chronic caffeine (20 mg/kg IP) alone usually produced a more moderate deficit or, for one animal, no deficit. Combined, chronic administration of these doses yielded a sustained synergism in motor performance impairment, which contrasted with the antagonism usually found between the benzodiazepines and methylxanthines when performance is evaluated by psychomotor tests not requiring fine motor control. The observed synergism was not explicable in terms of measured disposition of the drugs. The synergistic production of fine motor dyskinesia by the concurrent administration of caffeine and midazolam may be relevant to the triggering of anxiety attacks by caffeine observed in panic disorder patients.

Synergism by caffeine and by cocaine of the motor control deficit produced by midazolam

To evaluate the effects of caffeine and cocaine on the impairment of discriminative motor control produced by midazolam, rats were trained to hold a force transducer operated with a paw so that it remained between upper and lower limits of a force band for a continuous 1.5-s period to deliver each food pellet. Acute doses of 3 mg/kg midazolam SC impaired motor performance. Except for one animal, caffeine (10-40 mg/kg IP) had little or no effect on performance, while cocaine (3.75-22.5 mg/kg IP) produced dose-related impairment. When each dose of caffeine was combined with 3 mg/kg midazolam, a marked synergism in motor performance impairment occurred. Cocaine plus midazolam produced mainly an additive synergism. The conspicuous synergistic action of caffeine on the motor control deficit produced by midazolam contrasts with the typical antagonism found between the benzodiazepines and methylxanthines when performance is evaluated by psychomotor tests not requiring fine motor control.

Midazolam withdrawal and discriminative motor control: effects of FG 7142 and Ro 15-1788

Rats chronically drank either water or midazolam solution (0.1 mg/ml) in daily, 3-h schedule-induced polydipsia sessions and were evaluated in daily motor control sessions after polydipsia when midazolam metabolite levels had fallen to zero (withdrawal). Under midazolam polydipsia, animals orally self-administered between 21 and 38 mg/kg daily. The effect of acute drug administration [midazolam (0.75-3 mg/kg, SC), FG 7142 (1-8 mg/kg, IP), Ro 15-1788 (10-20 mg/kg, IP)] on motor control performance was similar after either chronic water or midazolam polydipsia. Thus chronic, oral midazolam self-administration did not lead to tolerance to the motor impairment produced by SC midazolam, nor did the daily discontinuation lead to impaired motor performance, nor had these performances, which occurred after daily elevated midazolam metabolite levels had reached zero (withdrawal), become sensitized to the effects of either the benzodiazepine inverse agonist FG 7142 or the agonist Ro 15-1788.

Behavioral tolerance to flurazepam

Rats were trained to earn 180 food pellets in daily, fixed-interval 1-min sessions. When performance had stabilized, a Before group was given a weekly 16 mg/kg flurazepam injection IP for 3 weeks immediately before the sessions, while an After group received their weekly injections immediately after the sessions. Then, the After group received 3 such weekly injections before the sessions. Behavioral tolerance developed by the 2nd flurazepam injection for the Before group, but for the After group, the 3 postsession flurazepam injections resulted in subsequent tolerance to presession flurazepam injection for session lever presses, but not for the time taken to earn 180 pellets. Dispositional tolerance to the serum elimination rate of flurazepam did not develop over the course of 3 injections. Behavioral suppression still evident in the initial portion of sessions with the 2nd and 3rd presession injection coincided with the duration of rising and high levels of serum flurazepam.

Chronic oral cocaine self-administration: pharmacokinetics and effects on spontaneous and discriminative motor functions

Rats receiving repeated doses of oral cocaine (15 mg/kg) showed replicable increases in large-movement and small-movement activity rates, but sensitization to the repeated doses did not develop. With a schedule-induction procedure, as the daily, 3-hr, oral dose of self-administered cocaine increased, marked dose-related increases occurred in both large-movement locomotor activity rate and the time for which these elevations were sustained during the following daily 2-hr activity session. Sensitization developed. At the highest levels of self-administered oral cocaine (about 80 mg/kg), post-administration serum cocaine levels remained undiminished for the activity-session period, as did the large-movement activities of most animals, indicating no development of acute tolerance. Rats receiving repeated doses of oral cocaine (15 mg/kg) showed discriminative motor control deficits as well as increases in work rate. These changes were dose-related in animals self-administering oral cocaine under the schedule-induction procedure. Upon withdrawal of cocaine from the schedule-induction animals, motor behavior returned to precocaine base-line performance for most animals. The behavior of the animal with the largest cocaine intake did not return. After a schedule-induced oral cocaine intake session, the tail-tip and trunk serum measures for cocaine and its metabolites were approximately equivalent, while brain cocaine and norcocaine levels remained markedly elevated over serum values.