Midazolam oral self-administration

Reinforcing effectiveness of midazolam, ethanol, and sucrose: behavioral economic comparison of a mixture relative to its component solutions

Benzodiazepines (BZs) are relatively safe when administered alone. However, these drugs can produce severe side effects when coadministered with ethanol. Despite these adverse consequences, rates of concurrent BZ and ethanol misuse are increasing, and it is unclear whether this behavior is maintained by an enhanced reinforcing effect of the mixture. To address this issue, the current study compared the reinforcing effectiveness of sucrose solutions mixed with midazolam, ethanol, or both. Eight male rats were trained to orally self-administer solutions of either sucrose (S), sucrose+midazolam (SM), sucrose+ethanol (SE), or sucrose+midazolam+ethanol (SME). The response requirement was increased between sessions until the number of reinforcers earned was zero and the relationship between response requirement and reinforcers earned was analyzed using the exponential model of demand. Although baseline intake was similar across drug conditions, consumption of SM was least affected by increases in cost, indicating that it possessed the highest reinforcing effectiveness (i.e. least elastic). The reinforcing effectiveness of S, SE, and SME did not differ significantly. The finding that the reinforcing effectiveness of the SME was less than that of SM does not support the supposition that BZ and ethanol coadministration is maintained by a higher reinforcing effectiveness of the mixture.

Sex differences in schedule-induced alcohol consumption

Six female and five male Wistar rats obtained food by pressing a lever on a fixed-interval (FI) 60-s schedule in the presence of a sipper tube which allowed access to an alcohol solution. Systematic manipulation of the alcohol concentration revealed that male rats consumed higher alcohol concentrations (v/v) than female rats to obtain maximum alcohol intake (g ETOH/kg) during an experimental session. Males also reached higher blood alcohol levels (BALs) than females. Individual lick rates varied over a small range when the FI schedule parameter was manipulated (20 s and 180 s) during probe sessions. Very few licks were observed when all pellets were presented at the beginning of the session, during extinction, or during sessions in which only a few pellets were presented. Reduction of the solution's alcohol concentration to half the maximum concentration or presentation of distilled water resulted in increased lick rates at most values of the FI schedule in four of the five male subjects, but not in female subjects. Alcohol intake during sessions in which half the maximum alcohol concentration was available was lower than that observed during sessions in which subjects consumed the maximum alcohol concentration. The lick rate data in conjunction with those on alcohol intake suggest that male rats, but not female rats, maintained blood alcohol concentrations at levels which they had also reached during sessions in which the maximum alcohol concentration was available from the sipper tube.

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.

Evidence for a role for dopamine in the diazepam locomotor stimulating effect

It is well known that benzodiazepines produce dependence in humans and locomotor stimulation in experimental animals. In this study the possible involvement of catecholamines in the diazepam-induced locomotor stimulation in mice were investigated. Diazepam was found to have a biphasic effect; increasing locomotor activity at a low dose (0.25 mg/kg), while decreasing it at higher doses (greater than 0.5 mg/kg). The locomotor stimulating effect of diazepam was effectively blocked by pretreatment with the benzodiazepine receptor antagonist flumazenil, as well as with the catecholamine synthesis inhibitor alpha-methyltryrosine and the dopamine receptor antagonists haloperidol, spiperone and SCH 23390. Taken together, these data indicate that the locomotor stimulating effect observed after low doses of diazepam is due to activation of brain dopaminergic systems involved in locomotor activity. The observations are discussed in relation to the hypothesis that dependence-producing drugs activate specific brain reward systems.

Intravenous self-administration of the short-acting benzodiazepine midazolam in the rat

Intravenous self-administration of the short-acting benzodiazepine midazolam was assessed in rats under conditions of unlimited access to the drug (24 hr/day). Evaluation of the temporal pattern of responding within a session revealed that maximal responding occurred during the dark phase of the 12 hr light/dark cycle. Upon attaining stable rates of responding for midazolam, the rats were tested under extinction conditions (saline). Responding progressively decreased with repeated extinction sessions. Over the course of subsequent reacquisition sessions responding increased, such that asymptotic levels of responding were similar to, or greater than, the rates obtained during the initial acquisition phase. In addition, the total number of sessions required to approach asymptote were fewer than during the initial acquisition phase. Thus, rats given continuous access to midazolam exhibited reliable and stable rates of intravenous self-administration of this short-acting benzodiazepine.

Oral self-administration of cocaine: chronic excessive intake by schedule induction

Rats were exposed to daily 3-hr schedule-induced polydipsia sessions (fixed-time 1 min food-pellet delivery) with a cocaine hydrochloride solution as the available session fluid. Cocaine intake level (mg/kg) was a direct function of solution concentration (0.02-0.2 mg/ml). In a second experiment with 0.15 mg/ml cocaine solution available, mean daily session cocaine intake remained constant at about 40 mg/kg over the 5-week period of the experiment. Post-session serum samples of animals drinking either 0.15 or 0.2 mg/ml cocaine solution yielded serum cocaine values that were similar to those producing subjective "highs" in coca-leaf chewers and experienced users of cocaine. The schedule-induction technique can be used to induce the intake of drug solutions, including those which perhaps taste bitter, so that chronic and pharmacologically significant consequences ensue.

Self-administration of barbiturates and benzodiazepines: a review

Studies of barbiturate and benzodiazepine self-administration are categorized by species and route of administration. Reinforcement, defined as self-administration of drug greater than of a non-drug control, has been demonstrated most often in studies employing the IV route, and there has been greater reliability in this result for a given drug among barbiturates rather than among benzodiazepines. Most studies of PO self-administration in rodents have not demonstrated reinforcement, despite a number of behavioral manipulations to induce drug intake. Studies of PO barbiturate self-administration in monkeys have demonstrated reinforcement but recent studies of PO benzodiazepine self-administration in baboons have not, although physical dependence was demonstrated. Reinforcement via the IG route has not been reliably demonstrated. Behavioral variables, including interreinforcement interval and drug self-administration history, appear to be important determinants of whether or not reinforcement will be demonstrated, particularly among the benzodiazepines; but the range of conditions under which behavioral and pharmacological variables interact to promote or lessen the likelihood of self-administration of these drugs remains to be determined experimentally.