The discriminative stimulus effects of midazolam are resistant to modulation by morphine, amphetamine, dizocilpine, and γ-butyrolactone in rhesus monkeys

Effects of amphetamine, morphine, and CP 55, 940 on Go/No-Go task performance in rhesus monkeys

In humans, impulsivity measured as false alarms in a Go/No-Go task is reportedly decreased by amphetamine and is not affected by oxycodone and delta(9)-tetrahydrocannabinol. To model these findings in animals, three rhesus monkeys were trained to perform a food-reinforced Go/No-Go task. In this task, amphetamine was found to decrease false alarms (i.e. responding during No-Go trials), but only at doses that also decreased hits (i.e. responding during Go trials). Morphine generally decreased hits but not false alarms. The cannabinoid receptor agonist CP 55, 940 decreased both false alarms and hits, but only at doses that also decreased the number of trials completed. Additional studies in animals and humans are necessary to delineate the conditions under which amphetamine and other psychoactive drugs affect impulsivity in Go/No-Go tasks.

Mechanisms for the Specific Properties of γ-Hydroxybutyrate in Brain

γ-Hydroxybutyrate (GHB) is both a natural brain compound with neuromodulatory properties at central GABAergic synapses (micromolar concentration range) and also a drug (Xyrem(R) ) clinically used for the treatment of various neurological symptoms (millimolar dose range). However, this drug has abuse potential and can be addictive for some patients. Here, we review the basic mechanistic role of endogenous GHB in brain as well as the properties and mechanisms of action for therapeutic clinical doses of exogenous GHB. Several hypotheses are discussed with a preference for a molecular mechanism that conciliates most of the findings available. This conciliatory model may help for the design of GHB-like drugs active at lower doses and devoid of major side effects.

Benzodiazepines

Benzodiazepines have been in clinical use since the 1960s. Benzodiazepines act through allosteric modulation of the GABA_A receptor to enhance the activity of GABA, an inhibitory neurotransmitter, resulting in a slowing of neurotransmission and sedative and anxiolytic effects. Initially benzodiazepines were thought to have low dependence liability, though over time there has been increasing evidence of benzodiazepine dependence. Benzodiazepines are commonly used to treat anxiety and insomnia, though increasingly they are considered second line treatments for most indications. Concerns about the effects of benzodiazepines on cognition, falls and their implication in opioid related mortality have emerged. Few pharmacological treatments for benzodiazepine dependence have been shown to be effective with gradual taper the most common treatment strategy for benzodiazepine dependence.

Quantitative pharmacological analyses of the interaction between flumazenil and midazolam in monkeys discriminating midazolam: Determination of the functional half life of flumazenil

The duration of action of a drug is commonly estimated using plasma concentration, which is not always practical to obtain or an accurate estimate of functional half life. For example, flumazenil is used clinically to reverse the effects of benzodiazepines like midazolam; however, its elimination can be altered by other drugs, including some benzodiazepines, thereby altering its half life. This study used Schild analyses to characterize antagonism of midazolam by flumazenil and determine the functional half life of flumazenil. Four monkeys discriminated 0.178mg/kg midazolam while responding under a fixed-ratio 10 schedule of stimulus-shock termination; flumazenil was given at various times before determination of a midazolam dose-effect curve. There was a time-related decrease in the magnitude of shift of the midazolam dose-effect curve as the interval between flumazenil and midazolam increased. The potency of flumazenil, estimated by apparent pA2 values (95% CI), was 7.30 (7.12, 7.49), 7.17 (7.03, 7.31), 6.91 (6.72, 7.10) and 6.80 (6.67, 6.92) at 15, 30, 60 and 120min after flumazenil administration, respectively. The functional half life of flumazenil, derived from potency estimates, was 57±13min. Thus, increasing the interval between flumazenil and midazolam causes orderly decreases in flumazenil potency; however, across a broad range of conditions, the qualitative nature of the interaction does not change, as indicated by slopes of Schild plots at all time points that are not different from unity. Differences in potency of flumazenil are therefore due to elimination of flumazenil and not due to pharmacodynamic changes over time.

Acute tolerance to chlordiazepoxide qualitatively changes the interaction between flumazenil and pregnanolone and not the interaction between flumazenil and midazolam in rhesus monkeys discriminating midazolam

Benzodiazepines and neuroactive steroids act at distinct binding sites on γ-aminobutyric acid(A) (GABA(A)) receptors where they positively modulate GABA, resulting in similar acute behavioral effects. Tolerance to benzodiazepines can develop with repeated treatment; however, cross tolerance to neuroactive steroids does not develop, perhaps due to conformational changes in benzodiazepine, and not neuroactive steroid, binding sites. Three monkeys discriminated 0.178 mg/kg midazolam while responding under a fixed-ratio 10 schedule of stimulus-shock termination. On separate occasions, dose-effect curves for midazolam and pregnanolone were determined when monkeys had not received chlordiazepoxide and when they received 10 mg/kg chlordiazepoxide 46 hours earlier; for some tests, flumazenil was given before determination of dose-effect curves. Midazolam and pregnanolone produced ≥80% midazolam-lever responding. When administered 46 h before sessions, chlordiazepoxide did not produce pregnanolone-lever responding; under those treatment conditions, midazolam dose-effect curves were shifted 2.8-fold rightward and pregnanolone dose-effect curves were not changed. Flumazenil antagonized midazolam; Schild (linear) analyses yielded slopes that were not different from unity and pA(2) values of 7.46 when monkeys had not received chlordiazepoxide and 7.44 when they received chlordiazepoxide 46 h earlier. Flumazenil did not alter the effects of pregnanolone in chlordiazepoxide-treated monkeys. Thus, interactions between flumazenil and midazolam were not qualitatively or quantitatively changed in monkeys acutely tolerant to chlordiazepoxide, suggesting that mechanisms other than alterations of benzodiazepine binding sites account for the development of acute tolerance.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).