Inhibition of GABA metabolism in rat brain synaptosomes by midazolam (RO-21-3981)

Spinal and supraspinal midazolam potentiates antinociceptive effects of isoflurane

The effects of lumbar intrathecal (i.t.) and intracerebroventricular (i.c.v.) midazolam on nociception during isoflurane anaesthesia were studied in rats using the tail-flick test. Rats received i.t. midazolam 2 and 4 microg or i.c.v. midazolam 4 and 8 microg during 1.1, 1.2 and 1.3% isoflurane or without isoflurane. Neither i.t. nor i.c.v. midazolam alone at doses studied influenced nociceptive responses. 1.1% isoflurane showed a minimum antinociceptive effect which was not influenced by i.t. or i.c.v. midazolam. 1.2 and 1.3% isoflurane produced moderate antinociception which was markedly potentiated by both i.t. and i.c.v. midazolam. The effects of midazolam shown in the present study are different from the reported effects of midazolam on opioid-induced antinociception; where spinally administered midazolam potentiates and supraspinal midazolam inhibits the antinociceptive effects of morphine. The present results suggest that midazolam potentiates isoflurane-induced antinociception at doses where no effect is seen alone.

Effects of midazolam on pain sensations in the face

OBJECTIVES

This study investigated the effects of midazolam, a sedative, on tactile and pain sensations on the skin of the chin.

STUDY DESIGN

Thirty-seven volunteers were segregated into four groups; the first group was the control group; the second to fourth groups were administered 0.025 mg/kg, 0.05 mg/kg, and 0.075 mg/kg of midazolam, respectively, as a bolus injection. All volunteers reclined in a dental chair for the experiment. Tactile and pain sensations were determined over time after injection of midazolam, the former using von Fray thread, the latter using an esthesiometer.

RESULTS

Thresholds of tactile sensitivity and of pain were statistically significantly different from control values at 10 minutes after injection of midazolam in the 0.05 mg/kg group and in the 0.075 mg/kg group.

CONCLUSION

Although 0.025 mg/kg of midazolam produced sedation, at least 0.05 mg/kg of this agent was required to alter the thresholds for perception of tactile and painful stimulation.

Midazolam for caudal analgesia in children: comparison with caudal bupivacaine

In a randomized, double-blind study we have examined the analgesic efficacy of caudal administration of midazolam, bupivacaine, or a mixture of both drugs in 45 children, undergoing inguinal herniotomy. They were allocated randomly into three groups (n = 15 in each) to receive a caudal injection of either 0.25% bupivacaine 1 ml.kg-1 with or without midazolam 50 micrograms.kg-1 or midazolam 50 micrograms.kg-1 with normal saline 1 ml.kg-1. There were no differences in quality of pain relief, postoperative behaviour or analgesic requirements between the midazolam group and the other two groups. Times to first analgesic administration (paracetamol suppositories) were longer (P < 0.001) in the bupivacaine-midazolam group than in the other two groups. Further, the bupivacaine-midazolam group received fewer (P < 0.05) doses of paracetamol than the bupivacaine group. Side effects such as motor weakness, respiratory depression or prolonged sedation were not observed in patients who received caudal epidural midazolam only. We conclude that caudal midazolam in a dose of 50 micrograms.kg-1 provides equivalent analgesia to bupivacaine 0.25%, when administered postoperatively in a volume of 1 ml.kg-1 for children following unilateral inguinal herniotomy.

Effects of thiopental, halothane and isoflurane on the calcium-dependent and -independent release of GABA from striatal synaptosomes in the rat

The effects of the anesthetic agents thiopental, halothane and isoflurane on the release of GABA induced by depolarization and/or reversal of the GABA carrier were investigated in a synaptosomal preparation obtained from the rat striatum. Veratridine (1 microM) and KCl (9 mM) elicited a significant, Ca(2+)-dependent release of [3H]GABA. The KCl-evoked release was not significantly modified in the presence of nipecotic acid (10(-5) M), a selective blocker of the neuronal GABA carrier. The [3H]GABA release was significantly decreased by omega-conotoxin (10(-7) M, a blocker of the N voltage-dependent Ca2+ channels, but was affected by neither nifedipine (10(-4) M) nor omega-Aga-IVA (10(-7) M which block the L and P Ca2+ channels, respectively. Thiopental application (10(-5) to 10(-3) M) was followed by a dose-related, significant, decrease in both the veratridine and KCl-induced releases, whether nipecotic acid was present or not. In contrast, halothane and isoflurane (1-3%) failed to alter [3H]GABA release. Altogether, these results suggest that reduction of the depolarization-evoked GABA release might contribute to thiopental anesthesia, but this seems unlikely for volatile anesthetics.

Awareness and general anaesthesia

A state exists after the induction of anaesthesia in which patients may be aware of their surroundings yet unable to communicate. This problem of awareness and recall during general anaesthesia is a recent one in the relatively short history of anaesthesia. Prior to the introduction of muscle relaxants in 1942 by Griffith and Johnson, it was felt that "light anaesthesia" would be signified by violent movements. Today, the concepts of anaesthetic depth, awareness, and recall have become more complicated with the addition of numerous newer, shorter-acting, intravenous anaesthetic agents with varying effects on the conscious state. Several methods have been described to detect awareness. None has yet been found to be totally reliable and numerous reports of awareness can be found in the literature. Light inhalation and total intravenous anaesthesia have been blamed for the majority of these case reports. However, awareness during total intravenous anaesthesia is avoidable with the proper use of a combination of a hypnotic and an analgesic such as midazolam and alfentanil for general anaesthesia.

Effects of acute barbiturate administration, tolerance and dependence on brain GABA system: comparison to alcohol and benzodiazepines

Central nervous system depressants, e.g., barbiturates, alcohol and benzodiazepines, have a wide spectrum of activity in humans and animals. Evidence accumulated suggests that some of the pharmacological actions exerted by these agents may be mediated through GABA system by mimicking GABAergic transmission. This review attempts to summarize the evidence available as to how the GABA system plays a part in the barbiturate actions and the development of tolerance to and physical dependence on barbiturates. The comparisons of the effects of alcohol, barbiturates and benzodiazepines at different steps of GABA synapse are also presented. Furthermore, the results which have been reported in the literature are inconsistent. This may be due to differences in: (a) animal models used; (b) brain regions used; (c) protocols (dose, duration, form and route of administration, etc.) used in treating animals and/or (d) techniques (pharmacological, biochemical, physiological, etc.) used.

Interaction of midazolam and morphine in the spinal cord of the rat

The antinociceptive properties, as measured by the tail-flick and hot-plate tests, and the motor effects of an intrathecally-administered benzodiazepine agonist midazolam, alone, and in combination with morphine, was examined in rats. Midazolam alone produced a weak but dose-dependent (20-60 micrograms) antinociceptive effect in addition to a clear motor dysfunction at larger doses (60-100 micrograms). An inactive dose of intrathecally-administered midazolam (20 micrograms) produced a leftward shift in the dose-response curve for intrathecally administered morphine, in the thermal antinociceptive tests. This supra-additive effect was antagonized by naloxone (1 mg/kg). The data suggest a synergistic interaction between mu- and GABAA-receptors in the spinal processing of thermally-evoked pain.

Partial reversal of the cerebral effects of isoflurane in the dog by the benzodiazepine antagonist flumazenil

Six dogs initially anaesthetized with isoflurane-N2O-O2 for surgery with cannulation of the sinus sagittalis for direct measurement of cerebral blood flow (CBF) and maintained with 1% isoflurane-N2-O2 anaesthesia, were given two 2-mg doses of the benzodiazepine antagonist flumazenil with an interval of 10 min. This was accompanied by a significant increase in the cerebral metabolic rate for oxygen (CMRO2) 10 min after the first 2-mg dose, and 5, 10 and 15 min after the second 2-mg dose, the increase varying in mean value from 9% to 14%. EEG was recorded in five of the six dogs and administration of flumazenil converted the EEG from a sleep pattern to an awake pattern in two of them. There were no significant changes in the CBF or mean arterial blood pressure (MABP) values. In the brain biopsies taken at the end of the study there were no significant changes in the adenylate energy charge (EC) and the lactate/pyruvate ratio (L/P) when compared to five other dogs not given flumazenil. It is concluded that flumazenil in this study demonstrated a partial antagonism of the cerebral effects of isoflurane.

A hypothetical model on the mechanism of anesthesia

An hypothesis on the mechanism of action of general anesthetic agents is proposed. It is based on a potentiation of chloride influx due to the action of anesthetic agents on the GABA-receptor complex at the lipid-protein interface. This hypothesis accommodates various observations such as lipid solubility of anesthetic molecules, their lack of stringent structural requirement, pressure reversal of anesthetic action, and other neurochemical and neuropharmacological data.

A comparison of amnesia in outpatients sedated with midazolam or diazepam alone or in combination with fentanyl during oral surgery

Midazolam is a new, water-soluble benzodiazepine that has been reported to produce a greater degree of amnesia than does diazepam. This study compares the amnesia produced in patients sedated with midazolam or diazepam or in combination with fentanyl during oral surgery. Twenty-four hours after surgery, each patient answered a questionnaire designed to assess his or her recall of events during and after surgery. Patients who received midazolam alone recalled significantly less than did patients in either diazepam group. The addition of fentanyl to midazolam did not produce any significant difference in recall.