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Effects of lorazepam tolerance and withdrawal on GABAA receptor-operated chloride channels
Abstract:
Mice were treated with 4 mg/kg of lorazepam for 7 days via implanted osmotic mini pumps. After chronic drug treatment, brains were assayed for GABA-mediated chloride flux (GABA-Cl-). Compared to control, brain membranes from lorazepam-tolerant mice were resistant to flunitrazepam stimulation of GABA-Cl-. Lorazepam tolerance did not affect [3H]diazepam binding affinity but did lower binding number slightly. Membranes from lorazepam-tolerant mice were cross-tolerant to both ethanol and phenobarbital stimulation of GABA-Cl-. Pentobarbital-stimulation of GABA-Cl- was equivalent in the two treatment groups. An increase in maximum inhibition of chloride flux produced by the benzodiazepine partial inverse agonist, n-methyl-beta-carboline-3-carboxamide (FG-7142) in membranes from lorazepam-tolerant mice was observed. FG-7142 was also found to be a more potent inhibitor of [3H]diazepam binding in membranes from lorazepam-tolerant mice. Withdrawal from chronic treatment by an acute injection with the benzodiazepine antagonist RO-15-1788 (flumazenil), restored functioning of the channel complex to control levels. There were no differences between membranes from control and lorazepam withdrawn mice in stimulation by flunitrazepam, ethanol, phenobarbital and pentobarbital or inhibition by FG-7142 of GABA-Cl-. [3H]Diazepam-saturated binding parameters and inhibition of binding by FG-7142 were similar. Chronic administration of lorazepam reduces the coupling between the benzodiazepine agonist site and the chloride channel and concomitantly increases coupling between the channel and the inverse agonist site. Furthermore, these findings offer neurochemical evidence for cross-tolerance to ethanol and phenobarbital after induction of lorazepam tolerance.
Insights
Chronic lorazepam treatment in mice induced tolerance to benzodiazepines, affecting GABA-mediated chloride flux and showing cross-tolerance to ethanol and phenobarbital.
Area of Science:
- Neuroscience
- Pharmacology
- Molecular Biology
Background:
- Benzodiazepines like lorazepam modulate GABA-A receptors, influencing neuronal excitability.
- Chronic drug exposure can lead to neurochemical adaptations and tolerance.
- Understanding these adaptations is crucial for managing drug dependence and withdrawal.
Purpose of the Study:
- To investigate the neurochemical changes underlying lorazepam tolerance in mice.
- To examine the effects of lorazepam tolerance on GABA-mediated chloride flux and receptor binding.
- To assess cross-tolerance to other CNS depressants and the role of specific receptor sites.
Main Methods:
- Mice received chronic lorazepam treatment (4 mg/kg/day) via osmotic mini-pumps.
- Brain membranes were analyzed for GABA-mediated chloride flux (GABA-Cl-) and [3H]diazepam binding.
- Responses to flunitrazepam, ethanol, phenobarbital, pentobarbital, and FG-7142 were measured.
- Effects of benzodiazepine antagonist RO-15-1788 (flumazenil) during withdrawal were assessed.
Main Results:
- Lorazepam tolerance rendered GABA-Cl- resistant to flunitrazepam stimulation.
- Tolerance slightly reduced [3H]diazepam binding number but not affinity.
- Cross-tolerance to ethanol and phenobarbital was observed, but not pentobarbital.
- The partial inverse agonist FG-7142 showed increased potency in tolerant mice.
- Withdrawal with flumazenil restored channel complex function to control levels.
Conclusions:
- Chronic lorazepam administration alters the coupling between benzodiazepine binding sites and the chloride channel.
- Tolerance involves decreased coupling to agonist sites and increased coupling to inverse agonist sites.
- Neurochemical evidence supports cross-tolerance to ethanol and phenobarbital following lorazepam tolerance.