Modulation of benzodiazepine agonist and inverse-agonist receptor binding by GABA during ethanol withdrawal

Acute in vivo effect of valproic acid on the GABAergic system in rat brain: A [11C]Ro15-4513 microPET study

γ-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the nervous system acting mainly through GABA_A receptors. In the presence of high levels of GABA, an allosteric shift in the GABA_A receptors can change the affinity of benzodiazepine (BZD) ligands. Valproic acid (VPA) is an anticonvulsant that enhances the level of endogenous GABA in the brain. The BZD ligand, Ro15-4513 has a high affinity for GABA_A receptors containing the α₅ subunit and can be used to investigate the GABA shift in the brains of living rats after VPA exposure. Seven Wistar rats were scanned using a Mediso NanoScan PET/MRI. A baseline 90-min dynamic [¹¹C]Ro15-4513 PET scan was acquired prior to an intravenous injection of 50 mg/kg VPA, and was followed by a second [¹¹C]Ro15-4513 PET scan. Standardized uptake values were obtained for regions of high GABA binding, including the hippocampus and amygdala, and low GABA binding such as the cerebellum. We showed a significant increase in [¹¹C]Ro15-4513 uptake in hippocampus and amygdala, but no significant differences in cerebellar uptake, after acute VPA exposure. In contrast to several in vitro studies, we demonstrated a positive allosteric change in the GABA_A receptors after pharmacologically enhanced GABA levels resulting in enhanced Ro15-4513 uptake. Knowledge of how subtypes of the GABA_A receptors react will provide us with information useful to fine-tune pharmacological interventions and design receptor subtype specific drugs.

5-HT2C and GABAB receptors influence handling-induced convulsion severity in chromosome 4 congenic and DBA/2J background strain mice

Progress towards elucidating the underlying genetic variation for susceptibility to complex central nervous system (CNS) hyperexcitability states has just begun. Genetic mapping analyses suggest that a gene(s) on mid-chromosome 4 has pleiotropic effects on multiple CNS hyperexcitability states in mice, including alcohol and barbiturate withdrawal and convulsions elicited by chemical and audiogenic stimuli. We recently identified Mpdz within this chromosomal region as a gene that influences alcohol and barbiturate withdrawal convulsions. Mpdz encodes the multi-PDZ domain protein (MPDZ). Currently, there is limited information available about the mechanism by which MPDZ influences drug withdrawal and/or other CNS hyperexcitability states, but may involve its interaction with 5-HT2C and/or GABAB receptors. One of the most useful tools we have developed thus far is a congenic strain that possesses a segment of chromosome 4 from the C57BL/6J (donor) mouse strain superimposed on a genetic background that is >99% from the DBA/2J strain. The introduced segment spans the Mpdz gene. Here, we demonstrate that handling-induced convulsions are less severe in congenic vs. background strain mice in response to either a 5-HT2C receptor antagonist (SB242084) or a GABAB receptor agonist (baclofen), but not a GABAA receptor channel blocker (pentylenetetrazol). These data suggest that allelic variation in Mpdz, or a linked gene, influences SB242084- and baclofen-enhanced convulsions. Our results are consistent with the hypothesis that Mpdz's effects on CNS hyperexcitability, including alcohol and barbiturate withdrawal, involve MPDZ interaction with 5-HT2C and/or GABAB receptors. However, additional genes reside within the congenic interval and may also influence CNS hyperexcitability.