PT  - JOURNAL ARTICLE
AU  - Damien E. Earl
AU  - Elizabeth I. Tietz
TI  - Inhibition of Recombinant L-Type Voltage-Gated Calcium Channels by Positive Allosteric Modulators of GABA<sub>A</sub> Receptors
AID  - 10.1124/jpet.110.178244
DP  - 2011 Apr 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 301--311
VI  - 337
IP  - 1
4099  - http://jpet.aspetjournals.org/content/337/1/301.short
4100  - http://jpet.aspetjournals.org/content/337/1/301.full
SO  - J Pharmacol Exp Ther2011 Apr 01; 337
AB  - Benzodiazepines (BDZs) depress neuronal excitability via positive allosteric modulation of inhibitory GABAA receptors (GABAAR). BDZs and other positive GABAAR modulators, including barbiturates, ethanol, and neurosteroids, can also inhibit L-type voltage-gated calcium channels (L-VGCCs), which could contribute to reduced neuronal excitability. Because neuronal L-VGCC function is up-regulated after long-term GABAAR modulator exposure, an interaction with L-VGCCs may also play a role in physical dependence. The current studies assessed the effects of BDZs (diazepam, flurazepam, and desalkylflurazepam), allopregnanolone, pentobarbital, and ethanol on whole-cell Ba2+ currents through recombinant neuronal Cav1.2 and Cav1.3 L-VGCCs expressed with β3 and α2δ-1 in HEK293T cells. Allopregnanolone was the most potent inhibitor (IC50, ∼10 μM), followed by BDZs (IC50, ∼50 μM), pentobarbital (IC50, 0.3–1 mM), and ethanol (IC50, ∼300 mM). Cav1.3 channels were less sensitive to pentobarbital inhibition than Cav1.2 channels, similar to dihydropyridine (DHP) L-VGCC antagonists. All GABAAR modulators induced a negative shift in the steady-state inactivation curve of Cav1.3 channels, but only BDZs and pentobarbital induced a negative shift in Cav1.2 channel inactivation. Mutation of the high-affinity DHP binding site (T1039Y and Q1043M) in Cav1.2 channels reduced pentobarbital potency. Despite the structural similarity between benzothiazepines and BDZs, mutation of an amino acid important for diltiazem potency (I1150A) did not affect diazepam potency. Although L-VGCC inhibition by BDZs occurred at concentrations that are possibly too high to be clinically relevant and is not likely to play a role in the up-regulation of L-VGCCs during long-term treatment, pentobarbital and ethanol inhibited L-VGCCs at clinically relevant concentrations.