BACKGROUND AND PURPOSE Pre-synaptic neurotransmitter release is largely dependent on Ca2+ entry through P/Q-type (CaV2.1) voltage-gated Ca2+ channels (PQCCs) at most mammalian, central, fast synapses. Barbiturates are clinical depressants and inhibit pre-synaptic Ca2+ entry. PQCC barbiturate pharmacology is generally unclear, specifically in man. The pharmacology of the barbiturate pentobarbital (PB) in human recombinant α1A PQCCs has been characterized.
EXPERIMENTAL APPROACH PB effects on macroscopic Ca2+(ICa) and Ba2+(IBa) currents were studied using whole-cell patch clamp recording in HEK-293 cells heterologously expressing (α1A)human(β2aα2δ-1)rabbit PQCCs.
KEY RESULTS PB reversibly depressed peak current (Ipeak) and enhanced apparent inactivation (fractional current at 800 ms, r800) in a concentration-dependent fashion irrespective of charge carrier (50% inhibitory concentration: Ipeak, 656 µM; r800, 104 µM). Rate of mono-exponential IBa decay was linearly dependent on PB concentration. PB reduced channel availability by deepening non-steady-state inactivation curves without altering voltage dependence, slowed recovery from activity-induced unavailable states and produced use-dependent block. PB (100 µM) induced use-dependent block during physiological, high frequency pulse trains and overall depressed PQCC activity by two-fold.
CONCLUSION AND IMPLICATIONS The results support a PB pharmacological mechanism involving a modulated receptor with preferential slow, bimolecular, open channel block (Kd= 15 µM). Clinical PB concentrations (<200 µM) inhibit PQCC during high frequency activation that reduces computed neurotransmitter release by 16-fold and is comparable to the magnitude of Ca2+-dependent facilitation, G-protein modulation and intrinsic inactivation that play critical roles in PQCC modulation underlying synaptic plasticity. The results are consistent with the hypothesis that PB inhibition of PQCCs contributes to central nervous system depression underlying anticonvulsant therapy and general anaesthesia.