Differential inhibitory effects of thiopental, thiamylal and phenobarbital on both voltage-gated calcium channels and NMDA receptors in rat hippocampal slices.

Although it is known that there are some pharmacological differences between the structurally similar barbiturates, the underlying mechanism of action remains unclear. We have compared the effects of thiopental, thiamylal and phenobarbital on both voltage-gated calcium channels (VGCC) and N-methyl-D-aspartate (NMDA) receptors in rat hippocampal slices by determining changes in intracellular calcium ([Ca2+]i). Experiments were performed in adult rat hippocampal slices perfused with Krebs solution (37 degrees C). Concentrations of [Ca2+]i in the pyramidal cell layer of the CA1 region were measured using a calcium indicator dye, fura-2. To activate VGCC and NMDA receptors, slices were exposed to K+ 60 mmol litre-1 (< or = 60 s) and NMDA 100 mumol litre-1 (30 s), respectively. Thiopental, thiamylal and phenobarbital were present 5 min before, during and 1 min after high K+ or NMDA application. Both thiamylal and thiopental (50-600 mumol litre-1) attenuated the increases in [Ca2+]i produced by high K+ or NMDA in a concentration-dependent manner, while phenobarbital 50-1000 mumol litre-1 only slightly attenuated the [Ca2+]i increase produced by high K+ at concentrations of more than 200 mumol litre-1 and was ineffective on the [Ca2+]i response produced by NMDA. Although the increases in [Ca2+]i caused by membrane depolarization with high K+ were reduced equally with thiamylal and thiopental, thiamylal was more effective in attenuating the increase in [Ca2+]i produced by NMDA receptor activation than thiopental. We conclude that the depressant effects of barbiturates on both VGCC and NMDA receptors varied between agents. Differential inhibition of both VGCC and NMDA receptors may determine the pharmacological properties of barbiturates and their ability to protect neurones against ischaemia.