A COMPARISON OF THE EFFECT OF CALCIUM-CHANNEL LIGANDS AND GABA-B AGONISTS AND ANTAGONISTS ON TRANSMITTER RELEASE AND SOMATIC CALCIUM-CHANNEL CURRENTS IN CULTURED NEURONS

Glutamate release has been examined from cultured cerebellar granule neurons in the rat using the technique of prelabelling the releasable pool of glutamate with [3H]glutamine. Glutamate release was stimulated in control neurons by 2-min incubation with 50 mM K +, or in neurons continuously depolarized in Ca2+ -free 50 mM K+ medium, by 2-min incubation with medium containing 5 mM Ca2+. The ability of the Ca2+-channel agonist (+)-202-791 to increase the stimulated release of [3H]glutamate was approximately doubled in the depolarized condition. The antagonist enantiomer (⊃-202-791 produced a small inhibition of K+-stimulated release, whereas (-)-202-791 completely inhibited Ca2+-stimulated release from depolarized neurons at concentrations greater than 10 nM. (-)-Baclofen (100 μM) inhibited transmitter release similarly (25-30%) under the two conditions. Calcium-channel currents were recorded from cultured dorsal root ganglion neurons under control conditions at a holding potential of -80 mV, or in neurons depolarized to -30 mV. (-)-202-791 produced a greater effect at -30 than at -80 mV although even at -30 mV the inhibition was slow in onset and incomplete. (-)-Baclofen (100 μM) inhibited the amplitude of the calcium-channel current at both holding potentials by 30-50%, although it did not clearly slow activation of the current at the depolarized holding potential. The GABAB receptors associated with inhibition of glutamate release and of calcium-channel currents were both markedly blocked by phaclofen but not by 2-OH-saclofen. These findings suggest that the GABAB receptor associated with inhibitory modulation of transmitter release, and that associated with inhibition of calcium-channel currents show pharmacological similarities, and are able to exert their action even at levels of steady depolarization at which most N-type channels should be inactivated. © 1990.