DIFFERENT EFFECTS OF DYNORPHIN A ON INVITRO GUINEA-PIG HIPPOCAMPAL CA3 PYRAMIDAL CELLS WITH VARIOUS DEGREES OF PAIRED-PULSE FACILITATION

We investigated the effects of dynorphin A (Dyn A), a heptadecapeptide, on the population spikes of the guinea pig hippocampal CA3 pyramidal neurons, in vitro, using paired-pulse stimulation of the mossy fibers. Dyn A produced facilitatory and inhibitory effects on the population spikes in the preparations with lower and higher degrees of paired-pulse facilitation, respectively. Morphine and D-Ala2,D-Leu5-enkephalin, .mu.- and .delta.-agonist, respectively, predominantly potentiated the population spikes, while .kappa.-agonists such as U-50,488H and bremazocine mainly caused an inhibition. These results suggest that Dyn A has two separate (excitatory and inhibitory) effects on the guinea pig hippocampal CA3 neurons through .mu.-(.delta.) and .kappa.-opioid receptors, respectively.