The excitatory response of in vitro hippocampal pyramidal cells to normorphine and methionine-enkephalin may be mediated by different receptor populations.

As shown previously, opiate agonists increase the excitability of hippocampal pyramidal cells in a naloxone-reversible manner. In the present study, the degree of excitability was measured by population spike size recorded from hippocampus slices (CA1) obtained from naive or chronically morphinized rats. Cross tolerance could not be demonstrated to occur between met-enkephalin and normorphine in hippocampal cells made tolerant to morphine: the potent stimulatory effect of met-enkephalin remains when applied to hippocampal slices removed from chronically morphinized rats, whereas normorphine was no longer effective. When these slices are washed or exposed to naloxone a diminution of the population spike occurs. These results suggest that while both opiate agonists increase neuronal excitability of hippocampal pyramidal cells, this effect is most likely mediated via different receptor populations.