Development of acute tolerance during exposure of hippocampal explants to an opioid peptide.

Bath application of FK 33-824, a stable enkephalin analogue, altered synaptic potentials and induced sustained depolarization shifts in hippocampal pyramidal cells. In a proportion of cells, these excitatory effects diminished and then disappeared within 6-8 min during FK 33-824 perfusion. This desensitization was temperature-sensitive. In contrast, no desensitization to similar excitatory effects of bicuculline was seen after acute application. This GABA-antagonist also excited cells which first had been rendered tolerant to the action of FK 33-824, but cross-tolerance occurred between FK 33-824 and (D-Ala)2-(D-Leu)5-enkephalin.