Effects of ohmefentanyl on CA1 field potentials in rat hippocampus slices

The effects of ohmefentanyl (OMF), a new opiate agonist with high affinity and high specificity for mu receptors, was examined on CA1 field potentials in the transverse hippocampal slices. OMF showed two effects upon the evoked population spikes (PS) recorded in stratum pyramidale: 1) a concentration-dependent increase in the amplitude of PS, which was largely reversed by naloxone, and 2) production of a naloxone-reversible additional PS at high stimulus intensities. No significant change was seen in field excitatory postsynaptic potential (EPSP) recorded simultaneously in stratum radiatum. The EC50 for OMF and morphine were 6.6 and 3700 nmol.L-1, respectively. Thus OMF was 560 times more potent than morphine. The mechanism of augmentation by OMF of PS could be attributed to disinhibition as judged from the paired-pulse paradigm.