MECHANISM OF MU-OPIOID RECEPTOR-MEDIATED PRESYNAPTIC INHIBITION IN THE RAT HIPPOCAMPUS IN-VITRO

1. The electrophysiological action of the mu-opioid receptor-preferrin g agonist D-Ala2, MePhe4, Met(O)5-ol-enkephalin (FK 33-824) on synapti c transmission has been studied in area CA3 of organotypic rat hippoca mpal slice cultures. 2. FK 33-824 (1 mum) had no effect on the amplitu de of pharmacologically isolated N-methyl-D-aspartate (NMDA) or non-NM DA receptor-mediated EPSPs. 3. FK 33-824 (10 nm to 10 mum) reduced the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) that were elicited in pyramidal cells with local stimulation after pha rmacological blockade of excitatory amino acid receptors. This effect was reversible, dose-dependent, and sensitive to naloxone and the mu-r eceptor antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP). FK 33-824 at 1 mu m caused a mean reduction in the amplitude of the monosynaptic IPSP of 70 %.4. Neither 8- nor K-receptor-preferring agonists had any effect on excitatory or inhibitory synaptic potentials. 5. The disinhibitory action of FK 33-824 was blocked by incubating the cultures with pertus sis toxin (500 ng/ml for 48 h) or by stimulation of protein kinase C w ith phorbol 12,13-dibutyrate (PDBu, 0.5 mum). 6. The depression of mon osynaptic IPSPs by FK 33-824 was unaffected by extracellular applicati on of the K+ channel blockers Ba2+ or Cs+ (1 mm each). 7. FK 33-824 pr oduced a decrease in the frequency of miniature, action potential-inde pendent, spontaneous inhibitory synaptic currents (mIPSCs) recorded wi th whole-cell voltage-clamp techniques, but did not change their mean amplitude. Application of the Ca2+ channel blocker Cd2+ (100 mum) or o f nominally Ca2+-free solutions did not alter either the frequency and amplitude of mIPSCs or the reduction of mIPSC frequency induced by FK 33-824. 8. The effect of FK 33-824 on spontaneous mIPSCs was prevente d by naloxone, and by incubation of cultures with pertussis toxin. 9. These results indicate that mu-opioid receptors decrease GABA release presynaptically by a G protein-mediated inhibition of the vesicular GA BA release process, and not by changes in axon terminal K+ or Ca2+ con ductances that are sensitive to extracellular Ba2+, Cs+ or Cd2+.