MET-ENKEPHALIN AND MORPHICEPTIN MODULATE A GABA-INDUCED INWARD CURRENT IN THE CNS OF LYMNAEA-STAGNALIS L

1. The interaction between GABA and opioid peptides (met-enkephalin an d morphiceptin) was studied on the identified, isolated and internally perfused neurons of Lymnaea stagnalis L. (Gastropoda, Basommatophora) . 2. GABA (10(-7)-10(-5) M) activated a Cl-dependent inward current wi th about -20 mV equilibrium potential. Slow and fast GABA-induced inwa rd currents were recorded with different kinetic parameters in distinc t identified neurons. 3. Both types of GABA-induced inward currents we re reduced or blocked by met-enkephalin (10(-7)-10(-5) M) and morphice ptin (10(-7)-10(-5) M) in a dose-dependent manner. GABA-activated fast inward current was modulated in a biphasic way in some neurons. Opioi d reduction of the GABA-activated slow inward current was reversible, whereas the fast current was not. 4. The reversible inhibition of the GABA-induced slow inward current produced by met-enkephalin or morphic eptin was naloxone (10(-5)-10(-4) M)-sensitive, whereas the irreversib le block of the fast GABA response was not antagonised by naloxone. So me additive effects between GABA and the peptides were also noted.5. T he modulatory effect of the opioid peptides on the GABA response alter ed the peak current, the time-to-peak and inactivation time-course of the GABA-induced current. 6. Thus, the identified, isolated and intern ally perfused neurons of Lymnaea stagnalis L. provide a useful model f or studying postsynaptic mechanisms of interaction between GABA and op ioid peptides. This interaction is a phenomenon of evolutionary signif icance because it is also found in mammals. Copyright (C) 1996 Elsevie r Science Inc.