Synaptic interactions in a newly identified excitatory synapse of Helix aspersa: concurrent enkephalinergic and...

Mollusc neurons, including those from Helix aspersa, have been extensely used for studies on synaptic plasticity and for pharmacological experiments. Synaptic interactions in H. aspersa are less well documented. In this work, I describe the identification and electrophysiological characteristics of a chemical synapse which is modulated by Met-Enkephalin (ME) and GABA. Low frequency repetitive presynaptic stimulation leads to a progressive amplitude reduction of the EPSPs (homosynaptic depression) of 45% after 20 stimuli and up to 80% after long-lasting repetitive stimulation (150-200 stimuli). The values of the smallest detectable EPSP after profound depression under normal neurotransmitter release levels corresponded to those obtained under high Mg(2+)-low Ca2+ conditions (0.33 +/- 0.045 mV) and to the spontaneous EPSPs (0.4 mV, sigma 2 = 0.02). Recovery after homosynaptic depression could occur by withdrawing the stimulation during 25 s after 15-20 stimuli or 2 min after 150-200 stimuli. Both, ME and GABA reduced the amplitude of the evoked EPSPs and augmented homosynaptic depression although this inhibition was independent of the homosynaptic depression trend. The amplitude of the spontaneous EPSPs was not affected. Naloxone alone did not produce any effect, making it unlikely that the enkephalinergic system could be recruited during homosynaptic depression. ME also shortened the duration of the presynaptic action potential and enhanced its afterhyperpolarization. These results suggest that ME and GABA exert a presynaptic modulation.