Postsynaptic induction of long-term synaptic facilitation in snail central neurones

Long-term facilitation in molluscs is believed to be induced due to purely presynaptic activations. We recorded excitatory postsynaptic potentials (EPSPs) simultaneously from two identified neurones of snail parietal ganglia. We report a non-decrementing facilitation induced by intracellular tetanization with concomitant presynaptic activation. The mean EPSP amplitude measured in 10 neurones 30-50 min after tetanization was 17% greater than in the non-tetanized control neurones. Only short-lasting (5-10 min) postsynaptic changes were found (post-tetanic hyperpolarization and resistance decrease). The facilitation was especially prominent (34%, 10 min post-tetanus) but decreased within 15 min in preparations with rapid wash-out of the external media. The data suggest that induction of long-term enhancement in molluscs depends on postsynaptic events and, like in mammals, may involve increased postsynaptic Ca2+ and subsequent release of retrograde messengers.