FAIRING-SPECIFIC, ACTIVITY-DEPENDENT PRESYNAPTIC FACILITATION AT APLYSIA SENSORY-MOTOR NEURON SYNAPSES IN ISOLATED CELL-CULTURE

Synapses made by Aplysia sensory neurons onto motor- and interneuron f ollowers in the intact nervous system exhibit an associative form of s ynaptic facilitation that is thought to contribute to classical condit ioning of the animal's gill and siphon withdrawal reflex (Hawkins et a l., 1983; Walters and Byrne, 1983). Here we demonstrate that a similar associative facilitation can be induced between individual sensory an d motor neurons isolated in culture. Fairing tetanic stimulation with either of two facilitatory transmitters, 5-HT or small cardioactive pe ptide, considerably prolongs facilitation compared to either tetanus o r transmitter alone. When corrected for the depression that occurs sim ply in response to low-frequency testing, the facilitation produced by one pairing trial does not decay for more than 20 min after training. This facilitation requires the temporal pairing (0.5 sec forward inte rstimulus interval) of the two stimuli, tetanus and 5-HT. Delivering t he same two stimuli in an unpaired fashion (1 min forward interval) fa ils to produce the long-lasting effect. Measurements of spontaneous tr ansmitter release during either paired or unpaired training reveal no changes in unitary mEPSP or mEPSC (''mini'') amplitude, indicating tha t the facilitation involves a presynaptic mechanism. While both forms of training dramatically increase the initial frequency of spontaneous release, mini frequency does not remain elevated as long as the evoke d EPSP following paired training, nor does paired training specificall y enhance spontaneous release frequency. Fairing-specific facilitation was not blocked by the protein kinase C inhibitor H7. In contrast, th e same training procedure produced pairing-specific increases of senso ry neuron excitability and action potential width, suggesting that cAM F-mediated processes are involved in the paired effect. Although Ca2influx is necessary for the associative effect (Abrams, 1985), we find that the facilitation does not require influx through L-type voltage- gated Ca2+ channels, since the effect was not blocked by the dihydropy ridine antagonist nitrendipine. Together, these findings indicate that the mechanism underlying associative, activity-dependent facilitation is intrinsic to the sensory neuron synapse, that it is presynapticall y mediated by processes unique to evoked synaptic transmission, and th at it appears to involve a pairing-specific broadening of the presynap tic action potential, allowing enhanced Ca2+ influx through the dihydr opyridine-insensitive channels responsible for release.