LATENT INHIBITORY CONNECTIONS BECOME FUNCTIONAL DURING ACTIVITY-DEPENDENT PLASTICITY

Simultaneous pre- and postsynaptic recordings from identified glyciner gic inhibitory interneurons and the Mauthner cell showed that 25% of t he afferents produced no or extremely small postsynaptic responses. Mo rphological determination of the number of contacts made by these cell s on the Mauthner cell revealed a connectivity similar to that of func tional neurons which always produce clear inhibitory postsynaptic pote ntials, suggesting that most of the endings made by weak interneurons are silent. Intraaxonal injection of 4-aminopyridine or Ca2+ greatly e nhanced transmission at functional connections but did not modify thos e which were ineffective. However, after eighth nerve tetanic stimuli, transmission at the weak connections was unmasked or enhanced for pro longed periods and was twice as likely to be potentiated, with a 6-fol d greater mean enhancement than the potent ones. This result provides additional support for long-term potentiation of inhibitory synapses. Furthermore, weakly functional junctions represent a ''reserve'' pool which can be critical for the expression of plasticity within a networ k, and, consequently, for setting the threshold of reflex activities s uch as the escape reaction mediated by the Mauthner cell.