Effect of holding potential on the dynamics of homosynaptic facilitation

We study a form of short-term synaptic plasticity that was originally described as a graded potentiating effect of holding potential on spike-mediated synaptic transmission (Shimahara and Tauc, 1975). This form of plasticity has recently generated considerable interest, as it has become apparent that it is present in the mammalian brain (Clark and Häusser, 2006; Marder, 2006). It has been suggested that it adds a previously unappreciated analog component to spike-mediated synaptic transmission (Alle and Geiger, 2006, 2008). A limitation of most previous research in this area is that effects of holding potential have been studied in relative isolation. Presynaptic neurons are stimulated at low frequencies so that a second form of plasticity (homosynaptic facilitation) is not induced. Under physiological conditions, however, both forms of plasticity are likely to be coinduced. In this report, we study the two types of plasticity together in an experimentally advantageous preparation (the mollusk Aplysia californica). Somewhat surprisingly, we find that effects of holding potential can be relatively modest when presynaptic neurons are activated at low frequencies. Interestingly, however, changes in membrane potential are highly effective when homosynaptic facilitation is induced. In this situation, PSPs facilitate at an increased rate. To summarize, our research suggests a novel view of the effect of holding potential on synaptic transmission. We propose that, under physiological conditions, it modifies the dynamics of homosynaptic facilitation.