Network plasticity arises in large part due to the effects of exogenous neuromodulators, many of which modify synaptic transmission. The rhythmically active pyloric network of the crab C. borealis is a well-characterized CPG and is subject to extensive neuromodulation. We investigate the effects of the neuropeptide proctolin on the dynamics of the inhibitory synapse from the LP to the PD neuron in the pyloric network of the crab C. borealis. This synapse has both spike-mediated and non-spike-mediated (graded) components and is the only chemical feedback to the pyloric pacemaker neurons. Previous studies in our lab have shown that, in the presence of proctolin, low-amplitude presynaptic pulses cause the graded component to facilitate (Atamturktur et al, SFN Abst 657.18, 2004). We show that this facilitation is correlated with the activation of a presynaptic inward current. This inward current is blocked by Mn++, suggesting that it is a slowly-accumulating Ca++ current activated by proctolin. We also charact...