Phorbol ester enhances synaptic transmission at crustacean neuromuscular junctions

Effects of phorbol ester (PE) (4β‐phorbol‐12,13‐dibutyrate) on transmitter release were studied in the deep extensor neuromuscular system of the prawn, Macrobrachium rosenbergii. Our findings show that PE enhances transmiter release as indicated by an increase in the quantal content. PE had no past‐synaptic effects. The increase in release is accompanied by a slight decline in twin pulse facilitation, suggesting a minor increase in Ca²⁺ entry. The fact that the increase in Ca²⁺ entry has a minor contribution to the PE effect is supported by the following observations: the duration of facilitation was not affected by PE, and 3,4‐diaminopyridine (3,4‐DAP), which by itself increased release, did not reduce the effect of PE. The time course of release was measured from synaptic delay histograms, upon which PE had no effect. The finding indicates that protein kinase C (PKC) is probably not involved in the rate limiting step of the process of secretion. The log/log plot of the initial part of the delay histogram is not affected by PE, suggesting a lach of effect on cooperativity of the release process. Increased release by loading the presynaptic terminal with Ca²⁺ either by pretreatment with Ca²⁺ ionophore or by frequent stimulation prevented further increase in release by PE. We conclude that the main effect of PE is confined to stages of release that are secondary to the first elevation in presynaptic Ca²⁺. PKC in this system probably plays a role in long term modulation of release, and it can be activated in processes leading to presynaptic Ca²⁺ accumulation.