Facilitation of Synaptic Transmission by EGL-30 Gqα and EGL-8 PLCβ DAG Binding to UNC-13 Is Required to Stimulate Acetylcholine Release

Therefore, in these cases the modulators regulate exo-cytosis at a step downstream from Ca for release, termed the readily releasable pool. Recent work suggests that the size of the readily releasable pool is a target for neuromodulation. The readily releas-Summary able pool is measured either by staining with the styril dyes (e.g., FM1–43) or by analysis of the kinetics of We show that neurotransmitter release at Caenorhab-ditis elegans neuromuscular junctions is facilitated by the rundown of postsynaptic responses following high-frequency stimuli, which is assumed to reflect depletion a presynaptic pathway composed of a G q ␣ By either erol-(DAG-) binding protein UNC-13. Activation of this pathway increased release of acetylcholine at neuro-of these methods, serotonin increases the size of this readily releasable pool of vesicles and facilitates release muscular junctions, whereas inactivation decreased release. Phorbol esters stimulated acetylcholine re-at the crayfish neuromuscular junction (Wang and Zucker, 1998). Phorbol esters also increase the size of lease, and this effect was blocked by a mutation that eliminates phorbol ester binding to UNC-13. Expres-the readily released pool in both neurons and chromaffin sion of a constitutively membrane-bound form of UNC-13 restored acetylcholine release to mutants lacking However, in other cases neuromodulators and phorbol esters appear to regulate exocytosis at a step down-the egl-8 PLC␤. Activation of this pathway with musca-rinic agonists caused UNC-13 to accumulate in punc-stream of Ca 2ϩ influx without altering the readily releas-able pool size (Isaacson and Hille, 1997; Yawo, 1999). tate structures in the ventral nerve cord. These results suggest that presynaptic DAG facilitates synaptic Together, these results suggest that the processes of synaptic vesicle exocytosis and recycling can be directly transmission and that part of this effect is mediated by UNC-13. regulated by neuromodulators in some manner but that different mechanisms may be utilized at different syn-apses. Introduction To better define the mechanisms of neuromodulation, we have carried out a genetic analysis of the inhibition G protein–coupled receptors often modulate synaptic and facilitation of neurotransmission in an in vivo sys-strength by regulating release of neurotransmitter from tem, the Caenorhabditis elegans neuromuscular junc-nerve terminals. These neuromodulatory pathways can tion. We previously showed that goa-1 G o ␣ inhibits ace-either facilitate or inhibit neurotransmitter release (Nicoll tylcholine release at these synapses by reducing the et al., 1990). In some cases, neuromodulators regulate levels of UNC-13 at release sites (Nurrish et al., 1999). synaptic transmission via regulation of presynaptic ion Here, …