Fig. 3. Loss of cca-1 function or reduction of MC neurotransmission compromises action potential initiation and reduces pharyngeal pumping rates. (A) Effect of cca-1 mutations on the pumping rates of intact worms in the presence of E. coli HB101. Results are shown as the mean number of pumps per minute. Values are means ± standard error of the mean (S.E.M.). N=20 worms for each data point. See Materials and methods for additional details. (B) In electropharyngeograms (EPGs) from wild-type worms, a small EPSP (excitatory post-synaptic potential) precedes each excitation (E-phase) spike. An EPSP is marked with a large black arrow while an E-spike is marked with a small black arrow. The action potential terminates with a large negative repolarization (R-phase) spike (large gray arrow). Smaller negative spikes between E and R represent inhibitory post-synaptic potentials (IPSPs) from the inhibitory motor neuron M3 (Dent et al., 1997; Raizen and Avery, 1994). (C) In EPGs from cca-1 mutants homozygous for either of two loss-of-function alleles (ad1650 or gk30), small, delayed E-phase spikes (small arrows) follow the EPSPs. EPGs from cca-1 mutants also contain interpump phase (I-phase) spikes between pumps (arrowheads). (D) Effect on EPG traces of defects in MC neurotransmission. unc-17 encodes a membrane transporter that loads acetylcholine into synaptic vesicles within the cholinergic neurons, including the MC motor neuron. e245 is a viable missense allele of unc-17 (Alfonso et al., 1993). Null mutations of unc-17 are lethal. EPGs from unc-17(e245) mutants contain occasional I-phase spikes (arrowheads), because the low acetylcholine content of synaptic vesicles reduces the success rate of MC neurotransmission. snt-1 encodes synaptotagmin, a vesicle-associated protein necessary for effective calcium-stimulated release of neurotransmitter. snt-1(md290) is a putative null allele of synaptotagmin (Nonet et al., 1993). EPGs from snt-1(md290) mutant worms contain many I-phase spikes (arrowheads), often occurring in clusters, as a result of uncoordinated neurotransmitter release. Because EPGs recorded from different individual worms show some variation, we have annotated the features that consistently differ between worms of different genotypes. Unmarked differences are likely to be due to individual variation.