Our current project focuses on the defecation behavior of this species. This behavior
is a rhythmic behavior that occurs every 45 seconds and can be used as an indicator of a
synaptic transmission activity. More details about these projects are described in the Defecation Motor Program, Defecation
Rhythm and IP3 Receptor, and Synaptic Transmission and
Defecation Motor Program.
The defecation motor program (DMP) consists of a stereotyped series of three muscle
contractions (Fig. 1). First, posterior body-wall muscles contract (pBoc) and then relax,
causing to accumulate the gut contents near the anus. Three to four seconds later,
anterior body-wall muscles contract (aBoc) to pressurize the gut contents. Finally,
specialized enteric muscles contract to expel the contents from the anus (Exp). These
motor program steps were recorded and displayed as an ethogram (Fig. 1), using a computer
program.
Figure 1. The Caenorhabditis elegans defecation motor program: (1)
Intercycle; (2) posterior body wall muscle contraction (pBoc); (3) anterior body wall
muscle contraction (aBoc); (4) enteric muscle contraction with expulsion of gut contents
(Exp); and return to intercycle.
In Caenorhabditis elegans, one of the best-studied rhythmic motor pattern is
defecation. In the presence of abundant food, the defecation motor program (DMP) is
activated every 45 seconds with little variation. Recently Erik Jorgensen's group found
that mutations in the IP3 Receptor gene cause defects in the defecation rhythm and that
the DMP activation correlates with Calcium oscillation in the intestine. These facts
indicate that the defecation clock is located in the intestine and transmits signals to
neurons and muscles for the DMP execution.
This rhythmic behavior has features similar to those of circadian clocks. (1) The
defecation rhythm can be reset by a sensory stimulus. When a touch stimulus is given to a
wild-type animal, the next DMP initiates approximately 45 seconds after the stimulus,
suggesting that the defecation clock has been reset to zero. (2) The defecation rhythm
stays approximately constant at temperatures ranging from 19?C to 30?C (temperature
compensation), while the rates of other behavioral processes show more typical temperature
dependence. (3) Defecation rhythm can be maintained without expressing the DMP. The DMP is
activated periodically on a bacterial lawn (food for this nematode). When an animal
spontaneously crawls out of the lawn it stops defecating, and when it returns to the lawn
defecation resumes. The phase of the resumed rhythm is usually the same as the phase
established before leaving the lawn, suggesting that the defecation clock runs without DMP
expression.
Figure 2. Ethograms (Ethological diagram, which describes behavior of
individual animals) of defecation behavior in the wild type animals. Each dot or character
represents one second. Seconds elapsed are indicated above each ethogram. p and x
represent pBoc and Exp, respectively.
aBoc and Exp seem to be particularly sensitive to loss of some synaptic transmission
genes. For example, a lack of either snt-1 (synaptotagmin) or aex-3 (Rab3 GDP/GTP exchange
factor) causes aBoc and Exp defects, and this phenotype is called Aex (Fig. 3). We use the
Aex phenotype as an indicator of neural activities to study novel genes in this synaptic
transmission pathway. Recently we identified the new gene cab-1 functioning in the
pathway. CAB-1 interacts with AEX-3 independent of RAB-3, which is a GTP binding protein
and regulates synaptic transmission. AEX-3 (Rab3 GEF) interacts with CAB-1 as well as
RAB-3 (Fig. 4).
Figure 3. aex-3 and cab-1 mutants fail to express the aBoc and Exp
steps.
Figure 4. AEX-3 interacts with RAB-3 through its N terminus and with
CAB-1 through its C terminus. RAB-3 and CAB-1 appear to regulate distinct pathways.
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