We are analyzing both how the nervous system
controls behavior and how genes specify the functioning of a neuromuscular
system. As described under "NEURAL
DEVELOPMENT," the C. elegans nervous system
is strikingly simple and exceptionally well described. We have studied
many aspects of the functioning of the C. elegans nervous system.
For example, we use a laser microbeam, pharmacology and mutations
to identify which neurons control specific behaviors. In this way,
we defined the complete GABAergic nervous system and the neural circuits
of and the behaviors controlled by 25 of the 26 GABAergic neurons.
We have analyzed a diversity of the animal’s behaviors, including
egg laying, feeding, defecation, locomotion, and foraging. We have
analyzed the animal’s chemosensory system and aspects of its
mechanosensory systems. We discovered that C. elegans has a
sense of smell and characterized its olfactory system. We have studied
simple motor outputs, reflex circuits, integrated circuits within
its central nervous system and behaviors that are modulated by the
environment and/or experience. We have also identified and are now
characterizing a two-pore potassium channel protein complex that regulates
muscle contraction.
One major current focus is to understand how both the environment
and experience modulate C. elegans behavior. For example, we
are examining how bacteria, the animal’s standard food source,
modulate locomotion and egg laying. We observed that when a worm encounters
bacteria it slows its rate of locomotion (the basal slowing response),
presumably to cause it to stay where the food supply is ample. We
found that this behavioral response can be modulated by experience:
worms that have been transiently deprived of food slow even more than
do well-fed worms (the enhanced slowing response), as if "hungry"
worms are taking no chances about leaving a food source. We showed
that the basal slowing response and the enhanced slowing response
are mediated by distinct mechanisms. For example, whereas the dopaminergic
nervous system controls the basal slowing response, the serotonergic
nervous system controls the enhanced slowing response. We are isolating
and characterizing mutants abnormal in the enhanced slowing response.
One of these mutants defined MOD-1, which encodes a novel class of
serotonin (5HT) receptor. Like the mammalian 5HT-3 receptor, MOD-1
is a ligand-gated ion channel, but unlike the 5HT-3 receptor, MOD-1
conducts chloride ions and hence its activity is likely to be inhibitory
rather than stimulatory. We plan to seek mammalian equivalents of
MOD-1. Such a molecule could have important implications for mammalian
neurobiology as well as for the therapeutic treatment of disorders
caused by serotonin dysfunction, which include disorders affecting
mood, sleep, pain, thermoregulation, appetite, sexual function, thirst,
blood pressure, respiration, heart rate and possibly memory.
The MOD-1 protein is a serotonin-gated
chloride channel, as exemplified by electrophysiological studies of
voltage-clamped Xenopus oocytes. MOD-1 responds specifically
to serotonin and shows a reversal potential that varies linearly with
the log of the chloride concentration.
A second gene that affects the enhanced slowing
response encodes MOD-5, a serotonin reuptake transporter similar to
the human protein that is the target of many of the pharmaceutical
agents used clinically to treat depression, including amitryptaline,
Prozac and Celexa. Such serotonin reuptake transporters mediate the
removal of serotonin from the synaptic cleft, so that when transporter
function is blocked, serotonin remains in the cleft longer, thereby
enhancing serotonergic function. Thus, whereas mod-1 mutations
block the enhanced slowing response, mod-5 mutations increase
the enhanced slowing response. We found that some but not all of the
effects of Prozac on C. elegans are mediated through serotonin
and MOD-5, and we plan to identify the other target(s) of Prozac,
with the hope that this knowledge will facilitate the development
of more specific therapeutic compounds. We are extending our studies
of the effects of acute food deprivation from locomotion to egg laying
with the goal of determining how experience and the environment coordinately
control multiple behaviors. In addition, by characterizing genes that
act in the enhanced slowing response downstream of the MOD-1 serotonin
receptor, we are attempting to define a complete pathway for serotonin
signal transduction. Finally, we are attempting to isolate mutants
with defects in the pathway that stores and retrieves the "memory"
of food deprivation. We hope that these studies will reveal where
and how C. elegans stores, retrieves and integrates information
concerning its prior exposure to food, and how it uses this information
to modulate serotonergic neurotransmission and control its behavior
in response to its past experience.
We are also pursuing the role of other biogenic amines, including
octopamine, as possible modulators of C. elegans behavior.
We found that exogenous octopamine antagonizes many of the behavioral
effects of exogenous serotonin. To analyze the role of octopamine,
we isolated a deletion allele of the putative tyramine-beta-hydroxylase
gene tbh-1, since this enzyme is essential for the biosynthesis
of octopamine. tbh-1 mutants, like wild-type animals exposed
to exogenous serotonin, have reduced locomotory rates, consistent
with the hypothesis that octopamine and serotonin act antagonistically.
The TBH-1 protein is specifically expressed in two head interneurons.
We are investigating the role of these neurons and the neural circuit
in which they act.
The repeated actions of neuromodulators, such as serotonin and octopamine,
can cause long-term changes in synaptic function. Studies of long-term
memory formation in the sea slug Aplysia, the fruit fly Drosophila
and mice have indicated a major role of the cyclic AMP response element
binding protein CREB. To explore the role of CREB in C. elegans,
we have isolated deletion alleles of the CREB-like gene crh-1.
These mutants display defects consistent with the hypothesis that
this CREB gene acts downstream of serotonin to mediate long-term changes
in both behavior and development dependent upon aspects of the animal’s
past experience.
Publications:
Behavior
Abstracts:
Behavior
