Our focus is to understand the molecular and cellular basis of behavioral plasticity. We are studying how the environment modulates behavior of the nematode Caenorhabditis elegans. The C. elegans nervous system is very simple and extraordinarily well described. The detailed knowledge of the C. elegans nervous system combined with its amenability to genetic analysis and laser microsurgery allows us to define neural circuits that control behavior and study behavior at the molecular and cellular level.
How does the nervous system translate sensory information into behavioral response? Facing the complexity of the mammalian nervous system this fundamental question presents daunting task. Some of the rare cases where we actually know the neural path, from sensory input to motor output, have come from the analyses of escape responses in mollusks, crayfish and goldfish (Korn and Faber, 2005; Edwards et al., 2002; Allen et al., 2006). Defining sensorimotor circuits requires detailed knowledge of the neural connectivity of the nervous system, and the ability to manipulate the functions of the component neurons and to define and quantify the behavioral outputs. The simplicity and completely defined synaptic connectivity of C. elegans nervous system provides unique opportunity to dissect how neural networks control behavior. Moreover, the combination of powerful genetic methods, calcium imaging and electrophysiology allows us to address how the nervous controls behavior with a cellular and molecular resolution that cannot be readily attained in other systems.
Gentle touch elicits an escape response C. elegans where the animal displays characteristic sequence of behaviors to get away from the stimulus. C. elegans moves on its side by propagating a sinusoidal wave of body wall muscle contractions along the length of its body. C. elegans locomotion is accompanied by oscillatory head movements during which the tip of the nose moves rapidly from side to side. First, in response to touch to the anterior half of the body of the animal reverses its direction of locomotion (Chalfie et al., 1985). During this reversal the animals suppresses its lateral head movements (Alkema et al., 2005). Second, the reversal is followed by a deep ventral head bend. Third, the animal makes a sharp turn where it slides the head down the ventral side of the body. This sharp turn (Omega turn) results in approximately 180° change in locomotion anterior. Fourth, the animal resumes forward locomotion and exploratory head movements. Based on the strength of the stimulus the animal has to decide whether to engage in an escape response. Once it does, the animal needs coordinate distinct motor programs, generate asymmetry in its locomotion pattern to allow it to make a sharp turn before it returns to a base state. Our goal is to elucidate what neurons, neurotransmitters and receptors define neural circuits that control these motor programs, and how these motor programs are linked temporally in the execution of the worm escape response.
Our previous work and that of others has provided some clues about the neurons that are required for these motor programs. The C. elegans neural wiring diagram and laser ablation experiments support a model in which the touch sensory neurons inhibit the forward locomotion command neurons and activate the backward command neurons causing the animal to move backward away from the stimulus. We have shown that the trace amine, tyramine, plays a crucial role in the coordination of the backing response and the suppression of head oscillations in the escape response. A pair of tyraminergic motorneurons is activated through gap junctions with the backward locomotion command neurons, triggering the release of tyramine (Alkema et al., 2005). Tyramine coordinates two motor programs by inhibiting the forward locomotion command neurons and directly hyperpolarizing neck muscles through the activation of a novel tyramine gated chloride channel, LGC-55 (Pirri et al., 2009).
In predator-prey experiments we have been able to show that the suppression of head movements allows the animal to escape from nematophagous fungi that entrap nematodes. Tyramine signaling mutants that fail to suppress head oscillations on response to touch are more likely to get caught in constricting hyphal rings that inflate upon contact (Maguire et al., 2011). Which neurons are required for the steep ventral head bend, and how the motor neurons in the ventral cord execute an omega turn is largely unknown. Moreover, it is not clear how a long reversal is coupled to an omega turn.
While we have shown that a fast acting ionotropic tyramine receptors is involved in the immediate suppression of head oscillations and reversal upon touch, the slower acting metabotropic tyramine receptors appear to be involved in the execution of the omega turn. We found that the G-protein coupled tyramine receptor, SER-2, is expressed in a subset of inhibitory GABAergic neurons that innervate body wall muscles on the ventral side of the animal. Our genetic and behavioral analyses indicate that SER-2 inhibits GABA release to allow the animal to hypercontract its ventral side during the execution of an omega turn. ser-2 mutants initiate a normal escape response but fail to touch head to tail during an omega turn. This suggests that aminergic modulation of ventral cord motorneurons may allow the animal to generate asymmetry in its locomotion pattern (Donnelly et al., 2013).
Ultimately, we hope that our studies will teach us more about the basic principles that underlie behavioral plasticity of more complex neural systems.