Area-restricted search (ARS) is a foraging strategy used by many animals to locate resources. The behavior is characterized by a timedependent reduction in turning frequency after the last resource encounter. This maximizes the time spent in areas in which resources are abundant and extends the search to a larger area when resources become scarce. We demonstrate that dopaminergic and glutamatergic signaling contribute to the neural circuit controlling ARS in the nematode Caenorhabditis elegans. Ablation of dopaminergic neurons eliminated ARS behavior, as did application of the dopamine receptor antagonist raclopride. Furthermore, ARS was affected by mutations in the glutamate receptor subunits GLR-1 and GLR-2 and the EAT-4 glutamate vesicular transporter. Interestingly, preincubation on dopamine restored the behavior in worms with defective dopaminergic signaling, but not in glr-1, glr-2, or eat-4 mutants. This suggests that dopaminergic and glutamatergic signaling function in the same pathway to regulate turn frequency. Both GLR-1 and GLR-2 are expressed in the locomotory control circuit that modulates the direction of locomotion in response to sensory stimuli and the duration of forward movement during foraging. We propose a mechanism for ARS in C. elegans in which dopamine, released in response to food, modulates glutamatergic signaling in the locomotory control circuit, thus resulting in an increased turn frequency.

How simple neuronal circuits control behavior is not well understood at the molecular or genetic level. In Caenorhabditis elegans, foraging behavior consists of long, forward movements interrupted by brief reversals. To determine how this pattern is generated and regulated, we have developed novel perturbation techniques that allow us to depolarize selected neurons in vivo using the dominant glutamate receptor mutation identified in the Lurcher mouse. Transgenic worms that expressed a mutated C. elegans glutamate receptor in interneurons that control locomotion displayed a remarkable and unexpected change in their behavior-they rapidly alternated between forward and backward coordinated movement. Our findings suggest that the gating of movement reversals is controlled in a partially distributed fashion by a small subset of interneurons and that this gating is modified by sensory input.

In almost all nervous systems, rapid excitatory synaptic communication is mediated by a diversity of ionotropic glutamate receptors. In Caenorhabditis elegans, 10 putative ionotropic glutamate receptor subunits have been identified, a surprising number for an organism with only 302 neurons. Sequence analysis of the predicted proteins identified two NMDA and eight non-NMDA receptor subunits. Here we describe the complete distribution of these subunits in the nervous system of C. elegans. Receptor subunits were found almost exclusively in interneurons and motor neurons, but no expression was detected in muscle cells. Interestingly, some neurons expressed only a single subunit, suggesting that these may form functional homomeric channels. Conversely, interneurons of the locomotory control circuit (AVA, AVB, AVD, AVE, and PVC) coexpressed up to six subunits, suggesting that these subunits interact to generate a diversity of heteromeric glutamate receptor channels that regulate various aspects of worm movement. We also show that expression of these subunits in this circuit is differentially regulated by the homeodomain protein UNC-42 and that UNC-42 is also required for axonal pathfinding of neurons in the circuit. In wild-type worms, the axons of AVA, AVD, and AVE lie in the ventral cord, whereas in unc-42 mutants, the axons are anteriorly, laterally, or dorsally displaced, and the mutant worms have sensory and locomotory defects. Key words: glutamate receptor; neuron; neural circuit; development; mechanosensation; homeodomain transcription factor; Caenorhabditis elegans; locomotion; glr-1; unc-42Glutamate is a neurotransmitter that is required for synaptic communication in vertebrate and invertebrate nervous systems. Signaling by glutamate is mediated by a large and diverse number of receptors that include ionotropic receptors that mediate rapid excitatory neurotransmission. Ionotropic glutamate receptors belong to either the NMDA family, which contains receptors that are selectively gated by the agonist NMDA, or the non-NMDA family, which contains receptors that are gated by the agonists AMPA and kainate (Dingledine et al., 1999;Hollmann, 1999). In vertebrates, 18 subunits have been identified, allowing for combinatorial complexity and the formation of heteromeric receptors that have different functional properties (Hollmann, 1999). Functional receptors are believed to be composed of either four or five subunits of the same subtype (AMPA, kainate, or NMDA) (Premkumar and Auerbach, 1997;Rosenmund et al., 1998;Dingledine et al., 1999). NMDA receptors contain the NR1 subunit and at least one NR2 subtype (Dunah et al., 1998), AMPA receptors contain from one to three different receptor subunits (Wenthold and Roche, 1998), and kainate receptors can function as homomers or heteromers (Paternain et al., 2000). Many of these receptor subunits are coexpressed in neurons, and in some neurons receptor subunits are differentially distributed at synapses (Petralia et al., 1999b). Presumably, individual neurons express different...

The C. elegans polymodal ASH sensory neurons detect mechanical, osmotic, and chemical stimuli and release glutamate to signal avoidance responses. To investigate the mechanisms of this polymodal signaling, we have characterized the role of postsynaptic glutamate receptors in mediating the response to these distinct stimuli. By studying the behavioral and electrophysiological properties of worms defective for non-NMDA (GLR-1 and GLR-2) and NMDA (NMR-1) receptor subunits, we show that while the osmotic avoidance response requires both NMDA and non-NMDA receptors, the response to mechanical stimuli only requires non-NMDA receptors. Furthermore, analysis of the EGL-3 proprotein convertase provides additional evidence that polymodal signaling in C. elegans occurs via the differential activation of postsynaptic glutamate receptor subtypes.

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is important for synaptic plasticity and nervous system development and function. We have used genetic and electrophysiological methods to demonstrate that NMR-1, a Caenorhabditis elegans NMDA-type ionotropic glutamate receptor subunit, plays a role in the control of movement and foraging behavior. nmr-1 mutants show a lower probability of switching from forward to backward movement and a reduced ability to navigate a complex environment. Electrical recordings from the interneuron AVA show that NMDA-dependent currents are selectively disrupted in nmr-1 mutants. We also show that a slowly desensitizing variant of a non-NMDA receptor can rescue the nmr-1 mutant phenotype. We propose that NMDA receptors in C. elegans provide long-lived currents that modulate the frequency of movement reversals during foraging behavior.

Small, high impedance neurons with short processes, like those found in the soil nematode C. elegans, are predicted to transmit electrical signals by passive propagation. We now show that certain neurons in C. elegans fire regenerative action potentials. These neurons resemble Schmitt triggers because their potential state appears to be bistable. Transitions between up and down states can be triggered by application of the neurotransmitter glutamate or brief current pulses.The nematode C. elegans is widely used for genetic studies of nervous system development and function. One attractive feature of C. elegans, a self-fertilizing hermaphrodite organism with 302 neurons, is the prospect of gaining a molecular understanding of how neural circuits control behavior 1 . In most of the vertebrate nervous system, information is coded by the frequency of action potentials-regenerative all-or-none changes in the membrane potential that allow the transmission of information over long distances without the decrease in information content that would occur with passive propagation. Although many molecules are conserved between C. elegans and vertebrate nervous systems, predicted channel proteins that would contribute to Na + -dependent action potentials have not been identified 2 , suggesting that C. elegans neurons may transmit information by simple passive propagation. This notion is supported by an in vivo electrophysiological study that found select neurons to be isopotential, with no evidence of classical action potentials, suggesting passive signal propagation along high impedance neurons 3 . In addition, electrical recordings have not revealed action potentials in the parasitic nematode Ascaris suum 4 . On the other hand, there is indirect evidence that C. elegans neurons may have intrinsic membrane properties that allow generation of action potentials 5 . Moreover, action potentials in neurons and muscles can be generated by the activation of voltage-gated calcium channels and several studies have suggested the possibility of active signaling in C. elegans muscles 5,6 and graded active responses in Ascaris 7 .We now find that at least one class of neurons in C. elegans fires regenerative action potentials that can lead to long-lived changes in membrane potential. C. elegans RMD class interneurons, which synapse with interneurons and muscle cells, and AVA command interneurons, which synapse with motor neurons, primarily contribute to the control of head movements and locomotion 8 . These neurons express ionotropic glutamate receptors (iGluRs), including the AMPA receptor subunit GLR-1 9 . Using previously described patch-clamp recording techniques 10 , we recorded from a neuron immediately adjacent to AVA in transgenic worms that expressed a GFP transgene under control of the glr-1 promoter. Based on position and GFP expression we believe this neuron was RMD. In 14 out of 16 RMD neurons, we found that the voltage response to depolarizing current ramps was linear from approximately −80 *Corresponding Author (E-...

Ionotropic glutamate receptors (iGluRs) mediate most excitatory synaptic signalling between neurons. Binding of the neurotransmitter glutamate causes a conformational change in these receptors that gates open a transmembrane pore through which ions can pass. The gating of iGluRs is crucially dependent on a conserved amino acid that was first identified in the 'lurcher' ataxic mouse. Through a screen for modifiers of iGluR function in a transgenic strain of Caenorhabditis elegans expressing a GLR-1 subunit containing the lurcher mutation, we identify suppressor of lurcher (sol-1). This gene encodes a transmembrane protein that is predicted to contain four extracellular beta-barrel-forming domains known as CUB domains. SOL-1 and GLR-1 are colocalized at the cell surface and can be co-immunoprecipitated. By recording from neurons expressing GLR-1, we show that SOL-1 is an accessory protein that is selectively required for glutamate-gated currents. We propose that SOL-1 participates in the gating of non-NMDA (N-methyl-D-aspartate) iGluRs, thereby providing a previously unknown mechanism of regulation for this important class of neurotransmitter receptor.

Summary The adult nervous system is plastic allowing us to learn, remember and forget. Experience-dependent plasticity occurs at synapses – the specialized points of contact between neurons where signaling occurs. However, the mechanisms that regulate the strength of synaptic signaling are not well understood. Here, we define a Wnt signaling pathway that modifies synaptic strength in the adult nervous system by regulating the translocation of one class of acetylcholine receptors (AChRs) to synapses. In C. elegans, we show that mutations in CWN-2 (Wnt ligand), LIN-17 (Frizzled), CAM-1 (Ror receptor tyrosine kinase), or the downstream effector DSH-1 (disheveled) result in similar subsynaptic accumulations of ACR-16/α7 AChRs, a consequent reduction in synaptic current, and predictable behavioral defects. Photoconversion experiments revealed defective translocation of ACR-16/α7 to synapses in Wnt signaling mutants. Using optogenetic nerve stimulation, we demonstrate activity-dependent synaptic plasticity and its dependence on ACR-16/α7 translocation mediated by Wnt signaling via LIN-17/CAM-1 heteromeric receptors.