No abstract

The frequencies of certain periodic behaviors of the nematode C. elegans are regulated in a dose-dependent manner by the activity of the gene egl-10. These behaviors are modulated oppositely by the activity of the G protein alpha subunit gene goa-1, suggesting that egl-10 may regulate a G protein signaling pathway in a dose-dependent fashion. egl-10 encodes a protein similar to Sst2p, a negative regulator of G protein signaling in yeast. EGL-10 protein is localized in neural processes, where it may function in neurotransmitter signaling. Two previously known and 13 newly identified mammalian genes have similarity to egl-10 and SST2, and we propose that members of this family regulate many G protein signaling pathways.

D1-like and D2-like dopamine receptors have synergistic and antagonistic effects on behavior. To understand the mechanisms underlying these effects, we studied dopamine signaling genetically in Caenorhabditis elegans. Knocking out a D2-like receptor, DOP-3, caused locomotion defects similar to those observed in animals lacking dopamine. Knocking out a D1-like receptor, DOP-1, reversed the defects of the DOP-3 knockout. DOP-3 and DOP-1 have their antagonistic effects on locomotion by acting in the same motor neurons, which coexpress the receptors and which are not postsynaptic to dopaminergic neurons. In a screen for mutants unable to respond to dopamine, we identified four genes that encode components of the antagonistic Galpha(o) and Galpha(q) signaling pathways, including Galpha(o) itself and two subunits of the regulator of G protein signaling (RGS) complex that inhibits Galpha(q). Our results indicate that extrasynaptic dopamine regulates C. elegans locomotion through D1- and D2-like receptors that activate the antagonistic Galpha(q) and Galpha(o) signaling pathways, respectively.

Four biogenic amines: octopamine, tyramine, dopamine and serotonin act in C. elegans to modulate behavior in response to changing environmental cues. These neurotransmitters act at both neurons and muscles to affect egg laying, pharyngeal pumping, locomotion and learning. A variety of experimental approaches including genetic, imaging, biochemical and pharmacological analyses have been used to identify the enzymes and cells that make and release the amines and the cells and receptors that bind them. Dopamine and serotonin act through receptors and downstream signaling mechanisms similar to those that operate in the mammalian brain suggesting that C. elegans will provide a valuable model for understanding biogenic amine signaling in the brain.

Like many behaviors, Caenorhabditis elegans egg laying alternates between inactive and active states. To understand how the underlying neural circuit turns the behavior on and off, we optically recorded circuit activity in behaving animals while manipulating circuit function using mutations, optogenetics, and drugs. In the active state, the circuit shows rhythmic activity phased with the body bends of locomotion. The serotonergic HSN command neurons initiate the active state, but accumulation of unlaid eggs also promotes the active state independent of the HSNs. The cholinergic VC motor neurons slow locomotion during egg-laying muscle contraction and egg release. The uv1 neuroendocrine cells mechanically sense passage of eggs through the vulva and release tyramine to inhibit egg laying, in part via the LGC-55 tyramine-gated Cl- channel on the HSNs. Our results identify discrete signals that entrain or detach the circuit from the locomotion central pattern generator to produce active and inactive states.DOI: http://dx.doi.org/10.7554/eLife.21126.001

To elucidate the cellular role of the heterotrimeric G protein G o , we have taken a molecular genetic approach in Caenorhabditis elegans. We screened for suppressors of activated GOA-1 (G o ␣) that do not simply decrease its expression and found mutations in only two genes, sag-1 and eat-16. Animals defective in either gene display a hyperactive phenotype similar to that of goa-1 loss-of-function mutants. Double-mutant analysis indicates that both sag-1 and eat-16 act downstream of, or parallel to, G o ␣ and negatively regulate EGL-30 (G q ␣) signaling. eat-16 encodes a regulator of G protein signaling (RGS) most similar to the mammalian RGS7 and RGS9 proteins and can inhibit endogenous mammalian G q /G 11 in COS-7 cells. Animals defective in both sag-1 and eat-16 are inviable, but reducing function in egl-30 restores viability, indicating that the lethality of the eat-16; sag-1 double mutant is due to excessive G q ␣ activity. Analysis of these mutations indicates that the G o and G q pathways function antagonistically in C. elegans, and that G o ␣ negatively regulates the G q pathway, possibly via EAT-16 or SAG-1. We propose that a major cellular role of G o is to antagonize signaling by G q .

Egg-laying behavior in Caenorhabditis elegans is activated by signaling through the G-protein G(rho)q and inhibited by signaling through a second G-protein, G(rho)o. Activation of egg laying depends on the serotonergic hermaphrodite-specific neurons (HSNs), but the neurotransmitter(s) and cell(s) that signal to inhibit egg laying are not known. Mutants for G-protein signaling genes have well characterized defects in egg laying. Here we present an analysis of mutants for other genes reported to lack inhibition of egg laying. Of the nine strongest, six have morphological defects in the ventral-type C (VC) neurons, which synapse onto both the HSNs and the egg-laying muscles and are thus the third cell type comprising the egg-laying system. Laser-ablating VC neurons could also disrupt the inhibition of egg laying. The remaining three mutants (unc-4, cha-1, and unc-17) are defective for synthesis or packaging of acetylcholine in the VCs. The egg-laying defects of unc-4, cha-1, and unc-17 were rescued by VC-specific expression of the corresponding cDNAs. In addition, increasing synaptic acetylcholine by reducing acetylcholinesterase activity, with either mutations or the inhibitor aldicarb, decreased egg laying. Finally, we found that a knock-out for the HSN-expressed receptor G-protein-coupled acetylcholine receptor 2 (GAR-2) shows a partial defect in the inhibition of egg laying and fails to respond to aldicarb. Our results show that acetylcholine released from the VC neurons inhibits egg-laying behavior. This inhibition may be caused, in part, by acetylcholine signaling onto the HSN presynaptic terminals, via GAR-2, to inhibit neurotransmitter release.

SUMMARY Little is known about how animals integrate multiple sensory inputs in natural environments to balance avoidance of danger with approach to things of value. Furthermore, the mechanistic link between internal physiological state and threat-reward decision making remains poorly understood. Here we confronted C. elegans worms with the decision whether to cross a hyperosmotic barrier presenting the threat of desiccation to reach a source of food odor. We identified a specific interneuron that controls this decision via top-down extrasynaptic aminergic potentiation of the primary osmosensory neurons to increase their sensitivity to the barrier. We also establish that food deprivation increases the worm’s willingness to cross the dangerous barrier by suppressing this pathway. These studies reveal a potentially general neural circuit architecture for internal state control of threat-reward decision making.