Worm Breeder's Gazette 12(5): 68 (February 1, 1993)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Sulston and Brenner (1975) have demonstrated that dopamine is present in the axonal processes of a set of C. elegans neurons. However, behavioral defects have not been observed either in mutant worms containing low levels of dopamine or in wild-type worms treated with exogenous neurotransmitter. Here we report that whereas up to 32 mM dopamine has no observable effect on worms in NG agar, dopamine exerts profound effects on several behaviors when worms are grown on a substrate containing only 1.5% agar and 16 mM dopamine.
At a concentration of 16 mM, dopamine has several major effects on worm behavior. The most apparent is the loss of motor activity and extended paralysis. Initially, dopamine-treated animals fail to move spontaneously, but will still move briefly if the plate is trapped Eventually, the animals stop moving altogether. Dopamine treatment also inhibits pharyngeal pumping and egg-laying. None of these effects are seen when the worms are grown on NG medium. Several of the components of NG appear to inhibit the action of dopamine, including sodium chloride, calcium chloride, phosphate and peptone. The reason that dopamine is ineffective in the presence of these compounds is unclear, perhaps the higher osmolarity of NG medium prevents the efficient uptake of dopamine through the cuticle.
The dopamine antagonist haloperidol also strongly affects the behavior of wild-type animals. Halopridol, added to the agar substrate at 4 mM, induces egg-laying in the absence of food, the converse of dopamine's effect. The ability of haloperidol to induce egg-laying can be blocked by a four-fold molar excess of dopamine; thus, in this physiological assay, dopamine is capable of competing away the effects of haloperidol, its presumed antagonist. Haloperidol has other effect on C. elegans; 4 mM halopridol leads to hypercontraction, particularly in the head and pharynx This concentration of halopridol is also toxic to worms, and eventually leads to death.
Since dopamine and haloperidol exert opposite effects on egg-laying, and since excess dopamine can block the action of halopridol in vivo, these experiments indicate that dopamine most likely negatively regulates egg-laying in the worm. This hypothesis is supported by the observation that cat-2 mutants, which contain low levels of dopamine, are weakly constitutive for egg-laying. To gain further insight into the possible role of dopamine in the control of egg-laying, we investigated the effects of dopamine and haloperidol on C. elegans mutants with defects related to egg-laying. egl-S mutants lack the serotonergic HSN neuron, which appears to stimulate egg-laying. The egg-laying defect in these animals can be rescued by serotonin treatment. 2 mM haloperidol induced egg-laying in egl-S mutants, indicating that dopamine's effect does not require HSN. In addition, dopamine did not block the induction of egg-laying by serotonin in egl-S animals. Taken together, these results suggest that dopamine may be acting through a parallel pathway from that which acts through HSN. The induction of egg-laying does not require the levamisole receptor, as haloperidol treatment induces -laying in unc-29 mutants, which lack functional receptors. Predictably, unc-54 mutants, with a defect in muscle myosin, are resistant to haloperidol's effects on egg-laying and head contraction, though the drug is still toxic to unc-54 animals.
Intriguingly, animals appear to adapt readily to the presence of dopamine; animals grown overnight on 16 mM dopamine recover both motility and egg-laying ability. These conditioned animals were resistant to paralysis and egg-laying inhibition by dopamine at concentrations up to 32 mM. Genetic screens are in progress to identify animals defective in response and adaptation to dopamine. It should be possible to use such mutants to gain an understanding of the mechanisms underlying adaptation to dopamine and the regulation of dopamine-influenced behaviors.