Worm Breeder's Gazette 12(5): 66 (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.
Foraging behavior consists of the rhythmic movements of the worm's nose, in an apparent attempt to explore the surrounding environment. We have found two environmental cues that modulate foraging behavior. Worms forage hyperactively both under starved conditions and after being touched with a platinum wire. To discover the mechanism of modulation we sought to identify the neurons and neurotransmitters involved in the control of foraging.
OLQ, IL1 ,and RMD neurons control foraging behavior. Laser ablation of any of these three classes of neurons results in an abnormally slow foraging rate, as well as an abnormal pattern of foraging movements. Laser operated animals bend their noses further dorsally and ventrally while foraging, which we call loopy foraging.
This neural circuit formed of the mechanosensory neurons IL1 and OLQ, and RMD motor neurons, also controls the head-withdrawal response. When a worm is touched on either the dorsal or ventral tip of its nose, it responds by an aversive head-withdrawal reflex (i.e., when touched ventrally, the head is pulled back dorsally). Laser ablation of the neurons IL1 ,OLQ, and RMD eliminate the head-withdrawal response, indicating that foraging and head-withdrawal are controlled by this single neural circuit.
Serotonin modulates foraging and head-withdrawal behaviors. Three pieces of information supports this belief. first, cat-l mutants, which have low levels of endogenous serotonin, forage hyperactively. Secondly, wild type foraging can be restored to cat-l animals by adding either imipramine (a serotonin uptake inhibitor), serotonergic agonists, or S-hydroxytryptophan (a serotonin precursor). Finally, serotonergic agonists or imipramine cause wild type worms to forage slowly and to fail to exhibit the headwithdrawal response.
These results suggest that serotonergic neurons modulate both foraging and the headwithdrawal behaviors. There are five classes of neurons that produce serotonin in C. elegans: NSM, ADF, HSN, RIG, and RIH (G. Garriga, personal communication). Laser ablation experiments should allow us to determine which specific serotonergic neurons modulate these behaviors.
Genetic analysis of modulation. We decided to conduct a genetic screen for mutations that disrupt modulation by serotonin. We reasoned that mutants with defects in modulation should resemble cat-l animals. Therefore, we are screening for mutants that forage hyperactively. So far, 4 foraging abnormal (fab) mutants corresponding to four different genes were found in a preliminary screen and an additional 1650 haploid genomes have been screened systematically and the mutants from this latter screen are being characterized. The fab mutants fall into three categories.
Presynaptic mutants The hyperforating mutants have been characterized pharmacologically using two drugs: imipramine, a serotonin re-uptake inhibitor, and CGS12066 B,a serotonergic agonist. These drugs cause wild type worms to forage abnormally slowly and to forage in a loopy pattern. Presynaptic mutants are resistant to the imipramine, but sensitive to the agonist. cat-l, cat-4 , fab-2 ( n2460 )L and fab-3 ( n2462 )m exhibit this presynaptic phenotype.
Pootsynaptic mutants. Mutants that have defects in the postsynaptic neuron fail to respond to both serotonergic agonists and uptake inhibitors. So far, two postsynaptic mutants have been found, 3AA2 and 4E1.
The final class of mutants are sensitive to both drugs. This class may contain weak presynaptic or postsynaptic defects, or defects in other pathways that regulate foraging. Four have been isolated thus far: fab-1 ( ky2 )I, fab-4 ( n2466 )VR,2E1, and 4H2.
We expect to screen between 10,000 - 15,000 haploid genomes in all. Hopefully, at that point we should have multiple alleles of these genes.