I am interested in understanding how motivation and behavioral states are regulated at the molecular and genetic level. My lab addresses this complex question in the well-studied nematode Caenorhabditis elegans. Several physical aspects of this worm make it convenient for integrating whole-organism-system biology studies with genetic/molecular analysis of neurobiology and behavior.

C. elegans, an anatomically simple organism, contains two sexes, males and self-fertilizing hermaphrodites. It is 1 mm in size, and it contains ~1,000 somatic cells, a third of which are neurons. The worm is also transparent, and thus every cell can be visualized by light microscopy. Behavioral mutants can be efficiently generated through standard chemical mutagenesis. In addition, gene functions involved in motivational and behavioral regulation can be identified and dissected by transgenic techniques.

My lab investigates the interplay between two ancient behaviors, feeding and sex-specific mating behavior. Our research is focused on how chemo/mechano-sensory and motor outputs are controlled under various environmental and physiological conditions. We use chemical mutagenesis/forward genetics to deconstruct male mating behavior into its fundamental sensory-motor components. We then use a combination of transgenics, pharmacology, classical genetics, and laser microsurgery to understand how individual motor sub-behaviors are coordinated to regulate male sexual behavior during periods when the animal is food deprived and when it is food satiated. (This work has been funded by the Searle Scholars Program and a grant from the National Institutes of Health.)

To study how food-related signaling through sensory neurons and insulin-like growth factor receptors regulate mating circuits, we have generated EMS (ethyl methanesulfonate)-induced mutations that cause spontaneous firing of the mating circuitry in well-fed males but induce little-to-no gross behavioral defects in hermaphrodites. One mutation affects UNC-103, the worm ortholog of the voltage-gated ERG (ether-a-go-go–related gene) K+ channel. In humans, genetic mutations in this channel cause a potentially lethal, prolonged action potential in cardiac muscle called long QT syndrome. We have characterized the gene structure and expression pattern of the UNC-103/ERG K+ channel in both hermaphrodites and males. The complexunc-103 gene contains six promoters with the alternative splicing capacity to encode 18 potential isoforms. In both sexes, the gene is widely expressed in neurons and muscles; however, defects in the gene predominantly disrupt the timing of reproductive behaviors. Another mutation we isolated affects the worm calcium-regulated protein kinase UNC-43/CaMKII, a molecule involved in the regulation of multiple developmental and neuronal processes. Similar to the UNC-103/ERG K+ channel, UNC-43/CaMKII is broadly expressed in neurons and muscles of both sexes. Work from others has demonstrated that global reduction of CaMKII will induce seizures in multiple C. elegans behaviors. However, we have identified alleles of CaMKII that preferentially affect the circuitry of male mating behavior.

In normal C. elegans males, K+ channels and CaMKII attenuate neurons and muscles in the male mating circuit until there is proper sex-specific stimulation by a mate. When genes of either one of these molecules are mutated, males spontaneously display precocious mating-like behaviors that superficially resemble seizures. Mutant males stop forward locomotion, change their posture, and protract their copulatory spicules from the cloacal opening in their tail. Under food-deprivation conditions, the percentage of ERG K+ channel mutant males displaying this abnormal behavior can be reduced; in contrast, most CaMKII mutant males still display the abnormal behavior. This suggests that signaling pathways involved in nutritional physiology can regulate excitability of specific behavioral circuits, possibly acting parallel to ERG K+ channel function and utilizing CaMKII molecules.

Through multiple approaches, we determined that a pair of chemosensory neurons (termed AWC) in the male senses the absence of food. Under food deprivation conditions, the neurons act with mating circuitry-expressed insulin-like growth factor receptors to stimulate CaMKII kinases. We found that CaMKII regulates a network of other potassium and calcium ion channels, including ryanodine receptor calcium channels, L-type voltage-gated calcium channels, BK channels, and ether-a-go-go K+ channels. These downstream molecules are regulated so that the excitability of neurons and muscles in specific behavioral circuits is reduced under caloric restriction conditions. Neurons and muscles in the ERG K+ channel mutant males do not spontaneously fire, and in wild-type animals, these mating circuits do not inappropriately respond to mating cues (when the male should be foraging for food instead of copulating). We are trying to understand other physiological changes that occur in the mating circuit under food deprivation conditions and how those changes are molecularly reversed when the animal becomes food satiated. With an understanding of how caloric restriction-induced pathways work, we hope to be able to manipulate them artificially in other circuits to reduce the effects of neuronal and muscle seizures and other consequences of channel dysfunction.

My lab is also investigating how, within food-satiated males, molecular mechanisms involved in executing motor programs are modulated to maintain behavioral persistence. Prior to ejaculation, the male must first breach his mate's vulva with his copulatory spicules. During mating, young adult hermaphrodites are not behaviorally receptive to mating. To achieve mating success, the male uses a network of cloacal cholinergic sensory/motor neurons to maintain his position over the vulva, as he repetitively attempts to penetrate the tightly closed vulval slit. He maintains this behavior until ejaculation is accomplished. The efficacy of these sensory-motor neurons to sustain their output is facilitated by the GAR-3/G protein–coupled M1/M3/M5-like muscarinic ACh receptor (mAChR). The GAR-3 receptor promotes persistence in executing behavioral programs, since males mutant in the gar-3 gene frequently cease spicule insertion attempts and move off the vulval area, if they do not immediately penetrate their mates. We are trying to understand how downstream effectors of GAR-3 receptor signaling integrate with nicotinic acetylcholine receptor signaling in various muscle and neuronal components of the mating circuit.