Ravi, Bhavya et al. (2015) International Worm Meeting "Development of activity in the Caenorhabditis elegans egg-laying circuit."

Ravi, Bhavya, Collins, Kevin

[International Worm Meeting, 2015]

Early activity has been shown to be important for the proper development and assembly of neural circuits. Activity is also required for the coordinated patterns in mature neural circuits that drive efficient behaviors. We are interested in studying the functional importance of early activity using the developing C. elegans egg-laying behavior circuit. The egg laying circuit contains two HSN serotonergic motor neurons which synapse onto vulval muscles and promote entry into the egg laying active phase. Six VC neurons release acetylcholine to regulate the activity of two pairs of vulval muscles that contract to lay eggs. We are using GCaMP calcium imaging in freely behaving animals to record activity in cells of the egg laying circuit at the late L4 stage and in adults after development. We found that HSN neurons have rhythmic calcium transients in the late L4 stage even before eggs are produced, but this activity lacked the characteristic burst-firing pattern of adults during egg laying. The vulval muscles also show spontaneous transients in the late L4 stage before they complete their development. In order to understand the functional consequences of circuit connectivity, we are analyzing how the observed activity correlates with morphological changes observed during circuit development. We hope to test how acute and/or developmental perturbations in cell activity drive functional and behavioral plasticity. Together, these results will help us to understand how normal patterns of cell activity allow for proper development of neural circuits that drive robust, stable behaviors.

Ravi, Bhavya et al. (2019) International Worm Meeting "C. elegans egg-laying behavior is controlled by an internal, stretch-dependent homeostat that is gated by external sensory input."

Kang, Lijun, Ravi, Bhavya, Collins, Kevin, Zhao, Jian

[International Worm Meeting, 2019]

The C. elegans egg-laying circuit is a simple, well-characterized neural circuit which drives a two-state behavior comprising ~20-minute inactive states that are punctuated by ~2-minute active states where eggs are laid. The circuit is comprised of two serotonergic Hermaphrodite Specific Neurons (HSNs) which release serotonin and NLP-3 neuropeptides to promote the egg-laying active state, and cholinergic ventral cord motoneurons which synapse onto a set of vulval muscles that contract to release eggs. We have been investigating how external and internal sensory signals converge on the circuit to determine when and where animals lay eggs. Using calcium imaging in behaving animals, we have recently found the accumulation of eggs in the adult uterus renders the postsynaptic vulval muscles sensitive to presynaptic HSN input (Ravi et al. 2018). Sterilization or acute electrical silencing of the vulval muscles inhibits presynaptic HSN activity. Reversal of muscle silencing triggers a homeostatic increase in HSN activity and egg release that maintains ~12-15 eggs in the uterus. These results show that egg-laying behavior in C. elegans is driven by an internal, stretch-dependent homeostat that scales serotonin motor neuron activity in response to postsynaptic muscle feedback. Parallel to mechanical activation, HSN activity is also regulated by external sensory input. Inhibitory neurotransmitters and peptides released from sensory neurons signal through receptors on HSN that activate the conserved, Pertussis Toxin-sensitive G protein, G?o/ GOA-1. In recent unpublished results, we find that mutations that increase inhibitory G?o signaling in HSN reduce the frequency of Ca2+ transients in both the HSNs and in the postsynaptic vulval muscles, extending the duration of the egg-laying inactive state. Conversely, mutations that eliminate inhibitory G?o signaling in HSN increase the frequency of Ca2+ transients in HSNs and vulval muscles, reducing the interval between egg-laying active states. Interestingly, animal sterilzation suppressed this increase of Ca2+ activity in the vulval muscles but not HSNs, showing the stretch homeostat acts outside of HSN. Patch clamp recordings show the HSNs are hyperpolarized in egl-10 mutants with increased G?o signaling and depolarized in transgenic animals where G?o is inhibited in HSN with Pertussis Toxin. Thus, inhibitory G?o signaling in HSN maintains a bi-stable state of cell electrical excitability that keeps the egg-laying circuit off in unfavorable environmental conditions and/or when the uterus is without eggs to lay. Ravi B, Garcia J, and Collins KM (2018). J. Neuroscience. 38 (28), 6283-6298.