A Cholinergic-Regulated Circuit Coordinates the Maintenance and Bi-Stable States of a Sensory-Motor Behavior during Caenorhabditis elegans Male Copulation

Abstract

Penetration of a male copulatory organ into a suitable mate is a conserved and necessary behavioral step for most terrestrial matings; however, the detailed molecular and cellular mechanisms for this distinct social interaction have not been elucidated in any animal. During mating, the Caenorhabditis elegans male cloaca is maintained over the hermaphrodite's vulva as he attempts to insert his copulatory spicules. Rhythmic spicule thrusts cease when insertion is sensed. Circuit components consisting of sensory/motor neurons and sex muscles for these steps have been previously identified, but it was unclear how their outputs are integrated to generate a coordinated behavior pattern. Here, we show that cholinergic signaling between the cloacal sensory/motor neurons and the posterior sex muscles sustains genital contact between the sexes. Simultaneously, via gap junctions, signaling from these muscles is transmitted to the spicule muscles, thus coupling repeated spicule thrusts with vulval contact. To transit from rhythmic to sustained muscle contraction during penetration, the SPC sensory-motor neurons integrate the signal of spicule's position in the vulva with inputs from the hook and cloacal sensilla. The UNC-103 K+ channel maintains a high excitability threshold in the circuit, so that sustained spicule muscle contraction is not stimulated by fewer inputs. We demonstrate that coordination of sensory inputs and motor outputs used to initiate, maintain, self-monitor, and complete an innate behavior is accomplished via the coupling of a few circuit components.

Cell Excitability Necessary for Male Mating Behavior inCaenorhabditis elegansIs Coordinated by Interactions Between Big Current and Ether-A-Go-Go Family K+ Channels

“…While C. elegans males have many more excitable cells than hermaphrodites, only a few of these express cat-2, which could account for the lack of difference between sexes. acr-18 is expressed in many male-specific neurons and muscles (Liu et al 2011); however, the acr-18 mRNA was slightly, but not significantly increased in males as compared to hermaphrodites. The acr-18 levels could be reduced in non-sex-specific cells in males, resulting in transcript levels that are not significantly different from hermaphrodites.…”

“…Protractor and retractor muscles are attached to the base of the spicules and control their proper position. Once the appropriate mating cues have been received and integrated by both neurons and muscles, the protractors contract, forcing the spicules out of the tail and into the vulva (Sulston et al 1980;Liu and Sternberg 1995;Liu et al 2011). Permanent spicule protraction in the absence of mating cues interferes with successful sperm transfer and thus reproduction.…”

“…To test this hypothesis, we measured the transcript levels of genes involved in regulating the excitability of cells used in male mating behavior and also of genes involved in more general cell processes. We analyzed acr-18 [nicotinic acetylcholine receptor (nAChR) a-subunit], gar-3 (ACh receptor coupled to ga q ), unc-29 (nAChR non-a-subunit), unc-43 (CaMKII), and cat-2 (tyrosine hydroxylase) as candidate genes that are enriched in excitable cells (Sulston et al 1975;Fleming et al 1997;Hwang et al 1999;Lints and Emmons 1999;Reiner et al 1999;Sze et al 2000;Garcia et al 2001;Mongan et al 2002;LeBoeuf et al 2007;Liu et al 2007bLiu et al , 2011. Neither acr-18 nor cat-2 transcript levels were increased in males as compared to hermaphrodites (Figure 3, C and D), although both were reduced in slo-1(rg432) males.…”

“…The SPC releases acetylcholine (ACh) onto the protractors, initiating contraction. Gap junctions between the protractors and other muscles and neurons in the circuit help reduce the excitability threshold of the protractors, so they can respond to the ACh released by the SPC neurons (Garcia et al 2001;Liu et al 2011). BK/SLO-1, ERG-like/ UNC-103, and EAG/EGL-2 K + channels attenuate circuit activity until sufficient cues have been received ( Figure 5A).…”

DOP-2 D2-Like Receptor Regulates UNC-7 Innexins to Attenuate Recurrent Sensory Motor Neurons during C. elegans Copulation

“…The 18 sensory ray neuron pairs (termed R1A/B through R9A/B) induce male tail contact with the hermaphrodite (Koo et al, 2011) and promote backward locomotion as he searches for the vulva (Sherlekar et al, 2013). When he approaches the vulva, the hook sensillum neurons HOA and HOB sense long-range vulva cues (Liu and Sternberg, 1995;Barr and Sternberg, 1999) and subsequently reduce the male's velocity, allowing the post cloacal sensilla (PCS) to detect short-range vulva features (Liu and Sternberg, 1995;Barr and Sternberg, 1999).…”