Neuropeptide Secreted from a Pacemaker Activates Neurons to Control a Rhythmic Behavior

Abstract

Summary Background Rhythmic behaviors are driven by endogenous biological clocks in pacemakers, which must reliably transmit timing information to target tissues that execute rhythmic outputs. During the defecation motor program in C. elegans, calcium oscillations in the pacemaker (intestine), which occur about every 50 seconds, trigger rhythmic enteric muscle contractions through downstream GABAergic neurons that innervate enteric muscles. However, the identity of the timing signal released by the pacemaker and the mechanism underlying the delivery of timing information to the GABAergic neurons are unknown. Results Here we show that a neuropeptide-like protein (NLP-40) released by the pacemaker triggers a single rapid calcium transient in the GABAergic neurons during each defecation cycle. We find that mutants lacking nlp-40 have normal pacemaker function, but lack enteric muscle contractions. NLP-40 undergoes calcium-dependent release that is mediated by the calcium sensor, SNT-2/synaptotagmin. We identify AEX-2, the G protein-coupled receptor on the GABAergic neurons, as the receptor of NLP-40. Functional calcium imaging reveals that NLP-40 and AEX-2/GPCR are both necessary for rhythmic activation of these neurons. Furthermore, acute application of synthetic NLP-40-derived peptide depolarizes the GABAergic neurons in vivo. Conclusions Our results show that NLP-40 carries the timing information from the pacemaker via calcium-dependent release and delivers it to the GABAergic neurons by instructing their activation. Thus, we propose that rhythmic release of neuropeptides can deliver temporal information from pacemakers to downstream neurons to execute rhythmic behaviors.

PKA Controls Calcium Influx into Motor Neurons during a Rhythmic Behavior

“…The period of the defecation cycle is controlled by a pacemaker in the intestine, and the Exp step is initiated by the release of a neuropeptide-like protein NLP-40 from the intestine. NLP-40 instructs the excitation of a pair of GABAergic neurons (AVL and DVB), which in turn release the neurotransmitter GABA to trigger the Exp step [20]–[23]. NLP-40 activates the GPCR AEX-2 on the GABAergic neurons, which is coupled to the heterotrimeric G protein α subunit GSA-1/Gαs, leading to the activation of adenylyl cyclase and the production of cAMP [5], [23].…”

“…NLP-40 instructs the excitation of a pair of GABAergic neurons (AVL and DVB), which in turn release the neurotransmitter GABA to trigger the Exp step [20]–[23]. NLP-40 activates the GPCR AEX-2 on the GABAergic neurons, which is coupled to the heterotrimeric G protein α subunit GSA-1/Gαs, leading to the activation of adenylyl cyclase and the production of cAMP [5], [23]. However, the molecular targets of cAMP in the GABAergic neurons are not known and how cAMP signaling impacts GABA release to mediate the Exp step is unclear.…”

Neurotransmitter signaling through heterotrimeric G proteins: insights from studies in C. elegans

“…For example, FLP-17 neuropeptides are released from BAG sensory neurons and signal via the EGL-6 neuropeptide receptor on the HSN motor neurons, which are not postsynaptic to BAG, to inhibit egg laying (Ringstad and Horvitz, 2008: PMID 18806786). The C. elegans defecation motor program is coordinated by NLP-40 peptides released from intestinal cells, which, being non-neuronal, make no synapses, and these NLP-40 peptides signal onto GABAergic motor neurons via the AEX-2 peptide receptor (Wang et al, 2013: PMID 23583549). Beyond these rigorous genetic demonstrations that specific peptides signal extrasynaptically via particular neuropeptide receptors, there are additional data showing that some neuropeptide receptors are expressed on cells that do not receive any synapses, so that any signaling they mediate on these cells must be extrasynaptic.…”

“…These mutants do not have obvious behavioral defects when animals are not bathed in a neurotransmitter, although defects have been detected when populations of mutant animals are put through very specific behavioral assays known to depend on serotonin or dopamine. There is perhaps only a single example of a neural GPCR for which loss-of-function mutants have been recovered from a forward genetic screen and for which the mutant phenotype is easily observable in single animals: mutants lacking AEX-2, the receptor for NLP-40 neuropeptides, are defective in the expulsion step of defecation (Mahoney et al, 2008: PMID 18852466; Wang et al, 2013: PMID 23583549).…”

Dense Core Vesicle Release: Controlling the Where as Well as the When

“…In addition, CAMKII mutations alter levels of serotonin synthesis, aging, neuronal asymmetries, and transcription (Sagasti et al 2001;Suo et al 2006;Tao et al 2013;Qin et al 2013). Most, if not all of these behaviors can be regulated by neuropeptides (de Bono and Bargmann 1998;Nelson et al 1998;Keating et al 2003;Hu et al 2011;Wang et al 2013); however, it remains unclear whether all, some, or none of the phenotypes associated with mutations in CaMKII depend on its role in control of DCV release. Suppressors of the reduced locomotion and egg-laying phenotypes of gain-of-function CaMKII mutants include two genes, goa-1 and dgk-1 (Robatzek et al 2001), mutations in which cause increased levels of DCV release (Ch'ng et al 2008).…”