INX-18 and INX-19 play distinct roles in electrical synapses that modulate aversive behavior in Caenorhabditis elegans

Abstract

In order to respond to changing environments and fluctuations in internal states, animals adjust their behavior through diverse neuromodulatory mechanisms. In this study we show that electrical synapses between the ASH primary quinine-detecting sensory neurons and the neighboring ASK neurons are required for modulating the aversive response to the bitter tastant quinine in C. elegans. Mutant worms that lack the electrical synapse proteins INX-18 and INX-19 become hypersensitive to dilute quinine. Cell-specific rescue experiments indicate that inx-18 operates in ASK while inx-19 is required in both ASK and ASH for proper quinine sensitivity. Imaging analyses find that INX-19 in ASK and ASH localizes to the same regions in the nerve ring, suggesting that both sides of ASK-ASH electrical synapses contain INX-19. While inx-18 and inx-19 mutant animals have a similar behavioral phenotype, several lines of evidence suggest the proteins encoded by these genes play different roles in modulating the aversive quinine response. First, INX-18 and INX-19 localize to different regions of the nerve ring, indicating that they are not present in the same synapses. Second, removing inx-18 disrupts the distribution of INX-19, while removing inx-19 does not alter INX-18 localization. Finally, by using a fluorescent cGMP reporter, we find that INX-18 and INX-19 have distinct roles in establishing cGMP levels in ASK and ASH. Together, these results demonstrate that electrical synapses containing INX-18 and INX-19 facilitate modulation of ASH nociceptive signaling. Our findings support the idea that a network of electrical synapses mediates cGMP exchange between neurons, enabling modulation of sensory responses and behavior.

Gap junctions: historical discoveries and new findings in the C aenorhabditis elegans nervous system

“…Optogenetic stimulation of cGMP production using blue light-inducible guanylyl cyclase (Ryu et al, 2010) in ADF neurons resulted in reduced quinine sensitivity, but not in inx-4 mutant, suggesting that cGMP transfer from ADF to ASH neurons dampens quinine sensitivity (Krzyzanowski et al, 2016). When endogenous cGMP level was measured using a fluorescent reporter FlincG3 (Woldemariam et al, 2019;Bhargava et al, 2013), loss of inx-19 resulted in increased cGMP levels in ASK neurons and reduced cGMP levels in ASH neurons (Voelker et al, 2019). However, calcium level was increased in ASH neurons in inx-19 mutant, suggesting that increased cGMP flow from ASK to ASH neurons dampens calcium levels in the ASH neurons, resulting in reduced quinine sensitivity (Voelker et al, 2019).…”

“…Second messenger cGMP in noxious stimuli sensing circuit ASH sensory neurons are the primary nociceptors responsible for quinine avoidance response. ASH is electrically connected to two other sensory neurons AFD, ADF, and interneuron AIA neurons via INX-4 (aka CHE-7) (Krzyzanowski et al, 2016), and also to ASK sensory neurons via homomeric INX-19 (aka NSY-5) gap junctions (Voelker et al, 2019) (Fig. 2E).…”

“…When endogenous cGMP level was measured using a fluorescent reporter FlincG3 (Woldemariam et al, 2019;Bhargava et al, 2013), loss of inx-19 resulted in increased cGMP levels in ASK neurons and reduced cGMP levels in ASH neurons (Voelker et al, 2019). However, calcium level was increased in ASH neurons in inx-19 mutant, suggesting that increased cGMP flow from ASK to ASH neurons dampens calcium levels in the ASH neurons, resulting in reduced quinine sensitivity (Voelker et al, 2019).…”

Positive interaction between ASH and ASK sensory neurons accelerates nociception and inhibits behavioral adaptation

“…Primary nociception and its modulations at the level of sensory neurons or the initial chain of sensory pathways provide more veridical and instantaneous information for animals to achieve rapid, more fine-tuned, and concentrated behavioral responses ( Baliki and Apkarian, 2015 ; Guo et al., 2015 ). Peripheral circuitry modulations of nociception include that nociceptive ASI neurons reciprocally inhibit nociceptive ASH ( Guo et al., 2015 ) and secondary nociceptive ASK neurons inhibit ASH nociceptive signal transduction by providing cGMP through gap junctions ( Voelker et al., 2019 ) in nematode Caenorhabditis elegans ( C. elegans ). The molecular and neuronal circuital mechanisms of nociception, especially pain perception, have been extensively studied.…”

“…ASH releases glutamate in a graded manner to elicit and modulate different models of nociceptive escape responses ( deBono and Maricq, 2005 ) and to regulate eating ( Zou et al., 2018 ) and egg laying ( Wen et al., 2020 ). ASH receives elaborate modulations from central and peripheral neurons by various signals of monoamines and peptides ( Ezcurra et al., 2016 ; Ghosh et al., 2016 ; Guo et al., 2015 ; Hapiak et al., 2013 ; Harris et al., 2010 ; Mills et al., 2012 ; Voelker et al., 2019 ; Zahratka et al., 2015 ). Such as central excitatory feedback from interneuron RIM via humoral tyraminergic signaling ( Ghosh et al., 2016 ), reciprocal inhibition from ASI via SER-5 signaling through the relay of ADF sensory neurons ( Guo et al., 2015 ), inhibition of signal transduction by cGMP from ASK ( Voelker et al., 2019 ), and inhibition from AUA interneurons via NPR-1 signaling when food availability is low ( Ezcurra et al., 2016 ).…”