## 9 PKA Signaling in Neurons Regulates _C. elegans_ Behaviors  
Several studies suggest PKA activity in neurons regulates locomotion. Activation of KIN-1/PKA-C, through depletion of _kin-_2, results in hyperactive movement of _C. elegans_; similar phenotypes are seen when the G\(\alpha_{\mathrm{s}}\) protein GSA-1 is activated or the phosphodiesterase PRE-4 is depleted [99]. Loss of function alleles in the two-pore-domain potassium (K\({}_{2}\)P) channel TWK-7 increase locomotion. Genetic evidence suggests TWK-7 is downstream of the GSA-1-KIN-1/PKA pathway in B- and D-type motor neurons [99]. Activation of KIN-1/PKA-C inhibits TWK-7. When PKA is active, TWK-7 is repressed, leading to increased locomotion. Further studies are necessary to understand the mechanism by which KIN-1/PKA-C regulates TWK-7 in motor neurons (Figure 4) [99]. The neurotransmitter acetylcholine stimulates _C. elegans_ locomotion. Several studies have revealed a role for PKA in acetylcholine release in motor neurons [100]. Exposure of _C. elegans_ to 0.1% ethanol also increases locomotion. Although the precise mechanism by which ethanol increases locomotion remains unclear, the Gos-cAMP-PKA signaling pathway can be activated by ethanol in the IL2 sensory neurons, which release acetylcholine and link to locomotor circuits by intermediary neurons [101]. This study identified a key downstream effector of PKA signaling, UNC-18/Sec1-Munc18. UNC-18 plays an essential role in synaptic vesicle exocytosis. G\(\alpha_{\mathrm{s}}\) signaling activates KIN-1/PKA-C, which phosphorylates UNC-18 thereby increasing neurotransmitter release and stimulating _C. elegans_ locomotion [101]. A similar pathway may be involved in the response to isoflurane, a general anesthetic, which results in erratic and diminished neuronal activity in motor neurons and physical quiescence of the nematode [102]. Sensitivity to isoflurane is related to levels of acetylcholine release. Activating KIN-1/PKA-C through loss of _kin-_2 or a gain  
Figure 4: **Schematic representation of proposed signaling pathway in B- and D- type motor neurons**. G\(\alpha_{\mathrm{s}}\) GSA-1 activates adenylyl cyclase ACY-1, producing more cAMP and therefore activating PKA. TWK-7 is a K\({}_{2}\)P channel that normally decreases locomotion. Through a mechanism that is not entirely understood (dotted line) active PKA inhibits TWK-7, therefore increasing locomotion. Adapted with permission from [99], 2022, Oxford University Press. Created with BioRender.com (accessed on 23 October 2022). of function mutation in adenylyl cyclase _acy-1(js127)_ results in resistance to isoflurane [103]. Aldicarb treatment increases acetylcholine release and leads to sustained muscle activation and eventual paralysis. Activation of KIN-1 further increases acetylcholine release and increases sensitivity to aldicarb-induced paralysis [104]. In addition to locomotion, PKA signaling in neurons regulates other behaviors, including wakefulness. In C. _elegans_, Drosophila, and mice, increased PKA-1 activity promotes wakefulness [105; 106; 107; 108], via the transcriptional activator cAMP response element-binding protein (CREB) [108]. When KIN-1/PKA-C activity is increased by deletion of KIN-2/PKA-R or ACY-1(GF), the worms are more active. PKA-C activates the transcription factor CRH-1/CREB and promotes neuropeptide release to promote active wakefulness. CRH-1 is the CREB ortholog in C. _elegans_[105; 109]. The Ca\({}^{2+}\)-dependent activator protein for secretion UNC-31/CAPS is necessary for neuropeptide release from dense core vesicles (DCV). By enhancing mobilization and priming, cAMP/PKA signaling augments synaptic vesicle (SV) fusion [110]. Activation of PKA can bypass the requirement for UNC-31 in the docking of DCVs in exocytosis. KIN-1/PKA-C phosphorylates the syntaxin-1-binding protein, TOM-1, which downregulates synaptic transmission and UNC-31/CAPS-dependent neuropeptide release, resulting in locomotion regulation and stabilized wakefulness [105; 111]. CREB is a common downstream effector of PKA.