Characterization of an A-Type Muscarinic Acetylcholine Receptor and Its Possible Non-neuronal Role in the Oriental Armyworm, Mythimna separata Walker (Lepidoptera: Noctuidae)

Abstract

Muscarinic acetylcholine receptor (mAChR) regulates many neurophysiological functions in insects. In this report, a full-length cDNA encoding an A-type mAChR was cloned from the oriental armyworm, Mythimna separata. Pharmacological properties studies revealed that nanomolar to micromolar concentrations of carbachol or muscarine induced an increase of intracellular Ca 2+ concentration ([Ca 2+ ] i), with the EC 50 values of 124.6 and 388.1 nM, respectively. The increases of [Ca 2+ ] i can be greatly blocked by the antagonist atropine, with an IC 50 value of 0.09 nM. The receptor mRNA is expressed in all developmental stages, with great differential expression between male and female adults. The tissue expression analysis identified novel target tissues for this receptor, including ovaries and Malpighian tubules. The distribution of Ms A-type mAChR protein in the male brain may suggest the neurophysiological roles that are mediated by this receptor. However, the receptor protein was found to be distributed on the membranes of oocytes that are not innervated by neurons at all. These results indicate that Ms A-type mAChR selectively mediates intracellular Ca 2+ mobilization. And the high level of receptor protein in the membrane of oocytes may indicate a possible non-neuronal role of A-type mAChR in the reproductive system of M. separata.

Comparative transcriptome analysis reveals the non‐neuronal cholinergic system in the ovary of the oriental armyworm, Mythimna separata Walker (Lepidoptera: Noctuidae)

“…However, there is no insecticide that targets insect mAChRs to date 46,47 . Our previous research showed the presence of A‐mAChR protein on the oocyte membrane of M. separata , which indicates a possible non‐neuronal function of A‐mAChR in the reproductive system 32 . However, nothing is known about the source of ACh and the main components of the cholinergic system in the ovary.…”

“…46,47 Our previous research showed the presence of A-mAChR protein on the oocyte membrane of M. separata, which indicates a possible non- neuronal function of A-mAChR in the reproductive system. 32 However, nothing is known about the source of ACh and the main components of the cholinergic system in the ovary. Thus, clarification of the difference between the neuronal and non-neuronal cholinergic systems is important to supplement our understanding of the non-neuronal role of ACh in insects and to lay the foundation for exploring novel insecticides targeting the nonneuronal cholinergic system.…”

“…In fact, the oocytes are enclosed in avascular follicular cells, which also completely lack any innervation. 32 ACh is a labile molecule and is subjected to rapid degradation by AChE. 61 Thus, it is impossible that ACh released from the cholinergic nerve endings diffuses into the oocyte membrane through hemolymph.…”

“…Since activation of A‐mAChR requires the presence of ACh, identification of the source of ACh in the ovary becomes a crucial issue. In fact, the oocytes are enclosed in avascular follicular cells, which also completely lack any innervation 32 . ACh is a labile molecule and is subjected to rapid degradation by AChE 61 .…”

“…22,31 Our previous research on M. separata suggested a possible non-neuronal function of A-mAChR in the female reproductive system. 32 As we know, most insecticides, such as the carbamates, neonicotinoids and organophosphorus insecticides, interfere with the cholinergic system. 22 There is an urgent need to learn more about the nonneuronal cholinergic system in insects.…”