Interaction of neurotransmitters on the proboscis muscles of the North American channeled whelk Busycon canaliculatum
As a contribution to the better understanding of neural control of rhythmically active muscle, this study examines the interaction of neurotransmitters on the proboscis muscles of the North American channeled whelk Busycon canaliculatum. 5-Hydroxytryptamine (5HT) modulates acetylcholine- and FMRFamide-induced contractions of the odontophore retractor (ORM) and radular protractor (RPM) muscles of Busycon canaliculatum by elevating intracellular cAMP concentration. A membrane permeant analogue of cAMP, dibutyryl cAMP, mimicked the modulatory actions of 5HT on ACh-induced contractions. Dibutyryl cAMP and an adenylate cyclase activator, forskolin, mimicked the effects of 5HT on FMRFamide-induced contractions. The actions of dibutyryl cAMP and forskolin were enhanced by a phosphodiesterase inhibitor isobutyryl methylxanthine. Lithium reduced the FMRFamide response whilst the antibiotic, neomycin, had no effect. It is suggested that FMRFamide acts to induce contraction via IP3, whilst the modulatory effect of 5HT is mediated by increases in intracellular cAMP concentration. ^ N-terminally extended FMRFamide-like peptides were examined as potential FMRFamide antagonists on the proboscis muscles. An activating peptide, N-Acetyl-F-Nle-RFamide applied alone, depolarized quiescent muscles and induced contraction, but at high concentrations also antagonized FMRFamide-induced contractions by an apparent receptor desensitization effect. The Helix aspersa heptapeptde, pGluDPFLRFamide, at concentrations equimolar to FMRFamide completely abolished FMRFamide-induced contractions. The heptapeptide acted as a FMRFamide receptor antagonist and not purely as a relaxing agent. It specifically abolished FMRFamide-induced responses leaving ACh- and KCl-induced responses unaffected. ^ An analysis of the innervation of the ORM by proboscis nerve 5 was undertaken utilizing a cholinergic antagonist, propanthelene, and the potential FMRFamide antagonist, pGluDPFLRFamide. It is proposed that proboscis nerve 5 contains cholinergic and peptidergic transmitters and acts in a cholinergic or cholinergic/peptidergic manner dependent on activity. With low frequency, long interburst interval stimulation parameters nerve 5 acted in a cholinergic manner. With high frequency, short interburst interval stimulation parameters nerve 5 acted in a cholinergic/peptidergic manner. It is suggested that the peptidergic transmitter is FMRFamide. ^
Biology, Animal Physiology|Biology, Zoology
Timothy John Fort,
"Interaction of neurotransmitters on the proboscis muscles of the North American channeled whelk Busycon canaliculatum"
Dissertations and Master's Theses (Campus Access).