Tyramine receptor (SER-2) isoforms are involved in the regulation of pharyngeal pumping and foraging behavior in Caenorhabditis elegans.
Octopamine regulates essential processes in nematodes; however, little is known about the physiological role of its precursor, tyramine. In the present study, we have characterized alternatively spliced Caenorhabditis elegans tyramine receptor isoforms (SER-2 and SER-2A) that differ by 23 amino acids within the mid-region of the third intracellular loop. Membranes prepared from cells expressing either SER-2 or SER-2A bind [3H]lysergic acid diethylamide (LSD) in the low nanomolar range and exhibit highest affinity for tyramine. Similarly, both isoforms exhibit nearly identical Ki values for a number of antagonists. In contrast, SER-2A exhibits a significantly lower affinity than SER-2 for other physiologically relevant biogenic amines, including octopamine. Pertussis toxin treatment reduces affinity for both tyramine and octopamine, especially for octopamine in membranes from cells expressing SER-2, suggesting that the conformation of the mid-region of the third intracellular loop is dictated by G-protein interactions and is responsible for the differential tyramine/octopamine affinities of the two isoforms. Tyramine reduces forskolin-stimulated cAMP levels in HEK293 cells expressing either isoform with nearly identical IC50 values. Tyramine, but not octopamine, also elevates Ca2+ levels in cells expressing SER-2 and to a lesser extent SER-2A. Most importantly, ser-2 null mutants (pk1357) fail to suppress head movements while reversing in response to nose-touch, suggesting a role for SER-2 in the regulation of foraging behavior, and fail to respond to tyramine in assays measuring serotonin-dependent pharyngeal pumping. These are the first reported functions for SER-2. These results suggest that C. elegans contains tyramine receptors, that individual SER-2 isoforms may differ significantly in their sensitivity to other physiologically relevant biogenic amines, such as octopamine (OA), and that tyraminergic signaling may be important in the regulation of key processes in nematodes.
Rex
Elizabeth
E
Department of Biological Sciences, University of Toledo, Toledo, Ohio3606-3390, USA.
Molitor
Scott C
SC
Hapiak
Vera
V
Xiao
Hong
H
Henderson
Megan
M
Komuniecki
Richard
R
D018759:Adrenergic Uptake Inhibitors / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D030801:Animals, Genetically Modified / Q000502:physiology; D001522:Behavior, Animal; D015306:Biogenic Monoamines / Q000493:pharmacokinetics; D017173:Caenorhabditis elegans / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002522:Chlorocebus aethiops; D003001:Cloning, Molecular / Q000379:methods; D000242:Cyclic AMP / Q000378:metabolism; D004274:DNA, Recombinant; D003952:Diagnostic Imaging / Q000379:methods; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004622:Embryo, Mammalian; D004625:Embryo, Nonmammalian; D005110:Extracellular Space / Q000378:metabolism; D005247:Feeding Behavior / Q000502:physiology*; D015870:Gene Expression / Q000502:physiology; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D008238:Lysergic Acid Diethylamide / Q000493:pharmacokinetics; D008958:Models, Molecular; D009666:Nose / Q000187:drug effects / Q000502:physiology; D009655:Octopamine / Q000494:pharmacology; D037342:Pertussis Toxin / Q000494:pharmacology; D010614:Pharynx / Q000502:physiology*; D010641:Phenotype; D010716:Phosphatidylinositols / Q000378:metabolism; D020033:Protein Isoforms / Q000502:physiology*; D012333:RNA, Messenger / Q000096:biosynthesis; D011869:Radioligand Assay / Q000379:methods; D017982:Receptors, Biogenic Amine / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods; D012701:Serotonin / Q000494:pharmacology; D013997:Time Factors; D014162:Transfection; D014316:Tritium / Q000493:pharmacokinetics; D014439:Tyramine / Q000494:pharmacology
D018759:Adrenergic Uptake Inhibitors; D000316:Adrenergic alpha-Agonists; D015306:Biogenic Monoamines; D004274:DNA, Recombinant; D010716:Phosphatidylinositols; D020033:Protein Isoforms; D012333:RNA, Messenger; D017982:Receptors, Biogenic Amine; C496144:SER-2 protein, C elegans; C066175:tyramine receptor; D014316:Tritium; D049452:Green Fluorescent Proteins; D009655:Octopamine; D012701:Serotonin; D008238:Lysergic Acid Diethylamide; D000242:Cyclic AMP; D037342:Pertussis Toxin; D002118:Calcium; D014439:Tyramine