Tyramine receptor (SER‐2) isoforms are involved in the regulation of pharyngeal pumping and foraging behavior in Caenorhabditis elegans

Abstract

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 [3 H]lysergic acid diethylamide (LSD) in the low nanomolar range and exhibit highest affinity for tyramine. Similarly, both isoforms exhibit nearly identical K i 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 IC 50 values. Tyramine, but not octopamine, also elevates Ca 2+ 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 serotonindependent 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.

Trace amine-associated receptor 1—Family archetype or iconoclast?

“…Furthermore, pharmacologic manipulations (Borison et al, 1974;Borison et al, 1975;Boulton, 1976a;Philips & Boulton, 1979;Stoff et al, 1984;Boulton et al, 1990;Juorio et al 1991a;Juorio et al 1991b) and lesioning studies Juorio & Jones, 1981;Greenshaw et al, 1985;Greenshaw et al, 1986;Greenshaw et al, 1986;Juorio et al, 1987; can significantly influence TA turnover and levels with physiological (Becu-Villalobos, 1987;Cheng, 1990;Hirashima, 1999;Lee et al, 2003) and behavioral (Dourish, 1982;Lapin, 1996;Rex et al, 2004;Suo et al, 2006) consequences.…”

“…Another benefit of being able to express a putative receptor at will is that it makes direct pharmacological and physiological analyses of expected (Evans, 1980;Evans, 1981) and unexpected novel receptor subtypes (Venter et al, 1988) more straightforward, with certain limitations, as mentioned above (Hiripi et al, 1994;Hirashima et al, 2003;Lee et al, 2003;Rex et al, 2004;Cazzamali et al, 2005;, Klaerke & Grimmelikhuijzen, 2005;Ohta et al 2005).…”

Dissecting a central flip-flop circuit that integrates contradictory sensory cues in C. elegans feeding regulation

“…We found ser-2, which encodes a TA receptor 33,34 , to be another interesting gene involved in ASH-mediated pumping inhibition (Fig. 3a, Table 1).…”

Tyramine and Octopamine Independently Inhibit Serotonin-Stimulated Aversive Behaviors inCaenorhabditis elegansthrough Two Novel Amine Receptors

“…In nematodes, TA and OA generally oppose the action of 5-HT. For example, in Caenorhabditis elegans, exogenous TA or OA causes hyperactivity and suppresses 5-HT-stimulated pharyngeal pumping and egg laying (Horvitz et al, 1982;Ségalat et al, 1995;Niacaris and Avery, 2003;Rex et al, 2004;Alkema et al, 2005).…”