Serotonin increases the rate of pharyngeal pumping and egg laying, decreases foraging behavior and is required for proper male mating behavior. Serotonin can increase pharyngeal pumping by stimulating MC, a motor neuron that stimulates pharyngeal muscle contraction, or by acting directly on pharyngeal muscle. To determine the receptor or receptors that mediate these responses, we have searched the genomic sequence for candidate genes having sequence identity to known vertebrate and invertebrate serotonin receptors. We found several candidate metabotropic serotonin receptors using this method:
ser-1,
ser-2 , ce-5HT-1, C09B7.1, C52B11.3, F01E11.5, F14D12.6, M03F4.3, T02E9.3, K02F2.6 and F15A8.5. Three of these receptors (
ser-1 , ce-5HT-1 , and M03F4.3) have been determined to be responsive to serotonin in heterologous expression systems (Ribeiro and Hamdan IWM97,
p278 and Olde and McCombie, 1997). In addition, we found two candidate ionotropic (5-HT 3 ) receptors in the genomic sequence (C31H5.3 and C40C9.2.) We have created GFP fusion constructs to ten of the thirteen candidate genes using overlap extension PCR (C52B11.3, T02E9.3 and F15A8.5 are in progress). Six of the genes,
ser-1 ,
ser-2 , C09B7.1, K02F2.6, C31H5.3 and C40C9.2 show pharyngeal expression of the fusion construct.
ser-1 expresses strongly in pharyngeal muscle cells of the procorpus (
pm3), isthmus (
pm5), and terminal bulb (
pm6,
pm7 and
pm8) (S. Nurrish pers. com. and my observations).
ser-2 pharyngeal muscle expression is specific for metastomal flap muscles (
pm1), and anterior terminal bulb muscles,
pm6. The
ser-2 ::GFP construct also expresses in muscles of the head, the pharyngeal serotonergic neuron NSM, the ventral nerve cord and unidentified neurons in the nerve ring, head and tail regions. In males,
ser-2 ::GFP expresses in dorsal and ventral body muscles of the tail region.
ser-2 ::GFP also expresses in the male-specific diagonal muscles and serotonergic CP neurons. The expression pattern of
ser-2 appears to be consistent with several known effects of serotonin. We are currently working to more carefully determine the pharyngeal cells that express C09B7.1, K02F2.6, C31H5.3 and C40C9.2. In addition, we are attempting to knockout the pharyngeal-expressing candidate receptors.