Migraine is a genetically complex, highly prevalent pain disorder that is functionally disabling in a subset of persons with this trait. Clinical and pharmacologic studies have linked migraine with abnormalities of serotonin (5HT) neurotransmission, and genetic studies have shown the P/Q-type alpha-1A calcium channel subunit, CACNA1A, also plays a role. In order to help define potential disease-related signaling effectors affected by CACNA1A we have examined the role of its C. elegans ortholog UNC-2 (Schafer and Kenyon, 1995) in the regulation of the critical migraine neurotransmitter, 5HT. Comparison of the anti-5HT staining pattern of wild-type and
unc-2 mutant animals revealed that
unc-2 has increased varicosity size in the processes from NSM, possibly due to a decreased efficiency of neurotransmitter vesicle release. In addition, a subset of the serotonergic neurons, the ADF neurons, is frequently absent in these assays. Three potential explanations for this variability in staining include deficiencies in tryptophan hydroxylase (TPH-1) synthesis, decreased 5HT synthesis rate, or deficient reuptake of 5HT. In order to address the first of these possibilities we compared the level of
tph-1 promoter activation in WT versus
unc-2 animals using a
tph-1::GFP reporter construct (Sze, et al., 2000) and found that the absence of 5HT staining correlated with a decrease in
tph-1::GFP expression in the ADF neurons. Isolation of suppressor mutations that restore normal levels of 5HT and
tph-1::GFP expression to
unc-2 mutants have identified a role for the TGF- pathway in the homeostatic maintenance of 5HT levels. Loss of function in the TGF- related Daf-C genes leads to an increase in the observed frequency of 5HT staining and
tph-1::GFP expression in the ADF neurons in
unc-2. Curiously, treatment of
unc-2 animals with the serotonergic antagonist, cyproheptadine, also leads to upregulation of
tph-1::GFP expression. Since expression of
daf-7 is positively regulated by 5HT level (Sze, et al., 2000), cyproheptadine treatment of
unc-2 may lead to a relative decrease in TGF- signal activation by DAF-7, leading to increased
tph-1::GFPexpression. The effects of cyproheptadine suggest the possibility that UNC-2 function is necessary for maintaining a normal basal level of 5HT receptor output in the ASI neuron. The UNC-2 channel could also be involved in activation of other Ca2+ dependent pathways that influence
tph-1 expression. We propose that UNC-2 plays a role in 5HT synthesis through its effects on a TGF- signal that inhibits
tph-1 expression.