Abstract
3-Iodothyronamine (3-T(1)AM) is an endogenous thyroid hormone metabolite. The profound pharmacological effects of 3-T(1)AM on energy metabolism and thermal homeostasis have raised interest to elucidate its signaling properties in tissues that pertain to metabolic regulation and thermogenesis. Previous studies identified G protein-coupled receptors (GPCRs) and transient receptor potential channels (TRPs) as targets of 3-T(1)AM in different cell types. These two superfamilies of membrane proteins are largely expressed in tissue which influences energy balance and metabolism. As the first indication that 3-T(1)AM virtually modulates the function of the neurons in hypothalamus, we observed that intraperitoneal administration of 50 mg/kg bodyweight of 3-T(1)AM significantly increased the c-FOS activation in the paraventricular nucleus (PVN) of C57BL/6 mice. To elucidate the underlying mechanism behind this 3-T(1)AM-induced signalosome, we used three different murine hypothalamic cell lines, which are all known to express PVN markers, GT1-7, mHypoE-N39 (N39) and mHypoE-N41 (N41). Various aminergic GPCRs, which are the known targets of 3-T(1)AM, as well as numerous members of TRP channel superfamily, are expressed in these cell lines. Effects of 3-T(1)AM on activation of GPCRs were tested for the two major signaling pathways, the action of Gα(s)/adenylyl cyclase and G(i/o). Here, we demonstrated that this thyroid hormone metabolite has no significant effect on G(i/o) signaling and only a minor effect on the Gα(s)/adenylyl cyclase pathway, despite the expression of known GPCR targets of 3-T(1)AM. Next, to test for other potential mechanisms involved in 3-T(1)AM-induced c-FOS activation in PVN, we evaluated the effect of 3-T(1)AM on the intracellular Ca(2+) concentration and whole-cell currents. The fluorescence-optic measurements showed a significant increase of intracellular Ca(2+) concentration in the three cell lines in the presence of 10 μM 3-T(1)AM. Furthermore, this thyroid hormone metabolite led to an increase of whole-cell currents in N41 cells. Interestingly, the TRPM8 selective inhibitor (10 μM AMTB) reduced the 3-T(1)AM stimulatory effects on cytosolic Ca(2+) and whole-cell currents. Our results suggest that the profound pharmacological effects of 3-T(1)AM on selected brain nuclei of murine hypothalamus, which are known to be involved in energy metabolism and thermoregulation, might be partially attributable to TRP channel activation in hypothalamic cells.