3 ',5 ' cyclic adenosine monophosphate mediates the salmon calcitonin-induced increase in hypothalamic tyrosine hydroxylase activity

This study examined the effect of salmon calcitonin (sCT) on hypothalamic t yrosine hydroxylase (TH) activity and evaluated the cellular signaling mech anisms involved in the response. Fetal hypothalamic cells were cultured in a defined medium and treated with sCT and/or specific protein kinase inhibi tors on day 14 in vitro, sCT (0.1-10 nM) increased both TH activity and cel lular cAMP content in a concentration-dependent manner. sCT (10 nM) increas ed TH activity to 150-175% of control values and resulted in a 10-fold incr ease in cellular cAMP content. Both the C1a and C1b CT receptor isoforms we re present in the cultures, as assessed by RT-PCR. Rp-adenosine 3',5'-cycli c monophosphothioate (Rp-cAMPS), a cAMP antagonist, and H-8, a cyclic nucle otide kinase inhibitor, blocked the sCT-induced increase in TH activity, wi th complete abolition of the response observed at concentrations of 1 mp6 a nd 5 mu M, respectively. sCT (10 nM) increased radiolabeled phosphate incor poration into TH protein to 169% of control values and 1 mM Rp-cAMPS comple tely blocked this effect. In contrast, neither Calphostin C, a protein kina se C inhibitor, nor U-73122, a phospholipase C inhibitor, significantly alt ered the ability of sCT to increase TH activity. Likewise, the sCT-induced increase in TH activity was observed after pretreating the cells with eithe r BAPTA/AM, an intracellular calcium chelator, or thapsigargin, an inhibito r of the endoplasmic reticulum calcium pump. These data indicate that sCT h as a profound stimulatory effect on TH activity in fetal hypothalamic cells and that enhanced phosphorylation of TH coincides with the sCT-induced inc rease in enzyme activity. Moreover, CT receptors, which are linked to cAMP production, are expressed in the hypothalamic cells and a cAMP-dependent me chanism mediates the sCT-induced activation and phosphorylation of TH.