Orexin-A Activates Hypothalamic AMP-Activated Protein Kinase Signaling through a Ca²⁺-Dependent Mechanism Involving Voltage-Gated L-Type Calcium Channel

Abstract

Hypothalamic AMP-activated protein kinase (AMPK) and orexins/ hypocretins are both involved in the control of feeding behavior, but little is known about the interaction between these two signaling systems. Here, we demonstrated that orexin-A elicited significant activation of AMPK in the arcuate nucleus (ARC) of the hypothalamus by elevating cytosolic free Ca 21 involving extracellular calcium influx. Electrophysiological results revealed that orexin-A increased the L-type calcium current via the orexin receptor-phospholipase C-protein kinase C signaling pathway in ARC neurons that produce neuropeptide Y, an important downstream effector of orexin-A's orexigenic effect. Furthermore, the L-type calcium channel inhibitor nifedipine attenuated orexin-A-induced AMPK activation in vitro and in vivo. We found that inhibition of AMPK by either compound C (6-[4-[2-(1-piperidinyl)ethoxy]phenyl]-3-(4-pyridinyl)-pyrazolo[1,5-a]pyrimidine) or the ATPmimetic 9-b-D-arabinofuranoside prevented the appetite-stimulating effect of orexin-A. This action can be mimicked by nifedipine, the blocker of the L-type calcium channel. Our results indicated that orexin-A activates hypothalamic AMPK signaling through a Ca 21 -dependent mechanism involving the voltage-gated L-type calcium channel, which may serve as a potential target for regulating feeding behavior.

Excitatory orexinergic innervation of rat nucleus incertus – Implications for ascending arousal, motivation and feeding control

“…The most abundant type of NI neuron -designated as 'type I', is characterised by a delayed return to baseline potential at the offset of a hyperpolarising current pulse, indicative of A-type potassium channel activity (Mei et al, 2000), and this characteristic was always present in relaxin-3 containing neurons. (Yang & Ferguson, 2002) and voltage-gated calcium channels (Uramura et al, 2001;Kohlmeier et al, 2004Kohlmeier et al, , 2008Wu et al, 2013). The increased conductance observed in our studies, as well as the lack of OXA influence on action potential shape, especially the AHP component of the spike, suggest that OXA does not act through closure of potassium channels in NI neurons.…”

“…The contribution of the L-type calcium current to the OXA-mediated depolarisation has been observed in other brain areas including the arcuate nucleus (Wu et al, 2013), prefrontal cortex (Xia et al, 2009), dorsal raphe and laterodorsal tegmentum (Kohlmeier et al, 2004(Kohlmeier et al, , 2008, and the ventral tegmental area (Uramura et al, 2001). Elevation of the intracellular calcium concentration is of particular importance since it regulates neuronal excitability, neurotransmitter release and links synaptic activity to gene expression.…”

“…Among the different types of voltage-gated calcium channels activated by orexins, L-type calcium channels have been shown to be involved in orexin actions in various brain areas (Kohlmeier et al, 2004;Xia et al, 2009;Wu et al, 2013). In the current study, we examined the possible involvement of L-type calcium channels in the OXA-mediated response in the NI, with the L-type current blocker, nifedipine (1 µM …”

The Ca 2+ /Calmodulin/CaMKK2 Axis: Nature's Metabolic CaMshaft

“…In the fasted state, the orexigenic peptide ghrelin is released from ghrelinergic cells of the intestine and binds its cognate receptor (GHSR, or growth hormone secretagogue receptor) on presynaptic neurons that express the orexigenic hormones Neuropeptide Y (NPY) and Agouti-Related Protein (AgRP), resulting in release of Ca 2+ from intracellular stores. Orexin A, an appetite-stimulating neuropeptide, acts via voltage-gated L-type Ca 2+ channels to increase intracellular Ca 2+ in NPY-producing hypothalamic neurons [11]. Together these Ca 2+ signals lead to activation of CaMKK2 and phosphorylation of AMPK, which are necessary for regulating the hypothalamic expression of NPY and AgRP.…”

“…Together these Ca 2+ signals lead to activation of CaMKK2 and phosphorylation of AMPK, which are necessary for regulating the hypothalamic expression of NPY and AgRP. These findings provide a mechanism for how two orexigenic signals, ghrelin and orexin A, function through CaMKK2 and its target AMPK in specific cell populations of the hypothalamus to control appetite and energy homeostasis [10, 11]. In fact, mice devoid of CaMKK2 fail to respond to ghrelin, have reduced NPY expression, and consequently consume less food when compared to pair-fed wild-type (WT) mice.…”