Activation of Orexin 1 Receptors in the Periaqueductal Gray of Male Rats Leads to Antinociception via Retrograde Endocannabinoid (2-Arachidonoylglycerol)-Induced Disinhibition

Abstract

Orexin A and B are hypothalamic peptides known to modulate arousal, feeding and reward via OX1 and OX2 receptors. Orexins are also antinociceptive in the brain but their mechanism(s) of action remain unclear. Here, we investigated the antinociceptive mechanism of orexin A in the rat ventrolateral periaqueductal gray (vlPAG), a midbrain region crucial for initiating descending pain inhibition. In vlPAG slices, orexin A (30-300 nM) depressed GABAergic evoked inhibitory postsynaptic currents (IPSCs). This effect was blocked by an OX1 (SB 334867), but not OX2 (Compound 29), antagonist. Orexin A increased the paired-pulse ratio of paired IPSCs, and decreased the frequency, but not amplitude, of miniature IPSCs. Orexin A-induced IPSC depression was mimicked by WIN 55,212-2, a cannabinoid 1 (CB1) receptor agonist. AM 251, a CB1 antagonist, reversed depressant effects by both agonists. Orexin A-induced IPSC depression was prevented by U73122 and tetrahydrolipstatin, inhibitors of phospholipase C (PLC) and diacylglycerol lipase (DAGL), respectively, and enhanced by URB602, which inhibits enzymatic degradation of 2-arachidonoylglycerol (2-AG). Moderate DAGLα, but not DAGLβ, immunoreactivity was observed in the vlPAG. Orexin A produced an overall excitatory effect on evoked postsynaptic potentials and hence increased vlPAG neuronal activity. Intra-vlPAG microinjection of orexin A reduced hot-plate nociceptive responses in rats in a manner blocked by SB 334867 and AM 251. Therefore, orexin A may produce antinociception by activating postsynaptic OX1 receptors, stimulating synthesis of 2-AG, an endocannabinoid, through a Gq-protein-mediated PLC-DAGLα enzymatic cascade culminating in retrograde inhibition of GABA release (disinhibition) in the vlPAG.

OX1 Orexin/Hypocretin Receptor Signaling through Arachidonic Acid and Endocannabinoid Release

“…These systems may be involved in the same pathways. A study indicated endocannabinoid involvement in orexin-mediated stimulation of appetite (Crespo et al, 2008), and similar findings are indicated for the analgesic action of orexin-A (Ho et al, 2011). In contrast, endocannabinoids inhibit orexin signaling in the lateral hypothalamus by reducing the excitatory glutamatergic drive to orexinergic neurons (Huang et al, 2007).…”

Autocrine Endocannabinoid Signaling through CB1Receptors Potentiates OX1Orexin Receptor Signaling

“…For instance, in rat dorsal raphe nucleus, orexin-B-mediated inhibition of glutamate release can be ascribed to 2-AG release and action on CB 1 receptors on presynaptic terminals (Haj-Dahmane and Shen, 2005). Orexin-A-induced analgesic response in rat periaqueductal gray relies on 2-AG-mediated retrograde inhibition of g-aminobutyric acid release (Ho et al, 2011). Heterologous coexpression of OX 1 and CB 1 receptors in Chinese hamster ovary (CHO) cells very strongly potentiates OX 1 receptor signaling to ERK (Hilairet et al, 2003).…”

Yohimbine Depresses Excitatory Transmission in BNST and Impairs Extinction of Cocaine Place Preference Through Orexin-Dependent, Norepinephrine-Independent Processes

“…The Gq-linked mGluR5 and a 1 -AR both induce depression of excitatory transmission yet enhance NMDA receptor function (Grueter and Winder, 2005;Kirkwood et al, 1999;McElligott and Winder, 2009;McElligott et al, 2010). Moreover, two recent studies reported that in the hippocampus and ventrolateral periaqueductal gray (vlPAG), orexin A elicited a depression of excitatory transmission in young animals (Ho et al, 2011;Selbach et al, 2010).…”

“…Two recent studies reported that in the hippocampus and ventrolateral periaqueductal gray (vlPAG), orexin A elicited a depression of excitatory transmission in young animals (Ho et al, 2011;Selbach et al, 2010). We thus investigated the effects of exogenous orexin A on excitatory transmission in the dlBNST.…”