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.

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the 'anti-reward' effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking. LINKED ARTICLESThis article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2 AbbreviationsBOLD, blood oxygen level-dependent; CeA, central nucleus of the amygdala; CPP, conditioned place preference; CRF, corticotropin-releasing factor; DAGL, DAG lipase; DMH, dorsomedial nucleus of the hypothalamus; LH, lateral hypothalamus; mPFC, medial prefrontal cortex; NAc, nucleus accumbens; PFA, perifornical area; VTA, ventral tegmental area IntroductionNeurons containing orexin, also known as hypocretin, are firmly established as regulators of reward seeking. Orexin-A and -B (hypocretin-1 and -2) are peptides produced in neurons residing in the lateral and dorsomedial hypothalamus and perifornical area Sakurai et al., 1998) that have widespread projections throughout the brain . Orexin containing neurons project from the lateral hypothalamus (LH) to many areas of the mesolimbic 'reward pathway' including the ventral tegmental area (VTA) and the nucleus accumbens (NAc) (Di Chiara and Imperato, 1985;Koob and Bloom, 1988;Wise and Rompre, 1989) and these neurons are primarily implicated in rewardseeking behaviour. Within the VTA, 20% of neuronal inputs originating in the LH show immunolabelling for orexin (...