Abstract
Orexins are associated with drug relapse in rodents. Here, we show that acute restraint stress in mice activates lateral hypothalamic (LH) orexin neurons, increases levels of orexin A and 2-arachidonoylglycerol (2-AG) in the ventral tegmental area (VTA), and reinstates extinguished cocaine-conditioned place preference (CPP). This stress-induced reinstatement of cocaine CPP depends on type 1 orexin receptors (OX1Rs), type 1 cannabinoid receptors (CB1Rs) and diacylglycerol lipase (DAGL) in the VTA. In dopaminergic neurons of VTA slices, orexin A presynaptically inhibits GABAergic transmission. This effect is prevented by internal GDP-β-S or inhibiting OX1Rs, CB1Rs, phospholipase C or DAGL, and potentiated by inhibiting 2-AG degradation. These results suggest that restraint stress activates LH orexin neurons, releasing orexins into the VTA to activate postsynaptic OX1Rs of dopaminergic neurons and generate 2-AG through a Gq-protein-phospholipase C-DAGL cascade. 2-AG retrogradely inhibits GABA release through presynaptic CB1Rs, leading to VTA dopaminergic disinhibition and reinstatement of cocaine CPP.
Highlights
Orexins are associated with drug relapse in rodents
Inhibitory GABAergic postsynaptic currents (IPSCs) after treatment with SB-334867 alone or in combination with orexin A were not significantly different (114.1±7.0 versus 127.1±11.3% of baseline, n 1⁄4 5, paired t-test, P 1⁄4 0.362, t(4) 1⁄4 1.027). These results suggest that orexin A depressed IPSCs in ventral tegmental area (VTA) dopaminergic neurons through OX1Rs, and not through OX2Rs
Orexin A (100 nM) significantly reduced the frequency (Fig. 2a,b), but not the amplitude (Fig. 2a,c), of miniature IPSCs, leading to a rightward shift in the cumulative distribution of mIPSC intervals (Po0.01, Kolmogorov–Smirnov test; Fig. 2b), but not amplitude (Fig. 2c). These results suggest that orexin A inhibits GABAergic transmission in VTA dopaminergic neurons by decreasing GABA release
Summary
Orexins are associated with drug relapse in rodents. Here, we show that acute restraint stress in mice activates lateral hypothalamic (LH) orexin neurons, increases levels of orexin A and 2-arachidonoylglycerol (2-AG) in the ventral tegmental area (VTA), and reinstates extinguished cocaine-conditioned place preference (CPP). The reinstatement of cocaine, alcohol, morphine or food seeking behaviours induced by cue, context, yohimbine or the rewarded drug was antagonized by an OX1R, but not OX2R, antagonist[6] It remains unclear whether orexins in the VTA are involved in stressinduced drug seeking[14,15]. Using VTA slices, Borgland et al.[16] have demonstrated that orexin A increases VTA dopaminergic activity via long-term potentiation of glutamatergic transmission onto VTA dopaminergic neurons This long-term plasticity in the VTA glutamatergic system may contribute to cocaine-induced sensitization[16] or other drug rewards[17], it may not be sufficient to stress induce drug relapse, since intra-VTA orexin A-induced reinstatement of cocaine seeking in rats is only partially glutamate-dependent[14]. It is likely that alternative mechanisms exist by which orexin A modulates GABAergic activity, and increases VTA dopaminergic activity
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