Conditioned place preference training prevents hippocampal depotentiation in an orexin-dependent manner

GPR55 activation prevents amphetamine-induced conditioned place preference and decrease the amphetamine-stimulated inflammatory response in the ventral hippocampus in male rats

Inflammatory response in the Central Nervous System (CNS) induced by psychostimulants seems to be a crucial factor in the development and maintenance of drug addiction. The ventral hippocampus (vHp) is part of the reward system involved in substance addiction and expresses abundant G protein-coupled receptor 55 (GPR55). This receptor modulates the inflammatory response in vitro and in vivo, but there is no information regarding its anti-inflammatory effects and its impact on psychostimulant consumption. The aim of the present study was to investigate whether vHp GPR55 activation prevents both the inflammatory response induced by amphetamine (AMPH) in the vHp and the AMPH-induced conditioned place preference (A-CPP). Wistar adult male rats with a bilateral cannula into the vHp or intact males were subjected to A-CPP (5 mg/kg). Upon the completion of A-CPP, the vHp was dissected to evaluate IL-1β and IL-6 expression through RT-PCR, Western blot and immunofluorescence. Our results reveal that AMPH induces both A-CPP and an increase of IL-1β and IL-6 in the vHp. The GPR55 agonist lysophosphatidylinositol (LPI, 10 μM) infused into the vHp prevented A-CPP and the AMPH-induced IL-1β increase. CID 16020046 (CID, 10 μM), a selective GPR55 antagonist, abolished LPI effects. To evaluate the effect of the inflammatory response, lipopolysaccharide (LPS, 5 μg/μl) was infused bilaterally into the vHp during A-CPP acquisition. LPS strengthened A-CPP and increased IL-1β/IL-6 mRNA and protein levels in the vHp. LPS also increased CD68, Iba1, GFAP and vimentin expression. All LPS-induced effects were blocked by LPI. Our results suggest that GPR55 activation in the vHp prevents A-CPP while decreasing the local neuro-inflammatory response. These findings indicate that vHp GPR55 is a crucial factor in preventing the rewarding effects of AMPH due to its capacity to interfere with proinflammatory responses in the vHp.

Stress induces reinstatement of extinguished cocaine conditioned place preference by a sequential signaling via neuropeptide S, orexin, and endocannabinoid

Neurons containing neuropeptide S (NPS) and orexins are activated during stress. Previously, we reported that orexins released during stress, via orexin OX₁ receptors (OX₁ Rs), contribute to the reinstatement of cocaine seeking through endocannabinoid/CB₁ receptor (CB₁ R)-mediated dopaminergic disinhibition in the ventral tegmental area (VTA). Here, we further demonstrated that NPS released during stress is an up-stream activator of this orexin-endocannabinoid cascade in the VTA, leading to the reinstatement of cocaine seeking. Mice were trained to acquire cocaine conditioned place preference (CPP) by context-pairing cocaine injections followed by the extinction training with context-pairing saline injections. Interestingly, the extinguished cocaine CPP in mice was significantly reinstated by intracerebroventricular injection (i.c.v.) of NPS (1 nmol) in a manner prevented by intraperitoneal injection (i.p.) of SHA68 (50 mg/kg), an NPS receptor antagonist. This NPS-induced cocaine reinstatement was prevented by either i.p. or intra-VTA microinjection (i.vta.) of SB-334867 (15 mg/kg, i.p. or 15 nmol, i.vta.) and AM 251 (1.1 mg/kg, i.p. or 30 nmol, i.vta.), antagonists of OX₁ Rs and CB₁ Rs, respectively. Besides, NPS (1 nmol, i.c.v.) increased the number of c-Fos-containing orexin neurons in the lateral hypothalamus (LH) and increased orexin-A level in the VTA. The latter effect was blocked by SHA68. Furthermore, a 30-min restraint stress in mice reinstated extinguished cocaine CPP and was prevented by SHA68. These results suggest that NPS is released upon stress and subsequently activates LH orexin neurons to release orexins in the VTA. The released orexins then reinstate extinguished cocaine CPP via an OX₁ R- and endocannabinoid-CB₁ R-mediated signaling in the VTA.

Orexin-mediated restoration of hippocampal synaptic potentiation in mice with established cocaine-conditioned place preference

Orexins (also called hypocretins) are implicated in reward and addiction, but little is known about their role(s) in the association between hippocampal synaptic plasticity and drug preference. Previously, we found that exogenous orexin via OX₁ and OX₂ receptors can impair low frequency stimulation-induced depotentiation, i.e. restoring potentiation of excitatory synaptic transmission (re-potentiation) in mouse hippocampal slices. Here, we found this re-potentiation in hippocampal slices from mice that had acquired conditioned place preference (CPP) to cocaine. Both 10 and 20 mg/kg of cocaine induced similar magnitudes of CPP in mice and re-potentiation in their hippocampal slices, but differed in their susceptibility to TCS1102, a dual (OX₁ and OX₂ ) orexin receptor antagonist. TCS1102 significantly attenuated CPP and hippocampal re-potentiation induced by cocaine at 10 mg/kg but not at 20 mg/kg. Nonetheless, SCH23390, an antagonist of dopamine D1-like receptors (D1-likeRs), inhibited the effects induced by both doses of cocaine. SKF38393, a D1-likeR-selective agonist, also induced hippocampal re-potentiation in vitro. Interestingly, this effect was attenuated by TCS1102. Conversely, SCH23390 prevented orexin A-induced hippocampal re-potentiation. These results suggest that endogenous orexins are released in mice during cocaine-CPP acquisition, which sustains potentiated hippocampal transmission via OX₁ /OX₂ receptors and may contribute to the addiction memory of cocaine. This effect of endogenous orexins, however, may be substituted by dopamine that may dominate hippocampal re-potentiation and CPP via D1-likeRs when the reinforcing effect of cocaine is high.

Orexin knockout mice exhibit impaired spatial working memory

Orexins play a crucial role in the maintenance of arousal and are involved in the modulation of diverse physiological process, including cognitive function. Recent data have suggested that orexins are involved in learning and memory processes. The purpose of this study was to assess the effects of orexin deficiency on working memory. A delayed non-matching-to-place T-maze task was used to evaluate spatial working memory in mice lacking orexin prepro-peptide (orexin knockout; KO) and wild-type controls. We demonstrated that the number of correct choices in the orexin KO mice became lower than that of the controls over training. In an object exploration task, the controls explored the displaced object more than the mutants did, whereas this difference was not observed for the nondisplaced objects in either group. The orexin KO mice showed locomotor activity comparable to the control mice in terms of total distance traveled across training in both the object exploration task and the open field test. These findings indicate that the orexin system plays an important role in working memory of spatial cues.