The role of orexin-1 receptors within the ventral tegmental area in the extinction and reinstatement of methamphetamine place preference

Involvement of D2-like dopamine receptors within the ventral tegmental area in the cannabidiol's inhibitory effects on the methamphetamine-seeking behavior

Cannabidiol (CBD) is a non-psychoactive substance derived from marijuana. Although a comprehensive understanding of CBD's mechanism of action is still lacking, it is well-established that CBD can effectively mitigate the addictive properties associated with drugs. This study examined how CBD inhibits the acquisition and expression of methamphetamine-induced conditioned place preference (CPP) through D2-like dopamine receptors (D2R) in the ventral tegmental area (VTA). After recovery from surgery, animals were subjected to bilateral intra-VTA administration of different dosages (0.25, 1, and 4 μg/0.3 μl DMSO per side) of a selective D2R antagonist, sulpiride before intracerebroventricular (ICV) injection of CBD, during the conditioning phase (10 μg/5 μl DMSO) or once in the post-conditioning phase (50 μg/5 μl DMSO) of methamphetamine-induced CPP (daily subcutaneous injections of methamphetamine at 1 mg/kg over 5-day conditioning period). Findings revealed that CBD inhibits the acquisition and expression of methamphetamine reward memory. At the same time, microinjection of D2R antagonists into the VTA significantly reduced CBD's suppressive effect on the acquisition (0.25 μg; P<0.05, 1 and 4 μg; P<0.001) and expression (1 and 4 μg; P<0.01) of methamphetamine place preference. Moreover, D2R blockage alone in the VTA did not affect the formation and expression of methamphetamine-induced CPP. In addition, the present study showed that administration of intra-VTA sulpiride and ICV injection of CBD together does not cause place preference in the CPP paradigm. In conclusion, pharmacological manipulation of D2Rs in the VTA may alter CBD's suppressive effects on the methamphetamine-context associations.

Chronic alcohol withdrawal-associated increases in VTA Hcrtr1 expression are associated with heightened nociception and anxiety-like behavior in female rats

Alcohol withdrawal is characterized by various symptoms that include pain and negative affect in the absence of the drug. The neural underpinnings of these behaviors are not entirely understood, but orexin has emerged as a candidate target for the treatment of substance use disorders. Here, we explored changes in orexin system-related gene expression in brain regions important for mediating reward and stress, including the ventral tegmental area (VTA) and extended amygdala (including the central amygdala, nucleus accumbens shell, and bed nucleus of the stria terminalis), in adolescent and adult female Wistar rats following chronic alcohol exposure. We observed higher numbers of Hcrtr1- (orexin receptor 1)-expressing neurons in the VTA of adolescent and adult female rats during withdrawal from chronic alcohol exposure. The number of Hcrt1+ VTA neurons was negatively correlated with thermal sensitivity in adolescent female rats and anxiety-like behavior in adult female rats. These data suggest that chronic alcohol effects on orexin receptor expression in the VTA are related to specific behaviors that manifest during withdrawal, highlighting potential avenues for targeting alcohol withdrawal-associated behaviors across development.

Sleep Loss and Substance Use Disorders: An Issue from Adolescents to Adults

Unsatisfactory sleep is a worldwide concern, as evidenced by the high prevalence of insomnia symptoms and diagnosis in the general population, and an issue that has also risen among adolescents. These circumstances are a cause of worry due to, among other factors, the observed bidirectional association of sleep disturbances and the risk of substance use disorder development. In this regard, across the globe, several reports indicate that substance consumption is at an all-time high, with alcohol, nicotine, and cannabis leading the charts. Additionally, the age of onset has dropped, with reports suggesting that first contact is usually during adolescence. Although the nature of the link between poor sleep and substance use disorder development is still not fully understood, it is possible that an overactive orexinergic system could play a role, as it has been observed that treatment with orexinergic antagonists improves insomnia symptoms and that postmortem studies show an increase in orexin immunoreactive neurons in sections obtained from habitual opioid consumers. We further argue that it is during adolescence that this maladaptive loop can be established, priming for the development of substance use disorders.

Orexin-mediated motivated arousal and reward seeking

The neuromodulator orexin has been identified as a key factor for motivated arousal including recent evidence that sleep deprivation-induced enhancement of reward behavior is modulated by orexin. While orexin is not necessary for either reward or arousal behavior, orexin neurons' broad projections, ability to sense the internal state of the animal, and high plasticity of signaling in response to natural rewards and drugs of abuse may underlie heightened drug seeking, particularly in a subset of highly motivated reward seekers. As such, orexin receptor antagonists have gained deserved attention for putative use in addiction treatments. Ongoing and future clinical trials are expected to identify individuals most likely to benefit from orexin receptor antagonist treatment to promote abstinence, such as those with concurrent sleep disorders or high craving, while attention to methodological considerations will aid interpretation of the numerous preclinical studies investigating disparate aspects of the role of orexin in reward and arousal.

The role of orexin receptors within the CA1 area in the acquisition and expression of methamphetamine place preference

Treatment of Methamphetamine (METH) use disorder has become a crucial public health issue. The orexin system manipulation has provided promising evidence to attenuate addictive-like behaviors. This study explored the role of the orexin 1 receptor and orexin 2 receptor (OX1R and OX2R) in the CA1 area of the hippocampal formation in the acquisition and expression of METH-induced place preference. Animals were subjected to bilateral administration of different dosages (1, 3, 10, and 30 nmol/0.5 μl DMSO per side) of a selective OX1R antagonist, SB334867, or selective OX2R antagonist, TCS OX2 29 into the CA1 area throughout the conditioning phase or once on the post-conditioning phase in separate control and experimental groups. Behavioral data revealed that both OX1R (10 nmol; P < 0.01 and 30 nmol; P < 0.001) and OX2R (10 nmol; P < 0.05 and 30 nmol; P < 0.001) antagonism during the conditioning phase could block the formation of METH place preference dose-dependently. In addition, intra-CA1 microinjection of SB334867 on the post-conditioning phase attenuated the expression of METH place preference in a dose-dependent manner (3 nmol; P < 0.05, 10 nmol; P < 0.01 and 30 nmol; P < 0.001) whereas intra-CA1 administration of TCS OX2 29 only at the highest dosage (30 nmol) declined the expression of METH place preference (P < 0.01). It was also indicated that the suppressive effects of orexin receptor blockade on the METH-seeking behavior in the CA1 area were anatomically specific to this area. These findings support the possibility of targeting the orexin system to develop novel and successful pharmacological options for the treatment of METH dependence.

Blockade of the orexin-2 receptors within the ventral tegmental area facilitates the extinction and prevents the reinstatement of methamphetamine-seeking behavior

Repeated use of methamphetamine (METH) causes severe effects on the central nervous system, associated with an increased relapse rate. The orexinergic system is highly implicated in the reward circuitry and may be a promising target for treating psychostimulant dependency. The present study aimed to investigate the involvement of the orexin system, mainly the orexin-2 receptors (OX2R) in the ventral tegmental area (VTA) in the extinction and reinstatement of METH-seeking behavior using a conditioned place preference (CPP) paradigm. To this end, animals received METH (1 mg/kg; sc) for a 5-day conditioning period. Then, in the first set of experiments, different groups of rats were given intra-VTA TCS OX2 29 (1, 3, 10, or 30 nmol/0.3 μl DMSO) as an OX2R antagonist over a 10-day extinction period. In another experiment, after the extinction period, a different set of animals received a single dose of TCS OX2 29 (1, 3, 10, or 30 nmol) before the priming dose of METH (0.25 mg/kg; sc) on the reinstatement day. The results revealed that TCS OX2 29 (10 and 30 nmol) remarkably facilitated the extinction of rewarding properties of METH (P < 0.001 for both doses). Furthermore, TCS OX2 29 (3, 10, or 30 nmol) significantly suppressed the METH-induced reinstatement (3 nmol; P < 0.05, 10 nmol; P < 0.01, and 30 nmol; P < 0.001). In conclusion, the current study revealed that the orexinergic system, specifically the VTA OX2R, is involved in METH-seeking behaviors and that manipulation of this system can be considered a potential therapeutics in treating METH dependency.