Role of intra-hippocampal orexin 1 and orexin 2 receptors in conditioned place preference induced by chemical stimulation of the lateral hypothalamus

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.

Involvement of orexin-2 receptors in the CA1 region of the hippocampus in the extinction and reinstatement of methamphetamine-induced conditioned place preference in the rats

The present study, regarding the orexin receptors having a pivotal role in reward-related psychostimulant use disorder (PUD), aimed to investigate the role of orexin-2 (OX2) receptors in the CA1 region of the hippocampus (HPC) in the extinction and reinstatement of methamphetamine (METH)-induced conditioned place preference (CPP). In the first set of investigations, to determine the role of OX2 receptors in the extinction of METH-induced CPP, rats were daily given (during the extinction) bilaterally intra-CA1 region different doses of TCS OX2 29 (1, 3, 10, and 30 nmol/0.5 μl 12% DMSO) as the selective OX2 receptor antagonist. Then, to demonstrate the role of OX2 receptors in the reinstatement of METH-induced CPP after the extinction was established, each rat bilaterally received TCS OX2 29 at the same doses in the CA1 region before injection of the sub-threshold (priming) dose of METH (0.25 mg/kg, sc) on the reinstatement day. The data revealed that the administration of TCS OX2 29 in the CA1 region reduces the mean extinction latency and suppresses the reinstatement of METH-seeking behavior in extinguished rats. Additionally, the potency of TCS OX2 29 to inhibit the reinstatement phase was higher compared to the potency of this drug to modulate the extinction phase of METH-induced CPP. Accordingly, it could be concluded that the blockade of the OX2 receptors in this area might be an essential application and potential therapeutics in treating METH use disorder.

Distinct roles for orexin-1 and orexin-2 receptors in the dentate gyrus of the hippocampus in the methamphetamine-seeking behavior in the rats

Because of the relapsing properties of psychostimulants such as methamphetamine (Meth), there is no established pharmacotherapy for Meth addiction. The orexinergic system is a promising target for treating psychostimulant use disorders and relapse. However, to the best of our knowledge, no investigation regarding the role of orexin receptors in the dentate gyrus (DG) region of the hippocampus has been conducted in the extinction and reinstatement of Meth-seeking behavior. Two stainless-steel guide cannulae were bilaterally implanted into the DG of the rats' brains. The unbiased conditioned place preference (CPP) procedure was conducted to induce Meth conditioning. Following the five days Meth injections (1 mg/kg; sc), animals received intra-DG microinjection of SB334867 or TCS OX2 29, as orexin 1 (OX1) or orexin 2 (OX2) receptor antagonists, respectively (without Meth administration) during extinction phase to elucidate the role of orexin receptors in the latency of the extinction period in the Meth-conditioned rats. To evaluate the role of orexin receptors in the DG region in the reinstatement of Meth-seeking behavior, the extinguished rats received SB334867 or TCS OX2 29 before injecting a priming dose of Meth (0.25 mg/kg; sc). The results indicated two distinct roles for the OX1 and OX2 receptors in the DG region. TCS OX2 29 attenuated the extinction latency, and SB334867 considerably reduced the reinstatement of Meth-seeking behavior in this region. Therefore, the DG region's orexinergic system might be a potential therapeutic target for psychostimulant use disorders.

Hippocampal orexin receptors: Localization and function

Orexin (hypocretin) is secreted from the perifornical/lateral hypothalamus and is well known for sleep regulation. Orexin has two, orexin A and B, transcripts and two receptors, type 1 and 2 (OX1R and OX2R), located in the plasma membrane of neurons in different brain areas, including the hippocampus involved in learning, memory, seizures, and epilepsy, as physiologic and pathologic phenomena. OX1R is expressed in the dentate gyrus and CA1 and the OX2R in the CA3 areas. Orexin enhances learning and memory as well as reward, stress, seizures, and epilepsy, partly through OX1Rs, while either aggravating or alleviating those phenomena via OX2Rs. OX1Rs activation induces long-term changes of synaptic responses in the hippocampus, an age and concentration-dependent manner. Briefly, we will review the localization and functions of hippocampal orexin receptors, their role in learning, memory, stress, reward, seizures, epilepsy, and hippocampal synaptic plasticity.

Reviewing the role of the orexinergic system and stressors in modulating mood and reward-related behaviors

In this review study, we aimed to introduce the orexinergic system as an important signaling pathway involved in a variety of cognitive functions such as memory, motivation, and reward-related behaviors. This study focused on the role of orexinergic system in modulating reward-related behavior, with or without the presence of stressors. Cross-talk between the reward system and orexinergic signaling was also investigated, especially orexinergic signaling in the ventral tegmental area (VTA), the nucleus accumbens (NAc), and the hippocampus. Furthermore, we discussed the role of the orexinergic system in modulating mood states and mental illnesses such as depression, anxiety, panic, and posttraumatic stress disorder (PTSD). Here, we narrowed down our focus on the orexinergic signaling in three brain regions: the VTA, NAc, and the hippocampus (CA1 region and dentate gyrus) for their prominent role in reward-related behaviors and memory. It was concluded that the orexinergic system is critically involved in reward-related behavior and significantly alters stress responses and stress-related psychiatric and mood disorders.

The role of dentate gyrus dopaminergic receptors in the lateral hypothalamic-induced antinociception during persistent inflammatory pain in male rats

The lateral hypothalamus (LH) is one of the key brain areas involved in pain modulation. Also, the dentate gyrus (DG) of the hippocampus expresses various receptors, including dopaminergic receptors. Dopaminergic receptors play a key role in pain transmission and modulation within the brain. The present study aimed to investigate the involvement of DG dopaminergic receptors in the LH-induced antinociception during the presence of inflammatory pain. Male Wistar rats were used in this study. Cannulae were unilaterally implanted in their skull for microinjections into the LH and DG. The LH was chemically stimulated by carbachol injection (250 nM/0.5 μl saline). In separate groups, different doses (0.25, 1, and 4 μg/0.5 μl vehicle) of the D1- and D2-like dopamine receptor antagonists (SCH23390 and Sulpiride, respectively) were microinjected into the DG, 5 min prior to intra-LH injection of carbachol. Five min after the second injection, formalin test as a persistent inflammatory pain model in animals was done in all rats. The results revealed that carbachol could induce antinociception following formalin injection into rat's hind paw. The 4 μg dose of both antagonists significantly reduced the LH stimulation-induced antinociception in both phases of formalin pain responses. Although the 1 μg dose of sulpiride significantly reduced antinociception during both phases, 1 μg SCH23390 could only reduce this antinociception during the late phase. These findings demonstrate the involvement of DG dopaminergic receptors in the LH-induced antinociception. The results also suggest that the effectiveness of DG dopaminergic receptors is more pronounced during the late phase of formalin-induced pain responses.

Blockade of orexin receptors in the hippocampal dentate gyrus reduced the extinction latency of morphine-induced place preference in male rats

Relapse to drugs such as opioids is a major challenge in addiction therapy. It has been known that the orexinergic system has a significant role in mediating reward processing and addiction, as shown by the conditioned place preference (CPP). The dentate gyrus (DG) of the hippocampus receives orexinergic projections from the lateral hypothalamus that has been approved as a critical area arbitrating the maintenance of drug-seeking behavior following the extinction. The present study aimed to investigate the effects of intra-DG administration of the orexin-1 receptor (OX1R) and orexin-2 receptor (OX2R) antagonists on the extinction of morphine-induced CPP in male rats. Animals received different doses of SB334867 (as OX1R antagonist) or TCS OX2 29 (as OX2R antagonist) (0.5, 2.5, and 12.5 nM/0.5 μl DMSO 12 %) bilaterally into the DG during the extinction phase, after CPP had been induced by subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. The conditioning scores were recorded during the test. The results demonstrated that intra-DG administration of the highest dose of OX1R antagonist (12.5 nM/0.5 μl DMSO 12 %) shortened the extinction latency of morphine-CPP compared to the DMSO group, while the OX2R antagonist did not significantly alter the latency. Findings imply that the blockade of OX1R, but not OX2R, within the DG facilitates the extinction of morphine-induced reward. In conclusion, the OX1R antagonist might be kept in mind as a convenient therapeutic factor in repressing drug-seeking behaviors in an optimum amount of treatment considering the low dose-treatments applied.

Blockade of orexin receptors in the ventral tegmental area reduced the extinction period of the lateral hypothalamic-induced conditioned place preference in rats

The orexinergic connection between the lateral hypothalamus (LH) and the ventral tegmental area (VTA) is involved in modulating the reward circuit. The conditioned place preference (CPP) can be induced by microinjection of carbachol, a cholinergic agonist, into the LH. The current research was conducted to understand whether intra-VTA orexin receptors (OXRs) could influence the duration of the extinction period or maintenance of the intra-LH carbachol-induced CPP. To this end, the rats unilaterally received intra-LH carbachol (250 nM) within a 3-day conditioning period. Animals that have already passed the conditioning test were unilaterally administered by intra-VTA microinjection of SB334867, an OX1R antagonist, or TCS OX2 29, an OX2R antagonist during the extinction phase of the LH stimulation-induced CPP. For the first time, our data indicated that daily intra-VTA administration of either SB334867 (30 nM) or TCS OX2 29 (10 and 30 nM) during the extinction period decreased the maintenance of intra-LH carbachol-induced CPP. In conclusion, OXRs in the VTA play crucial roles in the maintenance of reward processes.

Involvement of orexin receptors within the hippocampal dentate gyrus in morphine-induced reinstatement in food-deprived rats

The orexinergic system is found to cooperate in mediating stress-induced drug relapse. The orexinergic terminals innervate neurons of the hippocampal dentate gyrus (DG) which is a key structure in the maintenance and reinstatement of drug addiction. However, the specific contribution of intra-DG orexin receptors to stress-induced reinstatement has not been completely known. In the current study, the effects of intra-DG administration of SB334867, an orexin-1 receptor (OX1R) antagonist, and TCS OX2 29, an orexin-2 receptor (OX2R) antagonist, were investigated on the reinstatement induced by a sub-threshold dose of morphine and food deprivation (FD) stress. Adult male rats received different doses of SB334867 or TCS OX2 29 (3, 10, and 30 nM/0.5 μl DMSO 12%) bilaterally into the DG in separate groups, following the acquisition and extinction of morphine-induced conditioned place preference (CPP). Then, the reinstatement was evaluated by the 24 h FD stress and/or a sub-threshold dose of morphine (0.5 mg/kg, s.c.). CPP scores and locomotor activities were recorded during the test. The findings indicated that pre-treatment with the highest dose of SB334867 (30 nM) and two higher doses of TCS OX2 29 (10 and 30 nM) blocked the sub-threshold dose and FD stress-induced reinstatement of morphine. The effect of TCS OX2 29 on reduction of reinstatement was more pronounced than that of SB334867. It suggests a role for the orexin receptors, especially OX2R within the DG region in the stress-induced reinstatement of morphine-seeking behaviours in extinguished rats.

[Chronic alcoholism influences the mRNA level of the orexin receptor type 1 (OX1R) in emotiogenic structures of the rat brain]

Orexin and its receptors were shown to be involved into mechanisms of pathological craving to alcohol. This paper demonstrates that the orexin receptor type 1 (OX1R) mRNA level significantly decreased in the prefrontal cortex of rats chronically (during 6 months) consuming ethanol compared with intact control. The same results were observed on day 1 and day 7 of alcohol withdrawal after chronic alcoholization. On the contrary, in the hippocampus, the OX1R mRNA level increased on day 1 and day 7 of alcohol withdrawal. In the ventral tegmental area, the OX1R mRNA level did not change on the day 1 and day 7 of alcohol withdrawal compared with the groups of chronic alcoholization and intact control. These findings point out involvement of the prefrontal cortex and hippocampus first of all in mechanisms mediating chronic alcohol intoxication. The ventral tegmental area is described as a typical dopaminergic structure providing the executive mechanism of emotion reactions connected with alcohol abuse in particular. It is possibe, that the modulating action of orexins on dopaminergic neurons in this structure does not provide a significant effect on control of emotion reactions in alcoholism.

Functional crosstalk of nucleus accumbens CB1 and OX2 receptors in response to nicotine-induced place preference

In the present study, we have evaluated the existence of functional interaction between orexin-2 receptor (OX2R) and cannabinoid-1 receptor (CB1R) in the nucleus accumbens core (NAcc), in nicotine-induced conditioned place preference (CPP) of Wistar male rat. Nicotine (0.5 mg/kg; s.c.) in the course of conditioning, produced a significant place preference, without any effect on the locomotor activity. Intra-NAcc administration of ineffective and effective doses of TCS-OX2-29 (2 and 6 ng/rat), a selective OX2R antagonist and AM251 (10 and 50 ng/rat), a selective CB1R antagonist, showed a significant interaction between OX2R and CB1R in the acquisition of nicotine-induced CPP (p < 0.01), and the locomotor activity (p < 0.05). No significant interaction was observed between these two receptors in the expression of nicotine-induced CPP. Our findings provide insight into the possible interaction of OX2R and CB1R of the NAcc in nicotine addiction. We propose a potential interaction between cannabinoid and orexinergic systems within the NAcc, in producing the rewarding effects.

Intra-hippocampal administration of orexin receptor antagonists dose-dependently attenuates reinstatement of morphine seeking behavior in extinguished rats

It has been shown that the hippocampus plays an essential role in the regulation of reward and memory as indicated by the conditioned place preference (CPP) paradigm. Morphine-induced CPP is a common method to consider motivational properties of morphine in animals. Recently, this model has been used in many laboratories to investigate neuronal mechanisms underlying reinstatement of morphine seeking induced by drug re-exposure. Our previous studies indicate that the hippocampus especially CA1 region is involved in reinstatement of drug-seeking behaviors. Also, several studies have shown that orexin attenuates key functional and behavioral effects of its co-transmitter dynorphin. The present study evaluates the role of orexinergic receptors within the CA1 region of the hippocampus in the reinstatement of morphine-induced CPP. Therefore, after the extinction period, the different doses (SB 334867; 0.3, 3, and 30 nM/0.5 μl DMSO) of either orexin-1 or -2 receptor antagonists were bilaterally microinjected into the CA1, 15 min before receiving an effective priming dose of morphine (1 mg/kg). The results revealed that administration of both SB 334867 and TCS OX2 29 prior to injection of the priming dose of morphine significantly reduced the reinstatement of morphine-induced CPP without altering the animal's locomotor activity. Also, the 50% effective dose value of SB 334867 on the reinstatement of morphine seeking behavior was close three times more than that in TCS OX2 29 treatment group. Therefore, the consequences suggested that both orexin receptors in the CA1 play a considerable role in the reinstatement of morphine-induced CPP.

Role of orexin-1 receptors in the dorsal hippocampus (CA1 region) in expression and extinction of the morphine-induced conditioned place preference in the rats

Orexinergic system is involved in reward processing and drug addiction. Objectives here, we investigated the effect of intra-hippocampal CA1 administration of orexin-1 receptor (OX1r) antagonist on the expression, and extinction of morphine-induced place preference in rats. Conditioned place preference (CPP) was induced by subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. Two experimental plots were designed; SB334867 as a selective OX1r antagonist was dissolved in 12% DMSO, prepared in solutions with different concentrations (3, 30, and 300 nM), and microinjected into the CA1 and some neighboring regions (0.5 μl/side), bilaterally. CPP score and locomotor activity were recorded during the CPP test. Results demonstrated that intra-CA1 administration of the OX1r antagonist attenuates the expression of morphine-induced CPP. Furthermore, higher concentrations of SB334867 facilitated the extinction period of morphine-induced CPP and reduced its latency. Nevertheless, solely administration of DMSO did not have any influence on the CPP scores and locomotion in both phases. Our findings suggest that OX1rs in the CA1 region of the hippocampus are involved in the expression of morphine CPP. Moreover, blockade of OX1rs could facilitate extinction and may extinguish the ability of drug-related cues. It seems that the antagonist might be considered as a propitious therapeutic agent in suppressing drug-seeking behaviors.

Role of intra-accumbal orexin receptors in the acquisition of morphine-induced conditioned place preference in the rats

Orexin receptor shave essential role in the induction of reward-related behaviors to several drugs of abuse. In the present study, we investigated the effects of bilateral administration of SB334867, as an orexin-1 receptor antagonist, and TCS OX2 29, as an orexin-2 receptor antagonist, into the nucleus accumbens (NAc) on the acquisition of morphine-induced conditioned place preference (CPP) in the rats. Adult male Wistar rats (n=80; 220-250g) were entered in a CPP paradigm. Bilateral microinjections of different doses of SB334867 (1, 3, 10 and 30nM) or TCS OX2 29 (3, 10, 30 and 100nM) into the NAc (0.5μl/side) were done 5min before subcutaneous injection of morphine (5mg/kg) during 3-dayconditioning (acquisition) phase. The CPP scores and locomotor activity of animals were recorded by video tracking system and Ethovision software. The results demonstrated that intra-NAc microinjection of 3, 10 and 30nM solutions of SB334867 markedly decreased the acquisition of morphine-induced CPP in a dose-dependent manner. Intra-accumbal injection of 10, 30 and 100nM solutions of TCS OX2 29 significantly attenuated the acquisition of morphine CPP as well. In addition, contribution of orexin-1 receptors to development of morphine reward-related behaviors was more than orexin-2 receptors. Our results suggest that both orexin-1 and -2 receptors in the NAc are involved in the development of morphine-induced CPP. It seems that orexin-1 receptors in this region are more effective in development of drug seeking behaviors in the rats.

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

Background

Long-term potentiation (LTP) is well recognized as a cellular-correlated synaptic plasticity of learning and memory. However, its reversal forms of synaptic plasticity, depotentiation, is less studied and its association with behaviors is also far from clear. Previously, we have shown that nanomolar orexin A can prevent the depotentiation induced by low frequency stimulation (LFS) following theta burst stimulation-induced LTP, namely inducing re-potentiation, at hippocampal CA1 synapses in vitro. Here, we explored the functional correlate of this orexin-mediated hippocampal re-potentiation.

Methods and results

We found that intraperitoneal (i.p.) injection process-paired contextual exposures during the conditioned place preference (CPP) task in mice resulted in re-potentiation at CA1 synapses of hippocampal slices, regardless of whether the CPP behavior is expressed or not. Simply exposing the mouse in the CPP apparatus, or giving the mouse consecutive i.p. injections of saline in its home cage or a novel cage did not lead to hippocampal re-potentiation. Besides, this CPP training process-induced hippocampal re-potentiation was prevented when mice were pretreated with TCS1102, a dual orexin receptor antagonist. These results suggest that the expression of hippocampal re-potentiation is orexin-dependent and requires the association of differential spatial contexts and i.p. injections in the CPP apparatus.

Conclusions

Together, we reveal an unprecedentedly orexin-mediated modulation on hippocampal depotentiation by the training process in the CPP paradigm.

Electronic supplementary material

The online version of this article (10.1186/s12929-017-0378-0) contains supplementary material, which is available to authorized users.

Effects of dorsal hippocampal orexin-2 receptor antagonism on the acquisition, expression, and extinction of morphine-induced place preference in rats

RATIONALE

Orexinergic system is involved in reward processing and drug addiction.

OBJECTIVES

Here, we investigated the effect of intrahippocampal CA1 administration of orexin-2 receptor (OX2r) antagonist on the acquisition, expression, and extinction of morphine-induced place preference in rats.

METHODS

Conditioned place preference (CPP) was induced by subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. Three experimental plots were designed; TCS OX2 29 as a selective antagonist of orexin-2 receptors (OX2rs) was dissolved in DMSO, prepared in solutions with different concentrations (1, 3, 10, and 30 nM), and was bilaterally microinjected into the CA1 and some neighboring regions (0.5 μl/side). Conditioning scores and locomotor activities were recorded during the test.

RESULTS

Results demonstrate that intra-CA1 administration of the OX2r antagonist attenuates the induction of morphine CPP during the acquisition and expression phases. Effect of TCS OX2 29 on reduction of morphine CPP was dose-dependent and was more pronounced during the acquisition than the expression. Furthermore, higher concentrations of TCS OX2 29 facilitated the extinction of morphine-induced CPP and reduced extinction latency period. Nevertheless, administration of TCS OX2 29 solutions did not have any influence on locomotor activity of all phases.

CONCLUSIONS

Our findings suggest that OX2rs in the CA1 region of hippocampus are involved in the development of the acquisition and expression of morphine CPP. Moreover, blockade of OX2rs could facilitate extinction and may abrogate or extinguish the ability of drug-related cues, implying that the antagonist might be considered as a propitious therapeutic agent in suppressing drug-seeking behavior.

Orexin-A affects gastric distention sensitive neurons in the hippocampus and gastric motility and regulation by the perifornical area in rats

Orexin-A is mainly produced in the lateral hypothalamus (LHA) and the perifornical area (PeF). Here, we aim to elucidate the effects of orexin-A in the hippocampus (Hi) on gastric distention (GD)-sensitive neurons and gastric motility, and potential regulation mechanisms by the PeF. Retrograde tracing and fluorescent-immunohistochemical staining were used to determine orexin-A neuronal projections. Single unit discharges in the Hi were recorded extracellularly and gastric motility in conscious rats was monitored during administration of orexin-A to the Hi or electrical stimulation of the PeF. Orexin-A administration to the Hi excited most of the GD-excitatory (GD-E) neurons and GD-inhibitory (GD-I) neurons, and increased gastric motility in a dose-dependent manner. All of effects induced by orexin-A could be partly blocked by pretreatment with orexin-A antagonist, SB-334867. Electrical stimulation of the PeF excited the majority of the orexin-A-responsive GD neurons in the Hi and promoted gastric motility. Additionally, pretreatment with SB-334867 in the Hi increased the firing rate of GDI and GDE neurons following electrical stimulation of the PeF. These findings suggest that orexin-A could regulate activities of GD-sensitive neurons and gastric motility. Furthermore, the PeF may be involved in this regulatory pathway.