Role of dorsal hippocampal orexin-1 receptors in modulation of antinociception induced by chemical stimulation of the lateral hypothalamus

Contribution of the intra-hippocampal orexin system in the regulation of restraint stress response to pain-related behaviors in the formalin test

Stress-induced antinociception (SIA) is due to the activation of several neural pathways and neurotransmitters that often suppress pain perception. Studies have shown that the orexin neuropeptide system is essential in pain modulation. Therefore, this study aimed to investigate the role of orexinergic receptors in the hippocampal CA1 region in modulating SIA response during the formalin test as an animal model of inflammatory pain. The orexin-1 receptor (OX1r) antagonist, SB334867, at 1, 3, 10, and 30 nmol or TCS OX2 29 as an orexin-2 receptor (OX2r) antagonist at the same doses were microinjected into the CA1 region in rats. Five minutes later, rats were exposed to restraint stress (RS) for 3 h, and pain-related behaviors were monitored in 5-min blocks for the 60-min test period in the formalin test. Results showed that applying RS for 3 h reduced pain responses in the early and late phases of the formalin test. The main findings showed that intra-CA1 injection of orexin receptor antagonists reduced the antinociception caused by stress in both phases of the formalin test. In addition, the contribution of OX2r in mediating the antinociceptive effect of stress was more prominent than that of OX1r in the early phase of the formalin test. However, in the late phase, both receptors worked similarly. Accordingly, the orexin system and its two receptors in the CA1 region of the hippocampus regulate SIA response to this animal model of pain in formalin test.

Intra-accumbal orexinergic system contributes to the stress-induced antinociceptive behaviors in the animal model of acute pain in rats

Stress and pain are interleaved at numerous levels - influencing each other. Stress can increase the nociception threshold in animals, long-known as stress-induced analgesia (SIA). Orexin is known as a neuropeptide that modulates pain. The effect of stress on the mesolimbic system in the modulation of pain is known. The role of the intra-accumbal orexin receptors in the modulation of acute pain by forced swim stress (FSS) is unclear. In this study, 117 adult male albino Wistar rats (270-300 g) were used. The animals were unilaterally implanted with cannulae above the NAc. The antagonist of the orexin-1 receptor (OX1r), SB334867, and antagonist of the orexin-2 receptor (OX2r), TCS OX2 29, were microinjected into the NAc in different doses (1, 3, 10, and 30 nmol/0.5 µl DMSO) before exposure to FSS for a 6-min period. The tail-flick test was carried out as an assay nociception of acute pain, and the nociceptive threshold [tail-flick latency (TFL)] was measured for 60-minute. The findings demonstrated that exposure to acute stress could remarkably increase the TFLs and antinociceptive responses. Moreover, intra-accumbal microinjection of SB334867 or TCS OX2 29 blocked the antinociceptive effect of stress in the tail-flick test. The contribution of orexin receptors was almost equally modulating SIA. The present study's findings suggest that OX1r and OX2r within the NAc modulate stress-induced antinociceptive responses. The intra-accumbal microinjection of orexin receptors antagonists declares inducing antinociceptive responses by FSS in acute pain. Proposedly, intra-accumbla orexinergic receptors have a role in the development of SIA.

The stress-induced antinociceptive responses to the persistent inflammatory pain involve the orexin receptors in the nucleus accumbens

Stress suppresses the sense of pain, a physiological phenomenon known as stress-induced analgesia (SIA). Brain orexin peptides regulate many physiological functions, including wakefulness and nociception. The contribution of the orexinergic system within the nucleus accumbens (NAc) in the modulation of antinociception induced by forced swim stress (FSS) remains unclear. The present study addressed the role of intra-accumbal orexin receptors in the antinociceptive responses induced by FSS during the persistent inflammatory pain model in the rat. Stereotaxic surgery was performed unilaterally on 106 adult male Wistar rats weighing 250-305 g. Different doses (1, 3, 10, and 30 nmol/ 0.5 μl DMSO) of orexin-1 receptor (OX1r) antagonist (SB334867) or OX2 receptor antagonist (TCS OX2 29) were administered into the NAc five minutes before exposure to FSS for a 6-min period. The formalin test was carried out using formalin injection (50 μl; 2.5%) into the rat's hind paw plantar surface, which induces biphasic pain-related responses. The first phase begins immediately after formalin infusion and takes 3-5 min. Subsequently, the late phase begins 15-20 min after formalin injection and takes 20-40 min. The findings demonstrated that intra-accumbal microinjection of SB334867 or TCS OX2 29 attenuated the FSS-induced antinociception in both phases of the formalin test, with the TCS OX2 29 showing higher potency. Moreover, the effect of TCS OX2 29 was more significant during the early phase of the formalin test. The results suggest that OX1 and OX2 receptors in the NAc might modulate the antinociceptive responses induced by the FSS.

Blockade of orexin receptor type-1 by SB-334867 and activation of orexin receptor type-2 attenuate morphine tolerance in rats

Purpose

The interaction of orexinergic neurons with the opioidergic system and their effects on morphine analgesia and tolerance have not been fully elucidated. The purpose of the study was to evaluate the effects of the orexin-1 and orexin-2 receptor (OX1R and OX2R) agonist and antagonist on morphine analgesia and tolerance in rats.

Material and methods

A total of 90 Wistar albino male rats weighing 180–220 g were used in the experiments. To induce morphine tolerance, rats were injected with a single dose of morphine (50 mg kg−1, s.c.) for 3 days. Morphine tolerance was assessed on day 4 in randomly selected rats by analgesia tests. In order to evaluate morphine tolerance situation, orexin-A, SB-334867, orexin-B and TCS OX2 29 were administered together with morphine for 3 days. The analgesic effects of orexin-A (10 μg kg−1), OXR1 antagonist SB-334867 (10 mg kg−1), OXR2 agonist orexin-B (15 μg kg−1), OXR2 antagonist TCS OX2 29 (0.5 mg kg−1) and morphine (5 mg kg−1) were measured at 15 or 30-min intervals by tail-flick and hot-plate antinociceptive tests.

Results

The results suggested that the combination of orexin-1 receptor antagonist SB-334867 and orexin-B with morphine significantly increased the analgesic effect compared to morphine-tolerant rats. In addition, administration of orexin-A and -B alone showed significant analgesic effects compared to the saline group. However, co-administration of orexin-A and -B with morphine did not increase the analgesic efficacy of morphine.

Conclusions

The results of this study demonstrated that co-administration of SB-334867 and orexin-B with morphine attenuated morphine tolerance. Further studies are needed to elucidate the details of the interaction between orexin receptors and the opioidergic system.

The modulatory role of dopamine receptors within the hippocampal cornu ammonis area 1 in stress-induced analgesia in an animal model of persistent inflammatory pain

The intrinsic pain inhibitory mechanisms can be activated by fear, anxiety, and stress. Stressful experiences produce analgesia, referred to as stress-induced analgesia (SIA). Major components of the limbic system, including the ventral tegmental area, nucleus accumbens, amygdala, and hippocampus, are involved in the SIA. In this study, we tried to understand the role of dopamine receptors in the cornu ammonis area 1 (CA1) of the hippocampus in the forced swim stress (FSS)-induced analgesia. Stereotaxic surgery was unilaterally performed on 129 adult male Wistar rats weighing 220-280 g. SCH23390 (0.25, 1, and 4 μg/0.5 μl saline) or sulpiride (0.25, 1, and 4 μg/0.5 μl DMSO), as D1- and D2-like dopamine receptor antagonists, respectively, were microinjected into the CA1 area, 5 min before exposure to FSS for a 6-min period. The vehicle groups received saline or DMSO instead of SCH23390 or sulpiride, respectively. The formalin test was done using formalin injection (50 μl; 2.5%) into the plantar surface of the rat's hind paw immediately after exposure to FSS. The results demonstrated that FSS produces analgesia during the early and late phases of the formalin test. However, intra-CA1 microinjection of SCH23390 or sulpiride attenuated the FSS-induced analgesia in both phases of the formalin test. This study provides new insight into the role of D1- and D2-like dopamine receptors in the CA1 area in the FSS-induced analgesia during persistent inflammatory pain.

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.

Orexin receptors in the CA1 region of hippocampus modulate the stress-induced antinociceptive responses in an animal model of persistent inflammatory pain

Stress activates multiple neural pathways and neurotransmitters that often suppress pain perception, the phenomenon called stress-induced analgesia (SIA). Orexin neurons from the lateral hypothalamus project to entire brain structures such as the hippocampus. The present study examined this hypothesis that orexinergic receptors in the CA1 region of the hippocampus may play a modulatory role in the development of SIA in formalin test as an animal model of persistent inflammatory pain. One hundred-two adult male Wistar rats were administered with intra-CA1 orexin-1 receptor (OX1r) antagonist, SB334867, at the doses of 3, 10, 30, and 100 nmol or TCS OX2 29 as orexin-2 receptor (OX2r) antagonist at the doses of 1, 3, 10, and 30 nmol. Five min later, rats were exposed to forced swim stress (FSS) for a 6-min period. Then, pain-related behaviors induced by formalin injection were measured at the 5-min blocks during a 60-min period of formalin test. The current study indicated that solely stress exposure elicits antinociception in the early and late phases of the formalin test. The FSS-induced analgesia was prevented by intra-CA1 administration of SB334867 or TCS OX2 29 during either phase of the formalin test. Moreover, the contribution of the OX2r in the mediation of analgesic effect of stress was more prominent than that of the OX1r during both phases of the formalin test. It is suggested that OX1r and OX2r in the CA1 region of the hippocampus are involved in stress-induced analgesia in the animal model of persistent inflammatory pain.

Orexin-2 receptor antagonism in the cornu ammonis 1 region of hippocampus prevented the antinociceptive responses induced by chemical stimulation of the lateral hypothalamus in the animal model of persistent pain

Orexins are excitatory neuropeptides, mainly produced by neurons located in the lateral hypothalamus, which project to many brain areas. The orexinergic system plays a fundamental role in arousal, sleep/wakefulness, feeding, energy homeostasis, motivation, reward, stress and pain modulation. As a prominent part of the limbic system, the hippocampus has been involved in formalin-induced nociception modulation. Moreover, hippocampus regions express both orexin-1 (OX1) and orexin-2 (OX2) receptors. The present study investigated the role of OX2 receptors (OX2R) within the cornu ammonis 1 (CA1) region of the hippocampus in the mediation of lateral hypothalamus-induced antinociception. Fifty-three male Wistar rats were unilaterally implanted with two separate cannulae into the lateral hypothalamus and CA1. Animals were pretreated with intra-CA1 TCS OX2 29 as an OX2R antagonist before intra-lateral hypothalamus administration of carbachol (250 nM) as a muscarinic agonist for chemical stimulation of orexinergic neurons. Formalin test was used as an animal model of persistent pain, following intra-lateral hypothalamus carbachol microinjection. Results showed that the chemical stimulation of the lateral hypothalamus significantly attenuated formalin-evoked nociceptive behaviors during both phases of the formalin test, and administration of TCS OX2 29 into the CA1 blocked these antinociceptive responses in both phases, especially in the late phase. These findings suggest that OX2 receptors in the CA1 partially mediate the lateral hypothalamus-induced antinociceptive responses in persistent inflammatory pain.

Role of hippocampal orexin receptors in antinociception elicited by chemical stimulation of the lateral hypothalamus in the tail-flick test

The lateral hypothalamus (LH) orexinergic neurons project to numerous brain regions implicated in pain perception, including the CA1 part of the hippocampal formation. Moreover, the roles of orexin receptors (OXRs) in the CA1 in anti-analgesic consequences of the LH chemical stimulation by carbachol, muscarinic receptor agonist, in acute pain have not been clarified. The current research showed OXRs antagonist administration's effect in the CA1 on analgesia elicited by the LH chemical stimulation in a tail-flick test as an acute model of pain. The control groups, including vehicle-control groups, were given intra-LH administration of saline (0.5 μL), following intra-CA1 infusion of DMSO (12 %; 0.5 μL), and carbachol-control groups were treated with carbachol (250 nM/0.5 μL saline) into the LH following DMSO in the CA1. Treated groups received SB334867 (1, 3, 10, and 30 nM/0.5 μL DMSO) or TCS OX2 29 (0.1, 1, 10, and 20 nM/0.5 μL DMSO) as OX1R or OX2R antagonist, respectively, in the CA1 prior intra-LH administration of carbachol. After all injections, all rats underwent the tail-flick test over a 60-min time. Infusion of SB334867 or TCS OX2 29 in the CA1 impaired the analgesic consequences following chemical stimulation of the LH in acute pain. Meanwhile suppressive impact of the OX1R or OX2R antagonist on the analgesic impact of LH chemical stimulation was approximately identical. The current investigation provided a new document about the critical involvement of hippocampal orexinergic system in the modulatory role of the LH-CA1 path in pain perception.

Cannabidiol effectively reverses mechanical and thermal allodynia, hyperalgesia, and anxious behaviors in a neuropathic pain model: Possible role of CB1 and TRPV1 receptors

The incidence of chronic pain is high in the general population and it is closely related to anxiety disorders, which promote negative effects on the quality of life. The cannabinoid system has essential participation in the pain sensitivity circuit. In this perspective, cannabidiol (CBD) is considered a promising strategy for treating neuropathic pain. Our study aimed to evaluate the effects of sub-chronic systemic treatment with CBD (0.3, 3, 10, or 30 mg/kg, i.p.) in male in rats submitted to chronic constriction injury of the sciatic nerve (CCI) or not (SHAM) and assessed in nociceptive tests (von Frey, acetone, and hot plate, three days CBD's treatment) and in the open field test (OFT, two days CBD's treatment). We performed a screening immunoreactivity of CB1 and TRPV1 receptors in cortical and limbic regions tissues, which were collected after 1.5 h of behavioral tests on the 24th experimental day. This study presents a dose-response curve to understand better the effects of low doses (3 mg/kg) on CBD's antiallodynic and anxiolytic effects. Also, low doses of CBD were able to (1) reverse mechanical and thermal allodynia (cold) and hyperalgesia, (2) reverse anxious behaviors (reduction of the % of grooming and freezing time, and increase of the % of center time in the OFT) induced by chronic pain. The peripheral neuropathy promoted the increase in the expression of CB1 and TRPV1 receptors in the anterior cingulate cortex (ACC), anterior insular cortex (AIC), basolateral amygdala (BLA), dorsal hippocampus (DH), and ventral hippocampus (VH). CBD potentiated this effect in the ACC, AIC, BLA, DH, and VH regions. These results provide substantial evidence of the role of the ACC-AIC-BLA corticolimbic circuit, and BLA-VH for pain regulation. These results can be clinically relevant since they contribute to the evidence of CBD's beneficial effects on treating chronic pain and associated comorbidities such as anxiety.

Dopaminergic receptors in the ventral tegmental area modulated the lateral hypothalamic stimulation-induced antinociception in an animal model of tonic pain

The role of the ventral tegmental area (VTA) and the lateral hypothalamus (LH) in the modulation of formalin-induced nociception is well documented individually. The present study aimed to investigate the role of dopamine receptors of the VTA in the modulation of the LH stimulation-induced antinociception during both phases of the formalin test as an animal model of tonic pain. In this study, male Wistar rats were unilaterally implanted with two guide cannulae in the VTA and LH. In two separate groups, animals received different doses (0.25, 1, and 4 μg/rat) of D1- or D2-like dopamine receptor antagonists (SCH-23,390 or Sulpiride, respectively) into the VTA before intra-LH injection of carbachol (22.83 ng/rat) following formalin injection (50 μL; s.c.) into their contralateral hind paws. The blockade of these two receptors reduced intra-LH carbachol-induced antinociception during both phases of the formalin test. This reduction during the late phase of the formalin test was more than that of the early phase. The results indicated that LH stimulation-induced antinociception was mediated by D1- and D2-like dopamine receptors in the VTA, and so, the neural pathway projecting from the LH to the VTA contributes to the modulation of formalin-induced nociception in the rats.

Research progress on the mechanism of orexin in pain regulation in different brain regions

Orexin is a neuropeptide that is primarily synthesized and secreted by the lateral hypothalamus (LH) and includes two substances derived from the same precursor (orexin A [OXA] and orexin B [OXB]). Studies have shown that orexin is not only involved in the regulation of eating, the sleep–wake cycle, and energy metabolism, but also closely associated with various physiological functions, such as cardiovascular control, reproduction, stress, reward, addiction, and the modulation of pain transmission. At present, studies that have been performed both domestically and abroad have confirmed that orexin and its receptors are closely associated with pain regulation. In this article, the research progress on acute pain regulation involving orexin is reviewed.

Role of orexin receptors within the dentate gyrus in antinociception induced by chemical stimulation of the lateral hypothalamus in an animal model of inflammatory pain

Pain is a complex experience consisting of sensory, affective-motivational, and cognitive dimensions. Hence, identifying the multiple neural pathways subserving these functional aspects is a valuable task. The role of dentate gyrus (DG) as a relay station of neocortical afferents in the hippocampal formation (HF) in persistent pain is still controversial. The lateral hypothalamus (LH)-HF neural circuits are involved in numerous situations such as anxiety-like behavior, reward processing, feeding, orofacial as well as acute pain. Nonetheless, to our knowledge, the involvement of the LH-DG neural circuit in persistent pain has already remained unexplored. Adult male Wistar rats weighing 220-250 g were undergone stereotaxic surgery for unilateral implantation of two separate cannulae into the LH and DG. Intra-DG administration of the orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists, SB334867 and TCS OX2 29, respectively, was performed 5 min before intra-LH microinjection of carbachol. Animals were then undergone the formalin test using 50 μl formalin injection (2.5%) into the plantar surface of the hind paw. Microinjection of SB334867 or TCS OX2 29 into the DG region attenuated the antinociceptive effect produced by carbachol microinjection into the LH. The preventive effect of SB334867 and TCS OX2 29 on intra-LH carbachol-induced antinociception was approximately equal in both early and late phases of formalin nociception. The results suggest a neural pathway from the LH to the DG, which contributes to the modulation of formalin-induced inflammatory pain through the recruitment of OX1 and OX2 receptors within the DG.

Chronic Pain: Structural and Functional Changes in Brain Structures and Associated Negative Affective States

Chronic pain is a condition in which pain progresses from an acute to chronic state and persists beyond the healing process. Chronic pain impairs function and decreases patients’ quality of life. In recent years, efforts have been made to deepen our understanding of chronic pain and to develop better treatments to alleviate chronic pain. In this review, we summarize the results of previous studies, focusing on the mechanisms underlying chronic pain development and the identification of neural areas related to chronic pain. We review the association between chronic pain and negative affective states. Further, we describe the structural and functional changes in brain structures that accompany the chronification of pain and discuss various neurotransmitter families involved. Our review aims to provide guidance for the development of future therapeutic approaches that could be used in the management of chronic pain.