Role of orexin-2 and CB1 receptors within the periaqueductal gray matter in lateral hypothalamic-induced antinociception in rats.

Orexin plays an important role in pain modulation. Orexin-1 and orexin-2 receptors (Ox1r and Ox2r) are found at high density in the ventrolateral periaqueductal gray matter (vlPAG). Our previous study showed that chemical stimulation of the lateral hypothalamus with carbachol induces antinociception in the tail-flick test, a model of acute pain, and Ox1r-mediated antinociception in the vlPAG is modulated by the activity of vlPAG CB1 receptors. In the current study, TCS OX2 29, an Ox2r antagonist (5, 15, 50, 150, and 500 nmol/l), was microinjected into the vlPAG 5 min before the administration of carbachol (125 nmol/l). TCS OX2 29 dose dependently reduced carbachol-induced antinociception. In a second set of experiments, animals were treated with carbachol 5 min after intra-vlPAG administration of 15 nmol/l TCS OX2 29 and 1 nmol/l AM251 (a selective CB1 receptor antagonist), or 150 nmol/l TCS OX2 29 and 10 nmol/l AM251. The findings showed that the antinociceptive effect of orexin is partially mediated by activation of vlPAG Ox2 receptors. Furthermore, the administration of ineffective doses of Ox2 and CB1 receptor antagonists reduced the lateral hypothalamus-induced antinociception. It seems that Ox2 and CB1 receptors act through different pathways and Ox2r-mediated antinociception is not dependent on CB1 receptor activity.

Changes in the levels of p-ERK, p-CREB, and c-fos in rat mesocorticolimbic dopaminergic system after morphine-induced conditioned place preference: the role of acute and subchronic stress.

ERK pathway plays a critical role in the cellular adaptive responses to environmental changes. Stressful conditions can induce the activation of activate ERK, and its downstream targets, CREB and c-fos, in neural cells. Exposure to opioids has the same effect. In this study, we investigated the effects of morphine-induced conditioned place preference (CPP) on p-ERK/ERK ratio, p-CREB/CREB ratio and c-fos level in the mesocorticolimbic dopaminergic system including the nucleus accumbens (NAc), amygdala (AMY), striatum (Str), and prefrontal cortex (PFC).Our aim was to determine if acute and subchronic stress would affect these alterations. Male Wistar rats were divided into two saline- and morphine-treated groups. Each group contained of control, acute stress, and subchronic stress subgroups. The CPP procedure was performed for all of the rats. We dissected out the NAc, AMY, Str, and PFC regions and measured the mentioned ratios and c-fos level by Western blot analysis. The results revealed that in saline-treated animals, all factors enhanced significantly after performing acute and subchronic stress while there was an exception in p-ERK/ERK ratio in the Str and PFC; the changes were not significant during acute stress. Conditioning score decreased after applying the subchronic but not acute stress. In morphine-treated animals, all factors were increased after application of acute and subchronic stress, and conditioning scores also decreased after stress. Our findings suggest that in saline- or morphine-treated animals, acute and subchronic stress increases p-ERK, p-CREB, and c-fos levels in the mesocorticolimbic system. It has been shown that morphine induces the enhancement of the mentioned factors; on the other hand, our result demonstrates that stress can amplify these changes.

Cannabinoids induce apathetic and impulsive patterns of choice through CB1 receptors and TRPV1 channels.

Despite evidence from psychiatry and psychology clinics pointing to altered cognition and decision making following the consumption of cannabis, the effects of cannabis derivatives are still under dispute and the mechanisms of cannabinoid effects on cognition are not known. In this study, we used effort-based and delay-based decision tasks and showed that ACEA, a potent cannabinoid agonist induced apathetic and impulsive patterns of choice in rats in a dose-dependent manner when locally injected into the anterior cingulate cortex (ACC) and orbitofrontal cortex (OFC), respectively. Pre-treatment with AM251, a selective cannabinoid type 1 (CB1) receptor antagonist, reversed ACEA-induced impulsive and apathetic patterns of choice in doses higher than a minimally effective dose. Unlike CB1 receptor antagonist, pretreatment with capsazepine, a transient receptor potential vanilloid type 1 (TRPV1) channel antagonist, was effective only at an intermediary dose. Furthermore, capsazepine per se induced impulsivity and apathy at a high dose suggesting a basal tonic activation of TRPV1 channels that exist in the ACC and OFC to support cost-benefit decision making and to help avoid apathetic and impulsive patterns of decision making. Taken together, unlike previous reports supporting opposing roles for the CB1 receptors and TRPV1 channels in anxiety and panic behavior, our findings demonstrate a different sort of interaction between endocannabinoid and endovanilloid systems and suggest that both systems contribute to the cognitive disrupting effects of cannabinoids. Given prevalent occurrence of apathy and particularly impulsivity in psychiatric disorders, these results have significant implications for pharmacotherapy research targeting these receptors.

The Blockade of D1/D2-Like Dopamine Receptors within the Dentate Gyrus of Hippocampus Decreased the Reinstatement of Morphine-Extinguished Conditioned Place Preference in Rats.

INTRODUCTION: The hippocampus (HIP), the primary brain structure related to learning and memory, receives sparse but comprehensive dopamine innervations and contains dopamine D1/D2-like receptors. It is demonstrated that dopamine receptors in dentate gyrus (DG) region of HIP have a remarkable function in spatial reward processing. Much less is known about the involvement of HIP and its D1/D2 dopamine receptors in drug-seeking behaviors, more particularly, in the morphine extinguished conditioned place preference (CPP). METHODS: To find out the role of D1/D2-like receptors within the DG in morphine-seeking behaviors, forty adult male albino Wistar rats weighing 220-280g were unilaterally implanted by a cannula into the DG. The CPP paradigm was done; conditioning score and locomotors activity were recorded by Ethovision software. All drugs/vehicles were microinjected one day after extinction (just before the CPP test) into the DG as reinstatement day. RESULTS: The results showed that intra-DG administration of different dose of SCH23390 (0.25, 1 and 4µg/0.5µl saline), as a selective D1-like receptor antagonist and sulpiride (0.25, 1 and 4µg/0.5µl DMSO), as a selective D2-like receptor antagonist dose-dependently attenuated the morphine-extinguished CPP reinstated by priming injection of morphine (1 mg/kg;sc). DISCUSSION: It can be concluded that D1/D2-like receptors within this region have an important role in morphine-seeking behaviors in extinguished rats.

Involvement of D1/D2 dopamine receptors within the nucleus accumbens and ventral tegmental area in the development of sensitization to antinociceptive effect of morphine.

The nucleus accumbens (NAc) and the ventral tegmental area (VTA) are two major areas for the mesolimbic dopaminergic system which are strongly involved in the development of behavioral sensitization. In the present study, we investigated the role of D1/D2 dopaminergic receptors within the NAc or VTA in response to sensitization to morphine by the tail-flick test as a model of acute pain. Sensitization was induced by subcutaneous (SC) injection of morphine (5 mg/kg), once daily for three days followed by 5 days free of drug. After the sensitization period, antinociceptive responses induced by an ineffective dose of morphine (1 mg/kg; SC) were obtained by the tail-flick test, and represented as maximal possible effect (%MPE). In experimental groups, D1 and D2 receptor antagonists, SCH-23390 and sulpiride (0.25, 1 and 4 µg/rat), were separately microinjected into the NAc or VTA, 10 min before morphine administration during the sensitization period, respectively. Results showed that injection of morphine during the sensitization period (development of sensitization) increased %MPE of the ineffective dose of morphine from 2.43±1.4% in naive to 47.75±4.01% in sensitized animals (P<0.001). Unilateral microinjections of different doses of the D1/D2 receptor antagonists, SCH-23390 and sulpiride, into the NAc dose-dependently decreased %MPEs in morphine-sensitized animals. Nonetheless, %MPEs were only affected by intra-VTA administration of SCH-23390 in morphine-sensitized animals (P<0.05). Our findings suggest that both the D1/D2 dopamine receptors in the NAc and the D1 receptors in the VTA may be of more important in the development of sensitization to morphine in rats.

Interaction between the dopaminergic and opioidergic systems in dorsal hippocampus in modulation of formalin-induced orofacial pain in rats.

The hippocampus is a region of the brain that serves several functions. The dopaminergic system acts through D1- and D2-like receptors to interfere in pain modulation and the opioid receptors play major roles in analgesic processes and there are obvious overlaps between these two systems. The present study investigated the interaction between the opioidergic and dopaminergic systems in the dorsal hippocampus (CA1) region for formalin-induced orofacial pain. Two guide cannulae were stereotaxically implanted in the CA1 region and morphine (0.5, 1, 2 and 4 µg/0.5 µl saline) and naloxone (0.3, 1 and 3 µg/0.5 µl saline) were used as the opioid receptor agonist and antagonist, respectively. SKF-38393 (1 µg/0.5 µl saline) was used as a D1-like receptor agonist, quinpirole (2 µg/0.5 µl saline) as a D2-like receptor agonist, SCH-23390 (0.5 µg/0.5 µl saline) as a D1-like receptor antagonist and sulpiride (3 µg/0.5 µl DMSO) as a D2-like receptor antagonist. To induce orofacial pain, 50 µl of 1% formalin was subcutaneously injected into the left side of the upper lip. Our results showed that different doses of morphine significantly reduced orofacial pain in both phases induced by formalin. Naloxone (1 and 3 µg) reversed morphine induced analgesia in CA1. SKF-38393 and quinpirole with naloxone (1 µg) significantly decreased formalin-induced orofacial pain in both phases. SCH-23390 had no effect on the antinociceptive response of morphine in both phases of orofacial pain. Sulpiride reversed the antinociceptive effects of morphine only in the first phase, but this result was not significant. Our findings suggest that there is cross-talk between the opioidergic and dopaminergic systems. Opioidergic neurons also exerted antinociceptive effects by modulation of the dopaminergic system in the CA1 region of the brain.

Involvement of orexin-1 receptors in the ventral tegmental area and the nucleus accumbens in antinociception induced by lateral hypothalamus stimulation in rats.

Previous studies have demonstrated that chemical stimulation of the lateral hypothalamus (LH) with carbachol has an important role in the induction of antinociception in tail-flick test as a model of acute pain. In this study, we tried to evaluate the involvement of orexin-1 receptors in the ventral tegmental area (VTA) and the nucleus accumbens (NAc) on antinociceptive responses induced by LH stimulation in rats. One hundred twenty adult male albino Wistar rats weighing 200-250g were unilaterally implanted with two separate cannulae into the LH, and VTA or NAc. Antinociceptive effects for two doses of intra-LH carbachol (125 and 250nmol/0.5µl saline), as a cholinergic agonist, were evaluated in this study. In another set of experiments, animals received intra-VTA or -NAc infusions of SB334867 as a selective orexin-A receptor antagonist (0.3, 1, 3 and 10nmol/rat), just 5min before microinjection of an effective dose of carbachol into the LH. In the tail-flick test, antinociceptive responses of drugs were obtained by tail-flick analgesiometer and represented as maximal possible effects (%MPE) at 5, 15, 30, 45 and 60min after their administrations. The results showed that unilateral intra-LH administration of carbachol (125 and 250nmol/rat) induced antinociception in rats (P<0.01). There were no significant differences between the antinociceptive effects of these two doses. In the second part of our study, intra-VTA and intra-accumbal administrations of different doses of SB334867, 5min before microinjection of carbachol, could dose-dependently prevent the development of LH stimulation-induced antinociception in rats. However, this effect was less in the NAc. It is supposed that the orexinergic projections from the LH to the VTA and NAc are direct/indirectly involved in the antinociception induced by LH chemical stimulation, and orexin-1 receptors in the ventral tegmental area have a more substantial role in this phenomenon.

Involvement of orexin-2 receptors in the ventral tegmental area and nucleus accumbens in the antinociception induced by the lateral hypothalamus stimulation in rats.

Orexin, which is mainly produced by orexin-expressing neurons in the lateral hypothalamus (LH), plays an important role in pain modulation. Both kinds of orexin-1 (Ox1) and orexin-2 (Ox2) receptors have been found at high density in the ventral tegmental area (VTA) and nucleus accumbens (NAc). However, the quantity of Ox1 receptors in the VTA is more than that in the NAc. Additionally, it seems that the functional interaction between the LH, VTA and NAc implicates pain processing and modulation. In this study, we tried to examine the involvement of Ox2 receptors in the NAc and VTA using tail-flick test as an animal model of acute pain following microinjection of effective dose of carbachol (125nmol/0.5µl saline) into the LH. In this set of experiments, different doses of TCS OX2 29 as an Ox2 receptor antagonist were microinjected into the VTA (1, 7 and 20nmol/0.3µl DMSO) and the NAc (2, 10, 20 and 40nmol/0.5µl DMSO) 5min prior to carbachol administration. Administration of TCS OX2 29 into the VTA and NAc dose-dependently blocked intra-LH carbachol-induced antinociception. However, the inhibitory effect of TCS OX2 29 as an Ox2 receptor antagonist was more potent in the VTA than that in the NAc. It seems that VTA orexinergic receptors are more effective on LH stimulation-induced antinociception and the modulation of pain descending inhibitory system originated from the LH than those of the same receptors in the nucleus accumbens in rats.