Role of orexin-2 receptors in the nucleus accumbens in antinociception induced by carbachol stimulation of the lateral hypothalamus in formalin test.

Orexins, which are mainly produced by orexin-expressing neurons in the lateral hypothalamus (LH), play an important role in pain modulation. Previously, it has been established that the nucleus accumbens (NAc) is involved in the modulation of formalin-induced nociceptive responses, a model of tonic pain. In this study, the role of intra-accumbal orexin-2 receptors (OX2rs) in the mediation of formalin-induced pain was investigated. A volume of 0.5 µl of 10, 20, and 40 nmol/l solutions of TCS OX2 29, an OX2r antagonist, were unilaterally microinjected into the NAc 5 min before an intra-LH carbachol microinjection (0.5 µl of 250 nmol/l solution). After 5 min, animals received a subcutaneous injection of formalin 2.5% (50 µl) into the hind paw. Pain-related behaviors were assessed at 5 min intervals during a 60-min test period. The findings showed that TCS OX2 29 administration dose dependently blocked carbachol-induced antinociception during both phases of formalin-induced pain. The antianalgesic effect of TCS OX2 29 was greater during the late phase compared with the early phase. These observations suggest that the NAc, as a part of a descending pain-modulatory circuitry, partially mediates LH-induced analgesia in the formalin test through recruitment of OX2rs. This makes the orexinergic system a good potential therapeutic target in the control of persistent inflammatory pain.

Binding of Cimetidine to Balb/C Mouse Liver Catalase; Kinetics and Conformational Studies.

Catalase is responsible for converting hydrogen peroxide (H2O2) into water and oxygen in cells. This enzyme has high affinity for hydrogen peroxide and can protect the cells from oxidative stress damage. Catalase is a tetramer protein and each monomer contains a heme group. Cimetidine is a histamine H2 receptor blocker which inhibits acid release from stomach and is used for gasterointestinal diseases. In this research, effect of cimetidine on the activity of liver catalase was studied and the kinetic parameters of this enzyme and its conformational changes were investigated. Cell free extract of mouse liver was used for the catalase assay. The activity of the catalase was detected in the absence and presence of cimetidine by monitoring hydrogen peroxide reduction absorbance at 240 nm. The purified enzyme was used for conformational studies by Fluorescence spectrophotometry. The data showed that cimetidine could inhibit the enzyme in a non-competitive manner. Ki and IC50 values of the drug were determined to be about 0.75 and 0.85 uM, respectively. The Arrhenius plot showed that activation energy was 6.68 and 4.77 kJ/mol in the presence and absence of the drug, respectively. Fluorescence spectrophotometry revealed that the binding of cimetidine to the purified enzyme induced hyperchromicity and red shift which determined the conformational change on the enzyme. Cimetidine could non-competitively inhibit the liver catalase with high affinity. Binding of cimetidine to the enzyme induced conformational alteration in the enzyme.

Intra-accumbal Orexin-1 Receptors are Involved in Antinociception Induced by Stimulation of the Lateral Hypothalamus in the Formalin Test as an Animal Model of Persistent Inflammatory Pain.

Orexin, mainly produced by orexin-expressing neurons in the lateral hypothalamus (LH), plays an important role in pain modulation. Moreover, it is shown that the nucleus accumbens (NAc) is one of the important areas involved in this modulation. Orexin-1 (OX1) and orexin-2 (OX2) receptors are densely distributed in the NAc. The study investigated the involvement of OX1 receptors in the NAc on antinociception induced by intra-LH administration of carbachol in formalin test. Rats were unilaterally implanted by two separate cannulae into the LH and NAc. Different doses of SB334867, as an OX1 receptor antagonist, were microinjected into the NAc (1, 3 and 10 nM/0.5 µL DMSO) prior to intra-LH carbachol injection (250 nM/0.5 µL saline). Formalin test was applied as an animal model of persistent inflammatory pain. The animals received a subcutaneous injection of formalin into the hind paw, 5 min after SB334867 administration. Pain scores were calculated at 5-min blocks for a 60-min test period. Results showed that the administration of SB334867 into the NAc decreased LH chemical stimulation-induced antinociception dose-dependently in early and second phase of formalin test. Our findings showed that OX1 receptors in the NAc may be involved in modulation of inflammatory pain.

Functional interaction between OX2 and CB1 receptors in the ventral tegmental area and the nucleus accumbens in response to place preference induced by chemical stimulation of the lateral hypothalamus.

Orexinergic projections derived from the lateral hypothalamus (LH) to the ventral tegmental area (VTA) and the nucleus accumbens (NAc), play a key role in the acquisition of conditioned place preference (CPP) induced by LH stimulation. On the other hand, there are several studies which support the idea of the existence of a cross-talk between the orexinergic and cannabinoid systems. Nevertheless, the function and how both systems interact in the reward circuit remain unknown. In this study, the authors tried to clarify the role of orexin-2 receptor (OX2r) within the VTA and NAc in the development of reward-related behaviors after chemical stimulation of the LH and also find out the involvement of CB1 cannabinoid receptors in this phenomenon. Animals were implanted by two separate cannulae into the LH and VTA or NAc, unilaterally. The CPP paradigm was done; and conditioning scores were recorded. The results showed that administration of TCS OX2 29 as a selective OX2r antagonist (1, 3 and 10 nM/rat) into the VTA or NAc just 5 min before microinjection of carbachol (250 nM/0.5 µl saline), a cholinergic agonist, into the LH during the 3-day conditioning phase, could dose-dependently inhibit the development of LH stimulation-induced CPP. Furthermore, concurrent injection of ineffective doses of TCS OX2 29 and AM251, as a CB1 receptor antagonist, into the NAc could reduce conditioning scores. The findings of this study showed that the OX2 receptor has a critical role in modulating reward circuit in the VTA and NAc, when the LH was stimulated by carbachol. Moreover, we suggest the existence of an interaction between orexinergic and cannabinoid systems within the VTA and NAc in place preference induced by LH stimulation.