Restraint Stress Potentiated Morphine Sensitization: Involvement of Dopamine Receptors within the Nucleus Accumbens

Restraint stress potentiates sensitivity to the antinociceptive effect of morphine through orexin receptors in the ventral tegmental area

Orexin signaling in the ventral tegmental area (VTA) plays a critical role in stress and addictive behaviors. On the other hand, exposure to stress potentiates behavioral sensitization to drugs of abuse such as morphine. This study aimed to elucidate the role of orexin receptors within the VTA in restraint stress (RS)-induced morphine sensitization. Adult male albino Wistar rats underwent stereotaxic surgery, and two stainless steel guide cannulae were bilaterally implanted into the VTA. Different doses of SB334867 or TCS OX2 29 as orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists were microinjected into the VTA five min before exposure to RS, respectively. A duration of three hours was considered for applying the RS, and 10 min after RS exposure, animals received a subcutaneous injection of an ineffective dose of morphine (1 mg/kg) for three consecutive days followed by a five-day drug/stress-free period. On the ninth day, the tail-flick test evaluated the sensitivity to the antinociceptive effects of morphine. The results demonstrated that the sole application of RS or morphine (1 mg/kg) could not induce morphine sensitization; however, concurrent application of RS and morphine could induce morphine sensitization. Besides, intra-VTA administration of OX1 R or OX2 R antagonists before paired administration of morphine and RS blocked morphine sensitization. The role of OX1 R and OX2 R in the induction of stress-induced morphine sensitization was almost identical. This study provides new insight into the role of orexin signaling in the VTA in the potentiation of morphine sensitization induced by RS and morphine co-administration.

Effects of treadmill exercise and chronic stress on anxiety-like behavior, neuronal activity, and oxidative stress in basolateral amygdala in morphine-treated rats

The basolateral amygdala (BLA), which is sensitive to stress, is necessary for reward-seeking behavior and addiction. Regular exercise can produce various positive effects by affecting the BLA. Therefore, we aimed to investigate the effects of chronic stress and treadmill running (TR) on anxiety-like behavior, neuronal activity, lipid peroxidation (measured by malondialdehyde (MDA) levels, a marker for oxidative stress), and total thiol in BLA, in morphine-treated rats. Male Wistar rats were restricted in restraint stress and/or ran on the treadmill and treated with morphine (5 mg/kg) for 21 days. Anxiety-like behavior was evaluated using an elevated plus maze (EPM) and open field tests (OFTs), on day 22. On day 23, neuronal activity in BLA was assessed via single-unit recording. Finally, MDA and total thiol were assessed in BLA. Our results showed that chronic administration of morphine (5 mg/kg) did not affect anxiety-like behavior. However, the morphine-treated rats, subjected to chronic stress and exercise, showed fewer anxiety-like behaviors. Morphine increased BLA's MDA levels but it was prevented by TR. Glutamatergic and GABAergic basal neuronal activities were low in morphine-treated rats but after acute morphine application, there was a significant decrease in GABAergic neuronal activities in the morphine-exercise-stress (Mor-Exe-St) group. The results of this study showed that in morphine-treated rats, stress and exercise or their combination could have either co-directional or opposite effects to the chronic effects of morphine. These results indicate the existence of common pathways similar to endogenous opioids.

Role of orexinergic receptors within the ventral tegmental area in the development of morphine sensitization induced by forced swim stress in the rat

The ventral tegmental area (VTA) has been suggested as part of a common system for reward, stress, and morphine sensitization. Repeated exposure to stress enhances sensitivity to drugs such as morphine. The role of orexin receptor type 1 (OX1R) and type 2 (OX2R) within the VTA in cross-sensitization of morphine with stress was assessed in this study. Various doses of OX1R antagonist (SB334867) and OX2R antagonist (TCS OX2 29) were microinjected into the VTA of 134 adult male albino Wistar rats through cannulae, which had been bilaterally implanted above this region. Five min after microinjection, animals were forced to swim for 6 min, and 10 min after forced swim stress (FSS) termination, a low dose of morphine (i.e., ineffective dose for sensitization) was subcutaneously injected (1 mg/kg; sc). This procedure was repeated for three consecutive days as a sensitization period followed by a 5-day drug/stress-free period. On the 9th day, sensitivity to morphine was examined by measuring antinociceptive responses to the ineffective dose of morphine via tail-flick test. The obtained findings revealed that while concurrent administration of FSS and an ineffective dose of morphine (1 mg/kg; sc) for three consecutive days induced sensitivity to morphine, intra-VTA administration of OX1R- and OX2R antagonists, dose-dependently blocked this sensitization. These results suggested that both orexin receptors located in the VTA have a considerable role in morphine sensitization induced by concurrent administration of FSS and a low dose of morphine. So, there is a contribution of the orexin system partly to stress-induced sensitization to morphine.