Nicotine reward is a key driver of tobacco dependence; however, the mechanisms underlying reward induced by cigarette smoke, particularly under passive exposure conditions, remain incompletely understood. In the present study, we established an adjustable cigarette smoke exposure system to generate controlled smoke concentrations (0.5, 1.0, and 2.0 g/m3) and investigated the behavioral and molecular consequences of repeated smoke exposure in mice. Animals were exposed to cigarette smoke or air for 2 h daily for 14 or 28 days. Serum nicotine levels were quantified by UPLC-MS/MS, reward-related behavior was evaluated using a conditioned place preference (CPP) paradigm, and the expression of reward-associated signaling proteins in the nucleus accumbens (NAc) was assessed by Western blotting. Cigarette smoke exposure produced a dose-dependent increase in serum nicotine concentrations. No significant CPP was observed after 14 days of exposure; however, after 28 days, mice exhibited a significant and dose-dependent increase in preference for the smoke-paired compartment, accompanied by elevated preference index values. At the molecular level, repeated smoke exposure induced dose-dependent increases in p-ERK, p-CaMKII, and c-Fos expression in the NAc. Furthermore, administration of the nicotinic acetylcholine receptor antagonist mecamylamine significantly attenuated both CPP behavior and the activation of these signaling pathways. These findings demonstrate that repeated cigarette smoke exposure induces nicotine-dependent reward behavior through activation of NAc-associated signaling pathways. This model provides a reproducible and physiologically relevant platform for investigating the neurobiological mechanisms of smoke-induced nicotine reward under passive exposure conditions and may facilitate the development of therapeutic strategies for tobacco dependence.
The development of hyperalgesia (i.e., enhanced physical pain and/or psychological distress) during chronic fentanyl use plays a vital role in the continued and accelerated use of this potent synthetic opioid. Numerous biochemical changes in the brain, such as changes in redox status, are thought to contribute to the development of hyperalgesia to fentanyl and other opioids. This study addresses whether the clinically used antioxidant N-acetyl-L-cysteine (L-NAC) and its readily cell-penetrant analogue, L-NAC methyl ester (L-NACme) would affect the progressive loss of antinociception and appearance of hyperalgesia in adult male Sprague Dawley rats. We monitored changes in tail-flick latencies following twice daily co-injections over 5 days (8 AM, 8 PM) of fentanyl (125 μg/kg, IV) + vehicle or L-NAC (500 μmol/kg, IV) or L-NACme (500 μmol/kg, IV) in the rats. The first co-injections of fentanyl + vehicle elicited a long-lasting antinociception, whereas a robust hyperalgesia appeared with co-injections 5 and was even more dramatic following co-injections 6–10. The co-injections of fentanyl + L-NAC partially prevented the loss of antinociception and occurrence of hyperalgesia, and co-injections of fentanyl and L-NACme markedly reduced the loss of antinociception and prevented the appearance of hyperalgesia. This study provides evidence that L-NAC and L-NACme beneficially affect the alterations in pain processing that occurs with sub-chronic administration of fentanyl. The higher efficacy of L-NACme is likely to be due to its greater cell penetrability in brain regions and interaction with intracellular signaling cascades, which may include redox-dependent processes, that mediate fentanyl-induced antinociception and hyperalgesia.
Animals experience stressors in a variety of contexts that result in activation of neuroendocrine and cellular stress responses. Release of stress hormones can disrupt or restore redox homeostasis, and the resulting changes in oxidative states, physiology and behavior vary by an individual's stress coping style. However, oxidative stress can also directly modulate neuroendocrine stress signaling. To what extent antioxidant levels alter behavioral stress levels between individuals with different stress coping styles is not well understood. The present study investigated how N-acetylcysteine amide (NACA), an antioxidant, regulates stress behavior in zebrafish (Danio rerio) with different stress coping styles (proactive, reactive). Following 24-h exposure to NACA or control conditions, we quantified individual and composite stress behaviors using a Light–Dark Test. We observed significant main effects of stress coping style and NACA-treatment on individual and composite stress behaviors where reactive fish had lower stress behaviors than proactive fish and NACA treatment reduced stress behaviors relative to controls. We observed significant stress coping style by NACA treatment interaction effects on individual and composite stress behaviors. While there was no significant reduction in stress behavior between NACA-treated and control proactive fish, NACA-treated reactive fish showed lower stress behavior than controls. Overall, our data suggest that antioxidant mechanisms (e.g., glutathione system) may be key in facilitating the distinct behavioral and physiological responses to stressors that characterize alternative stress coping styles. The results underscore how stress coping style and redox state can influence behavioral responses to stress.
Investigating the neural mechanisms underlying social reward is important not only to further current understanding of adaptive social behaviors, but also for informing potential therapeutic strategies for disorders associated with impaired social functioning. A growing body of literature indicates that the neuropeptide oxytocin (OXT) plays a modulatory role in dynamic social behaviors and may influence reward processing to initiate and maintain them. The present study aimed to assess the reinforcing effects of social interaction in rats using a social conditioned place preference paradigm, in which male rats displayed social preference behavior. To investigate OXT's possible contribution in mediating this observed effect, a short-hairpin RNA (shRNA) was employed to selectively knockdown OXT receptors (OXTRs) within three reward-related brain structures: the ventral tegmental area (VTA), nucleus accumbens (NAc), or lateral septum (LS). Rats with region-specific OXTR knockdown failed to establish socially-conditioned place preference, suggesting OXTRs within the VTA, NAc, and LS are independently needed to drive the reinforcing effect of social interaction.
Excessive alcohol (ETOH) consumption is associated with alterations in the opioid system, including β-endorphin deficiency and differential expression of the mu opioid receptor (MOR) in brain areas involved in reward. This system also contributes to the rewarding properties of psychostimulants, such as cocaine and amphetamines; however, its role in this regard has been little studied, particularly with respect to sex differences. The aim of this study was to determine whether male and female Wistar rats with high ETOH consumption also exhibit higher intake of the psychostimulant phentermine (PHEN), and to analyze its relationship with MOR expression in the nucleus accumbens (Acb), ventral pallidum (VP), and ventral tegmental area (VTA). Rats of both sexes were classified as high or low consumers after a period of voluntary ingestion of 10% ethanol. Subsequently, self-administration of PHEN solution (0.1 mg/ml) was recorded. The results showed that females showed higher ETOH consumption, whereas males had higher PHEN consumption. High ETOH consumers did not show higher stimulant consumption. When MOR expression was examined, males showed higher density in the Acb and VP, while females showed higher density in the VTA. High ETOH consumers showed higher MOR expression in Acb than low consumers, regardless of sex. Taken together, these findings suggest that sex differences, such as MOR distribution, modulate the consumption of different drugs, which appear to be intrinsic differences that are not linked to a specific psychoactive substance use. Additionally, it was observed that high ETOH consumption does not necessarily indicate greater vulnerability to psychostimulant use.
