Neural circuit engagement within the nucleus accumbens (NAc) shell is implicated in the regulation of both negative and positive affect. Classically, the dynorphin/kappa opioid receptor (KOR) system in the NAc was believed to promote dysphoric behavior, while dopamine was viewed as interacting with reward behavior, and KOR activation was known to inhibit dopamine release. Recently, however, both the KOR and dopamine systems have, separately, been shown to have differential effects across the rostro-caudal axis of the NAc shell on hedonic responses. Whether or not this is due to interactions between KORs and dopamine, and if it extends to other affective behaviors, remains to be determined. In this study, we examined in rats the relationship between the KOR and dopamine systems in both the rostral and caudal NAc shell using ex vivo fast scan cyclic voltammetry and the impact of KOR activation on affective behavior using approach-avoidance assays. We report here that activation of KORs in the caudal NAc shell significantly inhibits dopamine release, stimulates novelty-induced rearing behavior, increases avoidance behavior, and reduces locomotor activity. In contrast, activation of KORs in the rostral NAc shell inhibits dopamine release to a lesser extent and instead increases approach behavior. Taken together, these results indicate that there is heterogeneity across the rostro-caudal axis of the NAc shell in the effects of KOR stimulation on affective behaviors, and they suggest that this might be due to differences in KOR control over dopamine release.

No abstract

Underage alcohol drinking increases the risk of developing alcohol use disorder (AUD). In rodents, adolescent ethanol exposure augments ethanol consumption and anxiety-like behavior while reducing social interaction. However, the underlying mechanisms driving these adaptations are unclear. The dopamine and kappa opioid receptor (KOR) systems in the nucleus accumbens (NAc) are implicated in affective disorders, including AUD, with studies showing augmented KOR function and reduced dopamine transmission in ethanol-dependent adult animals. Thus, here we examine the impact of adolescent intermittent ethanol (AIE) exposure on dopamine transmission and KOR function in the NAc. Rats were exposed to water or ethanol (4 g/kg, intragastrically) every other day during early (postnatal day (PD) 25–45) or late (PD 45–65) adolescence. While AIE exposure during early adolescence (early-AIE) did not alter dopamine release in male and female rats, AIE exposure during late adolescence (late-AIE) resulted in greater dopamine release in males and lower dopamine release in females. To determine the impact of AIE on KOR function, we measured the effect of KOR activation using U50,488 (0.01–1.00 µM) on dopamine release. Early-AIE exposure potentiated KOR-mediated inhibition of dopamine release in females, while late-AIE exposure attenuated this effect in males. Interestingly, no differences in KOR function were observed in early-AIE exposed males and late-AIE exposed females. Together, these data suggest that AIE exposure impact on neural processes is dependent on sex and exposure timing. These differences likely arise from differential developmental timing in males and females. This is the first study to show changes in KOR function following AIE exposure.

Lifelong social impairments are common in individuals with prenatal alcohol exposure (PAE), and preclinical studies have identified gestational day (G)12 as a vulnerable timepoint for producing social deficits following binge‐level PAE. While moderate (m)PAE also produces social impairments, the long‐term neuroadaptations underlying them are poorly understood. Activity of the projection from the basolateral amygdala to the prelimbic cortex (BLA → PL) leads to social avoidance, and the PL is implicated in negative social behaviours, making each of these potential candidates for the neuroadaptations underlying mPAE‐induced social impairments. To examine this, we first established that G12 mPAE produced sex‐specific social impairments lasting into adulthood in Sprague–Dawley rats. We then chemogenetically inhibited the BLA → PL using clozapine N‐oxide (CNO) during adult social testing. This revealed that CNO reduced social investigation in control males but had no effect on mPAE males or females of either exposure, indicating that mPAE attenuated the role of this projection in regulating male social behaviour and highlighting one potential mechanism by which mPAE affects male social behaviour more severely. Using whole‐cell electrophysiology, we also examined mPAE‐induced changes to PL pyramidal cell physiology and determined that mPAE reduced cell excitability, likely due to increased suppression by inhibitory interneurons. Overall, this work identified two mPAE‐induced neuroadaptations that last into adulthood and that may underlie the sex‐specific vulnerability to mPAE‐induced social impairments. Future research is necessary to expand upon how these circuits modulate both normal and pathological social behaviours and to identify sex‐specific mechanisms, leading to differential vulnerability in males and females.

Underage alcohol drinking increases the risk of developing alcohol use disorder (AUD). In rodents, adolescent ethanol exposure augments ethanol consumption and anxiety-like behavior while reducing social interaction. However, the underlying mechanisms driving these adaptations are not understood. The dopamine and kappa opioid receptor (KOR) systems in the nucleus accumbens (NAc) are implicated in affective disorders and AUD, with studies showing augmented KOR function and reduced dopamine transmission in ethanol-dependent adult animals. Thus, this study, we examined the impact of adolescent intermittent ethanol (AIE) exposure on dopamine transmission and KOR function. Rats were exposed to water or ethanol (4 g/kg, intragastrically) every-other-day during early (PD25-45) or late (PD45-65) adolescence. While AIE exposure during early-mid adolescence (early AIE) did not alter dopamine release in male and female rats, AIE exposure during late adolescence (late AIE) resulted in greater dopamine release in males and lower dopamine release in females. To determine the impact of AIE exposure on KOR function, we bath applied cumulative concentrations of KOR agonist, U50,488 (0.01-1.0 µM), and measured its effect on dopamine release. Early AIE exposure potentiated KOR-mediated inhibition of dopamine release in female rats, while late AIE exposure attenuated this effect in male rats. Together these data suggest that AIE-exposure impact on neural processes is dependent on sex and exposure timing. These differences likely arise from differential developmental timing in males and females. This is the first study to show changes in KOR function following AIE exposure.