Corticotropin releasing factor alters the functional diversity of accumbal cholinergic interneurons

Cholinergic interneurons (ChIs) provide the main source of acetylcholine in the striatum and have emerged as a critical modulator of behavioral flexibility, motivation, and associative learning. In the dorsal striatum, ChIs display heterogeneous firing patterns. Here, we investigated the spontaneous firing patterns of ChIs in the nucleus accumbens (NAc) shell, a region of the ventral striatum. We identified four distinct ChI firing signatures: regular single-spiking, irregular single-spiking, rhythmic bursting, and a mixed-mode pattern composed of bursting activity and regular single spiking. ChIs from females had lower firing rates compared to males and had both a higher proportion of mixed-mode firing patterns and a lower proportion of regular single-spiking neurons compared to males. We further observed that across the estrous cycle, the diestrus phase was characterized by higher proportions of irregular ChI firing patterns compared to other phases. Using pooled data from males and females, we examined how the stress-associated neuropeptide corticotropin releasing factor (CRF) impacts these firing patterns. ChI firing patterns showed differential sensitivity to CRF. This translated into differential ChI sensitivity to CRF across the estrous cycle. Furthermore, CRF shifted the proportion of ChI firing patterns toward more regular spiking activity over bursting patterns. Finally, we found that repeated stressor exposure altered ChI firing patterns and sensitivity to CRF in the NAc core, but not the NAc shell. These findings highlight the heterogeneous nature of ChI firing patterns, which may have implications for accumbal-dependent motivated behaviors.

Ranking the contribution of behavioral measures comprising oxycodone self-administration to reinstatement of drug-seeking in male and female rats

Introduction

Rates of relapse to drug use during abstinence are among the highest for opioid use disorder (OUD). In preclinical studies, reinstatement to drug-seeking has been extensively studied as a model of relapse-but the work has been primarily in males. We asked whether biological sex contributes to behaviors comprising self-administration of the prescription opioid oxycodone in rats, and we calculated the relative contribution of these behavioral measures to reinstatement in male and female rats.

Materials and methods

Rats were trained to self-administer oxycodone (8 days, training phase), after which we examined oxycodone self-administration behaviors for an additional 14 days under three conditions in male and female rats: short access (ShA, 1 h/d), long access (LgA, 6 h/d), and saline self-administration. All rats were then tested for cue-induced reinstatement of drug-seeking after a 14-d forced abstinence period. We quantified the # of infusions, front-loading of drug intake, non-reinforced lever pressing, inter-infusion intervals, escalation of intake, and reinstatement responding on the active lever.

Results

Both male and female rats in LgA and ShA conditions escalated oxycodone intake to a similar extent. However, males had higher levels of non-reinforced responding than females under LgA conditions, and females had greater levels of reinstatement responding than males. We then correlated each addiction-related measure listed above with reinstatement responding in males and females and ranked their respective relative contributions. Although the majority of behavioral measures associated with oxycodone self-administration did not show sex differences on their own, when analyzed together using partial least squares regression, their relative contributions to reinstatement were sex-dependent. Front-loading behavior was calculated to have the highest relative contribution to reinstatement in both sexes, with long and short inter-infusion intervals having the second greatest contribution in females and males, respectively.

Discussion

Our results demonstrate sex differences in some oxycodone self-administration measures. More importantly, we demonstrate that a sex- dependent constellation of self-administration behaviors can predict the magnitude of reinstatement, which holds great promise for relapse prevention in people.

Prenatal opioid exposure inhibits microglial sculpting of the dopamine system selectively in adolescent male offspring

The current opioid epidemic has dramatically increased the number of children who are prenatally exposed to opioids, including oxycodone. A number of social and cognitive abnormalities have been documented in these children as they reach young adulthood. However, little is known about the mechanisms underlying developmental effects of prenatal opioid exposure. Microglia, the resident immune cells of the brain, respond to acute opioid exposure in adulthood. Moreover, microglia are known to sculpt neural circuits during typical development. Indeed, we recently found that microglial phagocytosis of dopamine D1 receptors (D1R) in the nucleus accumbens (NAc) is required for the natural developmental decline in NAc-D1R that occurs between adolescence and adulthood in rats. This microglial pruning occurs only in males, and is required for the normal developmental trajectory of social play behavior. However, virtually nothing is known as to whether this developmental program is altered by prenatal exposure to opioids. Here, we show in rats that maternal oxycodone self-administration during pregnancy leads to reduced adolescent microglial phagocytosis of D1R and subsequently higher D1R density within the NAc in adult male, but not female, offspring. Finally, we show prenatal and adult behavioral deficits in opioid-exposed offspring, including impaired extinction of oxycodone-conditioned place preference in males. This work demonstrates for the first time that microglia play a key role in translating prenatal opioid exposure to changes in neural systems and behavior.

The neurobiology of abstinence-induced reward-seeking in males and females

Drugs of abuse and highly palatable foods (e.g. high fat or sweet foods) have powerful reinforcing effects, which can lead to compulsive and addictive drives to ingest these substances to the point of psychopathology and self-harm--specifically the development of Substance Use Disorder (SUD) and obesity. Both SUD and binge-like overeating can be defined as disorders in which the salience of the reward (food or drug) becomes exaggerated relative to, and at the expense of, other rewards that promote well-being. A major roadblock in the treatment of these disorders is high rates of relapse after periods of abstinence. It is common, although not universal, for cue-induced craving to increase over time with abstinence, often triggered by cues previously paired with the reinforcing substance. Accumulating evidence suggests that similar neural circuits and cellular mechanisms contribute to abstinence-induced and cue-triggered seeking of drugs and palatable food. Although much research has focused on the important role of corticolimbic circuitry in drug-seeking, our goal is to expand focus to the more recently explored hypothalamic-thalamic-striatal circuitry. Specifically, we review how connections, and neurotransmitters therein, among the lateral hypothalamus, paraventricular nucleus of the thalamus, and the nucleus accumbens contribute to abstinence-induced opioid- and (high fat or sweet) food-seeking. Given that biological sex and gonadal hormones have been implicated in addictive behavior across species, another layer to this review is to compare behaviors and neural circuit-based mechanisms of abstinence-induced opioid- or food-seeking between males and females when such data is available.

Effects of the estrous cycle and ovarian hormones on cue-triggered motivation and intrinsic excitability of medium spiny neurons in the Nucleus Accumbens core of female rats

Naturally occurring alterations in estradiol influence food intake in females. However, how motivational responses to food cues are affected by the estrous cycle or ovarian hormones is unknown. In addition, while individual susceptibility to obesity is accompanied by enhanced incentive motivational responses to food cues and increased NAc intrinsic excitability in males, studies in females are absent. Therefore, we examined basal differences in intrinsic NAc excitability of obesity-prone vs. obesity-resistant females and determined how conditioned approach (a measure of cue-triggered motivation), food intake, and motivation for food vary with the cycle in naturally cycling female obesity-prone, obesity-resistant, and outbred Sprague-Dawley rats. Finally, we used ovariectomy followed by hormone treatment to determine the role of ovarian hormones in cue-triggered motivation in selectively-bred and outbred female rats. We found that intrinsic excitability of NAc MSNs and conditioned approach are enhanced in female obesity-prone vs. obesity-resistant rats. These effects were driven by greater MSN excitability and conditioned approach behavior during metestrus/diestrus vs. proestrus/estrus in obesity-prone but not obesity-resistant rats, despite similar regulation of food intake and food motivation by the cycle in these groups. Furthermore, estradiol and progesterone treatment reduced conditioned approach behavior in obesity-prone and outbred Sprague-Dawley females. To our knowledge, these data are the first to demonstrate cycle- and hormone-dependent effects on the motivational response to a food cue, and the only studies to date to determine how individual susceptibility to obesity influences NAc excitability, cue-triggered food-seeking, and differences in the regulation of these neurobehavioral responses by the estrous cycle.

Enhanced anxiety-like behavior emerges with weight gain in male and female obesity-susceptible rats

Epidemiological data suggest that body mass index and obesity are strong risk factors for depression and anxiety. However, it is difficult to separate cause from effect, as predisposition to obesity may enhance susceptibility to anxiety, or vice versa. Here, we examined the effect of diet and obesity on anxiety-like behaviors in male and female selectively bred obesity-prone and obesity-resistant rats, and outbred Sprague-Dawley rats. We found that when obesity-prone and obesity-resistant rats do not differ in weight or fat mass, measures of anxiety-like behavior in the elevated plus maze and open field are similar between the two groups. However, once weight and fat mass diverge, group differences emerge, with greater anxiety in obesity-prone relative to obesity-resistant rats. This same pattern was observed for males and females. Interestingly, even when obesity-resistant rats were "forced" to gain fat mass comparable to obesity-prone rats (via prolonged access to 60% high-fat diet), anxiety-like behaviors did not differ from lean chow fed controls. In addition, a positive correlation between anxiety-like behaviors and adiposity were observed in male but not in female obesity-prone rats. Finally, diet-induced weight gain in and of itself was not sufficient to increase measures of anxiety in outbred male rats. Together, these data suggest that interactions between susceptibility to obesity and physiological alterations accompanying weight gain may contribute to the development of enhanced anxiety.

Functional and structural plasticity contributing to obesity: roles for sex, diet, and individual susceptibility

The role of cortico-striatal pathways in cue-triggered motivational processes have been extensively studied. However, recent work has begun to examine the potential contribution of plasticity in these circuits to obesity. Despite the inclusion of women in human obesity studies examining neurobehavioral alterations in cue-triggered motivation, preclinical studies have focused mainly on male subjects. This lack of female subjects in preclinical research had led to a gap in the basic understanding of the neural mechanisms underlying over-eating in females. In this review, we highlight recent work from our lab and others that has begun to elucidate how diet, obesity, and individual susceptibility to weight gain influence functional and structural plasticity within the nucleus accumbens and prefrontal cortex in adult rats. As is the case throughout neuroscience, studies of females or sex differences are largely lacking in this area. Thus, below we describe preliminary neurobehavioral results from female studies in our labs and point out areas for future investigation.