Increased negative affect when combining early-life maternal deprivation with adolescent, but not adult, cocaine exposure in male rats: regulation of hippocampal FADD

A gut (microbiome) feeling about addiction: Interactions with stress and social systems

In recent years, an increasing attention has given to the intricate and diverse connection of microorganisms residing in our gut and their impact on brain health and central nervous system disease. There has been a shift in mindset to understand that drug addiction is not merely a condition that affects the brain, it is now being recognized as a disorder that also involves external factors such as the intestinal microbiota, which could influence vulnerability and the development of addictive behaviors. Furthermore, stress and social interactions, which are closely linked to the intestinal microbiota, are powerful modulators of addiction. This review delves into the mechanisms through which the microbiota-stress-immune axis may shape drug addiction and social behaviors. This work integrates preclinical and clinical evidence that demonstrate the bidirectional communication between stress, social behaviors, substance use disorders and the gut microbiota, suggesting that gut microbes might modulate social stress having a significance in drug addiction.

Electroconvulsive seizures regulate various stages of hippocampal cell genesis and mBDNF at different times after treatment in adolescent and adult rats of both sexes

Electroconvulsive therapy, a fast-acting option for treatment-resistant depression, is modeled at the preclinical level through the induction of electroconvulsive seizures (ECS) in rodents. Recent studies from our group proved sex- and age-differences in the antidepressant-like response elicited by ECS in rats; while an antidepressant-like response was observed in male adolescent and adult rats (although with greater efficacy in adulthood), the same parameters rendered inefficacious in females of any age. To better understand the potential sex differences taking place at the molecular level that might be mediating these behavioral disparities, we evaluated the impact of a repeated treatment with ECS (95 mA for 0.6 s, 100 Hz, 0.6 ms) in adolescent and adult rats of both sexes. Several hippocampal markers of neuroplasticity, commonly regulated by most antidepressants, such as those of neurogenesis (cell proliferation, neurogenic differentiation, long-term cell survival) or mBDNF and associated signaling (e.g., mTOR and ERK1/2) were evaluated at different time-points after treatment (1-, 8-, 15- and up to 30-days post-treatment). The main results demonstrated that ECS improved the survival rate of new cells born in the dentate gryus before treatment. Moreover, ECS increased cell proliferation and neurogenic differentiation at different times post-treatment, paired with persistent increases in mBDNF, observed long after treatment. In general, effects were different for each sex and varied with the age of the animal (adolescent vs. adulthood). The present study is the first-one to demonstrate that such persistent molecular changes induced by ECS in hippocampus, some of them observed up to 30-days post-treatment, also occurred in female rats and adolescence. Although these molecular changes could not justify the lack of ECS efficacy described by these same parameters of ECS in female rats (vs. male rats), they proposed certain beneficial effects common to both sexes, and age periods studied, opening the avenue for further studies. Based on these neurochemical effects, ECS should have displayed similar efficacies for both biological sexes. Therefore, the reason behind these disparities should be further explored to better translate efficacious treatments specific and/or personalized for each sex to the clinic.

Aromatase inhibition and ketamine in rats: sex-differences in antidepressant-like efficacy

BACKGROUND

Ketamine has been recently approved to treat resistant depression; however preclinical studies showed sex differences in its efficacy. Sex steroids, such as estrogens and testosterone, both in the periphery and locally in the brain, are regarded as important modulators of these sex differences. Therefore, the present study evaluated how inhibiting the biosynthesis of estrogens with letrozole (an aromatase inhibitor) could affect the observed sex differences in ketamine's antidepressant-like-response.

METHODS

We performed several consecutive studies in adult Sprague-Dawley rats to evaluate potential sex differences in the antidepressant-like effects of ketamine (5 mg/kg, 7 days, i.p.), letrozole (1 mg/kg, 8 days, i.p.) and their combination (letrozole pre-treatment 3 h before ketamine). Acute and repeated antidepressant-like responses were ascertained in a series of behavioral tests (forced-swim, novelty-suppressed feeding, two-bottle choice for sucrose preference).

RESULTS

The main results proved clear sex differences in the antidepressant-like response induced by ketamine, which was observed following a repeated paradigm in adult male rats, but rendered inefficacious in female rats. Moreover, decreasing estrogens production with letrozole induced on itself an antidepressant-like response in female rats, while also increased ketamine's response in male rats (i.e., quicker response observed after only a single dose). Interestingly, both the antidepressant-like effects induced by ketamine in male rats or letrozole in female rats persisted over time up to 65 days post-treatment, suggesting long-term sex-directed benefits for these drugs.

CONCLUSIONS

The present results demonstrated a sex-specific role for aromatase inhibition with letrozole in the antidepressant-like response induced by ketamine in male rats. Moreover, letrozole itself presented as a potential antidepressant for females with persistent effects over time. Clearly, the production of estrogens is key in modulating, in a sex-specific manner, affective-like responses and thus deserve further studies.

Sex differences in the antidepressant-like response and molecular events induced by the imidazoline-2 receptor agonist CR4056 in rats

In searching for novel targets to design antidepressants, among the characterized imidazoline receptors (IR), I2 receptors are an innovative therapeutical approach since they are dysregulated in major depressive disorder and by classical antidepressant treatments. In fact, several I2 agonists have been characterized for their antidepressant-like potential, but the results in terms of efficacy were mixed and exclusively reported in male rodents. Since there are well-known sex differences in antidepressant-like efficacy, this study characterized the potential effects induced by two I2 drugs, CR4056 (i.e., most promising drug already in phase II clinical trial for its analgesic properties) and B06 (a compound from a new family of bicyclic α-iminophosphonates) under the stress of the forced-swim test in male and female rats exposed to early-life stress. Moreover, some hippocampal neuroplasticity markers related to the potential effects observed were also evaluated (i.e., FADD, p-ERK/ERK, mBDNF, cell proliferation: Ki-67 + cells). The main results replicated the only prior study reporting the efficacy of CR4056 in male rats, while providing new data on its efficacy in females, which was clearly dependent on prior early-life stress exposure. Moreover, B06 showed no antidepressant-like effects in male or female rats. Finally, CR4056 increased FADD content and decreased cell proliferation in hippocampus, without affecting p-ERK/t-ERK ratio and/or mBDNF content. Interestingly, these effects were exclusively observed in female rats, and independently of early-life conditions, suggesting some distinctive molecular underpinnings participating in the therapeutic response of CR4056 for both sexes. In conjunction, these results present CR4056 with an antidepressant-like potential, especially in female rats exposed to stress early in life, together with some neuronal correlates described in the context of these behavioral changes in females.

Exploring pharmacological options for adolescent depression: a preclinical evaluation with a sex perspective

There is an urgent need for developing novel pharmacological treatment options for adolescent depression, and to ensure an optimal translational outcome to the clinic, sex should be included as a biological variable in preclinical studies. In this context, the present study compared the antidepressant-like potential of ketamine and cannabidiol, with the clinical standard fluoxetine, in adolescent rats exposed to maternal deprivation (as a model of early-life stress), while including a sex perspective. Moreover, changes in drug efficacy over time were evaluated by re-exposing rats to the same dose regimens during adulthood. Antidepressant-like responses were scored through a battery of distinctive tests (forced-swim, novelty-suppressed feeding, and sucrose preference) across time. The main results proved an antidepressant-like potential for ketamine and cannabidiol in adolescent rats, although their efficacy was dependent on sex and prior stress exposure, as well as on treatment length and the behavioral feature analyzed. In general, while all tested antidepressants in male rats improved certain affective-like features, female rats were mainly unresponsive to the treatments performed (except for certain benefits induced by ketamine), demonstrating the need for further characterizing proper treatments for this particular sex. Moreover, when rats were re-exposed in adulthood to the same drug regimens as in adolescence, a drop in efficacy was observed. These findings may have translational ramifications in that ketamine or cannabidiol could be moved forward as antidepressants for the adolescent depressed population, but not before further characterizing their potential long-term safety and/or beneficial vs. harmful effects for both sexes.

Prepulse inhibition can predict the motivational effects of cocaine in female mice exposed to maternal separation

The prepulse inhibition (PPI) of the startle response can identify the rodents that are more sensitive to the effects of cocaine. Mice with a lower PPI presented a higher vulnerability to the effects of cocaine and a higher susceptibility to developing a substance use disorder (SUD). Maternal separation with early weaning (MSEW) is a relevant animal model to induce motivational alterations throughout life. Nevertheless, only a few studies on females exist, even though they are more vulnerable to stress- and cocaine-related problems. Hence, the aim of the present study was to evaluate the ability of PPI to identify females with a greater vulnerability to the long-term consequences of early stress on the motivational effects of cocaine. Female mice underwent MSEW and were classified according to their high or low PPI. They were then assessed in the cocaine-induced locomotor sensitization test, the conditioned place preference paradigm or the operant self-administration paradigm. Additionally, they were also evaluated in the passive avoidance task, the tail-suspension and the splash tests. The results revealed that the females with lower PPI presented higher consequences of MSEW on the effects of cocaine and showed an increase in anhedonia-like behaviours. Our findings support that a PPI deficit could represent a biomarker of vulnerability to the effects of cocaine induced by MSEW.

Electroconvulsive seizures protect against methamphetamine-induced inhibition of neurogenesis in the rat hippocampus

Following methamphetamine consumption and during abstinence many behavioral consequences emerge (i.e., cognitive deficits, ongoing episodes of psychosis, depression, severe cravings, brain neurotoxicity), which are likely linked to propensity to relapse. In this line of thought, we recently showed that binge methamphetamine administration enhanced negative affect and voluntary drug consumption in rats, while it induced persistent neurotoxic effects (i.e., impaired hippocampal neurogenesis), effects that emerged long after drug removal. To date, no pharmacological strategies have been proven to be effective for the treatment of methamphetamine toxicity. A few studies have evaluated the impact of combining methamphetamine pretreatment with electroconvulsive seizures (ECS) post-treatment, an alternative non-pharmacological option used in psychiatry for resistant depression that offers a safe and really potent therapeutic response. Against this background, the present study aimed at testing whether repeated ECS treatment could ameliorate some of the long-term neurotoxicity effects induced by adolescent methamphetamine exposure in rats and emerging after drug removal. At the behavioral level, the main results showed that methamphetamine administration did not alter negative affect immediate during adolescence or later on in adulthood. Interestingly, repeated ECS improved the negative impact of methamphetamine administration on reducing hippocampal neurogenesis, demonstrating that ECS can attenuate certain degree of methamphetamine-induced neurotoxicity in rats, and suggesting ECS as a good therapeutical candidate that deserves further studies.

Antidepressant-like effects of cannabidiol in a rat model of early-life stress with or without adolescent cocaine exposure

BACKGROUND

Further studies are needed to better understand the effects of potential novel antidepressants, such as cannabidiol, for the treatment of psychiatric disorders during adolescence. In this context, we evaluated in a rodent model of early-life stress (a single 24-h episode of maternal deprivation, PND 9), the antidepressant-like effects of adolescent cannabidiol alone and/or in combination with adolescent cocaine exposure (given the described beneficial effects of cannabidiol on reducing cocaine effects).

METHODS

Maternally deprived Sprague-Dawley male rats were treated in adolescence with cannabidiol (with or without concomitant cocaine) and exposed to a battery of behavioral tests (forced-swim, novelty-suppressed feeding, open field, sucrose preference) across time. Putative enduring molecular correlates (CB receptors, BDNF) were evaluated in the hippocampus by western blot.

RESULTS

Cannabidiol exerted antidepressant- and anxiolytic-like effects in rats exposed to early-life stress. Cocaine did not alter affective-like behavior during adolescence in rats exposed to early-life stress; however, a depressive- and anxiogenic-like phenotype emerged during adulthood, and cannabidiol exerted some behavioral improvements, along with the growing literature supporting its beneficial role for reducing cocaine intake and/or reinstatement in rodents. Finally, cannabidiol did not modulate hippocampal CB receptors or BDNF proteins, and although the data raised interesting questions about the possible role of CB1 receptors on modulating the long-term effects of cocaine, future research is needed to expand these findings.

CONCLUSION

Cannabidiol showed a promising therapeutic response in terms of ameliorating affect in a rat model of early-life stress during adolescence and up to adulthood.

The effects of cocaine exposure in adolescence: Behavioural effects and neuroplastic mechanisms in experimental models

Drug addiction is a devastating disorder with a huge economic and social burden for modern society. Although an individual may slip into drug abuse throughout his/her life, adolescents are at higher risk, but, so far, only a few studies have attempted to elucidate the underlying cellular and molecular bases of such vulnerability. Indeed, preclinical evidence indicates that psychostimulants and adolescence interact and contribute to promoting a dysfunctional brain. In this review, we have focused our attention primarily on changes in neuroplasticity brought about by cocaine, taking into account that there is much less evidence from exposure to cocaine in adolescence, compared with that from adults. This review clearly shows that exposure to cocaine during adolescence, acute or chronic, as well as contingent or non‐contingent, confers a vulnerable endophenotype, primarily, by causing changes in neuroplasticity. Given the close relationship between drug abuse and psychiatric disorders, we also discuss the translational implications providing an interpretative framework for clinical studies involving addictive as well as affective or psychotic behaviours.

Adolescent cocaine induced persistent negative affect in female rats exposed to early-life stress

RATIONALE

The combination of several risk factors (sex, a prior underlying psychiatric condition, or early drug initiation) could induce the emergence of negative affect during cocaine abstinence and increase the risk of developing addiction. However, most prior preclinical studies have been centered in male rodents, traditionally excluding females from these analyses.

OBJECTIVES

To ascertain the behavioral and neurochemical consequences of adolescent cocaine exposure when the combination of several risk factors is present (female, early-life stress).

METHODS

Whole litters of Sprague-Dawley rats were exposed to maternal deprivation for 24 h on postnatal day (PND) 9. Cocaine was administered in adolescence (15 mg/kg/day, i.p., PND 33-39). Negative affect was assessed by several behavioral tests (forced swim, open field, novelty-suppressed feeding, sucrose preference). Hippocampal cell fate markers were evaluated by western blot (FADD, Bax, cytochrome c) or immunohistochemistry (Ki-67; cell proliferation).

RESULTS

Maternal deprivation is a suitable model of psychiatric vulnerability in which to study the impact of adolescent cocaine in female rats. While adolescent cocaine did not alter affective-like behavior during adolescence, a pro-depressive-like state emerged during adulthood, exclusively in rats re-exposed to cocaine during abstinence. FADD regulation by cocaine in early-life stressed female rats might contribute to certain hippocampal neuroadaptations with some significance to the observed induced negative affect.

CONCLUSIONS

Adolescent cocaine induced persistent negative affect in female rats exposed to early-life stress, highlighting the risk of early drug initiation during adolescence for the emergence of negative reinforcement during abstinence likely driving cocaine addiction vulnerability, also in female rats.