Title: Peri-adolescent monoamine interference alters behavioral response to cocaine and associated dopamine dynamics in adulthood
Description: Adolescence is a sensitive developmental period encompassing neural maturation that is critical for an individual’s behavioral transition into adulthood. Due to widespread physiological changes attributed to this period, adolescents are also vulnerable to the initiation of risky behaviors, such as drug experimentation and use, as well as the emergence of various neuropsychiatric disorders. The mesocorticolimbic dopamine (DA) system in the brain undergoes a transient peak in activity and continues to mature during adolescence, potentially mediating adolescent hypersensitivity to social, appetitive, and drug-associated rewards. Simultaneously, the serotonin (5-HT) system exerts its influence on the dopamine system throughout adolescence, a process vital for proper impulse control and emotional regulation in adulthood. Substance use disorders (SUDs) are multifaceted and are comprised of diverse maladaptive behaviors that promote compulsive drug seeking, including loss of control and the propensity to engage in drug use irrespective of personal risks and/or consequences.

Drug addiction and substance use have a large negative impact globally both economically and with regards to public health outcomes, representing approximately 1.3% of the global burden of disease. Furthermore, dopamine reward pathways are heavily impacted by drug use, particularly with respect to stimulants, and the level of drug-induced dopamine release in the ventral striatum can be correlated with a drug’s perceived “high.” Certain experiences and adverse behaviors linked to refinement of monoamine connectivity in the brain during adolescence, such as heightened stress and sensation seeking, may predispose individuals for developing SUDs in adulthood. However, how drug reward sensitivity and associated activity of the dopamine system is altered in adulthood as a consequence of interfering with monoamine neurodevelopment during adolescence has not been clarified. To this end, I aim to understand how imbalanced monoamine development during adolescence contributes to stimulant-mediated behaviors in adulthood, specifically contextual reward associations, in relation to in vivo activity of the dopamine reward system.

I introduce a sensitive peri-adolescent (PA) period in mice, during which blockade of the serotonin transporter (SERT) via fluoxetine administration during postnatal (P) days 22-41 leads to inhibited adult aggression and locomotor response to stimulants. Conversely, I describe a more refined PA (P32-41) period during which systemic dopamine transporter (DAT) blockade via GBR12909 administration leads to enhanced aggression and stimulant-induced locomotor activity in adulthood. Utilizing these behaviorally opposing models characterized in our lab, I describe the diverging effects of systemic DAT and SERT blockade from P32-41 on cocaine-induced locomotor response as well as cocaine-mediated contextual preference. I administered cocaine intraperitoneally (i.p.) at doses of 5 and 10 mg/kg, applying the open field test and cocaine conditioned place preference (CPP) paradigm to assess stimulant-induced locomotor response and environmental reward associations, respectively. Potentiation of serotonergic tone during P32-41 via fluoxetine administration leads to decreased cocaine-induced locomotor response and a lack of preference for a cocaine-associated context in adulthood at a dose of 10 mg/kg, compared to controls and PA GBR treated subjects.

Conversely, potentiation of dopaminergic tone by administering GBR12909 during P32-41 is associated with enhanced cocaine-induced locomotor reactivity at 10 mg/kg and greater contextual preference at lower doses of cocaine (5 mg/kg), in comparison to PA fluoxetine treated mice and controls. To understand how in vivo VTA dopamine population activity is altered in both PA models during cocaine-associated behaviors in adulthood, I performed cocaine CPP while recording calcium signals in VTA dopamine neurons using fiber photometry in freely behaving subjects. Importantly, I utilize these recordings as a proxy for measuring changes in VTA dopamine activity as the subjects engage with a cocaine-paired environment. I found that PA DAT blockade was associated with greater baseline VTA dopamine activity in adulthood compared to controls, as well as heightened VTA dopamine activity while the subjects were in a cocaine-paired context during selected portions of the behavioral task compared to control subjects.

Additionally, we found a significant positive correlation between the magnitude of preference for a cocaine-associated context and the frequency of VTA dopamine calcium signals recorded while the subject is engaged with a cocaine-paired environment. Adult mice following PA DAT blockade displayed a greater frequency of recorded VTA dopamine calcium signals while in a cocaine-paired environment compared to PA fluoxetine treated mice. Supporting our correlational analysis, I detected a decreased preference for a cocaine-paired context in PA fluoxetine treated subjects compared to both controls and PA GBR12909 mice, when using a dose of cocaine in between the previous concentrations tested (7.5 mg/kg). Interestingly, PA fluoxetine treated subjects showed transition-dependent differences in VTA dopamine calcium activity during the final five-minute portion of our behavioral task, displaying less activity shortly post-entry into the cocaine-paired environment compared to pre-entry. In congruence, PA fluoxetine subjects showed enhanced VTA dopamine calcium activity on the saline-paired side shortly post-entry compared to pre-entry.

In collaboration with the Sulzer Lab, we also probed the effects of both PA manipulations on electrically evoked dopamine release in the ventral striatum of adult anesthetized mice using fast-scan cyclic voltammetry. Electrical stimulation was targeted to the midbrain and evoked dopamine release was recorded in the ventral striatum both at baseline and in response to cocaine injection, using the same 7.5 mg/kg dose applied in the calcium imaging study. Overall, we found a significant increase in dopamine release at baseline in the ventral striatum of adult PA GBR12909 treated subjects compared to both PA fluoxetine subjects and controls. Moreover, we found significantly greater cocaine-induced dopamine release in our PA GBR12909 mice compared to controls in adulthood. These findings are consistent with the imaging and behavioral data, highlighting the persistence of an elevated dopaminergic phenotype due to systemic PA DAT blockade. Conversely, systemic PA SERT blockade leads to behaviorally opposing effects and generally lower VTA DA activity dynamics in comparison to PA GBR12909 treated subjects in adulthood.

The unique, combinatorial approach applied in this dissertation work further our knowledge of how sensitive developmental periods influence the emergence of complex behaviors in adulthood, which is vital to improving treatment approaches for neuropsychiatric disorders.