Role of Tyrosine-Related Kinase B Inhibition in the Mesocorticolimbic Stress and Reward Circuitries of the Adolescent and Adult Brain Following a Heterotypic Stress Regimen

• Main Author: Azogu, Idu
• Other Authors: Plamondon, Hélène
• Language: en
• Published: Université d'Ottawa / University of Ottawa 2017
• Subjects: BDNF; TrkB; ANA-12; Heterotypic stress; Adolescence; Rats; HPA axis; Sex-specific differences; Corticosterone; Adulthood
• Online Access: http://hdl.handle.net/10393/36611; http://dx.doi.org/10.20381/ruor-20891

The mesocorticolimbic system is involved in fundamental processes that drive motivational behaviors essential for survival (feeding, reproduction and sexual behavior, etc.), as well as neurochemical activity involved in mood regulation. Stressful life events are an important cause of dysregulated psychological functioning, which in some leads to a pathophysiology of mood disorders. A source of such disorder could be, among other underlying factors, an impairment of synaptic plasticity induced by alterations in the levels of neurotrophins and/or aberrant glucocorticoid responses. The role of the brain derived neurotrophic factor (BDNF) and its high affinity receptor tyrosine-related kinase B (TrkB) in the mesocorticolimbic reward circuitry has been largely studied in adulthood, yet a possible role of this system in mediating memory and emotional responses induced by stress during the juvenile, adolescence period has not been elucidated. The proposed set of thesis studies are designed to investigate the roles of BDNF and TrkB signaling, via the selective and non-competitive TrkB antagonist, ANA-12 (N-[2-[[(Hexahydro-2-oxo-1H-azepin-3-yl)amino]carbonyl]phenyl] benzo[b]thiophene-2-carboxamide), in the expression of stress-induced changes in the brain stress circuitry (including the medial prefrontal cortex (mPFC), hypothalamic-pituitary-adrenal (HPA) axis, and hippocampus) and reward signaling systems of the brain (including the nucleus accumbens (NAc) and ventral tegmental area (VTA)). In addition, experiments aim to determine behavioral changes following stress exposure in male and female Wistar rats. Finally, the possible interplay between BDNF, dopamine, glutamate and orexins in response to repeated stress is examined. Articles 1 and 2, aimed to assess the biochemical and behavioral effects of direct ANA-12 infusion (0.25 µg/ 0.5µl) into the nucleus accumbens shell during exposure to a 10-day heterotypic stress paradigm in male rats. Specifically, Article 1 demonstrated a key role for BDNF/TrkB signaling to regulate stress-induced effects. Notably, the impact of ANA-12 to attenuate anxiety-like behavior in repeatedly stressed rats while increasing anxiety behavior in non-stress rats suggest an interesting behavioral and neurochemical state-dependent process induced by TrkB receptor signaling. Article 2 supports the key role for BDNF secretion in basal and stress-induced behaviors in rats suggesting an influence of TrkB in sociability, motivation and passive avoidance. Furthermore, this role of TrkB extended to increased expression of orexin A in the Perifornical area (PfA) and a decrease in the ventral CA1 of the hippocampus, and in stress-induced elevations in orexinergic projections to the VTA, of which reductions were observed in non-stress groups treated with ANA-12. Article 3 demonstrated gender-specific behavioral and biochemical responses in different developmental periods and the impact of TrkB activation, dependent on stress exposure, to affect the regulation of TrkB receptor isoforms (full length and truncated TrkB, TrkB.FL and TrkB.T1, respectively) in adulthood. Results revealed increased CORT responses in adolescent females relative to males and attenuated CORT secretions in both genders by TrkB inhibition. Elevated activity levels in young adult females and increased passive coping behavior in the forced swim in stress-naïve females were also noted, in addition to novel observations on brain region and sex differences in TrkB receptor isoforms. Taken together, thesis findings derived from applications of ANA-12, shall foster knowledge on the contribution of BDNF in regulation of mood upon stress exposure at times when the brain is undergoing important maturation and remodelling, as well as on the relationship of stress exposure during adolescence and lasting brain and behavioral disorders in adulthood.