Impact of Depressogenic-and Antidepressant-like Challenges on Monoamine System Activities: in vivo Electrophysiological Characterization Studies

Abstract

Introduction: major depressive disorder is a common psychiatric disorder associated with high economic cost, severe human suffering, and low remission rates. Imbalanced neurotransmission of the monoaminergic serotonin (5-HT), dopamine (DA) and norepinephrine (NE) systems is implicated in this disorder. However, the etiology underlying this presumed imbalance and the mechanism by which antidepressant strategies restore this imbalance requires further exploration. Accordingly, the present work assessed the effects of depressogenic and antidepressant-like conditions on these systems. Methodology: Electrophysiological extracellular single unit recordings from 5-HT, DA, NE, and hippocampal pyramidal neurons were obtained in adult male chloral hydrate anaesthetized Sprague-Dawley rats. Effects on relevant receptors were characterized using established electrophysiological and/or pharmacological strategies. Prenatal stress was used to model depressogenic-like conditions. The effects on monoamine systems of asenapine and brexpiprazole, two atypical antipsychotics with antidepressant potential, were characterized after acute (brexpiprazole) and sustained administration. These sustained regimens resulted in clinically relevant blood plasma levels. Results: Prenatal stress exposure altered monoamine system activities but did not produce detrimental effects on behavior. Asenapine and brexpiprazole had unique effects on the activities of monoamine systems. Unlike other antipsychotics, both agents did not produce a cessation of the firing of dopamine neurons in the ventral tegmental area, thereby providing novel insights in the role of this system in the treatment of mood disorders. Furthermore, both agents enhanced the tonic activation of postsynaptic 5-HT1A receptors, similarly to the effects of all antidepressant strategies. Conclusion: Prenatal stress altered the activities of 5-HT, NE and DA neurons. Since these central changes were obtained in animals displaying normal behavior, they presumably reflect adaptations to depressogenic-like conditions. The characterization of asenapine and brexpiprazole contributed to a further understanding of their mechanisms of action. Together, these studies provide insight in neural substrates presumably relevant to the antidepressant response.