Voltammetric Approach for Characterizing the Biophysical and Chemical Functionality of Human Induced Pluripotent Stem Cell-Derived Serotonin Neurons

Measurement of Real-Time Serotonin Dynamics from Human-Derived Gut Organoids

The importance of the gut in regulating the brain-body immune axis is becoming increasingly evident. Interestingly, the brain and gut share many common signaling molecules, with serotonin being one of the most notable. In fact, the gut is the primary source of serotonin in the body. However, studying serotonin dynamics in a human-specific context remains a challenge. Human stem cell-derived models provide a promising avenue for studying signal transmission in well-controlled, in vitro environments. In this study, we report the first fast-scan cyclic voltammetry (FSCV) measurements of serotonin signaling in a newly developed enterochromaffin cell (ECC)-enriched gut organoid model. First, we characterize the stem cell-derived gut organoids and confirm they are enriched with ECCs, the key cell type responsible for producing and releasing serotonin in the gut. We then optimize an in vitro buffer that maintains cell viability while supporting FSCV measurements. Using this system, we detect spontaneous release events, which increase in frequency and amplitude following stimulation with forskolin (FSK) and 3-isobutyl-1-methylxanthine (IBMX). Finally, we confirm the identity of the signal as serotonin using a selective serotonin reuptake inhibitor (SSRI), which significantly delayed the reuptake profile. Our study introduces the first real-time measurement of serotonin signaling in a human-derived gut model. We believe this system will be essential for future research on serotonin's role in the gut and for potential novel drug target identification.

Serotonin is a common thread linking different classes of antidepressants

Depression pathology remains elusive. The monoamine hypothesis has placed much focus on serotonin, but due to the variable clinical efficacy of monoamine reuptake inhibitors, the community is looking for alternative therapies such as ketamine (neurogenesis theory of antidepressant action). There is evidence that different classes of antidepressants may affect serotonin levels; a notion we test here. We measure hippocampal serotonin in mice with voltammetry and study the effects of acute challenges of escitalopram, fluoxetine, reboxetine, and ketamine. We find that pseudo-equivalent doses of these drugs similarly raise ambient serotonin levels, despite their differing pharmacodynamics because of differences in Uptake 1 and 2, rapid SERT trafficking, and modulation of serotonin by histamine. These antidepressants have different pharmacodynamics but have strikingly similar effects on extracellular serotonin. Our findings suggest that serotonin is a common thread that links clinically effective antidepressants, synergizing different theories of depression (synaptic plasticity, neurogenesis, and the monoamine hypothesis).

Kappa Opioid Receptors Reduce Serotonin Uptake and Escitalopram Efficacy in the Mouse Substantia Nigra Pars Reticulata

The serotonin and kappa opioid receptor (KOR) systems are strongly implicated in disorders of negative affect, such as anxiety and depression. KORs expressed on axon terminals inhibit the release of neurotransmitters, including serotonin. The substantia nigra pars reticulata (SNr) is involved in regulating affective behaviors. It receives the densest serotonergic innervation in the brain and has high KOR expression; however, the influence of KORs on serotonin transmission in this region is yet to be explored. Here, we used ex vivo fast-scan cyclic voltammetry (FSCV) to investigate the effects of a KOR agonist, U50, 488 (U50), and a selective serotonin reuptake inhibitor, escitalopram, on serotonin release and reuptake in the SNr. U50 alone reduced serotonin release and uptake, and escitalopram alone augmented serotonin release and slowed reuptake, while pretreatment with U50 blunted both the release and uptake effects of escitalopram. Here, we show that the KOR influences serotonin signaling in the SNr in multiple ways and short-term activation of the KOR alters serotonin responses to escitalopram. These interactions between KORs and serotonin may contribute to the complexity in the responses to treatments for disorders of negative affect. Ultimately, the KOR system may prove to be a promising pharmacological target, alongside traditional antidepressant treatments.

Diverse maturity-dependent and complementary anti-apoptotic brakes safeguard human iPSC-derived neurons from cell death

In humans, most neurons are born during embryonic development and have to persist throughout the entire lifespan of an individual. Thus, human neurons have to develop elaborate survival strategies to protect against accidental cell death. We set out to decipher the developmental adaptations resulting in neuronal resilience. We demonstrate that, during the time course of maturation, human neurons install a complex and complementary anti-apoptotic signaling network. This includes i.) a downregulation of central proteins of the intrinsic apoptosis pathway including several caspases, ii.) a shift in the ratio of pro- and anti-apoptotic BCL-2 family proteins, and iii.) an elaborate regulatory network resulting in upregulation of the inhibitor of apoptosis protein (IAP) XIAP. Together, these adaptations strongly increase the threshold for apoptosis initiation when confronted with a wide range of cellular stressors. Our results highlight how human neurons are endowed with complex and redundant preemptive strategies to protect against stress and cell death.