Activity-induced neurogenesis has been extensively studied in rodents but the lack of ante mortem accessibility to human brain at the cellular and molecular levels limits studies of the process in humans. Using cerebral spheroids derived from human induced pluripotent stem cells (iPSCs), we investigated the effects of 4-aminopyridine (4AP) on neuronal activity and associated neurogenesis. Our studies demonstrate that 4AP increases neuronal activity in 3-month-old cerebral spheroids while increasing numbers of new neurons and decreasing the population of new glial cells. We also observed a significant decrease in the expression of miR-135a, which has previously been shown to be decreased in exercise-induced neurogenesis. Predicted targets of miR-135a include key participants in the SMAD2/3 and BDNF pathways. Together, our results suggest that iPSC-derived cerebral spheroids are an attractive model to study several aspects of activity-induced neurogenesis.

The neurotransmitter acetylcholine (ACh) acting at its α4β2* subtype of nicotinic acetylcholine receptor (nAChR) within the cerebral cortex plays an important role to support cognitive functions. ACh normally initiates a robust excitatory response at α4β2* nAChRs, although this activity may be modulated by a number of mechanisms that alter receptor function. Negative receptor modulation may occur via desensitization by the competitive agonist nicotine or via presumed negative allosteric modulation by the endogenous progesterone‐derived neurosteroid 3α‐hydroxy‐5α‐prenan‐20‐one (allopregnanolone; ALLO). Conversely, positive modulation may occur via positive allosteric modulation by the alkaloid galantamine. Although each form of modulation has been demonstrated to occur in α4β2* nAChRs located on pyramidal neurons within layer VI of the medial prefrontal cortex (mPFC) in young postnatal male mice, the presence and relative strength of modulation in young postnatal female mice, and at maturity for both sexes, is unclear. Using whole‐cell electrophysiology in mFPC layer VI neurons, we measured the ability of nicotine (300 nM), ALLO (10 μM), and galantamine (0.1 μM) to modulate ACh activation of α4β2* nAChRs in male and female mice at postnatal day (P)15‐20 (young postnatal age) and P80‐120 (adulthood). For nicotine, nAChR desensitization at P15‐20 was greater than at P80‐120 and this result was driven by an effect of postnatal age in male mice only. In contrast, ALLO inhibited nAChR function to a similar degree at both P15‐20 and P80‐120. Galantamine potentiated nAChR function to a similar degree at both P15 and P80‐120, although this potentiation was significantly greater in females than in males at each age. Results from this study suggest that distinct factors are involved in each type of receptor modulation, which are differentially altered during postnatal maturation and across biological sex. Future studies will focus on mechanisms by which these factors influence the modulation of α4β2* nAChRs during postnatal maturation.

Acetylcholine (ACh) signalling within the medial prefrontal cortex (mPFC) is important for the normal development and function of prefrontal cognitive networks. Layer VI of the mPFC is highly populated with pyramidal neurons that express the α4β2* subtype of nicotinic acetylcholine receptor (nAChR), and the activation of these receptors by ACh is central to its function in this region. These receptors can be negatively modulated through several forms of inhibition. For example, nicotine potently desensitizes nAChRs to ACh activation, while the endogenous progesterone‐derived neurosteroid 3α‐hydroxy‐5α‐prenan‐20‐one (allopregnanolone; ALLO) inhibits nAChR activation by ACh through a presumed negative allosteric modulation. Both forms of inhibition mediated by nicotine and ALLO have been demonstrated for α4β2* nAChRs located on mPFC layer VI neurons in young postnatal mice. However, the relative strength for both forms of inhibition in mature mice, compared with the young postnatal age, is not clear. In addition, the mechanism of action for ALLO inhibition of α4β2* nAChRs is not well understood. Using whole‐cell electrophysiology, we measured the ability of both nicotine (300 nM) and ALLO (10 μM) to inhibit 1 mM ACh activation of α4β2* nAChRs located on mPFC layer VI neurons from male and female CD1‐strain mice at postnatal day (P)15‐20 (young postnatal age) and P80‐120 (adulthood). For nicotine, nAChR desensitization was greater at P15‐20 than at P80‐120, and this result was driven by an effect of postnatal age in male mice only. Conversely, ALLO inhibited nAChR function to a similar degree at both P15‐20 and P80‐120. Ongoing ACh binding experiments aim to determine the mechanism of action for ALLO inhibition of these receptors at both ages. Results from this study confirm that the endogenous neurosteroid ALLO inhibits mPFC α4β2* nAChRs in adulthood as it does in young postnatal life. Developmental changes to the magnitude of nicotine desensitization but not ALLO inhibition suggests that different factors regulate each type of negative modulation at α4β2* nAChRs, and that these factors are differentially altered during postnatal maturation of the mPFC. Support or Funding Information Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant 2019‐04989 to CDCB

Activity-induced neurogenesis has been extensively studied in rodents but the lack of ante mortem accessibility to human brain at the cellular and molecular levels limits studies of the process in humans. Using cerebral spheroids derived from human induced pluripotent stem cells (iPSCs), we investigated the effects of increased neuronal activity on neurogenesis. Our studies demonstrate that increasing neuronal activity with 4-aminopyridine in 3-month-old cerebral spheroids is associated with increases in the numbers of new neurons and decreases in the population of new glial cells. We also observed a significant decrease in the expression of miR-135a, which has previously been shown to be decreased in exercise-induced neurogenesis. Predicted targets of miR-135a include key participants in the SMAD2/3 and BDNF pathways. Together, our results suggest that iPSC-derived cerebral spheroids are an attractive model to study some aspects of activity-induced neurogenesis.