Neonatal involvement of the serotonergic system in hippocampal wiring: unraveling its role in gender-specific mood disorders

The hippocampus has been linked to a plethora of mood disorders. The monoamine neurotransmitter serotonin (5-HT) plays a critical role in the development of several of these mood and neuropsychiatric disorders. Serotonergic signaling via serotonin 1A receptor (5-HT1A-R) is crucial during the early postnatal days for later-life behavior, like anxiety and depression. Specifically, the forebrain 5-HT1A-R heteroreceptors have been implicated in several mood disorders. Intriguingly, the incidence of mood disorders is two-fold higher in women than men. Furthermore, the level of the serotonin 1A receptor (5-HT1A-R) is significantly higher in the brain of women than men, suggesting that the women may be more sensitive to a deficiency in 5-HT1A-R signaling. Taken together, all these studies also suggest that the serotonergic system operating via 5-HT1A-R in brain development may determine behavioral manifestation of mood disorders in adulthood. However, the mechanistic details of the 5-HT1A-R signaling pathway, especially how it operates in early developmental stages, are still unclear. The current study is aimed at bridging this gap. Current findings reveal that 5-HT1A-R signaling acting through PKC epsilon and Erk1/2 augments neuroproliferation and neurogenesis in the dentate gyrus (DG) in mice at postnatal day-6 (P6), which marks the peak of postnatal neuroproliferation. However, only the basal level of neuroproliferation was significantly stunted in the female but not male 5-HT1A-R (-/-) (KO) mice at P6. Subsequently, the neuroproliferation in the KO female mice could be restored to almost wild-type (WT) levels through the stimulation of the downstream PKC epsilon molecule using a selective activator, DCP-LA. Using Timm staining, a significant increase was observed in the arborization of the DG granule cell-derived mossy fiber (MF) axons and their connectivity with the CA3 pyramidal cells in the Stratum Oriens (SO) region in the female KO mice at P18. Such increased MF connectivity could lead to increased excitation and elevated anxiety. Confirming the importance of the identified signaling activity through PKC epsilon, this increased MF connectivity was restored to normal levels in the KO females treated with DCP-LA from P6-14. Finally, this treatment eliminated the significantly elevated anxiety levels in the adult female KO mice. Thus, a sex-specific effect of serotonergic signaling via the 5-HT1A-R plays an important role in hippocampal development and later-life behavior, which can be corrected by targeting a downstream signaling molecule, PKC epsilon. New-born and immature granule cells of the DG are resistant to inhibition by gamma-aminobutyric acid (GABA), and each granule cell-derived MF axon connects with about 50 inhibitory inter-neurons, which cause inhibition of the CA3 pyramidal neurons. In contrast, each MF axon connects directly with only 10-14 CA3 pyramidal neurons. Thus, the MF axons, especially the excitatory immature granule neurons, produce more feed-forward inhibition than excitation, which allows for only a limited level of activation of the CA3 neurons. Decreased neuroproliferation at P6 would yield less GABA-insensitive new granule neurons in the DG, which may then lead to a decrease in feed-forward inhibition, thereby eliciting an overall increase in excitation of the CA3 pyramidal neurons and the downstream Schaffer Collateral pathway of the hippocampus. While this may explain the extension of MF collaterals in the SO region, the sex-specific effect of 5-HT1A-R deficiency on neuroproliferation remains to be explained. Although sex-based differences have yet to be recorded for steroid levels in the neonatal hippocampus, it is known that neonatal neuroproliferation in the DG is significantly higher in male mice, and estradiol treatment boosts neuroproliferation only in the females. This study introduces the serotonergic system as a second signaling scheme with pronounced sex-specific effects in the neonatal DG and suggests a possible crosstalk between brain steroids and 5-HT during brain development. Current findings reveal that 5-HT1A--R signaling acting through PKC epsilon and Erk1/2 augments neuroproliferation and neurogenesis in the dentate gyrus (DG) in mice at postnatal day-6 (P6), which marks the peak of postnatal neuroproliferation. However, only the basal level of neuroproliferation was significantly stunted in the female but not male 5-HT1A--R (-/-) (KO) mice at P6. Subsequently, the neuroproliferation in the KO female mice could be restored to almost wild-type (WT) levels through the stimulation of the downstream PKC epsilon molecule using a selective activator, DCP-LA. Using Timm staining, a significant increase was observed in the arborization of the DG granule cell-derived mossy fiber (MF) axons and their connectivity with the CA3 pyramidal cells in the Stratum Oriens (SO) region in the female KO mice at P18. Such increased MF connectivity could lead to increased excitation and elevated anxiety. Confirming the importance of the identified signaling activity through PKC epsilon, this increased MF connectivity was restored to normal levels in the KO females treated with DCP-LA from P6-14. Finally, this treatment eliminated the significantly elevated anxiety levels in the adult female KO mice. Thus, a sex-specific effect of serotonergic signaling via the 5-HT1A-R plays an important role in hippocampal development and later-life behavior, which can be corrected by targeting a downstream signaling molecule, PKC epsilon. New-born and immature granule cells of the DG are resistant to inhibition by gamma-aminobutyric acid (GABA), and each granule cell-derived MF axon connects with about 50 inhibitory inter-neurons, which cause inhibition of the CA3 pyramidal neurons. In contrast, each MF axon connects directly with only 10-14 CA3 pyramidal neurons. Thus, the MF axons, especially the excitatory immature granule neurons, produce more feed-forward inhibition than excitation, which allows for only a limited level of activation of the CA3 neurons. Decreased neuroproliferation at P6 would yield less GABA-insensitive new granule neurons in the DG, which may then lead to a decrease in feed-forward inhibition, thereby eliciting an overall increase in excitation of the CA3 pyramidal neurons and the downstream Schaffer Collateral pathway of the hippocampus. While this may explain the extension of MF collaterals in the SO region, the sex-specific effect of 5-HT1A-R deficiency on neuroproliferation remains to be explained. Although sex-based differences have yet to be recorded for steroid levels in the neonatal hippocampus, it is known that neonatal neuroproliferation in the DG is significantly higher in male mice, and estradiol treatment boosts neuroproliferation only in the females. This study introduces the serotonergic system as a second signaling scheme with pronounced sex-specific effects in the neonatal DG and suggests a possible crosstalk between brain steroids and 5-HT during brain development.