(16) Hippocampal Interneuron Development: Sex Differences in population size and Ntrk2 Expression

Background: Hippocampal interneurons (INs) are essential for regulating excitatory-inhibitory balance during brain development. Nerve growth factor receptor, tyrosine kinase B receptor (TrkB), encoded by Ntrk2, implicated to plays a key role in IN maturation, yet its expression profiles across IN subtypes during postnatal development remains unknown. Understanding sex- and cell type-specific Ntrk2 expression is critical for revealing mechanisms underlying differences in the hippocampal circuit formation.

Objectives: Objective of this study is to investigate the hippocampal IN population sizes and associated Ntrk2 mRNA expressions. We hypothesize that there are baseline developmental sex differences in the Ntrk2 expression of the hippocampal INs during brain development.

Design/Methods: Naïve mice at postnatal day (P) 16 underwent perfusion fixation and 35 mm coronal brain sections were sampled at 1:7 intervals throughout the antero-posterior extent of hippocampus for immunohistochemistry (IHC; SST, DAPI) and RNAscope (Gad1, Lamp5) co-labeling (Fig 1). Design-based stereology was utilized to quantitate the hippocampal IN numbers and volume. Ntrk2 mRNA puncta within the INs were semi-quantitatively quantified from two randomly selected confocal images of the CA1 region of hippocampus. Cell density (cell/mm³) and Ntrk2 mRNA puncta counts are presented for the SST and Lamp5 INs that colocalize with Gad1 from P16 male and female mice. Multifactorial ANOVA was used for data analysis.

Results: No sex differences were detected in Gad1, Lamp5 and SST expressing cell numbers per mm³ in either of the right or left hippocampus at P16. The baseline Ntrk2 mRNA expression within the hippocampal CA1 SST INs was higher in male naïve mice compared to females (P=0.05). Additionally, SST expressing INs showed greater Ntrk2 mRNA expression than Lamp5 expressing INs in male naïve mice at P16 (P=0.02).

Conclusions: Hippocampal Gad1, Lamp5, and SST expressing interneuron cell populations are comparable between sexes at P16. Ntrk2 mRNA expression exhibits both sex-specific and interneuron subtype-specific differences in CA1 region of the hippocampus during early postnatal development. As future directions, same parameters will be examined at additional developmental stages (P6, P9 and P30) and other subregions of the hippocampus including CA3 and dentate gyrus. In addition, the parvalbumin (PV) expressing INs that appear later in development will be studied to better understand temporal and molecular dynamics of IN development and Ntrk2 expression.