A sexually dimorphic signature of activity-dependent BDNF signaling on the intrinsic excitability of pyramidal neurons in the prefrontal cortex

Abstract

Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders with strong genetic heterogeneity and more prevalent in males than females. We and others hypothesize that diminished activity-dependent neural signaling is a common molecular pathway dysregulated in ASD caused by diverse genetic mutations. Brain-derived neurotrophic factor (BDNF) is a key growth factor mediating activity-dependent neural signaling in the brain. A common single nucleotide polymorphism (SNP) in the pro-domain of the human BDNF gene that leads to a methionine (Met) substitution for valine (Val) at codon 66 (Val66Met) significantly decreases activity-dependent BDNF release without affecting basal BDNF secretion. By using mice with genetic knock-in of this human BDNF methionine (Met) allele, our previous studies have shown differential severity of autism-like social deficits in male and female BDNF^+/Met mice. Pyramidal neurons are the principal neurons in the prefrontal cortex (PFC), a key brain region for social behaviors. Here, we investigated the impact of diminished activity-dependent BDNF signaling on the intrinsic excitability of pyramidal neurons in the PFC. Surprisingly, diminished activity-dependent BDNF signaling significantly increased the intrinsic excitability of pyramidal neurons in male mice, but not in female mice. Notably, significantly decreased thresholds of action potentials were observed in male BDNF^+/Met mice, but not in female BDNF^+/Met mice. Voltage-clamp recordings revealed that the sodium current densities were significantly increased in the pyramidal neurons of male BDNF^+/Met mice, which were mediated by increased transcriptional level of Scn2a encoding sodium channel Na_V 1.2. Medium after hyperpolarization (mAHP), another important parameter to determine intrinsic neuronal excitability, is strongly associated with neuronal firing frequency. Further, the amplitudes of mAHP were significantly decreased in male BDNF^+/Met mice only, which were mediated by the downregulation of Kcnn2 encoding small conductance calcium-activated potassium channel 2 (SK2). This study reveals a sexually dimorphic signature of diminished activity-dependent BDNF signaling on the intrinsic neuronal excitability of pyramidal neurons in the PFC, which provides possible cellular and molecular mechanisms underpinning the sex differences in idiopathic ASD patients and human autism victims who carry BDNF Val66Met SNP.

Keywords: BDNF; Val66Met polymorphism; activity-dependent neural signaling; intrinsic neuronal excitability; sex.

Figure 1

Diminished activity-dependent BDNF signaling significantly increases intrinsic neuronal excitability of pyramidal neurons in the PFC of male mice, but not in female mice. (A) A representative confocal image showing layer V pyramidal neurons used for recording. (B) Quantification of the number of evoked action potentials in response to injected currents in the pyramidal neurons of PFC from BDNF^+/+ and BDNF^+/Met mice. n = 16–20 neurons/4–5 mice/group. (C) Representative action potential traces from regular spiking (black) and intrinsic bursting (red) pyramidal neurons. *p < 0.05, BDNF^+/Met versus BDNF^+/+; #p < 0.05, male versus female, three-way ANOVA.

Figure 2

Diminished activity-dependent BDNF signaling differentially alters properties of action potential of pyramidal neurons in the PFC of males and females. Bar graphs showing rheobase (A), first spike delay (B), threshold (C), and mAHP (D) in the pyramidal neurons of PFC from male and female BDNF^+/+ and BDNF^+/Met mice. *p < 0.05, **p < 0.01, BDNF^+/Met versus BDNF^+/+; #p < 0.05, ##p < 0.01, ###p < 0.001, male versus female, n = 16–20 neurons/4–5 mice/group, two-way ANOVA. (E) Representative traces of the first spike elicited with minimum electric current in regular spiking (black) and intrinsic bursting (red) pyramidal neurons.

Figure 3

Diminished activity-dependent BDNF signaling significantly increases voltage-gated sodium currents and the expression of Scn2a in male mice, but not in female mice. (A) Current density-voltage relationship showing sodium current density in the pyramidal neurons from PFC of BDNF^+/+ and BDNF^+/Met mice. n = 16 neurons/4 mice/group. (B) Bar graph showing the peak current density in the pyramidal neurons from PFC of BDNF^+/+ and BDNF^+/Met mice. ***p < 0.001, BDNF^+/Met versus BDNF^+/+; ###p < 0.001, male versus female, n = 16 neurons/4 mice/group, two-way ANOVA. (C) Representative traces of voltage-gated sodium currents recorded in response to voltage steps from −70 to +50 mV in the pyramidal neurons from PFC of BDNF^+/+ and BDNF^+/Met mice. Inset: TTX (0.5 µM) abolished the inward currents. (D) Quantitative real-time PCR showing the mRNA levels of Scn1a, Scn2a, Scn3a, and Scn8a in the PFC of BDNF^+/+ and BDNF^+/Met mice. ***p < 0.001, BDNF^+/Met versus BDNF^+/+; ###p < 0.001, male versus female, n = 8 mice/group, two-way ANOVA.

Figure 4

The impact of diminished activity-dependent BDNF signaling on the expression of SK channels, KCNQ channels, and HCN channels in the PFC. Quantitative real-time PCR showing the mRNA levels of SK channels (A), KCNQ channels (B), and HCN channels (C) in the PFC of BDNF^+/+ and BDNF^+/Met mice. ***p < 0.001, BDNF^+/Met versus BDNF^+/+; ##p < 0.01, male versus female, n = 8 mice/group, two-way ANOVA.