Enhanced Ba 2+ -sensitive inward rectifying potassium conductance reduces intrinsic excitability of layer 2/3 pyramidal neurons in the primary auditory cortex of Fmr1 knockout mice

ABSTRACT Fragile X syndrome (FXS) is frequently associated with auditory hypersensitivity and altered cortical processing, yet the intrinsic ionic mechanisms shaping auditory cortex excitability remain incompletely defined. Here, we tested whether subthreshold conductances contribute to compensatory changes in intrinsic excitability in layer 2/3 (L2/3) pyramidal neurons of the primary auditory cortex (AC) in Fmr1 knockout (Fmr1-KO) mice. We performed wholecell patchclamp recordings in acute slices from juvenile mice (P28–P42) and quantified passive properties, firing output, synaptic potentials, and subthreshold currents under pharmacological isolation. Compared with wild type (WT), Fmr1-KO L2/3 neurons displayed a more hyperpolarized resting membrane potential, reduced input resistance, elevated rheobase, prolonged firstspike latency, and reduced firing across depolarizing steps, consistent with a hypoexcitable state. Bath application of BaCl 2 (60 μM) depolarized the membrane, increased input resistance, and restored firing output and rheobase toward WT levels, indicating that a Ba 2+ -sensitive potassium conductance strongly constrains excitability in Fmr1-KO neurons. Voltageclamp recordings revealed a larger Ba 2+ -sensitive inwardly rectifying (Kir-like) current in Fmr1-KO neurons, supporting increased functional Ba 2+ -sensitive inwardly rectifying conductance. In contrast, blocking I h with ZD7288 (10 μM) produced modest changes in passive properties but induced genotype dependent effects on excitability and enhanced synaptic activity and EPSP summation preferentially in Fmr1-KO neurons, consistent with a role for I h in input filtering rather than setting basal conductance. Together, these findings identify enhanced Ba 2+ -sensitive potassium conductance as a primary determinant of the Fmr1-KO subthreshold conductance state in AC L2/3 pyramidal neurons, suggesting an intrinsic homeostatic mechanism that stabilizes output in the presence of elevated excitatory drive.