Abstract
The locus coeruleus (LC) is among the earliest brain regions affected in Alzheimer's disease (AD), yet the mechanisms underlying its early vulnerability remain poorly understood. Clarifying synaptic and intrinsic dysfunction in LC noradrenergic (NE) neurons is essential to determine how noradrenergic circuit failure contributes to prodromal AD. We combined whole-cell patch-clamp recordings with quantitative immunofluorescence to examine synaptic transmission, firing properties, voltage-gated currents, and synaptic organization in LC/NE neurons from young 3xTg mice prior to substantial neurodegeneration. Young 3xTg mice exhibited reduced excitatory and enhanced inhibitory synaptic input onto LC/NE neurons. Consistent with these functional shifts, structural analysis revealed decreased densities of VGLUT2, PSD95, and paired VGLUT2-PSD95 puncta, alongside increased VGAT, gephyrin, and paired VGAT-gephyrin puncta. Despite this net synaptic suppression, LC/NE neurons displayed elevated spontaneous firing rates but a markedly reduced capacity to generate spike output during sustained depolarization. Voltage-clamp recordings revealed a diminished total voltage-gated K⁺ current density alongside reduced voltage-gated Na⁺ current density, without significant shifts in their voltage-dependence or kinetics. These biophysical alterations provide a mechanistic basis for the divergent firing phenotype: reduced K current; conductance likely facilitates spontaneous pacemaker activity, whereas diminished Na current; current constrains repetitive, input-driven spike generation. Together, these findings demonstrate that LC/NE neurons undergo coordinated synaptic and ion-channel remodeling during prodromal AD. This unique decoupling of spontaneous baseline activity from stimulus-evoked excitability may impair dynamic noradrenergic modulation, positioning early LC cellular dysfunction as a candidate driver of network instability and a novel therapeutic target.