Heightened Hippocampal β-Adrenergic Receptor Function Drives Synaptic Potentiation and Supports Learning and Memory in the TgF344-AD Rat Model during Prodromal Alzheimer's Disease
Anthoni M. Goodman, Bethany M. Langner, Nateka L. Jackson, Capri Alex, Lori L. McMahon
Abstract
The central noradrenergic (NA) system is critical for the maintenance of attention, behavioral flexibility, spatial navigation, and learning and memory, those cognitive functions lost first in early Alzheimer's disease (AD). In fact, the locus coeruleus (LC), the sole source of norepinephrine (NE) for .90% of the brain, is the first site of pathologic tau accumulation in human AD with axon loss throughout forebrain, including hippocampus. The dentate gyrus is heavily innervated by LC-NA axons, where released NE acts on b-adrenergic receptors (ARs) at excitatory synapses from entorhinal cortex to facilitate long-term synaptic plasticity and memory formation. These synapses experience dysfunction in early AD before cognitive impairment. In the TgF344-AD rat model of AD, degeneration of LC-NA axons in hippocampus recapitulates human AD, providing a preclinical model to investigate synaptic and behavioral consequences. Using immunohistochemistry, Western blot analysis, and brain slice electrophysiology in 6-to 9-month-old wild-type and TgF344-AD rats, we discovered that the loss of LC-NA axons coincides with the heightened b-AR function at medial perforant path-dentate granule cell synapses that is responsible for the increase in LTP magnitude at these synapses. Furthermore, novel object recognition is facilitated in TgF344-AD rats that requires b-ARs, and pharmacological blockade of b-ARs unmasks a deficit in extinction learning only in TgF344-AD rats, indicating a greater reliance on b-ARs in both behaviors. Thus, a compensatory increase in b-AR function during prodromal AD in TgF344-AD rats heightens synaptic plasticity and preserves some forms of learning and memory.