Chemical ablation of locus coeruleus terminals in mice dysregulates behavioral features reminiscent of prodromal Alzheimer’s disease

Abstract

AbstractBackgroundThe prodromal stages of Alzheimer’s disease (AD), which precede cognitive impairment by decades, are characterized by neuropsychiatric symptoms such as anxiety, agitation, impulsivity, and sleep disturbances that significantly impair quality of life. The brainstem noradrenergic locus coeruleus (LC) modulates behaviors that are reminiscent of these prodromal symptoms. Furthermore, the loss of norepinephrine (NE) terminals in LC‐projecting regions is a prominent feature of early AD, implicating LC dysfunction as a possible contributor to prodromal symptoms. However, the behavioral and pathological consequences of LC terminal loss have not been fully investigated.MethodAdult male and female C57BL/6 mice aged 4‐6 months were administered either saline or the LC‐specific neurotoxin, N‐(2‐chloroethyl)‐N‐ethyl‐bromo‐benzylamine (DSP‐4) (50 mg/kg, i.p.) on day 1 and day 7 (12 animals/group). Performance in behavioral tests that assess arousal (sleep latency), locomotor activity (novelty‐induced and circadian), anxiety (novelty‐suppressed feeding), and associative memory (contextual fear conditioning) commenced one week following the last treatment. In addition, brain sections consisting of LC, hippocampus, and cortical areas were immunostained for tyrosine hydroxylase (TH) to evaluate LC integrity, NE transporter (NET) to determine fiber/terminal density to LC‐projecting regions, and glial fibrillary acidic protein (GFAP) to assess astrogliosis. Fluorescence immunoreactivity (IR) was analyzed with ImageJ.ResultAnimals treated with DSP‐4 displayed significantly increased latency to feed in a novel environment compared to saline‐treated animals, with no treatment differences in the home cage, suggesting elevated neophobic behavior. Furthermore, DSP‐4‐treated animals trended towards decreased locomotor activity in the center of the environment over 1 hour and 23 hours, indicating increased thigmotaxis. No behavioral treatment differences were observed for sleep latency and contextual fear conditioning paradigms. Immunofluorescence revealed no treatment differences in TH IR in the LC but reduced NET IR in LC‐projecting regions of DSP‐4‐treated animals, demonstrating preserved LC cell bodies and LC fiber loss. No significant treatment differences were found for GFAP.ConclusionThese data suggest that chemical ablation of LC terminals with DSP‐4 can contribute to neophobia and anxiogenic responses, implicating LC terminal loss in mediating prodromal symptoms relevant to AD. Further studies are warranted to identify the physiological and molecular mechanisms underlying LC dysfunction for alleviating prodromal AD symptoms.