• 3-Tesla
• Alzheimer's Disease
• amyloid
• biomarker
• EEG
• electroencephalography
• epilepsy
• locus coeruleus
• magnetic resonance imaging
• MRI
• noradrenaline
• norepinephrine
• pupillometry

Background: Although Alzheimer's disease (AD) is the most common cause of dementia, its pathophysiology has yet to be unraveled and this hampers the development of disease-modifying therapies. The pontine noradrenergic nucleus Locus Coeruleus (LC) has long been known to degenerate early in AD. This may contribute to the mechanisms involved in amyloid and Tau pathology, favoring, for example, neuroinflammation. Furthermore, it might predispose also to epileptic activity, which, apart from being an epiphenomenon frequently observed in AD, might also concur to AD physiopathology. Hence, studying LC function and morphology in vivo may have important pathophysiological, diagnostic, and, in perspective, therapeutic implications. However, this goal is complicated by LC small dimensions, its deep positioning and widespread connections. To get around these difficulties, LC structural evaluation is often performed through LC-specific magnetic resonance imaging (MRI), a validated technique for this purpose. Pupil diameter, as measured by pupillometry, has been shown to be associated with LC activity in animal models and humans, through MRI.

Objective: Thus, building on the results of LC structural and functional evaluation obtained, respectively, on elderly and young test populations, the present study aims at developing a protocol for LC morphological and functional evaluation in elderly subjects and AD patients.

Methods: To this end, we enrolled 26 healthy young volunteers (age range: 21–29 years, 12 females) and 54 cognitively healthy elderly volunteers (60–80 years, 33 females). Both groups completed neuropsychological tests evaluating, among others, attention and memory, e.g. digit span recall and delayed Rey auditory verbal learning tests. Elderly subjects underwent neuromelanin-sensitive MRI. Young volunteers underwent functional LC assessment, including EEG and pupillometry recording during execution of LC-activating tasks, such as a three-stimuli auditory oddball task.

Results: In elderly subjects, we found a positive correlation between LC volume and score at the digit span recall test (p= 0.039, Pearson’s r= 0.313), while a negative correlation was observed between LC average intensity and score at the delayed Rey auditory verbal learning test (p= 0.016, Pearson’s r = -0.362). No correlation was found between subjects’ age and LC MRI data. In young volunteers, the protocol for functional LC evaluation was successfully performed, the target stimulus during the oddball task effectively activated the LC, as measured by pupil dilation (p<0.05), and we found a negative correlation between amplitude of the EEG component P3b and pupil diameter change in response to the target stimulus during the auditory oddball. These results confirmed the feasibility of our protocol for functional LC evaluation and allowed us to design a similar protocol suitable for elderly subjects and AD patients. This protocol is a shorter and simpler version of the one we used in young volunteers, designed with the aim of acquiring maximal information with minimal effort for the subject. It accounts for the possibility of screening for EEG epileptic abnormalities, which will be crucial once the protocol is applied to AD patients.

Conclusions: Thus, we characterized the LC from a structural and functional viewpoint in elderly and young volunteers, respectively. This allowed us to design a protocol for functional LC evaluation in elderly subjects and AD patients. Experimental sessions for functional LC evaluation in the elderly volunteers, who underwent LC-sensitive MRI, are now postponed due to the SARS-CoV-2 pandemic. However, once we will have collected LC functional data from these subjects, we will be able to correlate them with the existing MRI data on LC structure, thus validating a protocol for LC structural and functional evaluation in elderly subjects, which will be of great interest for AD patients.