Comparing PET scans of more than 90 adults with and without mild cognitive impairment (MCI), Johns Hopkins Medicine researchers say relatively lower levels of the so-called “happiness” chemical, serotonin, in parts of the brain of those with MCI may play a role in memory problems including Alzheimer’s disease.
The findings, first published online Sept. 13 in the Journal of Alzheimer’s Disease, lend support to growing evidence that measurable changes in the brain happen in people with mild memory problems long before an Alzheimer’s diagnosis, and may offer novel targets for treatments to slow or stop disease progression.
The investigators cautioned that their study showed a correlation between lower serotonin transporter levels and memory problems in MCI, and was not designed to show causation or the role of serotonin in the progression from MCI to AD. To answer these questions, further research is needed to study over time healthy controls and individuals with MCI to demonstrate the role of serotonin in disease progression.
For the study, the Hopkins scientists recruited 49 volunteers with MCI, and 45 healthy adults ages 55 and older who underwent an MRI to measure changes in brain structure and two positron emission tomography (PET) scans of their brains at Johns Hopkins between 2009 and 2022. The research team used PET scans to look specifically at the serotonin transporter – a neurotransmitter, or brain chemical long associated with positive mood, appetite and sleep – and to look at the amyloid-beta protein (Aβ) distribution in the brain.
Researchers found that MCI patients had lower levels of the serotonin transporter, and higher levels of Aβ than healthy controls. The MCI patients had up to 25% lower serotonin transporter levels in cortical and limbic regions than healthy controls. In particular, they report, lower serotonin transporter levels were found in cortical, limbic, and subcortical regions of the brains in those with MCI, areas specifically responsible for executive function, emotion, and memory.
Researchers say future studies include longitudinal follow up of individuals with MCI to compare serotonin degeneration to the increase in and Aβ levels, as well as the increase in levels of the Tau protein that is also associated with AD compared to healthy adults. They are also studying multi-modal antidepressant drugs to treat depression and memory deficits in hopes of mitigating and halting symptoms.
