"The study is an exciting advancement in the development of a treatment for the cognitive and neuropsychiatric symptoms of AD," wrote Michele York, Baylor College of Medicine, Houston, Texas, to ARF in an e-mail. However, it is still in its early days, cautioned York, who was not involved in this study. "This avenue of research will require a large, well-controlled, and thoroughly described investigation ... prior to it being offered as a therapeutic intervention."

In 2010, Lozano's group implanted electrodes near the fornix—the fiber bundle that connects the hippocampus and hypothalamus—in six patients diagnosed with AD. After one year of stimulation with the electrodes, positron emission tomography scans using the radiotracer [18F]-2-deoxy-2-fluoro-D-glucose (FDG-PET) indicated that certain brain structures used more glucose at rest than they had before, meaning those regions were consuming more sugar and being more active (see ARF related news story on Laxton et al., 2010). In typical AD patients, glucose metabolism falls as the disease progresses, so this hinted at some improvement. Hippocampal volume also declined more slowly than expected in AD, or even grew a bit, as presented at the Society for Neuroscience meeting in 2011. A manuscript on those results is in preparation, said Smith.

In the current paper, first author Gwenn Smith, Johns Hopkins University, Baltimore, Maryland, and colleagues reanalyzed the FDG-PET data for five of those patients (one of the six was tested on a different scanner and was excluded from these analyses). The authors first looked for a correlation between baseline glucose use, improvement in glucose metabolism, and cognitive scores. All patients showed revved-up glucose metabolism after one year of DBS, but metabolism perked up more in those who started the study with the highest initial glucose use. These same high-baseline individuals either raised their scores on the Alzheimer’s Disease Assessment Scale-cognitive subscale (ADAS-cog), or at least declined less than expected in AD. This means that baseline glucose use might eventually predict the benefits of DBS, the authors wrote.

"That suggests the circuits you are targeting need to be in pretty good shape to be sustained; if they're already broken, DBS is not going to mend them," said Paul Frankland, University of Toronto, who was not involved with this study. In previous collaboration with Lozano, Frankland reported that spatial memory improved after DBS stimulation of the entorhinal cortex in wild-type mice (see ARF related news story). Another paper suggested that DBS applied in the fornix region overcomes memory impairment in rats (see Hescham et al., 2012), and Frankland will soon publish results that suggest similar benefits of DBS in AD mouse models.

The researchers further report that the structures with heightened glucose metabolism comprised two separate neural networks that showed strengthened functional connectivity. One was a frontal-temporal-parietal-striatal-thalamic network, the other a frontal-temporal-parietal-occipital-hippocampal network. Their glucose use rose simultaneously. "This shows a contrast with what we know about the course of Alzheimer's disease," said Smith. Previous studies have reported that functional connectivity breaks down over the course of AD (see ARF related news story on Sperling et al., 2009; Allen et al., 2007, and Damoiseaux et al., 2012). What mechanisms might underlie the apparent benefit of DBS? Candidates include neurogenesis and heightened neurotransmitter or neurotrophic factor release, the authors wrote. Future molecular imaging studies could be used to determine which is responsible.

Smith and her coauthors acknowledge that this is a tiny sample of patients without placebo controls. They are planning more rigorous clinical trials for people in early stages of AD. "In Alzheimer's, we are so anxious to find treatments that work," she said. "We want to be very careful. Before making any claims about using DBS on a larger scale or how effective it is, we really need to see more data." Other recent studies have suggested that reducing the brain's activity with pharmacologic agents may be a treatment for patients diagnosed with mild cognitive impairment (see ARF related news story and ARF news story). In Smith's, view, the two ideas are not at odds with each other, since each is targeting a different stage of the disease—hyperactivity in MCI and hypoactivity in AD.—Gwyneth Dickey Zakaib.

References:
Smith GS, Laxton AW, Tang-Wai DF, McAndrews MP, Diaconescu AO, Workman CI, Lozano AM. Increased Cerebral Metabolism After 1 Year of Deep-Brain Stimulation in Alzheimer Disease. Arch Neurol. 2012 May 7. Abstract