Declines in neuronal activity in Alzheimer disease are not surprising, given the loss of synapses and dendritic spines that occurs even in early stages of the disease. In agreement with this idea, previous studies examining Alzheimer disease animal models showed that amyloid-β (Aβ) inhibits synaptic currents. For example, Hsia et al. (1999) found that declines in synaptic density, neurons, and synaptic transmission precede amyloid deposition in APP transgenic mice. Chang et al. (2006) observed that AMPA receptor-mediated currents and spontaneous neuronal currents decline in double amyloid-precursor protein and presenilin knock-in mice. What made the current study different from these, and similar, investigations is that it suggests that amyloid deposition may not simply suppress neuronal activity, but may create an imbalance, specifically causing both hyperactivity and reduced activity in brain circuits.

The investigators simultaneously visualized cortical neurons and amyloid plaques by injecting the calcium-indicator dye Oregon Green 488 BAPTA-1 AM and fluorescent thioflavin S, which labels fibrillar Aβ. Injections were made into the cortex. Calcium imaging is known to indicate action potential firing in neurons, so by monitoring cortical calcium activity the researchers were able to infer increases or decreases in cortical neuronal activity. Stable cortical neuronal activity was detected in the cortices of wild-type mice, but in the double transgenic mice, cortical activity was quite different. Only about half of the neurons in the transgenic animals showed normal activity, whereas the remaining cortical cells were either abnormally silent or hyperactive. Hyperactive neurons were found only within very close proximity to the border of plaques, whereas silent cells and normally active cells were found throughout the cortex. Proportions of silent cells increased as the distance from the plaques increased.

The scientists wondered if the same type of transgenic animals in which amyloid plaques had not yet been deposited also had similar aberrant neuronal activity. They repeated their calcium imaging studies in animals between 1.5-2 months of age. These double transgenic mice had normal cortical activity, comparable to wild-type mice. They performed as well on tests of spatial memory—the Morris water maze and Y-maze—as wild-type animals. In contrast, six- or eight-month-old double transgenic mice with amyloid plaques performed poorly on these tests of memory.

Busche and colleagues were then interested in understanding the source of this hyperactivity in the vicinity of plaques. To determine that the increases in calcium activity (transients) were caused by action potential firing and not another source of calcium release, the researchers blocked calcium transients using the sodium-channel blocker, tetrodotoxin. This was effective in all four of the animals tested. They were also interested in knowing whether the hyperactive neurons possessed more intrinsic excitability or whether increased synaptic activity was causing the hyperactivity. Because ionotropic glutamate receptor blockers abolished all calcium transients in these neurons, the researchers concluded that increased synaptic activity was occurring, not increased intrinsic neuronal excitability. They speculated that decreased inhibition via the neurotransmitter GABA could be causing the hyperactivity, since the benzodiazepine, diazepam (which increases the opening of GABAA receptor channels) reduced the hyperactivity. Antagonizing GABAA receptors with gabazine increased calcium transients in the hyperactive neurons, further supporting the possibility that the cells were being disinhibited via decreased GABA activity.

Olga Garaschuk, one of the study investigators, stated in an e-mail to Alzforum that “…this study improves our understanding of the pathophysiological mechanisms of Aβ amyloidosis. It also identifies hyperactive cells as a possible target for pharmacological treatment. Fortunate location of these cells in the vicinity of Aβ plaques makes targeting of pharmacological substances easier, because many substances are binding or can be targeted to plaques.”

The physiological consequence of hyperactive neurons near plaques still remains to be understood, and similar activity in humans with Alzheimer disease needs to be confirmed. Interestingly, epileptic seizure incidence increases in people with Alzheimer disease. Similar hyperactive neurons may exist near neuronal plaques in the brains of people with Alzheimer disease, potentially causing seizures. Another physiological consequence implied by these findings could be that imbalances in cortical neuronal activity could contribute to cognitive dysfunction. This idea is particularly supported by the observation that memory impairments in the transgenic mice corresponded with Aβ deposition and the presence of hyperactive neurons. If either or both of these possibilities turn out to be the case, targeting hyperactive neurons might inhibit seizures or even improve memory.

Another study has, in fact, indicated possible changes in the balance between hyperactivity and reduced activity in an animal model of Alzheimer disease, and further suggests an association of this activity with seizures. Researchers led by Lennart Mucke described spontaneous, non-convulsive seizures in transgenic mice that overproduce the human amyloid precursor protein (see Palop et al., 2007 and ARF related news story). Seizures (in other words, hyperactive neurons), measured in both cortical and hippocampal neurons, were accompanied by increases in sprouting of GABAergic axons and synaptic inhibition, specifically in the dentate gyrus of the hippocampus, an area critical to formation of certain types of memory. Palop and colleagues proposed that activation of those inhibitory circuits was to compensate for the hyperactivity in the cortical and hippocampal networks.

Although the scientists who conducted the current study have not yet observed the hyperactive cortical neurons over the long term, future investigations examining the ultimate fate of these cells may be in the works. When asked about the prospect of imaging activity in these neurons over a longer duration, Garaschuk told ARF via an e-mail, “already this is now possible. However, the cells would need to be re-stained with a calcium indicator dye (by inserting a micropipette into the brain) each time before imaging.” She noted that a recent study published in Nature Methods (Mank et al., 2008) described the advance of a genetically encoded calcium indicator TN-XXL, which allowed calcium increases in neurons to be measured repeatedly, even up to several weeks. This technology could allow for long-term calcium imaging of the hyperactive neurons associated with amyloid plaques, further elucidating the role that these cells could potentially play in the brains of people with Alzheimer disease.—Alisa Woods.

Alisa Woods is a freelance writer in Brooklyn, New York.

Reference:
Busche MA, Eichhoff G, Adelsberger H, Abramowski D, Wiederhold K-H, Haass C, Staufenbiel M, Konnerth A, Garaschuk O. Clusters of Hyperactive Neurons Near Amyloid Plaques in a Mouse Model of Alzheimer’s Disease. Science. 2008 Sept 19;321:1686-9.