11 March 2010. “Use it or lose it” has become a self-help mantra for people trying to beat the odds of getting Alzheimer disease (AD). But exactly how does using one’s brain defend against the dreaded loss of faculties? One hint comes in a soon-to-be-published paper in PNAS online. Researchers led by Christine Gall at the University of California, Irvine, report that learning triggers biological pathways normally switched on by brain-derived neurotrophic factor (BDNF). This neurotrophin, of which there is a dearth in AD brain, supports neurons in the hippocampus and cerebral cortex, regions hit earliest and hardest in the disease. In piecing together a chain of events, the researchers also found that a specific frequency of electrical stimulation, at the so-called theta band, mimics the effect of learning. Interestingly, theta power markedly wanes as people age. The finding suggests that electrical and molecular activities unleashed during learning boost BDNF’s beneficial effects and might be one way that “using it” might prevent one from “losing it.”
First author Lulu Chen and colleagues used a combination of tissue and animal studies to unravel the relationship between learning and BDNF signaling. For one, they employed restorative deconvolution microscopy to detect active forms of TrkB, a BDNF receptor, at synapses. Borrowed from astronomers, the deconvolution methodology uses software that builds 3D montages from 2D images taken at successive (in this case, half-micron) depths in the field of view. Because the technique works best for puncta, it is particularly suited to studying synaptic spines and boutons, noted Gall.
Chen and colleagues deconvoluted images from hippocampal sections after young adult rats were allowed 30 minutes of unsupervised learning. (For that, they were placed in a novel, complex environment.) Averaging 8,000 synapses per image, and 500,000 per animal, the researchers found that the learning more than doubled the number of synapses in the hippocampal CA1b field that lit up as containing both the post-synaptic marker PSD-95 and the activated, phosphorylated form of TrkB (pTrkB). The increase was blocked if the researchers first gave the animals CPP ((R,S)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphoric acid), an NMDA glutamate receptor antagonist. CPP had no effect on pTrkB levels in control animals not exposed to the learning environment. These findings indicated that learning induced TrkB activation in post-synaptic compartments.
Next, the scientists tackled memory encoding at a different level of investigation. Unsupervised exploration in rats is usually accompanied by bursts of electrical activity in the theta frequency range (3-8 Hz), and there is evidence that this theta activity is important for laying down stable memories. The authors wondered if theta burst stimulation (TBS) alone could activate TrkB. Using CA1 hippocampal slices, Chen and colleagues found that a single train of 10 theta bursts was sufficient to induce not only long-term potentiation (LTP) but also TrkB activation. Lower (0.05 Hz) frequency stimulation had no effect on LTP or TrkB, while higher (100 Hz) frequencies had some effect, but weaker than TBS.
The findings indicate that both learning and TBS activate post-synaptic TrkB, so the next question was whether that is a direct result of BDNF. The authors found that an antibody that binds TrkB but cannot pass the cell membrane blocked TBS activation of TrkB in hippocampal slices. This experiment suggested that extracellular ligands are responsible for the receptor activation. Since BDNF is by far the most potent and abundant TrkB ligand in the brain, chances are that it is the trophin at play, suggested Gall. NT4 is another possibility, she noted, though this trophic factor exists but in tiny amounts in the hippocampus. “We are 99.5 percent certain that BDNF is the ligand we are dealing with,” Gall said.
Previous work by a Japanese group has suggested a link between learning and increases in BDNF mRNA at a tissue level (see Mizuno et al., 2000) and also between TrkB activation and NMDA receptors (see Mizuno et al., 2003). In this paper, Gall and colleagues show how individual synapses are affected. Interestingly, relatively few synapses change with learning. While the increase in the number of pTrkB-positive synapses was large (more than twofold), at about 8 percent the total number of PSD95-labeled synapses activated by learning remained relatively small. This limited activation fits with modeling data that predicts memory is encoded by a very small proportion of synapses. “It accords with the observation that we have a very large memory capacity,” said Gall.
“I think what is really interesting is the pattern of firing that stimulates the BDNF,” suggested Kathryn Nichol, who studied the relationship between BDNF and cognition when at Carl Cotman’s lab, also at the University of California, Irvine. Nichol, who is now at Lundbeck Pharmaceuticals, also thought the results fit with the use-it-or-lose-it idea, since something as potent as learning or exercise only raises BDNF activation slightly. “It suggests there really is a molecular explanation for this idea,” she said.
The results may turn out to be relevant in the context of cognitive decline seen in normal aging and memory disorders such as Alzheimer’s. Theta band electrical activity becomes less robust as people age (see Klimesch et al., 1999), and while it is not clear if BDNF levels change during normal aging, levels of the trophin have been reported to be lower in AD brain (see Murray et al., 1994). Boosting BDNF may be worth exploring to treat age- or disease-related cognitive decline, but Gall cautioned that it’s not as simple as flooding the brain with BDNF. That was tried in ALS clinical trials and had unintended consequences, including weight loss (see ARF related news story on BDNF dosing effects). The trophin also has roles in addition to acute synaptic effects, such as promoting neuron survival. “When you start thinking about AD, you definitely want both cell survival and acute cognitive function effects,” said Gall. “We really want to understand the processes at both levels, so we can perhaps tweak things to optimize at both levels,” she said. One potential therapy Gall considers worth exploring is electrical stimulation. “It is possible that TBS could prove to be a useful therapy for cognitive disorders,” she suggested.
Chen LY, Rex CS, Sanaiha Y, Lynch G, Gall CM. Learning induces neurotrophin signaling at hippocampal synapses. PNAS 2010, March 1. Abstract