Ottersen OP, Helm PJ.
How hardwired is the brain?.
Nature. 2002 Dec 19-26;420(6917):751-2.
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In this week’s issue of Nature, two different groups report a technique to observe neurons in the brains of living, intact animals. Both groups demonstrated the ability to image various neuronal structures with two-photon microscopy in the brains of transgenic mice that express fluorescent proteins in neurons. Particular attention was paid to the dynamic behavior of dendritic spines, small protrusions along dendrites that are the primary sites of post-synaptic contacts. Trachtenberg, et al. report that about 60 percent of spines in the barrel cortex of six- to 10-week-old mice are stable when observed over an eight-day period, with the remainder of spines being highly dynamic in regards to their formation and elimination. In their study, the number of stable spines dropped to about 50 percent when tracked over a duration of one month, with larger spines appearing to be the most stable. The authors conclude that the high degree of turnover in the number of dendritic spines may contribute to the adaptive remodeling of complex neural circuits. Grutzendler, et al. report that dendritic filopodia in the visual cortex of one month-old animals are highly dynamic structures, but dendritic spines are highly stable (99 percent of spines were stable over a three-day period). This apparent stability did not change appreciably over a one-to-two month period, and many spines persist during a significant portion of an animal’s life. The authors believe that the stability they see in dendritic spines could potentially provide a structural basis for the storage and maintenance of long-term memories.
On the surface, the two manuscripts appear to present opposing findings on the stability of dendritic spines. However, the differences in both the age of the animals, the areas of the brain that were analyzed and the criteria of differentiating between a spine and a filapodium could explain some of the conflict. Furthermore, the different transgenic mice (2 independent lines) used by each group may not be equivalent in their ability to label all dendritic spines. In spite of these apparent discrepancies, both groups conclude that axons and dendrites are extremely stable structures in vivo, that dendritic spines are dynamic structures during development, and at least a subset of theses structures become stable during adulthood. These are both very interesting papers that demonstrate the power of these types of fluorescent mice in allowing one to image living neurons in vivo over extended periods of time. Application of these mice to disease biology will also likely yield important future results in regard to brain plasticity under different conditions.