Don’t Panic! – Mice Aren’t Actually the Smartest
“Man had always assumed that he was more intelligent than dolphins because he had achieved so much — the wheel, New York, wars and so on — whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man — for precisely the same reasons….In fact there was only one species on the planet more intelligent than dolphins, and they spent a lot of their time in behavioural research laboratories running round inside wheels and conducting frighteningly elegant and subtle experiments on man. The fact that once again man completely misinterpreted this relationship was entirely according to these creatures’ plans.” – Douglas Adams, The Hitchhiker’s Guide to the Galaxy
As tempting as it may be to believe the science fiction version of the intelligence rankings, real-life science has spoken and suggests (much to my displeasure) that humans may actually be the highest on the intelligence scale.
Glia are non-neuronal cells found in the brain mainly described as performing “housekeeping” functions, for example, providing structural support to neurons, and providing them with nutrients. Astrocytes are a specific type of glia, and as one might hypothesize, they are bigger in humans than in mice. Was this just a consequence of humans having more complex brains, or do these astrocytes have different functions in humans beyond the basic housekeeping functions? To test this, scientists grafted human astrocyte progenitor cells into developing mouse brains to create chimeric mice.
Human astrocyte (green) and mouse astrocyte (red)
The human astrocytes that matured successfully matured as human cells; characteristics such as their size were unaffected by being in a mouse environment. But they did not remain completely foreign – they successfully formed electrical connections with the mouse cells. Their differing cellular properties were thus propagated into the mouse neural networks. Of particular interest is the hippocampus, the brain region important for learning and memory. Chimeric hippocampal slices had a higher level of baseline excitatory activity, and long-term potentiation (LTP), or synapse strengthening, was much greater. At the molecular level, this can be explained because the human cells express higher levels of a protein that promotes an increased number of glutamate receptors at the synapse.
There were also clear differences in the behavior of chimeric mice. Experiments were performed to test learning and memory abilities to corroborate the cellular results observed in the hippocampus. A classic fear conditioning experiment involves pairing a tone with a foot shock; mice learn to associate the two and exhibit freezing behavior after hearing a tone. Chimeras learned the association after only one tone/shock pairing. The learning persisted for several days, during which time control animals did not learn the initial association. The experiment was repeated as context fear conditioning, meaning that the mice were placed in different chambers that had varying floors and odors. Chimeric mice were able to differentiate between chambers significantly better than their control counterparts. In other learning and memory tasks, these mice learned their way through mazes faster and were better at familiar object recognition in novel contexts.
The results of this study show that glial cells have much more function beyond their basic housekeeping properties. A single cell graft manipulation was enough to significantly improve mouse performance on learning and memory tasks. Complexity of these cells has evolved with the brain, and this provides important new insight on how exactly this complexity has come to be. Future experiments could involve grafting chimpanzee or macaque glia, any differences observed could be key in outlining how our processing abilities evolved from our monkey fathers (I additionally support research with dolphin glia grafts, keeping on the theme of the three most intelligent species). Unfortunately, without the higher processing abilities made possible by human cells, mice likely cannot achieve the tasks and level of status they exhibit in the science fiction. It seems as though man has indeed correctly interpreted his relationship with the mouse.
So long, and thanks for all the fish.
-Reena Clements
References:
Human Brain Cells Boost Mouse Memory – ScienceNOW
Forebrain Engraftment by Human Glial Progenitor Cells Enhances Synaptic Plasticity and Learning in Adult Mice – Cell Stem Cell
