NEUROSCIENCE: Topographic Map Labels

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Science  14 Apr 2000:
Vol. 288, Issue 5464, pp. 229f-229
DOI: 10.1126/science.288.5464.229f

Some sensory input channels to the mammalian brain, such as vision or touch, originate as topographically organized information. In order to retain visual information as the signals are transferred from the retina to the superior colliculus, the axons from one region must connect to their target regions in a similarly organized manner. Indeed, axons originating from the temporal or nasal portions of the retina are destined to form connections in the anterior or posterior regions, respectively, of the superior colliculus. The signals that regulate the axonal map-forming capability include the ephrin ligands, expressed in gradients across the superior colliculus, and their corresponding Eph receptors, expressed in gradients across the retina.

Feldheim et al. have generated mice carrying mutations in ephrins A2 and A5, and analyzed the effects on retino-collicular connections. The results indicate that with the loss of the inhibitory ephrin signal, axons carrying the corresponding receptors are less restricted in their choice of targets. The outcome is not simply a lessening of inhibition, but rather a lessening of discriminating competition between axons. A corresponding pairing of ephrin A5 and the EphA4 receptor has been observed by Vanderhaeghen et al. in the somatosensory cortex and thalamus, respectively, and is thought to contribute to the topographic mapping of touch.—PJH

Neuron 25, 563 (2000); Nature Neurosci. 4, 358 (2000).

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