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Figure 1 EphA tyrosine kinase receptors are expressed in a nasotemporally increasing gradient on retinal ganglion cells and ephrin-A receptors are expressed in a reverse gradient within the SC. Repulsive interactions between ligand and receptor-containing neurons cause temporal retinal axons to be repelled from posterior SC. The combined action across the gradient sets up the rostrocaudal topography of the retinocollicular map. Adapted from Cheng, H. J., Nakamoto, M., Bergemann, A. D., and Flanagan, J. G. 1995. Complementary gradients in expression and binding of ELF-1 and Mek 4 in developement of the topographic retinotectal projection map. Cell 82, 371-381.

Figure 1 EphA tyrosine kinase receptors are expressed in a nasotemporally increasing gradient on retinal ganglion cells and ephrin-A receptors are expressed in a reverse gradient within the SC. Repulsive interactions between ligand and receptor-containing neurons cause temporal retinal axons to be repelled from posterior SC. The combined action across the gradient sets up the rostrocaudal topography of the retinocollicular map. Adapted from Cheng, H. J., Nakamoto, M., Bergemann, A. D., and Flanagan, J. G. 1995. Complementary gradients in expression and binding of ELF-1 and Mek 4 in developement of the topographic retinotectal projection map. Cell 82, 371-381.

1998; Frisen et al., 1998; Yates et al., 2001). This raises the question, if ephrin distributions in tectum define the termination sites of retinal axons, how can the retinocollicular map compress after caudal SC injury? It has been suggested that the ephrin gradients must be altered for compression to occur (Schmidt, 1982). Alternatively, retinal axons may be responding to relative rather than absolute expression levels of ephrins (Brown et al., 2000). To investigate this question, we quantified ephrin-A expression levels in normal and compressed retino-SC maps in hamsters. We found that the reduction in target size causes a compensatory alteration in ephrin expression such that the pitch of the ephrin-A gradient is steeper in the smaller target (Tadesse et al., 2004). Viewed from an evolutionary perspective, alterations in the size of a target area may produce changes in mapping cues that lead to compensatory alterations in topography, thus smoothly integrating the change in neuronal number.

1.10.3.1.2 Role of NMDA receptors in conservation of response properties It might be expected that compression of the map would result in compromised visual function. In a series of studies, we have shown that this is not the case. Instead, the circuitry within the SC compensates for the loss of target in a way that preserves the response properties of the component neurons. Rather than increasing the convergence ratio between ganglion cells and SC neurons, the retinal axon arbors are pruned in such a way that the receptive field size of individual SC neurons is conserved (Figure 2, top) (Pallas and Finlay, 1989,1991; Xiong etal., 1994). The mechanism underlying this conservation of receptive field size is the same as that operating during normal development of the map; through N-methyl-D-aspar-tate receptor (NMDAR)-dependent long term potentiation (LTP) (Constantine-Paton and Cline, 1998). Chronic blockade of NMDAR in normal SC prevents the activity-dependent refinement of retino-SC projections, resulting in stabilization of the initial, overlapped ganglion cell terminals. As a result, receptive fields of SC neurons remain larger than in normal adults (Huang and Pallas, 2001; see also Simon et al., 1992). Combining NMDAR blockade with map compression results in further enlargement of receptive fields, because both the normal refinement process and the compensation process are blocked (Figure 2, bottom).

1.10.3.1.3 Role of lateral inhibition in conservation of response properties Curiously, we found that response properties such as stimulus size and velocity tuning were unaffected by either the map

Early PT (Two models)

(1) Arborization reduction (2) Selective divergence

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