Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC292
Measuring the dynamics of retino-collicular map formation in the mouse
D. Lyngholm1, D. C. Sterratt2, I. D. Thompson1
1. MRC Centre for Developmental Neurobiology, King's College London, London, United Kingdom. 2. Institute for Adaptive and Neural Computation, School of Informatics, The University of Edinburgh, Edinburgh, United Kingdom.
Development of retinotopy in the superior colliculus is an area of longstanding investigation. Most studies, however, have yielded mainly qualitative descriptions of the temporal sequences of the topographic development. Here we provide a quantitative description of the temporal sequence of early postnatal topographic development in the superior colliculus of wild-type (C57BL/6J) and genetically manipulated mice to inform modelling studies of map formation. The study has been done mainly by utilizing retrograde transport of fluorescent microspheres administered by discrete injections into the superior colliculus in the neonate. All neonatal (P0-P7) animals were anaesthetised using intraperitoneal injection of Ketamine (100-130mg/kg) and Xylazine (10-13mg/kg). Older animals were anaesthetized using inhalation of Isoflurane (2%; 0.4-0.6l/min). After fixation perfusion under terminal anaesthesia (300mg/kg pentobarbital sodium), the retinae were removed, flat-mounted and the distributions of labeled ganglion cells plotted. As one measure of order, we take the collicular separation of paired tracer injections that result in separate retinal foci. We find that there are distinct phases of early postnatal refinement. At birth there is only order along the mediolateral (ML) collicular axis. Evidence of order in the anteroposterior (AP) collicular axis is first seen at P2. By P8 ordering in the retinal input has a precision of 315±13μm at P8. Precision refines gradually to reach an adult value of 120±16μm. Using mice lacking the beta2 subunit of the nicotinic acetylcholine receptor (Picciotto et al. 1995), we provide further evidence for a brief activity-dependent critical period for refinement between P4 and P8, showing that precision at P4 is of a similar magnitude as the wild-type, whereas the projection at P8 is significantly less ordered in nAChR-β2 -/- animals. While there is some subsequent refinement, the input remains significantly less ordered into adulthood.
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