BRAIN-STEM INPUTS TO THE FERRET MEDIAL GENICULATE-NUCLEUS AND THE EFFECT OF EARLY DEAFFERENTATION ON NOVEL RETINAL PROJECTIONS TO THE AUDITORY THALAMUS

Following specific neonatal brain lesions in rodents and ferrets, reti nal axons have been induced to innervate the medial geniculate nucleus (MGN). Previous studies have suggested that reduction of normal retin al targets along with deafferentation of the MGN are two concurrent fa ctors required for the induction of novel retino-MGN projections. We h ave examined, in ferrets, the relative influence of these two factors on the extent of the novel retinal projection. We first characterized the inputs to the normal MGN, and the most effective combination of ne onatal lesions to deafferent this nucleus, by injecting retrograde tra cers into the MGN of normal and neonatally operated adult ferrets, res pectively. In a second group of experiments, newborn ferrets received different combinations of lesions of normal retinal targets and MGN af ferents. The resulting extent of retino-MGN projections was estimated for each case at adulthood, by using intraocular injections of anterog rade tracers. We found that the extent of retino-MGN projections corre lates well with the extent of MGN deafferentation, but not with extent of removal of normal retinal targets. Indeed, the presence of at leas t some normal retinal targets seems necessary for the formation of ret ino-MGN connections. The diameters of retino-MGN axons suggest that mo re than one type of retinal ganglion cells innervate the MGN under a l esion paradigm that spares the visual cortex and lateral geniculate nu cleus. We also found that, after extensive deafferentation of MGN, oth er axonal systems in addition to retinal axons project ectopically to the MGN.These data are consistent with the idea that ectopic retino-MG N projections develop by sprouting of axon collaterals in response to signals arising from the deafferented nucleus, and that these axons co mpete with other sets of axons for terminal space in the MGN. (C) 1998 Wiley-Liss, Inc.