Effects of monocular enucleation on receptor binding and innervation pattern of the noradrenergic system in the superior colliculus of the pigmented rat

To examine visual activity-dependent regulation of the central noradrenergic (NAergic) system, we carried out in vitro autoradiography for α2-, β-adrenergic and NMDA, non-NMDA glutamatergic receptors together with immunohistochemical analysis for dopamine β-hydroxylase in the major central visual structures, including the lateral geniculate nucleus, superior colliculus (SC), and visual cortex of pigmented rats, that had received monocular enucleation. β-Adrenergic receptor (β-AR) binding in the SC contralateral to the enucleated eye was significantly decreased (82%, P<0.01) following monocular deprivation started at P12 and continued for 2 or 11 weeks. No significant change in β-AR binding was found in other structures examined. The number of varicosities in NAergic fibers was significantly increased following longer enucleation, i.e. for 11 weeks, in the contralateral SC (197%, P<0.001), whereas that in the ipsilateral SC was reduced (75%, P<0.001). Changes in α2-adrenergic, NMDA, and non-NMDA glutamatergic receptor binding were small in these animals. Changes in neither β-AR binding nor innervation pattern of NAergic fibers were found in one-year-old rats that had received a comparable period of monocular enucleation. Furthermore, neither unilateral ablation of the visual cortex to reduce a different set of major afferents nor neonatal enucleation, which induced anatomical reorganization of the afferents, was found to be effective. These findings suggest that β-AR binding and innervation pattern of NAergic fibers in the SC are modified only when massive imbalance of retinal afferent activity is imposed during a limited period in early postnatal life (i.e. the sensitive period).