Effects of neonatal enucleation on excitatory and inhibitory organizations of the albino rat lateral geniculate nucleus

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Abstract
Monocular enucleation in the neonatal period results in a marked expansion of the ipsilateral retinogeniculate projection in rats. In 26 young adult albino rats with the left eye enucleated within 24 h of birth, properties of relay (principal, P) cell responses to photic and electrical stimuli were studied for the dorsal lateral geniculate nucleus (LGD), ipsilateral (ipsi) and contralateral (contra) to the remaining eye. The P-cells of the contra LGD in neonatally enucleated rats were normal in response properties when compared with those of the LGD that receive contra retinogeniculate projections in normal rats. Physiological samples of P-cells that receive ipsi retinogeniculate projections were relatively easily obtained in neonatally enucleated rats as compared with those in normal rats. Though these P-cells of the ipsi LGD in enucleated rats functioned physiologically, their properties were different in various ways as compared with those of contra LGD: response latencies to single optic chiasm shocks were distributed, on average, in a slightly longer range; correlation was less clear between the latencies of orthodromic activation by single optic chiasm shocks and those of antidromic activation by visual cortical stimulation; and responses to stationary spots of light were weak and receptive-field center sizes were significantly larger in diameter. Representations of the remaining retina on the ipsi and contra LGD of enucleated rats were assessed by studying the distribution of receptive-field centers of the sampled P-cells from the 2 sides. The characteristic representation in enucleated rats as compared with normal rats was that the temporal retina is more heavily represented in the ipsi LGD, accompanied by a relatively weak representation of it in the contra LGD. The intrageniculate inhibition that was exerted on P-cells after the initial excitation by single optic chiasm shocks was also shown to be abnormal in the ipsi P-cells. Some P-cells showed only a short-lasting weak inhibition, others a strong long-lasting inhibition, in contrast to the relatively constant length of the inhibition (100-150 ms) in P-cells of the contra LGD. In the ipsi side, the response magnitude of perigeniculate reticular cells, which are known to be a main source of the intrageniculate inhibition was decreased when tested by optic chiasm shocks, while it was significantly enhanced when tested by visual cortical shocks. This also contrasts with responses of the perigeniculate reticular cells innervated by the contra projection. While the aberrant ipsi retinogeniculate projection functions to some extent, considerable disorganization was apparently induced within the LGD at both excitatory and inhibitory synapses. As a possible mechanism for the aberrant ipsi retinogeniculate projection, rerouting of ganglion cell axons from the contralateral to the ipsilateral hemisphere is suggested.