Plasticity and interaction after ablations of visual or somatosensory motor cortex or retina in neonatal rats

Changes of immunocytochemical localization of vesicular glutamate transporters in the rat visual system after the retinofugal denervation

To clarify which vesicular glutamate transporter (VGluT) is used by excitatory axon terminals of the retinofugal system, we examined immunoreactivities and mRNA signals for VGluT1 and VGluT2 in the rat retina and compared immunoreactivities for VGluT1 and VGluT2 in the retinorecipient regions using double immunofluorescence method, anterograde tracing, and immunoelectron microscopy. Furthermore, the changes of VGluT1 and VGluT2 immunoreactivities were studied after eyeball enucleation. Intense immunoreactivity and mRNA signal for VGluT2, but not for VGluT1 immunoreactivity, were observed in most perikarya of ganglion cells in the retina. Immunoelectron microscopy revealed that VGluT1- and VGluT2-immunolabeled terminals made asymmetrical synapses, suggesting that they were excitatory synapses, and that VGluT1-immunolabeled terminals were smaller than VGluT2-labeled ones in many retinorecipient regions, such as the dorsal lateral geniculate nucleus (LGd) and superior colliculus (SC). Double immunofluorescence study further revealed that almost no VGluT2 immunoreactivity was colocalized with VGluT1 in the retinorecipient regions. After wheat germ agglutinin (WGA) injection into the eyeballs, WGA immunoreactivity was colocalized in the single axon terminals of LGd and SC with VGluT2 but not VGluT1 immunoreactivity. After unilateral enucleation, VGluT2 immunoreactivity in the LGd, SC, nucleus of the optic tract, and nuclei of the accessory optic tract in the contralateral side of the enucleated eye was clearly decreased. Although only a small change of VGluT2 immunoreactivity was observed in the contra- and ipsilateral suprachiasmatic nuclei, olivary pretectal nucleus, anterior pretectal nucleus, and posterior pretectal nucleus, moderate reduction of VGluT2 was found in these regions after bilateral enucleation. On the other hand, almost no change in VGluT1 immunoreactivity was found in the structures examined in the present enucleation study. Thus, the present results support the notion that the retinofugal pathways are glutamatergic, and indicate that VGluT2, but not VGluT1, is employed for accumulating glutamate into synaptic vesicles of retinofugal axons.

Projection lines and the ipsilateral retino-geniculate pathway in the hooded rat

The organization of the hooded rat's dorsal lateral geniculate nucleus was studied with anatomical techniques, with particular regard to the representation of temporal retina and the binocular field. The ipsilateral and contralateral retinal terminal fields were examined in three stereotaxic planes following injections of horseradish peroxidase into the eye. Projections arising from the temporal crescent of the retina were studied with silver staining techniques for anterograde degeneration products. Following discrete retinal lesions there was clear evidence that the temporal retina projects in a topographic fashion both ipsilaterally and contralaterally. The orientation of the lines of projection in the dorsal lateral geniculate nucleus was assessed by retrograde labelling of cells after cortical implants of horseradish peroxidase. Although both the lines of projection and the ipsilateral terminal field extend rostro-caudally in the dorsal lateral geniculate nucleus, their paths are oblique rather than parallel. Their intersection appears to correspond to the representation in this nucleus of conjugate retinal points. This was confirmed by administering horseradish peroxidase by iontophoresis in either the binocular or monocular representation of the primary visual cortex, while one eye received an injection of [3H]proline. Only those cortical injections in the binocular region gave rise to labelled projection lines passing through the autoradiographically-labelled ipsilateral terminal field. The rat's dorsal lateral geniculate nucleus displays none of the cytoarchitectural lamination which is so prominent in the primate and cat. Even after labelling the input to the nucleus from one eye, there is still no obvious laminar relationship between the terminal fields from the two eyes. Despite the absence of lamination, the current results suggest that the principle of apposing the representation of conjugate retinal points in the dorsal lateral geniculate nucleus is the same in the rat as in cat and monkey.

Corticopontine remodelling after cortical and/or cerebellar lesions in newborn rats

The distribution of corticopontine projections was studied, primarily with the Fink-Heimer technique, in adult rats which at 2-3 days of age had sustained unilateral sensorimotor (SMC) or occipital (OC) cortical lesions and in animal that had received neonatal hemocerebellar (HCb) lesions singly or in combination with the neonatal SMC or OC lesions. Neonatal left SMC or left HCb lesions induced an anomalous increase in crossed sensorimotor corticopontine (SMP) projections originating from the right SMC. This increase appeared more dense after cortical lesions as compared to cerebellar lesions and appeared more dense in those animals receiving the combined SMC plus HCb lesions. No alterations in the distribution of SMP fibers were observed after neonatal OC lesions nor were any alterations of occipital corticopontine (OP) fibers observed in any of the neonatal lesion groups. The remodelling of right SMP projections after neonatal left SMC lesions demonstrates an interaction between corresponding pathways originating from opposite sides of the brain and which occurs in response to the partial removal of afferents to pontine gray neurons as a result of the SMC lesions. In contrast, the absence of SMP plasticity after OC lesions, and vice versa, demonstrates the absence of plasticity between pathways which differ anatomically and functionally. The substantial neuronal loss which occurs within the right PG after neonatal left HCb lesions is believed to influence the increase in crossed SMP fibers coursing from the right hemisphere to the larger left PG. No increase in crossed OP fibers from the right hemisphere was observed in these animals. The increase in crossed SMP fibers after neonatal SMC lesions combined with neonatal HCb lesions is more dense than found in the single lesion groups. These findings suggest that separate mechanisms of plasticity might be operational for each lesion and that these mechanisms may be additive.

The organization of subcortical projections of the hamster's visual cortex

The subcortical projections of the hamster's visual cortex were determined by use of injections of tritiated proline and heat lesions placed in different cortical loci. The brains were processed for autoradiography and silver impregnation of degenerating axons. Striate cortex was shown to project ipsilaterally to the dorsocaudal region of the caudate nucleus, a dorsolateral area within the thalamic reticular nucleus (RT), a laterodorsal region of the nucleus lateralis anterior (LA), the rostral half of nucleus lateralis posterior (LP), the whole territory of the dorsal (dLGN) and ventral (vLGN) geniculate nuclei, the anterior (PA) and posterior (PP) pretectal nuclei, the superior colliculus (SC), and the precerebellar pontine nuclei. In addition, the medial visual area (18b) was shown to project to a medial band of LA and part of the caudal half of LP, while the adjoining parietal cortex was seen to terminate in a lateral part of the caudate, a ventral band of LA, and the ventral half of rostral LP. Segregation of different cortical inputs was clear in LA, LP, caudate, and pons. The projections to dLGN, vLGN, SC, LP, and PA were retinotopically organized. Clear evidence of some topography was found within RT, PP, and the pons, although a consistent map could not be derived from the data.

Cortical projections to visual centres in the rat: an HRP study

We have investigated the relationships of the visual cortex to other visual centres in the rat: namely the lateral geniculate nucleus, the visually responsive part of the thalamic reticular nucleus and the superior colliculus. We injected horseradish peroxidase iontophoretically so as to restrict the injectate to each of the regions, and reacted sections using 3 different procedures. Areas 17, 18 and 18a project to both dorsal and ventral lateral geniculate nucleus as well as to the visually responsive part of the thalamic reticular nucleus and superior colliculus. Pyramidal cells in lamina VI project to the dorsal lateral geniculate nucleus and to the thalamic reticular nucleus, whereas cells of origin of the projection to the superior colliculus lie in lamina V; cells in lamina V also project to ventral lateral geniculate nucleus. The implications of these findings are discussed, particularly in terms of the functional relationships between the visual cortex, lateral geniculate nucleus and visual thalamic reticular nucleus.