Sprouting of ipsilateral retinal projections in the optic system of the albino rat

Sequential events of degeneration and synaptic remodelling in the viper optic tectum following retinal ablation. A degeneration, radioautographic and immunocytochemical study

The ultrastructural changes taking place in the retino-recipient layers of the viper optic tectum were examined between 5 and 122 days after retinal ablation. The initial degeneration of retinotectal terminals proceeds at widely different rates and is characterized by a marked degree of polymorphism in which a number of different patterns can be discerned. In the final stages of degeneration, either both the degenerating bouton and the distal portion of the postsynaptic element are engulfed by reactive glia, or, more frequently, only the degenerating terminal is eliminated and the postsynaptic differentiation remains. The free postsynaptic differentiations are reoccupied predominantly by boutons containing pleiomorphic vesicles and which are for the most part gamma-aminobutyric acid (GABA)ergic, thus forming heterologous synapses; less frequently these sites are occupied by boutons of the ipsilateral visual contingent to form homologous synapses. These two processes, both of which depend on terminal axonal sprouting, take place within the first 3 postoperative months. They are followed by a decrease in the number of heterologous synapses and a concurrent increase in the number of homologous synapses newly formed by optic boutons generated by collateral preterminal sprouting of ipsilateral retinotectal fibres. The data suggest that partial deafferentation of the optic tectum induces a transitory GABAergic innervation of free postsynaptic sites prior to the restoration of new retinal synaptic contacts.

Different metabolic changes in the lateral geniculate nucleus and the superior colliculus of adult rats after simultaneous or delayed double enucleation

By means of the [14C]deoxyglucose method the local cerebral glucose utilization (LCGU) was measured in the lateral geniculate nucleus and the superior colliculus of rats with a simultaneous double enucleation, animals with an enucleation of the right eye between neonatal and adult stages followed by an enucleation of the left eye some months later and adult control rats. The control animals show LCGU values within the limits published by other observers. The LCGU values of the simultaneously double-enucleated rats are reduced on both sides to the same extent by about 20%. The dorsal lateral geniculate nucleus shows the largest decrease. The LCGU values of the rats with the delayed double enucleation have decreased even more (up to 37%). The right sided visual brain regions have a significantly lower LCGU than the corresponding regions on the left side. These findings indicate that unilateral enucleation from neonatal to adult stages leads to adaptive changes resulting in a higher metabolic vulnerability, which is revealed by the second enucleation.

Effects of the unilateral striatal lesion on neurotransmitter markers in the contralateral striatum and both substantia nigrae of the rat

We investigated quantitative changes in possible neurotransmitters and their biosynthetic enzymes in the contralateral striatum and both substantia nigrae following unilateral electrothermic lesions of the striatum in the rat. Two types of changes were observed: (1) the first ones were long-lasting (up to 56 post-operative days) effects and consisted in a decrease of GABA content and tyrosine hydroxylase activity in the ipsilateral substantia nigra due to the anterograde and retrograde degeneration of striatal efferent and afferent fibres, respectively, and in a marked increase of glutamate and GABA contents in the contralateral striatum resulting possibly from a modified activity of the collaterals of the glutamatergic corticostriatal fibres and a subsequent secondary increase of GABA. The latter interpretation was supported by the finding that the changes observed were abolished by an additional callosal transection; (2) the second series of changes were transient (only found at 3-7 post-operative days) effects represented by an increase in GABA content, a decrease of tyrosine hydroxylase activity, and a decrease of dopamine content, which mostly appeared in the contralateral substantia nigra. The decrease of dopamine markers may be a subsequent secondary effect of the increase of GABA in the substantia nigra. These results suggest that the contralateral increase of the amino acid transmitters in the striatum and the increase followed by decrease of transmitter markers in the contralateral substantia nigra could be a "plastic" or "compensatory" biochemical response to the unilateral striatal lesions.

Changes of cytochrome oxidase activity in the rat subcortical visual centers after unilateral eye enucleation

Histochemical densitometric measurements of cytochrome oxidase (CO) activity in the dorsal and ventral lateral geniculate nuclei (LGNd and LGNv) and the superficial gray stratum of the superior colliculus (SC) were done between 2 and 12 weeks after unilateral enucleation in the adult albino rat. This deafferentation resulted in a transient, slight but statistically significant decrease in CO activity in the contralateral LGNd and LGNv. Whereas in the contralateral SC, a marked reduction in CO activity was observed, and this persisted over the postoperative periods studied. The regional difference in the recovery pattern of CO activity were discussed from the aspect of synaptic reorganization.

Expansion of the ipsilateral retinal projection to the medial terminal nucleus of the accessory optic system in rats with one eye removed

Expansion of ipsilateral retinal inputs into terminal regions of the accessory optic system was assessed in rats with one eye removed either at birth or as adults and in one animal with a congenital absence of one eye. The increase in the ipsilateral projection to the medial terminal nucleus in animals receiving unilateral enucleation at birth was greater than after eye removal as an adult. The greatest increase was observed in the animal with the congenital loss of one eye. No ipsilateral projection to other nuclei of the accessory optic system was observed.

Visual discrimination after early and late unilateral enucleation of the rabbit

Brightness and tilt discrimination were studied in rabbits in which one eye had been enucleated either 1 day or 3 months after birth. When tested at the age of 5 months no differences in performance were found between both groups of animals.

Ipsilateral projections to the terminal nuclei of the accessory optic system in the albino rat

The projection of the retinal ganglion cells to the terminal nuclei of the accessory optic system has been re-examined in the albino rat. After intravitreal injection of [3H]leucine the contralateral and ipsilateral terminal nuclei were labeled. This complete ipsilateral projection in the rat has, of yet, not been documented.

Synapse formation after injury in the adult rat brain: failure of fimbrial axons to reinnervate the bed nucleus of the stria terminalis

Selectivity in the reinnervation of denervated postsynaptic sites in the adult rat septal nuclei has been studied by both light and electron microscopic degeneration techniques after lesions of the fimbria and stria terminalis. In the mid-rostrocaudal septum the ventral border of the lateral septal nucleus is coextensive with the dorsal border of the strial bed nucleus. In the normal rat, fimbrial axons establish synapses throughout the lateral septal nucleus of the same side, and also in the dorsal part of the lateral septal nucleus on the opposite side. The stria terminalis establishes synapses in the ipsilateral but not in the contralateral bed nucleus at this level. Both the fimbria and the stria terminalis were completely severed on the left side, and after adequate survival for the removal of all degeneration, the distribution of the remaining fimbria was plotted. Interesting changes were found on the side contralateral to the second lesion, where the fimbria both increases the number of its synaptic terminals within its proper contralateral territory (the dorsal part of the lateral septal nucleus) and also extends its distribution into the ventral part of the lateral septal nucleus--the territory normally reserved for the ipsilateral fimbria. Although completely surrounding the strial bed nucleus, fimbrial axons fail to invade the bed nucleus, and fimbrial terminals are therefore unable to reinnervate denervated strial postsynaptic sites. Since there are no obvious structural barriers between the neuropil of the lateral septal nucleus and that of the strial bed nucleus it is suggested that this failure is most likely to be due either to some biochemical incompatibility between fimbrial axons and strial postsynaptic sites, or to the fact that the fimbrial axons are denied access because some other (unidentified) axonal system forms new presynaptic terminals which effectively pre-empt the sites in the strial bed nucleus.

Absence of sprouting by retinogeniculate axons after chronic focal lesions in the adult cat retina

Focal lesions were placed in the retina of adult cats in order to denervate partially the laminae of the lateral geniculate nucleus (LGN). Retinogeniculate projections were assessed after survival times of from 5 days to 2 years by means of either reduced silver staining for degeneration or autoradiographic labelling. Filling of the lesion-denervated zones by 'sprouts' from the intact retinofugal fibers was not observed, even in the brains of animals with long-term lesions. It was concluded that the retinogeniculate projection in adult cat does not display any significant ability to sprout into denervated regions.

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

The results obtained following eye enucleation and dorsal occipital cortical (DOC) lesion in normal adult animals are in general agreement with those reported in previous work. The details, however, vary somewhat. A DOC projection to the claustrum and reticularis thalami is reported here for the first time in the rat. Six experiments were performed: (1) DOC lesions followed by eye enucleations. (2) Eye enucleations followed by DOC lesions. (3) Eye enucleations followed by sensorimotor cortical (SMC) lesions. (4) SMC lesions followed by eye enucleations. (5) DOC lesions followed by SMC lesions. (6) SMC lesions followed by DOC lesions. The results obtained from the experimental animals demonstrate a certain degree of plasticity and interaction between the DOC and retinal efferents to the superior colliculus, lateral geniculate nucleus, the pretectal region and the superior colliculus. Thus, following a neonatal left dorsal occipital cortical lesion, the retinal efferents from the right eye produce a denser projection to the contralateral lateralis posterior (LP), nucleus lateralis of the optic tract (NLOT), and the caudal part of the lateral division of the lateral geniculate nucleus pars ventralis (VGN). Following neonatal left eye enucleation, increased ipsilateral projection from the right DOC was noted in the superficial part of the stratum griseum superficiale (SGS), NLOT, pretectal nucleus (PT) and the adjoining stratum griseum intermediate (SGI) of the superior colliculus, lateral geniculate nucleus, pars dorsalis (DGN), and both the medial and lateral halves of the caudal portion of VGN. Following a neonatal DOC lesion a denser projection from the remaining ipsilateral SMC was seen in the medial pretectal nucleus (PTM), PT and deep pretectal nucleus (PTP). Following a neonatal SMC lesion a denser projection from the ipsilateral DOC was seen in the SGI, PT, nucleus medialis of the optic tract (NMOT) and PTM. There was no change in the retinal efferents following neonatal SMC lesions. Nor was there any change in the SMC efferents following neonatal eye enucleations.