Quantitative analysis of the striate cortex in the mutant microphthalmic rat

The Effect of Onset Age of Visual Deprivation on Visual Cortex Surface Area Across-Species

Blindness early in life induces permanent alterations in brain anatomy, including reduced surface area of primary visual cortex (V1). Bilateral enucleation early in development causes greater reductions in primary visual cortex surface area than at later times. However, the time at which cortical surface area expansion is no longer sensitive to enucleation is not clearly established, despite being an important milestone for cortical development. Using histological and MRI techniques, we investigated how reductions in the surface area of V1 depends on the timing of blindness onset in rats, ferrets and humans. To compare data across species, we translated ages of all species to a common neuro-developmental event-time (ET) scale. Consistently, blindness during early cortical expansion induced large (~40%) reductions in V1 surface area, in rats and ferrets, while blindness occurring later had diminishing effects. Longitudinal measurements on ferrets confirmed that early enucleation disrupted cortical expansion, rather than inducing enhanced pruning. We modeled the ET associated with the conclusion of the effect of blindness on surface area at maturity (ETc), relative to the normal conclusion of visual cortex surface area expansion, (ETdev). A final analysis combining our data with extant published data confirmed that ETc occurred well before ETdev.

Altered expression of parvalbumin and calbindin in interneurons within the primary visual cortex of neonatal enucleated hamsters

In the present study, we tested the hypothesis that the expression of calcium binding proteins (CaBPs), parvalbumin (PV), calretinin (CR) and calbindin (CB), is dependent upon sensory experience as emphasized in visual deprivation and deafferentation studies. The expression of CaBPs was studied in interneurons within the primary and extrastriate visual cortices (V1, V2M, V2L) and auditory cortex (AC) of adult hamsters enucleated at birth. The effects of enucleation were mainly confined to area V1 where there was a significant volume reduction (26%) and changes in the laminar distribution of PV and CB immunoreactive (IR) cells. The density of PV-IR cell bodies was significantly increased in layer IV and reduced in layer V. Moreover, the density of CB-IR neurons was inferior in layer V of V1 in enucleated hamsters (EH) compared to controls. These results suggest that some features of the laminar distribution of specific CaBPs, in primary sensory cortices, are dependent upon or modulated by sensory input.

Optic nerve transection affects development and use-dependent plasticity in neocortex of the rat: Quantitative acetylcholinesterase imaging

We investigated the effects of neonatal optic nerve transection on cortical acetylcholinesterase (AChE) activity in hooded rats during postnatal development and following behavioral manipulation after weaning. AChE reaction product was quantified on digitized images of histochemically stained sections in layer IV of primary somatic sensory, primary visual and visual association cortex. Rats with optic nerve transection were compared to sham-operated littermates. In all cortical regions of both types of animal, AChE reaction product was increased to peak 2 weeks after birth and decreased thereafter, reaching adult levels at the end of the third postnatal week. During postnatal development, reaction product in primary visual cortex was lower in rats deprived of retinal input than in sham-operated littermates and the area delineated by reaction product was smaller. However, optic nerve transection did not modify the time course of postnatal development or statistically significantly diminish adult levels of AChE activity. Behavioral manipulations after weaning statistically significantly increased enzyme activity in sham-operated rats in all cortical areas examined. Compared with cage rearing, training in a discrimination task with food reward had a greater impact than environmental enrichment. By contrast, in the rats with optic nerve transection enrichment and training resulted in statistically significantly increased AChE activity only in lateral visual association cortex. Our findings provide evidence for intra- and supramodal influences of the neonatal removal of retinal input on neural activity- and use-dependent modifications of cortical AChE activity. The laminar distribution of the AChE reaction product suggests that the observed changes in AChE activity were mainly related to cholinergic basal forebrain afferents. These afferents may facilitate the stabilization of transient connections between the somatic sensory and the visual pathway.

The pattern of callosal connections in posterior neocortex of congenitally anophthalmic rats

In an effort to assess the innate capacity of the central visual system to specify corticocortical connectivity in the absence of retinal afferents, we examined the tangential distribution of callosal cells and terminations in posterior neocortex of congenitally anophthalmic rats. Although our results indicate that the callosal pattern is clearly anomalous in these rats, all features of the normal visual callosal pattern are recognizable in mutant rats, indicating that central visual pathways can generate many aspects of normal interhemispheric connectivity in the absence of input from the periphery. On the other hand, the presence of anomalies in the pattern indicates that the eyes are necessary to fine-tune the distribution of callosal connections at some developmental stage. Moreover, the fact that abnormalities in the callosal pattern of mutant rats are the same as those previously described in rats enucleated at birth suggests that the eyes begin to exert their influence on callosal development after birth.

Whole embryo morphometry in teratogen screening

Current methodology in embryo evaluation involves qualitative assessment of razor blade and paraffin serial sections. Presently, no one has applied existing computerized morphometric techniques to examine embryos. A technique has been developed that enables investigators to section embryos at 150 mu, thereby greatly reducing the number of sections and making morphometric analysis possible. This type of analysis permits the precise volumetric determination of several developing organ systems. The aim of this study was to evaluate the feasibility and sensitivity of whole embryo morphometry in teratogen screening. Cadmium chloride, a well-established teratogen, was chosen because of its ability to induce exencephaly in approximately one-half of offspring while having no observable effects on the remaining exposed embryos. It was found that both exencephalic and normal-appearing cadmium-exposed embryos had significantly smaller total cellular, neuroepithelial, otic vesicle, optic assembly, limb bud, and cardiac mesenchyme volumes when compared to controls. Also, the neuroepithelial volume of the exencephalic embryos was significantly smaller than the normal-appearing cadmium-exposed embryos. These results suggest that in addition to inducing exencephaly, cadmium chloride has an overall inhibitory effect on embryonic growth. We have shown that whole embryo morphometry is a sensitive means of evaluating embryonic growth that permitted determination of cadmium-induced aberrations not discernable by currently employed techniques. In light of these results, we feel this technique shows promise for future investigations of known and suspected teratogens.

Development of visual callosal connections in neonatally enucleated rats

The present report extends previous descriptions of the mature distributions of callosal cells and axonal terminations in rats monocularly or binocularly enucleated at birth. It also describes the time course of callosal development in these animals, and establishes the age at which eye removal ceases to alter the normal course of callosal development. Although our results indicate that the callosal pattern is anomalous in adult, neonatally enucleated rats, the major features of the normal callosal pattern are nonetheless clearly recognizable in both monocularly and binocularly enucleated rats. Thus, as in normally reared rats, there are dense accumulations of callosal cells and terminations at the 17/18a border region, at the lateral border of area 18a, and within area 18b in enucleated rats. In addition, several narrow bands of callosal connections bridge the width of area 18a at several rostrocaudal levels, and a ring-like callosal configuration is located anterolateral to area 17. In monocularly enucleated rats, the most prominent anomaly develops in the hemisphere ipsilateral to the remaining eye, where a dense band of callosal connections runs rostrocaudally through the center of area 17. Periodic fluctuations in the density of labeling along the length of this extra band give it a beaded appearance. The callosal pattern in the hemisphere contralateral to the remaining eye in these rats appears normal. Binocular enucleation causes the appearance of discrete regions of reduced labeling within the 17/18a callosal band and several densely labeled tongue-like regions that extend medially from this band well into area 17. The laminar distribution of callosal cells and terminations is not significantly altered by loss of one or both eyes at birth. Our data indicate that enucleation does not affect the time course of callosal development. Thus, in enucleated pups, all features of the mature callosal pattern can be recognized by 6-7 days of age, and by 12 days of age the patterns appear virtually mature. Finally, our data reveal that monocular or binocular enucleations performed at 6 days of age or later allow the callosal pattern to develop normally, whereas enucleations performed between birth and 5 days of age produce anomalies similar to those observed in rats enucleated at birth. Thus, at about 6 days of age--just as the earliest features of the mature callosal pattern become discernible, and long before rats first open their eyes--the developing callosal pathway is no longer susceptible to disruptions of visual input.

Quantitative analysis of the oculomotor nuclei in the mutant microphthalmic rat

A quantitative analysis of the oculomotor, trochlear, and abducens nuclei was carried out in the mutant microphthalmic rat. In this strain of rat, the eyeball is reduced to about one-third in diameter, and there is no optic nerve. Nevertheless, this strain possesses all the types of extraocular muscles; however, the volume of these muscles is reduced from 33 to 74% of the normal values. The oculomotor, trochlear, and abducens nuclei in this mutant rat were found in the same location as in the normal ones. Moreover, the neurons in these nuclei in the microphthalmic rat did not appear to be any different from those in the controls. The neuronal population of these nuclei was reduced by 63% (oculomotor), 50% (trochlear), and 61% (abducens) of normal values, respectively. The long axis of all neurons of these nuclei in the microphthalmic and normal rats was measured. In all three nuclei of both strains, the size histograms showed a unimodal distribution ranging from 10 to 35 micron, with peaks at 20 to 25 micron. There was no significant difference between the normal and mutant strains. Consequently, the only influence of the shrinkage of the muscles which they supply was seen in the reduction of the cell populations of these nuclei.

Neurons of the dorsal lateral geniculate nucleus of the hereditary microphthalmic rat: a Golgi study

Neurons of the dorsal lateral geniculate nucleus (LGNd) in the microphthalmic rat were examined by the Golgi-Cox method. LGNd neurons in the microphthalmic rat were classified into the multipolar (I) and bipolar (II) types as in the normal rat. The multipolar type was further divided into two subclasses (Ia and Ib) on the basis of their dendritic patterns. The proximal portion of their primary dendrites was thinner than in normal LGNd neurons. The Ia cells had 6-7 primary dendrites extending radially, while the Ib cells had 3-4 primary dendrites spreading primarily parallel to the optic tract. Type II cells had two or three primary dendrites emerging from the cell bodies. In both types, primary dendrites were shorter in length or less branched than usual. These results suggested that LGNd neurons in the microphthalmic rat had smaller dendritic fields than those in the normal rat.