G. Vrensen

Changes in size and shape of synaptic connections after visual training: An ultrastructural approach of synaptic plasticity

The effect of visual training30 on the synapses in the visual cortex of rabbits was quantitatively investigated. The number, the size and the fine structural organization of synaptic grids were analyzed in 0.5 μm E-PTA sections. The size of the pre- and postsynaptic terminals, the length of the apposition zones, the number of synaptic vesicles per terminal, the dimensions of the synaptic cleft and postsynaptic density and the frequency of mitochondria and spine apparatuses were studied in conventional OsO4 sections. As compared to control animals, the synaptic population of visually trained rabbits exhibits 3 important changes: 1. (1) the number of complex grids has significantly increased; (perforated synapses according to refs. 6 and 24), 2. (2) the size of the synaptic grids has decreased. As is deduced from careful electron microscopic observations, this decrease in size is the result of a focalization of large grids. 3. (3) a significant increase in thickness of the postsynaptic density and a decrease in synaptic cleft is observed. The changes are most pronounced in deep layers (L IV and V) and are almost absent in superficial layers (L I). On account of recent insights into the mechanism of synaptic transmission and of the intrinsic relation between the fine structural components of the synaptic junction and the process of transmitter release, the observed changes are interpreted as an expression of enhanced synaptic efficacy. It is further postulated that modification of synaptic efficacy must be considered as an important factor in the adaptation of the nervous system to changing environmental conditions and for the consolidation of memory traces.

Postnatal development of neurons and synapses in the visual and motor cortex of rabbits: A quantitative light and electron microscopic study

Abstract Introductory to a morphological investigation on the effects of early visual deprivation and on the critical periods in early postnatal life we have studied quantitatively the normal postnatal growth of neurons and synapses in the visual and motor cortex of rabbits. The major results of this analytical study are: (1) rapid decrease in neuron density and a rapid increase in neuronal volume are observed. They are almost completed at postnatal Day 10, i.e., before natural eye opening. The drop in neuron density is caused to a very large extent by an increase in cortical volume and not by a considerable disappearance of neurons; (2) the formation of synaptic contact zones starts at Day 6 to 7 and is most pronounced between Day 10 and Day 21, i.e., after natural eye opening. At Day 27 synaptic density has reached adult levels in the visual cortex and is in excess of the adult level in the motor cortex. In visual area I and in the motor cortex a significant difference in synaptic increase is observed between the left and right hemisphere, resulting in a lower synaptic density in the left counterparts at Day 27 and in adult animals [56,57]. In the visual cortex a small but highly correlated increase in synaptic vesicle density is observed. In the motor cortex no correlated relation between age and vesicle density is observed. In both cortical areas synaptic vesicle density has reached about 70 percent of the adult level at Day 27; and (3) in newborn and young rabbits the motor cortex seems to be more mature than the visual cortex.

The presynaptic grid: A new approach

A new electron microscopic facility is described which enables systematic visualization of E-PTA-stained presynaptic grids in full en face position. This EM-facility is used to analyze the size and the number of dense projections of synaptic grids in different brain areas of the rabbit. The observations support the view that dense projections form an intrinsic part of all central synapses and are organized in a hexagonal or triangular pattern. The observation of annulate and horseshoe-shaped synaptic grids is in agreement with previous observations on synaptic connections with subsynaptic plate perforations. A non-normal frequency distribution of dense projections per synaptic grid with distinct peaks is suggestive for the existence of distinct size classes of synaptic contacts. Analyses of the frequency distribution of dense projections in different areas and at different levels below the pial surface in adult animals point to a specificity of the distinct size classes related to the afferent origin or the target cell they are projecting on. Investigation of grid size and number of dense projections during a period of rapid synaptogenesis reveals that newly formed synapses also have a specific size. The complementarity of dense projections and vesicle attachment sites implicit in the model of Akert et al. has been used to calculate the number of vesicle attachment sites per sq. micrometer on account of the density of projections per grid. The agreement between our values and those of Akert and Peper, based on the analyses of vesicle attachment sites in freeze-etch specimen, is striking. The possible implications of these observations in relation to synaptic efficacy and neuronal microcicuitry are discussed.

The effect of dark rearing and its recovery on synaptic terminals in the visual cortex of rabbits. A quantitative electron microscopic study

Abstract The effect of dark rearing and its recovery on the visual and motor cortex of rabbits is studied. The number of synaptic contact zones per area (NA per 1000 sq.μm), their surface area (SV in sq.μm/1000 cu.μm) and mean length (L in μm) and the number of synaptic vesicles per terminal (Nves) are studied, using specific staining techniques and stereological principles. Dark rearing for 7 months from birth does not affect NA, SV and L, indicating that the formation of interneuronal connections is quantitatively normal in the absence of light. In the control and deprived animals a right-left dominance with respect to NA is observed in the motor cortex and the medial part of the visual cortex. The disappearance of this right-left dominance in deprived animals allowed to recover for 1 year, must be further investigated. No change in the synaptic density as a function of depth below the pial surface is observed. An obvious decrease in Nves is observed in the visual cortex after dark rearing. This decrease persists after recovery for 1 year. In the motor cortex there is no evident decrease. These results are related to recent studies regarding the significance of synaptic vesicles in the process of transmission of nerve impulses. It is hypothesized that light deprivation diminishes irreversibly the ability of synaptic terminals to synthetized and/or store transmitter substances. Electrophysiological and behavioral studies were performed on the same animals46,61,71–79. Concomitant with the changes in synaptic vesicles, a decrease in visual evoked response (VER) and visual acuity (VA) is observed. The decrease in VA is persistent for at least 5 months whereas VER recovered to normal after 1–2 months.

Quantitative stereology of synapses: a critical investigation.

Summary The quantitative characterization of synapses in O s O 4 - and ethanolic phosphotungstic acid acid (E-PTA)-treated nervous tissue is reinvestigated making use of recent progress an quantitative stereology. The surface area of synaptic junctions per volume and the mean length of the synaptic contact zones are introduced as parameters. It reveals that the E-PTA-stained synaptic junctions are qualitatively and quantitatively equivalent to the membrane thickenings of the O s O 4 -treated synaptic contact zones. The relevance of quantitative synaptology is discussed.

The effect of monocular deprivation on synaptic terminals in the visual cortex of rabbits. A quantitative electron microscopic study

The effect of prolonged (7 month) monocular deprivation (right eye sutured) on the density of synapses (NA) and the number of synaptic vesicles (Nves) in the visual cortex of rabbits is studied. The results can be summarized as follows: (1) No changes (NA and Nves) are observed in the non-deprived hemisphere (visual areas I and II, motor cortex) as compared to control animals. (2) NA has not changed, as compared to control animals, in the binocular field of the deprived visual cortex. (3) A significant rise in NA, as compared to control animals, is observed in the monocular field of area I and in the motor cortex of the deprived cortex. This rise is similar to that found after recovery from prolonged dark rearing. (4) The increase in NA is mainly restricted to the laminae I–III, indicating that, most likely, we are not dealing with an increase in specific visual afferents. (5) The deprived visual cortex shows a significant decrease (16%) in Nves. This decrease is less pronounced than after dark rearing (40%). This probably points to the fact that eye suturing does not fully prevent light penetration. The results are discussed in the light of recent discussions on the plasticity of synapses and the effect of retinal input on the development of the visual system.

SOME NEW ASPECTS OF EFFICIENCY OF ELECTRON MICROSCOPIC AUTORADIOGRAPHY WITH TRITIUM

Quantitative aspects of submicroscopic autoradiography, using the flat substrate technique, were studied. Using Kodak D19b as developer efficiency values of about 37 for the Ilford LA emulsion and about 17 for the Gevaert-Agfa Scientia Nuc 3.07 emulsion were obtained with H3-methacrylate sections of gray interference colors. Theoretical and experimental data revealed that the high standard deviations of efficiency values obtained are caused mainly by variations in section thickness within ribbons of sections, which on account of identical interference colors were considered to be equally thick. Measurements of efficiency on sections with thickness variations between 400 and 2500 Å revealed that self-absorption is an important factor in tritium autoradiography at the electron microscopic level.

The postnatal development of the presynaptic grid in the visual cortex of rabbits and the effect of dark-rearing

Abstract We have quantitatively investigated the postnatal development of synaptic contacts in the visual cortex of rabbits and the effects of long-term dark-rearing on this development. The synaptic contacts are visualized in full en face extension using 0.5 μm sections of E-PTA treated tissue21. In early postnatal life the mean size and the size distribution of the synaptic grids are much the same as in adult rabbits. Only minor changes are observed. The variations in size distribution at different levels below the pial surface, characteristic for the adult animal21, are also present in early postnatal life. Prior to eye opening many synaptic grids lack dense projections or are only partly covered. After eye opening the dense projections mature but are still different from the adult grids at postnatal days 21–27. The number of complex grids, annulate or horseshoe-shaped, gradually increases and is above adult level at days 21–27. Dark-rearing has no effect on size and size distribution of the synaptic grids and does not affect the frequency of complex grids. Careful observation reveals, however, that the dense projections do not develop normally. They remain small and fuzzy as in the visual cortex of young rabbits, and many grids are only partly covered. These observations demonstrate that the synaptic contacts are to a great extent genetically predetermined. They are also formed with a specific size which does not change during development and is not affected by dark-rearing. The gradual maturation of the dense projections and the retardation of their development during dark-rearing indicates that visual experience interferes with the development of these synaptic substructures. Because of the complementarity of dense projections and vesicle attachment sites the maturation of dense projections and the effect of dark-rearing can be interpreted as change in the efficacy of synaptic transmission.

Retinopretectal projections in albino and pigmented rabbits: An autoradiographic study

The ipsilateral and contralateral retinal projection was studied in pigmented rabbits and in 3 strains of albino rabbits by anterograde transport of [3H]proline and [3H]fucose combined with autoradiographic techniques. Special attention was paid to the terminals in the pretectal area of both the pigmented and albino strains. On the contralateral side terminal labeling was found in both pigmented and albino rabbits in the nucleus of the optic tract (NOT), the anterior pretectal nucleus (PA), the posterior pretectal nucleus (PP) and the pretectal olivary nucleus (PO). Ipsilaterally labeling was found only in the pigmented strain in small patches in the PP. Ipsilateral projection was not found in the albinos in the pretectal area. The results are in agreement with the findings of Scalia in pigmented rabbits. The absence of ipsilateral labeling in the pretectal region in albinos is in contrast with earlier findings of Giolli and Takahashi et al., in pigmented rabbits but is in agreement with the observations of Takahashi and Oyster. Since no radioactively labeled fibers were found to project to the NOT in either pigmented or albino rabbits, these results do not support the hypothesis of Collewijn that the inverted optokinetic nystagmus in albinos is due to misrouting of the ipsilateral retinal fibers to the NOT.

Quantitative aspects of the synaptic organization of the superior colliculus in control and dark-reared rabbits

Fine structural and quantitative aspects of the synaptic organization of the superior colliculus of the rabbit were studied. The synaptic density, the frequency of the distinct types of terminals, the length of the contact zones, the percentage of serial contacts and the number of synaptic vesicles per terminal were estimated in control, dark-reared and dark-reared-recovered animals. The ultrastructure of the synaptic terminals and the complex interconnectivity correspond to that described for other species. The depth distribution of terminals with round synaptic vesicles agrees quite well with the distribution of cortical and retinal afferents found in lesion experiments. Dark-rearing has little effect on the quantitative organization of the synaptic terminals. In contrast with previous observations in the visual cortex of the same animals37 no changes in the density of synaptic vesicles is observed in the superior colliculus after long term dark-rearing.