Neuronal plasticity is a central theme of modern neurobiology, from cellular and molecular mechanisms of synapse formation in Drosophila to behavioural recovery from strokes in elderly humans. Although the methods used to measure plastic responses differ, the stimuli required to elicit plasticity are thought to be activity-dependent. In this article, we focus on the neuronal changes that occur in response to complex stimulation by an enriched environment. We emphasize the behavioural and neurobiological consequences of specific elements of enrichment, especially exercise and learning.

Neural consequences of environmental enrichment.

Applications of the Morris water maze in the study of learning and memory.

This paper deals with isolated, countable items, often termed particles, in three-dimensional space. Its substance is the unbiased stereological estimation of the number, height, surface and volume of such particles without any assumptions about their shape. The full range of estimators is described, some of them for the first time, some in an improved form, several in more than one version, and all of them under the single, absolute requirement that one can in fact identify what one is quantifying on sections. In terms of the minimal number of sections for the analysis, the estimators may be classified as follows: On a single section it is possible to estimate vV, the mean volume of particles in the volume-weighted or 'sieving'-distribution. On two parallel sections, separated by a known distance, estimators exist of particle number and of all mean sizes (height, surface and volume) in the ordinary number distribution, as well as of SDN(v), the standard deviation in the number distribution of particle volumes. If the containing space is relatively transparent the sections may be two optical sections within one thick physical section. On a stack of parallel sections, at least as high as the largest particle, and separated by known distances, one can get twelve mean sizes and twelve distributions of individual sizes: all combinations of three sizes: height, surface and volume in four different types of distributions: number, height, surface and volume. Fulfilling the sampling requirements of the above two estimation principles it has been shown very recently that by combining them one may even estimate mean sizes and number of arbitrary particles in a stack of sections with constant but unknown separation. Finally, a unique, unbiased estimator of the total number of items in a specimen is described for the use of which one need not measure the distance between sections, nor their thickness, nor the volume of the specimen, nor assume anything about shrinkage/swelling, sectioning compression or lost caps. It is the fractionator.

Stereology of arbitrary particles. A review of unbiased number and size estimators and the presentation of some new ones, in memory of William R. Thompson

We have found that the density of synapses in the stratum radiatum of the hippocampal CA1 region in the adult female rat is sensitive to estradiol manipulation and fluctuates naturally as the levels of ovarian steroids vary during the 5 d estrous cycle. In both cases, low levels of estradiol are correlated with lower synapse density, while high estradiol levels are correlated with a higher density of synapses. These synaptic changes occur very rapidly in that within approximately 24 hr between the proestrus and estrus stages of the estrous cycle, we observe a 32% decrease in the density of hippocampal synapses. Synapse density then appears to cycle back to proestrus values over a period of several days. To our knowledge, this is the first demonstration of such short-term steroid-mediated synaptic plasticity occurring naturally in the adult mammalian brain.

https://www.jneurosci.org/content/jneuro/12/7/2549.full.pdf

Estradiol mediates fluctuation in hippocampal synapse density during the estrous cycle in the adult rat.

Identification of myelin-associated glycoprotein as a major myelin-derived inhibitor of neurite growth.

The abilities of embryonic and adult rat sensory neurons to regenerate were compared when cultured on cryostat sections of normal and lesioned sciatic nerve tissues. Differences in neurite growth, visualized by GAP-43 immunolabelling, were most pronounced on substrata consisting of longitudinal sections of normal versus predegenerated sciatic nerve. Adult dorsal root ganglion (DRG) neurons grew only on the lesioned nerves. Neurites extended along these sections in a characteristically longitudinal orientation, and this growth was not dependent on nerve growth factor. Embryonic DRG neurons extended neurites on sections from both types of nerves. These results highlight important differences in the regenerative abilities of embryonic and adult DRG neurons when grown on physiologically appropriate substrata.

Adult Rat Dorsal Root Ganglion Neurons Extend Neurites on Predegenerated But Not on Normal Peripheral Nerves In Vitro.

The Feulgen-DNA contents of human leukocytes, sperm, and oral squames were investigated by scanning and integrating microdensitometry, both with and without correction for residual distribution error and glare. Maximally stained sperm had absorbances which at lambdamax exceeded the measuring range of the Vickers M86 microdensitometer; this potential source of error could be avoided either by using shorter hydrolysis times or by measuring at an off-peak wavelength. Small but statistically significant apparent differences between leukocyte types were found in uncorrected but not fully corrected measurements, and some apparent differences disappeared when only one of the residual instrumental errors was eliminated. In uncorrected measurements, the apparent Feulgen-DNA content of maximally stained polymorphs measured at lambdamax was significantly lower than that of squames, while in all experimental series uncorrected measurements showed apparent diploid:haploid ratios significantly greater than two. In fully corrected measurements no significant differences were found between leukocytes and squames, and in four independent estimations the lowest diploid:haploid ratio found was 1.99 +/- 0.05, and the highest 2.03 +/- 0.05. Discrepancies found in uncorrected measurements could be correlated with morphology of the nuclei concerned. Glare particularly affected measurements of relatively compact nuclei such as those of sperm, polymorphs and lymphocytes, while residual distribution error was especially marked with nuclei having a high perimeter:area ratio (e.g. sperm and polymorphs). Uncorrected instrumental errors, especially residual distribution error and glare, probably account for at least some of the previously reported apparent differences between the Feulgen-DNA contents of different cell types. On the basis of our experimental evidence, and a consideration of the published work of others, it appears that within the rather narrow limits of random experimental error there seems little or no reason to postulate either genuine differences in the amounts of DNA present in the cells studied, or nonstoichiometry of a correctly performed Feulgen reaction.

https://rupress.org/jcb/article-pdf/71/1/68/1072207/68.pdf

Apparent anomalies in nuclear feulgen-DNA contents. Role of systematic microdensitometric errors.

Undernutrition during early life is known to affect the morphology of the hippocampal formation. Recent advances in stereological techniques have made it possible to make relatively unbiased estimates of total cell numbers in well‐defined brain regions. It was decided to use these methods to determine the effects of different levels of undernutrition during early postnatal life on the granule cells of the rat dentate gyrus. Male hooded Long Evans rats were undernourished between the 16th day of gestation and 30 postnatal days of age to two different levels. The daily food intake of level‐1 and level‐2 rats represented about 60 and 40%, respectively, of that eaten by well‐fed, age‐matched controls. Nutritional rehabilitation of the rats was commenced when they had reached 30 days of age by placing them on an ad libitum diet. Groups of control and experimental rats were killed at 70 and 212 days of age. The Cavalieri principle was used to determine the granule cell layer volume within the dentate gyrus, and the “disector” method was used to determine numerical densities of these granule cells. These estimates were used to calculate the total numbers of granule cells. There were between 260,000 and 320,000 granule cells within the dentate gyrus of 70‐day‐old control and experimental rats. By 212 days of age, well‐fed controls had an average of about 834,000 granule cells. The level‐1 and level‐2 previously undernourished rats had about 515,000 and 595,000 granule cells, respectively. Two‐way analysis of variance procedures showed significant main effects of nutrition and age as well as a significant interaction between them. These results provide strong evidence that a period of undernutrition during early life causes a long‐term deficit in the total number of dentate gyrus granule cells.

Effects of undernutrition during early life on granule cell numbers in the rat dentate gyrus.

1. Male rats were undernourished either during the gestational and suckling periods or for a period of time immediately following weaning. Some rats were killed at the end of the period of undernutrition; others were nutritionally rehabilitated for lengthy periods of time before examination. Two muscles, the extensor digitorum longus (EDL) and soleus (SOL) were studied from each rat. Histochemically-stained transverse sections of these muscles were used to determine total number of fibres, the fibre cross-sectional areas and the relative frequency of the various fibre types. 2. All rats killed immediately following undernutrition showed significant deficits in body-weight, muscle weight and fibre cross-sectional area compared to age-matched controls. 3. Animals undernourished during gestation and suckling and then fed normally for 5 months showed persistent and significant deficits in body-weight, muscle weight, muscle weight and total fibre number. There were also significant deficits in mean fibre cross-sectional area of each fibre type except for red fibres in the EDL. No difference in the volume proportion of connective tissue was found. 4. Rats undernourished after weaning and then fed ad lib. for approximately 7 months had normal body-and muscle weights. Their muscles showed no significant differences in total fibre number, relative frequency of the various fibre types, fibre size or volume proportion of connective tissue. 5. These results indicate that, although the effects on rat skeletal muscle of a period of undernutrition after weaning can be rectified, undernutrition before weaning causes lasting deficits.

/pdf/early-life-undernutrition-in-rats-1-quantitative-histology-10dhcwmvnl.pdf

Early life undernutrition in rats. 1. Quantitative histology of skeletal muscles from underfed young and refed adult animals.

Male rats undernourished from birth to 30 days were nutritionally rehabilitated till 160 days of age. Quantitative stereological procedures at the light and electron microscopical levels were employed to estimate, among other things, the synapse-to-neuron ratios in the frontal cortex and granular layer of the cerebellum.



In the frontal cortex, the 30-day-old undernourished rat had a mean ± SE of about 14,020 ± 1,540 synapses-per-neuron compared with 22,270 ± 3,250 for the controls. This was a deficit of 37% (p 0.1). This was due mainly to a fall in the synapse-to-neuron ratio with age. A much larger fall in the ratio occurred in the control than in the previously undernourished group.



In the granular layer of the cerebellum the 30-day-old undernourished rats had 341 ± 17 synapses-per-neuron compared with 495 ± 25 for the controls. This was a deficit of 31% (p 0.1).



These results show that the previously undernourished rats are capable of at least some (if not complete) “catch-up” with regard to the synapse-to-neuron ratio.

Kuldip S. Bedi

Papers

Adult Rat Dorsal Root Ganglion Neurons Extend Neurites on Predegenerated But Not on Normal Peripheral Nerves In Vitro.

Apparent anomalies in nuclear feulgen-DNA contents. Role of systematic microdensitometric errors.

Effects of undernutrition during early life on granule cell numbers in the rat dentate gyrus.

Early life undernutrition in rats. 1. Quantitative histology of skeletal muscles from underfed young and refed adult animals.

Synapse‐to‐neuron ratios of the frontal and cerebellar cortex of 30‐day‐old and adult rats undernourished during early postnatal life