Different effect of hippocampal granule cell destruction on amygdaloid kindling in Sprague-Dawley and Wistar rats

The hippocampal granule cells receive major inputs via the perforant path from other limbic structures such as the amygdala (AM). In this study, we examined Sprague-Dawley (SD) and Wistar rats, the effect of bilateral destructions of the hippocampal granule cells on the process of AM kindling and kindled AM seizures after completion of kindling. The granule cells were selectively and completely destroyed bilaterally by intra-hippocampal injections of colchicine. The left AM was used as the primary kindling site and the right AM as the secondary site. In SD rats, prior destruction of the granule cells caused a marked delay in the seizure development of both the primary AM kindling and subsequent secondary AM kindling. However, once AM kindling was established in SD rats, the destruction of granule cells was totally ineffective in preventing kindled seizures. In Wistar rats, unlike SD rats, prior destruction of the granule cells failed to change the rate of kindling at the primary and secondary sites. However, Wistar rats showed a transient and marked regression of kindled seizures when the granule cells were destroyed after the completion of AM kindling. In both strains, granule cell destruction had no effect on the re-establishment of kindled seizures at the time of primary-site re-test. These findings suggest that hippocampal granule cells of SD and Wistar rats play different roles in AM kindling.     

On the numbers of neurons in fields CA1 and CA3 of the hippocampus of Sprague-Dawley and Wistar rats

In a previous study it was found that there are significant differences in the numbers of granule cells in the dentate gyrus of adult Sprague-Dawley and Wistar rats and also that the continued postnatal addition of new cells to the dentate gyrus has quite different consequences in the two strains. We have now extended these observations to the two major cytoarchitectonic fields of the hippocampus (the regio superior or field CA1; and the regio inferior or field CA3). The mean number of pyramidal neurons in field CA1 of 1-month-old Sprague-Dawley rats is 420,000 (+/- 60,000 S.E.), while Wistar rats at the same age have 320,000 (+/- 20,000). The numbers of neurons in field CA3 in the two strains are: 330,000 (+/- 30,000) and 210,000 (+/- 20,000), respectively. Whether these strain differences reflect specific differences in the neural organization of the hippocampal formation in the two strains, or are related to more general differences in total body weight or brain weight, is unknown. Since during the first two days postnatally we estimate that there are between 358,000 and 491,000 cells in field CA1 of Sprague-Dawley rats, it would seem that there is no significant naturally-occurring neuronal death in this hippocampal field. This may be due to the extensive collateral projections of the hippocampal pyramidal neurons.     

Population dynamics of adult-formed granule neurons of the rat olfactory bulb

The population dynamics of internal granule cells in the rat olfactory bulb during adult life were analyzed in histological sections and in autoradiograms with (1) counts of granule cells, (2) counts of labeled granule cells 1 month after injection of 3H-thymidine at various ages, and (3) counts of labeled granule cells at varying survival times (up to 18 months) after injection at 3 months and 24 months. The total number of granule cells increases linearly throughout life, approximately doubling between 3 and 31 months. Autoradiographic studies show that the rate of production of new granule cells decreases from 3 to 12 months and then is approximately constant during the rest of the life span. The number of labeled cells found 6 months after injection at 3 and 24 months is about one-fourth and one-half, respectively, that of the number at a 1-month survival, suggesting that many of the cells produced to do not survive. However, at least some granule cells labeled at 3 months survive for 18 months. A model is suggested in which granule cells are produced continuously throughout life and control of the total number of granule cells is effected chiefly through the rate of cell death.     

Adrenalectomy-induced granule cell death is predicated on the disappearance of glucocorticoid receptor immunoreactivity in the rat hippocampal granule cell layer

In this study, we observed the changes of glucocorticoid receptor (GR)-immunoreactivity (ir) and cell death in the rat hippocampal granule cell layer at various periods after adrenalectomy (ADX). Our results revealed that all of the rats shortly after ADX showed a rapid loss of GR-ir and subsequent appearance of degenerating cells in the granule cell layer. One month after ADX, however, about 80% of the rats displayed a restoration of GR-ir and the absence of degenerating cells in the granule cell layer, and this phenomenon was successively noted for 6 months. Hippocampal structural destruction 3 and 6 months after ADX was found in about 20% of the rats with loss of GR-ir in the granule cell layer; the ADX rats with even weak GR-ir in this area had a normal hippocampus. The treatment of rats with synthetic GR agonist, dexamethasone, immediately after ADX prevented the loss of GR-ir and significantly reduced the number of degenerating cells in the granule cell layer. Our results clarified that granule cell death after ADX was necessarily accompanied by the disappearance of GR-ir in the granule cell layer, suggesting that ADX-induced granule cell death is predicated on the loss of GR-ir and that the presence of GR-ir in this area may be important for granule cell survival.     

Undernutrition during early life does not affect the number of granule cells in the rat olfactory bulb

Undernutrition during early life causes deficits and distortions of brain structure. However, whether or not this includes a diminution of the total numbers of neurones remains uncertain. Recent advances in stereological techniques have made it possible to obtain unbiased cells in well-defined biological structures. Rats were undernourestimates of total number of cells in well-defined biological structures. Rats were undernourished from conception to 90 postnatal days of age by standardised procedures. Groups of well-fed control and undernourished rats were anaesthetised and killed by intracardiac perfusion with fixatives at 30 and 90 days of age. Each olfactory bulb was serially sectioned at a nominal thickness of 100 pm on a vibratome. These sections were analysed by the Cavalieri principle to obtain estimates of the total volume of the olfactory bulb as well as the volume of its granule cell layer. The physical “disector” method was later used on serial 1-μm-thick toluidine-blue-stained sections to estimate the numerical density of granule cell neurones in the olfactory granule cell layer. These values were used to compute estimates of the total number of olfactory granule cell neurones for each animal. Thirty-day-old control and undernourished rats had between 2.6 and 3 million granule cell neurones in the olfactory bulb. By 90 days of age the number of granule cells had increased in both groups of animals to between about 4.2 and 5.2 million cells. Analysis of variance tests showed a significant main effect of age but not nutrition in these estimates. Although the interaction term did reach statistical significance, post hoc analysis did not reveal any differential effect of undernutrition between the two age groups examined. Therefore, undernutrition of rats from conception until 90 postnatal days of age does not seem to affect the total numbers of granule cell neurones in the olfactory bulbs. © 1994 Wiley-Liss, Inc.     

Neurons in the hilus region of the rat hippocampus are depleted in number by exposure to alcohol during early postnatal life

We have previously shown that exposing rats to a relatively high dose of ethanol during early postnatal life resulted in a deficit in spatial learning ability. This ability is controlled, at least in part, by the hippocampal formation. The purpose of the present study was to determine whether exposure of rats to ethanol during early postnatal life affected the number of specific neurons in the hippocampus. Wistar rats were exposed to a relatively high daily dose of ethanol between postnatal days 10 and 15 by placing them for 3 h each day in a chamber containing ethanol vapor. The blood ethanol concentration was about 430 mg/dl at the end of the exposure period. Groups of ethanol-treated (ET) rats, separation controls (SC), and mother-reared controls (MRC) were anesthetized and killed at 16 days of age by perfusion with phosphate-buffered glutaraldehyde (2.5%). The Cavalieri principle was used to determine the volume of various subdivisions of the hippocampal formation (CA1, CA2+CA3, hilus, and granule cell layer), and the physical disector method was used to estimate the numerical densities of neurons within each subdivision. The total number of neurons was calculated by multiplying estimates of the numerical density with the volume. There were, on average, about 441,000 granule cells in the granule cell layer and 153,000 to 177,000 pyramidal cells in both the CA1 and CA2+CA3 regions in all three treatment groups. In the hilus region, ET rats had about 27,000 neuronal cells. This was significantly fewer than the average of 38,000 such neurons estimated to be present in both MRC and SC animals. Thus, neurons in the hilus region may be particularly vulnerable to the effects of a high dose of ethanol exposure during early postnatal life.     

Early postnatal alcohol exposure acutely and permanently reduces the number of granule cells and mitral cells in the rat olfactory bulb: a stereological study.

This study demonstrates that exposure to alcohol during a period of rapid brain growth can lead to severe and permanent deficits in the number of granule cells and mitral cells in the main olfactory bulb. Sprague-Dawley rat pups were reared artificially and were administered alcohol over postnatal days (PD) 4 through 9, a period of brain development comparable to part of the human third trimester. The daily alcohol dose of 6.6 g/kg was concentrated into two of the twelve daily feedings, producing high peak blood alcohol concentrations followed by near total clearance. Pups were either sacrificed on PD10 or were allowed to grow to adulthood and sacrificed on PD115. The total number of granule cells and mitral cells in the main olfactory bulb were estimated with the aid of unbiased stereological principles and systematic sampling techniques. Exposure to alcohol resulted in significant reductions in the number of both granule cells and mitral cells on PD10. Significant deficits in both neuronal populations remained on PD115. The results support the hypothesis that alcohol exposure can kill developing neurons and lead to permanent neuronal deficits. Substantial developmental changes also occurred in the total number of mitral cells and granule cells between PD10 and PD115 in the control groups. In untreated rats, the number of granule cells increased from 2.20 x 10(6) on PD10 to 5.06 x 10(6) on PD115, while the number of mitral cells decreased from 5.30 x 10(4) to 4.33 x 10(4) over the same time period. These results demonstrate that there is a natural loss of mitral cells during postnatal development at the same time that granule cell number is increasing.     

Regionalization of Fos immunostaining in rat accessory olfactory bulb when the vomeronasal organ was exposed to urine.

The distribution of Fos-immunoreactive (Fos-ir) cells in the accessory olfactory bulb (AOB) of rats following vomeronasal organ exposure to urine was studied. Following exposure to male and female Wistar rat urine, Fos-ir cells were found in the mitral/tufted cell layer, granule cell layer and periglomerular cell layer of the AOB of female Wistar rat, with the highest number in the granule cell layer. Exposure to water or removal of the vomeronasal organ suppressed the expression of Fos-ir cells. These results suggest that female Wistar rats specifically detect urinary substances derived from male or female Wistar rats via the vomeronasal organ. Exposure of the vomeronasal organ of female Wistar rats to male Wistar urine induced the appearance of many more Fos-ir cells in all layers of the AOB than exposure to female Wistar urine. As for the mitral/tufted cell layer, the density of Fos-ir cells in the rostral portion (Gi2alpha-positive) of all regions of the AOB was about twice as high as that in the caudal portion when male urine was given. The distribution pattern of Fos-ir cells in response to female urine was not identical to that in response to male urine. That is, the density of Fos-ir cells in the caudal portion was slightly larger than that in the rostral portion in the lateral region, while in other regions the density in the rostral portion was higher than that in the caudal portion. It is likely that information from different pheromones is transmitted to the higher brain regions through the different regions of the AOB.     

Short-term and long-term survival of new neurons in the rat dentate gyrus

New neurons continue to be generated in the dentate gyrus throughout adulthood. Previous studies have shown that a significant proportion of new granule cells labeled with the thymidine analogue bromodeoxyuridine (BrdU) are lost from the adult dentate gyrus within 2 weeks. How long this loss continues and the extent to which it represents cell death, as opposed to dilution of label, is unclear. To address these questions, adult rats were injected with BrdU, and BrdU labeling in the dentate gyrus was compared at several survival time points. Double labeling with BrdU and the cell cycle marker Ki-67 showed that BrdU is detectable for up to 4 days in some cells that continue to divide, indicating that any decrease in the number of BrdU-labeled cells after 4 days is likely to reflect cell death rather than BrdU dilution. Death of new cells in the granule cell layer occurred at a steady rate between 6 and 28 days after labeling, resulting in loss of 50% of BrdU-labeled cells over this 22-day period. New granule cells that survived this first month lived for at least 5 additional months. In contrast, 26% of the granule cells labeled with BrdU at the peak of dentate gyrus development on postnatal day (P) 6 died between 1 and 6 months after labeling. These findings suggest that granule cells born during adulthood that become integrated into circuits and survive to maturity are very stable and may permanently replace granule cells born during development.     

Time course and distribution of neuronal degeneration in the dentate gyrus of rat after adrenalectomy: a silver impregnation study.

Recently, Sloviter et al. reported that adrenalectomy (ADX) of young adult rats after 3 months led to a selective loss of granule neurons in the dentate gyrus (DG) and that this loss could be prevented by low doses of corticosterone. In the present study, the ADX-induced neuronal degeneration was investigated in Wistar rats, using a silver impregnation method for degenerating neurons. To examine the time course and distribution of the ADX-induced degeneration, young adult male rats were allowed to survive 2, 3, and 5 days and 1, 2, and 3 weeks after ADX. Argyrophilic neurons were present in the dentate granule cell layer on the second day following ADX. Three days after ADX, the number of argyrophilic granule neurons was much more abundant, and it increased gradually with longer post-ADX survival times. Argyrophilia was specifically confined to dentate granule cells and was accompanied by the occurrence of pyknotic nuclei as observed in adjacent cresyl violet-stained sections. There were significant differences between individual rats in quantity of argyrophilia. About one fifth of the ADX rats showed sporadic or no argyrophilia, in spite of plasma corticosterone levels below the detection limit (10 ng/mL). Sham-operated rats and ADX rats receiving corticosterone (10 mg/L) or dexamethasone (15 mg/L) in their drinking water did not display any argyrophilic neurons in the dentate gyrus. The distribution of the argyrophilia within the DG was highly characteristic with the highest number of degenerating cells in the hidden blade of the middle and the temporal thirds of the DG.(ABSTRACT TRUNCATED AT 250 WORDS)