Aging and the human neocortex

The in vivo actions of insulin-like growth factor-I (IGF-I) on the growth and development of the hippocampal dentate gyrus were investigated in transgenic mice that overexpress IGF-I postnatally in the brain and in normal nontransgenic littermate controls. Stereological analyses of the dentate gyrus were performed by light and electron microscopy on days 7, 14, 21, 28, 35, and 130 to determine postnatal changes in the numerical density and total number of neurons and synapses. The volumes of both the granule cell layer and the molecular layer of the dentate gyrus were significantly increased by 27-69% in transgenic mice after day 7, with the greatest relative increases occurring by day 35. Although the numerical density of neurons in the granule cell layer did not differ significantly between transgenic and control mice at any age studied, the total number of neurons was significantly greater in transgenic mice by 29-61% beginning on day 14. The total number of synapses in the molecular layer was significantly increased by 42-105% in transgenic mice from day 14 to day 130. A transient increase in the synapse-to-neuron ratio was found in transgenic mice at postnatal days 28 and 35 but not at day 130. This finding indicates a disproportionate increase in synaptogenesis, exceeding that expected for the observed increase in neuron number. Our results demonstrate that IGF-I overexpression produces persistent increases in the total number of neurons and synapses in the dentate gyrus, indicating that IGF-I promotes both neurogenesis and synaptogenesis in the developing hippocampus in vivo.

The rat brain undergoes a period of rapid growth in the early postnatal period. During this time, the neocortex seems to be vulnerable to ethanol injury. Subdivisions of the neocortex develop in a temporospatial gradient that is likely to determine their vulnerability to ethanol-induced damage and whether damage is permanent. Therefore, the authors investigated the effect of postnatal ethanol exposure on the neocortex and specific subregions at the cessation of exposure and in the mature brain. Four-day-old rat pups with intragastric cannulae were artificially reared from postnatal day (PN) 4 through PN9. Of 12 daily feeds, two consecutive feeds contained either ethanol (4.5 g/kg) or an isocaloric maltose/dextrin solution. On PN10 or PN115, animals were perfused intracardially, and the brains were removed. Stereological methods were used to determine the total number of neurons and glial cells in, and the volume of, the neocortex, the parietal cortex, and the occipital cortex. Exposure to ethanol did not affect body or brain weight at PN10. In contrast, at PN115 forebrain weight was significantly lower in ethanol-exposed animals compared with control-treated animals. There was no effect of treatment on body weight at PN115. On PN10, neocortical volume was 15% smaller in the ethanol-exposed animals compared with controls, with no change in the total number of neurons or glial cells. Occipital cortical volume was reduced by 22% in the ethanol-exposed animals, with a significant deficit in the total number of neurons (ethanol-exposed, 2.62 x 10; gastrostomy control, 3.20 x 10). There was no effect of ethanol exposure on the total number of glial cells in the occipital cortex or on any parameter in the parietal cortex. There was also no significant effect of ethanol exposure on the occipital cortex on PN115. These findings provide support for the hypothesis that a specific area or cell population might be differentially vulnerable to ethanol exposure during the brain growth spurt and that cell deficits evident on PN10 may not be permanent.

The total numbers of neurons, glial cells, and endothelial cells in rat cerebral cortex were estimated using unbiased stereological counting techniques and systematic sampling. The reference volume chosen was the entire neocortex, most of the allocortex and parts of claustrum using the rhinal fissure as the macroscopical anatomical landmark. These regions are referred to collectively as syncortex. A method has been devised for reducing problems associated with the uncertainties that arise when distinguishing between various types of cells. At the light microscopic level, using the detailed criteria described in this article, the total numbers of neurons, glial cells, and endothelial cells, respectively, were estimated for the entire syncortex as the product of the estimate of the volume of the syncortex, made with point counting techniques, and the estimates of the numerical density for each group of cells, made with optical disectors. In a sample of three brains, the mean total number of cells (neurons, glial and endothelial) in the syncortex of the rat brain is 128 x 106. This number is made up of 47% neurons, 24% glial cells, 17% endothelial cells, and 11% uncertain cell types (probably mostly glial cells).

The medulla was sampled from nine cases of sudden infant death syndrome (SIDS) and from six age-matched control cases without neurological disease. Morphometric analyses were performed on serial Nissl sections through the hypoglossal nucleus on the left side of the medulla. The total volume of the nucleus and both the numerical density (Nv, cells per mm3) and total number of neurons were measured. Tissue from the remaining hypoglossal nucleus was prepared for electron microscopy using the ethanolic phosphotungstic acid method to stain synaptic contacts. Stereological analyses were performed to determine the Nv and total number of synapses. Total volume of the hypoglossal nucleus was significantly greater (36%) in SIDS cases than in controls. The Nv of neurons was significantly less than in controls (28%), although the total number of neurons did not differ significantly. The mean profile area of motor neuron cell bodies was significantly greater (30%) in SIDS cases, with no differences in the mean profile areas for interneurons or glia. The Nv of synapses did not differ significantly between SIDS cases and controls, although the total number of synapses was greater (61%) in SIDS. These abnormalities in growth indicate a greater volume of neuropil in a hypoglossal nucleus containing a normal complement of neurons. The greater number of synapses in SIDS cases is consistent with a failure to eliminate normally extraneous synapses during early development.

Variation in environmental conditions associated with differential selection on spatial memory has been hypothesized to result in evolutionary changes in the morphology of the hippocampus, a brain region involved in spatial memory. At the same time, it is well known that the morphology of the hippocampus might also be directly affected by environmental conditions. Understanding the role of environment-based plasticity is therefore critical when investigating potential adaptive evolutionary changes in the hippocampus associated with environmental variation. We previously demonstrated large elevation-related variation in hippocampus morphology in mountain chickadees over an extremely small spatial scale. We hypothesized that this variation is related to differential selection pressures associated with differences in winter climate severity along an elevation gradient, which make different demands on spatial memory used for food cache retrieval. Here, we tested whether such variation is experience based, generated by potential differences in the environment, by comparing the hippocampus morphology of chickadees from different elevations maintained in a uniform captive environment in a laboratory with those sampled directly from the wild. In addition, we compared hippocampal neuron soma size in chickadees sampled directly from the wild with those maintained in laboratory conditions with restricted and unrestricted spatial memory use via manipulation of food-caching experiences to test whether memory use can affect neuron soma size. There were significant elevation-related differences in hippocampus volume and the total number of hippocampal neurons, but not in neuron soma size, in captive birds. Captive environmental conditions were associated with a large reduction in hippocampus volume and neuron soma size, but not in the total number of neurons or in neuron soma size in other telencephalic regions. Restriction of memory use while in laboratory conditions produced no significant effects on hippocampal neuron soma size. Overall our results showed that captivity has a strong effect on hippocampus volume, which could be due, at least partly, to a reduction in neuron soma size specifically in the hippocampus, but it did not override elevation-related differences in hippocampus volume or in the total number of hippocampal neurons. These data are consistent with the idea of the adaptive nature of the elevation-related differences associated with selection on spatial memory, while at the same time demonstrating additional environment-based plasticity in hippocampus volume, but not in neuron numbers. Our results, however, cannot rule out that the differences between elevations might still be driven by some developmental or early posthatching conditions/experiences.

The rhesus monkey is widely used as an experimental animal model in the study of brain function and disease. While previous quantitative studies have provided knowledge of regional numbers, little is known of the total neocortical neuron and glial cell numbers in this species. The aim of this study is to establish quantitative norms. We use the optical fractionator and Cavalieri principle to examine the right hemisphere of eight young rhesus monkeys taken from the control group of an ongoing study. Applying these methods to agar-embedded and vibratome-sectioned tissue, we generate estimates of cell numbers and regional volumes of neocortical and hippocampal regions with coefficients of variance (CV) around 10%. The mean unilateral neocortical neuron number is 1.35 x 10(9) (CV +/- 0.10) and the mean unilateral neocortical glial cell number is 0.78 x 10(9) (CV +/- 0.17). Mean unilateral neocortical volume is found to be 8.5 (CV +/- 0.10) cm(3) after processing, or 19 cm(3) when correcting for shrinkage. The neuron/glia ratio is 1.77. The neurons are distributed with 18% in the frontal cortex, 57% in the temporal and parietal cortices, and 25% in the occipital cortex. In the hippocampal subregions, we found unilateral neuron number of 1.72 x 10(6) (CV +/- 0.13) and glial number of 2.25 x 10(6) (CV +/- 0.17) in CA1, and 0.80 x 10(6) (CV +/- 0.27) neurons and 1.05 x 10(6) (CV +/- 0.26) glial cells in CA2-3. Comparisons with related studies show quantitative variation, but also variations in methods and applications. The results are phylogenetically consistent, apart from the neuron/glia ratio, which is remarkably higher than what is found in other species.

Role of neural afferents as mediators of estrogen effects on the hypothalamic ventromedial nucleus

1. Intraplantar carrageenin (6 mg 150 microliters-1) evoked a high level of spinal c-Fos expression in the dorsal horn, of segments L4-L5 of the spinal cord, and an extensive peripheral oedema; both parameters were assessed 3 h after carrageenin. 2. Two series of experiments were performed, with the mean total number of Fos like-immunoreactive neurones (Fos-LI), after carrageenin, not being significantly different for the two series of experiments (266 +/- 17 and 332 +/- 31 Fos-LI neurones). For both series of experiments Fos-LI neurones were predominantly located in the superficial and deep laminae, only 10% of the total number of Fos-LI neurones were located in the nucleus proprius and 10% were located in the ventral horn. 3. Pre-administration of the N-methyl-D-aspartate (NMDA) receptor antagonist, (+)-HA966 (0.5 mg kg-1 and 2.5 mg kg-1, s.c.), 30 min before carrageenin, did not significantly influence the total number of Fos-LI neurones, as compared to control carrageenin expression. 4. Pre-administration of the highest dose of (+)-HA966 (10 mg kg-1) significantly reduced the number of deep laminae Fos-LI neurones (28 +/- 3% reduction of control number of Fos-LI neurones after carrageenin, P < or = 0.05), without influencing the number of superficial Fos-LI neurones. There was a tendency towards a reduction of the number of Fos-LI neurones in the nucleus proprius by the highest concentration of pre-administered (+)-HA966, (31 +/- 8% reduction), but this effect did not reach significance. 5. Pre-plus post-administered (+)-HA966 (0.5 mg kg-1), 30 min before and again 45 min afterintraplantar carrageenin, did not significantly influence the total number of Fos-LI neurones, as compared to control carrageenin expression.6. Pre- plus post-administration of 2.5 mg kg-1 (+)-HA966 significantly reduced the total number of Fos-LI neurones, as compared to control carrageenin expression. This effect was reflected by a significant reduction in the number of Fos-LI neurones in the nucleus proprius (36+/-7% reduction of control carrageenin c-Fos expression respectively, P<0.05).7. Pre-plus post-administration of 10 mg kg-1 of (+)-HA966 significantly reduced the number of Fos-LI neurones in the superficial laminae, nucleus proprius, deep laminae and ventral horn (33 +/-0.5%,55+/-6%, 40+/-4% and 51+/-4% reduction of control carrageenin c-Fos expression, respectively, P<0.05,for all areas).8. A single post-administration of (+)-HA966 (10 mg kg-1), 45 min after intraplantar carrageenin, did not significantly influence the number of Fos-LI neurones in the superficial, deep laminae or ventral horn, but significantly reduced the number of Fos-LI neurones in the nucleus proprius, as compared tocontrol carrageenin expression (39+-/8% reduction of control carrageenin c-Fos expression, P <0.05).9 None of the concentrations of (+)-HA966 studied, irrespective of the timing of administration,influenced the peripheral carrageenin oedema. Our results illustrate a contribution of central NMDA receptor activation to carrageenin-evoked spinal c-Fos expression. These results extend previous studies demonstrating the contribution of the NMDA receptor to central hyperalgesia and the expression of c-Fos.

No deficit in total number of neurons in the prefrontal cortex in schizophrenics

Data from transgenic mouse models of Huntington's disease (HD) suggest that dysfunction of the hypothalamic infundibular nucleus (INF) (in rodents, the arcuate nucleus) may contribute to unintended weight loss and insatiable appetite among HD patients. Using post-mortem paraffin-embedded tissue, we assessed the total number of INF neurones by thionin staining and four major regulatory neuropeptides in the INF of HD patients by immunocytochemistry and in situ hybridisation. In HD patients, the total number of neurones in the INF was unchanged compared to control subjects (P = 0.92), whereas it contained over 30% less neuropeptide Y-immunoreactive (IR) neurones (P = 0.016), as well as reduced peptide levels, in fibres to the paraventricular and ventromedial nucleus (P = 0.003, P = 0.005, respectively). Conversely, neuropeptide Y mRNA expression levels were increased three-fold (P = 0.047). No changes were observed in the number of neurones immunoreactive for α-melanocyte-stimulating hormone, agouti-related peptide, and cocaine- and amphetamine-regulated transcript (P ≥ 0.17). Our findings suggest changes in the pathology of the INF neuropeptide Y-expressing neurones in HD patients without changes in other (an)orexigenic neuropeptides and without neuronal cell loss. These findings indicate that unintended weight loss in patients suffering from this disease may be partly a result of neuropeptidergic alterations in the hypothalamic infundibular nucleus.

There have been numerous studies of neuronal hyperactivity and oxidative stress induced by iron. Moreover, it has been found that iron levels in the brain are markedly increased in some neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). We attempted to identify the toxic effect of iron on the hippocampus, a region of the brain known to be involved in learning and memory processes and which lends itself to stereological analysis. Rats were given iron chloride (FeCl35H2O), 200 mM in 2.5 μl) intracortically and were killed 10 days later. The total number of neurons in the hippocampus of control and iron‐treated rats was counted with an optical fractionator. Iron administration caused a significant decrease in the total number of pyramidal neurons in the left (31.6% decrease) and right (32.2% decrease) hippocampi (p&lt;0.001). The findings suggest that excess iron contributes to pyramidal cell death in the rat hippocampus.

The effects of age and sex on pain sensitivity in young lambs

Volume and neuron number of the mediodorsal thalamic nucleus in schizophrenia: A replication study

Cartilaginous fishes have large and elaborate olfactory organs, but only a small repertoire of olfactory receptor genes. Here, we quantitatively analyze the olfactory system of 21 species of sharks and rays, assessing many features of the olfactory organ (OOR) (number of primary lamellae, branches of the secondary folds, sensory surface area, and density and number of sensory neurons) and the olfactory bulb (OB) (number of neurons and non-neuronal cells), and estimate the ratio between the number of neuronsin the two structures. We show that the number of lamellae in the OOR does not correlate with the sensory surface area, while the complexity of the lamellar shape does. The total number of olfactory receptor neurons ranges from 30.5 million to 4.3 billion and the total number of OB neurons from 1.5 to 90 million. The number of neuronsin the olfactory epithelium is 16 to 158 times higher (median ratio is 46) than the number of neuronsin the OB. These ratios considerably exceed those reported in mammals. High convergence from receptor neurons to neurons processing olfactory information, together with the remarkably small olfactory receptor repertoire, strongly suggests that the olfactory system of sharks and rays is well adapted to detect a limited number of odorants with high sensitivity.

Serotonin (5-HT) is a regulator of morphogenetic activities during early brain development and neurogenesis, including cell proliferation, migration, differentiation, and synaptogenesis. The 5-HT transporter (5-HTT, SLC6A4) mediates high-affinity reuptake of 5-HT into presynaptic terminals and thereby fine-tunes serotonergic neurotransmission. Inactivation of the 5-HTT gene in mice reduces 5-HT clearance resulting in persistently increased concentrations of synaptic 5-HT. In the present study, we investigated the effects of elevated 5-HT levels on adult neurogenesis in the hippocampus of 5-HTT deficient mice, including stem cell proliferation, survival, and differentiation. Using an in vivo approach, we showed an increase in proliferative capacity of hippocampal adult neural stem cells in aged 5-HTT knockout mice (approximately 14.5 months) compared to wildtype controls. In contrast, in vivo and additional in vitro analyses of younger adult 5-HTT knockout mice (approximately 7 weeks and approximately 3.0 months) did not reveal significant changes in proliferation of neural stem cells or survival of newborn cells. We showed that the cellular fate of newly generated cells in 5-HTT knockout mice is not different with respect to the total number and percentage of neurons or glial cells from wildtype controls. Our findings indicate that elevated synaptic 5-HT concentration throughout early development and later life of 5-HTT deficient mice does not induce adult neurogenesis in adult mice, but that elevated 5-HT levels in aged mice influence stem cell proliferation.