Histogenesis of ferret somatosensory cortex

The ferret has emerged as an important animal model for the study of neocortical development. Although detailed studies of the birthdates of neurons populating the ferret visual cortex are available, the birthdates of neurons that reside in somatosensory cortex have not been determined. The current study used bromodeoxyuridine to establish when neurons inhabiting the somatosensory cortex are generated in the ferret; some animals also received injections of [³H]thymidine. In contrast to reports of neurogenesis in ferret visual cortex, most neurons populating the somatosensory cortex have been generated by birth. Although components of all somatosensory cortical layers have been produced at postnatal day 0, the layers are not distinctly formed but develop over a period of several weeks. A small number of neurons continue to be produced for a few days postnatally. The majority of cells belonging to a given layer are born over a period of approximately 3 days, although the subplate and last (layer 2) generated layer take somewhat longer. Although neurogenesis of the neocortex begins along a similar time line for visual and somatosensory cortex, the neurons populating the visual cortex lag substantially during the generation of layer 4, which takes more than 1 week for ferret visual cortex. Layer formation in ferret somatosensory cortex follows many established principles of cortical neurogenesis, such as the well-known inside-out development of cortical layers and the rostro-to-caudal progression of cell birth. In comparison with the development of ferret visual cortex, however, the generation of the somatosensory cortex occurs remarkably early and may reflect distinct differences in mechanisms of development between the two sensory areas. J. Comp. Neurol. 387:179–193, 1997. © 1997 Wiley-Liss, Inc.     

Visual cortex development in the ferret. I. Genesis and migration of visual cortical neurons

The production of ferret visual cortical neurons was studied using 3H-thymidine autoradiography. The genesis of cortical neurons begins on or slightly before embryonic day 20 (E20) of the 41 d gestational period, continues postnatally until 2 weeks after birth (P14), and follows an inside-out radial gradient with neurons for the deeper cortical layers being generated before those for the superficial layers. Layer I neurons are generated both early (E20-E30) and late (P1-P14) in the period of cortical neurogenesis and, thus, provide at least a partial exception to the inside-out gradient of cortical neurogenesis. Tangential gradients of cortical neurogenesis extend across areas 17 and 18 in both the anterior-to-posterior and lateral-to-medial directions. Neither of these gradients bears a meaningful relationship to the cortical representation of the visual field. Most infragranular and granular layer neurons are generated prenatally, while most supragranular layer neurons are produced postnatally. Neurons destined for a given layer are produced over a period of several days, and the neurons generated on any given day contribute to the formation of 2 or more cortical layers. In general, prenatally generated neurons complete their migration in 1 week or less, while most postnatally generated neurons require approximately 2 weeks to complete their migration.     

Times of generation of glutamic acid decarboxylase immunoreactive neurons in mouse somatosensory cortex

The birth dates of neurons showing glutamic acid decarboxylase (GAD) immunoreactivity have been determined in mouse somatosensory cortex. Pregnant C57Bl mice received pulse injections of (3H)thymidine from E10 through E17 (E0 being the day of mating). The distributions of thymidine-labeled, GAD-positive and nonimmunoreactive (non-GAD) cells as a function of depth under the pial surface were recorded in adult animals. The maximum rate of generation of GAD-positive neurons occurred at E14, whereas the generation of non-GAD neurons reached its maximum rate at E13. Except for those in layer I, GAD-positive neurons followed an inside-out sequence of positioning. GAD-positive neurons born at E12 and E13 were located in layers VI-IV. GAD-positive neurons born at E14 were found throughout the cortical thickness, with a maximum in layer IV. The GAD-positive neurons labeled after pulses at E15 or E16 or E17 were limited to the superficial strata, forming a band that became narrower as it moved toward the pial surface with increase in age of pulse labeling. GAD-positive neurons in layer I were generated at a constant rate during the whole embryonic period analyzed. Non-GAD neurons also followed an inside-out spatiotemporal gradient. Two partially overlapping phases were distinguished in non-GAD neurogenesis. During the first phase (from E12 to E14) neurons populating adult layers VI and V originated, while neurons located in layers IV through I were generated during the second phase (from E13 to E17). Since GAD-immunoreactive neurons form a heterogeneous population, we envisage further studies in order to test whether differences exist in birth dates among the classes.     

Time of origin of cortico-collicular projection neurons in the rat visual cortex.

The time of origin and the radial gradient of neurogenesis of cortico-collicular neurons have been studied in the rat visual area 17. We used a combined technique for the histochemical detection of the retrogradely transported horseradish peroxidase from the superior colliculus and the autoradiographic detection of the [3H]-thymidine administered during the gestational period. The cortico-collicular neurons of visual area 17 are located in layer V and are generated on gestational day (GD) 15 (59.78%), GD 16 (36.21%), and GD 17 (4.01%). This finding reveals that, for the cortico-collicular neuronal population, the birth date is well-correlated with the laminar position in the adult animal. To see whether the cortico-collicular neurons located at various radial levels of layer V are generated concurrently, or whether they follow an "inside-out" pattern of positioning, we divided layer V into three (upper, middle and lower) sublaminae. Most cortico-collicular neurons located in the lower two-thirds of layer V are generated on GD 15 (65%), whereas the neurons located in the upper third of the layer are generated both on GD 15 and GD 16 in almost equal proportions (52.53% and 44.39%, respectively).     

Neurogenesis of glutamic acid decarboxylase immunoreactive cells in the hippocampus of the mouse. I: Regio superior and regio inferior

The neurogenetic gradients of neurons showing glutamic acid decarboxylase (GAD) immunoreactivity were determined in the regio superior and in the regio inferior of the mouse hippocampus. Pregnant C57Bl mice received pulse injections of (3H)thymidine from E11 through E17 (E0 being the day of mating). Distributions of (3H)thymidine-labeled, GAD-positive neurons in the different strata of the hippocampus proper were recorded in adult animals. GAD-positive neurons in this region are generated prenatally. Radial gradients of neurogenesis of GAD-positive cells are characterized by two main features: 1) with the exception of the stratum lacunosum-moleculare and its interface with the stratum radiatum, GAD-positive neurons of the plexiform strata are generated before those destined for the pyramidal layer; 2) within the pyramidal layer, GAD-positive cells are positioned according to an inside-out sequence. In the transverse axis, neurogenesis of GAD-positive cells follows a regio inferior to regio superior gradient. This gradient is due to prolonged neurogenesis of GAD-positive cells for the pyramidal layer in the regio superior. Given the selective laminar disposition of the GABAergic interneurons in the hippocampus, the present authors explored whether or not the diverse types of these interneurons could have specific birth dates and concluded that no relationship exists between birth dates and adult phenotypes of GAD-immunoreactive cells in the mouse hippocampus proper.     

Laminar fate of cortical GABAergic interneurons is dependent on both birthdate and phenotype

Pioneering work indicates that the final position of neurons in specific layers of the mammalian cerebral cortex is determined primarily by birthdate. Glutamatergic projection neurons are born in the cortical proliferative zones of the dorsal telencephalon, and follow an "inside-out" neurogenesis gradient: later-born cohorts migrate radially past earlier-born neurons to populate more superficial layers. GABAergic interneurons, the major source of cortical inhibition, comprise a heterogeneous population and are produced in proliferative zones of the ventral telencephalon. Mechanisms by which interneuron subclasses find appropriate layer-specific cortical addresses remain largely unexplored. Major cortical interneuron subclasses can be identified based on expression of distinct calcium-binding proteins including parvalbumin, calretinin, or calbindin. We determined whether cortical layer-patterning of interneurons is dependent on phenotype. Parvalbumin-positive interneurons populate cortical layers with an inside-out gradient, and birthdate is isochronous to projection neurons in the same layers. In contrast, another major GABAergic subtype, labeled using calretinin, populates the cerebral cortex using an opposite "outside-in" gradient, heterochronous to neighboring neurons. In addition to birthdate, phenotype is also a determinant of cortical patterning. Discovery of a cortical subpopulation that does not follow the well-established inside-out gradient has important implications for mechanisms of layer formation in the cerebral cortex.     

Development of inhibitory circuitry in visual and auditory cortex of postnatal ferrets: immunocytochemical localization of GABAergic neurons.

The goal of this study was to describe the development of gamma-aminobutyric acid (GABA)-containing neurons in visual and auditory cortex of ferrets. The laminar and tangential distribution of neurons containing excitatory, inhibitory, and neuromodulatory substances constrain the potential circuits which can form during development. Ferrets are born at an early stage of brain development, allowing examination of inhibitory circuit formation in cerebral cortex prior to thalamocortical ingrowth and cortical plate differentiation. Immunocytochemically labelled nonpyramidal GABA neurons were present from postnatal day 1 throughout development, in all cortical layers, and generally followed the inside-out pattern of neuronal migration into the cortical plate. Prior to postnatal day 14, pyramidal neurons with transient GABA immunoreactivity were also observed. The density of Nissl-stained and GABA-immunoreactive neurons was high early in development, declined markedly by postnatal day 20, then remained relatively constant until adulthood. However, examination of the proportion of GABA neurons revealed an unexpected late peak at postnatal day 60, then a decrease in adulthood. Visual and auditory cortex were similar in most respects, but the peak at postnatal day 60 and the final proportion of GABA neurons was higher in auditory cortex. The late peak suggests that inhibitory circuitry is stabilized relatively late in sensory cortical development, and thus that GABA neurons could provide an important substrate for experience-dependent plasticity at late stages of development.     

Neurogenesis of the cat's primary visual cortex

The 3H-thymidine method of birth-dating was used to determine when the cells belonging to each of the principal cellular layers of the cat's primary visual cortex are generated. In order to detect systematic differences in the position of radioactively labeled cells following 3H-thymidine administration at different prenatal ages, a geometric method was devised to represent the distribution of labeled cells in the form of depth histograms. Results show that visual cortical neurogenesis occurs largely during the second half of gestation between embryonic day 31 (E31) and E57. Cells of layer 6 are generated early, between E31 and E38, whereas cells destined for successively more superficial layers are generated at progressively later times. Layer 4 cells, the principal targets of geniculocortical afferents, are generated between E37 and E44. In addition, a special population of cells embedded in the white matter below layer 6 was found to be produced throughout the week-long period immediately prior to the onset of layer 6 neurogenesis. Overall, this radial pattern of cortical neurogenesis closely resembles the inside-first, outside-last, spatiotemporal sequence of development described for the monkey's primary visual cortex (Rakic, '74). In addition to finding this pronounced gradient in the radial dimension, we were also able to detect a less pronounced gradient along the tangential dimension: neurons destined for any given layer in the anterior part of the cortex (inferior visual field representation) are generated slightly in advance of neurons destined for more posterior regions (superior visual field). However even our more quantitative histogram analysis failed to reveal a mediolateral (central to peripheral visual field) gradient within area 17. In the cat, layers 6, 5, and 4 each take about a week to be generated, although their total cell numbers and packing densities differ in the adult. About 2 weeks are required to produce the cells of layers 2 and 3 combined. Furthermore, we found that neurons belonging to different layers and different morphological classes can be generated simultaneously. This suggests that the identity of a cortical neuron is not solely a function of the time of neurogenesis.     

Development of GABA-immunoreactivity in the neocortex of the mouse.

The prenatal and postnatal development of GABAergic elements in the neocortex of the mouse was analyzed by GABA-immunocytochemistry. Radial distribution of cells and laminar numerical densities were calculated at each developmental stage to substantiate qualitative observations. The first immunoreactive neurons were observed in the cortical anlage at embryonic day 12-embryonic day 13 (E12-E13) in the primitive plexiform layer. At following prenatal stages (E14-E19), most GABA-positive neurons were present in the marginal zone, subplate, and subventricular zone. GABA-immunoreactivity in the cortical plate appeared early (E14), although the complete maturation of its derivatives was achieved postnatally. At prenatal stages we noted a well-developed system of immunopositive fibers in the subplate. As indicated by the direction of growth cones, most of these fibers had an extracortical origin and invaded the cortex laterally through the internal capsule and striatum. In rostral and middle telencephalic levels, fibers originating in the septal region contributed to the cingulate bundle. Presumably corticofugal fibers and callosal axons were also noticed. At postnatal stages the maturation of GABA-immunoreactivity appeared to be a complex, long-lasting process, in which the adult pattern was produced at the same time as the appearance of certain regressive phenomena. Thus, between postnatal day 0 and postnatal day 8 (P0-P8), GABA-positive populations disappeared from the subventricular zone, marginal zone and to a lesser extent from the subplate. At the same ages we noticed the presence of morphologically abnormal, GABA-immunoreactive neurons in the subventricular zone and subplate which are interpreted as correlates of neuronal degeneration. Most GABA-positive subplate fibers also disappeared whereas GABA-immunoreactive axons were seen in the cingulate bundle until the adult stage. In the derivatives of the cortical plate, the maturation of GABA-immunoreactive elements progressed according to the "inside-out" gradient of cortical development, with the important exception of layer IV, which was the last layer to exhibit an adult-like appearance. Within each layer deriving from the cortical plate (layers VIa to II-III), GABA-immunoreactivity showed a protracted maturation in which the first GABA-positive cells were detected a few days after cell birth but substantial numbers of neurons began to express GABA considerably later. The later phase occurred concurrently with the maturation of GABA-positive axonal plexuses. These results suggest that different GABA-positive populations show different developmental regulation of GABA expression during cortical ontogenesis.     

Patterns of γ-aminobutyric acid and glycine immunoreactivities reflect structural and functional differences of the cat lateral lemniscal nuclei

The three nuclei of the cat lateral lemniscus (dorsal, intermediate, and ventral) were distinguished by their immunoreactivities for the putative inhibitory transmitters, γ-aminobutyric acid (GABA) and glycine. Each nucleus had a distinct pattern of somatic and perisomatic labeling. The dorsal nucleus contained mostly GABA-immunoreactive neurons (85%), with moderate numbers of GABA- and glycine-immunoreactive puncta along their somata. The remaining neurons were nonimmunoreactive (15%). The intermediate nucleus contained mostly nonimmunoreactive neurons (82%), and these had numerous glycine-immunoreactive and few GABA-immunoreactive perisomatic puncta. The remaining neurons were immunoreactive for GABA only (10%), glycine only (2%), or both (6%). The ventral nucleus contained mostly glycine-immunoreactive neurons (81%), and about half of these were also GABA-immunoreactive. The remaining neurons were either nonimmunoreactive (8%) or GABA-immunoreactive only (11%). Neurons in the ventral nucleus had fewer immunoreactive perisomatic puncta than neurons in either the dorsal or the intermediate nuclei. These differences in neuronal immunoreactivity and in the relative abundance of GABA-and glycine-immunoreactive perisomatic puncta among the three nuclei of the lateral lemniscus support connectional and electrophysiological evidence that each nucleus has a different functional role in auditory processing. In particular, this study demonstrates that the intermediate nucleus of the cat is cytochemically distinct from the dorsal and ventral nuclei in terms of the somatic and perisomatic immunoreactivity of its neurons for these two important inhibitory transmitters and may provide novel inputs to the inferior colliculus. J. Comp. Neurol. 389:264–276, 1997. © 1997 Wiley-Liss, Inc.     

Expression of calbindin D-28k and parvalbumin in cerebral cortical dysgenesis induced by administration of ethylnitrosourea to rats at the stage of neurogenesis

It has been reported that transplacental administration of ethylnitrosourea (ENU), which is cytotoxic immediately after administration, to rat fetuses at the neurogenesis stage induces dysgenesis of the cerebral cortex, characterized by neuronal sparseness and architectural irregularity. In the present study, we examined the topographic distribution of neurons containing 5-bromo-2-deoxyuridine (BrdU), and those containing calbindin D-28k (CaBP) and parvalbumin (PV), most of latter two are considered to be interneurons located in particular layers of the normal cerebral cortex in rats with experimentally induced cerebral cortical dysgenesis. Pregnant Wistar albino rats were given a single transplacental administration of ENU on embryonic day 16, followed 4, 8, 16, 24, 36, or 48 h later by a single intraperitoneal injection of BrdU. The pups were killed 10 weeks after birth. In the normal cerebral cortex, BrdU-immunopositive neurons showed an inside-out pattern according to the time of BrdU injection, whereas in ENU-treated rats the topographic localization of the BrdU-immunopositive neurons was irregular and the inside-out pattern was disrupted. Although the number of CaBP- and PV-immunopositive neurons was lower in ENU-treated animals, no topographic difference was evident between the normal and the dysgenetic cerebral cortices. These findings indicate that the expression of CaBP and PV in the neurons of the rat cerebral cortex is extrinsic, and depends on the position of the neurons rather than on the time of their formation or on genetic control. This suggests the existence of re-regulation of the expression of CaBP and PV in the developing brain, which may be one of the effective mechanisms by which the cerebral cortex can maintain its normal function in spite of cytoarchitectural abnormality.     

Neurogenesis in a marsupial: the brush-tailed possum (Trichosurus vulpecula). I. Visual and auditory pathways

The times of origin of neurons in the visual and auditory systems were studied in a marsupial, the brush-tailed possum, using tritiated thymidine autoradiography. Within the subcortical visual pathways, most neurons are generated between postnatal days 5 and 21, and the neurons of the primary visual cortex up to postnatal day 68. In the subcortical auditory pathways, most neurons are generated between postnatal days 5 and 28, and all auditory cortex neurons have appeared by postnatal day 46. Neurons in a single layer of cerebral cortex are generated during a period of about 2 weeks. Thus cortical neurogenesis in marsupials extends over a period similar to that seen in primates.     

Early regionalisation of the neocortex and the medial ganglionic eminence

The two major functional classes of neurons that build the cerebral cortex are generated in two distinct parts of the telencephalon. Excitatory long distance projecting neurons are produced dorsally in the pallium, whereas local inhibitory interneurons are mainly produced in the medial ridge of the ventral telencephalon. These two parts of the telencephalon are molecularly regionalized from early embryonic stages, but cellular indices of regionalisation are observed only at later stages of development. We have looked for cellular indices of regionalisation in the cortical anlage at early embryonic stages, when the first efferent cortical neurons are generated. Similarly, we have looked for functional regionalisation of the medial ganglionic eminence at the same stages, when the future cortical interneurones are generated. Here, we summarize data showing that two regions in the mouse cortex embryo, the lateral and dorsal cortex, differ strongly in their early neurogenesis. Moreover, the two domains differ in their capacity to produce GABAergic neurons in vitro; this capacity is only observed in the dorsal cortex. The differentiation of the two domains appears to be independent of the laterorostral to mediocaudal gradient of maturation of the cortex. In the basal telencephalon too, the capacity to differentiate GABAergic neurons is not uniformly distributed across the medial ganglionic eminence. The neurogenesis of future cortical interneurons is seen to be highly active in a small area located in the rostral MGE, at mid dorso-ventral level.     

Embryonic cortical neurons differentiate into various types of interneurons when heterotopically transplanted into the adult rat brain

The adult cortex represents a heterogeneous mixture of different classes of pyramidal neurons and non-pyramidal interneurons. After grafting embryonic cortical anlage into the adult striatum, the present study investigated whether the development of different populations of interneurons in heterotopic cortical grafts is similar to the adult cortex. The presence of specific subpopulations of interneurons in grafts was assessed by immunocytochemistry using various antibodies against marker molecules for interneurons such as neuropeptides and calcium-binding proteins. These molecules are expressed to a different extent in specific subpopulations of cortical interneurons. Cortical primordia obtained on embryonic day 14 (1314) were stereotactically grafted into the center of the left striatum of adult recipient rats. After an 8-week differentiation period, host rats were perfusion fixed and immunocytochemistry was performed using antibodies against neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), somatostatin, parvalbumin and calbindin D-28k. Within the grafts, the number of immunopositive interneurons as well as the intensity of immunostaining for different marker molecules corresponded well with those of the adult cortex. In contrast, the expression pattern in the graft demonstrated clear differences when compared with the surrounding host striatum. The present study demonstrates, that at E14 at least some cells of the cortical anlage are primed to develop into different classes of interneurons independent of their normal environment and their regular synaptic connections. Thus, different interneuron progenitor cells survive transplantation and develop cell-specific morphological and cytochemical characteristics. Differentiation into various subpopulations of neurons may be a prerequisite for potential therapeutic approaches in humans.     

Visualization of neurogenesis in the central nervous system using nestin promoter-GFP transgenic mice

Neurons are generated from neural progenitor cells not only during development but also in the mature brain. To develop an in vivo system for analyzing neurogenesis, we generated transgenic mice expressing green fluorescent protein (GFP) under the control of regulatory regions of the nestin gene. GFP fluorescence was observed in areas and during periods connected with neurogenesis, including embryonic neuroepithelium, neonatal cerebellum, and hippocampal dentate gyrus and rostral migratory pathway from the subventricular zone to the olfactory bulb in the adult. GFP-positive cells in the adult brain included immature neuronal cells expressing polysialylated NCAM. BrdU labeling experiments revealed that newly generated interneurons which migrated rostrally from the subventricular zone expressed GFP until they reached the olfactory bulb. These results indicate that nestin promoter-GFP transgenic mice can be utilized to visualize the regions of neurogenesis throughout the life of the animals and to follow the migration and differentiation of newly generated neurons.     

Neurogenesis and stereological morphometry of calretinin-immunoreactive GABAergic interneurons of the neostriatum

We determined the neurogenesis characteristics of a distinct subclass of rat striatum gamma-aminobutyric acidergic (GABAergic) interneurons expressing the calcium-binding protein calretinin (CR). Timed-pregnant rats were given an intraperitoneal injection of 5-bromo-2'-deoxyuridine (BrdU), a marker of cell proliferation, on designated days between embryonic day 12 (E12) and E21. CR-immunoreactive (-IR) neurons and BrdU-positive nuclei were labeled in the adult neostriatum by double immunohistochemistry, and the proportion of double-labeled cells was quantified. CR-IR interneurons of the neostriatum show maximum birth rates (>10% double labeling) between E14 and E17, with a peak at E15. CR-IR interneurons occupying the lateral half of the neostriatum become postmitotic prior to medial neurons. In the precomissural neostriatum, the earliest-born neurons occupy the lateral quadrants and the latest-born neurons occupy the dorsomedial sector. No significant rostrocaudal neurogenesis gradient is observed. CR-IR neurons make up 0.5% of the striatal population and are localized in both the patch and the matrix compartments. CR-IR neurons of the patch compartment are born early (E13-15), with later-born neurons (E16-18) populating mainly the matrix compartment. CR-IR cells of the neostriatum are a distinct subclass of interneurons that are born at an intermediate time during striatal development and share common neurogenesis characteristics with other interneurons and projection neurons produced in the ventral telencephalon.     

Expression of TrkB and TrkC but not BDNF mRNA in neurochemically identified interneurons in rat visual cortex in vivo and in organotypic cultures

The mammalian visual cortex contains morphologically diverse populations of interneurons whose neurochemical properties are believed to be regulated by neurotrophic factors. This requires the expression of neurotrophin receptors. We have analysed whether brain-derived neurotrophic factor (BDNF), its receptor trkB and the NT-3 receptor trkC are expressed in interneurons of rat visual cortex in vivo, and in organotypic visual cortex cultures, paying particular attention to the subsets of neuropeptidergic neurons. In situ hybridization in combination with immunofluorescence for calcium-binding proteins and neuropeptides revealed that BDNF is not expressed in interneurons in vivo or in vitro. For the neurotrophin receptors we found in vivo at postnatal day 70 (P70) that approximately 80% of the parvalbumin-immunoreactive (-ir), but only 50% of the intensely calbindin-ir, and only 20% of the calretinin-ir neurons express trkB. Double labelling with neuropeptides revealed that approximately 50% of the neuropeptide Y-ir and approximately 50% of the somatostatin-ir neurons express trkB in a laminar-specific way. Only 25% of the vasoactive intestinal polypeptide (VIP)-ir neurons coexpress trkB. The coexpression of neuropeptide Y with trkB, but not with BDNF or trkC, was confirmed with a double in situ hybridization. In contrast, the percentages differed in the immature cortex; at P14 70% of the NPY-ir neurons and 46% of the calretinin-ir neurons revealed trkB expression, while the ratio for calbindin-ir cells was fairly constant (59%). From the interneuron populations studied, only 12% of the parvalbumin-ir neurons expressed trkC. A triple labelling revealed that some neurons coexpressed both trk mRNAs, while others had only trkC. The analysis of interneurons in organotypic cultures yielded very similar results. The results indicate that trkB ligands synthesized by pyramidal neurons influence neuropeptide or calcium-binding protein expression in a paracrine or transsynaptic manner. However, in contrast to current belief, in the adult only about half of all interneurons appear responsive to trkB ligands. Although the proportion is higher in the immature cortex, not all of the interneurons appear neurotrophin-receptive. With regard to the presence or absence of neurotrophin receptors, the molecular heterogeneity of GABAergic interneurons in the visual cortex is higher than currently assumed, and the responsiveness to neurotrophins changes with development in a cell type-specific way.     

Decreased proportion of GABA neurons accompanies age-related degradation of neuronal function in cat striate cortex

Electrophysiological studies indicate that a decline of GABAergic inhibition in the visual cortex may underlie age-related degradation of visual function [A.G. Leventhal, Y. Wang, M. Pu, Y. Zhou, Y. Ma, GABA and its agonists improved visual cortical function in senescent monkeys, Science 300 (2003) 812-815; M.T. Schmolesky, Y. Wang, M. Pu, A.G. Leventhal, Degradation of stimulus selectivity of visual cortical cells in senescent rhesus monkeys, Nat. Neurosci. 3 (2000) 384-390]. To date, there is little direct evidence to support this hypothesis. Using Nissl staining and immunohistochemical techniques, we quantitatively compared the density of total neurons (Nissl-stained neurons) and GABA-immunoreactive neurons as well as the proportion of GABA-immunoreactive neurons to total neurons in the primary visual cortex between 4 young adult (1-3 year old) cats and 4 old (12 year old) cats, which had been previously examined in a single-unit recording study [T. Hua, X. Li, L. He, Y. Zhou, Y. Wang, A.G. Leventhal, Functional degradation of visual cortical cells in old cats, Neurobiol. Aging 27 (2006) 155-162]. In that study, we found the function of V(1) (area 17) neurons in the old cats was significantly degraded relative to young adult cats. Our present results indicate that the density of total neurons in each cortical layer of V(1) exhibit no significant difference in the two age groups of cats. However, the density of GABA-immunoreactive neurons in old cats is significantly lower than in young adults. Further, the ratio of GABA-immunoreactive neurons to total neurons in each layer of V(1) in old cats is also significantly decreased when compared to young adult cats. These results provide direct morphological evidence of decreased GABAergic inhibition in the striate visual cortex of old animals, which accompany the functional degradation of visual cortical neurons.     

Neurogenesis in the mammalian neostriatum and nucleus accumbens: Parvalbumin-immunoreactive GABAergic interneurons

We study the neurogenesis of a distinct subclass of rat striatum γ-aminobutyric acid (GABA)ergic interneurons marked by the calcium-binding protein parvalbumin (PV). Timed pregnant rats are given an intraperitoneal injection of bromodeoxyuridine (BrdU), a marker of cell proliferation, on designated days between embryonic day (E) 11 and E22. Birthdate of PV neurons is determined in the adult neostriatum and nucleus accumbens by using a BrdU-PV double-labeling immunohistochemical technique. PV-immunoreactive interneurons of the neostriatum show maximum birthrates (>10% double-labeling) between E14–E17, whereas PV-immunoreactive interneurons of the nucleus accumbens show maximum double-labeling between E16–E19. In the neostriatum, caudal PV-immunoreactive neurons are born before those at rostral levels, and lateral PV-immunoreactive neurons become postmitotic before medial neurons. In the postcommissural striatum, ventral PV-immunoreactive neurons become postmitotic before dorsal neurons. In the precommissural striatum, ventral neurons are born before dorsal neurons laterally, but a dorsoventral gradient is seen medially. At corresponding coronal levels, PV-immunoreactive neurons of the nucleus accumbens are born shortly after PV neurons of the neostriatum. Analysis of BrdU labeling intensity in the nucleus accumbens shows that medium spiny projection neurons of the shell become postmitotic before neurons of the core. Similarly, PV-immunoreactive interneurons of the nucleus accumbens shell are born before PV interneurons of the core. Compared with cholinergic interneurons of the neostriatum, PV-immunoreactive interneurons are born later, but neurogenetic gradients are similar. The period of striatum PV interneuron genesis encompasses the period for somatostatin interneurons, although the latter neurons do not show neurogenetic gradients, possibly due to heterogeneous subtypes. Consideration of basal telencephalon neurogenesis suggests that subpopulations of striatum interneurons may share common neurogenetic features with phenotypically similar populations in the basal forebrain, with final morphology and connectivity depending on local cues provided by the host environment. J. Comp. Neurol. 389:193–211, 1997. © 1997 Wiley-Liss, Inc.     

Development of vasoactive-intestinal-polypeptide-immunoreactive neurons in the rat occipital cortex: a combined immunohistochemical-autoradiographic study

The postnatal development of vasoactive intestinal polypeptide (VIP)-immunoreactive neurons, previously labeled with [3H]thymidine on embryonic days E14-E22, has been studied in the rat occipital cortex. Immuno-histochemistry combined with autoradiography showed very little evidence of an "inside-out" pattern of maturation. Most VIP neurons are generated between E17 and E21 and are found in layers II-IV of the cortex, but their position within these layers is not dictated by their date of birth. There is evidence of a temporal maturation since E17 VIP neurons were seen first (at day 7) and E21 last. Peak numbers of VIP neurons were generated on E19. The numbers of VIP-immunoreactive neuronal somata detected in the cortex increased from the first week after birth to the third week and declined thereafter. However VIP-immunoreactive dendrites were still visible, suggesting that VIP levels in the cell bodies were very low, and not that there was a loss of neurons.