The quantitative effects of dark-rearing and light exposure on the laminar composition and depth distribution of neurons and glia in the visual cortex (area 17) of the rat

Summary The effects of dark-rearing and light-exposure on the distribution of neurons and glial cells types in the rat visual cortex (area 17) have been investigated. Three groups of animals were studied: i) rats reared in the dark until weaning at 21 days post natum (21 DPN) and subsequently light-exposed for 31 days (Group 21/31); ii) rats darkreared until 52 DPN and then exposed to light for 3 days (Group 3 dL); and iii) rats totally dark-reared until 52 DPN (Group 52 dD). Semithin sections tangential to the pial surface were obtained at sampling intervals 50 m apart throughout the depth of the left visual cortex. The volume numerical densities of neurons, astroglia, oligodendroglia, and microglia, at each sampling strata in the cortex were calculated using stereological techniques. The laminer density and distribution of neurons was not significantly different between the three groups. In comparison with group 21/31 there was a marked reduction in the densities of astroglia, oligodendroglia, and microglia in lower layer 5 of groups 3 dL and 52 dD. Additionally, the density of microglia in thalamorecipeint layer 4 was greatly increased in group 3 dL compared with groups 21/31 and 52 dD. These results indicate specific alterations in the glial cell composition of the rat visual cortex following periods of dark-rearing and light-exposure. Furthermore, changes in the density of glial cells in layer 5 may reflect functional modifications in neurons projecting to the superior colliculus.     

Quantitative distribution of GABA-immunoreactive neurons in the visual cortex (area 17) of the cat

Summary Cortical neurons using the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) are known to contribute to the formation of neuronal receptive field properties in the primary visual cortex (area 17) of the cat. In order to determine the cortical location of GABA containing neurons and what proportion of cortical neurons might use GABA as their transmitter, we analysed their distribution quantitatively using a post-embedding GABA immunohistochemical method on semithin sections in conjunction with stereological procedures. The mean total numerical density of neurons in the medial bank of the lateral gyrus (area 17) of five adult cats was 54,210±634 per mm³ (¯x±SD). An average of 20.60±0.48% (¯x±SEM) of the neurons were immunoreactive for GABA. The density of GABA-immunoreactive neurons was somewhat higher in layers II, III and upper VI, compared with layers I, IV, V and lower VI, with the lowest density being in layer V. The proportion of GABA-immunopositive cells relative to immunonegative neurons gradually decreased from the pia to the white matter. Layer I was different from other layers in that approximately 95% of its neurons were GABA-immunoreactive. The results allowed the calculation of the absolute numbers of GABAergic neurons in each layer under a given cortical surface area and could provide the basis for the quantitative treatment of cortical circuits.     

Receptive field plasticity of area 17 visual cortical neurons of adult rats

In contrast to somatosensory cortex (SI), where the pervasiveness of reorganizational capacities is well-established, plasticity of receptive fields (RFs) of adult primary visual cortex (VI) remains controversial. To investigate RF plasticity in VI of adult rats, we here used intracortical microstimulation (ICMS) to overcome particularities related to stimulus presentation and training procedures which limit comparison across modalities. Our results show that VI RFs can be altered by ICMS; however, changes depended on the pre-ICMS RF size. Initially small RFs expanded after 2 h of ICMS with little signs of recovery within the next hours, while initially large RFs remained unaffected. Inspection of the time course of neuron responses revealed, however, that in large RFs early response components were enhanced, while late response components were reduced resulting in changes of the spatiotemporal RF properties. Although plastic changes in VI showed a substantial heterogeneity, our results indicate a capacity of VI neurons to undergo plastic changes comparable to SI neurons. However, the magnitude and aspects of reversibility appeared to be different suggesting a significant modality-specificity of reorganizational changes of cortical sensory neurons.     

Cholinergic neurons and fibres in the rat visual cortex

Summary Choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, was localized immunocytochemically in neurons and fibres in the rat visual cortex using a monoclonal antibody. ChAT-labelled cells were non-pyramidal neurons, primarily of the bipolar form, distributed in layers II through VI but concentrated in layers II & III. Their perikarya contained a large nucleus and a small amount of perinuclear cytoplasm. The somata and dendrites of all labelled cells received Gray's type I and type II synapses.ChAT-stained axons formed a dense and diffuse network throughout the visual cortex and particularly in layer V. Electron microscopy revealed that the great majority formed type II synaptic contacts with dendrites of various sizes, unlabelled non-pyramidal somata and, on a few occasions, with ChAT-labelled cells. However, a very small number of terminals appeared to form type I synaptic contacts. This study describes the morphological organization of the cholinergic system in the visual cortex, the function of which has been under extensive investigation.     

Cytoplasmic inclusions of neurons in the monkey visual cortex (area 19).

Several unusual neuronal inclusions were found in certain cells of the rhesus monkey visual cortex (Area 19): 1. Filamentous bodies, present in the small stellate cells of layer IV, globoid, 0.3-0.6 mum in diameter, consisting of fine 50 A filaments in a hexagonal meshwork. These are often associated with the labyrinthine bodies. 2. Labyrinthine bodies found exclusively in the small stellate cells of layer IV, including certain neurons with dispersed ribosomes. These are 0.4-0.7 mum in diameter and consist of 900 A wide tubes which interconnect with one another. The walls of these tubes are continuous and made up of a sheet or honeycomb lacework of small hexagonal 150 A subunits. 3. This inclusion, an aggregate 0.3-0.7 mum in size, consists of small membrane-bounded vesicles with a single dense granule associated with other non-membrane bound small dense droplets. The inclusions are always associated with the maturing face of the Golgi complex of certain layer IV pyramidal cells; as such, they may be an unusual product of the Golgi apparatus. These observations were confirmed by examination of stereo pairs of electron micrographs. Speculations are made with regard to possible functions for these inclusions.     

Distribution of GABA-containing neurons in human frontal cortex: a quantitative immunocytochemical study

Fresh biopsy specimens of human cerebral cortex were collected from patients suffering from deep-seated tumors requiring resection. GABAergic neurons were revealed in 50-m-thick sections, for pre-embedding, and 1-m-thick sections, for post-embedding GABA immunocytochemistry. In both thick and thin sections, the reaction product was found in neuronal cell bodies and in small profiles in the neuropil. In both preparations, GABA-containing somata were distributed evenly throughout the depth of the cortex. As best appreciated in the thicker sections, GABA-immunoreactive neurons belonged to a variety of morphological cell types with multipolar, bitufted or bipolar, and horizontal dendritic arbors. In the semi-thin sections sampled in the frontal cortex, the proportion of these neurons could be accurately evaluated and was found to be 21.2%±4.8% of all cortical neurons. The average size of GABA-immunoreactive neurons was, in each layer, smaller than that of immunonegative neurons. The average soma size of both neuronal populations, immunoreactive and immunonegative for GABA, increased with depth. The comparison between the rat, cat, macaque monkey, and human GABAergic interneurons revealed similarities among primate brains, contrasting with the parameters (morphology, size, density) measured in rodents. These data are pertinent to the involvement of the GABAergic neurons in the shaping of receptive-field properties of cortical neurons in healthy brains and in pathologies involving the impairment of inhibitory neurotransmission.     

A Golgi deimpregnation study of neurons in the rhesus monkey visual cortex (Areas 17 and 18)

Summary The morhological features of 298 neurons impregnated according to Golgi-Kopsch in areas 17 and 18 of Macaca mulatta were analyzed, and the same neurons were deimpregnated to visualize structural details of the somata in different types of neurons. The following cell types were investigated: Pyramidal and pyramid-like cells, spiny stellate cells, double bouquet cells, bipolar cells, chandelier cells, neurogliaform cells, basket and related cells. This procedure allows the evaluation of the nuclear-cytoplasmic proportion and the position of the nucleus besides shape and size of the cell body. Pyramidal and pyramid-like cells (N=43), spiny stellate cells (N=26), basket and related cells (N=126) are variable in these features. A positive correlation between soma size and width of the cytoplasm is found in pyramidal, pyramid-like cells and spiny stellate cells. With the exception of some large somata in both these types of neurons the nucleus is found in a central position. Double bouquet cells (N=6), bipolar cells (N=13) and chandelier cells (N=11) exhibit small cytoplasmic rims and centrally located nuclei. The small somata of neurogliaform cells (N=37), however, and the small to very large somata of basket and related cells show broad cytoplasmic portions surrounding the eccentrically located nuclei. These findings allow the identification of different neuronal types in Nisslstained sections on the basis of these soma features. This is a prerequisite for further detailed quantitative studies on the laminar distribution of different neuronal types in the visual cortex of the monkey.     

Axosomatic synapses in the visual cortex of adult rat. A comparison between GABA-accumulating and other neurons

Summary Differences in axosomatic synapses between GABA-accumulating [G(+)N] or non-accumulating [G(–)N] neurons have been investigated in the visual cortex of adult rat. The neurons were classified and localized in light microscopic autoradiograms after [³H]GABA injections. The cells were then resectioned for electron microscopic identification of type 1 synapses (T1S) and type 2 synapses (T2S). A total of 167 neurons [45 G(+)N, 122 G(–)N] situated in laminae II-VI were evaluated. The two groups of neurons were not uniform populations. G(–)N included both pyramidal and non-pyramidal neurons, whereas no typical pyramidal neurons were found among G(+)N.A total of 691 synaptic contacts was evaluated for these groups of somata. The density of synapses was higher on G(+)N than on G(–)N. This was mainly due to a difference in the number of T1S. On G(–)N the frequency distributions of both types of synapses represented Poisson distributions, indicating that there were stochastic variations around mean values. In contrast, on G(+)N the distribution was exponential which suggests that G(+)N include several subpopulations with different densities of T1S.On all cortical neurons the average density of T2S was 50–60 T2S per 1000 m² of soma surface, which resembles the density in the neuropil. In contrast, T1S varied from zero to a mean of 12 per 1000 m² on G(–)N and to a mean of 51 per 1000 m² on G(+)N, i.e. the density of T1S, unlike T2S, is much smaller on neuronal somata than in the surrounding neuropil. It is suggested that the formation of axosomatic T1S, but not of T2S, is suppressed to a variable degree on almost all cortical neurons. Only on pyramidal neurons does the suppression of T1S seem to be complete.     

Quantitative morphological effects of dark-rearing and light exposure on the synaptic connectivity of layer 4 in the rat visual cortex (area 17)

Summary The quantitative effects of dark-rearing and light exposure on the ultrastructural characteristics of synapses and synaptic boutons in layer 4 of the rat visual cortex (area 17) have been investigated using stereological techniques. Two experimental groups (each containing 5 animals) were investigated i) animals dark-reared upto weaning at 21 days post natum (21DPN) and then light exposed until 52DPN (Group 21/31), and ii) littermate animals totally dark-reared until 52DPN (Group 52dD). The results indicate a significantly higher mean density of synapses in the neuropil of layer 4 in group 21/31 (3.58×10⁸ · mm^-3) compared with group 52dD (2.68×10⁸ · mm^-3). Although the density per unit volume of synapses with identified asymmetrical synaptic membrane specialisations was not significantly different in group 21/31 than in group 52dD (but was significantly lower than animals reared normally), the density of synapses with identified symmetrical synaptic membrane specialisations was about 200% higher in group 21/31 versus group 52dD. However, significant differences were detected in the number of asymmetrical synapses established by single synaptic boutons in group 21/31 (1.21 ± 0.11) compared with group 52dD (1.10 ± 0.09). On the basis of the numbers of post-synaptic targets contacted by an individual synaptic bouton, a significantly higher density of synaptic boutons was found in group 21/31 (2.32×10⁸ · mm^-3) compared with group 52dD (1.82×10⁸ · mm^-3). Furthermore, planar quantitative data indicated significant inter-group differences in the ultrastructure of asymmetrical and symmetrical synaptic boutons. The results of this study provide evidence indicating marked structural alterations in the synaptic connectivity of layer 4 of the rat visual cortex following the light exposure of rats dark-reared upto weaning. Indeed visual deprivation severely affected the inhibitory circuitry in the major thalamorecipient territory of the visual cortex.     

A morphological analysis of the cluster organization of the cortico-cortical neurons in area 17 of the cat visual cortex

Spatial organisation and quantitative features of cortico-cortical neurons in area 17 that project to area 21a of cat visual cortex were investigated. We used the HRP technique and two-dimensional reconstruction from serial sections of brain. Pattern of distribution of labelled cells in some regions of areas 17 and 18, on the lateral and medial surface of the brain were analysed. The data about cluster organisation of cortico-cortical neurones, localisation, layer distribution and density of neurons were obtained.     

Response properties of neurons in the visual cortex of the rat

Summary Response properties of neurons in the visual cortex, area 17, of Long Evans pigmented rats were investigated quantitatively with computer-controlled stimuli. Ninety percent of the cells recorded (296/327) were responsive to visual stimulation. The majority (95%, 281/296) responded to moving images and were classified as complex (44%), simple (27%), hypercomplex (13%) and non-oriented (16%) according to criteria previously established for cortical cells in the cat and monkey. The remaining 5% of the neurons responded only to stationary stimuli flashed on-off in their receptive field. Results of this study indicate that neurons of the rat visual cortex have properties similar to those of cells in the striate cortex of more visual mammals.Supported by grant EY02964, the Biological Humanics Foundation and the Bendix Corporation     

Subcortical projections to layer I of the visual cortex,area 17, of the rat

Summary The afferent connections to layer I of the visual cortex, area 17, of the albino rat were studied using the retrograde transport of horseradish peroxidase (HRP). In all animals examined, the majority of labeled cells were observed in the dorsal lateral geniculate nucleus and in the lateral posterior nucleus. In addition, in one-half of the animals examined labeled cells were also present in the posterior complex of the thalamus and in the ventromedial nucleus. Finally, in three cases a few HRP-positive neurons were observed in the locus coeruleus and in the dorsal raphe nucleus.This work was supported by USPHS grant EY029464 and by an Alfred P. Sloan Foundation Research Fellowship to John G. Parnavelas     

Maturation of rat visual cortex. I. A quantitative study of Golgi-impregnated pyramidal neurons

Summary The early postnatal maturation of pyramidal neurons in layers II/III and V of the rat visual cortex has been examined in an attempt to elucidate some determinants of their mature morphology. Three indices have been quantified using Golgi-impregnated pyramidal cells: densities of spines along apical dendrites, numbers of primary basal dendrites and volumes of cell bodies. The mean density of spines on the apical dendrites of all pyramidal neurons increases in a stepwise fashion. The first significant increase occurs between days 6 and 9 and the second, between days 12 and 15; these increases may correlate with the arrival of geniculate afferents and with the opening of the eyes, respectively. In younger animals, the distribution of spines along the apical shafts is relatively even, whereas in older animals, spine density increases significantly over the proximal 125 m portion and is relatively constant over the remaining distal portion. By day 21, layer V pyramidal cells have acquired more primary basal dendrites and larger somatic volumes than layer II/III cells. Furthermore, as the cells mature the rates of change in these characteristics are significantly different for neurons in layer II/III and in layer V. For both cell populations, the mean number of primary basal dendrites increases to a maximum before falling to a steady level, but for neurons in layer V, the maximum is higher and attained three days earlier than for layer II/III cells. Moreover, the increase in volume of cell bodies of layer V neurons begins three days before that of layer II/III cells. This three day phase difference in maturation may reflect the cell birth dates, since autoradiographic evidence indicates that layer V pyramidal neurons reach the cortical plate about three days prior to those which occupy layer II/III in the adult visual cortex.     

Organization and properties of neurons in a visual area within the insular cortex of the cat

1. Extracellular recordings from 304 neurons were obtained with carbon fiber-containing multibarrel micropipettes. The cells were isolated in the insula in cats anesthetized with barbiturate and immobilized with gallamine triethiodide. Cells were tested with visual stimuli in the form of bars of light, moving edges, and square-wave, drifting grating patterns. 2. The spatial extent of the visually responsive region of insular cortex was assessed and was found to be limited to a surface area of approximately 6-8 mm2, the perimeter being delimited caudally by visually unresponsive cortex of the anterior sylvian gyrus, rostrally by the cortex surrounding the posterior third of the orbital sulcus (ventral bank), dorsally by the rostral extension of the dorsal bank of the anterior ectosylvian sulcus, and ventrally by a visually unresponsive zone bounded by a region about 2 mm2 ventrolateral from the anterior ectosylvian sulcal infolding. Furthermore, a group of unimodal, visually responsive cells often was found in the upper bank of the anterior rhinal sulcus. 3. The possibility of there being a visuotopic organization of insular neurons was examined by analyzing the distribution of receptive-field representation of neurons in sequential penetrations, as well as by searching for spatial progressions in the locations of visually responsive areas within the region. No such clear-cut organization was found among the cells of the insula. 4. Visually responsive neurons were encountered in groups, within electrode penetrations. These groupings were roughly segregated into three distinct levels within the depth of the cortex: the first between the pial surface and 600 micron, the second between 1,100 micron and 1,800 micron, and the third between 2,000 micron and 2,500 micron. 5. Neurons were classified according to their velocity sensitivity, directional preference, orientation sensitivity, length preference, modality specificity, response to electrical stimulation of extrageniculostriate regions, and response to light stimulation in the presence of microiontophoretically administered bicuculline methiodide (BMI). 6. Cells of superficial layers tended to exhibit a preference for high-velocity movements of light bars (600 degrees s-1), whereas those of deeper laminae generally preferred relatively lower velocity movements (60 degrees s-1). The clear preferences of many cells for certain directions of movement within the 360 degrees arc suggested the presence of a dynamic orientation sensitivity. 7. Proportionately more cells preferred moving bars (57%) to small moving spots (43%).(ABSTRACT TRUNCATED AT 400 WORDS)     

Distribution of apelin-synthesizing neurons in the adult rat brain

The peptide apelin originating from a larger precursor preproapelin molecule has been recently isolated and identified as the endogenous ligand of the human orphan G protein-coupled receptor, APJ (putative receptor protein related to the angiotensin receptor AT(1)). We have shown recently that apelin and apelin receptor mRNA are expressed in brain and that the centrally injected apelin fragment K17F (Lys(1)-Phe-Arg-Arg-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe(17)) decreased vasopressin release and altered drinking behavior. Using a specific polyclonal antiserum against K17F for immunohistochemistry, the aim of the present study was to establish the precise topographical distribution of apelin immunoreactivity in colchicine-treated adult rat brain. Immunoreactivity was essentially detected in neuronal cell bodies and fibers throughout the entire neuroaxis in different densities. Cells bodies have been visualized in the preoptic region, the hypothalamic supraoptic and paraventricular nuclei and in the highest density, in the arcuate nucleus. Apelin immunoreactive cell bodies were also seen in the pons and the medulla oblongata. Apelin nerve fibers appear more widely distributed than neuronal apelin cell bodies. The hypothalamus represented, by far, the major site of apelin-positive nerve fibers which were found in the suprachiasmatic, periventricular, dorsomedial, ventromedial nuclei and in the retrochiasmatic area, with the highest density in the internal layer of the median eminence. Fibers were also found innervating other circumventricular organs such as the vascular organ of the lamina terminalis, the subfornical and the subcommissural organs and the area postrema. Apelin was also detected in the septum and the amygdala and in high density in the paraventricular thalamic nucleus, the periaqueductal central gray matter and dorsal raphe nucleus, the parabrachial and Barrington nuclei in the pons and in the nucleus of the solitary tract, lateral reticular, prepositus hypoglossal and spinal trigeminal nuclei.The topographical distribution of apelinergic neurons in the brain suggests multiple roles for apelin especially in the central control of ingestive behaviors, pituitary hormone release and circadian rhythms.