The termination of callosal fibres in the auditory cortex of the rat. A combined Golgi--electron microscope and degeneration study

When the corpus callosum of the rat is sectioned, the callosal fibres in the cerebral cortex undergo degeneration. In the auditory cortex (area 41) the degenerating axon terminals form asymmetric synapses, and the vast majority of them synapse with dendritic spines. Some other synapse with the shafts of both spiny and smooth dendrites, and a few with the perikarya of non-pyramidal cells. The degenerating axon terminals are contained principally within layer II/III, in which they aggregate in patches. Using a technique in which neurons within the cortex are Golgi-impregnated, then gold-toned and examined in the electron microscope, it has been shown that the dendritic spines of pyramidal neurons with cell bodies in different layers receive the degenerating callosal afferents. The spines arise from the main apical dendritic shafts and their branches, from the dendrites of the apical tufts, and in some cases from the basal dendrites of the pyramidal neurons. The shafts of some pyramidal cell apical dendrites also form asymmetric synapses with callosal afferents. Since we have encountered no spiny non-pyramidal neurons in Golgi preparations of rat auditory cortex, and because other types of non-pyramidal cells have few dendritic spines, it is concluded that practically all of the dendritic spines synapsing with callosal afferents originate from pyramidal neurons.     

The temporal evolution of the distribution of dendritic spines in the visual cortex of normal and dark raised mice

Summary A set of equations which define the distribution of spines along the apical dendrites have been developed. They are satisfied by the distribution of spines and its evolution with the age in the apicals of the layer V pyramidal cells of the visual cortex in normal and dark raised mice. The principal equation describes the distribution of the spines with three coefficients IF, B and K whose values have a functional relation with the age T of the animal. This relation has been defined by three additional equations whose coefficients were calculated. The equations have been used to predict the distribution of dendritic spines corresponding to age-groups of mice not previously studied and to find out the age of mice from the data of their known spine distribution resolving the inverse equations of IF (T) and B(T).     

Synaptic connections of morphologically identified and physiologically characterized large basket cells in the striate cortex of cat

Neurons were studied in the striate cortex of the cat following intracellular recording and iontophoresis of horseradish peroxidase. The three selected neurons were identified as large basket cells on the basis that (i) the horizontal extent of their axonal arborization was three times or more than the extent of the dendritic arborization; (ii) some of their varicose terminal segments surrounded the perikarya of other neurons. The large elongated perikarya of the first two basket cells were located around the border of layers III and IV. The radially-elongated dendritic field, composed of beaded dendrites without spines, had a long axis of 300-350 microns, extending into layers III and IV, and a short axis of 200 microns. Only the axon, however, was recovered from the third basket cell. The lateral spread of the axons of the first two basket cells was 900 microns or more in layer III and, for the third cell, was over 1500 microns in the antero-posterior dimension, a value indicating that the latter neuron probably fulfills the first criterion above. The axon collaterals of all three cells often branched at approximately 90 degrees to the parent axon. The first two cells also had axon collaterals which descended to layers IV and V and had less extensive lateral spreads. The axons of all three cells formed clusters of boutons which could extend up a radial column of their target cells. Electron microscopic examination of the second basket cell showed a large lobulated nucleus and a high density of mitochondria in both the perikarya and dendrites. The soma and dendrites were densely covered by synaptic terminals. The axons of the second and third cells were myelinated up to the terminal segments. A total of 177 postsynaptic elements was analysed, involving 66 boutons of the second cell and 89 boutons of the third cell. The terminals contained pleomorphic vesicles and established symmetrical synapses with their postsynaptic targets. The basket cell axons formed synapses principally on pyramidal cell perikarya (approximately 33% of synapses), spines (20% of synapses) and the apical and basal dendrites of pyramidal cells (24% of synapses). Also contacted were the perikarya and dendrites of non-pyramidal cells, an axon, and an axon initial segment. A single pyramidal cell may receive input on its soma, apical and basal dendrites and spines from the same large basket cell.(ABSTRACT TRUNCATED AT 400 WORDS)     

Maturation of pyramidal cell form in relation to developing afferent and efferent connections of rat somatic sensory cortex.

Golgi and axonal labeling methods were used to examine the maturation of pyramidal cells in layers III and V of the rat somatic sensory cortex. The material came from animals late in the gestation period, postnatal, ranging from 0 to 43 days of age and at maturity. Special attention was paid to the period (0–7 days of age) during which it is known that thalamic and callosal fibers grow into the cortex. It is shown that the basic features of the pyramidal cell form are established before the long afferent fibers arrive in layers III and V and before the large number of synapses are established in these layers. Nevertheless, considerable dendritic growth and spine formation occurs after the afferent fibers establish an adult-like pattern of distribution. It is also shown that even at 1 day of age, the axons of pyramidal cells in all layers have reached the vicinity of targets such as the striatum, thalamus, brainstem, spinal cord and contralateral cortex.At 0–1 day the immature pyramidal cells are essentially bipolar in the upper cortical plate, but in the developing infragranular layers they have a few short, almost spine-free, basal dendrites and, rarely, a few oblique branches of the apical dendrite. The apical dendrite extends to the pial surface and the dendritic branches end in growth cones. The dendrites of cells in all layers increase in size and complexity of branching over the first postnatal week; the maturation of dendrites in layer V leads that of dendrites in the supragranular layers by about 2–3 days. As maturation proceeds, basal dendrites acquire secondary and tertiary branches and more oblique branches appear on the apical dendrite. Dendritic spines appear after 4 days of age but remain sparse up to 7–8 days. At 14 days of age, the spine density is much higher than in 7-day-old animals but remains at a much lower density than in 4-week-old, 6-week-old, or adult animals. By 7–14 days, the difference in maturity between superficial (layer III) and deep (layer V) pyramidal cells is difficult to discern qualitatively. All the pyramidal cells now have relatively complex, highly branched dendritic trees when compared to younger cells, but the dendritic tree is still immature in terms of the number, length and complexity of branching of the apical and basal dendritic systems.It can be concluded that the growth of the long axon of cortical pyramidal neurons precedes the acquisition of afferent connections and when these afferent fibers arrive in the cortex the dendritic tree of the pyramidal cell is still highly immature. Thus it remains possible that the finer modeling of the dendritic tree and the formation of spines may be affected by extrinsic influences such as the afferent fibers.     

Ultrastructure and synaptic connections of vasoactive intestinal polypeptide-like immunoreactive non-pyramidal neurons and axon terminals in the rat hippocampus

In the hippocampus, antibody raised against vasoactive intestinal polypeptide (VIP) labeled perikarya and processes of non-pyramidal neurons whereas these structures remained unlabeled in pyramidal cells and granule cells. In the present study, VIP-immunostaining was used to investigate the fine structure and synaptic connections of identified non-pyramidal neurons and of imrnunoreactive axon terminals in the CA1 region of the rat hippocampus by means of electron microscopic immunocytochemistry.From a number of cells studied, two VIP-like imrnunoreactive non-pyramidal neurons in the regio superior were selected for an electron microscopic analysis of serial thin sections. These cells were different with regard to the location of their cell bodies and the orientation of their dendrites. One cell was located in the stratum lacunosum-moleculare with dendritic processes oriented parallel to the hippocampal fissure. The second neuron was found in the inner one-third of the stratum radiatum. The dendrites of this cell ran nearly parallel to the ascending apical dendrites of the pyramidal cells. Both cells had a round or ovoid perikaryon and an infolded nucleus. The aspinous dendrites of both neurons were densely covered with synaptic boutons. These terminals were small, filled with spherical vesicles and established asymmetric synaptic contacts. No variations in the fine structure of the presynaptic boutons were found along the course of the labeled dendrites through the various hippocampal layers, although different afferents are known to terminate in these layers.Vasoactive intestinal polypeptide-like immunopositive axon terminals course through all layers of the hippocampus. In the stratum pyramidale they established symmetric synaptic contacts with the perikarya of pyramidal cells. In the stratum radiatum they made symmetric contacts with the shafts of apical dendrites of pyramidal cells but never contacted dendritic spines.The symmetric contacts with pyramidal cell perikarya suggest an involvement of the VIP-like immunoreactive axon terminals in pyramidal cell inhibition.     

Structural changes in the area striata of the mouse after enucleation

Summary The optic pathways of the mouse have been studied by tracing of degenerating fibers after enucleation and coagulation of the lateral geniculate nucleus. The effects of unilateral enucleation at birth in the contralateral area striata of the mouse have been studied with the Golgi method. The number of spines on three different portions of the apical dendrites of layer V pyramidal cells have been counted in the affected area striata of mice 24 and 48 days old enucleated at birth. The results were compared with the countings obtained in the area striata homolateral to the enucleated side and with controls of the same ages. The results indicated that enucleation produces, through a series of transneuronal changes, significant diminution of the number of spines in the apical dendritic segments located in layer IV. The diminution of dendritic spines is more pronounced in younger animals. Specific variations in the orientation of dendrites of stellate cells with ascending axons have been observed in enucleated animals. The significance of these findings has been discussed suggesting the existence of compensatory mechanisms which affect significantly the intrinsic organization of the area striata.     

Quantification of synapses formed with apical dendrites of golgi-impregnated pyramidal cells: Variability in thalamocortical inputs,but consistency in the ratios of asymmetrical to symmetrical synapses

Five pyramidal cells from the posteromedial barrel subfield of mouse SmI cortex were labeled by Golgi impregnation and then gold-toned and de-impregnated (Fairén, Peters & Saldanha, 1977). Subsequently, 40 to 70 μm-long segments of their apical dendrites occurring in layer IV were graphically reconstructed from serial thin sections to determine the distribution of their synapses. Thalamocortical synapses onto these dendritic segments were identified by lesion-induced degeneration.The synaptic pattern of the pyramidal cell apical dendrites was consistent with previous reports in that most synapses occurred on spines and were asymmetrical and the smaller number of shaft synapses were primarily symmetrical. Some axospinous synapses were formed by degenerating thalamocortical axon terminals. The proportion of thalamocortical synapses onto reconstructed dendritic segments was different for different neurons. For example, thalamocortical axon terminals formed 15% of the synapses involving the spines of the reconstructed segment from a medium superficial layer V pyramidal cell and 10% of the synapses onto portions of the segment from a large layer VI pyramidal cell. In contrast, reconstructed dendritic segments of three other layer VI pyramidal cells formed no more than one thalamocortical synapse.An analysis of the distribution of synapses onto reconstructed dendritic segments revealed that the segments of 3 medium and large pyramidal cells had a ratio of about 12.5 asymmetrical synapses per symmetrical synapse, whereas the segments of 2 small pyramidal cells had ratios of only 6.5 asymmetrical synapses per symmetrical synapse. That these ratios fall into 2 distinct groups suggests that the relative number of asymmetrical and symmetrical synapses is stereotyped within populations of neurons.     

Total number and distribution of inhibitory and excitatory synapses on hippocampal CA1 pyramidal cells

The integrative properties of neurons depend strongly on the number, proportions and distribution of excitatory and inhibitory synaptic inputs they receive. In this study the three-dimensional geometry of dendritic trees and the density of symmetrical and asymmetrical synapses on different cellular compartments of rat hippocampal CA1 area pyramidal cells was measured to calculate the total number and distribution of excitatory and inhibitory inputs on a single cell.A single pyramidal cell has approximately 12,000 microm dendrites and receives around 30,000 excitatory and 1700 inhibitory inputs, of which 40 % are concentrated in the perisomatic region and 20 % on dendrites in the stratum lacunosum-moleculare. The pre- and post-synaptic features suggest that CA1 pyramidal cell dendrites are heterogeneous. Strata radiatum and oriens dendrites are similar and differ from stratum lacunosum-moleculare dendrites. Proximal apical and basal strata radiatum and oriens dendrites are spine-free or sparsely spiny. Distal strata radiatum and oriens dendrites (forming 68.5 % of the pyramidal cells' dendritic tree) are densely spiny; their excitatory inputs terminate exclusively on dendritic spines, while inhibitory inputs target only dendritic shafts. The proportion of inhibitory inputs on distal spiny strata radiatum and oriens dendrites is low ( approximately 3 %). In contrast, proximal dendritic segments receive mostly (70-100 %) inhibitory inputs. Only inhibitory inputs innervate the somata (77-103 per cell) and axon initial segments. Dendrites in the stratum lacunosum-moleculare possess moderate to small amounts of spines. Excitatory synapses on stratum lacunosum-moleculare dendrites are larger than the synapses in other layers, are frequently perforated ( approximately 40 %) and can be located on dendritic shafts. Inhibitory inputs, whose percentage is relatively high ( approximately 14-17 %), also terminate on dendritic spines.Our results indicate that: (i) the highly convergent excitation arriving onto the distal dendrites of pyramidal cells is primarily controlled by proximally located inhibition; (ii) the organization of excitatory and inhibitory inputs in layers receiving Schaffer collateral input (radiatum/oriens) versus perforant path input (lacunosum-moleculare) is significantly different.     

Distribution of synapses on an intracellularly labeled small pyramidal neuron in the cat motor cortex

Summary The morphological characteristics and distribution of synapses on a small pyramidal neuron in layer III of the cat motor cortex have been studied by combining intracellular HRP staining and electron microscopic examination. The stained neuron showed spiny apical and basal dendritic profiles under the light microscope, and exhibited the morphological features of a pyramidal neuron. Ultrastructural analysis indicated that about 80% of the presynaptic terminals formed asymmetrical synapses with spines of distal apical and basal dendrites. On proximal apical dendrites, 64% of the synapses were found to make contact with spines, and 16.7% of the synapses were of symmetrical type and formed with dendritic shafts. Two types of terminal could be identified on the soma; they were alternately located and established symmetrical and asymmetrical synaptic contacts respectively. Possible functional implications are discussed.This paper is dedicated to Professor Fred Walberg on the occasion of his 70th birthday.     

Apical dendritic spines of the visual cortex and light deprivation in the mouse

Summary The effects of light deprivation on the number of apical dendritic spines have been studied in the visual cortex of the mouse. In the portion of the apical dendrites of layer V-pyramidal cells traversing layer IV, dendritic segments of 50 in length from different cells were selected. The number of spines on each of 50 different segments per animal was counted. The countings were done in the areae striata and temporalis prima from mice raised in complete darkness since birth up to 22–25 days old. The observations were compared with the countings obtained in the areae striata and temporalis prima from mice raised under normal conditions. The results indicated that mice raised in darkness had a significant reduction in the number of spines per dendritic segment at the level of layer IV in area striata when compared with control animals. No significant difference was found in the number of spines per dendritic segment in layer IV between both groups of normal and dark raised mice in the area temporalis prima. The mean number of dendritic spines per consecutive segments along complete apical dendrites of layer V-pyramidal cells in area striata has been found to increase exponentially with the distance from the cell body. The same exponential relation, but with somewhat lower values, was obtained in the apical dendrites in area striata in mice raised in darkness. The significance of these findings were discussed. It was concluded that: First, visual sensory deprivation affect the fine structure of the central nervous system. Second, the results observed support the assumption that structural changes in the nerve cells occur as the result of experience.     

Morphological alterations of hippocampal pyramidal neurons heterotopically transplanted into the somatosensory cortex of adult rats: a quantitative Golgi study

A characteristic feature of hippocampal organization is the laminated termination of extrinsic and intrinsic afferents. At present, it is not known to what extent these layer-specific fiber projections modulate the development and final shape of the dendritic arbor of hippocampal target neurons. In the present study, pieces of late embryonic (E18) rat hippocampus were transplanted heterotopically into a cavity in the somatosensory cortex of 6–8 week-old recipient rats. Here, the transplanted neurons differentiated and survived up to several months in the absence of their specific extrinsic afferents. Moreover, tracing of transplant connections with the carbocyanine dye DiI revealed only a limited projection between the transplant and the host neocortex. Golgi-impregnated transplants were used to analyze the postsynaptic structures (dendrites and spines) of hippocampal pyramidal cells quantitatively. Compared with controls, the transplanted pyramidal neurons showed a significant reduction of apical primary dendrites and basal dendritic branches, i.e. of peripheral dendritic portions that originate farther from the soma. In contrast, the number of basal primary dendrites originating directly from the perikaryon was enhanced. Spine density on the main apical dendritic shaft was significantly lower in all peripheral dendritic segments in transplanted neurons. We conclude from our results that the absence of layerspecific extrinsic afferents that normally terminate on peripheral parts of the dendritic arbor of hippocampal pyramidal neurons caused a reduction of these peripheral dendrites and spines. In contrast, the increase of dendrites and spines near the cell body might be induced by intrinsic fibers that normally terminate on these proximal dendritic portions and are known to sprout under transplant conditions.     

Synaptic organization of intracellularly stained CA3 pyramidal neurons in slice cultures of rat hippocampus

Pyramidal cells of regio inferior in slice cultures of the rat hippocampus were impaled and intracellularly stained with horseradish peroxidase. A correlated light- and electron-microscopic analysis was then performed to study the properties of these neurons under culture conditions with particular emphasis on input synapses onto these cells. Like pyramidal cells in situ, CA3 pyramidal neurons in slice cultures had a triangular cell body with an apical stem dendrite emerging from it. Several basal dendrites and the axon arose from the basal pole of the cell body. The peripheral thin branches of both apical and basal dendrites were covered with small spines, whereas proximal thick dendritic segments and portions of the cell body exhibited large spines or excrescences. The axon gave off numerous fine varicose collaterals which projected to stratum radiatum of CA1 (Schaffer collaterals), to the alveus and to the hilar region. In one case a collateral could be followed to stratum moleculare of the fascia dentata. Electron-microscopic analysis of the injected pyramidal neurons revealed that their cell bodies, dendritic shafts and spines formed synaptic contacts with presynaptic terminals. Mossy fiber endings were identified by their large size and their numerous clear synaptic vesicles with some dense-core vesicles intermingled, and were observed to form synaptic contacts on the large spines or excrescences. Since extrinsic afferents degenerate in slice cultures, the numerous synaptic boutons on the identified pyramidal neurons probably arise from axons of intrinsic neurons that have sprouted in response to deafferentation. This assumption is supported by the finding that collaterals of the injected neurons formed abundant synaptic contacts on dendritic shafts and spines of other cells. These results suggest that, although pyramidal cells under culture conditions retain a remarkable number of their normal characteristics, considerable synaptic reorganization does take place.     

Long-term effects of postnatal undernutrition and maternal malnutrition on mouse cerebral cortex

Summary Quantitative analysis of the dendritic branchings of pyramidal cells from layers V and III, as well as of the number of spines on their apical dendrites, were performed on the visual cortex of postnatally undernourished mice (2nd–21st day) and on that of pups from malnourished mothers (gestation and lactation). Animals were followed till 180 days of age after more than 5 months of nutritional rehabilitation, and data were obtained at 10, 15, 21, 30, 60, and 180 days of age. The increase and maturation of dendritic branchings and spines were much more reduced in postnatal undernutrition than in maternal malnutrition. Furthermore, permanent damage still remained at 180 days in postnatal undernutrition while almost no damage was statistically detectable after maternal malnutrition.     

Fine structures of nerve fibers with corticotropin-releasing factor-like immunoreactivity in the rat lateral septum

The fine structures of nerve fibers with corticotropin-releasing factor (CRF)-like immunoreactivity in the rat lateral septum were investigated by means preembedding immunoelectron microscopy. A number of CRF axon terminals formed synapses with cell bodies of non-immunoreactive septal neurons. They occasionally had broad terminal bulges whose subregions showed little or no immunoreactivity for CRF. CRF axon terminals were also in synaptic contact with non-immunoreactive dendrites or dendritic spines. Some dendrites with CRF were postsynaptic to non-immunoreactive axon terminals.     

Morphological organization of the neuropil in laminae II-V of rabbit visual cortex

Summary A combined light- and electronmicroscopic study of tangential serial sections through the visual cortex of the rabbit has been performed in order to find out whether or not the vertical bundles of apical dendrites in laminae IV/V and II/III on one hand and the areas between these bundles on the other differ with respect to composition and/or spatial organization of the neuropil. Lightmicroscopically only thick profiles such as apical dendrites of pyramidal cells and myelinated axons contribute to the structural characteristics of the neuropil. The appearance of the areas between the dendrite bundles is determined by the presence or absence of radiate bundles of myelinated axons. Lamina-dependent variations were seen in the neuropil of the dendrite bundles as well as in that of the areas between them. Ultrastructurally, the dendrite bundles and the areas between them were observed to be different also with respect to the distribution of the various types of small dendritic profiles and thin axons. The neuropil within the dendrite bundles except for the shafts and thick branches of apical dendrites contains thin unmyelinated axons and numerous spines of apical dendrites contacted by axon terminals. Small irregularly shaped dendrites are few in number. The neuropil between the dendrite bundles contains a larger number of thin unmyelinated axons than that within the bundles, and, instead of spines of apical dendrites, small irregularly shaped smooth and spiny dendrites represent the prevailing postsynaptic structures. Hence, areas within dendrite bundles differ from areas between them 1. by the thickness and orientation of their profiles and 2. by the quantitative relation of the various kinds of processes accumulating in each compartment.     

Neocortical layers I and II of the hedgehog (Erinaceus europaeus). I. Intrinsic organization

Summary The intrinsic organization and interlaminar connections in neocortical layers I and II have been studied in adult hedgehogs (Erinaceus europaeus) using the Golgi method. Layer I contains a dense plexus of horizontal fibers, the terminal dendritic bouquets of pyramidal cells of layer II and of underlying layers, and varieties of intrinsic neurons. Four main types of cells were found in layer I. Small horizontal cells represent most probably persisting foetal horizontal cells described for other mammals. Large horizontal cells, tufted cells, and spinous horizontal cells were also found in this layer.Layer II contains primitive pyramidal cells representing the most outstanding feature of the neocortex of the hedgehog. Most pyramidal cells in layer II have two, three or more apical dendrites, richly covered by spines predominating over the basal dendrites. These cells resemble pyramidal cells found in the piriform cortex, hippocampus and other olfactory areas. It is suggested that the presence of these neurons reflects the retention of a primitive character in neocortical evolution.Cells with intrinsic axons were found among pyramidal cells in layer II. These have smooth dendrites penetrating layer I and local axons forming extremely complex terminal arborizations around the bodies and proximal dendritic portions of pyramidal cells. They most probably effect numerous axo-somatic contacts resembling basket cells. The similarity of some axonal terminals with the chandelier type of axonal arborization is discussed. Other varieties of cells located in deep cortical layers and having ascending axons for layers I and II were also studied. It is concluded that the two first neocortical layers represent a level of important integration in this primitive mammal.     

Axonal and dendritic arborization of an intracellularly labeled chandelier cell in the CA1 region of rat hippocampus

Summary During the course of an in vivo intracellular labeling study, a chandelier (axo-axonic) cell was completely filled with biocytin in the CA1 region of the hippocampus. Chandelier cells are known to provide GABAergic terminals exclusively to the axon initial segment of pyramidal cells. The lateral extent and laminar distribution of the dendritic arborization of the chandelier cell was very similar to that of pyramidal cells; the numerous basal and apical dendrites reached the ventricular surface and the hippocampal fissure, respectively. The dendrites, however, had very few spines. The neuron had an asymmetric axonal arbor occupying an elliptical area of 600 by 850 m in the pyramidal cell layer and stratum oriens, with over three-quarters of the axon projecting to the fimbrial side of the neuron. Counting all clusters of terminals, representing individually innervated axon initial segments, the chandelier cell was estimated to contact 1214 pyramidal cells, a number that exceeds previous estimations, based on Golgi studies, by several-fold. The findings support the view that chandelier cells may control the threshold and/or synchronize large populations of principal cells.     

Loss of dendritic spines in aging cerebral cortex

Summary Previous work has shown that the dendritic spines of pyramidal neurons of the cerebral cortex are sensitive to a wide variety of environmental and surgical manipulations. The present study shows that the normal aging process also affects these spines. The spines were studied with the light microscope in Golgi preparations from rats ranging in age from 3 to 29.5 months. Visible spines were counted on either 25 or 50 segments of the basal dendrites, apical dendrites, oblique branches, and terminal tufts of layer V pyramidal cells in area 17. A progressive loss of spines occurred at each of these loci. The smallest observed spine loss (24%) occurred on the dendrites of the terminal tuft, and the largest (40%) on the oblique branches. Age-related spine loss appears to affect all animals, and for animals of any one age the overall loss is similar. However, the cell-to-cell variability within an individual animal is pronounced, some cells with high spine densities being present at every age examined. As a general rule, there is a positive relationship between visible spine density along the apical dendrite as it traverses layer IV and the thickness of the dendrite. With advancing age, the relatively thick dendrites decrease in number so that the thinner dendrites make up an increasingly larger proportion of the total apical dendrite population. Questions that remain for the future include the genesis of the spine loss, its relation to other aging changes, and its functional significance for the neuron.Supported by United States Public Health Service Program Project Grant HDO-5796-03 and Research Grant NB-07016     

A light and electron microscopic study of betaine/GABA transporter distribution in the monkey cerebral neocortex and hippocampus

The present study aimed to elucidate the distribution of betaine/gamma-aminobutyric acid (GABA) transporter-1 (BGT-1) in the normal monkey cerebral neocortex and hippocampus by immunoperoxidase and Immunogold labelling. BGT-1 was observed in pyramidal neurons in the cerebral neocortex and the CA fields of the hippocampus. Large numbers of small diameter dendrites or dendritic spines were observed in the neuropil. These made asymmetrical synaptic contacts with unlabelled axon terminals containing small round vesicles, characteristic of glutamatergic terminals. BGT-1 label was observed in an extra-perisynaptic region, away from the post-synaptic density. Immunoreactivity was not observed in portions of dendrites that formed symmetrical synapses, axon terminals, or glial cells. The distribution of BGT-1 on dendritic spines, rather than at GABAergic axon terminals, suggests that the transporter is unlikely to play a major role in terminating the action of GABA at a synapse. Instead, the osmolyte betaine is more likely to be the physiological substrate of BGT-1 in the brain, and the presence of the transporter in pyramidal neurons suggests that these neurons utilize betaine to maintain osmolarity.