Regional Distribution of Neurofilament and Calcium-binding Proteins in the Cingulate Cortex of the Macaque Monkey

The cingulate cortex is composed of morphologically and functionallydistinct areas. It is considered to be a major component ofthe limbic system and has been shown to subserve a wide rangeof autonomic and somatic motor functions. The anterior and posteriorregions of the cingulate cortex can be differentiated accordingto their thalamic afferents as well as their patterns of corticocorticalconnectivity. The primate cingulate cortex is traditionallydivided into a series of cytoarchitec tonic zones that can bedistinguished along a ventral-dorsal axis of differentiationin both the anterior (areas 25, 24a, 24b, and 24c), and posterior(areas 29, 30, 23a, 23b, and 23c) regions. However, little isknown about the precise cellular organization of these subareas.In the present study, we attempt to define the neuronal morphologicaland biochemical composition of the different cingulate cortexsubareas, using antibodies to the neurofilament triplet proteinand calcium-binding proteins. Results indicate that there isa strong correlation between the structure and functions ofthe cingulate cortex and the immunostaining patterns. For instance,distribution of neurofilament-rich pyramidal neurons parallelsthat of specific corticocortical and corticosubcortical systemsand is a useful marker to delineate the cingulate motor area.Calcium-binding protein-containing neurons display a high degreeof regional and laminar specialization. In particular, parvalbumin-positiveinterneurons are codistributed with neurofilament-immunoreactivepyramidal cells along the ventrodorsal and rostrocaudal axesof the cingulate cortex. Calbindin- and calretinin-positiveimmunostaining show more monotonous laminar and regional patterns,although they exhibit a particular labeling in area 29 thatmay correspond to the termination of select thalamocorticalafferents. These chemoarchitectural patterns of regional andlaminar neuronal specialization may be envisioned as the reflectionof the richness of cortical diversity in the cingulate gyrus,and make it an ideal place to explore the interplay of the distributionsof various neuron types in cortical areas of known function.     

The pyramidal cell of the sensorimotor cortex of the macaque monkey: phenotypic variation

Recent studies have revealed striking differences in pyramidal cell structure among cortical regions involved in the processing of different functional modalities. For example, cells involved in visual processing show systematic variation, increasing in morphological complexity with rostral progression from V1 through extrastriate areas. Differences have also been identified between pyramidal cells in somatosensory, motor and prefrontal cortex, but the extent to which the pyramidal cell phenotype may vary between these functionally related cortical regions remains unknown. In the present study we investigated the structure of layer III pyramidal cells in somatosensory and motor areas 3b, 4, 5, 6 and 7b of the macaque monkey. Cells were intracellularly injected in fixed, flat-mounted cortical slices and analysed for morphometric parameters. The size of the basal dendritic arbours, the number of their branches and their spine density were found to vary systematically between areas. Namely, we found a trend for increasing complexity in dendritic arbour structure through areas 3b, 5 and 7b. A similar trend occurred through areas 4 and 6. The differences in arbour structure may determine the number of inputs received by neurons and may thus be an important factor in determining function at the cellular and systems level.     

Age-related dendritic and spine changes in corticocortically projecting neurons in macaque monkeys

Alterations in neuronal morphology occur in primate cerebral cortex during normal aging, vary depending on the neuronal type, region and cortical layer, and have been related to memory and cognitive impairment. We analyzed how such changes affect a specific subpopulation of cortical neurons forming long corticocortical projections from the superior temporal cortex to prefrontal area 46. These neurons were identified by retrograde transport in young and old macaque monkeys. Dendritic arbors of retrogradely labeled neurons were visualized in brain slices by intracellular injection of Lucifer Yellow, and reconstructed three-dimensionally using computer-assisted morphometry. Total dendritic length, numbers of segments, numbers of spines, and spine density were analyzed in layer III pyramidal neurons forming the projection considered. Sholl analysis was used to determine potential age-related changes in dendritic complexity. We observed statistically significant age-related decreases in spine numbers and density on both apical and basal dendritic arbors in these projection neurons. On apical dendrites, changes in spine numbers occurred mainly on the proximal dendrites but spine density decreased uniformly among the different branch orders. On basal dendrites, spine numbers and density decreased preferentially on distal branches. Regressive dendritic changes were observed only in one particular portion of the apical dendrites, with the general dendritic morphology and extent otherwise unaffected by aging. In view of the fact that there is no neuronal loss in neocortex and hippocampus in old macaque monkeys, it is possible that the memory and cognitive decline known to occur in these animals is related to rather subtle changes in the morphological and molecular integrity of neurons subserving identifiable neocortical association circuits that play a critical role in cognition.     

The effects of morphine self-administration on cortical pyramidal cell structure in addiction-prone lewis rats

The consumption of drugs of abuse provokes sensitization, the development of tolerance, dependency, and eventually addiction. It is thought that these events are partially a consequence of drug-induced alterations in the organization of neuronal circuits in specific areas of the brain. In the present study, we have used intracellular injections of lucifer yellow to examine the alterations that may occur in cortical pyramidal neurons of addiction-prone Lewis rats following 15 days of self-administration of morphine. Specifically, the effects of morphine on the structure, size and branching complexity of the basal dendrites, and spine density were determined in the basal dendritic arbors of layer III pyramidal neurons in both the prelimbic and motor cortex. We found that following morphine self-administration, there was a reduction in the size and branching complexity of the dendritic arbors of pyramidal cells in the motor cortex. In contrast, prelimbic pyramidal neurons from these morphine-treated animals had larger and longer basal dendritic arbors. Furthermore, the spine density on pyramidal neurons was higher in both cortical regions of morphine self-administered rats. These results suggest that at least part of the behavioral changes produced by repeated opiate administration may be attributed to alterations in pyramidal cell structure.     

A study of pyramidal cell structure in the cingulate cortex of the macaque monkey with comparative notes on inferotemporal and primary visual cortex

Recent studies have revealed a marked degree of variation in the pyramidal cell phenotype in visual, somatosensory, motor and prefrontal cortical areas in the brain of different primates, which are believed to subserve specialized cortical function. In the present study we carried out comparisons of dendritic structure of layer III pyramidal cells in the anterior and posterior cingulate cortex and compared their structure with those sampled from inferotemporal cortex (IT) and the primary visual area (V1) in macaque monkeys. Cells were injected with Lucifer Yellow in flat-mounted cortical slices, and processed for a light-stable DAB reaction product. Size, branching pattern, and spine density of basal dendritic arbors was determined, and somal areas measured. We found that pyramidal cells in anterior cingulate cortex were more branched and more spinous than those in posterior cingulate cortex, and cells in both anterior and posterior cingulate were considerably larger, more branched, and more spinous than those in area V1. These data show that pyramidal cell structure differs between posterior dysgranular and anterior granular cingulate cortex, and that pyramidal neurons in cingulate cortex have different structure to those in many other cortical areas. These results provide further evidence for a parallel between structural and functional specialization in cortex.     

Morphological alteration and reduction of MAP2-immunoreactivity in pyramidal neurons of cerebral cortex in a rat model of focal cortical compression

Subdural hematoma causes cortical damage including brain tissue disruption, often resulting in neuronal dysfunction and neurological impairment. The aim of the present study was to identify the relationship between cerebral compression and neuronal injury. In this report, we investigated time-dependent morphological alterations within layers II, III, and V pyramidal neurons in the cerebral cortex, using Golgi-Cox staining and immunohistochemistry for microtubule-associated protein 2 (MAP2) in a rat model of focal cortical compression. An acryl pole was used to experimentally induce chronic cerebral compression by continuous pressure on the cortical surface. Changes in cellular morphology were examined at five survival time periods: 12?h and 1, 2, 3, and 4 weeks. The Golgi-Cox method revealed time-dependent alterations in dendritic length of apical and basilar dendrites of pyramidal neurons. The number of dendritic branch segments and spines of basilar dendrites were decreased in cells in layers II, III, and V. Immunohistochemical staining for MAP2 revealed changes in the intracellular distribution of immunoreactive materials. A significant reduction in MAP2 immunostaining was found in nerve cell bodies and apical dendrites of ipsilateral cortical neurons. The number of MAP2-immunoreactive neurons was significantly decreased at 12?h compared with the contralateral cerebral cortex in the same animal. Dendritic changes in layers II, III, and V pyramidal neurons were accompanied by reductions in intracellular MAP2-immunoreactive materials. The present results suggest that cortical compression causes alteration of cellular morphology as a consequence of injury, and that these morphological changes may be related to reductions in MAP2-immunoreactive materials.     

Catecholaminergic innervation of pyramidal neurons in the human temporal cortex

In the human neocortex, catecholaminergic connections modulate the excitatory inputs of pyramidal neurons and are involved in higher cognitive functions. Catecholaminergic fibers form a dense network in which it is difficult to distinguish whether or not target specificity exists. In order to shed some light on this issue, we set out to quantify the catecholaminergic innervation of pyramidal cells in different layers of the human temporal cortex (II, IIIa, IIIb, V and VI). For this purpose, pyramidal cells were labeled in human cortical tissue by injecting them with Lucifer Yellow, and then performed immunocytochemistry for the rate limiting catecholamine synthesizing enzyme tyrosine hydroxylase (TH) to visualize catecholaminergic fibers in the same sections. Injected cells were reconstructed in three dimensions and appositions were quantified (n = 1503) in serial confocal microscopy images of each injected cell (n = 71). We found TH-immunoreactive appositions (TH-ir) in all the pyramidal cells analyzed, in both the apical and basal dendritic regions. In general, the density of TH-ir apposition was greater in layers II, V and VI than in layers IIIa and IIIb. Furthermore, TH-ir appositions showed a regular distribution in almost all dendritic compartments of the apical and basal dendritic arbors across all layers. Hence, it appears that all pyramidal neurons in the human neocortex receive catecholaminergic afferents in a rather regular pattern, independent of the layer in which they are located. Since pyramidal cells located in different layers are involved in different intrinsic and extrinsic circuits, these results suggest that catecholaminergic afferents may modify the function of a larger variety of circuits than previously thought. Thus, this aspect of human cortical organization is likely to have important implications in cortical function.     

The early differentiation of the neocortex: a hypothesis on neocortical evolution.

During development, a cerebral cortex appears in the wall of the telencephalic vesicle in reptiles and mammals. It arises from a cell-dense cortical plate, which develops within a primordial preplate. The neurons of the preplate are essential for cortical development; they regulate the neuronal migration of the cortical plate neurons and form the first axonal connections. In the reptilian cortex and in the hippocampus of the mammalian cerebral cortex, most ingrowing afferent axons run above the cortical plate, in the zone where the receptive tufts of apical dendrites of the cortical pyramidal neurons branch extensively. In the mammalian neocortex, however, axons enter mainly from below the cortical plate where they do not encounter the apical tufts of these pyramidal neurons. In this paper, we discuss the idea that this difference in cortical development has relieved a functional constraint in the expansion of the cortex during evolution. We hypothesize that the entrance of axons below the cell-dense cortical plate, together with the inside-out migration of cortical neurons, ensures that the neocortex remains an "open" system, able to differentiate into new (sub)layers and more cortical areas.     

The occipitoparietal pathway of the macaque monkey: comparison of pyramidal cell morphology in layer III of functionally related cortical visual areas

The dendritic morphology of pyramidal cells located at the base of layer III in the primary visual area (V1), the second visual area (V2), the middle temporal area (MT), the ventral portion of the lateral intraparietal area (LIPv) and in the portion of cytoarchitectonic area 7a within the anterior bank of the superior temporal sulcus was revealed by injecting neurons with Lucifer Yellow in fixed, flattened slices of macaque monkey visual cortex. These areas correspond to different levels of the occipitoparietal cortical 'stream', which processes information related to motion and spatial relationships in the visual field. The tissue was immunocytochemically processed to obtain a light-stable diaminobenzidine reaction product, revealing the dendritic morphology in fine detail. Retrogradely labelled MT- projecting neurons in supragranular V1 (layer IIIc of Hassler's nomenclature, corresponding to Brodmann's layer IVb) were predominantly pyramidal, although many spiny multipolar (stellate) cells were also found. The average basal dendritic field area of pyramidal neurons in sublamina IIIc of V1 was significantly smaller than that in the homologous layer of V2, within the cytochrome oxidase-rich thick stripes. Furthermore, the average basal dendritic field areas of V1 and V2 pyramidal neurons were significantly smaller than those of neurons in MT, LIPv and area 7a. There was no difference in basal dendritic field area between layer III pyramidal neurons in areas MT, LIPv and 7a. While the shape of most basal dendritic fields was circularly symmetrical in the dimension tangential to the cortical layers, there were significant biases in complexity, with dendritic branches tending to cluster along particular axes. Sholl analysis revealed that the dendritic fields of neurons in areas MT, LIPv and 7a were significantly more complex (i.e. had a larger number of branches) than those of V1 or V2 neurons. Analysis of basal dendritic spine densities revealed regional variations along the dendrites, with peak densities being observed 40-130 microns from the cell body, depending on the visual area. The peak spine density of layer III pyramidal neurons in V1 was lower than that observed in V2, MT or LIPv, which were all similar. Pyramidal neurons in area 7a had the greatest peak spine density, which was on average 1.7 times that found in V1. Calculations based on the average spine density and number of dendritic branches at different distances from the cell body demonstrated a serial increase in the total number of basal dendritic spines per neuron at successive stations of the occipitoparietal pathway. Our observations, comparing dendritic fields of neurons in the homologous cortical layer at different levels of a physiologically defined 'stream', indicate changes in pyramidal cell morphology between functionally related areas. The relatively large, complex, spine-dense dendritic fields of layer III pyramidal cells in rostral areas of the occipitoparietal pathway allow these cells to sample a greater number of more diverse inputs in comparison with cells in 'lower' areas of the proposed hierarchy.      

Early postnatal migration and development of layer II pyramidal neurons in the rodent cingulate/retrosplenial cortex

The cingulate and retrosplenial regions are major components of the dorsomedial (dm) limbic cortex and have been implicated in a range of cognitive functions such as emotion, attention, and spatial memory. While the structure and connectivity of these cortices are well characterized, little is known about their development. Notably, the timing and mode of migration that govern the appropriate positioning of late-born neurons remain unknown. Here, we analyzed migratory events during the early postnatal period from ventricular/subventricular zone (VZ/SVZ) to the cerebral cortex by transducing neuronal precursors in the VZ/SVZ of newborn rats/mice with Tomato/green fluorescent protein-encoding lentivectors. We have identified a pool of postmitotic pyramidal precursors in the dm part of the neonatal VZ/SVZ that migrate into the medial limbic cortex during the first postnatal week. Time-lapse imaging demonstrates that these cells migrate on radial glial fibers by locomotion and display morphological and behavioral changes as they travel through the white matter and enter into the cortical gray matter. In the granular retrosplenial cortex, these cells give rise to a Satb2+ pyramidal subtype and develop dendritic bundles in layer I. Our observations provide the first insight into the patterns and dynamics of cell migration into the medial limbic cortex.     

Regional dendritic and spine variation in human cerebral cortex: a quantitative golgi study

The present study explored differences in dendritic/spine extent across several human cortical regions. Specifically, the basilar dendrites/spines of supragranular pyramidal cells were examined in eight Brodmann's areas (BA) arranged according to Benson's (1993, Behav Neurol 6:75-81) functional hierarchy: primary cortex (somatosensory, BA3-1-2; motor, BA4), unimodal cortex (Wernicke's area, BA22; Broca's area, BA44), heteromodal cortex (supple- mentary motor area, BA6beta; angular gyrus, BA39) and supramodal cortex (superior frontopolar zone, BA10; inferior frontopolar zone, BA11). To capture more general aspects of regional variability, primary and unimodal areas were designated as low integrative regions; heteromodal and supramodal areas were designated as high integrative regions. Tissue was obtained from the left hemisphere of 10 neurologically normal individuals (M(age) = 30 +/- 17 years; five males, five females) and stained with a modified rapid Golgi technique. Ten neurons were sampled from each cortical region (n = 800) and evaluated according to total dendritic length, mean segment length, dendritic segment count, dendritic spine number and dendritic spine density. Despite considerable inter-individual variation, there were significant differences across the eight Brodmann's areas and between the high and low integrative regions for all dendritic and spine measures. Dendritic systems in primary and unimodal regions were consistently less complex than in heteromodal and supramodal areas. The range within these rankings was substantial, with total dendritic length in BA10 being 31% greater than that in BA3-1-2, and dendritic spine number being 69% greater. These findings demonstrate that cortical regions involved in the early stages of processing (e.g. primary sensory areas) generally exhibit less complex dendritic/spine systems than those regions involved in the later stages of information processing (e.g. prefrontal cortex). This dendritic progression appears to reflect significant differences in the nature of cortical processing, with spine-dense neurons at hierarchically higher association levels integrating a broader range of synaptic input than those at lower cortical levels.     

The distribution of chandelier cell axon terminals that express the GABA plasma membrane transporter GAT-1 in the human neocortex

Chandelier cells represent a unique type of cortical GABAergic interneuron whose axon terminals (Ch-terminals) form synapses exclusively with the axon initial segments of pyramidal cells. In this study, we have used immunocytochemistry for the high-affinity plasma membrane transporter-1 (GAT-1) to analyze the distribution and density of Ch-terminals in various cytoarchitectonic and functional areas of the human neocortex. The lowest density of GAT-1-immuoreactive (-ir) Ch-terminals was detected in the primary and secondary visual (areas 17 and 18) and in the somatosensory areas (areas 3b and 1). In contrast, an intermediate density was observed in the motor area 4 and the associative frontolateral areas 45 and 46, whereas the associative frontolateral areas 9 and 10, frontal orbitary areas 11, 12, 13, 14, and 47, associative temporal areas 20, 21, 22, and 38, and cingulate areas 24 and 32 displayed the highest density of GAT-1-ir Ch-terminals. Despite these differences, the laminar distribution of GAT-1-ir Ch-terminals was similar in most cortical areas. Hence, the highest density of this transporter was observed in layer II, followed by layers III, V, VI, and IV. In most cortical areas, the density of GAT-1-ir Ch-terminals was positively correlated with the neuronal density, although a negative correlation was detected in layer III across all cortical areas. These results indicate that there are substantial differences in the distribution and density of GAT-1-ir Ch-terminals between areas and layers of the human neocortex. These differences might be related to the different functional attributes of the cortical regions examined.     

Morphological variation of layer III pyramidal neurones in the occipitotemporal pathway of the macaque monkey visual cortex

We compared the morphological characteristics of layer III pyramidal neurones in different visual areas of the occipitotemporal cortical 'stream', which processes information related to object recognition in the visual field (including shape, colour and texture). Pyramidal cells were intracellularly injected with Lucifer Yellow in cortical slices cut tangential to the cortical layers, allowing quantitative comparisons of dendritic field morphology, spine density and cell body size between the blobs and interblobs of the primary visual area (V1), the interstripe compartments of the second visual area (V2), the fourth visual area (V4) and cytoarchitectonic area TEO. We found that the tangential dimension of basal dendritic fields of layer III pyramidal neurones increases from caudal to rostral visual areas in the occipitotemporal pathway, such that TEO cells have, on average, dendritic fields spanning an area 5-6 times larger than V1 cells. In addition, the data indicate that V1 cells located within blobs have significantly larger dendritic fields than those of interblob cells. Sholl analysis of dendritic fields demonstrated that pyramidal cells in V4 and TEO are more complex (i.e. exhibit a larger number of branches at comparable distances from the cell body) than cells in V1 or V2. Moreover, this analysis demonstrated that the dendrites of many cells in V1 cluster along specific axes, while this tendency is less marked in extrastriate areas. Most notably, there is a relatively large proportion of neurones with 'morphologically orientation-biased' dendritic fields (i.e. branches tend to cluster along two diametrically opposed directions from the cell body) in the interblobs in V1, as compared with the blobs in V1 and extrastriate areas. Finally, counts of dendritic spines along the length of basal dendrites revealed similar peak spine densities in the blobs and the interblobs of V1 and in the V2 interstripes, but markedly higher spine densities in V4 and TEO. Estimates of the number of dendritic spines on the basal dendritic fields of layer III pyramidal cells indicate that cells in V2 have on average twice as many spines as V1 cells, that V4 cells have 3.8 times as many spines as V1 cells, and that TEO cells have 7.5 times as many spines as V1 cells. These findings suggest the possibility that the complex response properties of neurones in rostral stations in the occipitotemporal pathway may, in part, be attributed to their larger and more complex basal dendritic fields, and to the increase in both number and density of spines on their basal dendrites.      

Dual role of substance P/GABA axons in cortical neurotransmission: synaptic triads on pyramidal cell spines and basket-like innervation of layer II-III calbindin interneurons in primate prefrontal cortex

In spite of accumulating evidence on the potent neuromodulatory, neuroprotective, trophic and memory-enhancing effects of the neuropeptide substance P (SP) in the cerebral cortex, the excitatory or inhibitory nature of the cortical SP innervation remains unclear and the postsynaptic targets of SP fibers are not defined. To obtain further insight into these issues, we have examined SP-containing axons and their postsynaptic targets in the prefrontal cortex of adult monkeys with single- and double label immunocytochemistry combined with light and correlated electron microscopy. SP fibers in the primate prefrontal cortex, unlike those in the rat cortex, preferentially innervate cortical layers I, II and upper layer III. Our results demonstrate for the first time that all SP-immunoreactive boutons in all cortical layers contain GABA. Of the entire sample of SP boutons, 53% synapse on dendritic shafts, 39% on dendritic spines and 8% on cell bodies. Another new finding is that synapse-forming SP boutons, in addition to their known innervation of pyramidal cells, form pericellular baskets around interneurons in layers II and upper III, a subpopulation of which contains calbindin D28k. Finally, the study also revealed that SP boutons frequently participate in 'synaptic triads' with spines which receive another (asymmetric, putatively excitatory amino acid-utilizing) synapse. Our findings indicate that SP/GABA axons in the primate prefrontal cortex modulate excitatory amino acid- mediated neurotransmission and control feed-forward disinhibitory GABAergic circuits in supragranular cortical layers.      

Altered interneuron development in the cerebral cortex of the flathead mutant.

One approach to defining mechanisms essential to neocortical development is to analyze the phenotype of novel spontaneous mutations that dramatically affect the generation and differentiation of different neocortical neurons. Previously we have shown that there is a large decrease in the total number of cortical neurons in the flathead mutant rat, and in this paper we show that the flathead (fh/fh) mutation causes an even larger decrease in the number of interneurons. The decrease in relative interneuron number is different in different cortical lamina and for different interneuron subtypes. Specifically, the percentage of GABA and calretinin- positive cells in upper layers of somatosensory cortex is not appreciably decreased in homozygous mutants, while other interneuron subtypes in somatosensory cortex and all GABA-positive interneuron types in entorhinal cortex are decreased. In addition, the soma and dendritic arbors of interneurons in flathead are greatly hypertrophied, while those of pyramidal neurons are not. Furthermore, we found that at embryonic day 14, flathead mutants display high levels of cell death throughout both the neocortical and ganglionic eminence (GE) proliferative zones with a larger increase in cell death in the GE than in the neocortical VZ. In addition, we provide evidence that there is widespread cytokinesis failure resulting in binucleate pyramidal cells and interneurons, and the number of binucleate interneurons is greater than the number of binucleate pyramidal neurons. Together, these results demonstrate that the fh mutation causes dramatic alterations in interneuron development, and suggest that the flathead mutation causes differential cytokinesis failure and cell death in different types of neocortical progenitors.     

Experience-induced changes of dendritic spine densities in the prefrontal and sensory cortex: correlation with developmental time windows

The present study provides evidence for the hypothesis that the extent and the direction of experience-induced synaptic changes in cortical areas correlates with time windows of neuronal as well as endocrine development. Repeated brief exposure to maternal separation prior to the stress hyporesponsive period (SHRP) of the hypothalamic-pituitary-adrenal (HPA) axis induced significantly reduced dendritic spine density (-16%) in layer II/III pyramidal neurons of the anterior cingulate cortex (ACd) of 21-day-old rats, whereas separation after termination of the SHRP resulted in increased spine densities (+16%) in this neuron type. In addition, rats of both groups displayed elevated basal plasma levels of corticosterone at this age. Separation during the SHRP (postnatal days 5-7) did not influence spine density in the ACd, and basal corticosterone levels remained unchanged. In contrast, pyramidal neurons in the somatosensory cortex (SSC) displayed significantly enhanced spine densities (up to 52% increase) independent from the time of separation. These results indicate that alterations in the synaptic balance in limbic and sensory cortical regions in response to early emotional experience are region-specific and related to the maturational stage of endocrine and neuronal systems.     

Embryonic and postnatal development of the layer I-directed ("matrix") thalamocortical system in the rat

Inputs to the layer I apical dendritic tufts of pyramidal cells are crucial in "top-down" interactions in the cerebral cortex. A large population of thalamocortical cells, the "matrix" (M-type) cells, provides a direct robust input to layer I that is anatomically and functionally different from the thalamocortical input to layer VI. The developmental timecourse of M-type axons is examined here in rats aged E (embryonic day) 16 to P (postnatal day) 30. Anterograde techniques were used to label axons arising from 2 thalamic nuclei mainly made up of M-type cells, the Posterior and the Ventromedial. The primary growth cones of M-type axons rapidly reached the subplate of dorsally situated cortical areas. After this, interstitial branches would sprout from these axons under more lateral cortical regions to invade the overlying cortical plate forming secondary arbors. Moreover, retrograde labeling of M-type cell somata in the thalamus after tracer deposits confined to layer I revealed that large numbers of axons from multiple thalamic nuclei had already converged in a given spot of layer I by P3. Because of early ingrowth in such large numbers, interactions of M-type axons may significantly influence the early development of cortical circuits.     

Estrogen replacement increases spinophilin-immunoreactive spine number in the prefrontal cortex of female rhesus monkeys

While studies have shown that estrogen affects hippocampal spine density and function, behavioral studies in humans and nonhuman primates have also implicated the prefrontal cortex in the effects of estrogen on cognition. However, the potential for similar estrogen-induced increases in spines and synapses in the prefrontal cortex has not been investigated in primates. Moreover, it is not known if such an estrogen effect would be manifested throughout the neocortex or primarily in the regions involved in cognition. Therefore, we investigated the effects of estrogen on dendritic spines in the prefrontal and primary visual cortices of young rhesus monkeys. Young female monkeys were ovariectomized and administered either estradiol cypionate or vehicle by intramuscular injection. Using an antibody against the spine-associated protein, spinophilin, spine numbers were estimated in layer I of area 46 and in layer I of the opercular portion of area V1 (V1o). Spine numbers in layer I of area 46 were significantly increased (55%) in the ovariectomy + estrogen group compared to the ovariectomy + vehicle group, yet spine numbers in layer I of area V1o were equivalent across the two groups. The present results suggest that estrogen's effects on synaptic organization influence select neocortical layers and regions in a primate model, and provide a morphological basis for enhanced prefrontal cortical functions following estrogen replacement.     

Retinoic acid synthesis in the postnatal mouse brain marks distinct developmental stages and functional systems

Retinoic acid (RA) affects development and function of the brain, but little is known about how much is made locally and where it is distributed. To identify RA-sensitive neural processes, we mapped the RA-synthesizing retinaldehyde dehydrogenases (RALDHs) during postnatal brain formation of the mouse. High and stable RALDH expressions mark the basal ganglia, olfactory bulbs, hippocampus and auditory afferents as major sites of RA actions in the functional brain. During the early postnatal period, transient and very high RALDH3 expressions distinguish two developmental events: (i) the colonization of the nucleus accumbens and the olfactory bulbs by neuronal precursors and (ii) the maturation of selected parts of the cerebral cortex. In the cortex, RALDH3 is transiently activated in postmigratory layer II/III neurons during formation of their dendritic arbors and it is transported in their axons across the corpus callosum. RALDH3-expressing cortical regions include most of the limbic lobe, with strongest expression in the anterior cingulate cortex, medial and lateral secondary visual cortices, auditory cortical areas, the secondary motor cortex and some association areas. The transient cortical expression points to a brief RA-critical period during differentiation of the cortical network that serves in the coordination of sensory-motor activity with emotional and recently learned information.