The emergence of architectonic field structure and areal borders in developing monkey sensorimotor cortex

Mapping vascular network architecture in primate brain using ferumoxytol-weighted laminar MRI

Mapping the vascular organization of the brain is of great importance across various domains of basic neuroimaging research, diagnostic radiology, and neurology. However, the intricate task of precisely mapping vasculature across brain regions and cortical layers presents formidable challenges, resulting in a limited understanding of neurometabolic factors influencing the brain's microvasculature. Addressing this gap, our study investigates whole-brain vascular volume using ferumoxytol-weighted laminar-resolution multi-echo gradient-echo imaging in macaque monkeys. We validate the results with published data for vascular densities and compare them with cytoarchitecture, neuron and synaptic densities. The ferumoxytol-induced change in transverse relaxation rate (ΔR2*), an indirect proxy measure of cerebral blood volume (CBV), was mapped onto twelve equivolumetric laminar cortical surfaces. Our findings reveal that CBV varies 3-fold across the brain, with the highest vascular volume observed in the inferior colliculus and lowest in the corpus callosum. In the cerebral cortex, CBV is notably high in early primary sensory areas and low in association areas responsible for higher cognitive functions. Classification of CBV into distinct groups unveils extensive replication of translaminar vascular network motifs, suggesting distinct computational energy supply requirements in areas with varying cytoarchitecture types. Regionally, baseline R2* and CBV exhibit positive correlations with neuron density and negative correlations with receptor densities. Adjusting image resolution based on the critical sampling frequency of penetrating cortical vessels, allows us to delineate approximately 30% of the arterial-venous vessels. Collectively, these results mark significant methodological and conceptual advancements, contributing to the refinement of cerebrovascular MRI. Furthermore, our study establishes a linkage between neurometabolic factors and the vascular network architecture in the primate brain.

Brain expression and song regulation of the cholecystokinin gene in the zebra finch (Taeniopygia guttata)

The gene encoding cholecystokinin (Cck) is abundantly expressed in the mammalian brain and has been associated with such functions as feeding termination and satiety, locomotion and self-stimulation, the modulation of anxiety-like behaviors, and learning and memory. Here we describe the brain expression and song regulation of Cck in the brain of the adult male zebra finch (Taeniopygia guttata), a songbird species. Using in situ hybridization we demonstrate that Cck is highly expressed in several discrete brain regions, most prominently the caudalmost portion of the hippocampal formation, the caudodorsal nidopallial shelf and the caudomedial nidopallium (NCM), the core or shell regions of dorsal thalamic nuclei, dopaminergic cell groups in the mesencephalon and pons, the principal nucleus of the trigeminal nerve, and the dorsal raphe. Cck was largely absent in song control system, a group of nuclei required for vocal learning and song production in songbirds, although sparse labeling was detected throughout the striatum, including song nucleus area X. We also show that levels of Cck mRNA and the number of labeled cells increase in the NCM of males and females following auditory stimulation with conspecific song. Double labeling further reveals that the majority of Cck cells, excluding those in the reticular nucleus of the thalamus, are non-GABAergic. Together, these data provide the first comprehensive characterization of Cck expression in a songbird, and suggest a possible involvement of Cck regulation in important aspects of birdsong biology, such as perceptual processing, auditory memorization, and/or vocal-motor control of song production.

Singing under the influence: examining the effects of nutrition and addiction on a learned vocal behavior

The songbird model is widely established in a number of laboratories for the investigation of the neurobiology and development of vocal learning. While vocal learning is rare in the animal kingdom, it is a trait that songbirds share with humans. The neuroanatomical and physiological organization of the brain circuitry that controls learned vocalizations has been extensively characterized, particularly in zebra finches (Taeniopygia guttata). Recently, several powerful molecular and genomic tools have become available in this organism, making it an attractive choice for neurobiologists interested in the neural and genetic basis of a complex learned behavior. Here, we briefly review some of the main features of vocal learning and associated brain structures in zebra finches and comment on some examples that illustrate how themes related to nutrition and addiction can be explored using this model organism.

Molecular analysis of neocortical layer structure in the ferret

Molecular markers that distinguish specific layers of rodent neocortex are increasingly employed to study cortical development and the physiology of cortical circuits. The extent to which these markers represent general features of neocortical cell type identity across mammals, however, is unknown. To assess the conservation of layer markers more broadly, we isolated orthologs for 15 layer-enriched genes in the ferret, a carnivore with a large, gyrencephalic brain, and analyzed their patterns of neocortical gene expression. Our major findings are: 1) Many but not all layer markers tested show similar patterns of layer-specific gene expression between mouse and ferret cortex, supporting the view that layer-specific cell type identity is conserved at a molecular level across mammalian superorders; 2) Our panel of deep layer markers (ER81/ETV1, SULF2, PCP4, FEZF2/ZNF312, CACNA1H, KCNN2/SK2, SYT6, FOXP2, CTGF) provides molecular evidence that the specific stratifications of layers 5 and 6 into 5a, 5b, 6a, and 6b are also conserved between rodents and carnivores; 3) Variations in layer-specific gene expression are more pronounced across areas of ferret cortex than between homologous areas of mouse and ferret cortex; 4) This variation of area gene expression was clearest with the superficial layer markers studied (SERPINE2, MDGA1, CUX1, UNC5D, RORB/NR1F2, EAG2/KCNH5). Most dramatically, the layer 4 markers RORB and EAG2 disclosed a molecular sublamination to ferret visual cortex and demonstrated a molecular dissociation among the so-called agranular areas of the neocortex. Our findings establish molecular markers as a powerful complement to cytoarchitecture for neocortical layer and cell-type comparisons across mammals.

Mouse auditory cortex differs from visual and somatosensory cortices in the laminar distribution of cytochrome oxidase and acetylcholinesterase

Cytochrome oxidase (CYO) and acetylcholinesterase (AChE) staining density varies across the cortical layers in many sensory areas. The laminar variations likely reflect differences between the layers in levels of metabolic activity and cholinergic modulation. The question of whether these laminar variations differ between primary sensory cortices has never been systematically addressed in the same set of animals, since most studies of sensory cortex focus on a single sensory modality. Here, we compared the laminar distribution of CYO and AChE activity in the primary auditory, visual, and somatosensory cortices of the mouse, using Nissl-stained sections to define laminar boundaries. Interestingly, for both CYO and AChE, laminar patterns of enzyme activity were similar in the visual and somatosensory cortices, but differed in the auditory cortex. In the visual and somatosensory areas, staining densities for both enzymes were highest in layers III/IV or IV and in lower layer V. In the auditory cortex, CYO activity showed a reliable peak only at the layer III/IV border, while AChE distribution was relatively homogeneous across layers. These results suggest that laminar patterns of metabolic activity and cholinergic influence are similar in the mouse visual and somatosensory cortices, but differ in the auditory cortex.

Short-term variations in response distribution to cortical stimulation

Patterns of responses in the cerebral cortex can vary, and are influenced by pre-existing cortical function, but it is not known how rapidly these variations can occur in humans. We investigated how rapidly response patterns to electrical stimulation can vary in intact human brain. We also investigated whether the type of functional change occurring at a given location with stimulation would help predict the distribution of responses elsewhere over the cortex to stimulation at that given location. We did this by studying cortical afterdischarges following electrical stimulation of the cortex in awake humans undergoing evaluations for brain surgery. Response occurrence and location could change within seconds, both nearby to and distant from stimulation sites. Responses might occur at a given location during one trial but not the next. They could occur at electrodes adjacent or not adjacent to those directly stimulated or to other electrodes showing afterdischarges. The likelihood of an afterdischarge at an individual site after stimulation was predicted by spontaneous electroencephalographic activity at that specific site just prior to stimulation, but not by overall cortical activity. When stimulation at a site interrupted motor, sensory or language function, afterdischarges were more likely to occur at other sites where stimulation interrupted similar functions. These results show that widespread dynamic changes in cortical responses can occur in intact cortex within short periods of time, and that the distribution of these responses depends on local brain states and functional brain architecture at the time of stimulation. Similar rapid variations may occur during normal intracortical communication and may underlie changes in the cortical organization of function. Possibly these variations, and the occurrence and distribution of responses to cortical stimulation, could be predicted. If so, interventions such as stimulation might be used to alter spread of epileptogenic activity, accelerate learning or enhance cortical reorganization after brain injury.

The roles of growth factors and neural activity in the development of the neocortex

Previous research on primarily the peripheral nervous system has shown that soluble growth factors help control key developmental events by contributing to dynamic autocrine and paracrine signalling systems. Much less is known about the roles of these substances in neocortical development. Using cell and tissue culture paradigms, we have demonstrated that soluble growth factors are produced by the neocortex and its subcortical targets, and that these tissues can respond to them. There are several possible functions for these factors in neocortical development in vivo: they may initiate axonal growth from neocortical neurons and/or their afferents; accelerate or guide that growth; and/or play a role in the later refinement of connections. Although none of these possibilities can be excluded, the existing evidence strengthens the hypothesis that soluble growth factors are important for the early postnatal growth and refinement of neocortical connections, when their levels of release may be regulated by neocortical activity. At present we do not know which growth factors are involved in these processes, but the results of preliminary experiments indicate that neurotrophins and fibroblast growth factor are prime candidates.

Exposure to Ethanol during Gastrulation Alters Somatosensory-Motor Cortices and the Underlying White Matter in the Macaque

The present study tests the hypothesis that a critical window for cortical development coincides with the period of neural stem cell proliferation (during the first 6 weeks of gestation), specifically, gastrulation (on embryonic day [E] 19 and E20). Pregnant female macaques were exposed to ethanol 1 day/week for 6 or 24 weeks such that it included E19 or E20 or a time before or after the time of gastrulation. Total forebrain size was increased in macaques exposed to ethanol on E19 or E20. Thus, various features of the gray and white matter of the paracentral lobule of adolescent offspring were examined. Ethanol exposure affected the gray matter, for example, the 1.63 billion neurons in somatosensory cortex of controls (areas 3a and 3b) was 32% lower in ethanol-exposed monkeys, but neither duration nor timing of the episodic exposure had a differential effect. In contrast, the timing of the exposure during the third week critically affected the amount of white matter (the mass of myelopil, but not cell number). Therefore, fetal exposure to ethanol unveils a normal programming mechanism wherein neural stem cells appear to be a target and a critical window for forebrain development concurs with gastrulation.

Effect of prenatal exposure to ethanol on glutamate and GABA immunoreactivity in macaque somatosensory and motor cortices: Critical timing of exposure

The present study explored the effects of gestational ethanol exposure on enduring changes in the distribution of projection neurons and local circuit neurons in somatosensory/motor cortex. Critical events in corticogenesis occur during macaque gestation: the first six weeks of gestation include the period of primary stem cell production and the next 18 weeks are marked by the birth, migration, early differentiation, and death of cortical neurons. Monkeys were exposed to ethanol (or saline) one day per week during the first six or during the entire 24 weeks of gestation. Offspring were killed as adolescents. Projection neurons and local circuit neurons were identified immunohistochemically with antibodies directed against glutamate and anti-GABA, respectively. In all animals, both projection neurons and local circuit neurons were distributed in all laminae of both somatosensory and motor cortices. Ethanol did not affect the size of Cresyl Violet-stained, glutamate-positive, or GABA-immunolabeled somata, however, it did decrease neuronal density. The total density of Cresyl Violet-stained neurons was reduced in monkeys treated with ethanol (or saline) one day per week during the first six weeks of gestation and during the entire 24 weeks of gestation. Similar reductions were detected for glutamate- and GABA-positive neurons. The densities of Cresyl Violet-stained and of glutamate- and GABA-expressing neurons were reduced in all cortical layers. The only exception was layer V which was unaffected in monkeys treated with ethanol (or saline) one day per week during the first six weeks of gestation and during the entire 24 weeks of gestation. Thus, the parallel effects on both neuronal subpopulations suggest that ethanol targets a population of undetermined neuronal precursors.

Correlation between the sequential ingrowth of afferents and transient patterns of cortical lamination in preterm infants

Transient patterns of regional, laminar, modular, neuronal, and functional organization are essential features of the developing cerebral cortex in preterm infants. Analysis of cytological, histological, histochemical, functional, and behavioral parameters revealed that transient cerebral patterns develop and change rapidly between 24 weeks post ovulation (W) and birth. The major afferent fibers (thalamocortical, basal forebrain, and corticocortical) grow through the transient "waiting" subplate zone (SP) compartment and accumulate below the cortical plate (CP) between 22 and 26 W. These afferent fibers gradually penetrate the CP after 26 W. The prolonged process of dissolution of the SP can be explained by prolonged growth and maturation of associative connections in the human cerebral cortex. The neurons and circuitry elements of the transient layers are the substrate for transient functional and behavioral patterns. The predominance of deep synapses and deep dendritic maturation underlies the immaturity and different polarity of the cortical electrical response in the preterm infant. The significant changes in the transient SP, together with profound changes in the transient architecture of the neocortical plate, parallel the changes observed in recent MRI studies. The role of the SP in the formation of cortical connections and functions is an important factor in considering the pathogenesis of cognitive deficits after brain lesions in the preterm infant.

Neurobiological mechanisms of the onset of puberty in primates

An increase in pulsatile release of LHRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in LHRH release is still unclear. In primates the LHRH neurosecretory system is already active during the neonatal period but subsequently enters a dormant state in the juvenile/prepubertal period. Neither gonadal steroid hormones nor the absence of facilitatory neuronal inputs to LHRH neurons is responsible for the low levels of LHRH release before the onset of puberty in primates. Recent studies suggest that during the prepubertal period an inhibitory neuronal system suppresses LHRH release and that during the subsequent maturation of the hypothalamus this prepubertal inhibition is removed, allowing the adult pattern of pulsatile LHRH release. In fact, y-aminobutyric acid (GABA) appears to be an inhibitory neurotransmitter responsible for restricting LHRH release before the onset of puberty in female rhesus monkeys. In addition, it appears that the reduction in tonic GABA inhibition allows an increase in the release of glutamate as well as other neurotransmitters, which contributes to the increase in pubertal LHRH release. In this review, developmental changes in several neurotransmitter systems controlling pulsatile LHRH release are extensively reviewed.

Temporal modulation of GABA(A) receptor subunit gene expression in developing monkey cerebral cortex

In situ hybridization histochemistry was used to examine the expression of 10 GABA(A) receptor messenger RNAs corresponding to the alpha1-alpha5, beta1-beta3, gamma1 and gamma2 subunits in primary somatosensory and visual areas of macaque monkey cerebral cortex from embryonic day (E) 125 to postnatal day (P) 125. Results were compared with expression patterns in adults. In the sensorimotor cortex at E125, overall levels of all subunit transcripts were low. At E137, there was a major lamina-specific increase in all subunit messenger RNAs except gamma1. For alpha1, alpha2, alpha4, beta2, beta3 and gamma2 subunit transcripts, this increase was highest in areas 3a and 3b, particularly in layers III/IV and VI. Postnatally, there were significant decreases in all transcripts. Alpha1, alpha5, beta2 and gamma2 subunit transcripts, while still at significantly lower levels than at E137, remained expressed at levels higher than other transcripts. Unlike in rodents, there was no obvious "switch" in the major subunits expressed in fetal and adult cortex, alpha1, alpha5, beta2 and gamma2 remaining highest throughout. In area 17, the most prominently expressed subunits at earliest ages were alpha2, alpha5, beta1, beta2, beta3 and gamma2, especially in layers II/III and VI. At E150, expression for alpha2, alpha3, beta1 and beta3 subunit transcripts in these layers decreased, but levels for alpha1, alpha4, alpha5, beta2, gamma1 and gamma2 transcripts increased, particularly within layer IV. The increase at E150 was particularly marked for alpha5 transcripts, which were expressed at levels more than four times those of other transcripts. Alpha1, beta2 and gamma2 remain highest into aduthood. Fetal area 17 displayed lamina-specific patterns of expression not found in adult animals. In particular, alpha3 messenger RNAs were present in layer IVA and gamma1 transcripts were present in layer IVC at E150, despite a lack of expression in these layers in the adult. These data demonstrate increased expression of GABA(A) receptors during the period of establishment of thalamocortical and intracortical connections, and a temporal regulation that may be associated with the period of developmental plasticity.

Primary sensory and forebrain motor systems in the newborn brain are preferentially damaged by hypoxia-ischemia

Cerebral hypoxia-ischemia causes encephalopathy and neurologic disabilities in newborns by unclear mechanisms. We tested the hypothesis that hypoxia-ischemia causes brain damage in newborns that is system-preferential and related to regional oxidative metabolism. One-week-old piglets were subjected to 30 minutes of hypoxia and then seven minutes of airway occlusion, producing asphyxic cardiac arrest, followed by cardiopulmonary resuscitation and four-day recovery. Brain injury in hypoxic-ischemia piglets (n = 6) compared to controls (n = 5) was analyzed by hematoxylin-eosin, Nissl, and silver staining, relationships between regional vulnerability and oxidative metabolism were evaluated by cytochrome oxidase histochemistry. Profile counting-based estimates showed that 13% and 27% of neurons in layers II/III and layers of somatosensory cortex had ischemic cytopathology, respectively; CA1 neuronal perikarya appeared undamaged, and < 10% of CA3 and CA4 neurons were injured; and neuronal damage was 79% in putamen, 17% in caudate, but nucleus accumbens was undamaged. Injury was found preferentially in primary sensory neocortices (particularly somatosensory cortex), basal ganglia (predominantly putamen, subthalamic nucleus, and substantia nigra reticulata), ventral thalamus, geniculate nuclei, and tectal nuclei. In sham piglets, vulnerable region generally had higher cytochrome oxidase levels than less vulnerable areas. Postischemic alterations in cytochrome oxidase were regional and laminar, with reductions (31-66%) occurring in vulnerable regions and increases (20%) in less vulnerable areas. We conclude that neonatal hypoxia-ischemia causes highly organized, system-preferential and topographic encephalopathy, targeting regions that function in sensorimotor integration and movement control. This distribution of neonatal encephalopathy is dictated possibly by regional function, mitochondrial activity, and connectivity.

Laminar patterns of expression of GABA-A receptor subunit mRNAs in monkey sensory motor cortex

Radioactive complementary RNA probes, made from monkey-specific cDNAs specific for the alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 2 subunits of the gamma-aminobutyric acid A (GABAA) receptor were used for in situ hybridization histochemistry of the primary motor, somatosensory, and anterior parietal areas of the cerebral cortex in macaque monkeys. mRNAs for the alpha 1, beta 2, and gamma 2 subunit polypeptides, which form receptors with the full range of classical properties, are expressed at much higher levels in all areas and show laminar- and sublaminar-specific concentrations. alpha 2, alpha 4, alpha 5, and beta 1 subunit transcripts are expressed at much lower levels but also display individual, laminar-specific concentrations; alpha 5 expression, in particular, is highly expressed in layer IV in the somatosensory and parietal areas and in a layer IV-like band in the motor cortex. In layers in which expression of a particular transcript is high, all neurons may express the gene, but in layers in which expression is moderate, it is possible to detect differences in the degree of labeling of individual neurons for a particular mRNA, and some neurons may not express certain subunit transcripts in detectable amounts. These findings indicate the variability in expression of different GABAA receptor subunits in the cerebral cortex. Laminar differences may indicate the assembly of functional receptors from different arrangements of available subunits in different classes of cells.

Somatotopy of monkey premotor cortex examined with microstimulation

We reinvestigated the organization of the premotor cortex (PM) using intracortical microstimulation. Movements of forelimb, hindlimb, and orofacial structures were evoked from broad regions of PM that appeared to be contiguous with other motor areas. There were two principal findings: (1) the somatotopy of PM lies roughly parallel to that of the primary motor cortex (MI). Forelimb movements were evoked from sites deep in the caudal bank of the arcuate sulcus and throughout the adjacent cortex bounded by a face representation (laterally) and a hindlimb representation (medially and caudally); (2) unlike the MI, the PM forelimb representation overlaps significantly with its own face representation. PM hindlimb movement sites overlap only slightly with PM forelimb sites, in a manner similar to the MI. There was no obvious boundary between PM, MI, or supplementary motor area hindlimb representations. The present findings are discussed in relation to recently identified subdivisions of the PM.

Postnatal maturation of GABA-immunoreactive neurons of rat medial prefrontal cortex

A light microscopic immunocytochemical approach has been used to examine the distribution and maturation of gamma-aminobutyric acid- (GABA) containing cells in rat medial prefrontal cortex (mPFC) at progressive postnatal stages. Between P1 and P5, labeled cells in the cortical plate show less differentiated morphological characteristics when compared to cells in the deeper laminae. By P10, however, most labeled cells in superficial laminae show more differentiated characteristics with some having a distinctive multipolar appearance. Between P1 and P5, there is a significant increase (50%) in the density of GABA-containing cells in the superficial laminae, while concurrently there is an overall decreases in the subjacent deeper laminae. As the cortex continues to expand, there is a corresponding decrease in the density of GABA-immunoreactive cells in the outer two-thirds of the cortical mantle until approximately P15, stabilizing at 20-25 cells/100,000 microns2 for all laminae. Between P1 and P15, there is also a significant increase (133%) in the average size of labeled cells, followed by a gradual decrease of 30% between P15 and P41. During P1-7, there is a marked increase in the density of labeled axosomatic terminals in both the superficial (200%) and deep laminae (116%). In the superficial layers, however, the density of labeled terminals again increases by 86% between P12 and P18. In general, the present findings are consistent with the idea that there is a progressive maturation of the intrinsic GABAergic system in rat mPFC in a classic "inside-out" pattern, and this involves extensive postnatal changes occurring during the first 3 postnatal weeks.

Getting there and being there in the cerebral cortex

The mammalian neocortex is composed of functional areas that are specified to process particular aspects of information. How is this specification achieved during development? Since cells migrate to their final positions in the developing nervous system, a central issue is the relation between cellular migration and positional information. This review combines evidence for early positional specification in the developing cortex with evidence for cellular dispersion during migration. A model is suggested whereby stable cues provide positional information and minorities of 'displaced' cells are respecified accordingly. Comparison with other parts of the CNS reveals that cellular dispersal is ubiquitous and has to be included in any mechanism relaying positional specification. Ontogenetic and phylogenetic considerations suggest that radial glial cells might provide the positional information in the developing nervous system.

Early development of the somatotopic map and barrel patterning in rat somatosensory cortex

Several lines of evidence implicate a crucial role for thalamic afferents from the ventroposterior nucleus (VP) in the development of barrels and their characteristic pattern in the primary somatosensory cortex (S1) of rodents. We sought to determine the stage in development when VP thalamocortical afferents are first distributed in a periphery-related pattern and the sequence of events that culminate in a mature pattern. Using acetylcholinesterase (AChE) histochemistry, an early marker for VP thalamocortical afferents, and the anterograde axon tracer DiI, we show that VP thalamocortical afferents become distributed into a periphery-related pattern earlier than was previously reported, including their parcellation into a barrel-related pattern that mirrors the distribution of sensory hairs on the face. The earliest periphery-related patterning observed is transiently present in the deep cortical layers prior to the emergence of layer 4, the layer in which barrels later develop. AChE histochemistry reveals a clear sequence of maturation of the barrel pattern in the distribution of VP afferents: An initially patternless distribution of AChE-reactive afferents is followed by their distribution in a nascent trigeminal representation, from which rows subsequently emerge; barrel-related clusters of afferents then emerge from the rows. This process begins before birth, and the transition from row-related to barrel-related distributions of VP afferents is evident during the first postnatal day (P0). This demonstration of a periphery-related pattern in developing rat S1 precedes by about 2 days that revealed by any other marker reported to delineate barrels. These findings confirm that VP thalamocortical afferents are the first barrel component to have a periphery-related pattern and support the hypothesis that thalamocortical afferents provide to immature S1 the patterning information that initiates the formation of barrels and their characteristic array. Furthermore because these findings show an earlier onset for barrel formation than was previously realized, they necessitate a reevaluation of conclusions drawn from experiments examining developmental plasticity in barrel patterning.

Neurochemical development of the hippocampal region in the fetal rhesus monkey. II. Immunocytochemistry of peptides, calcium-binding proteins, DARPP-32, and monoamine innervation in the entorhinal cortex by the end of gestation

Material for the study came from one 126 day-old rhesus monkey fetus and two 3 day-old neonates. The immunocytochemical detection of somatostatin, neurotensin (NT), parvalbumin, calbindin D-28K, DARPP-32 as well as tyrosine hydroxylase (TH), dopamine-beta-hydroxylase and serotonin (5-HT), was carried out on serial cryostat sections of the entorhinal cortex. The authors reported in a previous paper the precocious differentiation of the entorhinal cortex in rhesus monkey fetuses and featured the conspicuous expression of calbindin D-28K, somatostatin, neurotensin, and the monoaminergic innervation during the first half of gestation. The present study shows distinct temporal profiles of neurochemical development during the second half of gestation: the dense neuropeptidergic innervation remained a constant feature; the three aminergic systems gradually increased in density; parvalbumin, unlike calbindin D-28K, was primarily expressed during the last quarter of gestation. Three other prominent features of the last quarter of gestation are illustrated: the refinement of the modular neurochemical organization of the lamina principalis externa, the delayed chemoanatomical development of the rhinal sulcus area, and the establishment of a distinct rostrocaudal pattern of neurochemical distribution. In correspondence with the cluster-like organization of the lamina principalis externa, the authors observed in the olfactory, rostral, and intermediate fields of the neonate monkey entorhinal cortex, a particular subset of pyramidal-shaped neurons: located in layer III, they were characterized by fasciculated apical dendrites ascending between the cellular islands of the discontinuous layer II and the coexpression of calbindin D-28K and DARPP-32. Besides, most of the other chemical systems displayed a distinct, area-specific, patchy distribution, except for the homogeneously distributed noradrenergic innervation. In the olfactory and rostral fields, TH positive dopaminergic fibers accumulated on the neuronal islands of layers II-III, and parvalbumin labeled fibers on those of layer III, whereas patches of 5-HT and NT-like reactive terminals were segregated between the cellular islands, overlapping the DARPP-32/calbindin D-28 K labeled dendritic bundles. At the opposite, in the intermediate field, 5-HT positive terminals overlapped the cellular islands of layer II and thin fascicles of dopaminergic fibers ran in the inter island spaces. The somatostatin-LIR innervation was apparently too dense to reveal a patchy distribution that existed at earlier developmental stages. In the caudal field, the patchy pattern was replaced by a predominant bilaminar type of distribution of NT, 5-HT, and TH-like positive afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental expression of brain derived neurotrophic factor mRNA by neurons of fetal and adult monkey prefrontal cortex

In situ hybridization histochemistry with labeled cRNA probes complementary to monkey brain derived neurotrophic factor (BDNF) mRNAs has been used to study the cellular localization and expression of this neurotrophin in the prefrontal cerebral cortex of fetal and adult monkeys. Expression could not be detected in prefrontal cortex before the 121st fetal day. Thereafter, in fetal life and in adulthood BDNF mRNA could be detected primarily in large, putative pyramidal cells of layers III and VI throughout the prefrontal cortex. The temporal course and cellular localization of BDNF expression suggests its association with the development and stabilization of specific connections in regions of cortex that display marked functional plasticity.