Adult Primate Brain Research Articles

Dopaminergic Substantia Nigra Neurons Project Topographically Organized to the Subventricular Zone and Stimulate Precursor Cell Proliferation in Aged Primates

The subventricular zone of the adult primate brain contains neural stem cells that can produce new neurons. Endogenous neurogenesis might therefore be used to replace lost neurons in neurodegenerative diseases. This would require, however, a precise understanding of the molecular regulation of stem cell proliferation and differentiation in vivo. Several regulatory factors, including dopamine, have been identified in rodents, but none in primates. We have, therefore, studied the origin and function of the dopaminergic innervation of the subventricular zone in nonhuman primates. Tracing experiments in three macaques revealed a topographically organized projection from the substantia nigra pars compacta (SNpc), but not the adjacent retrorubral field, to the subventricular zone: the anteromedial SNpc projects to the anteroventral subventricular zone, the posterolateral SNpc to the posterodorsal subventricular zone. Double immunolabeling for tyrosine hydroxylase and BrdU (5-bromo-2'deoxyuridine) incorporated into the DNA of proliferating cells showed that dopaminergic fibers approach proliferating cells in the subventricular zone. We investigated the effect of this nigro-subventricular projection on cell proliferation in six aged macaques, because the rate of neurogenesis differs between young adult and aged primates and because neurodegenerative diseases mainly affect aged humans. Three macaques were treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) to decrease dopaminergic innervation of the subventricular zone. A significant decrease in the number of PCNA+ (proliferating cell nuclear antigen-positive) proliferating cells (-44%) and PSA-NCAM(+) (polysialylated neural cell adhesion molecule-positive) neuroblasts (-59%) was found in the denervated regions of the subventricular zone, suggesting that an intact dopaminergic nigro-subventricular innervation is crucial for sustained neurogenesis in aged primates.

Ephrin/Eph receptor expression in brain of adult nonhuman primates: Implications for neuroadaptation

In developing brain, Eph receptors and their ephrin ligands (Ephs/ephrins) are implicated in facilitating topographic guidance of a number of pathways, including the nigrostriatal and mesolimbic dopamine (DA) pathways. In adult rodent brain, these molecules are implicated in neuronal plasticity associated with learning and memory. Cocaine significantly alters the expression of select members of this family of axonal guidance molecules, implicating Ephs, ephrins in drug-induced neuroadaptation. The potential contribution of Ephs, ephrins to cocaine-induced reorganization of striatal circuitry brain in primates [Saka, E., Goodrich, C., Harlan, P., Madras, B.K., Graybiel, A.M., 2004. Repetitive behaviors in monkeys are linked to specific striatal activation patterns. J. Neurosci. 24, 7557–7565] is unknown because there are no documented reports of Eph/ephrin expression or function in adult primate brain. We now report that brains of adult old and new world monkeys express mRNA encoding EphA4 receptor and ephrin-B2 ligand, implicated in topographic guidance of dopamine and striatal neurons during development. Their encoded proteins distributed highly selectively in regions of adult monkey brain. EphA4 mRNA levels were prominent in the DA-rich caudate/putamen, nucleus accumbens and globus pallidus, as well as the medial and orbitofrontal cortices, hippocampus, amygdala, thalamus and cerebellum. Immunocytochemical localization of EphA4 protein revealed discrete expression in caudate/putamen, globus pallidus, substantia nigra, cerebellar Purkinje cells, pyramidal cells of frontal cortices (layers II, III and V) and the subgranular zone of the hippocampus. Evidence for EphA4 expression in dopamine neurons emerged from colocalization with tyrosine-hydroxylase-positive terminals in striatum and substantia nigra and ventral tegmental area cell bodies. The association of axonal guidance molecules with drug-induced reorganization of adult primate brain circuitry warrants investigation.

Experience induces structural and biochemical changes in the adult primate brain

Primates exhibit complex social and cognitive behavior in the wild. In the laboratory, however, the expression of their behavior is usually limited. A large body of literature shows that living in an enriched environment alters dendrites and synapses in the brains of adult rodents. To date, no studies have investigated the influence of living in a complex environment on brain structure in adult primates. We assessed dendritic architecture, dendritic spines, and synaptic proteins in adult marmosets housed in either a standard laboratory cage or in one of two differentially complex habitats. A month-long stay in either complex environment enhanced the length and complexity of the dendritic tree and increased dendritic spine density and synaptic protein levels in the hippocampus and prefrontal cortex. No differences were detected between the brains of marmosets living in the two differentially complex environments. Our results show that the structure of the adult primate brain remains highly sensitive even to modest levels of experiential complexity. For adult primates, living in standard laboratory housing may induce reversible dendritic spine and synapse decreases in brain regions important for cognition.

Enhanced proliferation of progenitor cells in the subventricular zone and limited neuronal production in the striatum and neocortex of adult macaque monkeys after global cerebral ischemia

Cerebral ischemia in adult rodent models increases the proliferation of endogenous neural progenitor cells residing in the subventricular zone along the anterior horn of the lateral ventricle (SVZ a) and induces neurogenesis in the postischemic striatum and cortex. Whether the adult primate brain preserves a similar ability in response to an ischemic insult is uncertain. We used the DNA synthesis indicator bromodeoxyuridine (BrdU) to label newly generated cells in adult macaque monkeys and show here that the proliferation of cells with a progenitor phenotype (double positive for BrdU and the markers Musashi 1, Nestin, and beta III-tubulin) in SVZ a increased during the second week after a 20-min transient global brain ischemia. Subsequent progenitor migration seemed restricted to the rostral migratory stream toward the olfactory bulb and ischemia increased the proportion of adult-generated cells retaining their location in SVZ a with a progenitor phenotype. Despite the lack of evidence for progenitor cell migration toward the postischemic striatum or prefrontal neocortex, a small but sustained proportion of BrdU-labeled cells expressed features of postmitotic neurons (positive for the protein Neu N and the transcription factors Tbr 1 and Islet 1) in these two regions for at least 79 days after ischemia. Taken together, our data suggest an enhanced neurogenic response in the adult primate telencephalon after a cerebral ischemic insult.

Distribution of the NMDA receptor NR3A subunit in the adult pig-tail macaque brain

The NMDA subtype of glutamate receptors are heteromeric complexes comprised of multiple subunits encoded by at least seven different genes (NR1, NR2A–2D and NR3A–3B), and differential expression of these subunits alters the pharmacological and electrophysiological properties of NMDA receptors. NR3A is a recently identified unique modulatory subunit that decreases NMDA receptor current and calcium influx. In rodents, NR3A is developmentally expressed, displaying robust expression early in development that declines with age, reaching low levels in the adult brain. A distinct and highly selective pattern of expression is observed in the developing and mature rodent brain, suggesting that NR3A may play a very specific role in NMDA receptor-mediated processes. NR3A expression in other species, however, is unknown. Therefore, we examined the expression of NR3A mRNA and protein in the adult macaque brain. Our results indicate that NR3A mRNA is expressed throughout much of the adult primate brain, and at high levels in specific brain regions including the neocortex, substantia nigra par compacta and cerebellum, as well as select areas of the hippocampus, amygdala, thalamus and hypothamalamus. Western blot analysis reflects that this protein is translated and expressed in multiple brain regions. In contrast to the rat mRNA, our results suggest that NR3A transcript is widely expressed in the adult primate brain. Particular enrichment in some brain areas may reflect brain-region or circuit-specific functions for this NMDA receptor subunit.

Autologous transplantation of expanded neural precursor cells into the demyelinated monkey spinal cord

The objective of this study was to establish if neural precursor cells could safely be developed from biopsy of the subventricular zone (SVZ) in the non-human primate (marmoset), and to determine their myelinating potential after autologous transplantation into a demyelinated lesion. Small amounts of tissue were safely collected from the subventricular–subependymal zone of the adult primate brain under ultrasonography without any neurological deficit. Neural precursor cells were isolated and expanded in the presence of mitogen in vitro. The dorsal columns of the adult marmoset spinal cord were demyelinated by X-irradiation and intraspinal injections of ethidium bromide in the center of the radiation field. Cell suspensions of the neural precursors were microinjected through a micropipette into the demyelinated lesion site in the spinal cord. Lesions were histologically examined 3 weeks after transplantation. Light and electron microscopic examination of plastic embedded sections revealed a significant number of myelinating profiles in the transplantation zone; no myelination was observed in control lesions. The myelinated axons had predominantly peripheral patterns of myelination. These results demonstrate that autologous transplantation of neural precursor cells in the adult nonhuman primate can remyelinate demyelinated central nervous system (CNS) axons, thus suggesting the potential utility of such an approach in demyelinating lesions in humans.

Reelin immunoreactivity in the adult primate brain: intracellular localization in projecting and local circuit neurons of the cerebral cortex, hippocampus and subcortical regions.

Reelin is a large secretable protein which is widely expressed by specific neuronal populations. In the embryonic brain, Reelin plays a signaling role critical for the correct positioning of migrating neuroblasts. Reelin is also expressed in the adult mammalian brain, including humans; however, its function/s there remain poorly understood. To gain insight into which neuronal populations and specific circuits may be influenced by Reelin in the adult, we have conducted a light and electron microscope analysis of Reelin-immunoreactive neuron types in the cerebral cortex and subcortical regions of adult macaque monkeys. Results show that the great majority of brain neurons, including interneurons and projection neurons, are immunoreactive for Reelin although some neuronal populations do not contain Reelin. The immunoreactive protein is located intracellularly, mainly in neuronal somata. Reelin is also present in gray matter neuropil as well as in some long axonal pathways and their terminal arborizations, suggesting that it can be axonally transported over long distances. The staining patterns in the labeled neurons are remarkably diverse. Our observations reveal a wider distribution of Reelin in the adult macaque brain than in any other species investigated to date. The data show that Reelin is in a position to influence most brain circuits in the adult primate brain.

Pre- and post-developmental neurogenesis in primates

The [ 3H]TdR or bromodeoxyuridine (BrdU) methods for marking DNA replication were used to determine time of origin of over 125 classes of neurons in the 34 structures of the macaque monkey central nervous system. The analysis revealed that each neuronal class is generated in a precisely defined developmental period, the duration of which did not depend on the size and final number of neurons in a given structure. An exception was the granule cell class of the olfactory bulb and dentate gyrus that continues genesis at a low rate even after sexual maturity. Thus, most neurons of the primate brain, including cerebral cortex last the entire life span and are normally not renewable. Overcoming resistance of the adult primate's brain to acquisition of functionally competent neurons as a form of therapy would benefit from insight into why neurogenesis normally ceases at a specific developmental period as well as why there are species-specific and regional differences in the capacity for neurogenesis.

An Update on Estrogen: Higher Cognitive Function, Receptor Mapping, Neurotrophic Effects

An Update on Estrogen: Higher Cognitive Function, Receptor Mapping, Neurotrophic Effects

The generation, migration, and differentiation of olfactory neurons in the adult primate brain.

In adult rodents, neural progenitor cells in the subependymal (SZ) zone of the lateral cerebral ventricle generate neuroblasts that migrate in chains via the rostral migratory stream (RMS) into the olfactory bulb (OB), where they differentiate into interneurons. However, the existence of this neurogenic migratory system in other mammals has remained unknown. Here, we report the presence of a homologue of the rodent SZ/RMS in the adult macaque monkey, a nonhuman Old World primate with a relatively smaller OB. Our results-obtained by using combined immunohistochemical detection of a marker for DNA replication (5-bromodeoxyuridine) and several cell type-specific markers-indicate that dividing cells in the adult monkey SZ generate neuroblasts that undergo restricted chain migration over an extended distance of more than 2 cm to the OB and differentiate into granule interneurons. These findings in a nonhuman primate extend and support the use of the SZ/RMS as a model system for studying neural regenerative mechanisms in the human brain.

Long-term methamphetamine-induced decreases of [(11)C]WIN 35,428 binding in striatum are reduced by GDNF: PET studies in the vervet monkey.

The effects of glial cell line-derived neurotrophic factor (GDNF) pretreatment on methamphetamine (METH)-induced striatal dopamine system deficits in the vervet monkey were characterized with [(11)C]WIN 35,428 (WIN)-positron emission tomography (PET). WIN, a cocaine analog that binds to the dopamine transporter (DAT), was used to provide an index of striatal dopamine terminal integrity. In two subjects, GDNF (200 microg/40 microl) was injected into the caudate and putamen unilaterally vs. saline contralaterally. After 1-2 weeks, + and -GDNF striatal WIN-PET binding values were equivalent as calculated by multiple time graphic analysis, suggestive of an absence of unilateral DAT up-regulation. Three other subjects (n = 3) received GDNF injections into the caudate and putamen unilaterally and one week later, were administered METH HCl (2 x 2 mg/kg; i.m., 24 hours apart; a neurotoxic dosage for this species). At 1 week post-METH, WIN-PET studies showed that mean WIN binding was decreased by 72% in the +GDNF and by 92% in the -GDNF striatum relative to pre-drug assessment values. Thus, GDNF pretreatment reduced the extent of METH-induced decreases in WIN binding. Subsequent WIN-PET studies (1.5-9-month range) showed a protracted recovery of WIN binding in each striatum, indicative of long-term but partially reversible METH neurotoxicity. Further, at each time point, WIN binding remained relatively higher in the +GDNF vs. -GDNF striatum. These results provide further evidence that the adult non-human primate brain remains responsive to exogenously administered GDNF and that this pharmacotherapy approach can counteract aspects of neurotoxic actions associated with methamphetamine.

Long-term methamphetamine-induced decreases of [11C]WIN 35,428 binding in striatum are reduced by GDNF: PET studies in the vervet monkey

The effects of glial cell line-derived neurotrophic factor (GDNF) pretreatment on methamphetamine (METH)-induced striatal dopamine system deficits in the vervet monkey were characterized with [11C]WIN 35,428 (WIN)-positron emission tomography (PET). WIN, a cocaine analog that binds to the dopamine transporter (DAT), was used to provide an index of striatal dopamine terminal integrity. In two subjects, GDNF (200 μg/40 μl) was injected into the caudate and putamen unilaterally vs. saline contralaterally. After 1–2 weeks, + and –GDNF striatal WIN-PET binding values were equivalent as calculated by multiple time graphic analysis, suggestive of an absence of unilateral DAT up-regulation. Three other subjects (n = 3) received GDNF injections into the caudate and putamen unilaterally and one week later, were administered METH HCl (2 × 2 mg/kg; i.m., 24 hours apart; a neurotoxic dosage for this species). At 1 week post-METH, WIN-PET studies showed that mean WIN binding was decreased by 72% in the +GDNF and by 92% in the –GDNF striatum relative to pre-drug assessment values. Thus, GDNF pretreatment reduced the extent of METH-induced decreases in WIN binding. Subsequent WIN-PET studies (1.5–9-month range) showed a protracted recovery of WIN binding in each striatum, indicative of long-term but partially reversible METH neurotoxicity. Further, at each time point, WIN binding remained relatively higher in the +GDNF vs. –GDNF striatum. These results provide further evidence that the adult non-human primate brain remains responsive to exogenously administered GDNF and that this pharmacotherapy approach can counteract aspects of neurotoxic actions associated with methamphetamine. Synapse 35:243–249, 2000. © 2000 Wiley-Liss, Inc.

Estrogen receptor immunoreactivity in the adult primate brain: Neuronal distribution and association with p75, trkA, and choline acetyltransferase

The neuroactive steroid hormone, estrogen, has been implicated in both the prevention and treatment of Alzheimer's disease. Interactions between estrogen and neurotrophic systems may partially explain the beneficial effects of estrogen therapy. Previous studies have identified estrogen binding sites colocalized with neurotrophin-related proteins and mRNA within the rodent brain. Extending these studies to a model more relevant to human systems, we have mapped the distribution of estrogen receptor alpha (ER-α)-immunoreactive neurons in adult nonhuman primate brains. In addition, we used double-label immunohistochemistry to examine colocalization of ER-α with the low- and high-affinity neurotrophin receptors, p75 and trkA, and with the cholinergic marker choline acetyltransferase. Large numbers of ER-α-immunoreactive cells were detected in several amygdaloid and hypothalamic nuclei. ER-α-labeled cells were also found in the lateral septum, nucleus of the stria terminals, subfornical organ, and periaqueductal gray. Only rare, scattered ER-α-immunoreactive cells were noted in the cholinergic basal forebrain. In contrast to rodents, no cells exhibited ER-α and p75 or ER-α and trkA double-labeling. However, ER-labeled neurons in the amygdala, a region containing putative nerve growth factor-producing cells and exhibiting a role in memory, were densely and specifically invested with cholinergic terminals projecting from the basal forebrain. Estrogen-labeled neurons were also present in the lateral septal nucleus, a system that receives hippocampal inputs and projects to the neurotrophin-sensitive medial septum. Thus, interactions between neurotrophin-sensitive neurons and ER-bearing neurons exist in the primate brain, providing a potential paracrine basis for estrogen-state modulation of vulnerability to Alzheimer's disease. J. Comp. Neurol. 405:529–542, 1999. © 1999 Wiley-Liss, Inc.

Estrogen receptor immunoreactivity in the adult primate brain: Neuronal distribution and association with p75,trkA, and choline acetyltransferase

The neuroactive steroid hormone, estrogen, has been implicated in both the prevention and treatment of Alzheimer's disease. Interactions between estrogen and neurotrophic systems may partially explain the beneficial effects of estrogen therapy. Previous studies have identified estrogen binding sites colocalized with neurotrophin-related proteins and mRNA within the rodent brain. Extending these studies to a model more relevant to human systems, we have mapped the distribution of estrogen receptor alpha (ER-alpha)-immunoreactive neurons in adult nonhuman primate brains. In addition, we used double-label immunohistochemistry to examine colocalization of ER-alpha with the low- and high-affinity neurotrophin receptors, p75 and trkA, and with the cholinergic marker choline acetyltransferase. Large numbers of ER-alpha-immunoreactive cells were detected in several amygdaloid and hypothalamic nuclei. ER-alpha-labeled cells were also found in the lateral septum, nucleus of the stria terminals, subfornical organ, and periaqueductal gray. Only rare, scattered ER-alpha-immunoreactive cells were noted in the cholinergic basal forebrain. In contrast to rodents, no cells exhibited ER-alpha and p75 or ER-alpha and trkA double-labeling. However, ER-labeled neurons in the amygdala, a region containing putative nerve growth factor-producing cells and exhibiting a role in memory, were densely and specifically invested with cholinergic terminals projecting from the basal forebrain. Estrogen-labeled neurons were also present in the lateral septal nucleus, a system that receives hippocampal inputs and projects to the neurotrophin-sensitive medial septum. Thus, interactions between neurotrophin-sensitive neurons and ER-bearing neurons exist in the primate brain, providing a potential paracrine basis for estrogen-state modulation of vulnerability to Alzheimer's disease.

Large-scale plasticity of the human motor cortex.

The adult primate brain is capable of modifying rapidly the size of cortical receptive fields or motor output modules in response to altered synaptic input. We used positron emission tomography (PET) to map the regional cerebral blood flow changes related to voluntary finger movements in patients with tumours occupying the hand area of motor cortex. All patients showed activations solely outside the tumour. Compared with the unaffected side, the activations were shifted by 9-43 mm either along the mediolateral body representation of motor cortex or into premotor or parietal somatosensory cortex. These results provide evidence that slowly developing lesions can induce large-scale reorganization that is not confined to changes within the somatotopic body representation in motor cortex.

Nerve growth factor infusion in the primate brain reduces lesion- induced cholinergic neuronal degeneration

NGF is a protein that promotes survival, differentiation, and process extension of selected neuronal populations during development and, in some cases, in the mature organism. Previous lesion and aging studies in the rat have shown that intracerebroventricular NGF infusions can prevent degenerative changes in basal forebrain cholinergic neurons. We sought to determine whether salutory effects of NGF occur in the primate brain. Cholinergic fibers of the septohippocampal projection in the primate were surgically transected, followed by infusion of either a vehicle or an NGF solution into the ventricular system for a 4-week period. Quantification of cholinergic neurons in the medial septal nucleus at the end of the infusion period demonstrated that only 45 +/- 5% of cholinergic neurons could be identified after fornix lesions in vehicle-infused animals, whereas 80 +/- 6% of neurons were visible in NGF-treated animals. Thus, NGF substantially reduced lesion-induced cholinergic neuronal degeneration in the adult primate brain. This finding may be relevant to the hypothesis that NGF has potential use as a cholinergic "neurotrophic-factor therapy," given that loss of basal forebrain cholinergic neurons is common in Alzheimer's disease.