Nonhuman Primate Brain Research Articles

Preclinical evaluation of a promising C-11 labeled PET tracer for imaging phosphodiesterase 10A in the brain of living subject

Phosphodiesterase 10A (PDE10A) plays a key role in the regulation of brain striatal signaling. A PET tracer for PDE10A may serve as a tool to evaluate PDE10A expression in vivo in central nervous system disorders with striatal pathology. Here, we further characterized the binding properties of a previously reported radioligand we developed for PDE10A, [11C]TZ1964B, in rodents and nonhuman primates (NHPs). The tritiated counterpart [3H]TZ1964B was used for in vitro binding characterizations in rat striatum homogenates and in vitro autoradiographic studies in rat brain slices. The carbon-11 labeled [11C]TZ1964B was utilized in the ex vivo autoradiography studies for the brain of rats and microPET imaging studies for the brain of NHPs. MicroPET scans of [11C]TZ1964B in NHPs were conducted at baseline, as well as with using a selective PDE10A inhibitor MP-10 for either pretreatment or displacement. The in vivo regional target occupancy (Occ) was obtained by pretreating with different doses of MP-10 (0.05–2.00mg/kg). Both in vitro binding assays and in vitro autoradiographic studies revealed a nanomolar binding affinity of [3H]TZ1964B to the rat striatum. The striatal binding of [3H]TZ1964B and [11C]TZ1964B was either displaced or blocked by MP-10 in rats and NHPs. Autoradiography and microPET imaging confirmed that the specific binding of the radioligand was found in the striatum but not in the cerebellum. Blocking studies also confirmed the suitability of the cerebellum as an appropriate reference region. The binding potentials (BPND) of [11C]TZ1964B in the NHP striatum that were calculated using either the Logan reference model (LoganREF, 3.96±0.17) or the simplified reference tissue model (SRTM, 4.64±0.47), with the cerebellum as the reference region, was high and had good reproducibility. The occupancy studies indicated a MP-10 dose of 0.31±0.09mg/kg (LoganREF)/0.45±0.17mg/kg (SRTM) occupies 50% striatal PDE10A binding sites. Studies in rats and NHPs demonstrated radiolabeled TZ1964B has a high binding affinity and good specificity for PDE10A, as well as favorable in vivo pharmacokinetic properties and binding profiles. Our data suggests that [11C]TZ1964B is a promising radioligand for in vivo imaging PDE10A in the brain of living subject.

Calorie restriction improves brain health in aging rhesus macaques

Calorie restriction (CR) without malnutrition slows the aging and disease process, prolonging median and maximum lifespan in yeast, worms, fish, flies, and rodents. Studies of CR in non-human primates (Macaca mulatta) indicate a protective effect of CR against aging-related diseases and a salutary effect on brain health. Our group has studied the specific effects of CR on the non-human primate brain using neuroimaging, behavioral, and post-mortem histological techniques and this talk will provide an overview of these findings. Animals in this study were part of the longitudinal “Dietary Restriction and Aging Study” at the Wisconsin National Primate Research Center (WNPRC). Animals were either fed a normal diet or maintained on a moderately restricted diet (approximately 30% reduced intake from baseline), with both groups receiving comparable diet supplements. Using volumetric imaging we demonstrated that CR preserves gray matter cortex in limbic and heteromodal association areas, indicating a positive effect of CR against age-related brain atrophy. Diffusion tensor imaging showed preservation of white matter integrity in the corpus callosum and fronto-occipital fasciculus, superior longitudinal fasciculus, external capsule, and brainstem. CR also attenuated age-related iron accumulation in the basal ganglia, red nucleus, and parietal, temporal, and perirhinal cortex. Decreased iron accumulation was in turn associated with faster performance on fine motor function tests, signifying a protective effect against motor slowness that results during aging. CR moderated the effect of important plasma-based inflammatory markers (e.g. IL-6) on gray and white matter changes in several brain areas, including the parietal and temporal gray matter regions that are sensitive to aging. CR improved glucoregulatory profiles and positively influenced gray matter volume in the hippocampus. Histopathology studies reveal that CR monkeys express significantly lower (∼30%) levels of microglial activation in the hippocampus. Energy metabolism in the hippocampus as indexed by PGC1alpha and GSK3B was preserved in CR. Number of MTL amyloid plaques was however equivalent between groups. Overall, these results recapitulate the neuroprotective effects of CR from lower animal models. Taken together, these findings point to an overall beneficial effect of CR on the brain in this non-human primate model of aging.

Incidence angle and gray-to-white-matter ratio dependence of the focused-ultrasound induced blood-brain barrier opening in non-human primates

FUS coupled with the systemic administration of microbubbles has been proved to open the Blood-Brain Barrier (BBB) locally, transiently and non-invasively in non-human primates (NHP). However, the complexity of the NHP brain constitutes an obstacle in relating the volume size of the induced opening (VBBB) to the geometric aspects of the method as well as the physiologic characteristics of the targeted areas. The objective of the current study is to quantify the correlation between the VBBB, the FUS pressure and the incidence angle. Additionally, the dependence of the opening shift on the gray-to-white-matter ratio at the targeted area is studied.

Olanzapine Reversed Brain Gene Expression Changes Induced by Phencyclidine Treatment in Non-Human Primates

The NMDA receptor antagonist phencyclidine (PCP) creates schizophrenia-like symptoms in normal controls. The effect of PCP on non-human primate brain gene expression was examined and compared to changes induced by olanzapine treatment. Experimental studies of PCP and antipsychotic drugs have direct relevance to understanding the pathophysiology and treatment of schizophrenia. Genome-wide changes in prefrontal cortex gene expression revealed alterations of 146 transcripts in the PCP treatment group compared to vehicle controls. Dysregulated genes were enriched in identified classes implicated in neurological and genetic disorders, including schizophrenia genes from the Psychiatric Genomics Consortium 108 loci as well as cell death in PCP-treated primates. Canonical pathway analysis revealed a significant overrepresentation of several groups including synaptic long-term potentiation and calcium signaling. Olanzapine coadministered with PCP normalized 34% of the 146 PCP-induced probe set expression changes, and a network of 17 olanzapine-normalized genes was identified enriched in schizophrenia candidate genes containing RGS4, SYN1 and AKT as nodes. The results of this study support the use of PCP administration in non-human primates as a glutamatergic model of schizophrenia and suggest that a large number of PCP-induced expression differences can be reversed by olanzapine. The results of this study may be informative for identification of potential candidates for pharmacogenetics and biomarker research related to the treatment of schizophrenia.

Synthesis and biological characterization of a promising F-18 PET tracer for vesicular acetylcholine transporter

Nine fluorine-containing vesicular acetylcholine transporter (VAChT) inhibitors were synthesized and screened as potential PET tracers for imaging the VAChT. Compound 18a was one of the most promising carbonyl-containing benzovesamicol analogs; the minus enantiomer, (−)-18a displayed high potency (VAChT Ki=0.59±0.06nM) and high selectivity for VAChT versus σ receptors (>10,000-fold). The radiosynthesis of (−)-[18F]18a was accomplished by a two-step procedure with 30–40% radiochemical yield. Preliminary biodistribution studies of (−)-[18F]18a in adult male Sprague-Dawley rats at 5, 30, 60 and 120min post-injection (p.i.) were promising. The total brain uptake of (−)-[18F]18a was 0.684%ID/g at 5min p.i. and by 120min p.i. slowly washed out to 0.409 %ID/g; evaluation of regional brain uptake showed stable levels of ∼0.800 %ID/g from 5 to 120min p.i in the VAChT-enriched striatal tissue of rats, indicating the tracer had crossed the blood brain barrier and was retained in the striatum. Subsequent microPET brain imaging studies of (−)-[18F]18a in nonhuman primates (NHPs) showed high striatal accumulation in the NHP brain; the standardized uptake value (SUV) for striatum reached a maximum value of 5.1 at 15min p.i. The time–activity curve for the target striatal region displayed a slow and gradual decreasing trend 15min after injection, while clearance of the radioactivity from the cerebellar reference region was much more rapid. Pretreatment of NHPs with 0.25mg/kg of the VAChT inhibitor (−)-vesamicol resulted in a ∼90% decrease of striatal uptake compared to baseline studies. HPLC metabolite analysis of NHP plasma revealed that (−)-[18F]18a had a good in vivo stability. Together, these preliminary results suggest (−)-[18F]18a is a promising PET tracer candidate for imaging VAChT in the brain of living subjects.

Effects of abstinence from chronic cocaine self-administration on nonhuman primate dorsal and ventral noradrenergic bundle terminal field structures.

Repeated exposure to cocaine is known to dysregulate the norepinephrine system, and norepinephrine has also been implicated as having a role in abstinence and withdrawal. The goal of this study was to determine the effects of exposure to cocaine self-administration and subsequent abstinence on regulatory elements of the norepinephrine system in the nonhuman primate brain. Rhesus monkeys self-administered cocaine (0.3mg/kg/injection, 30 reinforcers/session) under a fixed-interval 3-min schedule of reinforcement for 100 sessions. Animals in the abstinence group then underwent a 30-day period during which no operant responding was conducted, followed by a final session of operant responding. Control animals underwent identical schedules of food reinforcement and abstinence. This duration of cocaine self-administration has been shown previously to increase levels of norepinephrine transporters (NET) in the ventral noradrenergic bundle terminal fields. In contrast, in the current study, abstinence from chronic cocaine self-administration resulted in elevated levels of [(3)H]nisoxetine binding to the NET primarily in dorsal noradrenergic bundle terminal field structures. As compared to food reinforcement, chronic cocaine self-administration resulted in decreased binding of [(3)H]RX821002 to α2-adrenoceptors primarily in limbic-related structures innervated by both dorsal and ventral bundles, as well as elevated binding in the striatum. However, following abstinence from responding for cocaine binding to α2-adrenoceptors was not different than in control animals. These data demonstrate the dynamic nature of the regulation of norepinephrine during cocaine use and abstinence, and provide further evidence that the norepinephrine system should not be overlooked in the search for effective pharmacotherapies for cocaine dependence.

Mutant alpha-synuclein causes age-dependent neuropathology in monkey brain.

Parkinson's disease (PD) is an age-dependent neurodegenerative disease that often occurs in those over age 60. Although rodents and small animals have been used widely to model PD and investigate its pathology, their short life span makes it difficult to assess the aging-related pathology that is likely to occur in PD patient brains. Here, we used brain tissues from rhesus monkeys at 2-3, 7-8, and >15 years of age to examine the expression of Parkin, PINK1, and α-synuclein, which are known to cause PD via loss- or gain-of-function mechanisms. We found that α-synuclein is increased in the older monkey brains, whereas Parkin and PINK1 are decreased or remain unchanged. Because of the gain of toxicity of α-synuclein, we performed stereotaxic injection of lentiviral vectors expressing mutant α-synuclein (A53T) into the substantia nigra of monkeys and found that aging also increases the accumulation of A53T in neurites and its associated neuropathology. A53T also causes more extensive reactive astrocytes and axonal degeneration in monkey brain than in mouse brain. Using monkey brain tissues, we found that A53T interacts with neurofascin, an adhesion molecule involved in axon subcellular targeting and neurite outgrowth. Aged monkey brain tissues show an increased interaction of neurofascin with A53T. Overexpression of A53T causes neuritic toxicity in cultured neuronal cells, which can be attenuated by transfected neurofascin. These findings from nonhuman primate brains reveal age-dependent pathological and molecular changes that could contribute to the age-dependent neuropathology in PD.

Brain/MINDS: brain-mapping project in Japan.

There is an emerging interest in brain-mapping projects in countries across the world, including the USA, Europe, Australia and China. In 2014, Japan started a brain-mapping project called Brain Mapping by Integrated Neurotechnologies for Disease Studies (Brain/MINDS). Brain/MINDS aims to map the structure and function of neuronal circuits to ultimately understand the vast complexity of the human brain, and takes advantage of a unique non-human primate animal model, the common marmoset (Callithrix jacchus). In Brain/MINDS, the RIKEN Brain Science Institute acts as a central institute. The objectives of Brain/MINDS can be categorized into the following three major subject areas: (i) structure and functional mapping of a non-human primate brain (the marmoset brain); (ii) development of innovative neurotechnologies for brain mapping; and (iii) human brain mapping; and clinical research. Brain/MINDS researchers are highly motivated to identify the neuronal circuits responsible for the phenotype of neurological and psychiatric disorders, and to understand the development of these devastating disorders through the integration of these three subject areas.

Serotonin transporter occupancy by escitalopram and citalopram in the non-human primate brain: a [(11)C]MADAM PET study.

A number of serotonin receptor positron emission tomography (PET) radioligands have been shown to be sensitive to changes in extracellular serotonin concentration, in a generalization of the well-known dopamine competition model. High doses of selective serotonin reuptake inhibitors (SSRIs) decrease serotonin receptor availability in monkey brain, consistent with increased serotonin concentrations. However, two recent studies on healthy human subjects, using a single, lower and clinically relevant SSRI dose, showed increased cortical serotonin receptor radioligand binding, suggesting potential decreases in serotonin concentration in projection regions when initiating treatment. The cross-species differential SSRI effect may be partly explained by serotonin transporter (SERT) occupancy in monkey brain being higher than is clinically relevant. We here determine SERT occupancy after single doses of escitalopram or citalopram by conducting PET measurements with [(11)C]MADAM in monkeys. Relationships between dose, plasma concentration and SERT occupancy were estimated by one-site binding analyses. Binding affinity was expressed as dose (ID50) or plasma concentration (K i) where 50% SERT occupancy was achieved. Estimated ID50 and K i values were 0.020mg/kg and 9.6nmol/L for escitalopram and 0.059mg/kg and 9.7nmol/L for citalopram, respectively. Obtained K i values are comparable to values reported in humans. Escitalopram or citalopram doses nearly saturated SERT in previous monkey studies which examined serotonin sensitivity of receptor radioligands. PET-measured cross-species differential effects of SSRI on cortical serotonin concentration may thus be related to SSRI dose. Future monkey studies using SSRI doses inducing clinically relevant SERT occupancy may further illuminate the delayed onset of SSRI therapeutic effects.

Whole brain mapping of visual and tactile convergence in the macaque monkey

The proposal that sensory processing is achieved in segregated anatomical pathways has been profoundly revisited following the description of cross-modal anatomical connections both at higher and at lower processing levels. However, an understanding of the cortical extent of these long range cross-modal functional influences has been missing. Here, we use functional magnetic resonance imaging (fMRI) to map, in the non-human primate brain, the cortical regions which are activated by both visual and tactile stimulations. We describe an unprecedented pattern of functional visuo-tactile convergence, encompassing both low-level visual and somatosensory areas and multiple higher-order associative areas. We also show that the profile of this convergence depends on the physical properties of the mapping stimuli, indicating that visuo-tactile convergence is most probably even more prevailing than what we actually describe. Overall, these observations substantiate the view that the brain is massively multisensory.

The brain's default mode network.

The brain's default mode network consists of discrete, bilateral and symmetrical cortical areas, in the medial and lateral parietal, medial prefrontal, and medial and lateral temporal cortices of the human, nonhuman primate, cat, and rodent brains. Its discovery was an unexpected consequence of brain-imaging studies first performed with positron emission tomography in which various novel, attention-demanding, and non-self-referential tasks were compared with quiet repose either with eyes closed or with simple visual fixation. The default mode network consistently decreases its activity when compared with activity during these relaxed nontask states. The discovery of the default mode network reignited a longstanding interest in the significance of the brain's ongoing or intrinsic activity. Presently, studies of the brain's intrinsic activity, popularly referred to as resting-state studies, have come to play a major role in studies of the human brain in health and disease. The brain's default mode network plays a central role in this work.

477. Three-Dimensional Imaging of Multiple AAV Serotype/Mutant Vector Transductions in the Non-Human Primate Brain By Massively Parallel Sequencing

Global therapeutic gene delivery to the central nervous system (CNS) offers a promising approach to treat CNS diseases and preclinical studies using rodents have shown AAV9 as a potential candidate to achieve this goal. However, recent work has highlighted the existence of nonhuman primate (NHP)-rodent differences in AAV vector biology and transduction profiles in the CNS after gene delivery, making imperative a more rigorous evaluation of various AAV serotype and capsid mutant vectors in NHPs. It is of particular importance to understand how each AAV strain (serotype or mutant) is distributed and transduces cells in the brain of a primate species closely related to humans when injected into the bloodstream or cerebrospinal fluids (CSF). Here we report a novel approach to generate three-dimensional (3D) vector transduction heat maps of more than one hundred of AAV strains in the rhesus monkey brain when vectors are delivered via the cisterna magna. In this approach, we integrate the vector transduction data obtained by AAV Barcode-Seq (an Illumina sequencing-based high-throughput vector transduction analysis) into an MRI image. To show proof-of-principle of the method, a barcoded-AAV library containing 145 different AAV strains was directly injected into CSF via the cisterna magna of three rhesus macaques (4×1012 vg/kg) and their brains were subjected to MRI analysis before collection of the organ 6 weeks post-injection. Six mm-thick coronal sections of the brain were made and photographed, and the right half of each section slice was diced into 6 × 7 × 7 mm3 cubes using a matrix, generating more than 100 cubes from a cerebral hemisphere. Total DNA was then extracted from each cube using an automated magnetic particle processor and subjected to AAV Barcode-Seq analysis to determine transduction efficiency of each AAV strain in each cube by multiplexed Illumina sequencing. The coordinates of each cube were determined by aligning the photo images of each coronal section to the corresponding MRI images using an image alignment algorithm. This strategy allowed us to create 3D heat maps showing distribution and transduction of each AAV strain in the entire NHP brain. A preliminary analysis using one of the three monkeys revealed that AAV1 vector spreads globally and transduces all brain regions including the cerebral cortex while the distribution of AAV2 was limited locally. Unexpectedly, we found that many other serotypes and mutants transduced the NHP brain better than AAV9 when delivered into CSF. Thus, imaging of AAV vector distribution and transduction throughout the NHP brain by AAV Barcode-Seq combined with MRI provides a powerful approach to comprehensively study AAV vector-mediated CNS gene delivery in clinically relevant animal models. Our results emphasize the large difference in AAV distribution and transduction efficiency that exists in the brain of higher primates compared to rodent models.

The effects of chronic alcohol self-administration in nonhuman primate brain networks.

Long-term alcohol abuse is associated with change in behavior, brain structure, and brain function. However, the nature of these changes is not well understood. In this study, we used network science to analyze a nonhuman primate model of ethanol self-administration to evaluate functional differences between animals with chronic alcohol use and animals with no exposure to alcohol. Of particular interest was how chronic alcohol exposure may affect the resting state network. Baseline resting state functional magnetic resonance imaging was acquired in a cohort of vervet monkeys. Animals underwent an induction period where they were exposed to an isocaloric maltose dextrin solution (control) or ethanol in escalating doses over three 30-day epochs. Following induction, animals were given ad libitum access to water and a maltose dextrin solution (control) or water and ethanol for 22h/d over 12months. Cross-sectional analyses examined region of interests in hubs and community structure across animals to determine differences between drinking and nondrinking animals after the 12-month free access period. Animals were classified as lighter (<2.0g/kg/d) or heavier drinkers (≥2.0g/kg/d) based on a median split of their intake pattern during the 12-month ethanol free access period. Statistical analysis of hub connectivity showed significant differences in heavier drinkers for hubs in the precuneus, posterior parietal cortices, superior temporal gyrus, subgenual cingulate, and sensorimotor cortex. Heavier drinkers were also shown to have less consistent communities across the brain compared to lighter drinkers. The different level of consumption between the lighter and heavier drinking monkeys suggests that differences in connectivity may be intake dependent. Animals that consume alcohol show topological differences in brain network organization, particularly in animals that drink heavily. Differences in the resting state network were linked to areas that are associated with spatial association, working memory, and visuomotor processing.

Kinetic Analysis and Quantification of [¹¹C]Martinostat for in Vivo HDAC Imaging of the Brain.

Epigenetic mechanisms mediated by histone deacetylases (HDACs) have been implicated in a wide-range of CNS disorders and may offer new therapeutic opportunities. In vivo evaluation of HDAC density and drug occupancy has become possible with [(11)C]Martinostat, which exhibits selectivity for a subset of class I/IIb HDAC enzymes. In this study, we characterize the kinetic properties of [(11)C]Martinostat in the nonhuman primate (NHP) brain in preparation for human neuroimaging studies. The goal of this work was to determine whether classic compartmental analysis techniques were appropriate and to further determine if arterial plasma is required for future NHP studies. Using an arterial plasma input function, several analysis approaches were evaluated for robust outcome measurements. [(11)C]Martinostat showed high baseline distribution volume (VT) ranging from 29.9 to 54.4 mL/cm(3) in the brain and large changes in occupancy (up to 99%) with a blocking dose approaching full enzyme saturation. An averaged nondisplaceable tissue uptake (VND) of 8.6 ± 3.7 mL/cm(3) suggests high specific binding of [(11)C]Martinostat. From a two-tissue compartment model, [(11)C]Martinostat exhibits a high K1 (averaged K1 of 0.65 mL/cm(3)/min) and a small k4 (average of 0.0085 min(-1)). Our study supports that [(11)C]Martinostat can be used to detect changes in HDAC density and occupancy in vivo and that simplified analysis not using arterial blood could be appropriate.

Improving Single Injection CSF Delivery of AAV9-mediated Gene Therapy for SMA: A Dose–response Study in Mice and Nonhuman Primates

Spinal muscular atrophy (SMA) is the most frequent lethal genetic neurodegenerative disorder in infants. The disease is caused by low abundance of the survival of motor neuron (SMN) protein leading to motor neuron degeneration and progressive paralysis. We previously demonstrated that a single intravenous injection (IV) of self-complementary adeno-associated virus-9 carrying the human SMN cDNA (scAAV9-SMN) resulted in widespread transgene expression in spinal cord motor neurons in SMA mice as well as nonhuman primates and complete rescue of the disease phenotype in mice. Here, we evaluated the dosing and efficacy of scAAV9-SMN delivered directly to the cerebral spinal fluid (CSF) via single injection. We found widespread transgene expression throughout the spinal cord in mice and nonhuman primates when using a 10 times lower dose compared to the IV application. Interestingly, in nonhuman primates, lower doses than in mice can be used for similar motor neuron targeting efficiency. Moreover, the transduction efficacy is further improved when subjects are kept in the Trendelenburg position to facilitate spreading of the vector. We present a detailed analysis of transduction levels throughout the brain, brainstem, and spinal cord of nonhuman primates, providing new guidance for translation toward therapy for a wide range of neurodegenerative disorders.

Long-Term Survival and Differentiation of Human Neural Stem Cells in Nonhuman Primate Brain with no Immunosuppression

Cellular fate of human neural stem cells (hNSCs) transplanted in the brain of nonhuman primates (NHPs) with no immunosuppression was determined at 22 and 24 months posttransplantation (PTx) regarding survival, differentiation, and tumorigenesis. Survival of hNSCs labeled with magnetic nanoparticles was successfully detected around injection sites in the brain at 22 months PTx by MRI. Histological examination of brain sections with H&E and Prussian blue staining at 24 months revealed that most of the grafted hNSCs were found located along the injection tract. Grafted hNSCs were found to differentiate into neurons at 24 months PTx. In addition, none of the grafted hNSCs were bromodeoxyuridine positive in the monkey brain, indicating that hNSCs did not replicate in the NHP brain and did not cause tumor formation. This study serves as a proof of principle and provides evidence that hNSCs transplanted in NHP brain could survive and differentiate into neurons in the absence of immunosuppression. It also serves as a preliminary study in our scheduled preclinical studies of hNSC transplantation in NHP stroke models.

Vesicular Monoamine Transporter 2 Loss in Human Cocaine Abusers Confirmed in Nonhuman Primate Brain

Vesicular Monoamine Transporter 2 Loss in Human Cocaine Abusers Confirmed in Nonhuman Primate Brain

3-D Printing of a Non-Human Primate Brain Slicer for Accurate, High Resolution Sampling of Specific Regions of Fresh or Frozen Brain

The non-human primate (NHP) brain closely parallels the human brain in terms of both circuitry, and of the specific functional regions which form connections within the circuits. Using conventional manufacturing technology, we constructed a Brain Block Slicer tailored to the structure of the NHP brain, and used it to make precise and reproducible coronal slices for next generation sequencing-based transcriptome analysis. Here we use computed tomography to scan the original brain block, and to use additive manufacturing to produce a 3-D printed NHP Brain Slicer that is fully comparable to the original. The block is easily scalable to primate brains of different sizes, and different slicing geometries can be deployed in silico, including sagittal and horizontal, in addition to the coronal geometry shown in the present printed brain block.

Genetic sculpture of fine-grained human cortical regionalization

Genetic sculpture of fine-grained human cortical regionalization

A multi-site array for combined local electrochemistry and electrophysiology in the non-human primate brain

A multi-site array for combined local electrochemistry and electrophysiology in the non-human primate brain