Adolescent Rhesus Research Articles

Individual and sex differences in the behavior of adolescents Macaque Rhesus (Macaca mulatta) united into a new social group

Introduction. Primates living in groups in the laboratory can exhibit a wide range of behaviors consistent with their natural behavioral repertoire. The study of adolescent macaques’ behavior grouped together in new social group is of interest for understanding the social organization of preschool children, as well as for anthropology in general. The goal of the work is to reveal the individual and sex differences of behavioral activity of unrelated adolescent rhesus monkeys (Macaca mulatta), grouped in new laboratory conditions. Materials and methods. We conducted focal observations recording the behavior activity of each macaque in the first two months of their cohabitation. We analyzed individual and sex differences in the manifestation of locomotor, exploratory and social behavior. Results. Individual characteristics of macaques for each type of behavior were identified. The group turned out to be the most heterogeneous in the degree of manifestation of exploratory behavior. The exploratory activity of females was significantly higher than that of males. Social activity was significantly higher in males due to the predominance of friendly reactions, although aggression was equally demonstrated in both sexes. However, females were significantly more often exposed to aggression from other individuals than they showed it themselves. As for males, aggression towards other individuals and aggression from other individuals did not differ significantly. Most individuals in the group demonstrated more friendliness reactions than aggression, but two macaques showed these types of behavior equally. There were no differences in the number of locomotor reactions between males and females. Conclusion. The analysis made it possible to trace the basic forms of behavioral activity of adolescent macaques in context of the new social group formation. The obtained results give us better understanding of the mechanisms of primates’ social behavior organization.

P138. Prefrontal Connections to the Extended Amygdala and Their Role in Early-Life Inhibited Temperament in Adolescent Rhesus Monkeys

The extended amygdala has been implicated in the expression of fear and anxiety, and is thought to mediate early-life risk for psychopathology. How these regions interact with newly expanded areas of the primate frontal cortex remains largely unknown. Using a well-validated primate model, we leverage a multimodal neuroimaging to explore the contributions of prefrontal-connections in early-life inhibited temperament, a risk factor for the future development of stress-related psychopathologies

Induction of core symptoms of autism spectrum disorder by in vivo CRISPR/Cas9-based gene editing in the brain of adolescent rhesus monkeys

Although CRISPR/Cas9-mediated gene editing is widely applied to mimic human disorders, whether acute manipulation of disease-causing genes in the brain leads to behavioral abnormalities in non-human primates remains to be determined. Here we induced genetic mutations in MECP2, a critical gene linked to Rett syndrome (RTT) and autism spectrum disorders (ASD), in the hippocampus (DG and CA1–4) of adolescent rhesus monkeys (Macaca mulatta) in vivo via adeno-associated virus (AAV)-delivered Staphylococcus aureus Cas9 with small guide RNAs (sgRNAs) targeting MECP2. In comparison to monkeys injected with AAV-SaCas9 alone (n = 4), numerous autistic-like behavioral abnormalities were identified in the AAV-SaCas9-sgMECP2-injected monkeys (n = 7), including social interaction deficits, abnormal sleep patterns, insensitivity to aversive stimuli, abnormal hand motions, and defective social reward behaviors. Furthermore, some aspects of ASD and RTT, such as stereotypic behaviors, did not appear in the MECP2 gene-edited monkeys, suggesting that different brain areas likely contribute to distinct ASD symptoms. This study showed that acute manipulation of disease-causing genes via in vivo gene editing directly led to behavioral changes in adolescent primates, paving the way for the rapid generation of genetically engineered non-human primate models for neurobiological studies and therapeutic development.

Recovery from impaired working memory performance during chronic Δ-9-tetrahydrocannabinol administration to adolescent rhesus monkeys.

The relationship between adolescent cannabis use and susceptibility to persistent cognitive impairments is poorly understood. We examined the effects of repeated exposure to Δ-9-tetrahydrocannabinol (THC) on reinforcement-related learning and performance of spatial working memory (WM) tasks of varying difficulty in adolescent monkeys. Seven pairs of male adolescent rhesus monkeys, matched for baseline cognitive performance, received vehicle or THC intravenously 5 days/week for 12 months. Performance on 4-item spatial WM trials was assessed throughout the 12-month study period. At the 6-month time point, more difficult novel and distractor 8-item spatial WM trials were added. Residual effects on performance were determined 23 or 71 h after THC or vehicle administration throughout the study. Relative to vehicle-exposed animals, repeated THC exposure was initially associated with significantly slower improvement in performance accuracy on 4-item spatial WM trials; however, this performance difference gradually diminished such that by month 12, accuracy did not significantly differ between vehicle and THC groups. Similarly, for the novel and distractor 8-item trials introduced at month 6, performance accuracy improved more slowly in the THC than in the vehicle group, despite comparable performance between groups on the 4-item task during this same period. These findings suggest that compared to vehicle exposure, THC exposure during adolescence impairs the reinforcement-related learning process required for improved performance on spatial WM tasks, but this impairment might be overcome with continued training, even in the face of ongoing THC exposure.

Early adversity contributes to chronic stress induced depression-like behavior in adolescent male rhesus monkeys

Chronic stress is an important cause for depression. However, not everyone who is exposed to chronic stress will develop depression. Our previous studies demonstrated that early adversity can cause lasting changes in adolescent rhesus monkeys, but depressive symptoms have not been observed. Compared to adults, it is still unknown that whether adolescent rhesus monkeys experiencing early adversity are more likely to develop depressive symptoms. In this study, we investigated the long term relationship between early adversity, chronic stress and adolescent depression for the first time. Eight male rhesus monkeys were reared in maternal separation (MS) or mother-reared (MR) conditions. All of them went through unpredictable chronic stress for two months at their age four. The stressors included space restriction, intimidation, long illumination and fasting. Behavioral and physiological data were collected during the experiment. The results showed that, compared with the MR group, the locomotor activity of MS group was significantly decreased after one month of chronic stress while huddling up and stereotypical behaviors were significantly increased. Moreover, this trend continued and even worsened at the second month. Significantly higher hair cortisol levels and lower body weight were observed in MS group after two months of stress. These results indicate that early adversity is one of the environmental factors which can increase the susceptibility of depression when experiencing chronic stress in the later life. This will further clarify the important roles of early environmental factors in the development of adolescent depression and children rearing conditions should receive more attention.

Pharmacologically and Edinger–Westphal stimulated accommodation in rhesus monkeys does not rely on changes in anterior chamber pressure

This study was undertaken to understand the role of anterior chamber pressure (ACP) during pharmacological and Edinger–Westphal (EW) stimulated accommodation in anesthetized monkeys. Experiments were performed on one iridectomized eye each of 7 anesthetized adolescent rhesus monkeys. Accommodation was induced by EW stimulation (n = 2) and intravenous administration of 0.25–4.0 mg/kg pilocarpine (n = 6). Accommodative refractive and biometric changes were measured with continuous 60 Hz infrared photorefraction (n = 6) and 100 Hz A-scan ultrasound biometry (n = 1). An ocular perfusion system was used to measure and manipulate ACP. Pressure was recorded via a 27-gauge needle in the anterior chamber connected to a pressure transducer (n = 7). The needle was also connected to a fluid reservoir to allow ACP to be manipulated and clamped (n = 4) by raising or lowering the fluid reservoir. In all six pharmacologically stimulated monkeys ACP increased during accommodation, from 0.70 to 2.38 mmHg, four of which showed pressure decreases preceding the pressure increases. Two eyes also showed increases in ACP during EW-stimulated accommodation of 2.8 and 7.2 mmHg. ACP increased with increasing EW stimulus amplitudes (n = 2). Clamping or externally manipulating ACP had no effect on resting refraction or on EW and pharmacologically stimulated accommodation in four eyes. The results show that EW stimulated and pharmacologically stimulated accommodation do not rely on ACP in rhesus monkeys.

Repeated Δ9-Tetrahydrocannabinol Exposure in Adolescent Monkeys: Persistent Effects Selective for Spatial Working Memory

Epidemiological findings suggest that, relative to adults, adolescents are more vulnerable to the adverse persistent effects of cannabis on working memory. However, the potential confounds inherent in human studies preclude direct determination of a cause-and-effect relationship between adolescent cannabis use and heightened susceptibility to persistent working memory impairments. Consequently, the authors examined the effects of repeated exposure to Δ9-tetrahydrocannabinol (THC) on performance of spatial and object working memory tasks in adolescent monkeys. Seven pairs of male adolescent rhesus monkeys, matched for baseline cognitive performance, received vehicle or THC intravenously 5 days/week for 6 months. Performance on spatial and object memory tasks was assessed 23 or 71 hours after drug administration throughout the study. In addition, acute effects on working memory were also assessed at the beginning and end of the 6-month period. Relative to the vehicle-exposed control animals, those with repeated THC exposure had a blunted trajectory of accuracy improvements on the spatial working memory task in a delay-dependent manner. Accuracy improvements on the object working memory task did not differ between groups. Relative to the acute effects of THC on working memory at the beginning of the study, neither sensitivity nor tolerance was evident after 6 months of THC exposure. Because maturation of performance is later for spatial than for object working memory, these findings suggest that persistent effects of THC on cognitive abilities are more evident when exposure coincides with the developmental stage during which the underlying neural circuits are actively maturing.

How Heavy Marijuana Use May Hurt the Developing Brain: An Animal Study

With legalization of recreational marijuana use accomplished in two states and likely in many more, concerns about how it may affect youth are becoming more prominent than ever. To examine cognitive issues, investigators intravenously administered vehicle or Δ9-tetrahydrocannabinol (THC) at a dose equivalent to one or two marijuana cigarettes 5 days per week for 6 months to seven matched pairs of male adolescent rhesus monkeys. The …

Brain white matter microstructure alterations in adolescent rhesus monkeys exposed to early life stress: associations with high cortisol during infancy

BackgroundEarly adverse experiences, especially those involving disruption of the mother-infant relationship, are detrimental for proper socioemotional development in primates. Humans with histories of childhood maltreatment are at high risk for developing psychopathologies including depression, anxiety, substance abuse, and behavioral disorders. However, the underlying neurodevelopmental alterations are not well understood. Here we used a nonhuman primate animal model of infant maltreatment to study the long-term effects of this early life stress on brain white matter integrity during adolescence, its behavioral correlates, and the relationship with early levels of stress hormones.MethodsDiffusion tensor imaging and tract based spatial statistics were used to investigate white matter integrity in 9 maltreated and 10 control animals during adolescence. Basal plasma cortisol levels collected at one month of age (when abuse rates were highest) were correlated with white matter integrity in regions with group differences. Total aggression was also measured and correlated with white matter integrity.ResultsWe found significant reductions in white matter structural integrity (measured as fractional anisotropy) in the corpus callosum, occipital white matter, external medullary lamina, as well as in the brainstem of adolescent rhesus monkeys that experienced maternal infant maltreatment. In most regions showing fractional anisotropy reductions, opposite effects were detected in radial diffusivity, without changes in axial diffusivity, suggesting that the alterations in tract integrity likely involve reduced myelin. Moreover, in most regions showing reduced white matter integrity, this was associated with elevated plasma cortisol levels early in life, which was significantly higher in maltreated than in control infants. Reduced fractional anisotropy in occipital white matter was also associated with increased social aggression.ConclusionsThese findings highlight the long-term impact of infant maltreatment on brain white matter structural integrity, particularly in tracts involved in visual processing, emotional regulation, and somatosensory and motor integration. They also suggest a relationship between elevations in stress hormones detected in maltreated animals during infancy and long-term brain white matter structural effects.

Objective and Subjective Refractive Error Measurements in Monkeys

To better understand the functional significance of refractive-error measures obtained using common objective methods in laboratory animals, we compared objective and subjective measures of refractive error in adolescent rhesus monkeys. The subjects were 20 adolescent monkeys. Spherical-equivalent spectacle-plane refractive corrections were measured by retinoscopy and autorefraction while the animals were cyclopleged and anesthetized. The eye's axial dimensions were measured by A-Scan ultrasonography. Subjective measures of the eye's refractive state, with and without cycloplegia, were obtained using psychophysical methods. Specifically, we measured spatial contrast sensitivity as a function of spectacle lens power for relatively high spatial frequency gratings. The lens power that produced the highest contrast sensitivity was taken as the subjective refraction. Retinoscopy and autorefraction consistently yielded higher amounts of hyperopia relative to subjective measurements obtained with or without cycloplegia. The subjective refractions were not affected by cycloplegia and on average were 1.42 ± 0.61 D and 1.24 ± 0.62 D less hyperopic than the retinoscopy and autorefraction measurements, respectively. Repeating the retinoscopy and subjective measurements through 3 mm artificial pupils produced similar differences. The results show that commonly used objective methods for assessing refractive errors in monkeys significantly overestimate the degree of hyperopia. It is likely that multiple factors contributed to the hyperopic bias associated with these objective measurements. However, the magnitude of the hyperopic bias was in general agreement with the "small-eye artifact" of retinoscopy.

Delay- and Dose-Dependent Effects of Δ9-Tetrahydrocannabinol Administration on Spatial and Object Working Memory Tasks in Adolescent Rhesus Monkeys

Among adolescents, the perception that cannabis can cause harm has decreased and use has increased. However, in rodents, cannabinoid administration during adolescence induces working memory (WM) deficits that are more severe than if the same exposure occurs during adulthood. As both object and spatial WM mature in a protracted manner, although apparently along different trajectories, adolescent cannabis users may be more susceptible to impairments in one type of WM. Here, we evaluate the acute effects of a range of doses (30-240 μg/kg) of intravenous Δ⁹-tetrahydrocannabinol (THC) administration on the performance of spatial and object WM tasks in adolescent rhesus monkeys. Accuracy on the object WM task was not significantly affected by any dose of THC. In contrast, THC administration impaired accuracy on the spatial WM task in a delay- and dose-dependent manner. Importantly, the THC-induced spatial WM deficits were not because of motor or motivational impairments. These data support the idea that immature cognitive functions are more sensitive to the acute effects of THC.

Acquisition and baseline performance of working memory tasks by adolescent rhesus monkeys

Adolescence is a transitional stage of development characterized by protracted refinements in the neural circuits required for adult level proficiency of working memory. Because impaired working memory is a hallmark feature of several psychiatric disorders that have their onset during adolescence, model systems that can be used to assess the maturation of working memory function, and of disease-related risk factors that disrupt its development, are of particular importance. However, few studies have investigated the maturation of working memory in nonhuman primates. Thus in the present study, we adapted two working memory tests that are among the most widely used in human and adult nonhuman primates, for adolescent rhesus monkeys. Using a touch-screen apparatus, monkeys were trained on a spatial delayed-response task to assess spatial working memory and a delayed match-to-sample task to assess object working memory. The results indicate that adolescent rhesus monkeys readily and efficiently acquire the ability to perform touch-screen based, complex tests of working memory. These data establish that distinct components of adult prefrontal cortex-dependent cognitive functions can be effectively modeled and evaluated in adolescent monkeys. As such, this approach should be useful for assessing the influence of environmental risk factors on the protracted maturation of working memory in adolescent macaques.

Effects of confined space and near vision stimulation on refractive status and vitreous chamber depth in adolescent rhesus monkeys

This study aimed to investigate the effects of sustained near vision stimulation, on the refractive development and elongation of the vitreous chamber in adolescent rhesus monkeys. A total of 12 adolescent rhesus monkeys (1.5-2.0 years old) were randomly assigned to 3 groups. In groups A (n=4) and B (n=4), monkeys were reared in close-vision cages for 8 and 4 h d(-1), respectively; tiny granules were added on the cage floor to avoid visual deprivation and to encourage near gaze. In group C (n=4), monkeys were reared in open-vision cages, with non-granule food as a control. Vitreous chamber depth, refractive status, and corneal refractive power were assessed over 18 months. Paired t-test was used to compare the differences and a P-value<0.05 was considered to be statistically significant. In group A, vitreous chamber depth and optical axis elongated significantly, and refractive error shifted towards myopia during the observation period. In group B, vitreous chambers and optical axis elongated but the refractive power did not show significant changes. In group C, there was no significant elongation in vitreous chambers and optical axis, and the refractive power changed slightly towards hypermetropia. There were no significant changes in corneal refractive power in each group. Sustained near vision can promote vitreous chamber growth and induce myopic shifts in refractive power in adolescent monkeys. Our results demonstrate the potential for a primate model of near-work-related myopia.

Chronic alcohol consumption impairs visuo-spatial associative memory in periadolescent rhesus monkeys

Alcohol abuse in the adult is often preceded by high alcohol consumption during adolescence. Profound changes in brain structure and function occur during this developmental period, therefore alcohol may impact essential cognitive skill development during the formal educational years. The objective of this study was to determine if chronic oral alcohol intake slows acquisition and performance of cognitive tasks in male adolescent rhesus monkeys. Treatment groups (Alcohol, N=4; Control, N=3) were evaluated on bimanual dexterity and tests of visuo-spatial memory and learning adapted from the Cambridge Neuropsychological Test Automated Battery. Animals were trained daily in 30min sessions and had subsequent access to alcohol/Tang® solutions (Alcohol group) or Tang® only (Control group) Monday through Friday for 11 months. Recordings of brainstem auditory evoked potentials (BSAEP) were conducted periodically before and during the chronic drinking. Results: Chronic alcohol drinking (ave of 1.78g/kg alcohol per session) impaired behavioral performance assessed ∼22h after the prior drinking session. The Alcohol group required more trials than the Control group to reach criterion on the visuo-spatial memory task and showed increased sensitivity to trial difficulty and retention interval. Alcohol animals also had slowed initial acquisition of the bimanual task. The latency of P4 and P5 BSAEP peaks were also delayed in the Alcohol group. Chronic alcohol consumption impaired the acquisition and performance of a spatial memory task and disrupted brainstem auditory processing, thus these results show that repeated alcohol exposure in adolescence interferes with a range of brain functions including complex visuo-spatial mnemonic processing.

Subgenual Prefrontal Cortex Activity Predicts Individual Differences in Hypothalamic-Pituitary-Adrenal Activity Across Different Contexts

Hypothalamic-pituitary-adrenal (HPA) system activation is adaptive in response to stress, and HPA dysregulation occurs in stress-related psychopathology. It is important to understand the mechanisms that modulate HPA output, yet few studies have addressed the neural circuitry associated with HPA regulation in primates and humans. Using high-resolution F-18-fluorodeoxyglucose positron emission tomography (FDG-PET) in rhesus monkeys, we assessed the relation between individual differences in brain activity and HPA function across multiple contexts that varied in stressfulness. Using a logical AND conjunctions analysis, we assessed cortisol and brain metabolic activity with FDG-PET in 35 adolescent rhesus monkeys exposed to two threat and two home-cage conditions. To test the robustness of our findings, we used similar methods in an archival data set. In this data set, brain metabolic activity and cortisol were assessed in 17 adolescent male rhesus monkeys that were exposed to three stress-related contexts. Results from the two studies revealed that subgenual prefrontal cortex (PFC) metabolism (Brodmann's area 25/24) consistently predicted individual differences in plasma cortisol concentrations regardless of the context in which brain activity and cortisol were assessed. These findings suggest that activation in subgenual PFC may be related to HPA output across a variety of contexts (including familiar settings and novel or threatening situations). Individuals prone to elevated subgenual PFC activity across multiple contexts may be individuals who consistently show heightened cortisol and may be at risk for stress-related HPA dysregulation.

Ozone and allergen exposure exacerbates the role of 5‐HT heteroreceptors in cholinergic mediated airway smooth muscle contraction in a non‐human primate model

Exposure of adolescent rhesus monkeys to ozone (O3) and/or house dust mite allergen (HDMA) has been shown to contribute to post natal remodeling of the neural, muscular and immunological components of the airway. Similar exposure regimens have associated O3 and HDMA with airway hyper responsiveness. One possible mechanism contributing to the increase in contractility has been attributed to the post‐ganglionic, pre‐synaptic influence of serotonin (5‐HT) on cholinergic mediated airway smooth muscle contraction. Twenty‐four adolescent rhesus monkeys were exposed to 11 episodes of filtered air (FA), HDMA aerosol, O3, or HDMA + O3 (5 days each followed by 9 days of FA). Ozone was delivered for 8 h/day at 0.5 ppm. All monkeys were sensitized to HDMA and were exposed to HDMA aerosol for 2 h/day on days 3‐5 of either FA (n = 6) or O3 (n = 6) exposure. Necropsies were performed at one year of age and 6 conducting airway rings were harvested for evaluation by electrical field stimulation. All four treatment groups exhibited an increase in contractility when exposed to increasing concentrations of 5‐HT (1uM‐100uM). The O3 treated group exhibited the greatest increase in contraction tension in response to 5‐HT (433% of control). Further evaluation of contractility utilizing specific 5‐HT receptor agonists revealed that the 5‐HT3 receptor subtype played a prominent role in exacerbating the cholinergic mediated contraction in the O3 treated group. The influence of 5‐HT3 receptors was also substantiated due to the attenuation of 5‐HT modulated contraction with the addition of a 5‐HT3 receptor antagonist.

The distribution of D2/D3 receptor binding in the adolescent rhesus monkey using small animal PET imaging

PET imaging of the neuroreceptor systems in the brain has earned a prominent role in studying normal development, neuropsychiatric illness and developing targeted drugs. The dopaminergic system is of particular interest due to its role in the development of cognitive function and mood as well as its suspected involvement in neuropsychiatric illness. Nonhuman primate animal models provide a valuable resource for relating neurochemical changes to behavior. To facilitate comparison within and between primate models, we report in vivo D2/D3 binding in a large cohort of adolescent rhesus monkeys. Methods: In this work, the in vivo D2/D3 dopamine receptor availability was measured in a cohort of 33 rhesus monkeys in the adolescent stage of development (3.2–5.3 years). Both striatal and extrastriatal D2/D3 binding were measured using [F-18]fallypride with a high resolution small animal PET scanner. The distribution volume ratio (DVR) was measured for all subjects and group comparisons of D2/D3 binding among the cohort were made based on age and sex. Because two sequential studies were acquired from a single [F-18]fallypride batch, the effect of competing (unlabeled) ligand mass was also investigated. Results: Among this cohort, the rank order of regional D2/D3 receptor binding did not vary from previous studies with adult rhesus monkeys, with: putamen > caudate > ventral striatum > amygdala ∼ substantia nigra > medial dorsal thalamus > lateral temporal cortex ∼ frontal cortex. The DVR coefficient of variation ranged from 14%–26%, with the greatest variance seen in the head of the caudate. There were significant sex differences in [F-18]fallypride kinetics in the pituitary gland, but this was not observed for regions within the blood-brain barrier. Furthermore, no regions in the brain showed significant sex or age related differences in DVR within this small age range. Based on a wide range of injected fallypride mass across the cohort, significant competition effects could only be detected in the substantia nigra, thalamus, and frontal cortex, and were not evident above intersubject variability in all other regions. Conclusion: These data represent the first report of large cohort in vivo D2/D3 dopamine whole brain binding in the adolescent brain and will serve as a valuable comparison for understanding dopamine changes during this critical time of development and provide a framework for creating a dopaminergic biochemical atlas for the rhesus monkey.

Changes in crystalline lens radii of curvature and lens tilt and decentration during dynamic accommodation in rhesus monkeys

Dynamic changes in crystalline lens radii of curvature and lens tilt and decentration were measured during centrally stimulated accommodation in four iridectomized eyes of two adolescent rhesus monkeys. Phakometry measurements were performed dynamically using a custom-built, video-based, Purkinje-image instrument. Lens anterior and posterior radii were calculated from reflections of paired light sources from the ocular surfaces (Purkinje images PI, PIII, and PIV). Lens tilt and decentration were calculated assuming linearity between Purkinje image positions, eye rotation, lens tilt, and decentration. Because the monkey eyes were iridectomized, Purkinje images were referred to the mid-point of the double first Purkinje image (PI). Mean unaccommodated values of anterior and posterior lens radii of curvature were 11.11 +/- 1.58 mm and -6.64 +/- 0.62 mm, respectively, and these decreased relatively linearly with accommodation in all eyes, at a rate of 0.48 +/- 0.14 mm/D and 0.17 +/- 0.03 mm/D for anterior and posterior lens surfaces, respectively. Tilt and decentration did not change significantly with accommodation except for tilt around the horizontal axis, which changed at a rate of 0.147 +/- 0.25 deg/D. These results are important to fully characterize accommodation in rhesus monkeys.

The distribution of D 2/D 3 receptor binding in a large cohort of the adolescent rhesus monkey using small animal PET imaging

The distribution of D 2/D 3 receptor binding in a large cohort of the adolescent rhesus monkey using small animal PET imaging

Role of the Primate Orbitofrontal Cortex in Mediating Anxious Temperament

Excessive behavioral inhibition during childhood marks anxious temperament and is a risk factor for the development of anxiety and affective disorders. Studies in nonhuman primates can provide important information related to the expression of this risk factor, since threat-induced freezing in rhesus monkeys is a trait-like characteristic analogous to human behavioral inhibition. The orbitofrontal cortex (OFC) and amygdala are part of a circuit involved in the processing of emotions and associated physiological responses. Earlier work demonstrated involvement of the primate central nucleus of the amygdala (CeA) in mediating anxious temperament. This study assessed the role of the primate OFC in mediating anxious temperament and its involvement in fear responses. Twelve adolescent rhesus monkeys were studied (six lesion and six control monkeys). Lesions were targeted at regions of the OFC that are most interconnected with the amygdala. Behavior and physiological parameters were assessed before and after the lesions. The OFC lesions significantly decreased threat-induced freezing and marginally decreased fearful responses to a snake. The lesions also resulted in a leftward shift in frontal brain electrical activity consistent with a reduction in anxiety. The lesions did not significantly decrease hypothalamic-pituitary-adrenal (HPA) activity or cerebrospinal fluid (CSF) concentrations of corticotrophin-releasing factor (CRF). These findings demonstrate a role for the OFC in mediating anxious temperament and fear-related responses in adolescent primates. Because of the similarities between rhesus monkey threat-induced freezing and childhood behavioral inhibition, these findings are relevant to understanding mechanisms underlying anxious temperament in humans.