Steven E. Shelton

The Role of the Central Nucleus of the Amygdala in Mediating Fear and Anxiety in the Primate

Numerous studies demonstrate that the rhesus monkey is an excellent species with which to investigate mechanisms underlying human emotion and psychopathology. To examine the role of the central nucleus of the amygdala (CeA) in mediating the behavioral and physiological responses associated with fear and anxiety, we used rhesus monkeys to assess the effects of excitotoxic lesions of the CeA. Behavioral and physiological responses of nine monkeys with bilateral CeA destruction (ranging from 46 to 98%) were compared with five animals with asymmetric lesions (42-86.5% destruction on the most affected side) and with 16 unoperated controls. Results suggest that similar to rodent species, the primate CeA plays a role in mediating fear- and anxiety-related behavioral and endocrine responses. Compared with controls and the asymmetric-lesion group, bilaterally lesioned monkeys displayed significantly less fear-related behavior when exposed to a snake and less freezing behavior when confronted by a human intruder. In addition, bilaterally lesioned monkeys had decreased levels of CSF corticotrophin-releasing factor (CRF), and both lesioned groups had decreased plasma ACTH concentrations. In contrast to these findings, patterns of asymmetric frontal brain electrical activity, as assessed by regional scalp EEG, did not significantly differ between control and lesioned monkeys. These findings suggest that in primates, the CeA is involved in mediating fear- and anxiety-related behavioral and pituitary-adrenal responses as well as in modulating brain CRF activity.

Asymmetric frontal brain activity, cortisol, and behavior associated with fearful temperament in rhesus monkeys.

The authors examined the hypothesis that rhesus monkeys with extreme right frontal electroencephalographic activity would have higher cortisol levels and would be more fearful compared with monkeys with extreme left frontal activity. The authors first showed that individual differences in asymmetric frontal electrical activity are a stable characteristic. Next, the authors demonstrated that relative right asymmetric frontal activity and cortisol levels are correlated in animals 1 year of age. Additionally, extreme right frontal animals had elevated cortisol concentrations and more intense defensive responses. At 3 years of age, extreme right frontal animals continued to have elevated cortisol concentrations. These findings demonstrate important relations among extreme asymmetric frontal electrical activity, cortisol levels, and trait-like fear-related behaviors in young rhesus monkeys.

Opiate modulation of separation-induced distress in non-human primates.

Infant rhesus monkeys respond to separation from their mothers with a dramatic increase in vocalizations and activation of autonomic and pituitary-adrenal systems. Using the mother-infant separation paradigm in rhesus monkeys, we focused on the role of opiate systems in modulating the behavioral and neuroendocrine consequences of a brief, naturally occurring stressor. In the first experiment, morphine 0.1 mg/kg significantly decreased separation-induced vocalizations without affecting activity levels. In the second experiment, naloxone 1.0 mg/kg increased distress vocalizations but lower doses had no effect. In the third experiment we blocked the effect of morphine 0.1 mg/kg with naloxone 0.1 mg/kg, a dose of naloxone that had no intrinsic effects of its own. This suggests that the reduction of separation-induced vocalizations by morphine is mediated by opiate receptors. The last experiment demonstrated that separation-induced increases in pituitary-adrenal hormones can also be modulated by opiate agonists and antagonists. These findings are consistent with work in non-primate species and support the hypothesis that opiate receptors are specifically involved in mediating separation-induced vocalizations and pituitary-adrenal activation in primates.

Amygdalar and hippocampal substrates of anxious temperament differ in their heritability

Anxious temperament (AT) in human and non-human primates is a trait-like phenotype evident early in life that is characterized by increased behavioural and physiological reactivity to mildly threatening stimuli. Studies in children demonstrate that AT is an important risk factor for the later development of anxiety disorders, depression and comorbid substance abuse. Despite its importance as an early predictor of psychopathology, little is known about the factors that predispose vulnerable children to develop AT and the brain systems that underlie its expression. To characterize the neural circuitry associated with AT and the extent to which the function of this circuit is heritable, we studied a large sample of rhesus monkeys phenotyped for AT. Using 238 young monkeys from a multigenerational single-family pedigree, we simultaneously assessed brain metabolic activity and AT while monkeys were exposed to the relevant ethological condition that elicits the phenotype. High-resolution 18F-labelled deoxyglucose positron-emission tomography (FDG–PET) was selected as the imaging modality because it provides semi-quantitative indices of absolute glucose metabolic rate, allows for simultaneous measurement of behaviour and brain activity, and has a time course suited for assessing temperament-associated sustained brain responses. Here we demonstrate that the central nucleus region of the amygdala and the anterior hippocampus are key components of the neural circuit predictive of AT. We also show significant heritability of the AT phenotype by using quantitative genetic analysis. Additionally, using voxelwise analyses, we reveal significant heritability of metabolic activity in AT-associated hippocampal regions. However, activity in the amygdala region predictive of AT is not significantly heritable. Furthermore, the heritabilities of the hippocampal and amygdala regions significantly differ from each other. Even though these structures are closely linked, the results suggest differential influences of genes and environment on how these brain regions mediate AT and the ongoing risk of developing anxiety and depression.

Brain Regions Associated with the Expression and Contextual Regulation of Anxiety in Primates

BACKGROUND A key to successful adaptation is the ability to regulate emotional responses in relation to changing environmental demands or contexts. METHODS High-resolution PET 18fluoro-deoxyglucose (FDG) scanning in rhesus monkeys was performed during two contexts (alone, and human intruder with no eye contact) during which the duration of anxiety related freezing behavior was assessed. Correlations between individual differences in freezing duration and brain activity were performed for each of the two conditions, as well as for the contextual regulation between the two conditions. RESULTS In both conditions, activity in the basal forebrain, including the bed nucleus of the stria terminalis and the nucleus accumbens were correlated with individual differences in freezing duration. In contrast, individual differences in the ability to regulate freezing behavior between contexts were correlated with activity in the dorsal anterior cingulate cortex, the thalamus and the dorsal raphe nucleus. CONCLUSIONS These findings demonstrate differences in the neural circuitry mediating the expression compared to the contextual regulation of freezing behavior. These findings are relevant since altered regulatory processes may underlie anxiety disorders.

Cerebrospinal fluid corticotropin-releasing hormone levels are elevated in monkeys with patterns of brain activity associated with fearful temperament

BACKGROUND Asymmetric patterns of frontal brain activity and brain corticotropin-releasing hormone (CRH) systems have both been separately implicated in the processing of normal and abnormal emotional responses. Previous studies in rhesus monkeys demonstrated that individuals with extreme right frontal asymmetric brain electrical activity have high levels of trait-like fearful behavior and increased plasma cortisol concentrations. METHODS In this study we assessed cerebrospinal fluid (CSF) CRH concentrations in monkeys with extreme left and extreme right frontal brain electrical activity. CSF was repeatedly collected at 4, 8, 14, 40, and 52 months of age. RESULTS Monkeys with extreme right frontal brain activity had increased CSF CRH concentrations at all ages measured. In addition, individual differences in CSF CRH concentrations were stable from 4 to 52 months of age. CONCLUSIONS These findings suggest that, in primates, the fearful endophenotype is characterized by increased fearful behavior, a specific pattern of frontal electrical activity, increased pituitary-adrenal activity, and increased activity of brain CRH systems. Data from other preclinical studies suggests that the increased brain CRH activity may underlie the behavioral and physiological characteristics of fearful endophenotype.

Plasma ACTH and cortisol concentrations before and after dexamethasone

Alteration in the hypothalamic-pituitary-adrenal (HPA) axis occurs in up to 50% of depressed patients and is demonstrated by the failure to suppress cortisol concentrations after dexamethasone administration. Evidence suggesting that these cortisol abnormalities reflect hypothalamic-pituitary dysfunction has been inconsistent. We administered the dexamethasone suppression test to 28 psychiatric inpatients, including 17 cortisol suppressors and 11 nonsuppressors. Adrenocorticotropic hormone (ACTH) concentrations at 8 a.m. pre- and postdexamethasone were significantly greater in cortisol nonsuppressors than in suppressors. Our data support the hypothesis that pituitary ACTH secretion is altered in depressed patients who have HPA axis abnormalities demonstrated by plasma cortisol measurements.

Corticotropin-releasing factor administered intraventricularly to rhesus monkeys ☆

Synthetic ovine corticotropin-releasing factor (CRF) administered intraventricularly (ICV) to rhesus monkeys resulted in endocrine and behavioral changes. At doses of 20 and 180 micrograms, CRF stimulated the pituitary-adrenal axis in four chair-restrained monkeys. These monkeys showed concomitant increases in arousal. To study these animals in a less restrictive setting, three of the monkeys later received CRF ICV (20 and 180 micrograms) in their home cages. At the 180-micrograms dose the monkeys exhibited a combination of huddling and lying down behavior. These behavioral effects did not seem to be due to alterations in blood pressure.

Role of the primate orbitofrontal cortex in mediating anxious temperament.

BACKGROUND 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. METHODS 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. RESULTS 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). CONCLUSIONS 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.

Opiate systems in mother and infant primates coordinate intimate contact during reunion

In humans, and non-human primates, reunion following a separation results in a positive emotional state, and an increase in affiliative behaviors. To examine the role of opiate systems, in mothers and infants in mediating reunion behavior, morphine and naltrexone were administered after a brief separation. Infants administered morphine (0.1 mg/kg IM) showed a significant reduction in clinging and girning, a vocalization emitted during close physical contact. Naltrexone (5 mg/kg IM) had opposite effects. When administered to mothers, again morphine decreased and naltrexone increased clinging. Morphine administered to mothers had a more transient behavioral effect which could not be accounted for by lower morphine blood levels. These results demonstrate that during reunion, the amount of intimate contact between a mother and her infant is regulated by the reciprocal activation of their opiate systems. This activation of opiate systems may reinforce the infants need for attachment and the mothers role in care giving.