Overview of the past contributions of animal models and their changing place in psychiatry.

Research utilizing animal models has been responsible for major advances in understanding psychiatric disorders. The first data based integrative theories of psychopathology grew largely out of animal research. A variety of animal models that have been developed have been critical in broadening our understanding of human development and in providing empirical support for the importance of early experiences for behavioral and neurobiologic development. The development of many widely used clinical psychopharmacologic agents has depended on the use of animal models. The above examples represent substantive contributions of animal models to investigations of fundamental aspects of psychopathology. There is no "perfect," complete or comprehensive single animal model for any specific psychiatric disorder and contentious debates about the validity and/or usefulness of specific animal models persist. Animal models of diseases in medicine need to be understood in a historical and evolutionary perspective and their advantages as well as limitations recognized. There will likely never be an animal model in any field of medicine that is a perfect fit with the human condition, rather the emphasis in the development and study of disease models in animals needs to be on specific components of the human illness. Neither overextended cross-species comparisons nor unjustified negativism about animal models seems defensible. A major challenge in the continuing development and use of biobehavioral animal models in psychiatry is their relationship to the molecular neurosciences, including genetics, in understanding the mechanisms of mental disorders.

Biogenic amine activity in response to fluoxetine and desipramine in differentially reared rhesus monkeys.

BACKGROUND: It has been hypothesized that adverse early experience may be a mechanism by which children become vulnerable to later psychopathology via alteration of neurochemical or hormonal systems associated with such disorders. Such effects may in turn affect later responses to pharmacologic agents that act on these systems. METHODS: In this study, 18 mother-reared (MR) and 18 peer-reared (PR) rhesus monkeys experienced six 1-week separations from cagemates interspersed with 1-week reunions, while housed in like-reared groups of 3. Within rearing groups, equal numbers of animals received either fluoxetine (2 mg/kg), desipramine (5 mg/kg) or placebo delivered daily beginning 4 weeks before the first separation. Levels of norepinephrine (NE), the NE metabolite MHPG, the dopamine metabolites DOPAC and HVA, and the serotonin metabolite 5HIAA were measured in CSF samples collected approximately every 2 to 3 weeks during these procedures. RESULTS: Following treatment, DMI increased NE and decreased MHPG in the DMI-treated groups, while 5HIAA was decreased in the fluoxetine-treated groups following treatment. The increase in NE was followed by a sharp decline over the course of treatment, which was accompanied by an increase in MHPG. The rearing groups did not show a differential response to the drug treatments, and the separation manipulation itself had few effects. The mother-reared group showed higher levels of NE and DOPAC over all samples and higher levels of HVA in most samples. CONCLUSIONS: These rearing effects on biogenic amine activity were observed even in the presence of pharmacologic treatments that effectively altered the activity of these systems, and are consistent with previous findings from the same subject. The higher NE values observed in mother-reared infants over separations and reunions may have been due to higher basal levels of NE than peer-reared monkeys or to greater responsiveness to the stress of repeated social disruption or both. These findings agree with other primate studies showing that rearing differences persist beyond the infancy period and add to growing evidence of the important influence of the early social environment on neurobiologic development in primates.

Rearing experience and biogenic amine activity in infant rhesus monkeys.

In this report we present evidence that early social experience influences aspects of the function of brain biogenic amine systems, most notably the noradrenergic system. Biogenic amine activity was studied in mother- vs. peer-reared monkey infants over the first 6 months of life and in response to two housing transitions. Norepinephrine (NE), 3-methoxy-4-hydroxyphenylglycol (MHPG), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) levels in cerebrospinal fluid (CSF) were measured. Peer-reared monkeys showed significantly higher CSF levels of norepinephrine and MHPG than mother-reared animals over early development, but showed an attentuated NE response to separation and group formation compared to mother-reared animals. Peer-reared monkeys showed a greater developmental decline in 5-HIAA levels than mother-reared monkeys. There were no rearing effects for DOPAC or HVA over early development; however, peer-reared monkeys showed significantly lower HVA and DOPAC concentrations at 6-8 months of age. The results add to evidence for the influence of primate mothers on the psychobiological development of central nervous system neurotransmitter systems in their infants, and suggest that the noradrenergic system is among the more sensitive of these to early experience.

Evidence for heritability of biogenic amine levels in the cerebrospinal fluid of rhesus monkeys.

Susceptibility to several human psychopathological disorders is under partial genetic influence, and many of these disorders have biological correlates that may form part of the basis of this vulnerability. In humans, alterations in cerebrospinal fluid (CSF) metabolite levels of the amine transmitters norepinephrine, dopamine, and serotonin have been associated with several forms of psychopathology, and altered levels of these metabolites have been found in healthy probands with a familial history of such illnesses. We report evidence for heritability of CSF levels of biogenic amine measures in rhesus monkeys, Macaca mulatta. In a pilot study of 54 monkeys with known pedigrees, significant differences among sire families were found for CSF levels of norepinephrine (p = 0.04), homovanillic acid (p = 0.02), and 5-hydroxyindoleacetic acid (p = 0.04). These data indicate that variation in bioaminergic measures is associated with pedigree, and that model systems incorporating both genetic and environmental factors can contribute to the understanding of the function of aminergic systems implicated in vulnerability to psychopathology.

The noradrenergic innervation density of the monkey paraventricular nucleus is not altered by early social deprivation.

A series of neuroanatomic analyses have been undertaken to identify potential neuropathological changes seen in monkeys exposed to early social deprivation, which leads to psychopathology, inappropriate responses to stress and appetitive disorders. The animals used in this study were either socially reared or maternal- and peer-deprived. Within this framework, the distribution and density of noradrenergic (and adrenergic) varicosities was assessed in the hypothalamic paraventricular nucleus of rhesus monkeys using dopamine-beta-hydroxylase immunohistochemistry combined with laser scanning microscopy. Quantitative analysis of dopamine-beta-hydroxylase-immunoreactive varicosity density within magnocellular and parvicellular regions revealed no significant differences between rearing conditions, suggesting that this chemically identified afferent input to the paraventricular nucleus was not affected by the early environmental insult of social deprivation. The apparent lack of vulnerability of the paraventricular nucleus to differential rearing conditions contrasts with the neuropathological changes observed in several discrete brain regions.

Effects of social deprivation in prepubescent rhesus monkeys: immunohistochemical analysis of the neurofilament protein triplet in the hippocampal formation.

Social deprivation during early postnatal life has profound and long-lasting effects on the behavior of primates, including prolonged and exaggerated responses to stress as well as impaired performance on a variety of learning tasks. Although the cellular changes that underlie such alterations in behavior are unknown, environmentally induced psychopathology may involve morphologic or biochemical changes in select neuronal populations. The hippocampal formation of both socially deprived and socially reared prepubescent rhesus monkeys was selected for immunocytochemical investigation because of its association with the behavioral stress response and learning. Immunocytochemical analysis using antibodies specific for the neurofilament protein triplet was performed since these proteins are modified within degenerating neurons in a variety of neurodegenerative disorders. Results from optical density measurements indicate an increase in the intensity of non-phosphorylated neurofilament protein immunoreactivity in the dentate gyrus granule cell layer of socially deprived monkeys in comparison with that of socially reared animals, suggesting that early social deprivation may result in an increase in the amount of non-phosphorylated neurofilament protein in these cells. This phenotypic difference in dentate granule cells between differentially reared monkeys supports the notion that specific subpopulations of neurons in brain regions that subserve complex behaviors may undergo long-term modifications induced by environmental conditions. Furthermore, the data suggest that constitutive chemical components related to structural integrity may be as susceptible to early environmental manipulations as the more traditionally viewed measures of cellular perturbations, such as neurotransmitter dynamics, cell density and the establishment of connectivity. The observed modifications may serve as an anatomical substrate for behavioral abnormalities that persist in later life.

Quantitative analysis of tuberoinfundibular tyrosine hydroxylase- and corticotropin-releasing factor-immunoreactive neurons in monkeys raised with differential rearing conditions.

A major goal in assessing biological determinants of behavior lies in studying the effect(s) of rearing on the development of the central nervous system. Specifically, a series of neuroanatomic analyses have been undertaken to identify potential neuropathological changes seen in monkeys exposed to early social deprivation, which leads to profound psychopathology and inappropriate responses to stress. The animals used in this study were either raised with their mother and peers (socially reared) or raised without maternal/peer contact (socially deprived). Within this context, the distribution of tuberoinfundibular dopaminergic neurons in the hypothalamic paraventricular nucleus and arcuate nucleus of rhesus monkeys was determined by immunohistochemistry using an antibody against the enzyme tyrosine hydroxylase, a marker for dopamine-containing systems. Additionally, the distribution of corticotropin-releasing factor-containing neurons in the paraventricular nucleus was assessed immunohistochemically. The majority (97.5%) of dopaminergic neurons in the paraventricular nucleus were parvicellular, with a small (2.5%), but consistently observed population of magnocellular neurons immunoreactive for tyrosine hydroxylase. Within the arcuate nucleus, tyrosine hydroxylase-immunoreactive neurons were similar in morphology to the parvicellular neurons of the paraventricular nucleus. Qualitative assessment of corticotropin-releasing factor-immunoreactive neurons in the paraventricular nucleus revealed a parvicellular population of neurons located in medial aspects of the nucleus, similar to what has been observed in the rat. Quantitative analysis revealed no differences in the number of tyrosine hydroxylase- and corticotropin-releasing factor-immunoreactive neurons between rearing conditions, suggesting that these neurons were not affected, in terms of overall cell counts, by the early environmental insult of social deprivation.

Strangers in a strange land: a psychobiological study of infant monkeys before and after separation from real or inanimate mothers.

Some rhesus monkey (Macaca mulatta) infants have a "despair" or depression-like response to mother-infant separation, while others do not. The presumed interrelation between early rearing conditions and the neurobiological status of the infant that might lead to increased risk for despair is not understood. In this study, the characteristics of the "mother" were controlled by rearing infant rhesus monkeys with their biological mothers, or with inanimate mothers. Behavioral data were collected before and after separation at 6-7 months of age. The neurobiological status of the infants was evaluated by measuring the concentration of norepinephrine, its major metabolite, and the metabolites of dopamine and serotonin in cerebrospinal fluid. The results suggest that despair is not simply a behavioral response to separation. Instead, despair may reflect the inability to cope with the separation environment. Coping with the separation environment appears to depend on neurobiological and behavioral characteristics of the infant that are related to, if not determined by, characteristics of the mother.

A longitudinal study of the effect of different social rearing conditions on cerebrospinal fluid norepinephrine and biogenic amine metabolites in rhesus monkeys.

The purpose of this study was to determine whether disruption of early social attachment alters the activity of brain biogenic amine systems in rhesus monkeys (Macaca mulatta). Male rhesus monkey infants were deprived of maternal interaction, peer interaction, or both, during the first 22 months of life. Cerebrospinal fluid (CSF) was collected under rigorously controlled conditions approximately every month and assayed for levels of norepinephrine (NE), its major metabolite, and the metabolites of dopamine and serotonin. Mother-Deprived infants had lower levels of CSF NE than Mother-Reared infants. Mother-Deprived infants also failed to develop the same pattern of intercorrelations between compounds and month-to-month stability in levels of neurotransmitter and metabolites in CSF as the Mother-Reared infants. Finally, there were changes in CSF NE levels associated with social separation and social group formation. The brain NE system appears to be sensitive to changes in the social environment. Its level of activity, as reflected in levels of NE in CSF, appears to depend on both the prevailing social environment and the prior rearing environment.

Effects of alcohol on cerebrospinal fluid norepinephrine in rhesus monkeys.

Alcohol (1-3 g/kg) significantly increased the concentration of cerebrospinal fluid (CSF) norepinephrine (NE) in rhesus monkeys. This effect is consistent with the previously demonstrated activational and possible antidepressant effect of low doses of alcohol. The greatest increase was observed in subjects with low baseline levels of CSF NE. Individual differences in activation or euphoria could be related to differential increases in CSF NE following alcohol consumption.

Social separation increases alcohol consumption in rhesus monkeys.

This study used 16 socially reared juvenile rhesus monkeys as subjects to test the hypothesis that social separation promotes alcohol consumption in this species. In the first part of the study, 12 monkeys were intermittently separated from their social groups, while 4 were separated before the beginning of the study and remained continuously separated. Refrigerated water or aspartame-sweetened water (vehicle) containing 6% alcohol (w/v) were presented after 4.5 h of fluid deprivation. Intermittently separated monkeys drank more alcohol during separation than when they were socially housed, and more than the continuously separated monkeys. Stable individual differences in consumption rate developed over repeated separations. These differences were not correlated with consumption of refrigerated water or vehicle, or with differential behavioral (locomotor) responses to social separation. This suggested that some monkeys were predisposed to drink more alcohol than others. The second part of the study determined whether established alcohol/vehicle consumption rates for all 16 monkeys were altered when the monkeys were not water deprived, and then when water and the vehicle were available at the same time as alcohol/vehicle. Among monkeys that drank the most (mean of 2.4 g/kg/h) and the least (mean of 0.8 g/kg/h), alcohol consumption was not affected. These results, combined with previous reports, suggest a neurobiological linkage between genetically based social attachment mechanisms, social stressors, and vulnerability to alcohol abuse and addiction in primates.

Cerebrospinal fluid measures of neurotransmitter changes associated with pharmacological alteration of the despair response to social separation in rhesus monkeys.

Social separation is a risk factor for major depressions that can be modeled in nonhuman primates. Changes in central monoamine neurotransmission are also likely to be involved in major depression. This study examined the relationship between separation-induced depressive-like behavior and central monoamine neurotransmitter changes in rhesus monkeys. Measures of cerebrospinal fluid (CSF) norepinephrine (NE), 5-hydroxyindoleacetic acid (5HIAA), and homovanillic acid (HVA) were used to assess the neurobiological impact of social separation and drug treatments alone or in combination. alpha-Methyl-p-tyrosine exacerbated, and fusaric acid ameliorated, the depressive-like response to separation. Probenecid-induced accumulations of HVA and 5HIAA reflected changes in behavior, but were not consistently affected by drug treatment. In contrast, pretreatment CSF NE was comparatively stable across repeated sampling, and drug-induced changes in this measure were correlated with changes in behavior. Low CSF NE, whether drug-induced or naturally occurring, was associated with a more severe depressive-like response to separation.

Hypersensitivity to d-amphetamine several years after early social deprivation in rhesus monkeys.

Social deprivation of rhesus monkeys in infancy results in increased sensitivity to psychotic-like behavioral effects of low doses of d-amphetamine given 2-3 years later. These behavioral effects are associated with increased levels of CSF norepinephrine. These data suggest that social developmental factors could be partially responsible for variation in neurochemical responses and long-lasting differential sensitivity of primates to the psychosis-inducing effects of d-amphetamine.

Animal models of depression: an overview.

Although animal models cannot replicate human psychopathology in every detail, they should properly be conceived as experimental systems in which selected and specific questions can be investigated in ways impossible to do in humans. In considering the general kinds of animal models, distinctions must be drawn among those designed to simulate specific signs or symptoms, those designed to test a specific etiological theory, those designed to study underlying behavioral and neurobiological mechanisms, and those whose principal purpose is to permit pre-clinical drug evaluation. If, for example, drug evaluation is the first concern, the empirical validity of the model in predicting the therapeutic efficacy of drugs is primary, whereas the mechanisms responsible for inducing the syndrome, and behavioral similarity issues become secondary. The available models of depression are reviewed in the light of their specific advantages and limitations, including those induced pharmacologically, maternal and peer separation, learned helplessness, chronic stress, changes in dominance hierarchy, intra-cranial self-stimulation, conditioned motionlessness and behavioral despair models. Since multiple variables are involved in the etiology of depressions, animal models offer the possibility of evaluating their main effects and interactions in a controlled prospective manner. While caution is required in cross-species reasoning, there are nevertheless guidelines, and the continuing development of a comparative approach in Psychiatry has great potential.

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.

Associated endocrine, physiological and behavioral changes in rhesus monkeys after intravenous corticotropin-releasing factor administration.

The intravenous (IV) administration of synthetic ovine corticotropin-releasing factor (CRF) (10 and 125 micrograms/kg) to chair restrained rhesus monkeys stimulated the pituitary-adrenal axis. At these doses, increases in plasma concentrations of adrenocorticotropic hormone (ACTH) and cortisol were associated with blood pressure decreases and behavioral effects. These data demonstrate that synthetic ovine CRF (10 and 125 micrograms/kg) administered IV to the rhesus monkey results in associated endocrine, physiological, and behavioral changes.