Dynamic alterations of serotonergic metabolism and receptors during social isolation of low- and high-active mice

A role for the serotonin 2A receptor in the expansion and functioning of human transmodal cortex

Integrating independent but converging lines of research on brain function and neurodevelopment across scales, this article proposes that serotonin 2A receptor (5-HT2AR) signalling is an evolutionary and developmental driver and potent modulator of the macroscale functional organization of the human cerebral cortex. A wealth of evidence indicates that the anatomical and functional organization of the cortex follows a unimodal-to-transmodal gradient. Situated at the apex of this processing hierarchy-where it plays a central role in the integrative processes underpinning complex, human-defining cognition-the transmodal cortex has disproportionately expanded across human development and evolution. Notably, the adult human transmodal cortex is especially rich in 5-HT2AR expression and recent evidence suggests that, during early brain development, 5-HT2AR signalling on neural progenitor cells stimulates their proliferation-a critical process for evolutionarily-relevant cortical expansion. Drawing on multimodal neuroimaging and cross-species investigations, we argue that, by contributing to the expansion of the human cortex and being prevalent at the apex of its hierarchy in the adult brain, 5-HT2AR signalling plays a major role in both human cortical expansion and functioning. Owing to its unique excitatory and downstream cellular effects, neuronal 5-HT2AR agonism promotes neuroplasticity, learning and cognitive and psychological flexibility in a context-(hyper)sensitive manner with therapeutic potential. Overall, we delineate a dual role of 5-HT2ARs in enabling both the expansion and modulation of the human transmodal cortex.

Pivotal mental states

This paper introduces a new construct, the 'pivotal mental state', which is defined as a hyper-plastic state aiding rapid and deep learning that can mediate psychological transformation. We believe this new construct bears relevance to a broad range of psychological and psychiatric phenomena. We argue that pivotal mental states serve an important evolutionary function, that is, to aid psychological transformation when actual or perceived environmental pressures demand this. We cite evidence that chronic stress and neurotic traits are primers for a pivotal mental state, whereas acute stress can be a trigger. Inspired by research with serotonin 2A receptor agonist psychedelics, we highlight how activity at this particular receptor can robustly and reliably induce pivotal mental states, but we argue that the capacity for pivotal mental states is an inherent property of the human brain itself. Moreover, we hypothesize that serotonergic psychedelics hijack a system that has evolved to mediate rapid and deep learning when its need is sensed. We cite a breadth of evidences linking stress via a variety of inducers, with an upregulated serotonin 2A receptor system (e.g. upregulated availability of and/or binding to the receptor) and acute stress with 5-HT release, which we argue can activate this primed system to induce a pivotal mental state. The pivotal mental state model is multi-level, linking a specific molecular gateway (increased serotonin 2A receptor signaling) with the inception of a hyper-plastic brain and mind state, enhanced rate of associative learning and the potential mediation of a psychological transformation.

Neural mechanisms of social homeostasis

Social connections are vital to survival throughout the animal kingdom and are dynamic across the life span. There are debilitating consequences of social isolation and loneliness, and social support is increasingly a primary consideration in health care, disease prevention, and recovery. Considering social connection as an "innate need," it is hypothesized that evolutionarily conserved neural systems underlie the maintenance of social connections: alerting the individual to their absence and coordinating effector mechanisms to restore social contact. This is reminiscent of a homeostatic system designed to maintain social connection. Here, we explore the identity of neural systems regulating "social homeostasis." We review findings from rodent studies evaluating the rapid response to social deficit (in the form of acute social isolation) and propose that parallel, overlapping circuits are engaged to adapt to the vulnerabilities of isolation and restore social connection. By considering the neural systems regulating other homeostatic needs, such as energy and fluid balance, we discuss the potential attributes of social homeostatic circuitry. We reason that uncovering the identity of these circuits/mechanisms will facilitate our understanding of how loneliness perpetuates long-term disease states, which we speculate may result from sustained recruitment of social homeostatic circuits.

The lonely mouse - single housing affects serotonergic signaling integrity measured by 8-OH-DPAT-induced hypothermia in male mice

Male BALB/c mice single-housed for a period of three weeks were found to respond with a more marked hypothermia to a challenge with a selective serotonergic agonist (8-OH-DPAT) than their group-housed counterparts. This effect of single housing was verified by screening a genetically heterogeneous population of male mice on a C57BL/6 background from a breeding colony. Enhanced activity of the implicated receptor (5-HT1A) leading to an amplified hypothermic effect is strongly associated with depressive states. We therefore suggest that the 8-OH-DPAT challenge can be used to demonstrate a negative emotional state brought on by e.g. long-term single housing in male laboratory mice. The study emphasizes the importance of social housing, and demonstrates that male mice deprived of social contact respond with altered serotonergic signaling activity. Male mice not only choose social contact when given the option, as has previously been shown, but will also, when it is deprived, be negatively affected by its absence. We propose that the 8-OH-DPAT challenge constitutes a simple, but powerful, tool capable of manifesting the effect of social deprivation in laboratory mice. It potentially allows not only for an unbiased, biochemical evaluation of psychological stressors, but may also allow for determining whether the effect of these can be counteracted.

A model of alcohol drinking under an intermittent access schedule using group-housed mice

Here, we describe a new model of voluntary alcohol drinking by group-housed mice. The model employs sensor-equipped cages that track the behaviors of the individual animals via implanted radio chips. After the animals were allowed intermittent access to alcohol (three 24 h intervals every week) for 4 weeks, the proportions of licks directed toward bottles containing alcohol were 50.9% and 39.6% for the male and female mice, respectively. We used three approaches (i.e., quinine adulteration, a progressive ratio schedule and a schedule involving a risk of punishment) to test for symptoms of compulsive alcohol drinking. The addition of 0.01% quinine to the alcohol solution did not significantly affect intake, but 0.03% quinine induced a greater than 5-fold reduction in the number of licks on the alcohol bottles. When the animals were required to perform increasing numbers of instrumental responses to obtain access to the bottle with alcohol (i.e., a progressive ratio schedule), they frequently reached a maximum of 21 responses irrespective of the available reward. Although the mice rarely achieved higher response criteria, the number of attempts was ∼10 times greater in case of alcohol than water. We have developed an approach for mapping social interactions among animals that is based on analysis of the sequences of entries into the cage corners. This approach allowed us to identify the mice that followed other animals in non-random fashions. Approximately half of the mice displayed at least one interaction of this type. We have not yet found a clear correlation between imitative behavior and relative alcohol preference. In conclusion, the model we describe avoids the limitations associated with testing isolated animals and reliably leads to stable alcohol drinking. Therefore, this model may be well suited to screening for the effects of genetic mutations or pharmacological treatments on alcohol-induced behaviors.

Have studies of the developmental regulation of behavioral phenotypes revealed the mechanisms of gene-environment interactions?

This review addresses the recent convergence of our long-standing knowledge of the regulation of behavioral phenotypes by developmental experience with recent advances in our understanding of mechanisms regulating gene expression. This review supports a particular perspective on the developmental regulation of behavioral phenotypes: That the role of common developmental experiences (e.g. maternal interactions, peer interactions, exposure to a complex environment, etc.) is to fit individuals to the circumstances of their lives within bounds determined by long-standing (evolutionary) mechanisms that have shaped responses to critical and fundamental types of experience via those aspects of gene structure that regulate gene expression. The phenotype of a given species is not absolute for a given genotype but rather variable within bounds that is determined by mechanisms regulated by experience (e.g. epigenetic mechanisms). This phenotypic variation is not necessarily random, or evenly distributed along a continuum of description or measurement, but often highly disjointed, producing distinct, even opposing, phenotypes. The potentiality for these varying phenotypes is itself the product of evolution, the potential for alternative phenotypes itself conveying evolutionary advantage. Examples of such phenotypic variation, resulting from environmental or experiential influences, have a long history of study in neurobiology, and a number of these will be discussed in this review: neurodevelopmental experiences that produce phenotypic variation in visual perception, cognitive function, and emotional behavior. Although other examples will be discussed, particular emphasis will be made on the role of social behavior on neurodevelopment and phenotypic determination. It will be argued that an important purpose of some aspects of social behavior is regulation of neurobehavioral phenotypes by experience via genetic regulatory mechanisms.

Geissoschizine methyl ether, an alkaloid in Uncaria hook, is a potent serotonin ₁A receptor agonist and candidate for amelioration of aggressiveness and sociality by yokukansan

Yokukansan (YKS), a traditional Japanese medicine, is composed of seven kinds of dried herbs. It is widely prescribed in clinical situation for treating psychiatric disorders such as aggressiveness in patients with dementia. We previously demonstrated that YKS and Uncaria hook (UH), which is a constituent herb of YKS, had a partial agonistic effect to 5-HT(1A) receptors in vitro. However, it has still been unclear whether this in vitro effect is reflected in in vivo, and what the active ingredients are. The purpose of the present study is to find the active ingredient in YKS and to demonstrate the effect in in vivo. In the present study, we first studied the effect of YKS and UH on aggressiveness and sociality in socially isolated mice. YKS and UH ameliorated the isolation-induced increased aggressiveness and decreased sociality, and these ameliorative effects were counteracted by coadministration of 5-HT(1A) receptor antagonist WAY-100635, or disappeared by eliminating UH from YKS. These results suggest that the effect of YKS is mainly attributed to UH, and the active ingredient is contained in UH. To find the candidate ingredients, we examined competitive binding assay and [(35)S] guanosine 5'-O-(3-thiotriphosphate) (GTPγS) binding assay of seven major alkaloids in UH using Chinese hamster ovary cells expressing 5-HT(1A) receptors artificially. Only geissoschizine methyl ether (GM) among seven alkaloids potently bound to 5-HT(1A) receptors and acted as a partial agonist. This in vitro result on GM was further demonstrated in the socially isolated mice. As did YKS and UH, GM ameliorated the isolation-induced increased aggressiveness and decreased sociality, and the effect was counteracted by coadministration of WAY-100635. These lines of results suggest that GM in UH is potent 5-HT(1A) receptor agonist and a candidate for pharmacological effect of YKS on aggressiveness and sociality in socially isolated mice.

Consequences of post-weaning social isolation on anxiety behavior and related neural circuits in rodents

Exposure to adverse experiences in early-life is implicated in the later vulnerability to development of psychiatric disorders, including anxiety and affective disorders in humans. Adverse early-life experiences likely impart their long-term consequences on mental health by disrupting the normal development of neural systems involved in stress responses, emotional behavior and emotional states. Neural systems utilizing the neurotransmitters serotonin, dopamine and the neuropeptide corticotropin-releasing factor (CRF) are implicated in mediating emotive behaviors, and dysfunction of these neurochemical systems is associated with mood/anxiety disorders. These neural systems continue maturing until early or mid-adolescence in humans, thus alterations to their development are likely to contribute to the long-term consequences of adverse early-life experiences. A large body of literature suggests that post-weaning isolation rearing of rodents models the behavioral consequences of adverse early-life experiences in humans. Overall, the majority findings suggest that post-weaning social isolation that encompasses pre-adolescence produces long-lasting alterations to anxiety behavior, while measures of monoaminergic activity in various limbic regions during social isolation suggest alterations to dopamine and serotonin systems. The goal of this review is to evaluate and integrate findings from post-weaning social isolation studies specifically related to altered fear and anxiety behaviors and associated changes in neuroendocrine function and the activity of monoaminergic systems.

Effects of chronic citalopram treatment on 5-HT1A and 5-HT2A receptors in group- and isolation-housed mice

Selective serotonin reuptake inhibitors (SSRI) are characterized by high clinical effectiveness and good tolerability. A 2-3 week delay in the onset of effects is caused by adaptive mechanisms, probably at the serotonergic (5-HT) receptor level. To analyze this in detail, we measured 5-HT(1A) and 5-HT(2A) receptor bindings in vitro after 3 weeks of citalopram treatment (20 mg/kg i.p. daily) in group-housed as well as isolation-housed mice, reflecting neurobiological aspects seen in psychiatric patients. Isolation housing increased somatodendritic (+52%) and postsynaptic (+30-95%) 5-HT(1A) as well as postsynaptic 5-HT(2A) receptor binding (+25-34%), which confirms previous findings. Chronic citalopram treatment did not induce alterations in raphe 5-HT(1A) autoreceptor binding, independent of housing conditions. Housing-dependent citalopram effects on postsynaptic 5-HT(1A) receptor binding were found with increases in group- (+11-42%) but decreases in isolation-housed (-11 to 35%) mice. Forebrain 5-HT(2A) receptor binding decreased between 11 and 38% after chronic citalopram administration, independent of housing conditions. Citalopram's long-term action comprises alterations at the postsynaptic 5-HT(1A) and 5-HT(2A) receptor binding levels. Housing conditions interact with citalopram effects, especially on 5-HT(1A) receptor binding, and should be more strongly considered in pharmacological studies. In general, SSRI-induced alterations were more pronounced and affected more brain regions in isolates, supporting the concept of a higher responsiveness in "stressed" animals. Isolation-induced receptor binding changes were partly normalized by chronic citalopram treatment, suggesting the isolation housing model for further analyses of SSRI effects, especially at the behavioral level.

Inhibition of neuronal nitric oxide reduces anxiety-like responses to pair housing

Many psychological disorders are characterized by anxiety and alterations in social interactions. Recent studies demonstrate that the chemical messenger nitric oxide (NO) can regulate both anxiety and social behaviours. We tested whether an enzyme that produces NO in the brain, neuronal nitric oxide synthase (nNOS), serves as an interface between social interactions and anxiety-like behaviour. Several investigators have observed that mice increase anxiety-like responses in the elevated plus-maze after pair housing. nNOS gene deletion and 3-Bromo-7-Nitroindazole were used to inhibit the production of neuronal NO. Similar to previous studies, pair housing reduced open arm exploration in the elevated plus-maze. Pair housing also increased corticotropin-releasing hormone (CRH) immunoreactive cells in the paraventricular nucleus (PVN) of the hypothalamus. Inhibition of NO production increased open arm exploration in pair-housed mice but decreased open arm exploration in individually housed mice. These results suggest that the effect of nNOS inhibition on anxiety-like responses is context dependent and that behavioural responses to social housing are altered after nNOS inhibition. This research suggests that NO may play an important role in mediating the effect social interactions have on anxiety.

Anxiolytic-like effects of oleamide in group-housed and socially isolated mice

Oleamide (cis-9,10-octadecenoamide) is an endogenous sleep-inducing lipid and prototypic member of a new class of biological signaling molecules identified in recent years. In the present study, the anxiolytic-like effect of oleamide was studied in several experimental models of anxiety in group-housed and socially isolated mice. As the results show, socially isolated mice exhibited an anxiogenic-like profile in the elevated plus-maze test, the light/dark test, and the hole-board test, which could be significantly reversed by oleamide (10 or 20 mg/kg, i.p.). Moreover, oleamide significantly reduced the anxiety levels in grouped-housed mice. In the isolation-induced aggressive test, oleamide markedly reduced the attacking duration and increased the attacking latency. It is concluded that oleamide has an anxiolytic-like effect in socially isolated or group-housed mice, which suggests that fatty acid amides might be involved in the regulation of anxiety-related behavior in mice.

Social isolation and expression of serotonergic neurotransmission-related genes in several brain areas of male mice

Early-life events influence brain development and evoke long-lasting behavioral consequences. Postweaning social isolation in rodents induces emotional and neurochemical alterations similar to those observed among some human psychopathologies. Central serotonergic neurotransmission is intimately involved in the observed adjustments, but the impact of social deprivation on serotonergic gene expression is unknown. We investigated the effects of prolonged early social isolation on emotion-related behaviors and 5-hydroxytryptamine (5-HT)-related gene transcription in mice. After weaning, male C57BL/6J mice were reared singly or in groups of four for 6 weeks. Gene expression of 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(2C), 5-HT(3A), 5-HT(6) and 5-HT(7) receptors and of 5-HT transporter and tryptophan hydroxylase-2 was determined by quantitative real-time polymerase chain reaction in distinct brain areas. Single-housed mice were hyperactive in a novel environment and showed signs of aggressive behavior. Housing condition did not alter weight gain or body temperature. Isolation markedly reduced transcription of all postsynaptic 5-HT receptors in the prefrontal cortex and reduced 5-HT(1B), 5-HT(2A) and 5-HT(2C) in both hypothalamus and midbrain. In contrast, the only alteration in the hippocampus was 5-HT(6) overexpression. Neither 5-HT transporter nor synthetic enzyme gene transcription differed between housing conditions. In conclusion, early social isolation in mice induces robust changes in postsynaptic 5-HT receptors gene transcription, motor hyperactivity and behavioral disinhibition. The overall pattern of decreased gene expression in the prefrontal cortex highlights its high vulnerability to environment. Furthermore, this is the first study to present a general representation of 5-HT-related gene expression in specific brain areas after social isolation and identifies novel candidates that may be critical for underlying molecular mechanisms.

Impaired nitric oxide synthase signaling dissociates social investigation and aggression

A combination of social withdrawal and increased aggression is characteristic of several mental disorders. Most previous studies have investigated the neurochemical bases of social behavior and aggression independently, as opposed to how these behaviors are regulated in concert. Neuronal nitric oxide synthase (nNOS) produces gaseous nitric oxide, which functions as a neurotransmitter and is known to affect several types of behavior including mating and aggression. Compared with wild-type mice, we observed that nNOS knockout mice showed reduced behavioral responses to an intruder behind a wire barrier. Similar results were observed in mice treated with the selective nNOS inhibitor 3-bromo-7-nitroindazole (3BrN). In habituation-dishabituation tests, treatment with 3BrN did not block recognition of male urine but did attenuate investigation time compared with oil-treated animals. Finally, nNOS knockout mice and 3BrN treated mice were significantly more aggressive than wild-type and oil-treated males, respectively. In general, these behavioral effects are less pronounced in pair-housed males compared with singly-housed males. Thus, nNOS inhibition results in a phenotype that displays reduced social investigation and increased aggression. These data suggest that further study of nNOS signaling is warranted in mental disorders characterized by social withdrawal and increased aggression.

A combined marble burying–locomotor activity test in mice: A practical screening test with sensitivity to different classes of anxiolytics and antidepressants

Over the last decades, the inhibition of spontaneous burying of glass marbles by mice has been used as an index of anxiolytic drug action in the so-called marble burying test. Indeed, acute administration of rapid-onset (e.g. diazepam) and slow-onset (e.g. fluoxetine) anxiolytics inhibit marble burying. However, non-anxiolytic compounds such as classical antipsychotics also reduce marble burying thus suggesting that the predictive validity of this procedure for anxiety may be limited. In the present study, after having selected a strain of mice (C57BL/6J) that showed spontaneous avoidance of glass marbles, we tried to improve the predictive validity of the marble burying test for anxiety by measuring locomotor activity during the marble burying test and--if needed--in control experiments by using a videotracking system. Twenty-four reference compounds were tested including anxiolytics, anxiogenics, antidepressants, antipsychotics and other classes. By comparing marble burying scores with locomotor measures, we found that, based on our criteria, most of the anxiolytics and antidepressants selectively inhibited marble burying in contrast to most of the other compounds (e.g. haloperidol, morphine). Two putative anxiolytics, i.e. the nociceptin orphanin FQ peptide receptor agonist Ro 64-6198 and the metabotropic glutamate 5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine, also showed a selective profile. We propose this modified procedure, requiring only a limited number of animals, as a valuable screening test for the detection of compounds having anxiolytic effects.

The influence of sex and social isolation housing on pre- and postsynaptic 5-HT1A receptors

Serotonergic (5-HT) receptors are crucial for different brain functions and play an important role in several pathological conditions. We analysed [3H]8-OH-DPAT-specific binding to 5-HT1A receptors in male and female mice after group or isolation housing by in vitro autoradiography (n = 6 per group). Females displayed higher postsynaptic 5-HT1A receptor binding compared to males, especially in the cortex. In contrast, lower [3H]8-OH-DPAT-specific binding was found in the female hippocampus. No sex difference was seen for the somatodendritic 5-HT1A autoreceptor. Sex differences in postsynaptic 5-HT1A receptor binding should be relevant to behavioural sex differences, especially in locomotor activity and hippocampus-dependent behaviours. Six weeks isolation housing caused an increase in 5-HT1A receptor binding in most of the brain regions analysed and was more pronounced in males. In isolated males, the increases were detected in the CA1 field of the hippocampus (+16.8%), in the septum (+76.8%), in the cortical amygdala (+24.6%), in the periaqueductal gray (+67.2%) and in the different cortical regions analysed (+61.8-81.4%). [3H]8-OH-DPAT-specific binding increased significantly in the dentate gyrus (+47.1%), the supramammillary nucleus (+31.2%) and in the ventromedial hypothalamus (+34.4%) of isolated females. Sex-dependent isolation-induced alterations in [3H]8-OH-DPAT-specific binding were also found in the raphe nuclei. Isolation-induced increases in 5-HT1A receptor binding could be relevant to the behavioural disinhibition with heightened arousal, impulsivity and activity often observed in isolates. The male-specific alterations in the corticolimbic system as well as in the midbrain could be crucial for isolation-induced aggression.

Serotonin 1A and 2A receptor densities, neurochemical and behavioural characteristics in two closely related mice strains after long-term isolation

Knowledge about individual differences in behavioural traits and their neurostructural and neurochemical correlates should improve therapeutic approaches of corresponding psychopathology. The presented investigations are aimed to reveal interrelationships between central nervous serotonergic [5-HT] receptor densities and neurochemical as well as behavioural traits in two mice strains. Male AB-Halle [ABH] and AB-Gatersleben [ABG] mice differing in aggression were investigated after 6 weeks of isolation housing. 5-HT1A and 5-HT2A receptors were analysed in different brain regions by in vitro autoradiography. HPLC determinations of aminergic transmission in the cortex, hippocampus, striatum as well as in the raphe-region and radioimmunoassay determination of serum corticosterone were done before (basal condition) and after behavioural tests (challenge condition). Receptor autoradiography revealed higher 5-HT1A receptor densities, especially in limbic regions, and lower 5-HT2A receptor densities in the basal ganglia of ABH mice. Furthermore, ABH mice characterized as behaviourally more active in the open field and plus maze as well as more reactive and aggressive during the social interaction test showed lower basal 5-hydroxyindolacetic acid [5-HIAA] concentrations in the hippocampus, cortex and raphe-region as well as a different activation pattern in serotonergic, dopaminergic and noradrenergic brain systems after challenge in comparison to ABG mice. Additionally lower corticosterone concentrations were found in ABH mice. Lower basal serotonergic and striatal dopaminergic, but higher basal cortical dopaminergic metabolism in contrast to enhanced challenge-induced central nervous serotonergic and cortical dopaminergic reactivities are discussed to be crucial for an enhanced reactive behavioural trait, which could secondarily result in aggression-related behaviours, where higher 5-HT1A receptor and lower 5-HT2A receptor densities may be essential.

Immunodetection of the serotonin transporter protein is a more valid marker for serotonergic fibers than serotonin

Tracking serotonergic pathways in the brain through immunodetection of serotonin has widely been used for the anatomical characterization of the serotonergic system. Immunostaining for serotonin is also frequently applied for the visualization of individual serotonin containing fibers and quantification of serotonin positive fibers has been widely used to detect changes in the serotonergic innervation. However, particularly in conditions with enhanced serotonin metabolism the detection level of serotonin may lead to an underestimation of the true number of serotonergic fibers. The serotonin transporter (SERT) protein, on the other hand, is less liable to metabolism and for that reason we hypothetized that SERT immunostaining is a more stable marker of serotonergic fibers. Rats were pretreated with monoamine oxidase (MAO) inhibitor and compared with placebo treated rats. Brains were double immunostained for serotonin and SERT protein and colocalization was quantified in several brain areas by confocal microscopy. In comparison with untreated rats, MAO inhibitor treated rats had a significantly higher number (almost 200% increase) of serotonin immunopositive fibers whereas no difference was observed in the number of the SERT positive fibers. Colocalization between serotonin and SERT positive fibers was close to 100% in MAO inhibitor treated animals but only 30% in untreated rats. We conclude that the rapid metabolism of serotonin leads to an underestimation of immunodetected serotonergic fibers and that in many instances, SERT immunostaining may be a better indicator of serotonergic fibers.

Effect of early isolation on signal transfer in the entorhinal cortex–dentate–hippocampal system

Deprivation of socio-sensory interactions during early life impairs brain function in adulthood. In previous investigations we showed that early isolation severely affects neuron development in several structures of the hippocampal region, including the entorhinal cortex. In the present study we investigated the effects of early isolation on signal processing along the entorhinal cortex-dentate-CA3-CA1 system, a major memory circuit of the hippocampal region. Male and female guinea-pigs were assigned at 6-7 days of age to either a social or an isolated environment. At 90-100 days of age the animals were anesthetized and field potentials were recorded from the entorhinal cortex-dentate-CA3-CA1 circuit, driven by dorsal psalterium commissural volleys. Analysis of the input-output function in the different structures showed that in isolated males there was a small reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in layer II of the entorhinal cortex. No changes occurred in isolated females. In isolated males and females there was a reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in the dentate gyrus, CA3 and CA1, but this effect was larger in males. In isolated males, but not in females, the population spike/population excitatory postsynaptic potential ratio was reduced in all investigated structures, indicating that in males the size of the discharged neuron population was reduced more than due to the decreased input. Results show that isolation reduces the synaptic function in the whole entorhinal cortex-dentate gyrus-CA3-CA1 system. While the entorhinal cortex was moderately impaired, the dentate-hippocampal system was more severely affected. The impairment in the signal transfer along the entorhinal cortex-dentate gyrus-CA3-CA1 system was heavier in males, confirming the larger susceptibility of this sex to early experience. This work provides evidence that malfunctioning of a major hippocampal network may underlie the learning deficits induced by impoverished surroundings during early life.

Repeated Catha edulis oral administration enhances the baseline aggressive behavior in isolated rats

The effects of repeated oral administration of the psychostimulant plant, Catha edulis and its active principle, cathinone on rats were studied using isolation induced aggression paradigm. The behavioral responses were videotaped and scored later by offline data analyses. Rats were decapitated at the end of the behavioral experiment and in the relevant brain regions, monoamines were assessed. The results demonstrate that isolation of male rats produces a baseline aggression. Treatments with the psychostimulant plant, Catha edulis or commercial S-(--)-cathinone enhanced significantly: The locomotor activities and the baseline aggression behaviors compared with vehicle treated rats. Neurochemical correlates revealed a significant depletion of serotonin (5-HT) and its corresponding metabolites (5-HIAA) in both the anterior and posterior striatum. There was also a reduction in the level of homovanillic acid (HVA) in the hippocampus. Additionally, elevation of dopamine level was observed in the nucleus accumbens, especially, in those rats treated with Catha edulis extract. Cathinone, on the other hand, increased the level of HVA in the posterior striatum and decreased HVA in the nucleus accumbens. In conclusion, the present data demonstrate that repeated administration of Catha edulis or S-(--)-cathinone enhances aggression in rats, presumably by decreasing the level of serotonin and its corresponding metabolites. Besides, the data obtained do not rule out the involvement of dopamine in aggression behavior.

Social isolation stress-induced aggression in mice: a model to study the pharmacology of neurosteroidogenesis

Long-term social isolation of laboratory animals is a model to study the behavioral and neurochemical consequences of the absence of social interaction in rodents. Many of the symptoms induced by isolation resemble depression and anxiety disorder symptomatology. Our studies have revealed that male mice socially isolated for more than 4 weeks, exhibit increased aggressiveness, a reduced responsiveness to GABA(A) receptor acting drugs, and a downregulation of brain levels of 3alpha,5alpha-tetrahydroprogesterone (allopregnanolone: 3alpha,5alpha-THP), a neurosteroid endowed with potent positive allosteric modulatory activity of the action of GABA at various GABA(A) receptor subtypes. This downregulation of 3alpha,5alpha-THP appeared to be associated with the reduction of brain type I 5alpha-reductase mRNA and protein expression. Systemic administration of the selective serotonin reuptake inhibitor fluoxetine and its metabolite norfluoxetine normalized brain 3alpha,5alpha-THP content and reduced responsiveness to GABA(A) mimetic drugs in a stereospecific manner. These drugs in nanomolar doses also reduced social isolation-induced aggressiveness with the same stereospecificity as detected in their action on 3alpha,5alpha-THP brain content, while their ex vivo inhibition of serotonin reuptake occurred at high micromolar doses and lacked stereospecificity. From these results we infer that the brain 3alpha,5alpha-THP content physiologically upregulates GABA(A) receptor responsiveness to GABA and that social isolation induces a reduction of brain 3alpha,5alpha-THP content that is probably causally related to the onset of aggression.

Interactions between anxiety, social support, health status and buspirone efficacy in elderly patients

Psychosocial factors are among the etiological factors of anxiety, and have been shown to affect the anxiolytic efficacy of buspirone in laboratory rodents. Disparate human studies suggest that a similar interaction may be valid for anxious patients. However, this interaction is poorly known at present. It was hypothesized that social support and health status are especially relevant psychosocial problems in elderly, and as such, have a large impact on both anxiety and the efficacy of anxiolytic treatment with buspirone. The hypothesis was assessed by three independent studies performed in a total number of 384 elderly in-patients (109 males, 275 females, age approximately 80 years). A low number of social contacts associated with a large number of diseases proved to be a strong risk factor for anxiety, whereas the reverse condition (many contacts/few diseases) was associated with considerably lower Hamilton Rating Scale for Anxiety (HAM-A) scores. Buspirone ameliorated anxiety significantly in general, but the "many contacts/many diseases" condition was associated with twice as much improvement as the "few contacts/few diseases" condition. The patient's self-evaluation of health status was predicted strongly by the disease score used in the above two studies. Taken conjointly, data suggest that the major Axis-IV problems faced by the age class studied (social support and health status) have a strong effect on both anxiety and buspirone responsiveness in elderly patients. Thus, drug responses appear to be modulated by nonpharmacological factors, and research directed towards identifying such factors would provide information important to a more appropriate patient targeting of certain medications.

Reduced aggression in AMPA-type glutamate receptor GluR-A subunit-deficient mice

The importance of AMPA-type glutamate receptors has been demonstrated in neuronal plasticity and in adaptation to drugs of abuse. We studied the involvement of AMPA receptors in social interaction and anxiety and found that in several paradigms of agonistic behavior naïve male mice deficient for the GluR-A subunit- containing AMPA receptors are less aggressive than wild-type littermates. GluR-A deficient mice and wild-type littermates exhibited similar basic behavior and reflexes as monitored by observational Irwin's test, but they tended to be less anxious in elevated plus-maze and light-dark tests. Maternal aggression or male-female encounters were not affected which suggests that male hormones are involved in the expression of suppressed aggressiveness. However, testosterone levels and brain monoamines can be excluded and found to be similar between GluR-A deficient and wild-type littermates. The reduced AMPA receptor levels caused by the lack of the GluR-A subunit, and measured by a 30% reduction in hippocampal [3H]-S-AMPA binding, seem to be the reason for suppressed male aggressiveness. When we analyzed mice with reduced number of functional AMPA receptors mediated by the genomic introduced GluR-A(Q582R) channel mutation, we observed again male-specific suppressed aggression, providing additional evidence for GluR-A subunit-containing AMPA receptor involvement in aggression.

Genetic and environmental (inter)actions in male mouse lines selected for aggressive and nonaggressive behavior

The purpose of this study was to investigate the effects of genetic and environmental factors, as well as their interaction, in the etiology of aggressive behavior in two mouse lines bidirectionally selected for offensive aggression. To this end, we raised the Finnish TA (aggressive) and TNA (nonagressive) selection lines either in isolation or in cohabitation with a female after weaning. At the age of 3 months we determined their aggressive behavior in three paradigms (intruder resident, neutral cage, resident intruder) against a male standard opponent. We also determined the animals' aggressive behavior against a female mouse. The results show genetic and environmental effects, as well as gene-environment interaction. We see prominent genotype effects under all conditions but each test is sensitive to a specific combination of environmental effects. A particularly noteworthy result is that variation in the unusual behavior of aggression towards a female is largely explained by the interaction of genotype with isolation. We also examined whether test experience influenced the outcome of an encounter between an experimental animal and an opponent, and found that this factor should not be underestimated, its effect size and direction depending on the type of paradigm and way of housing. These data suggest that the identification of genes underlying aggressive behavior in mice is by no means straightforward and that the result of this search will depend on the environmental design of the study (type of paradigm, housing conditions). These data also suggest that the use of 'test battery' mice might produce different results than the use of test-naïve animals.

Region specific changes in forebrain 5-hydroxytryptamine1a and 5-hydroxytryptamine2a receptors in isolation-reared rats: an in vitro autoradiography study

The neurochemical correlates of the behavioural consequences of isolation rearing of rats are complex and involve many neurotransmitters, including the serotonergic system. Impaired functioning of the ascending serotonergic system has been implicated in many neuropsychiatric syndromes, including attention deficit hyperactivity disorder and schizophrenia. In the present investigation serotonergic function was assessed using in vitro receptor autoradiography. The 5-hydroxytryptamine(2A) (5-HT(2A)) receptor antagonist [(3)H]ketanserin and the 5-HT(1A) receptor antagonist, [(3)H]WAY100, 635 were used to compare 5-HT receptor subtype densities in the forebrains of socially and isolation-reared rats. Regions of highest receptor density were observed in the frontal cortex for 5-HT(2A) receptors and in the frontal cortex, dorsal hippocampus and lateral septum for 5-HT(1A) receptors. In isolation-reared rats, 5-HT(2A) receptor binding site densities were significantly increased by between 36 and 67% in the prelimbic, motor and cingulate cortices compared with socially reared controls. By contrast, 5-HT(1A) receptor binding site densities were significantly reduced by 22% in the prelimbic cortex, and significantly increased by between 10 and 50% in the motor cortex, somatosensory cortex, dentate gyrus and CA fields of the hippocampus. These data demonstrate that isolation-rearing produces significant effects on forebrain 5-HT(1A) and 5-HT(2A) receptor densities in the adult rat. It is hypothesised that altered serotonergic function, particularly in the hippocampus and prefrontal cortex, may underlie some of the behavioural abnormalities associated with isolation-rearing.

Sex difference in psychological behavior changes induced by long-term social isolation in mice

Social isolation can induce psychological behavior changes. It is interesting to know whether there is sex difference in responding to social isolation or not. The present study compared the behavior difference between male and female mice isolated for 1-4 months. The results showed that the isolated male mice had higher accounts of locomotor activity than the isolated female and group-housed ones. Both isolated male and female mice spent shorter time in the dark box than the group-housed mice in the light/dark test, and isolated male mice spent less time in the closed arms than isolated female and group-housed mice when isolated for 2, 3 and 4 months in the elevated plus-maze test. These results suggest that isolation induce an anxiolytic-like effect. The immobile time in the forced swimming test was shortened in male mice isolated for 1 and 2 months. Both isolated male and female mice showed shorter time in pentobarbital-induced loss of righting reflex and less body weight gain. These results demonstrated that there was a sex difference in psychological behavior changes in mice undergoing social isolation and the male mice were more easily affected by isolation.

Serotonin fibre densities in subcortical areas: differential effects of isolated rearing and methamphetamine

Serotoninergic neurons interact with dopaminergic cells on all levels and are physiologically affected by both isolated rearing (IR) and a single early methamphetamine (MA) injection. We therefore checked for anatomical effects of both interventions by immunohistochemically staining serotonin fibres and assessing fibre densities in the caudate-putamen (CPu), nucleus accumbens (NAc) and amygdala of Mongolian gerbils. IR led to significantly increased 5-HT fibre densities in the dorsal part of the CPu and in the central and basolateral amygdala. No effects were seen in the ventral CPu, in the NAc and in the lateral amygdala. The early MA injection resulted in a denser 5-HT innervation in the dorsomedial and ventromedial CPu, in the NAc shell of animals reared in an enriched environment and in the NAc core of both rearing conditions, leaving the lateral CPu and the amygdala unaffected. Thus, the single pharmacological versus the environmental challenge exerts an almost complementary effect on the 5-HT innervation in different areas of the brain, which demonstrates that systemic interactions, e.g. with dopaminergic and glutamatergic afferents, must be taken into account when the seemingly uniform 5-HT projections are investigated.

The effect of social factors on the anxiolytic efficacy of buspirone in male rats, male mice, and men

Earlier findings suggest that housing conditions in laboratory animals and life events in humans influence the efficacy of anxiolytic drugs. Here we report on the impact of social isolation on buspirone efficacy in male mice and rats as assessed by the elevated plus-maze. In addition, the impact of social support on buspirone efficacy was assessed in male patients. When administered 30 min before testing and irrespective of housing conditions, buspirone significantly suppressed locomotor activity both in mice (6 mg/kg) and rats (10 mg/kg) and, as such, other behavioral changes observed at this time point must be seen as behaviorally nonselective. However, these locomotor disruptive effects of buspirone were not evident in either species at longer injection-test intervals (2 and 4 h). When given 2 h prior to testing, a low (3 mg/kg) but not high (10 mg/kg) dose of buspirone increased the frequency of open arm exploration in rats (but not mice) irrespective of housing conditions. At the longest injection-test interval used (4 h), buspirone increased the duration of open arm exploration in individually housed, but not group-housed, rats. Similar, though somewhat less robust, effects were observed in male mice at this time. In a double-blind placebo-controlled study with male patients, chronic buspirone treatment (3 x 10 mg daily for 6 weeks) produced a highly significant reduction in scores on the Hamilton Rating Scale for Anxiety (HAM-A). Multiple regression analysis of social support received by patients indicated that the support of nonrelatives (but not of family or other relatives) was a strong positive predictor of buspirone efficacy. Taken together, our data support the hypothesis that social conditions affect the anxiolytic efficacy of buspirone. Results are discussed in relation to differences in the social organization of the three species investigated.

Interaction of nitric oxide and serotonin in aggressive behavior

Nitric oxide (NO) modulates many behavioral and neuroendocrine responses. Genetic or pharmacological inhibition of the synthetic enzyme that produces NO in neurons evokes elevated and sustained aggression in male mice. Recently, the excessive aggressive and impulsive traits of neuronal NO synthase knockout (nNOS-/-) mice were shown to be caused by reductions in serotonin (5-HT) turnover and deficient 5-HT1A and 5-HT1B receptor function in brain regions regulating emotion. The consistently high levels of aggression observed in nNOS-/- mice could be reversed by 5-HT precursors and by treatment with specific 5-HT1A and 5-HT1B receptor agonists. The expression of the aggressive phenotype of nNOS-/- knockout mice requires isolated housing prior to testing. The effects of social factors such as housing condition and maternal care can affect 5-HT and aggression, but the interaction among extrinsic factors, 5-HT, NO, and aggression remains unspecified. Taken together, NO appears to play an important role in normal brain 5-HT function and may have significant implications for the treatment of psychiatric disorders characterized by aggressive and impulsive behaviors.

Autoradiographic analyses of 5-HT1A and 5-HT2A receptors after social isolation in mice

Social isolation of rodents is used to model human psychopathological processes. In the present study, the effects of intermediate and long term isolation housing on postsynaptic 5-HT(1A) and 5-HT(2A) receptors were analyzed in male mice housed in groups or isolation for 4 and 12 weeks. [3H]8-OH-DPAT and [3H]ketanserin were used to label 5-HT(1A) and 5-HT(2A) receptors. Four representative sagittal sections (planes 1-4) were scored by in vitro autoradiography. Whereas after 4 weeks of housing both receptor densities were lowered significantly in isolated mice, after 12 weeks of housing only marginal isolation effects were seen. Intermediate isolation reduced 5-HT(1A) receptors especially in the lateral frontal, parietal and entorhinal cortex (-63%), in the lateral CA1-3 and dentate gyrus region of the hippocampus (-68%), in the basolateral, basomedial, central and medial amygdaloid nuclei (between -38 and -66%), and in the hypothalamus (-28%). 5-HT(2A) receptors were strongly reduced in the frontal cortex (between -47 and -74%), in the hippocampus (between -47 and -95%), in the striatum (between -66 and -76%), and in the accumbens nucleus (between -59 and -73%) in comparison to group housed control mice. After 12 weeks of isolation in the hippocampus continuously decreased 5-HT(1A) receptor densities were demonstrated (between -24 and -61%). But increased 5-HT(2A) receptor densities were seen in the lateral striatum (+86%) compared to control mice. Age-dependent effects were also found. After 12 weeks of group housing the 5-HT(1A) and 5-HT(2A) receptor densities were decreased (between -28 and -54%) in all analyzed brain regions in comparison to 4 weeks of group housing. Isolated animals showed diminished 5-HT(1A) receptor densities in the cortex (-14%) and hippocampus (-15%), but increased 5-HT(1A) receptor densities in the amygdala (+33%) after 12 weeks. The 5-HT(2A) receptor densities were increased in all analyzed regions (between +31 and +96%) after 12 weeks of isolation compared to 4 weeks. To explain these dynamic, time-dependent pattern of isolation-induced changes different regulation processes are supposed regarding 5-HT(1A) and 5-HT(2A) receptors. Besides metabolism-related adaptation processes also neurotransmitter and hormonal (e.g., glucocorticoid) interactions especially in limbic regions have to be considered.

Anxiolytic effects of acute tryptophan depletion in anorexia nervosa

OBJECTIVE

Recent studies have raised the question as to whether a dysregulation of the neurotransmitter serotonin may contribute to the alterations in mood seen in anorexia nervosa (AN). People with AN tend to be anxious, obsessional, perfectionistic, and harm avoidant. These traits are premorbid and persist after recovery. It has been suggested that increased activity of brain serotonin systems could contribute to this pathologic condition. Dieting in AN, which serves to reduce plasma levels of tryptophan (TRP), may serve to reduce symptoms of dysphoric mood.

METHOD

Fourteen women currently symptomatic with AN (ILL AN), 14 women recovered from AN (REC AN), and 15 healthy control women (CW) underwent acute tryptophan depletion (ATD). Measures of psychological state were self-assessed at baseline and hourly after ATD to determine whether ATD would reduce negative mood.

RESULTS

ILL AN and REC AN had significantly higher mean baseline TRP/LNAA (tryptophan/large neutral amino acids) ratios compared with CW. In contrast to placebo, the ATD challenge demonstrated a significantly greater reduction in the TRP/LNAA ratio for ILL AN (-95%) and REC AN (-84%) compared with CW (-70 %). Both the ILL AN and REC AN had a significant reduction in anxiety on the ATD day compared with the placebo day.

DISCUSSION

These data demonstrate that a dietary-induced reduction of TRP, the precursor of serotonin, is associated with decreased anxiety in people with AN. Restricting dietary intake may represent a mechanism through which individuals with AN modulate a dysphoric mood.

In socially isolated mice, the reversal of brain allopregnanolone down-regulation mediates the anti-aggressive action of fluoxetine

Social isolation (SI) of male mice lasting >4 weeks is associated with aggression toward intruders and a down-regulation of brain allopregnanolone (Allo) content. SI of female mice fails to down-regulate brain Allo content or to induce aggressiveness. Fluoxetine (Prozac in clinical use) is an S- and R-fluoxetine (FLX) mixture, which in mammals is metabolized into S- and R-norfluoxetine (NFLX). The S isomers of FLX and NFLX are more active than their respective R isomers in normalizing brain Allo down-regulation and in reducing the aggressiveness induced by SI. Thus, FLX stereospecifically reduces brain Allo down-regulation and the aggressiveness induced by SI, whereas serotonin (5-HT) uptake inhibition lacks stereospecificity. The doses of S-FLX and S-NFLX that reduce aggressiveness and Allo brain content down-regulation induced by SI are at least one order of magnitude lower than the doses that block 5-HT reuptake. Doses of imipramine that inhibit 5-HT uptake neither reduce aggressiveness nor normalize brain Allo down-regulation. We conclude that Allo brain content normalization is a better candidate than 5-HT reuptake inhibition to explain the reduction of aggressiveness elicited by S-FLX and S-NFLX.

Amine neurochemistry and aggression in crayfish

A primary goal of our research is to explore proximate mechanisms important in recruiting adaptive social behaviors. For instance, if one of three different behaviors may be expressed in a particular set of circumstances, how do neurochemical mechanisms bias behavior towards the expression of one act in lieu of the other possibilities? In this article, we review recent results suggesting that serotonin may play such a role in the control of aggression in crayfish. First, we summarize techniques that have been optimized for sensitive characterization of neurochemical profiles in crayfish. Then, borrowing concepts from behavioral ecology, we review a framework for quantitative investigation, which regards behavior as a set of individual decisions, each with a particular probability for occurrence, a motivational context, and controlled by its own distinct neurochemical mechanisms.

Rewarding effects of the optical isomers of 3,4-methylenedioxy-methylamphetamine (‘Ecstasy’) and 3,4-methylenedioxy-ethylamphetamine (‘Eve’) measured by conditioned place preference in rats

3,4-methylenedioxy-methylamphetamine (MDMA) ('Ecstasy') and its analogue 3,4-methylenedioxy-methylamphetamine (MDE) ('Eve') are well known illicit street drugs mainly abused by young people. In spite of the actual research going on, the classification of their abuse potential remains unclear. Since secondary reinforcers are the main factors responsible for craving and relapse, the aim of our study was to assess the potency of MDMA and MDE in a second order reinforcement paradigm, i.e. conditioned place preference (CPP). For the general assessment of our study conditions, we compared MDMA with amphetamine. Unexpectedly, no significant CPP for MDMA was found in contrast to amphetamine. Detailed analysis of current literature led us to the working hypothesis that social environment is crucial for the development of CPP. In a subsequent experiment we tested the influence of housing conditions on CPP using MDMA and demonstrated that isolated animals show significant CPP compared to group-housed ones. In order to better understand the rewarding mechanisms of Ecstasy-derivatives, we tested both the racemic drugs and the pure isomers in the CPP paradigm. Both MDMA's optical isomers and racemic MDMA showed significant CPP without notable differences, while MDE and its isomers completely failed to show any significant CPP. In conclusion, the mechanism by which MDMA induces addiction is much more complicated than assumed so far and more pronounced in isolated animals. The fact that both optical isomers of MDMA led to CPP implies that at least two pathways by which MDMA induces craving behaviour exist.

Behavioral and neurochemical effects of anpirtoline and citalopram in isolated and group housed mice

Acute effects of serotonergic drugs acting via different mechanisms were investigated by a social interaction test and subsequent determination of serotonin and dopamine metabolisms in mice housed in groups or isolated for 6 weeks. A resident/intruder test was performed with anpirtoline (5-HT1B receptor agonist in rodents; 1 mg/kg), citalopram (SSRI; 0.5 mg/kg) and saline treatment before animals were decapitated and different brain regions were frozen for subsequent HPLC-analyses. Behavioral investigations indicated a strong increase of aggressive behavior after 6 weeks of isolation housing. Acute citalopram treatment did not influence behavioral parameters of isolated and group housed mice. In contrast, anpirtoline antagonized isolation induced aggressive behavioral components in a specific manner. Analysis of dopamine and serotonin metabolism revealed that citalopram treatment did not affect dopamine metabolism, but reduced serotonin metabolism in the striatum, hippocampus, cortex and midbrain independent of housing conditions. In contrast, anpirtoline treatment increased dopamine metabolism in cortex, striatum and midbrain as well as influenced serotonin metabolism in a structure- and state-specific manner. Whereas anpirtoline decreased serotonin metabolism in the cortex, the midbrain and the hippocampus independent of housing conditions, in the striatum anpirtoline abolished the isolation induced decrease of serotonin metabolism. These results indicate that anpirtoline might induce antiaggressive effects via postsynaptic receptor- and structure-specific activation of serotonergic but also dopaminergic processes, whereas structure independent increase of synaptic serotonin via citalopram was ineffective to reverse aggressivity in isolated mice.

Long-term memory underlying hippocampus-dependent social recognition in mice

The ability to learn and remember individuals is critical for the stability of social groups. Social recognition reflects the ability of mice to identify and remember conspecifics. Social recognition is assessed as a decrease in spontaneous investigation behaviors observed in a mouse reexposed to a familiar conspecific. Our results demonstrate that group-housed mice show social memory for a familiar juvenile when tested immediately, 30 min, 24 h, 3 days, and 7 days after a single 2-min-long interaction. Interestingly, chronic social isolation disrupts long-term, but not 30-min, social memory. Even a 24-h period of isolation disrupts long-term social memory, a result that may explain why previous investigators only observed short-term social memory in individually housed rodents. Although it has no obvious configural, relational, or spatial characteristics, here we show that social memory shares characteristics of other hippocampus-dependent memories. Ibotenic acid lesions of the hippocampus disrupt social recognition at 30 min, but not immediately after training. Furthermore, long-term, but not short-term social memory is dependent on protein synthesis and cyclic AMP responsive element binding protein (CREB) function. These results outline behavioral, systems, and molecular determinants of social recognition in mice, and they suggest that it is a powerful paradigm to investigate hippocampal learning and memory.

Open field locomotion and neurotransmission in mice evaluated by principal component factor analysis-effects of housing condition, individual activity disposition and psychotropic drugs

Effects of housing condition and individual disposition on dopaminergically and GABAergically influenced open field locomotion and neurochemistry were studied in mice. Mice characterized as high active (HAM) and low active (LAM) by a running-wheel test were housed in groups or isolated for 1 day, 1 week, 3, 6, 12 or 18 weeks before an open field test was performed with saline, apomorphine (0.75 mg/kg) or diazepam (1.00 mg/kg) administration. Immediately afterwards animals were decapitated and brain sections were frozen for subsequent HPLC-analysis of dopaminergic and serotonergic transmitter metabolism. Principal component factor analysis (PCA) of locomotion variables provided three factors explaining 78.5% of total variance. Variables related to the amount of locomotion loaded highly on Factor 1 (F1-Activity), variables related to place utilization loaded highly on Factor 2 (F2-Exploration) and variables related to immobility and place preference loaded highly on Factor 3 (F3-Irritation). Apomorphine decreased F1-Activity with smaller effects in HAM and without changes in F2-Exploration and F3-Irritation independent on housing conditions. Diazepam exerted a decrease in F2-Exploration with a small increase in FI-Activity and no effects in F3-Irritation. Diazepam induced changes depended on housing conditions and were especially pronounced in isolated HAM. PCA of considerable locomotion and neurochemical data revealed interrelationships between striatal dopamine metabolism and F1-Activity, between cortical dopamine and serotonin metabolism and F2-Exploration as well as between cerebellar, hippocampal and striatal serotonin metabolism and F3-Irritation. The authors concluded that the application of PCA is a useful method to provide functionally relevant characteristics of behaviors and functionally relevant descriptions of interrrelationships between behavior and appropriate central nervous mechanisms. Furthermore the received behavioral characteristics (F1, F2, F3) of open field locomotion were sensitive to reveal housing and drug effects.