Nonhuman behavioral models in the genetics of disturbed behavior

Animal models of behavioral dysfunctions: Basic concepts and classifications, and an evaluation strategy

In behavioral neurosciences, such as neurobiology and biopsychology, animal models make it possible to investigate brain-behavior relations, with the aim of gaining insight into normal and abnormal human behavior and its underlying neuronal and neuroendocrinological processes. Different types of animal models of behavioral dysfunctions are reviewed in this article. In order to determine the precise criteria that an animal model should fulfill, experts from different fields must define the desired characteristics of that model at the neuropathologic and behavioral level. The list of characteristics depends on the purpose of the model. The phenotype-abnormal behavior or behavioral dysfunctions-has to be translated into testable measures in animal experiments. It is essential to standardize rearing, housing, and testing conditions, and to evaluate the reliability, validity (primarily predictive and construct validity), and biological or clinical relevance of putative animal models of human behavioral dysfunctions. This evaluation, guided by a systematic strategy, is central to the development of a model. The necessity of animal models and the responsible use of animals in research are discussed briefly.

Behavioural phenotyping of mouse mutants

Behavioural phenotyping of mouse mutants is not a goal in itself but serves to characterise the behavioural effects of naturally occurring or experimentally induced mutations. Genetically engineered mouse mutants are valuable tools to elucidate the genetic control of behaviour and the interaction between genetic and environmental factors. However, a prerequisite for their use is the ability to assess different elements of behaviour. To this end, a battery of tests, which should be flexible enough to meet the needs of a particular study, should be used to characterise the behavioural phenotype. Detailed and extensive information about the effects of gene mutations is crucial for model building and model evaluation. Model building is an iterative process, switching between experimental data and theory formation. In order to facilitate this process and to allow comparison of results within and between laboratories, the standardisation of breeding, housing, and testing conditions is essential. The development and standardisation of sensitive, valid behavioural tests which are suited to phenotype mouse mutants is both a responsibility and a challenge to investigators of mouse behaviour.

Stress promotes major changes in dopamine receptor densities within the mesoaccumbens and nigrostriatal systems

This study investigated the effects of stress on brain dopamine receptor densities in two inbred strains of mice. Analysis of [3H]SCH23390 binding by quantitative autoradiography revealed that repeated restraint stress significantly increases D1-like receptor density in the nucleus accumbens of mice of the DBA/2 strain whist reducing it in the caudate-putamen of C57BL/6 mice. No significant changes in D2-like receptor quantified by [3H](-)-sulpiride binding were observed in caudate, substantia nigra and accumbens of stressed C57BL/6 mice. Instead, in DBA/2 mice, stress significantly increased D2-like receptor density in the nucleus accumbens whilst reducing it in the substantia nigra. Finally, stress significantly increased D2-like receptor density within the ventral tegmental area of C57BL/6 mice whilst significantly reducing it in mice of the DBA/2 strain. These results indicate that stress promotes major changes in mesoaccumbens and nigrostriatal dopamine receptor densities. The direction of these changes depends on receptor subtype, brain area and strain. Moreover, the opposite changes of D2-like receptor densities promoted by stress in the ventral tegmental area of the two inbred strains of mice suggest that mesoaccumbens dopamine autoreceptors density might be controlled by a major genotype x stress interaction.

Psychopharmacology of dopamine: the contribution of comparative studies in inbred strains of mice

Comparative studies of behavioral responses to centrally acting drugs in inbred strains of mice which show differences in brain neurotransmitter activity represent a major strategy in the investigation of the neurochemical bases underlying behavioural expression. Moreover, these studies represent a preliminary stage in behavioral genetic research since they allow quantitative scales to be established and suggest correlations to be tested in recombinant inbred strains. The present review evaluates results obtained in mice of the C57BL/6 (C57) and DBA/2 (DBA) inbred strains which have been used for studies of the behavioral pharmacology of dopamine (DA) and investigated for the functional and anatomical characteristics of their brain DA systems. Differences between C57 and DBA strain involve susceptibility and sensitivity as well as qualitative differences in the type or direction of the behavioral effects of DA agonists. Moreover, data on strain-dependent differences for DA metabolism, release and receptor densities and distribution provide important indications about the relationship between behavioral and central effects of DA agonists and, more generally, about the involvement of brain DA in behavior. Comparative studies in C57 and DBA mice have also revealed differences in susceptibility to context-dependent, context-independent and stress-induced behavioral sensitization to psychostimulants. Consequently, they support the view that the term "behavioral sensitization" may define different phenomena in which different, independent genotype-related factors play a major role. Finally, studies on the behavioral and central effects of stressful experiences in C57 and DBA mice together with psychopharmacogenetic analyses, indicate that different symptomatological profiles may derive from genotype-dependent adaptation of brain DA receptors to environmental pressure.

Parallel strain-dependent susceptibility to environmentally-induced stereotypies and stress-induced behavioral sensitization in mice

Food restriction (9 days) promoted stereotyped behavior in drug-free mice of the DBA/2 (DBA), but not in those of the C57BL/6 (C57), inbred strain. Indeed, behavior presented by food-restricted mice of the DBA strain within the home cage was characterised by a very high response rate within a single response: cage cover climbing. Moreover, enhanced climbing in food-restricted mice of the DBA strain was also observed in a test designed to detect stereotypic effects of drugs in mice. Stereotypic behavior in DBA mice did not depend on nutritional status because: 1. No stereotypies were observed in DBA mice food-deprived for 15 h; 2. no strain-dependent differences in weight loss were observed; and 3. enhanced cage cover climbing was still evident in DBA mice following 24 h of free feeding. Finally, food-restricted DBA mice showed long-lasting sensitization to the locomotor effects of systemic amphetamine, indicating stress-induced behavioral sensitization in this strain of mice. By contrast, no sign of behavioral sensitization was observed in food-restricted mice of the C57 strain. These results indicate that restricted feeding promotes stereotyped behavior, as well as behavioral sensitization to amphetamine, in mice. Moreover, the observed parallelism between strain-dependent susceptibility to environmentally-induced stereotypies and behavioral sensitization supports the hypothesis that these phenomena share common neuro-biological bases.

Different effects of repeated stressful experiences on mesocortical and mesolimbic dopamine metabolism

The effects of repeated stressful experiences (10 min restraint, daily) on the levels of dopamine and metabolites in the nucleus accumbens septi and frontal cortex were evaluated. In naive mice, restraint stress increased 3-4-dihydroxyphenylacetic acid, homovanillic acid, and 3-methoxytyramine levels in the nucleus accumbens and 3-4-dihydroxyphenylacetic acid levels in the frontal cortex. The effects of stress on 3-methoxytyramine and homovanillic acid levels in the nucleus accumbens septi disappeared within five days of daily restraint experiences and the increase in 3-4-dihydroxyphenylacetic acid levels was no longer evident by the 10th day. By contrast, the response of mesocortical dopamine system to restraint (increased 3-4-dihydroxyphenylacetic acid levels) was unaffected by either five or 10 days of exposure to the stressor. Moreover, 10 min of restraint were still able to increase 3-4-dihydroxyphenylacetic acid levels in the frontal cortex of mice repeatedly exposed (nine days) to 120 min restraint. These results indicate that the mesolimbic and the mesocortical dopamine systems adapt differently to repeated exposure to a stressor.

Dose-dependent aversive and rewarding effects of amphetamine as revealed by a new place conditioning apparatus

Amphetamine-induced place conditioning was evaluated in mice using a newly designed apparatus. It was demonstrated that this apparatus provides a neutral set of cues devoid of rewarding or aversive properties and can reveal place preference or aversion after pairings with drugs (amphetamine and morphine for preference and naloxone for aversion) known to produce such effects. Moreover, repeated pairings of environmental cues with either 2 or 3 mg/kg d-amphetamine resulted in significant conditioned place preference on a drug free test, whilst repeated pairings with a lower dose of the drug (1 mg/kg) resulted in significant conditioned place aversion. Finally, a small number of mice showed opposite responses in comparison with group means at low as well as at high doses of amphetamine. These results suggest that amphetamine may promote conditioned place preference or avoidance depending on dosage and individual susceptibility.

Strain-dependent effects of dopamine agonists on acetylcholine release in the hippocampus: an in vivo study in mice

The effects of selective D1 or D2 dopamine receptor agonists and the indirect dopamine agonist cocaine on hippocampal acetylcholine release in mice of the C57BL/6 and DBA/2 inbred strains were investigated using intracerebral microdialysis. The D1 SKF 38393 (10, 20, 30 mg/kg, i.p.), the D2 agonist LY 171555 (0.5, 1, 2 mg/kg, i.p.) and cocaine (5, 10, 15 mg/kg, i.p.) all increased, dose-dependently, acetylcholine release in the hippocampus of C57BL/6 mice. Both the D1 agonist and cocaine did not produce any significant effect in DBA/2 mice. In the latter strain, however, LY 171555 produced a decrease in acetylcholine release that was evident after 60 min from injection of the doses of 0.5 and 1 mg/kg, but not at the dose of 2 mg/kg. The effects observed in C57BL/6 mice as well as those produced by low doses of LY 171555 in the DBA/2 strain were consistent with previous results obtained in rats. The present results indicate major strain-dependent differences in the effects of dopamine agonists on hippocampal acetylcholine release in mice. Moreover, they suggest a complex genotype-related neural organization of dopamine-acetylcholine interactions in the mesolimbic system. Finally, the strain differences in the effects of the dopamine agonists on hippocampal acetylcholine release parallel previously reported strain differences in the effects of these substances on memory consolidation.

Gene expression in mental illness: a navigation chart to future progress

An initial course in disentangling complex causal interactions in psychiatric illnesses, we suggest, is finding co-familial traits with classical Mendelian segregation. Starting with non-Mendelian traits, three methods can be used to find underlying Mendelian phenotypes. (1) Statistically-inferred latent traits, with more nearly Mendelian transmission than the measures from which they are derived, can serve as pointers to concrete Mendelian phenotypes. (2) Linkage of non-Mendelian traits to genetic markers, if it can be established, can be followed by searching for phenotypes that discriminate carriers from non-carriers of the imputed trait gene. (3) In the long run, the most successful method is likely to be direct refinement of non-Mendelian behavioral and physiological traits into more fundamental components.