Genetic regulation of neurotransmitter enzymes and receptors: relationship to the inheritance of psychiatric disorders

Genetic studies at the receptor level: investigations in human twins and experimental animals

In receptors, as in enzymes, quantitative as well as qualitative genetic variation may exist. Studies in inbred strains of mice have shown for various receptors that the receptor density as determined by Bmax values is under genetic control. In healthy adult twins we have shown that the density of alpha-adrenoceptors on platelets is also influenced by genetic factors, since monozygotic twins were much more similar to one another than dizygotic twins. However, Bmax values are up-regulated and down-regulated by endogenous neurotransmitters and pharmacologically active agents. Thus, receptor densities are under considerable regulatory influences. Bmax values therefore reflect regulatory mechanisms rather than innate characteristics of the receptor protein. In another twin study we failed to find evidence for a genetic influence on the density of imipramine-binding sites on platelets. Since qualitative variation (polymorphism) is well known in enzymes, it may also apply to receptors. Qualitative differences in the receptor protein within one species would be of particular interest because of possible functional implications. As a first approach we examined central benzodiazepine receptors by photoaffinity labelling and sodium dodecyl sulphate-polyacrylamide gel electrophoresis. A comparison of fish, frog, chicken, mouse, rat and calf led to the detection of variation between species. Investigations in five inbred mouse and rat strains have not so far revealed genetic variation in benzodiazepine receptors. Nevertheless variation may be detectable by more sensitive methods such as peptide mapping after limited proteolysis or two-dimensional electrophoresis.

Indication of a genetic basis of stereotypies in laboratory-bred bank voles (Clethrionomys glareolus)

The development of stereotypies was studied in two successive laboratory-bred generations of bank voles representing F1 (n=248) and F2 (n=270) of an originally wild caught stock. It was shown that the propensity to develop stereotypies under barren housing conditions strongly relates to the same propensity of the parents. Stereotypies were approximately seven times more frequent in the offspring of stereotyping parents than in the offspring of permanent non-stereotypers. This held true even when only one of the parents was stereotyper. The paternal and maternal contributions to stereotypies in the offspring appeared to be equal. Males showing stereotypies but prevented from any physical contact with the offspring were as potent as stereotyping females in producing stereotyping offspring. Moreover, the specific type of stereotypy appearing in the offspring after isolation was very much related to the type of stereotypy developed in the mothers. We found no support for the possible importance of social facilitation from littermates, in that the development of stereotypies was independent of the length of time the voles were kept socially with littermates before isolation. We suggest that the possible genetic basis of individual differences in the propensity to develop stereotypies in captivity may result from differences in genetic predispositions and their interactions with discrete frustrating stimuli early in life and/or to genetically different predispositions to cope with frustrating experiences later in life.

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.

L-Tyrosine-3-hydroxylase regulation in the brain: genetic aspects

L-tyrosine-3-hydroxylase (TH) is the first and rate limiting enzyme in the biosynthetic pathway of catecholamine neurotransmitters (dopamine, noradrenaline, adrenaline). Implication of dopamine (DA) in various psychopathological phenomena, such as schizophrenia, has considerably contributed to the intensity of investigation of basic biochemical regulation of TH by activation and induction. Here we consider a third, constitutional (genotypic) aspect of regulation and present evidence that differences in mesencephalic (TH/SN), striatal (TH/CS), and hypothalamic (TH/HT) TH activity between virtually isogeneic strains of mice can be explained by segregating genetic factors. Biometrical genetic analysis of progenitor strains and their crosses indicated significant additive gene effects for TH/SN, TH/CS, and TH/HT, whereas dominance effects were statistically non-significant. A monogenic model of inheritance for TH/SN and TH/CS could not be rejected, while more than one gene was indicated for TH/HT. Significant positive phenotypic correlations were found in genetically segregating populations among mesencephalic, striatal and hypothalamic TH activities. This would suggest that some common genetic factors (or linked genes) are involved in the genetic variation of all three traits. A genetic selection experiment to elucidate the cellular and biochemical mechanisms underlying these variations is in progress.

Nonhuman behavioral models in the genetics of disturbed behavior

The development of the association method in which genetic markers match quantitative traits had led to quantitative trait loci (QTL) interval mapping. The association method has been extensively used in animal behavior genetics. Animal research allows more suitable linkage studies and detailed assessment of cellular and subcellular components of the central nervous system that may play a crucial role in the development susceptibility to behavioral disorders. Moreover, experimental designs in the laboratory setting allow genotype x environment interactions to be controlled, thus possibly providing more information on the role of nongenetic factors in gene expression. Experimental results are discussed which indicate that animal studies will provide a sort of test for hypotheses arising in clinical settings, allowing gene-product and product-behavior pathways to be examined at molecular levels when the gene accounts for a very small amount of genetic variance. In such a perspective, new molecular biology approaches and behavior genetics in nonhuman species could provide useful tools in the assessment of the genetic as well as nongenetic factors that lead to psychopathology.

Role of genotype in brain dopamine metabolism and dopamine-dependent behavior of mice

In mice of eight inbred strains--BALB/c, AKR/J, DBA/2, CBA, C57B1/6, DD, CC57Br, and C3H/He--brain dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in striatum and nucleus accumbens with tuberculum olfactorium, the structures of two main dopaminergic systems--nigrostriatal and mesolimbic--were determined. In both dopaminergic regions, no strain effect on either dopamine or DOPAC levels was found, while for HVA content a highly significant hereditary determination was shown. Influences of selective D1 and D2 dopamine receptor agonists--SK&F 38393 and quinpirole, respectively--as well as that of a mixed D1/D2 agonist, apomorphine, on general locomotor activity and stereotypic climbing were studied. By that, marked genotypic differences in dopamine-dependent behavior and dopamine receptor sensitivity were observed. Although both SK&F 38393 (5 mg/kg) and apomorphine (0.25 mg/kg) decreased locomotion, the effect being genotype dependent, in all strains of mice quinpirole (2.5 mg/kg) proved more potent in locomotor inhibition. SK&F 38393 (10 mg/kg) induced climbing, but 2.5 mg/kg apomorphine in most strains was much more effective. At the same time, quinpirole (up to 8 mg/kg) failed to induce this behavior. This suggests the crucial role of D1 receptors in the generation of climbing, attracting, at the same time, attention to the importance of D1/D2 interaction. The observed drastic interstrain differences in dopamine receptor sensitivity demonstrate the essential role of genotype in the effects of dopaminergic drugs.

Genotype-dependent effects of chronic stress on apomorphine-induced alterations of striatal and mesolimbic dopamine metabolism

After 10 daily consecutive restraint experiences, DBA/2 (DBA) mice showed an increase of climbing behavior after injection of 0.25 mg/kg of the dopamine (DA) agonist apomorphine (APO), while no changes were observed following vehicle or 1 mg/kg of APO. By contrast, chronically stressed C57BL/6 (C57) mice showed a clear-cut decrease of climbing behavior at the dose of 0.25 mg/kg of APO and a similar, although less pronounced, effect of stress on the behavior of mice injected either with vehicle or with 1 mg/kg APO. The DA agonist at these same doses decreased 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) concentrations in the caudatus putamen (CP) and nucleus accumbens septi (NAS) of both strains. Higher DOPAC, HVA and 3-MT concentrations were evident in stressed DBA mice receiving 0.25 mg/kg but not 1 mg/kg of APO, in both CP and NAS. Concerning C57 mice, lower concentrations of the 3 metabolites were present at both doses of APO in the NAS of stressed mice in comparison with non-stressed animals, while no significant stress-related effects were evident in the CP. Non-significant differences between control and stressed mice of both strains were evident as regards DA concentrations in CP and NAS. These results suggest that repeated stressful experiences lead to a hyposensitivity of DA presynaptic receptors in DBA mice while they produce a sensitization of mesolimbic DA presynaptic receptors possibly accompanied by down-regulation of postsynaptic DA receptors in the C57 strain.

Role of genotype in the adaptation of the brain dopamine system to stress

Behavioral and biochemical analysis of the effects of stress on brain dopamine (DA) functioning in two inbred strains of mice reveals opposite patterns of adaptation to chronic stress. Chronically stressed mice of the C57BL/6 (C57) strain are characterized by hypersensitive mesolimbic DA autoreceptors and by a dramatic increase of D1/D2 DA receptor ratio (possibly postsynaptic) in the nucleus accumbens septi (NAS) as revealed by in vivo binding of 3H-spiperone and 3H-SCH23390. Chronically stressed DBA/2 (DBA) mice present, on the contrary, hyposensitive DA autoreceptors and no changes in the D1/D2 DA receptors ratio in this brain area. The analysis of the behavioral responses of chronically stressed mice of the C57 strain to the mixed D1/D2 receptor agonist apomorphine, to the selective D2 agonist LY171555 and to the selective D1 agonist SKF 38393 suggest a close relationship between the behavioral alterations produced by chronic stress and the alterations of sensitivity of D2 pre- and postsynaptic receptors in the mesolimbic system. Furthermore, chronically stressed C57 mice present a marked decrease of spontaneous-climbing behavior which is not observed in the mice of the DBA strain and is dependent on the alteration of the biphasic evolution of this behavior during exposure to the test situation which, for these mice, represents a novel environment. Acute exposure to aversive environmental conditions induces a biphasic alteration of DA transmission (initial increase of DA release followed by a decrease under control levels) in the NAS.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral and biochemical changes monitored in two inbred strains of mice during exploration of an unfamiliar environment

Mice of the C57BL/6 (C57) and DBA/2 (DBA) strains were introduced individually in an unfamiliar environment (a large cage where food, water and sawdust had been removed). Over a 90-min period of observation, both strains presented a time-dependent decrease of locomotion and leaning and an increase of grooming. C57 mice were characterized by more cage cover climbing than DBA mice during the first 15-min stay in the new cage and by a significant decrease of this behavior after 90 min. During the first 60 min in the new environment, the DBA mice were less active than C57 mice, and both strains presented a significant increase of immobility after 90 min of test. After 30 min in the test situation, C57 presented a larger increase of plasma corticosterone levels than DBA mice. The plasma corticosterone levels were back to control values after 60 min of test in mice of the C57 strain and after 90 min in the DBA strain. Finally, both C57 and DBA mice presented a significant increase of homovanillic acid concentrations in the nucleus accumbens, but not in the striatum at 30, 60 and 90 min of testing. These results are discussed in terms of the possible involvement of mesolimbic dopaminergic system in mouse behavioral responses to an unfamiliar environment and of possible habituation to the stressful properties of this experience.

Effects of immobilization stress on dopamine and its metabolites in different brain areas of the mouse: role of genotype and stress duration

Immobilization stress induced, in mice of both C57BL/6 (C57) and DBA/2 (DBA) strains, an increase in dihydroxyphenylacetic acid (DOPAC)/dopamine (DA) and homovanillic acid (HVA)/DA ratios and a reduction of 3-methoxytyramine (3-MT)/DA ratio in the caudatus putamen (CP) and nucleus accumbens septi (NAS). These effects were already evident after 30 min stress in the NAS, while in the CP 120 min were needed in order to show the effects of stress. Immobilization did not produce any effects on dopaminergic metabolism in the frontal cortex (FC) of the C57 strain either after 30 or after 120 min stress while in mice of the DBA strain a time-dependent effect of stress on the HVA/DA ratio was evident. When B6D2F1 hybrids were considered, the effects produced by 120 min immobilization in the CP and the NAS paralleled those observed in parental strains, while in the FC 120 min stress induced the same increase of HVA observed in DBA mice, thus suggesting that the pattern of response in the FC that characterizes the DBA strain may be inherited through a dominant pattern of inheritance.

Behavior and brain neurotransmitters: correlations in different strains of mice

Correlations of behavioral patterns in a social setting with catecholamines, serotonin, and several metabolites and precursors in three brain regions were examined in the DeFries H2, C1, and L1 strains of mice. In Experiment I, behavioral observations were recorded for two 15-min sessions in same-sex, same-strain pairs at about 65 days of age. In Experiment II, sex and strain groups were subdivided into 4% and 24% protein diet groups about 1 week before a second set of behavioral observations at about 120 days of age. Brain tissue content of neurotransmitters, precursors, and metabolites was determined by high-performance liquid chromatography after the second set of observations. Significant multivariate strain differences were shown for behavioral variables (both experiments) as well as concentrations of various neurochemicals. Strain H2 showed relatively high levels of locomotion, while rearing and social investigation were high in strain C1 and self-grooming in strain L1. Significant neurochemical differences were found in the following sets of variables: dopamine variables in the cortex, norepinephrine variables and serotonin variables in the combined diencephalon and midbrain, and norepinephrine and serotonin variables in the hindbrain. Effects of diet were found only on serotonin and tryptophan in the subcortical regions. Significant multivariate correlation with the behavioral variables was demonstrated for the catecholamines but not for serotonin. The results suggest that these strain differences in behavior may be mediated by catecholamine systems.

Different effects of apomorphine on climbing behavior and locomotor activity in three strains of mice

Apomorphine (0.1, 0.25, 0.5, 1, 3 mg/kg, SC), induces a dose-dependent reduction of locomotor activity in DBA/2(DBA) and BALB/c(BALB) mice, while it enhances locomotor activity in a biphasic way in C57BL/6(C57) mice. On the other hand, apomorphine is ineffective in modifying climbing behavior in DBA mice while it increases climbing behavior in C57 and BALB mice. The results, taken together, suggest that these are two different behaviors, possibly controlled by different dopaminergic mechanisms depending on the genetic makeup.

A genetic analysis of stereotypy in the mouse: dopaminergic plasticity following chronic stress

After repeated stressful experiences, DBA/2 (DBA) mice showed an increase in apomorphine-induced climbing while C57BL/6 (C57) mice showed a clear-cut decrease of this behavior. Genetic analysis involving F1 and F2 hybrids and the backcross populations (F1 X C57; F1 X DBA) indicated complete dominance of the C57 genotype and a significant genotype X environment interaction. These findings are discussed in terms of dopaminergic plasticity and of the heuristic value of this animal model in relation to disturbed behaviors triggered by stressful experiences.