Catecholamines: strain differences in biosynthetic enzyme activity in mice

Does Developmental Variability in the Number of Midbrain Dopamine Neurons Affect Individual Risk for Sporadic Parkinson's Disease?

Parkinson’s disease (PD) is a slowly progressing neurodegenerative disorder that is coupled to both widespread protein aggregation and to loss of substantia nigra dopamine (DA) neurons, resulting in a wide variety of motor and non-motor signs and symptoms. Recent findings suggest that the PD process is triggered several years before there is sufficient degeneration of DA neurons to cause onset of overt motor symptoms. According to this concept, the number of DA neurons present in the substantia nigra at birth could influence the time from the molecular triggering event until the clinical diagnosis with lower number of neurons at birth increasing the risk to develop the disease. Conversely, the risk for diagnosis would be reduced if the number of DA neurons is high at birth. In this commentary, we discuss the genetic and epigenetic factors that might influence the number of nigral DA neurons that each individual is born with and how these may be linked to PD risk.

Cocaine-Induced Sensitization is Linked to Distal Chromosome 6 Region in Congenic Mouse Model

OBJECTIVE

Previously we mapped QTL Eac2 to mouse Chr6 and identified the first gene (Grm7) as accounting for alcohol consumption in a mammalian model. Despite the central role of glutamate receptors in addiction, the effects of Grm7 gene variants are not well known. Here we test the hypothesis that genetic variation of the distal mouse Chr6 Eac2 region, location of Grm7, controls cocaine-induced locomotor sensitization.

METHOD

C57BL/6By background and B6.C6.327.54 congenic mice were subjected to whole-genome SNP genotyping. Isogeneic (C57BL/6ByXB6.C6.327.54)F2 mice homozygous for SNPs in the BALB/c-type Eac2 region were selected to create a subcongenic strain (B6By.C6.108-120). In a 2-strain x 2-sex 2-treatment factorial design (n = 6-10) C57BL/6By and B6By.C6.108-120 mice received repeated daily cocaine or saline intraperitoneal injections, and locomotor activity was recorded for 90 minutes immediately after injection.

RESULTS

C57BL/6By females with the G/G genotype of SNP rs3723352 of Grm7 responded to cocaine with significantly higher activity and greater cocaine-induced sensitization than those with the BALB/cJ-type T/T genotype in the congenic strain.

CONCLUSION

The results are consistent with a large body of accumulated mechanistic evidence for a role of the mGlu7 receptor in the control of neurobiological responses to cocaine, and are consistent with the hypotheses that (1) natural variants of the Grm7 gene show pleiotropy and can modulate cocaine-induced behaviors in addition to alcohol consumption, (2) interactions between mGluR7 expression, estrogen receptors, and estradiol may explain phenotypic variation in females. Heritable variation of GRM7 may affect vulnerability to substance abuse in women.

Molecular genetic approaches to investigate individual variations in behavioral and neuroendocrine stress responses

A large response range can be observed in both behavioral and neuroendocrine responses to environmental challenges. This variation can arise from central mechanisms such as those involved in the shaping of general response tendencies (temperaments) or involves only one or the other output system (behavioral vs. endocrine response). The participation of genetic factors in this variability is demonstrated by family and twin studies in humans, the comparison of inbred strains and selection experiments in animals. Those inbred strains diverging for specific traits of stress reactivity are invaluable tools for the study of the molecular bases of this genetic variability. Until recently, it was only possible to study biological differences between contrasting strains, such as neurotransmitter pathways in the brain or hormone receptor properties, in order to suggest structural differences in candidate genes. The increase of the power of molecular biology tools allows the systematic screening of significant genes for the search of molecular variants. More recently, it was possible to search for genes without any preliminary functional hypothesis (mRNA differential expression, nucleic acid arrays, QTL search). The approach known as quantitative trait loci (QTL) analysis is based on the association between polymorphic anonymous markers and the phenotypical value of the trait under study in a segregating population (such as F2 or backcross). It allows the location of chromosomal regions involved in trait variability and ultimately the identification of the mutated gene(s). Therefore, in a first step, those studies skip the 'black box' of intermediate mechanisms, but the knowledge of the gene(s) responsible for trait variability will point out to the pathway responsible for the phenotypical differences. Since variations in stress-related responses may be related to numerous pathological conditions such as behavioral and mood disorders, drug abuse, cardiovascular diseases or obesity, and production traits in farm animals, these studies can be expected to bring significant knowledge for new therapeutic approaches in humans and improved efficiency of selection in farm animals.

HPA axis function and drug addictive behaviors: insights from studies with Lewis and Fischer 344 inbred rats

Much research supports a link between stress and its concomitant hypothalamic-pituitary-adrenal (HPA) axis responses with behavioral sensitivity to psychoactive drugs. Our research demonstrates that Lewis inbred rats more readily acquire drug self-administration than Fischer 344 (F344) inbred rats and, compared to this strain, Lewis rats have hyporesponsive HPA axis responses to stress exposure. This association appears to conflict with investigations using outbred rats and suggests that the relationship between drug sensitivity and HPA axis responsiveness is more complicated than originally thought. It is essential to better understand this relationship because of its relevance to vulnerability and relapse to drug abuse. Thus, this paper reviews the literature in which these two inbred strains have been compared. We discuss strain differences in HPA axis function, in characteristics of the mesolimbic dopamine system, and in behaviors thought to reflect emotionality. Strain differences in unconditioned and conditioned effects of psychoactive drugs are then reviewed. Next, we discuss the possible role of sex and gonadal hormones on responsiveness to psychoactive drugs in these strains. Finally, a comparison of results obtained from these strains to three other comparator groups (e.g., high and low responders) suggests that a non-monotonic relationship between behavioral sensitivity to drugs and HPA axis responsiveness can explain much of the discrepancies in the literature.

Analysis of the mesotelencephalic dopamine system by quantitative-trait locus introgression

One of the significant factors that affect brain dopamine function is the activity of tyrosine hydroxylase (TH), the first and rate-limiting enzyme in catecholamine biosynthesis. For the analysis of the genetically determined role of dopamine function and TH in behavior and in the regulatory mechanisms of the mesotelencephalic dopamine system we devised a novel genetic strategy (Vadasz; Mouse Genome 88:16-18; 1990). We hypothesized that phenotypic introgression and recombinant fixation could ensure the transfer of Quantitative Trait Loci (QTL) from one strain onto the genetic background of another strain, and new, genetically very similar quasi-congenic strains could be created that would carry individual QTLs, or QTLs in various combinations. Here we summarize the construction of the first set of QTL Introgression strains, and present evidence that QTLs that are responsible for the continuous variation of mesencephalic tyrosine hydroxylase activity (TH/MES), have been transferred onto the C57BL/6By (B6) strain background from BALB/cJ (C) and CXBI (I) donor strains with high and low TH/MES, respectively. The QTL transfer was carried out in two directions by repeated backcross-intercross cycles with concomitant selection for the extreme high and low expressions of TH/MES in replicates, resulting in four QTL Introgression lines. Analysis of regional brain TH activities in the course of the QTL introgression indicated that (a) TH activity in B6.I lines showed quite limited heritability, (b) TH/MES was not highly correlated with striatal TH, and (c) the control of hypothalamic and olfactory tubercle TH activities was largely independent from that of TH/MES. Examination of the open-field (OF) behavior data demonstrated that TH activity did not correlate significantly with OF behavior. After 5 backcross-intercross cycles, TH/MES in each replicate line was still significantly different from that of the B6 background strain. A genomewide scanning of microsatellite markers in the QTL introgression lines demonstrated that about 96% of the markers were of background (B6) type. These results indicate the successful transfer of TH/MES QTLs. After the QTL transfer phase of the experiment altogether more than 100 new RQI strains were initiated in the QTL Introgression lines by strict brother x sister mating. After fixing the introgressed QTLs, ten of the inbred RQI strains were tested for TH/MES. The results showed that in one of the new RQI strains TH/MES was restored to a level that is characteristic to the C donor strain, while TH/MES values in some other strains were between those of the background and donor strains, confirming our hypothesis that phenotypic introgression and recombinant fixation can ensure a virtually complete transfer of QTLs. We conclude from this study that complex, continuously distributed neural traits can successfully be subjected to QTL introgression, and the results raise the possibility that the RQI method can be efficiently applied for gene mapping of complex neural and behavioral traits even if their phenotypic expression is sensitive to confounding developmental and environmental variations, genetic interactions, and genotype-environment interactions.

Differences in the liability to self-administer intravenous cocaine between C57BL/6 x SJL and BALB/cByJ mice

Application of animal models of psychostimulant abuse for experimentation in mice is becoming increasingly important for studying the contribution of genetic differences, as well as the roles of selected (targeted) genes, in specific behaviors. The purpose of this study was to investigate strain differences in cocaine self-administration behavior between C57BL/6 x SJL hybrid mice and BALB/cByJ mice. These two strains were chosen because BALB/cByJ mice have a well-developed behavioral pharmacological profile, and hybrid strains on a C57BL/6 background are commonly used for generating transgenic expressing and knockout mutant mice. C57BL/6 x SJL mice dose-dependently acquired cocaine self-administration (1.0 mg/kg/injection but not 0.25 mg/kg/injection) by responding selectively in the active nose-poke hole and maintaining stable levels of daily drug intake; they also exhibited a characteristic inverted-U-shaped cocaine dose-effect function. BALB/cByJ mice failed to acquire cocaine self-administration at either dose under the same test conditions. The strain differences observed in self-administration did not seem to be attributed to other behavioral differences because the two strains exhibited similar amounts of spontaneous nose-poking in the absence of reinforcers, and BALB/cByJ mice responded more than C57BL/6 x SJL mice in a food-reinforced nose-poke operant task. Importantly, the dose-effect function for the motor stimulating effects of cocaine (3.8-30 mg/kg intraperitoneally) suggests enhanced sensitivity but reduced efficacy of cocaine in stimulating motor activity in BALB/cByJ mice relative to the C57BL/6 x SJL hybrid mice. These results indicate that the decreased liability of BALB/cByJ mice to acquire cocaine self-administration is not the result of differences in spontaneous activity or performance, but may reflect different sensitivities to the reinforcing, or rate-disrupting, properties of cocaine. The data support an influence of genetic background in the liability to self-administer cocaine. Thus, a hypothesis is proposed that the decreased liability of BALB/cByJ mice to acquire cocaine self-administration is related to differences in brain monoamine systems linked to the high "emotionality" profile of BALB/c mice in novel or fearful situations, including perhaps cocaine administration.

Immunohistochemical studies of mesolimbic dopaminergic neurons in Fischer 344 and Lewis rats

Previous work has shown that the inbred Lewis and Fischer 344 rat strains differ in several behavioral measures related to mesolimbic dopamine function. Moreover, the ventral tegmental area (VTA) of the Lewis rat has been shown to contain higher levels of tyrosine hydroxylase compared to that of the Fischer rat by blot immunolabeling procedures. To investigate structural correlates of this biochemical difference, an immunohistochemical study of VTA dopaminergic neurons in these two strains was undertaken. Results show that the density and total number of tyrosine hydroxylase-positive neurons in the VT 4 of the Lewis rat is about 50% of that found in the Fischer rat. In contrast, examination of the substantia nigra in the same sections revealed no differences in the density and number of tyrosine hydroxylase-positive cells between these strains. Fischer-Lewis strain differences were also evident for cholecystokinin immunoreactivity in the VTA, with much lower levels seen in the Lewis rat, consistent with the known colocalization of this neuropeptide in many VTA dopamine neurons. The finding of 50% fewer tyrosine hydroxylase-positive neurons in the VTA of the Lewis rat, along with our earlier results showing 45% higher levels of tyrosine hydroxylase by blot immunolabeling, would suggest much higher levels of tyrosine hydroxylase per VTA neuron in this strain. However, no obvious strain difference in the cellular intensity of tyrosine hydroxylase immunoreactivity could be detected by immunohistochemistry. Finally, the density of VTA dopamine neurons was assessed in 1-week-old Fischer and Lewis rats. In contrast to the results obtained for adult animals, no difference in the number of tyrosine hydroxylase-positive neurons was apparent in these young animals, indicating that the Fischer-Lewis strain difference in VTA dopamine neurons appears later in postnatal development. These anatomical findings shed new light on the differences in the mesolimbic dopamine system between Fischer and Lewis rats that may contribute to the behavioral differences exhibited by these animals.

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.

Motor activity and the mesotelencephalic dopamine function. II. Multivariate analysis of genetically segregating generations

Previous experiments on genetically different inbred strains of mice demonstrated parallel variations between the activity of regional brain tyrosine hydroxylase (TH) and locomotor behavior. Based on these associations, it was hypothesized that genetic variations in mesotelencephalic TH activity, an index of dopamine neurotransmitter function, would correlate positively with exploratory and locomotor behavior. In order to test this hypothesis, open-field motor behaviors and mesencephalic and striatal TH activities were analyzed by multivariate statistical methods in genetically segregating (C57BL/6ByJ X BALB/cJ)F2 and (C57BL/6ByJ X CXBI/ByJ)F2 generations. Factor analysis, based on correlation matrices of variables with significant genetic dominance or additive effects, demonstrated that locomotor activity and frequency of occurrence of various motor patterns were not correlated with mesencephalic and striatal TH activity. These results indicate that the assumption of a positive phenotypic correlation between spontaneous motor activity and mesotelencephalic TH activity does not hold in genetically segregating populations. Strategies and problems in revealing the behavioral consequences of genetically based variations in the mesotelencephalic DA system are briefly discussed.

Genotype differences in catecholamine concentrations in hypothalamus, intramedial hyperstriatum ventrale, and optic tectum of newly hatched chicks

This study was designed to compare catecholamine concentrations among three brain areas of four pureline populations of visually isolated chicks. The purelines used were a commercial male line, a fertility selected line, an unselected fertility control line, and unselected White Jersey Giants. In general, male chicks had significantly larger brain weights than females. Six catecholamine-related compounds (norepinephrine, epinephrine, L-DOPA, dopamine, DOPAC, and MHPG) were measured via HPLC-ECD. No significant differences in neurochemical concentration were observed for any line or brain area due to sex of the chick. The hypothalamus (HT) contained the greatest concentration of catecholamines in all lines, followed by the intramedial hyperstriatum ventrale (IMHV) and optic tectum (OT). The HT exhibited consistent lateralization in all lines with the right HT containing ca. 30% more catecholamines than the left HT. While no consistent lateralization was observed among the other brain areas, the IMHV exhibited significantly different degrees of lateralization among the populations. Neuronal activity, as measured by MHPG:NE and DOPAC:DA ratio varied by line within each brain area. There were line differences for MHPG:NE in the HT, IMHV, and OT, while line differences for DOPAC:DA were observed in the HT. Since differences among purelines have been demonstrated in this study, care must be given to precisely define the genotype of chicks used in behavioral and neurochemical research.

Met-enkephalin levels in midbrain dopamine regions of inbred mouse strains which differ in the number of dopamine neurons

The Met-enkephalin content in tissue micropunches of the substantia nigra zona compacta, ventral tegmental area and the caudate nucleus in BALB/c and CBA mouse strains was measured by radioimmunoassay. The CBA strain had significantly higher Met-enkephalin levels in all 3 regions than the BALB/c strain. These differences should be taken into account when relating the intensity of dopamine-induced behaviors to the number of dopamine neurons.

Genetic determination of striatal tyrosine hydroxylase activity in mice

An additive major gene effect is described for tyrosine hydroxylase activity in mouse corpus striatum (CS). Quantitative genetic analysis indicated the presence of a segregating Mendelian factor with robust additive effect in F2 generations derived from crossing two highly inbred mouse strains, C57BL/6ByJ and BALB/cJ, with intermediate (INT) and high (HI) TH activity in CS. Significant positive correlation was found between striatal and mesencephalic TH activity in the segregating generations, raising the possibility that a common single gene may express its effect through pleiotropy or linkage. Genetic preparations taking advantage of the major gene effect should serve well as animal models of DA-mediated neuropsychiatric disorders.

Differences in acquisition of discrete lever-press and shuttle avoidance responses in 6 strains of mice

Characteristics of the acquisition processes of discrete lever-press and shuttle avoidance responses (intertrial interval = 25 sec, warning duration = 5 sec with an escape contingency, and 1 session = 1 hr training/day) as well as those of the ambulatory activity were investigated in 6 strains of mice (dd, ICR, BALB/c, C57BL/6, C3H/He and DBA/2). In the lever-press avoidance situation, the dd, BALB/c and DBA/2 strains demonstrated a good avoidance response, showing average avoidance rates of higher than 80% within 10-15 sessions of the training. The C3H/He also demonstrated a good avoidance response, though 20 sessions or more of the training were required to attain to the level. The ICR and C57BL/6 strains demonstrated a poorer avoidance response than the other 4 strains, and achieved average avoidance rates of 30-40%. The dd, BALB/c, C3H/He and DBA/2 strains also demonstrated a good shuttle avoidance response when they were trained in an experimental box of 50 cm in width. In contrast, the ICR and C57BL/6 strains demonstrated an extremely poor avoidance response in this situation, showing an average avoidance rate of less than 10%. However, when the dd, ICR and C57BL/6 strains were trained in a shuttle box of 30 cm in width, they rapidly acquired the avoidance response, and the average avoidance rates finally achieved were much higher than those of the mice trained in the shuttle box of 50 cm in width. In particular, the C57BL/6 strain exhibited the best avoidance response among the 3 strains. The ambulatory activity of the C57BL/6 strain was less than those of the other 5 strains.

Genetic effects and sexual dimorphism in tyrosine hydroxylase activity in two mouse strains and their reciprocal F1 hybrids

This study further analyzed the environmental and genetic mechanisms underlying the previously reported strain differences in tyrosine hydroxylase (TH) activity in the nigrostriatal and hypothalamic dopamine system of the BALB/cJ (B) and CBA/J (C) inbred mouse strains and related behavioral processes using parental and reciprocal F1 hybrid generations. Significant strain differences were found in both sexes in all the measured characters. Comparing males and females, sexual dimorphisms were found in TH activity of substantia nigra (SN), corpus striatum (CS) and hypothalamus (H), and in exploratory behavior (IE). Presence of sexual dimorphism was genotype dependent, with the exception of TH activity in H. Major components of strain differences, maternal effects and additive gene effects, were separated by biometrical genetic methods. The analysis indicated that significant maternal effects were present in TH activity of TH and CS with a trend towards this phenomenon in the SN. Additive gene effects were significant in all characters and various degrees of dominance were expressed in the hybrids in TH activity of SN and CS, as well as in behavioral traits, IE and spontaneous locomotion (SL). All the biochemical and behavioral parameters were expressed at lower levels in CBA/J than in BALB/cJ mice and reciprocal F1 hybrids took intermediate positions between the two parental strains for all phenotypes examined, with the exception of IE, where complete dominance was found in (CXB)F1 females. These results are consistent with the hypothesis that some of the genes affecting TH activity in brain dopamine systems contribute to the expression of dopamine mediated behaviors. Our analysis also indicates the possibility that the maternal effects on TH activity in CS and SN are the consequences of X-chromosome linked gene effects. We suggest that the influence of the X-chromosome linked gene(s) is dependent upon the action of gonadal steroids during the critical period of ontogenesis, and X-chromosome linked gene(s) play a major role in the genotype dependent expression of sexual dimorphism in TH activity.

Number of dopamine neurons predicts prolactin levels in two inbred mouse strains

Mice of the BALB/cJ strain have more dopamine neurons than mice of the CBA/J strain. We now report that BALB/cJ mice have less circulating and pituitary prolactin than CBA/J mice, a relationship expected from the difference in tuberoinfundibular dopamine neuron number.

Dopamine synthesis in inbred mouse strains which differ in numbers of dopamine neurons

BALB/cJ and CBA/J mice have been shown to have different numbers of dopamine (DA) neurons in the central nervous system, with BALB/cJ mice having 20-50% more DA neurons in each dopaminergic cell group which is reflected in a difference in tyrosine hydroxylase activity in these cell groups. The present study compared the levels of DA and the rate of DA synthesis between these two inbred mouse strains. Three measures were used to reflect the rate of DA synthesis: the levels of DA metabolites (DOPAC and HVA) in the striatum, the rate of disappearance of DA following inhibition of tyrosine hydroxylase with alpha-methyl-P-tyrosine, and the rate of accumulation of DOPA following inhibition of aromatic amino acid decarboxylase with NSD-1015. Striatal DA levels were slightly higher in CBA/J mice than BALB/cJ mice. The rate of DA synthesis in the striatum, as estimated from the accumulation of DOPA following NSD-1015 injection or from the decline of DA levels following alpha-methyl-p-tyrosine injection, was from 30-50% greater in the BALB/cJ mice compared to the CBA/J mice. In striatum, DOPAC levels were higher, HVA levels lower, and DOPAC plus HVA levels equal in CBA/J mice compared to BALB/cJ mice. The results show that BALB/cJ mice, with more DA neurons than CBA mice, also synthesize more DA. In addition, the data suggest that DA levels do not necessarily reflect numbers of DA neurons, and that catecholamine metabolite levels are not a good measure for comparing catecholamine synthesis between inbred animal strains.

Genetic control of apomorphine-induced climbing behavior in two inbred mouse strains

Over a wide dose range (1-32 mg/kg), apomorphine-induced climbing behavior was significantly greater in the AKR/J than in the DBA/2J inbred mouse strain. A similar difference was found when apomorphine-induced stereotypy was examined. A cross-fostering study demonstrated that the strain difference in apomorphine-induced climbing behavior did not result from differences in post-natal rearing environment. After apomorphine administration, brain levels of the drug were similar in the two strains indicating that the difference in behavioral response to apomorphine in the two strains was not due to differences in metabolism or distribution of the drug. The climbing response to apomorphine was examined in the F1 cross of the two strains (AKD2F1/J) and in 10 AKXD recombinant inbred strains. Results suggested that the trait was partially dominant and not X-linked; furthermore, a few and possibly one locus was responsible for the differences in apomorphine-induced climbing behavior observed in the AKR/J and DBA/2J mice.

Age- and genotype-dependent differences in catecholamine concentrations in the porcine caudate nucleus

Catecholamine concentrations of the caudate nucleus were determined by radioenzymatic assay in stress-susceptible and stress-resistant pigs at five different ages. Dopamine concentrations increased with age up to the time of puberty and declined thereafter in both genotypes of pigs. Generally, epinephrine concentrations increased with increasing age in both genotypes of pigs. Norepinephrine concentrations remained relatively constant for all ages of pigs except the oldest pigs. For most age groups, dopamine concentrations were lower in pigs of stress-susceptible genotype than in stress-resistant genotype.

Strain differences in sympathetic-adrenal medullary responsiveness and behavior

Three experiments confirmed and extended previous findings from this laboratory regarding strain differences in physiological and behavioral responses of rats to stressful stimulation. In the first experiment, adult male Wistar-Kyoto (WKY) rats had greater plasma levels of norepinephrine and epinephrine and higher mean arterial pressures following 5 min of intermittent footshock (2.0 mA, 0.5-sec duration, every 6 sec) compared to adult male Brown-Norway (B-N) rats. In contrast, basal plasma levels of both catecholamines and resting values of mean arterial pressure and heart rate did not differ between strains when rats were undisturbed in their home cages. The second experiment involved a behavioral comparison of adult male and female WKY and B-N rats during 3 consecutive daily tests in an open field arena. B-N males and females were dramatically more active and reared more frequently during each open field test compared to WKY rats. For the third experiment, adult male and female rats of the two strains were trained in a one-trial passive avoidance task and median crossover latencies were similar for all strain-sex comparisons. However, median 24-hr retention latencies were much greater for WKY male and female rats. These findings indicate that strain differences in the physiological and behavioral responses of WKY and B-N rats are consistent across sexes. Moreover, our studies with these inbred strains of rats provide a convenient model for examining the relationship between sympathetic-adrenal medullary activity and behavioral responses to stressful stimulation.

Age and strain comparisons of neurotransmitter synthetic enzyme activities in the mouse

Activities of the neurotransmitter synthetic enzymes, choline acetyltransferase (EC 2.3.1.6; ChAT), glutamic acid decarboxylase (EC 4.1.1.15; GAD), and tyrosine hydroxylase (EC 1.14.3.2; TH), were assayed in four brain regions of A/J and C57BL/6J mice at three ages (4, 18, and 24 months). The brain regions assayed were the fronto-parietal cortex, hippocampus, striatum, and cerebellum. Strain effects: In some brain regions, at several ages, ChAT activity did not differ among the two strains. However, ChAT was higher in the C57BL/6J strain in the cortex at 18 months, the hippocampus at 18 and 24 months, the striatum at 24 months, and the cerebellum at 4 months. The reverse was true in the cerebellum at 24 months, where ChAT was higher in A/J mice. GAD activity in C57BL/6J mice compared to that of A/J mice was higher in the striatum and cortex, and lower in the hippocampus and cerebellum. TH activities in all four regions were generally higher in C57BL/6J mice than in A/J mice. Age effects: Age differences in enzyme activities varied with the genetic strain. ChAT activity generally was higher in brain regions of older mice of both strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Amphetamine-induced hypolocomotion in mice with more brain D2 dopamine receptors

The relationship between brain D2 dopamine receptors and locomotor response to amphetamine was investigated in eight strains of mice. The D2 receptor is defined as that dopaminergic site with high affinity (nanomolar) for neuroleptics and low affinity (micromolar) for agonists. D2 receptors were measured in the striatum and olfactory tubercle using [3H]spiperone and 10 microM sulpiride to define specific binding. Four inbred strains of mice (CBA/J; C57BL/6J; DBA/2J; SEC/1ReJ) had low receptor densities of about 380 and 160 fmoles/mg protein in the striatum and olfactory tubercle, respectively; all these mice were essentially nonresponsive (i.e., locomotion) to low doses of amphetamine (0.5 and 1.0 mg/kg i.p.) or showed hyperlocomotion to high doses (5 mg/kg). Three other mouse strains (BALB/cJ; A/J; C3H/HeJ) had higher densities of about 600 and 230 fmoles/mg protein in the striatum and olfactory tubercle, respectively, and these mice all responded with hypolocomotion to the low doses and hyperlocomotion to the high dose of amphetamine. The two genetically different populations, one of which responded to amphetamine with hypolocomotion while the other did not, are analogous to hyperactive children, only 70% of whom respond to amphetamine-like drugs. Thus, the mice with high receptor density may serve as a model for studying the hyperactivity syndrome which may be associated with dopaminergic dysfunction.

Endogenous serotonin (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) levels in large regions and in discrete brain areas of C57BL and BALBc mice at three times of the day

The serotoninergic system has been analysed in the brainstem and the forebrain of adult C57BL and BALBc mice. Endogenous 5-HT and 5-HIAA levels have also been measured in the different raphe nuclei and in the caudate nucleus and the hippocampus of these strains, using the micropunch technique and the radioenzymatic method (for 5-HT) and a new radioimmunoassay (for 5-HIAA). This investigation was performed at three times of the day: 9:00 hr; 14:00 hr; 20:00 hr to underline the differences between the strains. Most of the time, higher 5-HT and 5-HIAA levels are found in the brainstem and the raphe nuclei of the BALBc strain compared to C57BL. In the forebrain the differences are reversed, except in the caudate nucleus and the hippocampus where the levels of 5-HT are identical in the two strains. Moreover the 5-HIAA/5-HT ratio shows a higher turn-over of the amine in the C57BL strain.

The inheritance and genetic correlation of tyrosine hydroxylase activities in the substantia nigra and corpus striatum in the C x B recombinant inbred mouse strains

In mouse, strain differences in the activity of tyrosine hydroxylase (TH) in ventral midbrain dopamine systems of substantia nigra-A10 (SN) region of mouse brain and in a terminal field, the striatum (CS), can be entirely attributed to variations in the number of dopaminergic neurons2. To obtain further information about the complexity of the genetic systems influencing phenotypes for regional TH activity, we examined TH activity in the SN and CS of 7 recombinant inbred (RI) mouse strains, their progenitor strains (BALB/cBy and C57BL/6By), their reciprocal F1 hybrids and a CB6F2 segregating generation. Genetic analysis indicated that TH activity in both brain regions seems unlikely to be controlled by single gene effects. However, the mode of inheritance is presumably not very complex. Estimates of the degree of genetic determination for TH activity in SN and CS were relatively high with significant and positive correlations with respect to either the RI lines (r = 0.82) or the CB6F2 generation (r = 0.53). These positive correlations suggest that some of the genes of two gene sets influencing TH activities in the SN and CS are the same. However, additional non-shared genes may also be present. Assuming that our two measures reflect the number of dopaminergic neurons in SN-A10 area and density of their processes in corpus striatum, our results lead us to the hypothesis that the number of dopaminergic neurons and the axonal arborization of these neurons in the nigrostriatal system are in part under a common genetic control but that other genes may contribute to branching of SN neurons.

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

This report begins with a summary of the evidence for genetic involvement in certain major psychiatric syndromes. The relation of these disorders to deficits in central nervous system neurotransmitters is also summarized. These reviews serve as an introduction to our studies on the genetic regulation of neurotransmitters and their enzymes and receptors in inbred mice. The steady-state levels of the adrenal catecholamine biosynthetic enzymes are controlled genetically; not only is each enzyme regulated by a single locus, but also there is statistical evidence that the phenotypic expression of the entire pathway is regulated by a single gene. Studies on the biochemical mechanism of gene action suggest that genetic regulation is exerted on proteolysis of the enzymes, rather than their synthesis. In addition, we have examined the genetic control of dopamine receptors in inbred mice. Dopaminergic receptors in the nigrostriatal and mesolimbic pathways are under genetic control. Preliminary evidence suggests that the pathways are regulated by different genetic systems. If this early speculation proves true, it would have important clinical implications.

An overview of the genetics of psychopathology

The suggestion that psychopathologies are in part mediated by aberrant catecholamine metabolism has resulted in one of the more rapidly growing areas of pharmacogenetics. Collectively, the studies conducted to date indicate that psychopathological conditions have multiple causes which cannot be related to single genetic or biochemical deficits. However, through multidisciplinary research integrating behavioral, genetic, and biochemical approaches, a great deal of insight may be gained concerning the causes of psychopathological disorders and the use of drug therapy to modify the course of these illnesses.

Strain-dependency in motor activity and in concentration and turnover of catecholamines in synchronized rats

Circadian rhythm in motor activity was studied with an Animex motimeter in six strains of rats (ACI, BH, BS, DA, LEW, TNO) synchronized by a 12 hr light: 12 hr dark cycle. ANOVA revealed significant interstrain differences in motor activity as well as in the concentration and turnover of central noradrenaline and dopamine. Strain-dependent differences were also found with regard to tyrosine hydroxylase inhibition on motor activity. However, no significant interstrain correlations were found between endogenous concentration and/or turnover rates of the catecholamines and motor activity in normal and drug-treated rats.

Variations in brain cortical polysome translation products among rats of the same strain

Studies were undertaken to determine whether there exist variations among the translation products of polysomes from different brains of animals of the same strain. Polysomes were prepared from individual rat cortices and translated in a reticulocyte protein-synthesizing system containing rabbit reticulocyte factors and L-[35S]methionine; he resulting radioactive proteins were analyzed by two-dimensional polyacrylamide gel electrophoresis autoradiography. Comparison of the autoradiographs revealed that two acidic proteins, A and B, of apparent 54,000 M. W. occur as three phenotype: A only, B only, or A plus B. These proteins were not detectable by Coomassie brilliant blue staining of two-dimensional electrophoretograms of brain protein preparations. Messenger RNA was extracted from pooled cortices and translated in a wheat germ extract, and both A and B proteins were detected among the products of translation. Cyclic AMP affinity chromatography of the translation products of cortical polysomes showed that both A and B proteins bind to cyclic AMP. Our data are consistent with the conclusion that there are qualitative differences in the polysome translation products that bind to cyclic AMP among individual cortices of rats of the same strain.

An experimental geneticist looks at catecholamine metabolism

The contributions of genetic variation, including that at unstable and duplicated loci, interactions between alleles at different loci and genotype-environment interaction to observable phenotypic variation in experimental animals are discussed with particular reference to catecholamine metabolism. The possibilities and complexities involved in extrapolating to human populations, and to behavioural consequences of biochemical variation, are outlined.

Biochemical genetics of neurotransmitter enzymes and receptors: relationships to schizophrenia and other major psychiatric disorders

Genetic control of catecholamine biosynthetic enzymes and dopamine receptors is described. The steady-state levels of each of the catecholamine biosynthetic enzymes in the adrenal is regulated by a single genetic locus. The entire biosynthetic pathway gives the appearance of concerted inheritance under the control of a single locus. Mesolimbic and nigrostriatal dopamine receptors are also genetically regulated. Preliminary evidence suggests that agonist binding sites differ from antagonist sites in both brain regions, and that the genetic controls, which are expressed on receptor site number, are independent in the two brain regions.

An overview of neurobiological comparisons in mouse strains

A literature review of research dealing with mouse strain comparisons along neurobiological parameters is presented in tabular form which provides a rank-ordering of strains and estimates of percentage differences in individual parameters. Drawing from 86 references, three tables deal respectively, with comparisons of neurochemical, neuroanatomical, and neuropsychophysiological parameters.

Regional tyrosine hydroxylase and choline acetyltransferase concentrations in the brains of lymphocytic choriomeningitis-infected mice

The neurotransmitter biosynthesis enzymes tyrosine hydroxylase and choline acetyltransferase were investigated in selected brain areas of Nya : NYLAR mice infected with lymphocytic choriomeningitis (LCM) virus. Statistically significant alterations in the concentrations of both enzymes occurred in the olfactory, caudate, and neocortical regions at 5 days postinfection. No such alterations occurred in mice given cytoxan (150 mg/kg) 3 days postinfection and examined 5 days postinfection. At 10 days postinfection, however, the cytoxan-treated animals had significantly altered enzyme concentrations in the olfactory region, though not in the caudate or neocortex. This alteration appeared to be transitory, since it was not found in cytoxan-treated animals 60 days postinfection. A possible explanation is that virus production or interference in a brain region cycles over a period of hours or days. Still undetermined is whether these neurochemical changes are a primary effect of the virus or a secondary effect due to the immune response. It is noteworthy that cytoxan caused a marked increase in the enzyme activities studied in most of the brain areas.

Genetic control of number of midbrain dopaminergic neurons in inbred strains of mice: relationship to size and neuronal density of the striatum

The activity of tyrosine hydroxylase [TyrHase; tyrosine-3-monooxygenase; L-tyrosine, tetrahydropteridine: oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] is 20% less in whole midbrain of CBA/J mice than BALB/cJ mice and is paralleled by a comparable difference in the number of dopaminergic neurons in which the enzyme can be detected immunocytochemically. The strain-dependent difference in numbers of TyrHase-containing neurons and of TyrHase activity is not homogeneous in the midbrain but is restricted (along the rostral-caudal axis) to the medial one-third, where almost 2-fold variations are found. The volume of the striatum, a major projection field of midbrain dopamine neurons, is 20% smaller in CBA/J than in BALB/cJ mice; the difference is regional and is concentrated in the caudal half. Because the packing density of intrinsic neurons of the striatum is similar in both strains, CBA/J mice contain 20% fewer neurons than do BALB/cJ mice. The activities of TryHase and of choline acetyltransferase (ChoAcTase; acetyl-CoA:choline-O-acetyltransferase, EC 2.3.1.6) in the whole striatum of CBA/J mice are less than in BALB/cJ. The strain-dependent differences in midbrain TyrHase activity are due to variations in the number of dopamine neurons and directly correlate with differences in the number of striatal cholinergic neurons. There is genetic control of the number of neurons of a neurochemically specific class in the mammalian brain.

Genetic studies of daily variations of first-step enzymes of monoamines metabolism in the brain of inbred strains of mice and hybrids. I. Daily variations of tryptophan hydroxylase activity in the nuclei raphe dorsalis, raphe centralis and in the striatum

A daily variation of tryptophan hydroxylase (TrH) activity was observed in the raphe dorsalis (RD), raphe centralis (RC) and striatum (St) of 3 inbred strains of mice (BALB/c, C57BL/6, C57BR) and of the reciprocal hybrids obtained from Balb/c and C57BL6. Significant differences of the characteristics of these rhythms have been found in the same strain between different structures and for the same structure between different strains. In RD and RC hybridization led to less defined daily variations which, in the striatum, remained well synchronized and could be controlled by a dominant genetic mechanism part. These results help to discuss evidence for selective control mechanisms of regulation responsible for daily variation of TrH in the 5-HT cell bodies and terminals and their relative independence.

Genetic studies of daily variations of first-step enzymes of monoamines metabolism in the brain of inbred strains of mice and hybrids. II. Daily variations of tyrosine hydroxylase activity in the locus coeruleus

Daily variations of tyrosine hydroxylase (TH) activity in the locus coeruleus of 3 inbred strains of mice (BALB/c; C57BL6; C57Br) and the F1 hybrids obtained from BALB/c and C57BL/6 are discussed. Precise characteristics of the circadian rhythms were observed in each strain. They were found significantly different in two genetically pure parents (BALB/c and C57BL6). In their two F1 hybrids the daily variation of TH activity was similar to that observed in one of the parents (C57BL6). This strongly suggests selective and genetically controlled mechanisms of regulation responsible for the daily variation of TH activity in the locus coeruleus of mice.

Urinary excretion of catecholamines and metabolites in the Lyon strain of genetic hypertensive rats

Urinary catecholamines (Norepinephrine : NE, Epinephrine : E) methoxylated amines (Metanephrine : MN, Normetanephrine : NMN) and 3-methoxy-4-hydroxyphenylglycol (MHPG) have been studied in the Lyon hypertensive strain of rats (LHS) and their normotensive controls (LNS) at three ages (5, 9 and 21 weeks) characteristic of the development of hypertension. In 5-week-old rats, increased excretion of E, MN, NMN and early elevation of the ratio E/E+NE were observed, thus suggesting a sympatho-adrenal hyperactivity, with accelerated maturation of E secretion. At adult age, the excretion of most adrenergic metabolites was similar in the two strains of rats. However, the data from adult LHS are difficult to interpret because of a potential renal impairment at this age. Furthermore, there was a positive correlation between systolic blood pressure and urinary catecholamines at 5 weeks and 9 weeks of age but not at 21 weeks. It is suggested that the early sympathoadrenal hyperactivity, which disappears with age, may be related to the onset of hypertension in the Lyon strain of hypertensive rats.

Strain differences in amphetamine sensitivity in mice. II. Overcompensation of paradoxical sleep after deprivation in two C57 strains

d-Methylamphetamine (1--4 mg/kg) induced wakefulness for 3--5 h followed by paradoxical sleep (PS) rebound in C57BR, C57BL/6, and BALB/c mice. Slow-wave sleep (SWS) and PS suppression time were proportional to the given dose and different among strains. It was also dependent on the time of injection. Later PS rebound was significant in C57 strains, and in certain cases greater than PS loss, whereas rebound was small in BALB/c mice and PS loss was not completely compensated.

Regional acetylcholine turnover rates in the brains of three inbred strains of mice: correlation with some interstrain behavioural differences

The hypothesis that the genetically determined behavioural differences which exist between the inbred mouse strains Balb/c, DBA/2 and C57Bl/6 may be related to differences in acetylcholine metabolism in certain regions of the brain has been tested. In vivo ACh turnover rates have been measured in three regions (hippocampus, caudate nucleus and frontal-parietal cortex) of the brains of each strain by following the rate of formation of labelled ACh, in these regions, after a pulse intravenous injection of a tracer dose of 3H labelled choline. Focused microwave procedures were used for the rapid fixation of brain tissue and Ch and ACh radioactivities were determined following their electrophoretic separation. Steady-state concentrations of Ch and ACh were measured by a sensitive radio-enzymatic method. Significant interstrain differences in ACh turnover rates are reported for each of the brain regions studied with the order of metabolic activity being Balb/c greater than DBA/2 greater than C57 Bl/6 in each case. These results are interpreted as being in agreement with previous reports on correlations between learning ability or locomotor activity and regional activities of choline acetyltransferase in the brains of these inbred strains. The correlations between the in vivo ACh turnover rates and (1) interstrain differences in behavioural measures and (2) regional choline acetyltransferase activities are discussed.