A genetic study of neostriatal cholinergic neurones in C57BL/6 and DBA/2 mice

In the present study Butcher's pharmaco-histochemical technique for acetylcholinesterase has been employed for a morphometrical analysis of striatal cholinergic neurones in crossbred C57BL/6 x DBA/2 F1 mice. The general organization of neostriatal cholinergic systems in hybrid mice was similar to that of parent strains. However, as shown by morphometry, the size of neostriatum in hybrids was larger than that of both parental strains, and the density of striatal cholinergic neurones was significantly lower than that of DBA/2 mice, being close to that of the C57BL/6 strain. The present data indicate that a reduced number of striatal cholinergic neurones is inherited as a dominant trait by these hybrid mice.     

Spatial learning and memory, maze running strategies and cholinergic mechanisms in two inbred strains of mice

The acquisition process of the radial maze task was studied in two inbred strains of mice, C57BL/6 and DBA/2. A quantitative and qualitative evaluation of performance was performed and the pretest level of activity was measured. The results showed a significant correlation between activity and performance since the highly active C57BL/6 mice exhibited better performance of the radial maze task than the less active DBA/2 mice. Moreover, for correct trials, strain-dependent maze-running strategies were observed: while both strains displayed about the same percentage of clockwise and spatial strategies, it was observed that among the spatial strategies C57BL/6 used a larger number of different correct solutions. Subsequently, the effect of scopolamine administration on working memory processes was assessed in sequential and discrete trials. A different reactivity of each strain to anti-cholinergic treatment was found in discrete trials since only DBA/2 mice were impaired. The effect of scopolamine is discussed in relation to the different models of information processing involved in learning and memorizing the experimental rule.     

Genotypic variation in striatal calmodulin content

CBA/J and BALB/cJ mice have quantitative differences in the nigrostriatal projection. The number of nigral tyrosine hydroxylase reactive neurons, nigral and striatal tyrosine hydroxylase activity and the density of striatal D-2 dopamine receptors are all less in the CBA/J compared to the BALB/cJ mouse. An unrelated strain, the C57BL/6J, has a striatal D-2 dopamine receptor density that is intermediate to that of CBA/J and BALB/cJ mice. CBA/J mice also show deficits in the ability of brain monoaminergic receptor systems to develop supersensitivity. Calmodulin may participate in several striatal dopaminergic receptor mechanisms. Thus, striatal calmodulin was examined in CBA/J, C57BL/6J and BALB/cJ mice. Striatal calmodulin was greater in CBA/J mice than in C57BL/6J or BALB/cJ. In all three strains, cerebral cortical calmodulin was similar. The percent distribution of total striatal calmodulin between soluble and particulate fractions was similar in the three strains. Calcium redistributed soluble striatal calmodulin into the particulate fraction and EGTA shifted calmodulin from the particulate into the soluble fraction. The percent of total striatal calmodulin redistributed by either treatment was similar in all three strains. Gel filtration chromatography of heat-treated soluble extracts from CBA/J and BALB/cJ striatum was similar in elution pattern, although more calmodulin was observed in extracts from the CBA/J. Possible mechanisms for the strain differences in calmodulin are discussed along with their relationship to strain differences in striatal dopamine receptor subtypes.     

N(2)O stimulates NOS enzyme activity in C57BL/6 but not DBA/2 mice

Exposure to 70% N(2)O produces a prominent antinociception in C57BL/6 mice but not DBA/2 mice. N(2)O exposure also increases conversion of [14C]L-arginine to [14C]L-citrulline in homogenates prepared from whole brains of C57BL/6 mice; there was no such increase in NOS activity in the DBA/2 whole brain. A differential N(2)O effect on brain NOS in these inbred strains might explain why the C57BL/6 but not DBA/2 mice are responsive to N(2)O antinociception.     

Neural mechanisms distinguishing the neocortical EEG of C57BL/6 mice from that of DBA/2 mice

C57BL/6 inbred mice lack the 1-5 sec bursts of 6-7 cps spindles characteristic of the neocortical EEG of DBA/2 mice during waking. C57BL/6 mice (1) may be unable to generate any synchronized cortical EEG activity, (2) may lack the thalamocortical circuitry required to generate these brief spindle episodes (BSEs), (3) may lack mechanisms that can activate this circuitry or (4) may possess a potent mechanism to suppress BSE initiation and generation. Possibilities 1 and 2 have been eliminated because C57BL/6 mice generate pentobarbital, rostropontine-induced and sleep spindles, and because certain C57BL/6 sleep spindles resembled the BSEs seen in DBA/2 mice. Possibilities 3 and 4 were examined in the experiments reported here. In DBA/2 mice, pentylenetetrazol activates BSEs at subconvulsant doses. In contrast, neither 20 nor 50 mg/kg, IP, pentylenetetrazol activated BSEs in C57BL/6 mice, although the higher dose provoked 4-5 cps slow waves and myoclonic jerks. In DBA/2 mice, the beta-noradrenergic antagonist propranolol has been reported to powerfully release BSEs. In C57BL/6 mice, 10 and 15 mg/kg propanolol weakly released BSEs; fewer than 3 per hour occurred. Hence neither possibilities 3 and 4 are sufficient in themselves to explain the lack of BSEs during waking in C57BL/6 mice. However, simultaneous administration of 10 mg/kg propranolol and 20 mg/kg pentylenetetrazol provoked numerous BSEs in C57BL/6 mice. This suggests that perhaps C57BL/6 mice, as compared to DBA/2 mice, possess both a more powerful noradrenergic mechanism to suppress spindles and a more weakly functioning mechanism to activate BSEs. Hence possibilities 3 and 4 may both be correct.(ABSTRACT TRUNCATED AT 250 WORDS)     

Developmental differences of antinociceptive effects of oxotremorine in two inbred strains of mice

During development the C57BL/6 and DBA/2 mouse strains present morphological variations in cholinergic forebrain structures correlated with different behavioral reactivities to cholinergic agents. The present research assessed that these strain-dependent differences are also present in cholinergic-mediated analgesia. The administration of oxotremorine (0.0025, 0.005 and 0.01 mg/kg) to 30- and 60-day-old C57 and DBA mice resulted in dose- age- and strain-dependent analgesia. In particular oxotremorine is more effective in DBA/2 than in C57BL/6 mice and the latter strain showed a significant decrease of analgesic response in adulthood.     

Prenatal development of the hippocampus in two strains of inbred mice

In this paper we present some morphological observations on the morphogenesis of the hippocampus in two inbred strains of mice, C57BL/6J (C57) and DBA/2J (DBA), from day 16 (E16) to day 17 (E17) of prenatal life. At E16 some differences in the histoarchitecture of the hippocampal anlage begin to be detectable. At E17 the differences become more relevant, particularly those concerning the lamination pattern. The more relevant differences concern the differentiation of the pyramidal layer, which appears well defined in C57 mice, while in DBA mice there is no clearcut delimitation between the pyramidal layer and the intermediate zone, where numerous migrating neurons are present. These observations may be interpreted in terms of different migration rates for the neurons in the two strains.     

Distinct patterns of Fos expression induced by systemic amphetamine in the striatal complex of C57BL/6JICo and DBA/2JICo inbred strains of mice

Mice from the inbred strains C57BL/6 and DBA/2 are characterized by striking differences in their behavioral response to addictive drugs. We used Fos expression as a tool to reveal strain differences in the postsynaptic effects of amphetamine (AMPH; 2.5 mg/kg) within the nucleus accumbens (NAc) (core and shell) and the dorsal caudate (dorsomedial and dorsolateral). AMPH stimulated Fos expression in all striatal regions of mice from both strains. However, while C57BL/6 showed a higher Fos response than DBA/2 mice in both NAc shell and core, the opposite was true for the dorsolateral caudate. The effects of AMPH were prevented by D1 blockade in all striatal regions of both strains and mimicked by the D1 agonist, SKF82958 (0.1 mg/kg), in both regions of the caudate and in the NAc shell, but not in the core. Our results suggest that the functional heterogeneity of the striatal complex is under genetic control and that this control may implicate DA transmission and corticostriatal interactions.     

Association between striatal accumulation of FosB/ΔFosB and long-term psychomotor sensitization to amphetamine in mice depends on the genetic background

Previous results demonstrated association between increased FosB/ΔFosB immunostaining in the ventromedial striatum and behavioral sensitization to amphetamine promoted by repeated stress or by repeated pairings of the psychostimulant and the testing cage in mice of the C57BL/6J strain. The present experiments tested this association in an additional protocol, its stability following the end of the sensitizing procedure and its generalization to mice from a different inbred strain. Eleven days after repeated administration of amphetamine within their home-cages, mice of the C57BL/6J strain expressed sensitization to the psychomotor effects of the psychostimulant when tested in a novel cage. At this time-point the same mice showed increased FosB/ΔFosB immunostaining in the ventromedial striatum. Instead, mice of the genetically unrelated DBA/2J inbred strain expressing robust sensitization in the same protocol did not show changes in FosB/ΔFosB immunostaining throughout the striatal complex. Lack of effects in FosB/ΔFosB immunostaining was also observed in DBA/2J mice behaviorally sensitized by repeated pairings of amphetamine with the test cage. These results demonstrate that mice, depending on the genetic background, can develop robust and long-lasting behavioral sensitization to amphetamine in the absence of striatal ΔFosB accumulation.     

Effects of genetic background and environmental novelty on wheel running as a rewarding behaviour in mice

Recent studies suggest running wheel activity to be naturally rewarding and reinforcing; considering the shared neuro-behavioural characteristics with drug-induced reward situations, wheel running behaviour gains interest as a tool to study mechanisms underlying reward-sensitivity. Previously, we showed that wheel running has the potential to disrupt the daily organization of home cage behaviour in female C57BL/6 [de Visser L, van den Bos R, Spruijt BM. Automated home cage observations as a tool to measure the effects of wheel running on cage floor locomotion. Behav Brain Res 2005;160:382-8]. In the present study, we investigated the effects of novelty-induced stress on wheel running and its impact on home cage behaviour in male C57BL/6 and DBA/2 mice. Our aim was to determine whether wheel running may be used as a tool to study both genetic and environmentally induced differences in sensitivity to rewarding behaviour in mice. One group of male mice was placed in an automated home cage observation system for 2 weeks with a wheel integrated in the cage. A second group of mice was allowed to habituate to this cage for 1 week before a running wheel was introduced. Results showed a pronounced sensitising effect of novelty on the level of wheel running in C57Bl/6 mice but not in DBA mice. Overall levels of wheel running were higher in DBA/2 mice. Furthermore, wheel running affected circadian rhythmicity in DBA/2 mice but not in C57BL/6 mice. From these findings we tentatively suggest that wheel running behaviour could serve as a tool to study the interaction between genetic and environmental factors in sensitivity to rewarding behaviour in mice. As it is displayed spontaneously and easy to monitor, wheel running may be well suitable to be included in high-throughput phenotyping assays.     

Differences in basal levels of CREB and NPY in nucleus accumbens regions between C57BL/6 and DBA/2 mice differing in inborn alcohol drinking behavior

An increasing body of evidence suggests that genetic factors play a role in alcohol drinking behaviors. C57BL/6J (C57) mice innately consume larger amounts of alcohol compared to that consumed by DBA/2J (DBA) mice. Furthermore, alterations in cAMP-responsive element binding (CREB) protein function in the brain have been implicated in alcohol drinking behaviors. The present investigation examined innate expression and phosphorylation of CREB in various brain structures of C57 and DBA mice. It was found that CREB expression and phosphorylation was lower, specifically in the shell structure of the nucleus accumbens, in C57 mice compared to that in DBA mice. CREB expression and phosphorylation were similar in other brain regions such as the nucleus accumbens core and the cortical, amygdaloid, hippocampal, and striatal structures of C57 and DBA mice. The expression of a cAMP-inducible gene, neuropeptide Y (NPY), was also investigated in the nucleus accumbens region of C57 and DBA mice. It was found that in C57 mice, NPY protein levels were lower in the shell but not in the core structure of the nucleus accumbens compared to that in DBA mice. It was also found that C57 mice are not innately anxious, but they consume larger amounts of alcohol than do DBA mice. Because the shell structure of the nucleus accumbens has been implicated in reward mechanisms of alcohol, it is possible that lower CREB function in this brain structure may be in part associated with the excessive alcohol drinking behavior of C57 mice.     

FOS and FOSB expression in the medial preoptic nucleus pars compacta of maternally active C57BL/6J and DBA/2J mice

C57BL/6J and DBA/2J inbred mice differ in aspects of maternal behavior and in the morphology of the medial preoptic nucleus (MPO), suggesting a possible association. DBA/2J mice have a compact subnucleus in the MPO, the MPOpc, that is sexually dimorphic and absent in C57BL/6J mice. To determine whether MPOpc cells are activated by maternal behavior, FOS and FOSB immunohistochemistry was performed on brain sections of C57BL/6J and DBA/2J mothers following the return of their pups after a separation of 2 days. In both light and dark phases of the daily cycle, stimulation of DBA/2J mothers evoked an increase in FOS- and FOSB-immunoreactivity in the MPOpc. Stimulated C57BL/6J mice, which lack the MPOpc, did not show an increase in cellular activity in the corresponding MPO region. Cells immediately lateral to the MPOpc were activated by pup stimulation, in both strains. These results suggest that MPOpc cells are active during maternal behavior, and that strain differences in maternal behavior are related to anatomical differences in the MPO.     

Long-term potentiation persistence greater in C57BL/6 than DBA/2 mice: Predicted on basis of protein kinase C levels and learning performance

The gamma isoform of protein kinase C (γ-PKC) activity is elevated and learning is superior in the inbred C57BL/6 mouse when compared to the DBA/2 mouse strain. Given the proposed link between PKC and long-term potentiation (LTP) on the one hand and PKC and learning on the other, it was predicted that LTP persistence would be greater in C57BL/6 mouse. When suprathreshold levels of tetanic stimulation were used, similar persistent LTP was observed in both C57BL/6 and DBA/2 strains. However, when tetanus was at threshold, the response in DBA/2 mice decayed to baseline in 30 min. similar to short-term potentiation (STP). Using this same paradigm with C57BL/6 mice. LTP persisted for 4 h, the longest time tested. The time course of the results parallels those observed in rat when phorbol ester, a potent PKC activator, converts STP to LTP. The present findings thus confirm the predicted difference between the two mouse strains. Moreover, the present findings are consistent with a role for γ-PKC in LTP. Since such results call attention to the need for γ-PKC interventive procedures, the relative utility of current PKC inhibitors, null mutants and antisense methods are discussed.     

Mouse strain- and age-dependent effects of binge methamphetamine on dopaminergic signaling

We have shown that a single "binge" dose of methamphetamine (Meth) in mice has long-lasting effects on open-field behavior dependent on mouse strain and age. Here we further investigated the impact of genotype and age on tyrosine hydroxylase (TH) loss and dopamine (DA) metabolism due to a high binge dose of Meth (4 × 5 mg/kg × 2 h × 2 days). Administration of high dose Meth or saline (Sal) to adolescent (PND 40) and adult (PND 80) C57BL/6 (B6), DBA/2 (DBA), and 129S6SvEv/Tac (129) mice was followed by a 1mg/kg Meth or Sal (control) challenge 40 days later. Striatal and prefrontal cortex tissues were collected 1h following the challenge. Meth-pretreated adolescent B6 and DBA mice exhibited losses in striatal DA concentrations; DBA adolescents also showed losses in striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and increased DA turnover. Pre-exposed B6 and 129 adults demonstrated significant decreases in striatal DA, DOPAC, and increased DA turnover; DBA adults showed significant losses in striatal DA and increased DA turnover. 129 and DBA adults exhibited increases and decreases, respectively, in prefrontal cortex DA. Adult pretreated B6 mice produced significant losses in striatal TH. The results again show age and genotype dependent differences in Meth-induced DA alterations.     

Mouse strain differences in locomotor,sensitisation and rewarding effect of heroin; association with alterations in MOP‐r activation and dopamine transporter binding

There is growing agreement that genetic factors play an important role in the risk to develop heroin addiction, and comparisons of heroin addiction vulnerability in inbred strains of mice could provide useful information on the question of individual vulnerability to heroin addiction. This study examined the rewarding and locomotor‐stimulating effects of heroin in male C57BL/6J and DBA/2J mice. Heroin induced locomotion and sensitisation in C57BL/6J but not in DBA/2J mice. C57BL/6J mice developed conditioned place preference (CPP) to the highest doses of heroin, while DBA/2J showed CPP to only the lowest heroin doses, indicating a higher sensitivity of DBA/2J mice to the rewarding properties of heroin vs C57BL/6J mice. In order to investigate the neurobiological substrate underlying some of these differences, the effect of chronic ‘intermittent’ escalating dose heroin administration on the opioid, dopaminergic and stress systems was explored. Twofold higher μ‐opioid receptor (MOP‐r)‐stimulated [³⁵S]GTPγS binding was observed in the nucleus accumbens and caudate of saline‐treated C57BL/6J mice compared with DBA/2J. Heroin decreased MOP‐r density in brain regions of C57BL/6J mice, but not in DBA/2J. A higher density of dopamine transporters (DAT) was observed in nucleus accumbens shell and caudate of heroin‐treated DBA/2J mice compared with heroin‐treated C57BL/6J. There were no effects on D₁ and D₂ binding. Chronic heroin administration decreased corticosterone levels in both strains with no effect of strain. These results suggest that genetic differences in MOP‐r activation and DAT expression may be responsible for individual differences in vulnerability to heroin addiction.     

Effect of Milacemide on Audiogenic Seizures and Cortical (Na+, K+)-ATPase of DBA/2J Mice

Milacemide (MLM, CP 1552 S, 2-N-pentylaminoacetamide), a glycinamide derivative, is currently being evaluated clinically for antiepileptic activity. Anticonvulsant properties have been shown in various animal models, but the mechanism of action of MLM is unclear. We studied its activity in audiogenic seizures of DBA/2J mice. MLM was effective in inhibiting the convulsions induced by sound with a biphasic dose-effect relation. The ED50 was 109 mg/kg orally against tonic extension. Higher doses were necessary to abolish clonic convulsion and running response. Because impaired cerebral (Na+, K+)-ATPase activity is supposed to play a role in epileptogenesis, we tested MLM on in vitro cortical enzymatic activity of DBA/2J mice. Basal (Na+, K+)-ATPase activity was unchanged by several concentrations of MLM in normal C57BL/6J and audiogenic DBA/2J mice. K+ activation (from 3 to 18 mM) of (Na+, K+)-ATPase is abolished in DBA/2J mice as compared with C57BL/6J mice, suggesting impaired glial (Na+, K+)-ATPase. In the presence of MLM (from 30 to 1000 mg/L), cortical (Na+, K+)-ATPase of DBA/2J mice is activated by high concentrations of K+, as in C57BL/6J mice. Results suggest that the antiepileptic activity of MLM in audiogenic mice may be secondary to an activation of a deficient glial (Na+, K+)-ATPase.     

Synaptic plasticity in the hippocampus of awake C57BL/6 and DBA/2 mice: interstrain differences and parallels with behavior.

C57BL/6 mice consistently outperform DBA/2 mice in a range of hippocampal-dependent spatial learning behaviors. We recorded evoked responses from the dentate gyrus of awake, freely-moving mice and measured synaptic plasticity (LTP) and performance in a hippocampal-dependent task in individual animals from these two inbred strains. Spatial alternation tasks confirmed the behavioral divergence between the two strains, with C57BL/6 mice demonstrating more robust alternation than DBA/2 mice. Recording changes in field potentials in the dentate gyrus following three different high-frequency stimulation paradigms in the same groups of animals revealed differences in neural plasticity: both strains were able to support long-term potentiation (LTP) at perforant path synapses, but brief high-frequency stimulation induced larger and longer potentiation of the population spike in C57BL/6 than in DBA/2 mice. This greater propensity for population-spike potentiation in the strain that performed better in a hippocampal-dependent task is in accord with the different neurochemical profiles of C57BL/6 and DBA/2 mice.     

Cross-tolerance between D-amino acids and morphine in mice

The effects of morphine administration on analgesia and running activity were studied in DBA/2 (DBA) and C57BL/6 (C57) mice respectively, injected withd-amino acids (d-AA) for 5 days. The results indicate a clear cross-tolerance betweend-AA and morphine in DBA mice when analgesia is considered, while the stimulating effect of the opiate is not modified afterd-AA pretreatment in the C57 strain.     

Sex-specific hemispheric differences in cortical activation to a bimodal odor

C57BL/6 and DBA/2 were compared in the 5-choice serial reaction time task for differences in performance related to attention and impulsivity. The goal was to examine behavioural processes in mice that may relate to ADHD in humans. Groups of male mice were trained to nose-poke in response to a stimulus light presented randomly in one of five holes; correct responses were reinforced with food. During training the stimulus duration (SD) was reduced progressively from 60 to 0.5 s. The C57BL/6 and DBA/2 mice did not differ during early stages of training when attentional demands were low (SD of 60, 10 or 5 s). As task demands increased, strain differences emerged; C57BL/6 mice were more accurate than DBA/2 mice with stimuli of 2, 1 and 0.5 s. DBA/2 mice also made more anticipatory (impulsive) responses during inter-trial intervals than C57BL/6 mice at SD of 5, 2, 1 and 0.5 s. The ability to carry out the task was present in both strains of mice but they differed significantly in the levels of performance that were achieved. It is argued that the differences in accuracy and anticipatory responding were closely related and that the primary difference between the strains may be in impulsivity.     

Genes on distal chromosome 18 determine vulnerability to excitotoxic neurodegeneration following status epilepticus, but not striatal neurodegeneration induced by quinolinic acid

Previous studies have provided evidence that a quantitative trait locus (QTL) on the distal part of chromosome 18 (chr18) is a major determinant of vulnerability to hippocampal neurodegeneration following kainic acid (KA)-induced seizures in inbred mouse strains. We assessed excitotoxic vulnerability in two congenic, “genome tagged” mouse strains carrying segments of either distal or proximal/medial chr18 from vulnerable DBA/2J mice on a resistant C57BL/6 background. Systemic KA injections triggered brain-wide neurodegeneration in the distal chr18 congenic strain, and specifically in the hilus of the dentate gyrus, but not in CA3. In contrast, the proximal/medial chr18 congenic strain exhibited enhanced degeneration in CA1 and CA3, but little neurodegeneration elsewhere. Both strains exhibited low levels of QUIN-induced striatal neurodegeneration comparable to what is seen in C57BL/6 mice. These results suggest that gene(s) on distal chr18 are important determinants of vulnerability to KA-induced hippocampal neurodegeneration, but not QUIN-induced striatal neurodegeneration.