Stereological estimation of the total number of neurons in the murine hippocampus using the optical disector.

Using a stereological method, the optical disector, we examined three inbred strains of mice (NZB/BINJ, DBA/2, and C57BL/6J) for morphological differences in volume, neuronal number, and density of the pyramidal cell and dentate gyrus granule cell layers of the hippocampus. We found significant differences in volume and neuronal number for both regions between the three strains at 9 weeks of age, but only modest differences in neuronal density. The left dentate volume was 90% larger in the NZB strain and 70% greater in the DBA strain (P<0.0001), and the left pyramidal cell layer was 144% larger in the NZB strain and 150% larger in the DBA strain, than in the B6 strain (P<0.0001). Neuron number in the left dentate was 81% greater in NZB and 37% greater in DBA (P<0.001), and in the left pyramidal cell layer 118% greater in the NZB and 92% greater in the DBA (P<0.01). Differences in neuronal density of the left dentate were not significant (P = 0.060, ns). For the left pyramidal cell layer, neuronal density was 14% greater in B6 and 34% greater in NZB than the DBA strain (P = 0.016). No significant differences were found in left-right laterality, or according to sex. We found that strain accounted for 60% of the variance in hippocampal volume and 44% of neuron number. These differences thus mainly reflect genetic variation in hippocampal volume and may have important implications for brain evolution, behaviour, and human diseases where hippocampal degeneration is involved.     

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.     

Strain background influences neurotoxicity and behavioral abnormalities in mice expressing the tetracycline transactivator

The tet-off system has been widely used to create transgenic models of neurological disorders including Alzheimer's, Parkinson's, Huntington's, and prion disease. The utility of this system lies in the assumption that the tetracycline transactivator (TTA) acts as an inert control element and does not contribute to phenotypes under study. Here we report that neuronal expression of TTA can affect hippocampal cytoarchitecture and behavior in a strain-dependent manner. While studying neurodegeneration in two tet-off Alzheimer's disease models, we unexpectedly discovered neuronal loss within the dentate gyrus of single transgenic TTA controls. Granule neurons appeared most sensitive to TTA exposure during postnatal development, and doxycycline treatment during this period was neuroprotective. TTA-induced degeneration could be rescued by moving the transgene onto a congenic C57BL/6J background and recurred on reintroduction of either CBA or C3H/He backgrounds. Quantitative trait analysis of B6C3 F2 TTA mice identified a region on Chromosome 14 that contains a major modifier of the neurodegenerative phenotype. Although B6 mice were resistant to degeneration, they were not ideal for cognitive testing. F1 offspring of TTA C57BL/6J and 129X1/SvJ, FVB/NJ, or DBA/1J showed improved spatial learning, but TTA expression caused subtle differences in contextual fear conditioning on two of these backgrounds, indicating that strain and genotype can interact independently under different behavioral settings. All model systems have limitations that should be recognized and mitigated where possible; our findings stress the importance of mapping the effects caused by TTA alone when working with tet-off models.     

Comparison of seizure phenotype and neurodegeneration induced by systemic kainic acid in inbred, outbred, and hybrid mouse strains

We assessed inbred, outbred and hybrid mouse strains for susceptibility to seizures and neurodegeneration induced by systemic administration of kainic acid (KA). Each strain showed a unique pattern of susceptibility to seizures as assessed by the dose necessary to induce continuous tonic clonic seizures, progression through six seizure levels, the number of mice that failed to satisfy seizure criteria, and seizure-induced mortality. In general, the C57BL/6, ICR, FVB/N, and BALB/c strains were resistant to seizures while the C57BL/10, DBA/2 J, and F1 C57BL/6*CBA/J strains were vulnerable. Neuronal cell death was quantified in four subfields of the hippocampus: CA3, the hilus of the dentate gyrus, CA1, and the dentate granule cell layer. Neurodegeneration was also semiquantitatively assessed in other brain regions including the neocortex, striatum, thalamus, hypothalamus and amygdala. Although there was variability in the extent of cell death within strains, there were significant differences in the amount of hippocampal cell death between strains and also different patterns of neurodegeneration in affected brain areas. In general, the C57BL/6, C57BL/10, and F1 C57BL/6*CBA/J strains were resistant to neurodegeneration while the FVB/N, ICR and DBA/2 J strains were vulnerable. The BALB/c strain was unique in that neurodegeneration was confined to the hippocampus. Consistent with previous findings, the resistant neurodegeneration phenotype was dominant in an F1 cross of resistant and vulnerable inbred strains. Our results, using a large number of mouse strains, definitively demonstrate that a mouse strain's seizure phenotype is not related to its neurodegeneration phenotype.     

Testosterone manipulation protects motoneurons from dendritic atrophy after contralateral motoneuron depletion

Dendritic morphology is reactive to many kinds of injuries, including axotomy and deafferentation. In this study, we examined the response of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), an androgen-dependent population of motoneurons in the lumbar spinal cord of the rat, to partial motoneuron depletion. We depleted SNB motoneurons on one side only of the spinal cord by unilateral intramuscular injection of a retrogradely transported form of saporin, and examined the morphology of contralateral SNB motoneurons. Motoneuron morphology was assessed in normal control males, gonadally intact saporin-treated males, and saporin-treated males who had been castrated 6 weeks previously and given testosterone replacement beginning at the time of saporin injection. Untreated castrated males served as an additional control group. Four weeks after saporin treatment, SNB motoneurons contralateral to the saporin injection were retrogradely labeled with horseradish peroxidase conjugated to the cholera toxin B subunit and reconstructed in three dimensions. In gonadally intact males, unilateral motoneuron depletion caused regressive changes in contralateral SNB motoneurons: Soma size and dendritic length were both decreased. However, testosterone manipulation (i.e., castration followed by testosterone replacement) completely prevented the dendritic retraction. These data suggest a therapeutic role for testosterone in preventing, or accelerating recovery from, dendritic atrophy induced by motoneuron injury.     

Differential effects of morphine on histamine metabolism in brain and spinal cord of mice

The effects of morphine on the levels of histamine (HA), its metabolite tele-methylhistamine (t-MH) and on t-MH synthesis rates (thought to be indicative of neuronal HA release) were investigated in brain regions and spinal cords of DBA/2J (DBA) and C57/BL6 (C57) mice, two strains known to differ in their sensitivity to morphine. In DBA (a strain highly sensitive to morphine antinociception), morphine (10 mg/kg, s.c.) had no effect on brain regional t-MH or HA levels, but produced a generalized inhibition of regional t-MH synthesis rates ranging from 11 to 53%. The monoamine oxidase (MAO) inhibitor pargyline (used to estimate t-MH synthesis rates) had no effect on HA or t-MH levels in the DBA or C57 spinal cord, indicating the absence of detectable spinal HA turnover. Morphine (10 mg/kg) had no effect on DBA or C57 spinal cord HA or t-MH levels, but significantly increased t-MH synthesis in the DBA but not in the C57 spinal cord. These results suggest that in DBA mice, antinociceptive doses of morphine inhibit HA release in brain, and promote the release of HA from spinal cord. Neither effect was found in C57 mice, which are resistant to morphine antinociception. The relevance of these actions to previous studies showing the blockade of opiate antinociception by H2 antagonists remains to be established.     

5-HT2 receptor binding and 5-HT uptake in mouse brain: developmental changes and the relationship to audiogenic seizure susceptibility in DBA/2J mice

Abnormal function of serotonergic neurones may be involved in the age-related susceptibility of DBA/2J mice to generalised convulsions induced by auditory stimulation. We have measured 5-HT2 receptor binding sites and synaptosomal 5-HT uptake in 5 brain regions of DBA/2J mice at ages before, during and after their maximal susceptibility to audiogenic seizures and in age-matched C57 B1/6 mice, a strain resistant to audiogenic seizures at all ages. The number of 5-HT2 binding sites was 20% higher in the cerebral cortex of DBA/2J than C57 B1/6 mice at the time of maximal susceptibility of DBA/2J mice to audiogenic seizures but did not differ at other ages. The number of 5-HT2 binding sites did not differ between the two strains at the ages studied in forebrain, mid-brain, hippocampus and pons-medulla. A marked reduction in the number of 5-HT2 binding sites was apparent in the mid-brain, hippocampus and pons-medulla of both strains of mice between 13-15 days of age and 21-23 days of age. Synaptosomal 5-HT uptake did not differ significantly between DBA/2J and C57 B1/6 in any of the brain regions at the ages studied. The higher density of cortical 5-HT2 binding sites in DBA/2J mice may contribute to their susceptibility to sound-induced seizures.     

Functional analysis of neurovascular adaptations to exercise in the dentate gyrus of young adult mice associated with cognitive gain

The discovery that aerobic exercise increases adult hippocampal neurogenesis and can enhance cognitive performance holds promise as a model for regenerative medicine. This study adds two new pieces of information to the rapidly growing field. First, we tested whether exercise increases vascular density in the granular layer of the dentate gyrus, whole hippocampus, and striatum in C57BL/6J mice known to display procognitive effects of exercise. Second, we determined the extent to which new neurons from exercise participate in the acute neuronal response to high levels of running in B6D2F1/J (F1 hybrid of C57BL/6J female by DBA/2J male). Mice were housed with or without a running wheel for 50 days (runner vs. sedentary). The first 10 days, they received daily injections of BrdU to label dividing cells. The last 10 days, mice were tested for performance on the Morris water maze and rotarod and then euthanized to measure neurogenesis, c‐Fos induction from running and vascular density. In C57BL/6J, exercise increased neurogenesis, density of blood vessels in the dentate gyrus and striatum (but not whole hippocampus), and enhanced performance on the water maze and rotarod. In B6D2F1/J, exercise also increased hippocampal neurogenesis but not vascular density in the granular layer. Improvement on the water maze from exercise was marginal, and no gain was seen for rotarod, possibly because of a ceiling effect. Running increased the number of c‐Fos positive neurons in the granular layer by fivefold, and level of running was strongly correlated with c‐Fos within 90 min before euthanasia. In runners, ～3.3% (±0.008 S.E.) of BrdU‐positive neurons in the middle of the granule layer displayed c‐Fos when compared with 0.8% (±0.001) of BrdU‐negative neurons. Results suggest that procognitive effects of exercise are associated with increased vascular density in the dentate gyrus and striatum in C57BL/6J mice, and that new neurons from exercise preferentially function in the neuronal response to running in B6D2F1/J. © 2009 Wiley‐Liss, Inc.     

GABA-gated chloride ion influx and receptor binding studies in C57BL6J and DBA2J mice

GABA-gated chloride ion influx was measured in brain 'microsac' preparations of young (20-22-day-old) and older (40-42-day-old) C57BL6J and DBA2J mice. The young DBA2J mice are susceptible to audiogenic seizures. GABA sensitivity was reduced in young DBA2J mice as compared to age-matched C57BL6J mice or older mice of either strain. Age and strain differences in ligand binding to GABA/benzodiazepine receptor complex and glutamate receptor could not account for this finding. These results provide evidence for a defect in GABA-gated chloride ion influx in audiogenic seizure-susceptible DBA2J mice.     

Synaptic changes in the perineal motoneurons of aged male rats

Cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles of young (2 months of age) and old (19–20 months of age) male rats, and animals were killed 2 days later. The spinal cords containing the spinal nucleus of the bulbocavernosus (SNB) were dissected, processed with a modified tetramethylbenzidine method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 120 CT-HRP–labeled SNB motoneurons were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts, synaptoid contacts, and neuron-neuron contacts. Most of the neuronal structures in the young and old SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic membranes covered by synapses in old rats was significantly smaller than that in young ones. Size and number of synaptic contacts per unit length of somatic membranes in old animals were also significantly reduced. Plasma levels of testosterone in old males were significantly smaller than those in young ones. These age-related changes in synaptic inputs to SNB motoneurons and plasma levels of androgen seem to correlate with aging of the SNB system. J. Comp. Neurol. 400:103–109, 1998. © 1998 Wiley-Liss, Inc.     

C57BL/6J and DBA/2J mice differ in extinction and renewal of extinguished conditioned fear

While a number of studies have examined the acquisition and expression of conditioned fear in inbred mice, very few have examined extinction of conditioned fear in inbred mice and few attempts have been made to compare extinction learning between inbred strains. Because inbred strains differ in a number of physiological and biochemical variables, differences in extinction learning may provide insight into the genetic influence of extinction learning. The purpose of this study was to examine extinction and renewal of conditioned fear in two common inbred strains of mice. C57BL/6J and DBA/2J mice were conditioned with pairings of either a tone or light and foot shock in a single session. On the following 4 days, mice were given extinction training, consisting of tone or light alone trials (Experiment 1A). C57 mice exhibited robust spontaneous recovery between sessions, but did extinguish both within and between sessions. DBA mice extinguished more quickly relative to C57 mice, and this extinction was stable between sessions (i.e., DBA mice did not exhibit spontaneous recovery). The rapid loss of fear in DBA relative to C57 mice was extinction-dependent and not merely due to poor long-term memory (Experiment 1B). Renewal testing (Experiment 2) replicated the strain difference in extinction and also showed that DBA mice have a deficit in the context specificity of extinction. C57 mice, but not DBA mice showed renewal of extinguished fear when tested in a context different from the one in which extinction training took place. These data suggest that the nature of extinction learning is influenced by characteristics of the inbred mouse strain.     

Steroid regulation of calcitonin gene-related peptide mRNA expression in motoneurons of the spinal nucleus of the bulbocavernosus

Motoneurons express calcitonin gene-related peptide (CGRP). Previous studies have shown that CGRP immunoreactivity is regulated by testosterone in the androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB). In this research the effect of plasma levels of testosterone on the expression of alpha CGRP mRNA in the SNB motoneurons of adult male rats was studied with in situ hybridization. The number of motoneurons expressing alpha CGRP mRNA and the level of alpha CGRP mRNA expression was significantly higher in the SNB of castrated male rats than in the SNB of gonadally intact rats. Using a 5x background labeling criterion in castrated rats 88.1 +/- 4.5% while in intact rats 75.3 +/- 6.4% of SNB motoneurons expressed alpha CGRP mRNA. Testosterone replacement at the time of castration prevented the effect of castration on the expression of alpha CGRP mRNA in SNB motoneurons. In castrated rats, the increase in the number of SNB cells expressing CGRP was the result of increased steady state levels of alpha CGRP mRNA in all SNB neurons.     

Murine strain differences in taste responsivity and organization of the rostral nucleus of the solitary tract

Taste responsivity and organization of fungiform papillae, geniculate ganglion neurons and gustatory recipient zones of the nucleus of the solitary tract (NST) were examined in C57BL/6NCrlBR (C57) mice, BALB/c6NCrlBR (BALB/c) mice and CB6F1/CrlBr (CB6) mice, an F1 hybrid cross between BALB/c and C57 mice. Results from behavioral studies confirm that C57 and CB6 mice exhibit higher preferences to sucrose and lower preferences to NaCl, as compared to BALB/c mice. No strain differences were confirmed for aversion responses to citric acid or quinine HCl taste stimuli. Anatomical analyses show that the number and organization of fungiform papillae do not reliably differ between C57, BALB/c, and CB6 mice, nor do volumes of glossopharyngeal terminal fields in the NST. However, strain-specific differences exist in the number of neurons contained in the geniculate ganglion, volume of chorda tympani (CT) terminal fields in the rostral NST, and number of NST neurons contained in CT terminal fields. BALB/c and CB6 mice possess a greater number of geniculate ganglion neurons and larger CT terminal fields, as compared to C57 mice. However, strain differences in the number of geniculate ganglion neurons and terminal field volume are not obviously correlated with strain differences in gustatory responsivity. The only reliable relationship confirmed between taste responsivity and neuroanatomical organization of the rostral NST relates to the absolute number of neurons contained in CT terminal fields, and corresponding neuronal density within CT terminal fields. Chorda tympani terminal fields of C57 and CB6 mice contain an average of 379 neurons, whereas CT terminal fields of BALB/c mice contain an average of 531 neurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dissociation of seizure traits in inbred strains of mice using the flurothyl kindling model of epileptogenesis

Previous seizure models have demonstrated genetic differences in generalized seizure threshold (GST) in inbred mice, but the genetic control of epileptogenesis is relatively unexplored. The present study examined, through analysis of inbred strains of mice, whether the seizure characteristics observed in the flurothyl kindling model are under genetic control. Eight consecutive, daily generalized seizures were induced by flurothyl in mice from five inbred strains. Following a 28-day rest period, mice were retested with flurothyl. The five strains of mice demonstrated inter-strain differences in GST, decreases in GST across seizure trials, and differences in the behavioral seizure phenotypes expressed. Since many of the seizure characteristics that we examined in the flurothyl kindling model were dissociable between C57BL/6J and DBA/2J mice, we analyzed these strains in detail. Unlike C57BL/6J mice, DBA/2J mice had a lower GST on trial 1, did not demonstrate a decrease in GST across trials, nor did they show an alteration in seizure phenotype upon flurothyl retest. Surprisingly, [C57BL/6J × DBA/2J] F1-hybrids had initial GST on trial 1 and GST decreases across trials similar to what was found for C57BL/6J, but they did not undergo the alteration in behavioral seizure phenotype that had been observed for C57BL/6J mice. Our data establish the significance of the genetic background in flurothyl-induced epileptogenesis. The [C57BL/6J × DBA/2J] F1-hybrid data demonstrate that initial GST, the decrease in GST across trials, and the change in seizure phenotype differ from the characteristics of the parental strains, suggesting that these phenotypes are controlled by independent genetic loci.     

Androgen regulates synaptic input to motoneurons of the adult rat spinal cord

Adult male rats (Sprague-Dawley) were castrated and implanted subcutaneously with Silastic capsules containing testosterone or nothing. Sham-castrated males served as controls. Four weeks following castration, cholera toxin-horseradish peroxidase (CT-HRP) was injected bilaterally into the bulbocavernosus muscles and animals were sacrificed 2 d later. The spinal cords containing the spinal nucleus of the bulbocavernosus (SNB) were dissected, processed with a modified tetramethylbenzidine (TMB) method for visualization of retrogradely transported CT-HRP, and examined at the ultrastructural level. Neuronal structures apposing the membranes of 150 TMB-labeled SNB neurons were analyzed by measuring the percentage of somatic and proximal dendritic membranes covered by synaptic contacts, synaptoid contacts, and neuron-neuron contacts. Most of the neuronal structures in the control and experimental SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic and proximal dendritic membranes covered by synapses 4 weeks after castration was reduced to approximately 30% of those in control animals. However, treatment with testosterone for 4 weeks after castration prevented this decline. Castration and testosterone treatment also influenced the size and number of synaptic contacts per unit length of somatic and proximal dendritic membranes, and the incidence of neuron-neuron contacts and double synapses onto SNB motoneurons. These results indicate that androgen is critical for maintaining the organization of synaptic inputs to these spinal motoneurons in adult male rats.     

Heterosis for brain cerebroside synthesis in mice

The (C57BL/6J X DBA/2J) F1 or B6D2F1 hybrid mice are known to exhibit a transitory hypermyelinating activity compared with their parental strains B6 and D2. These mice exhibit an elevated accumulation of cerebrosides in the brain which can be explained by an increase in their synthesis. Analysis of the two major cerebroside species indicated that the elevated content of total cerebrosides in the cerebellum and cerebrum of B6D2F1, as well as D2B6F1 hybrids, reflected an increased accumulation of the hydroxylated species. The UDP-galactose:ceramide galactosyltransferase (CgalT) activities in B6, D2 and hybrid mice were studied using as substrates alpha-hydroxy fatty acid (HFA)-containing ceramides or normal fatty acid (NFA)-containing ceramides (HFA- and NFA-CgalT activities, respectively). Both CgalT activities were found to be about 2 times higher in the cerebellum than in the cerebrum for all the strains studied. Before 20 days of age, the HFA-CgalT activities in B6D2F1, D2B6F1 and D2 mice were higher than in B6. However, at 20 days, there was no difference between B6 and D2 while the HFA-CgalT activity in the hybrids remained about 20% higher than in the parental strains. In contrast, no strain differences could be detected for the NFA-CgalT activity at all ages. The data suggest that the increased synthesis of brain cerebrosides in the B6D2F1 and D2B6F1 strains of mice could be largely accounted for by an increased HFA-CgalT activity.     

Correlation between choline acetyltransferase activity and learning ability in different mice strains and their offspring

The enzymes related to acetylcholine metabolism were investigated in several cortical regions of 3 inbred strains of mice (SEC, DBA and C57) differing in learning ability, and in their F1 offsprings. A higher choline acetyltransferase (ChAc) activity was found in the temporal lobe of mice with high learning ability (SEC and DBA) than in C57 mice which score poorly. The genetic behavioural analysis conducted on the F1 offsprings of these strains¹⁸ demonstrated that F1 hybrids of SEC and C57 crosses were able to reach the same high learning levels as the parental SEC strain. In contrast, the DBA × C57 offsprings performed as poorly as the C57 strain which had low levels of performance. We showed that ChAc activity in the temporal lobe of the SEC × C57 hybrids was close to that of the SEC strain and in the F1 offspring of DBA × C57 crosses, near the values of the parental line C57.It was suggested that these differences may constitute a neurochemical correlate of the differences in learning abilities of these mice strains.     

Total RNA content of brain cells of inbred mice reared under different conditions.

AB/Jena, DBA/2Jena, C57B1/6 Jena inbred mice and ABD2F1- and B6D2F4-hybrid mice either were reared communally and weaned at day 30 or were reared by isolated mothers and weaned at day 21 (I21). On day 50 the total RNA content of pyramidal brain cells of male offspring was determined by cytophotometry. The studied brain areas were the frontal pole of cerebral cortex (FCC) and the CA1 region of the dorsal hippocampus (DHI). The DBA, C57 and ABD2 mice had a reduced FCC and DHI RNA-content under I21-rearing. Under these conditions the AB strain has the highest RNA-content in both substrates. The results call for the awareness of possible genetic influences when studying the effects of environmental enrichment/impoverishing.     

Home cage activity and behavioral performance in inbred and hybrid mice

Locomotor activity is a key component in many behavioral tests, suggesting that genetic differences in activity levels may be a critical consideration when comparing mouse strains. In order to assess the relationship between activity and performance, we recorded home cage activity, and locomotion and defecation, a non-activity-linked behavior, in tests of anxiety in inbred (C57BL/6J (B6), n = 25; BALB/cJ (C), n = 24; DBA/2J (D2), n = 28) and hybrid (CB6F1/6J (CB6: B6 x C) n = 19) mice. Under our test conditions, the strains showed significant differences in home cage activity levels: C > B6 > D2. The CB6 mice were similar to the B6 mice in horizontal activity and were intermediate between the parental strains in vertical movement. Based on measures of locomotion and defecation in the open field, emergence and novel object tests, and the elevated zero maze, the C mice appeared to be the most anxious and the B6 were the least anxious. The D2 mice were intermediate on some measures but more similar to B6 mice on others, making ranking them more difficult. In addition, the CB6 mice displayed characteristics of both parental strains. They had greater similarity to B6 mice in measures of horizontal movement in the home cage and locomotion in the open field and emergence tests, but exhibited defecation responses similar to those of C mice in the novel object test and elevated zero maze. The results suggest that strain differences in spontaneous locomotion should be considered when interpreting strain differences in behavioral tests, and that home cage activity may be a useful interpretive aid.     

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.