Postnatal maturation of cortical serotonin lateral asymmetry in gerbils is vulnerable to both environmental and pharmacological epigenetic challenges

Epigenome Defines Aberrant Brain Laterality in Major Mental Illnesses

Brain-hemisphere asymmetry/laterality is a well-conserved biological feature of normal brain development. Several lines of evidence, confirmed by the meta-analysis of different studies, support the disruption of brain laterality in mental illnesses such as schizophrenia (SCZ), bipolar disorder (BD), attention-deficit/hyperactivity disorder (ADHD), obsessive compulsive disorder (OCD), and autism. Furthermore, as abnormal brain lateralization in the planum temporale (a critical structure in auditory language processing) has been reported in patients with SCZ, it has been considered a major cause for the onset of auditory verbal hallucinations. Interestingly, the peripheral counterparts of abnormal brain laterality in mental illness, particularly in SCZ, have also been shown in several structures of the human body. For instance, the fingerprints of patients with SCZ exhibit aberrant asymmetry, and while their hair whorl rotation is random, 95% of the general population exhibit a clockwise rotation. In this work, we present a comprehensive literature review of brain laterality disturbances in mental illnesses such as SCZ, BD, ADHD, and OCD, followed by a systematic review of the epigenetic factors that may be involved in the disruption of brain lateralization in mental health disorders. We will conclude with a discussion on whether existing non-pharmacological therapies such as rTMS and ECT may be used to influence the altered functional asymmetry of the right and left hemispheres of the brain, along with their epigenetic and corresponding gene-expression patterns.

Age- and sex-dependent effects of methamphetamine on cognitive flexibility and 5-HT2C receptor localization in the orbitofrontal cortex of Sprague-Dawley rats

Adolescents and females experience worse outcomes of drug use compared to adults and males. This could result from age- and sex-specific consequences of drug exposure on brain function and cognitive behavior. In the current study, we examined whether a history of intravenous methamphetamine (METH) self-administration impacted cognitive flexibility and 5-HT_2CR localization in the orbitofrontal cortex (OFC) in an age- and sex-dependent manner. Strategy shifting was assessed in male and female Sprague-Dawley rats that had self-administered METH (0.08 mg/kg/inf) or received non-contingent infusions of saline during periadolescence or young adulthood. After all rats reached adulthood, they were tested in an operant strategy shifting task and their brains were subsequently analyzed using immunofluorescence to quantify co-localization of 5-HT_2C receptors with parvalbumin interneurons in the OFC. We found that adolescent-onset females were the only group impaired during discrimination and reversal learning, but they did not exhibit changes in localization of 5-HT_2C receptors. In contrast, adult-onset males exhibited a significant increase in co-localization of 5-HT_2C receptors within parvalbumin interneurons in the left hemisphere of the OFC. These studies reveal that age and sex differences in drug-induced deficits in reversal learning and 5-HT_2CR co-localization with parvalbumin interneurons are dissociable and can manifest independently. In addition, these data highlight the potential for certain treatment approaches to be more suitable in some populations compared to others, such as alleviating drug-induced cognitive deficits as a focus for treatment in adolescent females.

Cerebral correlates of visual lateralization in Sepia

The common cuttlefish, Sepia officinalis (cephalopod mollusc) has recently become a relevant model for studying the setting-up of brain asymmetry among invertebrates. As the animals age from 3 to 30 days post hatching, they progressively develop a left-turning bias resulting from an eye-use preference. The aim of this study is to investigate whether anatomical (vertical, peduncle, inferior buccal, and optic lobes) or neurochemical (monoamines in optic lobes) brain asymmetries are present in the cuttlefish brain at 3 or at 30 post hatching days; and whether these correlate with side-turning preferences. We here find brain and behavioral asymmetry only at 30 post hatching days. Cuttlefish displayed a significant population bias towards a larger right peduncle lobe, and higher monoamine concentration in the left optic lobe (i.e. serotonin, dopamine and noradrenaline). None of these brain asymmetries were correlated to the studied side-turning bias. However, we found individual variation in the magnitude of the vertical and optic lobes asymmetry. A striking correlation was found with the behavioral results: the larger the right optic lobe and the right part of the vertical lobe, the stronger the bias to turn leftwards. To our knowledge, this is the first study to demonstrate a relationship at the individual level between brain and behavioral asymmetries in invertebrates.

Hemispheric differences in basilar dendrites and spines of pyramidal neurons in the rat prelimbic cortex: activity- and stress-induced changes

Pyramidal neurons of the rat medial prefrontal cortex have been shown to react to chronic stress by retracting their apical dendrites and by spine loss. We extended these findings by focusing on the basilar dendritic tree of layer III pyramidal neurons in both hemispheres of the rat prelimbic cortex. Animals were subjected to daily restraint stress for 1 week (6 h/day), during either the resting or the activity period. The morphology of basilar dendrites and spines of Golgi-Cox-stained neurons in the left and right hemispheres was digitally reconstructed and analyzed. We observed the following: (i) there was an inherent hemispheric asymmetry in control rats during the resting period: the number of spines on proximal dendrites was higher in the left than in the right hemisphere; (ii) basal dendrites in controls displayed a diurnal variation: there was more dendritic material during the resting period than in the activity period; (iii) chronic stress reduced the length of basal dendrites in only the right prelimbic cortex; (iv) chronic stress reduced spine density on proximal basal dendrites; (v) restraint stress during the activity period had more pronounced effects on the physiological stress parameters than restraint stress during the resting period. Our results show dynamic hemisphere-dependent structural changes in pyramidal neurons of the rat prelimbic cortex that are tightly linked to periods of resting and activity. These morphological alterations reflect the capacity of the neurons to react to external stimuli and mirror presumptive changes in neuronal communication.

Disruption of orbitofrontal cortex laterality in offspring from multiplex alcohol dependence families

BACKGROUND

Increased susceptibility for developing alcohol dependence (AD) might be related to structural differences in brain circuits that influence the salience of rewards and/or modify the efficiency of information processing. The role of the orbitofrontal cortex (OFC) in regulating emotional processing is increasingly being recognized along with its association with impulsive behavior.

METHODS

Magnetic resonance imaging was used to measure the OFC in 107 high- and low-risk offspring (mean age 17.6 +/- 4.69 years) from either multiplex AD families or control families. Region of interest measures including segmented values were obtained by reliable raters using BRAINS2 software. Statistical analyses were adjusted for intracranial volume, age, socioeconomic status (SES), IQ, and handedness. The Multidimensional Personality Questionnaire (MPQ) was administered to determine scale scores for Control. Genotyping was performed for the serotonin transporter (5-HTT) gene and the brain-derived neurotrophic factor (BDNF) gene.

RESULTS

High-risk offspring from multiplex for AD families showed decreased right/left OFC volumes in comparison with control subjects. Smaller volume in the right hemisphere was significantly associated with variation in the 5-HTT and BDNF genes. White matter (WM) ratios showed a positive correlation with MPQ Control scale scores, indicating that reduced OFC WM is related to greater impulsivity.

CONCLUSIONS

Offspring from multiplex families for AD manifest genetic susceptibility by exhibiting disruption in the laterality of the OFC volume that is related to greater impulsivity (lower Control scale scores). This disruption in OFC laterality is related to variation in genes associated with neuronal growth.

Effects of social isolation rearing on the limbic brain: a combined behavioral and magnetic resonance imaging volumetry study in rats

Rearing rats in social isolation from weaning induces robust behavioral and neurobiological alterations resembling some of the core symptoms of schizophrenia, such as reduction in prepulse inhibition of acoustic startle (PPI) and locomotor hyperactivity in a novel arena. The aim of this study was to investigate whether social isolation rearing induces volumetric remodeling of the limbic system, and to probe for anatomical structure-behavioral interrelations. Isolation- (n=8) and group-reared (n=8) rats were examined by magnetic resonance (MR) volumetry using high-resolution T2-weighted imaging at 7 T. Volumes of medial prefrontal cortex (mPFC), anterior cingulate cortex (ACC), retrosplenial cortex (RSC) and hippocampal formation were compared between groups and with behavioral measures, i.e. PPI and locomotor activity in a novel arena. Isolation rearing induced locomotor hyperactivity and impaired PPI compared with group-housed rats. The right mPFC was significantly reduced (5.4%) in isolation-reared compared with group-reared rats, with a similar trend on the left side (5.2%). mPFC volumes changes were unrelated to behavioral abnormalities. No significant volume changes were observed in ACC, RSC or hippocampal formation. Hippocampal volumes were associated with the magnitude of PPI response in control but not in isolation-reared rats. Rearing rats in social isolation induced remodeling of the limbic brain with selective prefrontal cortex volume loss. In addition, a dissociation of the interrelation between hippocampal volume and PPI was noted in the isolation-reared rats. Taken together, limbic morphometry is sensitive to the effects of social isolation rearing but did not reveal direct brain-behavior interrelations, calling for more detailed circuitry analysis.

Early preweaning methamphetamine and postweaning rearing conditions interfere with the development of peripheral stress parameters and neural growth factors in gerbils

Adrenal steroid hormones and neuronal growth factors are two interacting systemic factors that mediate the environment's influence on the brain's structure and function. In order to further elucidate their role and relationship in the effects of early stressful experience and isolated rearing (IR), this study measured blood corticosterone titres and relative adrenal weights and assessed nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) concentrations in brain regions of both hemispheres of young adult Mongolian gerbils injected on postnatal day 14 with a single high dose of methamphetamine (MA) or saline and raised after weaning either in an enriched or an impoverished environment. Irrespective of MA challenge, IR decreased corticosterone titres to about half, but increased relative adrenal weights. BDNF concentrations were decreased by IR in saline-injected animals in the left prefrontal and parietal cortices and right entorhinal and hippocampal cortices, and in the subcortical regions of both hemispheres. NGF concentrations were unaltered by IR in saline-injected animals, but increased in MA challenged animals in the entorhinal/hippocampal cortices and subcortical areas of both hemispheres. MA application induced shifts of the lateral asymmetry in NGF contents in prefrontal and entorhinal cortices. The results suggest that an early pharmacological traumatization can set a switch for further brain development, and that growth factor concentrations might possibly be influenced by peripheral stress hormones.

Administration of oral methylphenidate during adolescence prevents suppressive development of dopamine projections into prefrontal cortex and amygdala after an early pharmacological challenge in gerbils

The enduring effects of postweaning subchronic methylphenidate (MP) treatment and/or previous early preweaning methamphetamine (MA) application on dopamine (DA) fiber density were investigated in multiple cortical and subcortical areas of the gerbil brain. The study aimed to explore three questions: (1) is the development of DA fiber innervation in control animals sensitive to a clinically relevant subchronic treatment with MP? (2) Is the development of DA fiber innervation in the forebrain altered by a single early MA challenge? (3) If so, might the subsequent institution of a therapeutically relevant MP application scheme interfere with such early induced alternative developmental trajectories for DA fiber innervation? For this purpose, gerbils pretreated both with saline and MA (50 mg/kg, i.p.) on day 14 received either H(2)O or MP (5 mg/kg) orally on days 30 to 60. On day 90, DA fibers were immunohistochemically detected and quantified. As a result, MP on its own did not have any significant influence on the postnatal development of the DA fiber systems, whereas it prevented a previously MA triggered suppressive development of DA fiber innervation in the prefrontal cortex and amygdala complex (30% less fiber innervation in both areas). Thus, MP prevented previously initiated miswiring of DA fibers from actually being implemented in the gerbil forebrain. During earlier studies, rather complex miswiring has been documented in response to an early preweaning MA challenge. This miswiring was associated with functional deficits resembling some of the symptoms of patients with ADHD. Therefore, morphogenetic properties of MP need further attention.

Developmental pattern changes of prefrontal efferents in the juvenile gerbil (Meriones unguiculatus)

Previous findings of our group showed that early traumatisation leads to a dysfunctional organisation of prefrontocortical efferents in adulthood. To identify vulnerable time windows during maturation, we labelled either layer III- or layer V/VI-pyramidal cells with biocytin in the prefrontal cortex of gerbils (Meriones unguiculatus) from the age of postnatal day (PD) 15 up to adulthood (PD 90). The density of passing fibres and axonal terminals in distinct cortical columns in specific prefrontal projection areas was assessed by digital image analysis. Following layer III injections, fibre densities reached adult values between adolescence (PD 60) and adulthood (PD 90). However, layer V/VI-fibre densities decreased after eye-opening (PD 15), followed by an increase to adult values after weaning (PD 30). These findings are the first to describe dynamic structural changes even beyond adolescence of functionally diverse prefrontal output systems. External interventions might exert adverse influences on the establishment of integrated prefrontal networks especially during the early phase of re-arranging.

Transmitter balances in the olfactory cortex: adaptations to early methamphetamine trauma and rearing environment

The olfactory cortex, comprising the anterior olfactory cortex (AOC) and the anterior piriform cortex (PirC), is a model system for the study of neural plasticity. We investigated the structural imbalances of different transmitter systems induced in this area by an early traumatisation (methamphetamine [MA] intoxication) and/or environmental deprivation (isolated rearing [IR]), with the working hypothesis that such alterations will not occur in an isolated fashion, but in mutual interaction. Indeed, acetylcholine fibre density is increased by IR in both hemispheres of the PirC (left: +22%, p<0.01, right: +21%, p<0.05) and the left hemisphere of the AOC (+13%, p<0.05), while an early MA intoxication increases it in afterwards enriched-reared animals in the PirC (+14%/+17%, p<0.05), but decreases it in the AOC (-18%/-22%, p<0.001). The serotonin fibre density is increased by IR in the right PirC of saline-treated (+13%, p<0.01), but not of MA-traumatised gerbils. GABA and dopamine in the AOC show an inverse correlation, with dopamine innervation density being increased by IR (+30%, p<0.001) and MA (+26%, p<0.01), and GABA neuropil density being reduced. Furthermore, switches in hemispheric laterality occur in the AOC. These results demonstrate the complex recursive interactions in structural cortical plasticity.

Contralateral prefrontal projections in gerbils mature abnormally after early methamphetamine trauma and isolated rearing

As previously shown, a miswiring of ipsilateral prefrontal projections after methamphetamine (MA) intoxication and/or isolated rearing (IR) may serve as a model of so-called "dysconnection" in human schizophrenia. We here find that deep prefrontal projections to contralateral targets were drastically reduced by both MA and IR alone, but remained equally dense if both impairments cumulated. Projections from superficial layers were not altered by MA and/or IR. These findings confirm that the normal intercortical integration of information is compromised in this animal model of schizophrenia.

Long-term effects of a single adult methamphetamine challenge: minor impact on dopamine fibre density in limbic brain areas of gerbils

BACKGROUND

The aim of the study was to test long-term effects of (+)-methamphetamine (MA) on the dopamine (DA) innervation in limbo-cortical regions of adult gerbils, in order to understand better the repair and neuroplasticity in disturbed limbic networks.

METHODS

Male gerbils received a single high dose of either MA (25 mg/kg i.p.) or saline on postnatal day 180. On postnatal day 340 the density of immunoreactive DA fibres and calbindin and parvalbumin cells was quantified in the right hemisphere.

RESULTS

No effects were found in the prefrontal cortex, olfactory tubercle and amygdala, whereas the pharmacological impact induced a slight but significant DA hyperinnervation in the nucleus accumbens. The cell densities of calbindin (CB) and parvalbumin (PV) positive neurons were additionally tested in the nucleus accumbens, but no significant effects were found. The present results contrast with the previously published long-term effects of early postnatal MA treatment that lead to a restraint of the maturation of DA fibres in the nucleus accumbens and prefrontal cortex and a concomitant overshoot innervation in the amygdala.

CONCLUSION

We conclude that the morphogenetic properties of MA change during maturation and aging of gerbils, which may be due to physiological alterations of maturing vs. mature DA neurons innervating subcortical and cortical limbic areas. Our findings, together with results from other long-term studies, suggest that immature limbic structures are more vulnerable to persistent effects of a single MA intoxication; this might be relevant for the assessment of drug experience in adults vs. adolescents, and drug prevention programs.

The maturation of the acetylcholine system in the dentate gyrus of gerbils (Meriones unguiculatus) is affected by epigenetic factors

The current study investigated the influence of impoverished rearing (IR) conditions and a single early methamphetamine challenge (MA; 50 mg/kg i.p.) on day 14 on the postnatal maturation of acetylcholinesterase-positive (AChE+) fibres in the hippocampal dentate gyrus (DG) of gerbils (Meriones unguiculatus). The layer-specific densities of histochemically stained AChE+ fibres were quantified in two planes of the left and right DG in young adults (day 90). Compared to enriched reared (ER) animals, the AChE+ fibre densities turned out to be higher in both the septal and the temporal plane of both hemispheres in saline treated IR and MA treated ER gerbils. The temporal plane was slightly more affected than the septal plane. In IR animals, MA treatment selectively diminished the AChE+ fibre densities in the subgranular layer of both left and right temporal DG. In conclusion, the maturation of AChE+ fibres is vulnerable to both rearing conditions and early MA challenge. The results correlate with our previous studies on the dentate cell proliferation rates and the serotonergic innervation, two parameters which are similarly affected by the experimental design. Thus, disturbances of the ACh system may impair the hippocampal plasticity and hippocampus-related cognitive and emotional function.

Isolation rearing or methamphetamine traumatisation induce a "dysconnection" of prefrontal efferents in gerbils: implications for schizophrenia

A miswiring of prefrontal efferents is generally discussed by the name of "dysconnection" as the anatomical substrate of schizophrenia. Since direct histological confirmation of this hypothesis can hardly be obtained in humans, we used an animal model of schizophrenia to trace prefrontal efferents to distal cortical fields. Mongolian gerbils were intoxicated with a single high dose of methamphetamine on postnatal day 14 and reared in isolation after weaning (day 30). Controls received a saline injection and/or were reared under enriched conditions. Upon reaching adulthood (day 90), biocytin was injected into the medial prefrontal cortex into either deep or superficial laminae. The density of passing fibres and terminal fields in the frontal, parietal and insular cortices was assessed by digital image analysis. Isolation rearing or methamphetamine treatment alone reduced the projections from lamina V/VI to the frontal and from lamina III to the insular cortex, and from both laminae to the parietal cortex. In contrast, isolation rearing of methamphetamine-intoxicated gerbils significantly increased the projections from the deep laminae to the frontal and parietal cortices, compared to isolation-reared controls, with no difference in the efferents from superficial laminae. These results are the first to demonstrate a miswiring of prefrontal efferents in response to adverse systemic influences. They might give a hint at the anatomical basis of "dysconnection" in schizophrenia.

Hemisphere-specific effects on serotonin but not dopamine innervation in the nucleus accumbens of gerbils caused by isolated rearing and a single early methamphetamine challenge

The aim of this study was twofold: We examined whether serotonin (5-HT) and dopamine (DA) innervations of the nucleus accumbens are lateralised and whether the environment or the combination with an early pharmacological impact might interfere with the postnatal maturation of the monoaminergic innervation. Male gerbils were assigned to either enriched rearing (ER) or isolated rearing (IR). Animals from both rearing conditions additionally received a single dose of either methamphetamine [MA (50 mg/kg ip)] or saline on postnatal day 14. DA and 5-HT fibres of the adult animals (postnatal day 90-110) were immunocytochemically stained and fibre densities were quantified in nucleus accumbens core and shell of both the left and right hemisphere. Our data demonstrate that the DA and 5-HT innervation is not lateralised in saline-treated animals of both rearing conditions. IR increases the DA fibre density in both hemispheres of saline controls, whereas an additional MA treatment reverses this effect. In both ER and IR groups, MA provokes an excessive 5-HT fibre in growth of only the right hemisphere. The combination of IR with MA induces right-side asymmetries of the 5-HT fibre density in both the core and shell. From the data obtained, we conclude that the maturation of the monoaminergic innervation of the nucleus accumbens is vulnerable to postnatal stimuli. The subtle "innervation imbalance" observed in our studies is consistent with previously reported effects in other brain regions of this animal model and may be causative for behavioural disturbances.