Differences in extracellular glutamate levels in striatum of rats with high and low exploratory activity

Active vs passive novelty-related strategies: Sex differences in exploratory behaviour and monoaminergic systems

Sex differences are apparent in numerous behavioural characteristics. In order to compare and characterise male and female variability of exploratory behaviour, 365 male and 401 female rats were assessed in a task where a bimodal response distribution had previously been established in males. Female rats had significantly higher exploratory activity, and presented normal distribution of the behaviour, very differently from the bimodal distribution of males. No major effect of litter or oestrous cycle was detected. Several differences between male and female rats were found in monoamine metabolism measured ex vivo. Male rats had lower levels of dopamine (DA) in frontal cortex, and higher levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in raphe area; higher levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in dorsal striatum but lower levels of 5-HT and 5-HIAA in locus coeruleus area, 5-HIAA levels were also lower in hippocampus as compared to females. Males had higher noradrenaline (NA) levels in hippocampus and lower normetanephrine (NMN) levels in striatum, in both brain regions male animals had lower NMN/NA ratio. No sex difference was found in accumbens. The only brain region with an interaction between sex and the expression of exploratory activity was raphe: Here 5-HT levels were lower, and DOPAC levels and DOPAC/DA and 5-HIAA/5-HT ratios higher in low exploring male but not female rats. Conclusively, female rats not only display higher levels of exploration but the population distribution of this behaviour is distinct; this may be related to differences in the monoaminergic systems between female and male animals.

Aspalathus linearis infusion affects hole-board test behaviour and amino acid concentration in the brain

Rooibos tea, brewed using Aspalathus linearis leaves, is a popular South African herbal infusion, but its everyday intake is not fully described in terms of the neuropsychopharmacological outcomes. The cell-protective activity of A. linearis is connected with the ability of reducing glycaemia, inflammation as well as oxidative stress. It was already shown that "fermented" rooibos herbal tea (FRHT), which is rich in phenolic compounds, improves the cognitive performance of rats in the water maze and impacts dopaminergic striatal transmission. The present research was taken to extend the knowledge about the feasible behavioural and neurochemical implications of sustained oral FRHT consumption. We hypothesized that it might affect brain amino acid content and thus induce behaviour and neuroprotection. FRHTs of different leaf to water ratios (1:100, 2:100 and 4:100), analysed by chromatographic methods as regards their flavonoid characteristics, were given to rats as only liquid for 3 months. Their behaviour was evaluated in the hole-board test (HBT). Brain amino acids concentration was analysed in the striatum, hippocampus and prefrontal cortex by HPLC-ECD. The rats drinking rooibos tea presented increased motor activity defined as time spent on moving in the HBT. Their exploration measured by head-dipping and rearing was enhanced. Longer time of the testing-box central zone occupation indicated to reduction in anxiety-related behaviour. Excitatory amino acids (aspartate and glutamate) content was decreased in the striatum of animals drinking the infusions whereas taurine level was increased both in the striatum and hippocampus. In conclusion we suggest that long-term FRHT intake affects exploration and anxiety-related behaviour of the rats as well as exerts biochemical outcomes in the brain that support the neuroprotective impact of rooibos tea.

Cocaine-induced changes in behaviour and DNA methylation in rats are influenced by inter-individual differences in spontaneous exploratory activity

BACKGROUND

Individual differences in behavioural traits influence susceptibility to addictive disorders. Drug addiction involves changes in gene expression, proposed to occur via DNA methylation (DNAm).

AIMS

To investigate DNAm changes in reward-related brain structures (nucleus accumbens (NAc), lateral habenula (LHb)) in response to cocaine exposure in rats differing in spontaneous exploratory activity.

METHODS

Rats were observed in the exploration box and categorised as high- (HE) or low explorers (LE). Rats were administered vehicle or cocaine (12 mg/kg, i.p.) for 7 days, followed by a 14-day withdrawal period and cocaine challenge (7 mg/kg); horizontal locomotor activity was recorded. Brain tissue was dissected after 24 h; we analysed messenger RNA (mRNA) and activity levels of epigenetic DNA modifiers (DNMTs and TETs) as well as mRNA and promoter methylation levels at selected genes previously linked to addictive behaviours.

RESULTS

The cocaine challenge dose stimulated locomotor activity in both LE- and HE rats only when administered after a repeated cocaine schedule, suggesting development of behavioural sensitisation. Quantitative polymerase chain reaction analyses demonstrated higher basal expression of Dnmt3a, Tet2 and Tet3 in the LHb of HE- vs. LE rats, and we observed differential effects of cocaine exposure on the expression and activity of epigenetic DNA modifiers in the NAc and LHb of HE- and LE rats. Furthermore, cocaine exposure differentially altered promoter methylation levels of A_2AR, Ppp1cc, and Taar7b in the NAc and LHb of HE- and LE rats.

CONCLUSIONS

DNAm might play a role in the HE- and LE phenotypes as well as mediate behavioural effects of LE- and HE rats in response to drugs of abuse.

Anticonvulsants lamotrigine and riluzole disrupt maternal behavior in postpartum female rats

Maternal behavior is a highly motivated and well-organized social behavior. Previous studies have reported that anticonvulsants are frequently used in postpartum bipolar disorder. However, the maternal disruptive effect of the anticonvulsants has not been explored. The purpose of the present study was to examine the effect of anticonvulsants lamotrigine and riluzole on maternal behavior in postpartum female rats. On postpartum Day 3, Sprague-Dawley mother rats were given a single intraperitoneal injection of vehicle, lamotrigine (15, 25, 35 mg/kg), or riluzole (2, 4, 8 mg/kg). Maternal behavior was tested 30 min before and after injection. Animals treated with lamotrigine or riluzole had a longer pup retrieval latency, retrieved fewer pups into the nest, spent less time on nursing pups, as well as on building the disturbed nest, and animals treated with riluzole spent less time on pup licking. Whereas, the drugs in the tested doses did not shorten the total duration of behavior unrelated to maternal behavior. Overall, these data indicate that lamotrigine and riluzole disrupt major components of maternal behavior in postpartum female rats, but do not inhibit the behaviors unrelated to maternal behavior, which indicates that the maternal disruptive effect is not due to nonspecific sedative effect.

Impaired novelty acquisition and synaptic plasticity in congenital hyperammonemia caused by hepatic glutamine synthetase deficiency

Genetic defects in ammonia metabolism can produce irreversible damage of the developing CNS causing an impairment of cognitive and motor functions. We investigated alterations in behavior, synaptic plasticity and gene expression in the hippocampus and dorsal striatum of transgenic mice with systemic hyperammonemia resulting from conditional knockout of hepatic glutamine synthetase (LGS-ko). These mice showed reduced exploratory activity and delayed habituation to a novel environment. Field potential recordings from LGS-ko brain slices revealed significantly reduced magnitude of electrically-induced long-term potentiation (LTP) in both CA3-CA1 hippocampal and corticostriatal synaptic transmission. Corticostriatal but not hippocampal slices from LGS-ko brains demonstrated also significant alterations in long-lasting effects evoked by pharmacological activation of glutamate receptors. Real-time RT-PCR revealed distinct patterns of dysregulated gene expression in the hippocampus and striatum of LGS-ko mice: LGS-ko hippocampus showed significantly modified expression of mRNAs for mGluR1, GluN2B subunit of NMDAR, and A1 adenosine receptors while altered expression of mRNAs for D1 dopamine receptors, the M1 cholinoreceptor and the acetylcholine-synthetizing enzyme choline-acetyltransferase was observed in LGS-ko striatum. Thus, inborn systemic hyperammonemia resulted in significant deficits in novelty acquisition and disturbed synaptic plasticity in corticostriatal and hippocampal pathways involved in learning and goal-directed behavior.

Middle-range exploratory activity in adult rats suggests higher resilience to chronic social defeat

OBJECTIVE

Stressful life events play an important role in the aetiology of human mood disorders and are frequently modelled by chronic social defeat (SD) in rodents. Exploratory phenotype in rats is a stable trait that is likely related to inter-individual differences in reactivity to stress. The aim of the study was to confirm that low levels of exploratory activity (LE) are, in rodents, a risk factor for passive stress coping, and to clarify the role of medium (ME) and high (HE) exploratory disposition in the sensitivity to SD.

METHODS

We examined the effect of SD on male Wistar rats with LE, ME, and HE activity levels as measured in the exploration box. After SD, the rats were evaluated in social preference, elevated zero maze, and open-field tests. Brain tissue levels of monoamines were measured by high-performance liquid chromatography.

RESULTS

Rats submitted to SD exhibited lower weight gain, higher sucrose consumption, showed larger stress-induced hyperthermia, lower levels of homovanillic acid in the frontal cortex, and higher levels of noradrenaline in the amygdala and hippocampus. Open-field, elevated zero maze, and social preference tests revealed the interaction between stress and phenotype, as only LE-rats were further inhibited by SD. ME-rats exhibited the least reactivity to stress in terms of changes in body weight, stress-induced hyperthermia, and sucrose intake.

CONCLUSION

Both low and high novelty-related activity, especially the former, are associated with elevated sensitivity to social stress. This study shows that both tails of a behavioural dimension can produce stress-related vulnerability.