Antidepressants differentially affect striatal amphetamine-stimulated dopamine and serotonin release in rats with high and low novelty-oriented behaviour.

In the studies of depression pathogenesis and antidepressant action, the monoaminergic hypothesis of depression has mainly focused on the serotonergic and noradrenergic mechanisms. However, dopaminergic neurotransmission is also linked to both depressive symptomatology as well as antidepressant effects. We have previously shown that persistent inter-individual differences in the rat behavioural activity in novel environments is associated with differences in the striatal extracellular levels of dopamine and serotonin, depressive-like behaviour and the expression of several depression-related genes. The aim of the current study was to investigate the relative potency of the tricyclic antidepressant imipramine, the selective serotonin re-uptake inhibitor fluoxetine, and the selective noradrenaline re-uptake inhibitor reboxetine (all drugs administered in the dose of 10mg/kg, i.p.) to enhance amphetamine-stimulated dopamine and serotonin release in the striatum using in vivo microdialysis in awake, freely-moving rats, categorized into high explorers (HE) and low explorers (LE) based on their spontaneous novelty-oriented behaviour. The basal extracellular dopamine and serotonin concentration in the striatum did not differ between the LE- and HE-rats. None of the antidepressants alone were able to modify baseline striatal dopamine levels, but the amphetamine-stimulated dopamine release was significantly higher in the HE-rats after acute and chronic imipramine (but not fluoxetine or reboxetine). Acute imipramine and fluoxetine, but not reboxetine, increased both the basal and amphetamine-stimulated levels of serotonin in the striatum. Again, the HE-rats had higher amphetamine-stimulated serotonin release after fluoxetine administration. These findings suggest that rats with depressive-like phenotype are less sensitive to the neurochemical effects of antidepressants in the striatum. These results may have relevance in understanding the neurobiological bases for inter-individual differences in antidepressant treatment response in humans and development of novel medicines.

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.

Regulation of extracellular serotonin levels and brain-derived neurotrophic factor in rats with high and low exploratory activity.

Serotonin (5-HT) system has a significant role in anxiety- and depression-related states and may be influenced by brain-derived neurotrophic factor (BDNF). This study examined extracellular 5-HT levels and expression of BDNF in rats with persistently low or high levels of exploratory activity (LE and HE, respectively). Baseline extracellular levels of 5-HT as assessed by in vivo microdialysis in conscious animals were similar in both groups in medial prefrontal cortex (PFC) and dentate gyrus (DG). No differences were found in parachloroamphetamine-induced 5-HT release in either region. However, LE animals had significantly higher levels of 5-HT transporter (5-HTT) binding in PFC and a larger increase in extracellular 5-HT levels after administration of citalopram (1 microM) into this area by retrograde dialysis. No difference in 5-HTT levels was found in hippocampus, while perfusion with citalopram was accompanied by a greater increase in extracellular 5-HT in the HE group in this brain region. LE-rats had higher levels of BDNF mRNA in the PFC but not hippocampus. In contrast, levels of nerve growth factor mRNA were similar in these brain regions of LE- and HE-rats. The differential regulation of 5-HT-ergic system in LE- and HE-rats in PFC and hippocampus may form the basis for their distinct anxiety-related behaviours.

Chronic variable stress prevents amphetamine-elicited 50-kHz calls in rats with low positive affectivity.

The relationship between stress response and positive affective states is thought to be bidirectional: whilst stress can lead to a blunted hedonic response, positive affect reduces the negative effects of stress. We have previously shown that persistently high positive affectivity as measured by 50-kHz ultrasonic vocalizations (USVs) is protective against chronic variable stress (CVS). The present study examined the effect of CVS on 50-kHz USVs elicited by amphetamine administration, simultaneously considering the stable inter-individual differences in positive affectivity. Forty juvenile male Wistar rats were categorised as of high (HC) or low (LC) positive affectivity based on their 50-kHz USV response to imitation of rough-and-tumble play ('tickling'). As adults, the rats were subjected to four weeks of CVS, after which D-amphetamine was administered in five daily doses followed by a challenge dose (all 1mg/kg IP) nine days later. CVS reduced sucrose preference in LC-rats only. After CVS, amphetamine-elicited 50-kHz USVs were significantly reduced in LC-rats, the effect of stress in HC-rats being smaller and less consistent. In previously stressed and amphetamine-treated LC-rats, locomotor response to amphetamine was attenuated. In stressed LC-rats, DOPAC levels and dopamine turnover were increased in striatum after amphetamine treatment, and dopamine D1 receptor levels were upregulated in nucleus accumbens. LC-rats had lower isoleucine levels in frontal cortex. These results show that stress-related changes in response to amphetamine are dependent on inter-individual differences in positive affectivity both at neurochemical and behavioural levels, and further support the notion of higher vulnerability of animals with low positive affect.

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

BACKGROUND: Major inter-individual differences exist in vulnerability to anxiety and affective disorders, and the underlying neurobiology could help in understanding the predisposition to these disorders and treatment resistance. Recently the glutamatergic system has become a target in the development of novel antidepressants. METHODS: We compared extracellular glutamate levels in low (LE-) and high exploring (HE-) rats in hippocampus and striatum at baseline and after inhibition of re-uptake by perfusion with l-trans-pyrrolidine-2,4-dicarboxylate (PDC, 4mM). Glutamate levels in microdialysates were measured by HPLC after derivatization. RESULTS: In the striatum, baseline glutamate levels in young adult LE-rats and HE-rats were not significantly different, but the response to uptake inhibition was: perfusion with PDC increased extracellular glutamate levels in both LE- and HE-rats, but to a significantly lower extent in LE-rats. Although the characteristic levels of exploration of LE- and HE-rats had previously been shown to be stable up to 8 months of age, we identified a subgroup of HE-rats whose exploration levels had drastically dropped by age of 11 months (formerly HE-rats, HEF), and analyzed their data separately. There were no differences in the PDC-evoked striatal glutamate release between the three groups; however, the baseline glutamate levels were higher in the HEF-subgroup compared to the HE- and LE-animals. In the CA1 area of hippocampus, there were no differences in extracellular glutamate levels between the LE- and HE-rats either at baseline or after inhibition of uptake. CONCLUSION: These results suggest that inter-individual differences in exploratory behaviour may be related to striatal glutamatergic neurotransmission.

The effect of denervation of the locus coeruleus projections with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) on cocaine-induced locomotion and place preference in rats.

The potential contribution of locus coeruleus (LC)-derived noradrenaline (NA) in the motor activating and rewarding effects of cocaine (15 mg/kg) were assessed following administration of the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4). In Experiment 1, administration of 10 mg/kg of DSP-4 similarly to substantial denervation with 50 mg/kg of DSP-4 significantly attenuated the activating effects of cocaine during the first cocaine-paired training session (30 min) in the conditioned place preference (CPP) apparatus. Only administration of the higher dose (50 mg/kg) of DSP-4 attenuated line crossings during the last training, while both doses reduced rearings. Thus, both minor and substantial denervation of LC reduced but did not abolish locomotion activating effect of cocaine. Cocaine CPP as measured by increment of time spent in the previously cocaine-paired chamber during drug-free conditions before and after cocaine-paired trainings was clearly revealed only in animals with intact projections from the LC, and was entirely absent after a large lesion of LC projections by DSP-4 (50 mg/kg). Because recovery of noradrenaline levels by the end of experiment did not allow assessment of the efficacy of the neurotoxin, the effect of DSP-4 pre-treatment on the acute psychomotor effect of cocaine was re-examined in an independent experiment (Experiment 2). Near complete denervation of the LC projections again reduced the effect of cocaine, but the lower dose of DSP-4 had no effect, suggesting that small lesions of the LC do not have a robust impact. Overall, this study demonstrates that both unconditioned and conditioned effects of cocaine depend upon the integrity of LC projections.

Ethanol-induced effects on the dopamine and serotonin systems in adult Wistar rats are dependent on early-life experiences.

Some individuals control their ethanol consumption throughout life, but others escalate their intake to levels that increase the risk for addiction. The early environment influences the individual response to ethanol and affects the underlying physiological processes that lead to a transition from a voluntary to a compulsive use of ethanol. However, the neurobiological substrates for these processes are not understood. The present study aimed to test the hypothesis that early environmental experiences affect the neurobiological effects that are induced by voluntary ethanol consumption. Rat pups were subjected to three different rearing environments: conventional animal facility rearing or separation from dam and littermates for either 15 or 360min. In adulthood, the rats were exposed to a two-bottle free choice between ethanol and water for seven weeks. Tissue levels of dopamine, 5-hydroxytryptamine (5-HT) and their metabolites were measured in brain areas that have been implicated in reward and addiction processes. Differences in ethanol-induced effects were noted in 5-HT-related measurements in the nucleus accumbens and ventral tegmental area and in dopamine-related measurements in the dorsal raphe nucleus (DRN). These results provided evidence of an early environmental impact on interactive neuronal circuits between the DRN and reward pathways. The amygdala, a key area in addiction processes, was particularly sensitive to early-life conditions. The animals that experienced the longest separation differed from the others; they had low basal 5-HT levels and responded with an increase in 5-HT after ethanol. These altered responses to initial ethanol consumption as a result of early environmental factors may affect the transition from habitual to compulsive drinking and contribute to individual vulnerability or resilience to addiction.