Kadri Kõiv

Tickling-induced 50-kHz ultrasonic vocalization is individually stable and predicts behaviour in tests of anxiety and depression in rats

Manipulation of juvenile rats in a way that mimics the rough-and-tumble play and resembles tickling elicits 50-kHz ultrasonic vocalizations (USVs) that have been proposed as a measure of positive affect. In the present experiments the stability of the 50-kHz USV response (chirping) over 1.5 months of daily manipulation and the effect of tickling was studied. By the second week of tickling rats of both sexes developed a level of 50-kHz USVs that remained individually characteristic. During tickling the rats also emitted low levels of 22-kHz USVs. No correlation was found between the two types of USVs. In tests used in anxiety and depression research, tickling on its own had an anxiolytic effect in many experimental settings. Significantly lower levels of [(35)S]GTPgammaS binding to the dopamine-activated receptor-G protein complex in striatum and serotonin transporter levels in the frontal cortex were found in female control rats as compared to males. These differences were eliminated by tickling. Rats which expressed high level of chirping (HC-rats) were similar to low-chirping (LC) rats in anxiety measures but had lower activity in an exploration test and lower sucrose preference. LC-rats adopted more active coping strategies in the forced swimming test. These findings suggest that there are individually characteristic 50-kHz USV response levels to tickling in rats, and that HC- and LC-rats are similar with regard to anxiety levels but have different coping strategies to novelty. The anxiolytic-like changes in behaviour that were brought about by tickling could be mediated by changes in dopamine- and serotonergic systems.

Rats with persistently low or high exploratory activity : Behaviour in tests of anxiety and depression, and extracellular levels of dopamine

Behaviour in novel environments is influenced by the conflicting motivators fear and curiosity. Because changes in both of these motivational processes are often simultaneously involved in human affective disorders, we have developed the exploration box test which allows separation of animals belonging to clusters with inherent high neophobia/low motivation to explore and low neophobia/high motivation to explore (LE and HE, respectively). In a novel home-cage, no behavioural differences were found between LE- and HE-rats, suggestive that it is not the general locomotor activity but specific features of the exploration box test that bring about the differences. In studies on both Wistar and Sprague-Dawley rats we found that the trait of exploratory activity remains stable over long periods of time and that LE and HE animals display differences in many other behavioural tests related to mood disorders. Namely, LE animals were found to display enhanced anxiety-like behaviour and to be generally less active in the elevated plus-maze, used more passive coping strategies in the forced swimming test, and acquired a more persistent association between neutral and stressful stimuli in fear conditioning test. LE animals consumed more sucrose solution in non-deprived conditions. We also found that both at baseline and in response to d-amphetamine (0.5mg/kg) administration, LE-rats had lower extracellular dopamine levels in striatum but not in nucleus accumbens. In conclusion, LE-rats appear more inhibited in their activity in typical animal tests of anxiety and are more susceptible to acute stressful stimuli.

Rat behavior after chronic variable stress and partial lesioning of 5-HT-ergic neurotransmission: effects of citalopram.

Deficits in serotonergic (5-HT-ergic) neurotransmission and stressful life events have been implicated in affective disorders, and chronic variable stress (CVS) can elicit behavioral changes reminiscent of increased emotionality, anxiety and atypical depression after partial 5-HT depletion. This study examined the effect of chronic citalopram treatment (10 mg/kg daily) on these changes. Parachloroamphetamine (PCA) (2 mg/kg) reduced the levels of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex, increased anxiety in the social interaction test, and increased activity in the open field. CVS reduced social activity in the social interaction test and immobility time in the forced swimming test. Reduction of excrements left during immobilization indicated partial adaptation with the CVS. Specific stressors had different effects on body weight gain, shorter lasting stressors having a smaller effect in general than those that lasted longer. Combination of CVS and PCA increased sucrose intake after two weeks of stress. In addition, combination of the two treatments reduced diving in the forced swimming test. Citalopram prevented the increase in sucrose consumption in the PCA+CVS rats, and in 5-HT-depleted animals blocked the increase in struggling and reduced the number of defecations in the forced swim test. In conclusion, citalopram treatment prevented several effects of either 5-HT depletion or combined PCA+CVS treatment, suggesting that these behavioral changes could be used in studies on the neural mechanisms underlying emotional behavior that may have relevance to the neurobiology of depression.

Effect of chronic stress on behavior and cerebral oxidative metabolism in rats with high or low positive affect

The 50 kHz ultrasonic vocalizations (USVs) in rats have been associated with positive affect and rewarding experience. We have previously reported that stable inter-individual differences exist in the expression of these USVs (chirps). We have examined the effect of four weeks of chronic variable stress on cerebral oxidative metabolism, and depression and anxiety related behavior in male and female high (HC) and low (LC) chirping rats. Significant differences in regional oxidative metabolic activity as measured by cytochrome c oxidase (COX) histochemistry were found between male and female rats: Females had lower oxidative metabolism in several brainstem areas such as dorsal and median raphe and pontine nucleus, some cortical areas, and reward-related forebrain regions such as striatum and nucleus accumbens, but higher oxidative metabolism in amygdala and related limbic regions. Chronic stress increased oxidative metabolism in several depression-related brain regions in male but not female LC-rats such as amygdala, hippocampus and anterior thalamus. No systematic behavioral effect of stress was evident in females. In LC males, stress elicited increased levels of 22-kHz USVs, earlier and more stable reduction of weight gain, persistently lower sucrose intake and preference, and higher levels of immobility in the forced swimming test. These behavioral changes, accompanied by increased oxidative metabolism in limbic brain regions, indicate greater vulnerability to stress of male LC-rats, and suggest that in males low inherent positive affectivity predisposes to anxiety and affective disorders.

Brain responses to chronic social defeat stress: effects on regional oxidative metabolism as a function of a hedonic trait, and gene expression in susceptible and resilient rats.

Chronic social defeat stress, a depression model in rats, reduced struggling in the forced swimming test dependent on a hedonic trait-stressed rats with high sucrose intake struggled less. Social defeat reduced brain regional energy metabolism, and this effect was also more pronounced in rats with high sucrose intake. A number of changes in gene expression were identified after social defeat stress, most notably the down-regulation of Gsk3b and Map1b. The majority of differences were between stress-susceptible and resilient rats. Conclusively, correlates of inter-individual differences in stress resilience can be identified both at gene expression and oxidative metabolism levels.

Effects of low dose N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine administration on exploratory and amphetamine-induced behavior and dopamine D2 receptor function in rats with high or low exploratory activity

Individual differences in behavioral traits are associated with sensitivity to various neurochemical and psychopharmacological manipulations. In this study exploratory and amphetamine-induced behavior in rats with persistently high or low exploratory activity (HE and LE, respectively) was examined before and after a partial denervation of the locus coeruleus (LC) projections with the selective neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine; 10 mg/kg). Partial LC denervation prevented the increase in exploratory activity over repeated test sessions in the LE animals, but had no effect in HE-rats. Amphetamine- (0.5 mg/kg) induced locomotor activity was attenuated by DSP-4 pretreatment only in HE-rats. These results suggest differential involvement of LC noradrenergic transmission in novelty- and amphetamine-induced behavior in animals with persistent differences in novelty-related behavior. In addition to partial noradrenaline depletion in the frontal cortex and hippocampus, which occurred in both HE- and LE-rats, DSP-4 treatment also decreased the content of dopamine and its metabolites in the nucleus accumbens, and the metabolite levels in striatum, but only in the LE-animals. 5-HIAA levels were also reduced in the nucleus accumbens and striatum in LE-rats by the neurotoxin. D(2) receptor function, as determined by dopamine-stimulated [(35)S]GTPgammaS binding, was increased by DSP-4 treatment in the striatum of LE-rats, but reduced in HE-rats. No effect of partial LC denervation was found on dopamine-stimulated [(35)S]GTPgammaS binding in the nucleus accumbens. Together these findings suggest that LC noradrenergic neurotransmission is differently involved in dopaminergic mechanisms which mediate novelty-related vs amphetamine-induced behavior.

Differential gene expression in a rat model of depression based on persistent differences in exploratory activity

Affective disorders are often accompanied by changes in motivation and anxiety. We investigated the genome-wide gene expression patterns in an animal model of depression that separates Wistar rats belonging into clusters of persistently high anxiety/low motivation to explore and low anxiety/high motivation to explore (low explorers and high explorers, LE and HE, respectively), in three brain regions previously implicated in mood disorders (raphe, hippocampus and the frontal cortex). Several serotonin-, GABA-, and glutamatergic genes were differentially expressed in LE- and HE-rats. The analysis of Gene Ontology biological process terms associated with the differentially regulated genes identified a significant overrepresentation of genes involved in the neuron development, morphogenesis, and differentiation; the most enriched pathways from the Kyoto Encyclopedia of Genes and Genomes were the Wnt signalling, MAPK signalling, long-term potentiation, and long-term depression pathways. These findings corroborate some expression data from other models of depression, and suggest additional targets.

Rats with high or low sociability are differently affected by chronic variable stress

Depression is strongly related to social behavior. We have previously shown that social behavior of rats is individually stable. The purpose of the present study was to compare the sensitivity of animals with different sociability to chronic variable stress (CVS). Four social interaction tests were performed with 60 single-housed male Sprague-Dawley rats. Twenty rats with the lowest and 20 with the highest average social activity time were selected as low sociability (LS) and high sociability (HS) rats, respectively. Both groups were further divided into control and stress groups with equal average body weight. The CVS procedure lasted for 3 weeks. The stressors applied were cold water and wet bedding, imitation of injection, stroboscopic light, movement restriction in a small cage, tail pinch with a clothespin, and strong illumination during the predicted dark phase. In HS-rats, but not in LS-rats, CVS reduced sucrose intake compared with baseline after 3 weeks, suggesting that HS-rats are more vulnerable to anhedonia elicited by CVS. LS-animals were more anxious in the social interaction and open field tests, but stress eliminated differences with HS-animals in the social interaction test and increased their activity in the forced swimming test. In LS-rats stress increased ex vivo dopamine levels and reduced 5-HT levels in the frontal cortex, suggesting that the increased behavioral activity after stress may be related to increased impulsivity. This study thus revealed that animals with high sociability trait are more vulnerable to anhedonia elicited by chronic stress in conditions of single housing.

Changes in regional long-term oxidative metabolism induced by partial serotonergic denervation and chronic variable stress in rat brain

Stressful experiences and genetic predisposition have both independent and interactive contributions to the development of depression. The serotonergic system is involved in the development of depression, and administration of neurotoxins that specifically compromise its function leads to symptoms of affective disorders. In order to find out which brain regions are most affected by stress, partial serotonergic denervation and their combination, chronic variable stress (CVS) was applied for 3 week. Serotonergic denervation was elicited by parachloroampetamine (PCA, 2mg/kg), and cytochrome oxidase histochemistry was used to characterize the long-term levels of neuronal oxidative energy metabolism. PCA pretreatment blocked the increase in oxidative activity in chronically stressed rats in medial preoptic area, cortical and medial amygdala. PCA raised oxidative activity compared to control animals in substantia nigra and ventrolateral division of laterodorsal thalamus. CVS reduced the oxidative activity induced by PCA in suprachiasmatic hypothalamus, anteroventral thalamus, hippocampal CA3 region and cortical amygdala. In the dorsal part of the anterior olfactory nucleus chronic stress blocked the decrease in oxidative activity evoked by PCA. Conclusively, partial serotonergic denervation with PCA and chronic variable stress both had independent effects on long-term energy metabolism in several rat brain structures, tending to increase it. However, partial serotonergic denervation by parachloroampetamine and chronic variable stress had in many brain regions an interactive effect on energy metabolism, each factor reducing the effect of the other, which could reflect the weakening of adaptive mechanisms.

Rats with persistently high exploratory activity have both higher extracellular dopamine levels and higher proportion of D(2) (High) receptors in the striatum.

Increases in both striatal dopamine release and the proportion of the D2 receptors in the high affinity state (D  2High ) accompany the behavioral sensitization to psychostimulants, but it is not known whether the physiological substrate of the interindividual differences locomotor and exploratory behavior is similar. Thus, we examined whether persistently high spontaneous exploratory activity is associated with extracellular dopamine as well as the proportion of D  2High in the striatum. Extracellular dopamine levels were found to be significantly higher in rats with high exploratory activity (high explorers, HE) as compared with low explorers (LE) in baseline conditions as well as after administration of amphetamine (0.5 mg/kg, i.p.). Also, the HE animals had significantly higher proportion of striatal D  2High receptors than the LE‐rats (43.8 ± 4.4% and 22.5 ± 1.5%, respectively). Thus, the present findings support the notion that concomitant higher extracellular dopamine levels and the proportion of D  2High receptors in the striatum, whether naturally occurring and persistent or pharmacologically induced, are causally related to high behavioral activity. Synapse 63:443–446, 2009.