Kertu Rünkorg

Nitric oxide is involved in the regulation of marble-burying behavior

The aim of this study was to characterize the behavioral effect of modulators of NO synthesis in the mouse marble-burying test. We found that the non-selective NOS inhibitor 7-nitroindazole (7-NI) as well as the more selective neuronal and inducible NOS inhibitor 1-(2-trifluoromethylphenyl)imidazole (TRIM) decreased the number of marbles buried. Treatment with NO precursor l-arginine alone (500mg/kg) had no effect in this paradigm, but counteracted the effects of paroxetine (10mg/kg) and citalopram (10mg/kg). Moreover, agmatine (20 and 50mg/kg i.p), a molecule related to l-arginine metabolism, inhibited the marble-burying behavior. This effect of agmatine was not related to inhibition of NO synthesis as the drug did not decrease the nitrate and nitrite level in the brain tissue.We conclude that NOS inhibitors and agmatine dose-dependently inhibit the marble-burying behavior in mice. Inhibition of nNOS seems to play a key role in the behavioral effects of NOS inhibitors, whereas agmatine seems to have a different mechanism of action. Moreover, enhancement of NO synthesis by l-arginine reversed the effect of SSRI antidepressants, further demonstrating the role of NO in regulating the marble-burying behavior.

Distinct changes in the behavioural effects of morphine and naloxone in CCK2 receptor-deficient mice

The effects of morphine, mu-opioid receptor agonist, and naloxone, a non-selective opioid receptor antagonist, in the locomotor activity and place conditioning tests were studied in the CCK(2) receptor-deficient male mice. The exposure of mice to the motility boxes for 3 consecutive days induced a significant inhibition of locomotor activity in the wild-type (+/+) mice compared to homozygous (-/-) animals. The administration of naloxone (10 mg/kg i.p.) to animals, adapted to the motility boxes, induced a significant reduction of locomotor activity in the homozygous (-/-), but not in the wild-type (+/+) mice. Treatment of habituated mice with morphine (10 mg/kg i.p.) caused a stronger increase of locomotor activity in the wild-type (+/+) mice compared to the homozygous (-/-) littermates. In the place preference test the pairing of the preferred side with naloxone (1 and 10 mg/kg i.p.) induced a dose-dependent place aversion in the wild-type (+/+) mice. The treatment with naloxone was less effective in the homozygous (-/-) mice, because the high dose of naloxone (10 mg/kg) tended to shift the preference. The pairing of morphine (3 mg/kg i.p.) injections with the non-preferred side induced a significant place preference both in the wild-type (+/+) and homozygous (-/-) mice. The increased density of opioid receptors was established in the striatum of homozygous (-/-) mice, but not in the other forebrain structures. In conclusion, the targeted invalidation of CCK(2) receptors induces a dissociation of behavioural effects of morphine and naloxone. Morphine-induced place preference remained unchanged, whereas hyper-locomotion was less pronounced in the mutant mice compared to the wild-type (+/+) littermates. By contrast, naloxone-induced place aversion was weaker, but naloxone caused a stronger inhibition of locomotor activity in the homozygous (-/-) mice than in the wild-type (+/+) animals. These behavioural alterations can be explained in the light of data that the targeted mutation of CCK(2) receptors induces distinct changes in the properties of opioid receptors in various brain structures.

GLP-1 receptor agonists have a sustained stimulatory effect on corticosterone release after chronic treatment

Objective Glucagon-like peptide 1 (GLP-1) receptor agonists are a new group of antidiabetic medications quickly gaining popularity. We aimed to examine behavioural and neuroendocrine changes following chronic treatment with GLP-1 receptor agonists in animal models. Methods The effects of chronic treatment with GLP-1 receptor agonists were determined on behavioural parameters [anxiety level in the light–dark compartment test, the motor activity in automated activity cages, immobility in the forced swimming test (FST)] and on corticosterone release in mice. The possible antidepressant effect of chronic liraglutide treatment was also studied in Flinders Sensitive Line (FSL) rats, a genetic model of depression. Results Two weeks of treatment with exenatide (10 µg /kg twice daily) or liraglutide (1200 µg/kg once daily) did not affect the anxiety level in a light–dark compartment test nor induce an antidepressant-like effect in the FST in mice. Moreover, chronic treatment with liraglutide had no effect on depression-related behaviour in FSL rats. Interestingly, hypolocomotion induced by the drugs in mice disappeared after chronic dosing. Both of the GLP-1 receptor agonists induced robust increases in corticosterone levels in mice under basal conditions as well as in the case of combination with swimming stress. Remarkably, exenatide was as potent a stimulator of corticosterone release after 2 weeks as after acute administration. Conclusions The increases in corticosterone release seen after acute exenatide or liraglutide treatment do not abate after 2 weeks of treatment demonstrating that tolerance does not develop towards this particular effect of GLP-1 agonists.

Differences in behavioural effects of amphetamine and dopamine-related gene expression in wild-type and homozygous CCK2 receptor deficient mice

Neuropeptide cholecystokinin (CCK) interacts with dopamine in the regulation of motor activity and motivations. Therefore, in CCK(2) receptor deficient mice the behavioural effects of repeated amphetamine administration and changes in dopamine-related gene expression were studied. Four-day amphetamine (1 mg/kg) treatment induced a significantly stronger motor sensitization in homozygous mice compared to their wild-type littermates. However, in the conditioned place preference test the action of amphetamine was more pronounced in wild-type animals. As opposed to wild-type mice, amphetamine (1-3 mg/kg) did not cause a significant conditioned place preference in homozygous mice. The expression of Tyhy gene was elevated in the mesolimbic structures and Drd2 gene was down-regulated in the mesencephalon of saline-treated homozygous mice in comparison with respective wild-type group. Four-day treatment with amphetamine induced a significant increase in the expression of Tyhy in the mesencephalon, striatum and mesolimbic structures of wild-type mice, whereas in homozygous mice a similar change was evident only in the mesencephalon. Also, the expression of Drd1 gene in the striatum and Drd2 gene in the mesolimbic structures of wild-type mice were up-regulated under the influence of amphetamine. In conclusion, the present study established differences in the behavioural effects of amphetamine in wild-type and homozygous mice. The increased tone of dopaminergic projections from the mesencephalon to mesolimbic structures is probably related to increased amphetamine-induced motor sensitization in homozygous mice. The lack of development of up-regulation of Drd1 and Drd2 genes after repeated treatment with amphetamine probably explains the reduced place conditioning in CCK(2) receptor deficient mice.

Exenatide Is an Effective Antihyperglycaemic Agent in a Mouse Model of Wolfram Syndrome 1

Wolfram syndrome 1 is a very rare monogenic disease resulting in a complex of disorders including diabetes mellitus. Up to now, insulin has been used to treat these patients. Some of the monogenic forms of diabetes respond preferentially to sulphonylurea preparations. The aim of the current study was to elucidate whether exenatide, a GLP-1 receptor agonist, and glipizide, a sulphonylurea, are effective in a mouse model of Wolfram syndrome 1. Wolframin-deficient mice were used to test the effect of insulin secretagogues. Wolframin-deficient mice had nearly normal fasting glucose levels but developed hyperglycaemia after glucose challenge. Exenatide in a dose of 10 μg/kg lowered the blood glucose level in both wild-type and wolframin-deficient mice when administered during a nonfasted state and during the intraperitoneal glucose tolerance test. Glipizide (0.6 or 2 mg/kg) was not able to reduce the glucose level in wolframin-deficient animals. In contrast to other groups, wolframin-deficient mice had a lower insulin-to-glucose ratio during the intraperitoneal glucose tolerance test, indicating impaired insulin secretion. Exenatide increased the insulin-to-glucose ratio irrespective of genotype, demonstrating the ability to correct the impaired insulin secretion caused by wolframin deficiency. We conclude that GLP-1 agonists may have potential in the treatment of Wolfram syndrome-related diabetes.

Acute administration of GLP-1 receptor agonists induces hypolocomotion but not anxiety in mice

Krass M, Rünkorg K, Vasar E, Volke V. Acute administration of GLP-1 receptor agonists induces hypolocomotion but not anxiety in mice. Objective: The aim of this study was to compare the behavioural and hormonal effects of systemic (subcutaneous) treatment with glucaemically equipotent doses of exenatide and liraglutide in mice. Methods: The effects of glucagon-like peptide-1 (GLP-1) receptor agonists were determined on anxiety level in the light–dark compartment test, the motor activity in automated activity cages and finally the forced swimming test was performed. Results: Both exenatide (1–20 µg/kg) and liraglutide (200–1200 µg/kg) decreased the glucose levels up to 30% in freely fed animals. In glucaemically equipotent doses the drugs induced very similar behavioural and hormonal effects: there was no change on anxiety level or immobility time, however, both drugs suppressed motor activity and increased corticosterone levels. Conclusion: We conclude that the two clinically approved GLP-1 receptor agonists induce very similar suppression of motor activity and stimulation of corticosterone release in mice.

Rimonabant attenuates amphetamine sensitisation in a CCK2 receptor-dependent manner.

In this behavioural and pharmacological study in male CCK2 receptor-deficient mice (CCK2(-/-)), we evaluated the role of the interaction of endocannabinoids (eCBs) and cholecystokinin (CCK) on the regulation of anxiety-related and motor behaviours. Repeated treatment with amphetamine (2mg/kg daily for four days) induced slightly weaker motor sensitisation in CCK2(-/-) mice compared to their wild-type (CCK2(+/+)) littermates. Co-administration of rimonabant (1mg/kg) with amphetamine antagonised the development of motor sensitisation in CCK2(+/+) mice. However, we did not find a similar effect of rimonabant in CCK2(-/-) mice. We did not find any differences between the behaviour of CCK2(+/+) and CCK2(-/-) mice in models designed to assess emotional behaviours (dark/light exploration, marble burying and conditioned place aversion). This study supports the hypothesis that eCBs play a role in the development of amphetamine-induced sensitisation. Moreover, we have demonstrated that intact CCK2 receptors are necessary for the development of eCB-mediated sensitisation to amphetamine.