Neurochemical factors underlying individual differences in locomotor activity and anxiety-like behavioral responses in zebrafish.

Variation among individuals may arise for several reasons, and may have diverse underlying mechanisms. Individual differences have been studied in a variety of species, but recently a new model organism has emerged in this field that offers both sophistication in phenotypical characterization and powerful mechanistic analysis. Recently, zebrafish, one of the favorites of geneticists, have been shown to exhibit consistent individual differences in baseline locomotor activity. In the current study, we further explore this finding and examine whether individual differences in locomotor activity correlate with anxiety-like behavioral measures and with levels of dopamine, serotonin and the metabolites of these neurotransmitters. In addition, we examine whether individual differences in locomotor activity are also associated with reactivity to the locomotor stimulant effects of and neurochemical responses to acute ethanol exposure (30min long, 1% v/v ethanol bath application). Principal component analyses revealed a strong association among anxiety-like responses, locomotor activity, serotonin and dopamine levels. Furthermore, ethanol exposure was found to abolish the locomotion-dependent anxiety-like behavioral and serotonergic responses suggesting that this drug also engages a common underlying pathway. Overall, our results provide support for an important role of the serotonergic system in mediating individual differences in anxiety-like responses and locomotor activity in zebrafish and for a minor modulatory role of the dopaminergic system.

MK-801 increases locomotor activity in a context-dependent manner in zebrafish.

Zebrafish have become a popular animal model for behavioral neuroscience with an increasing number of studies examining the effects of pharmacological compounds targeting the brain. Exposure to MK-801, a non-competitive N-methyl-d-aspartate receptor antagonist has been shown to increase locomotor activity in zebrafish. However, others have failed to replicate this finding as several contradicting studies report no changes in locomotor activity following exposure to similar doses. In the current study we reconcile these behavioral reports by demonstrating that zebrafish do not exhibit changes in locomotor activity during exposure to non-sedative doses of MK-801. Interestingly, zebrafish do exhibit significant increases in locomotion if pre-treated with MK-801 followed by subsequent testing in a novel environment, which suggests the effects of MK-801 are context-dependent. In addition, we examine the potential role of the dopaminergic system in mediating MK-801's locomotor stimulant effect by quantifying the levels of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the brains of zebrafish following a 30 min exposure to 10 µM of MK-801 (the dose found to induce the largest increase in locomotor activity). Our findings indicate that the MK-801-induced increase in locomotor activity is not accompanied by changes in whole-brain levels of dopamine or DOPAC. Overall, our results suggest that MK-801's context-dependent locomotor stimulant effect may be independent of whole-brain dopaminergic activation.

Differential effects of dopamine D1 and D 2/3 receptor antagonism on motor responses.

RATIONALE: The zebrafish dopaminergic system is thought to be evolutionarily conserved and may be amenable to pharmacological manipulation using drugs developed for mammalian receptors. However, only few studies have examined the role of specific receptor subtypes in behaviour of adult zebrafish. OBJECTIVES: The objectives of this study are to determine the translational relevance of the zebrafish and examine the psychopharmacology of specific dopamine receptors in this species. METHODS: Using a behavioural pharmacological approach, we examine the effect of D1 and D2/3 receptor antagonisms on motor patterns of adult zebrafish during acute drug exposure and withdrawal. RESULTS: Acute exposure to SCH-23390 (D1 receptor antagonist) decreased total distance travelled in a dose-dependent manner. Exposure to amisulpride (D2/3 receptor antagonist) induced a biphasic dose-response in total distance travelled and in angular velocity. The results provide support for the existence of structurally and functionally conserved postsynaptic D1 and D2 receptors, as well as presynaptic D2 autoreceptors in the zebrafish brain. The behavioural effects of the employed antagonists did not persist following 30 min of withdrawal. CONCLUSION: The results suggest that zebrafish, a cheaper and simpler model organism compared to the rat and the mouse, may be an efficient translationally relevant tool for the analysis of the psychopharmacology of receptors of the vertebrate dopaminergic system.

Differential effects of acute administration of SCH-23390, a D1 receptor antagonist, and of ethanol on swimming activity, anxiety-related responses, and neurochemistry of zebrafish.

RATIONALE: The zebrafish has become an increasingly popular animal model for investigating ethanol's actions in the brain and its effects on behavior. Acute exposure to ethanol in zebrafish has been shown to induce a dose-dependent increase of locomotor activity, to reduce fear- and anxiety-related behavioral responses, and to increase the levels of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC). OBJECTIVES: The objective of the present study was to investigate the role of dopamine D1 receptors (D1-R) in ethanol-induced locomotor activity in zebrafish. METHODS: Zebrafish were pre-treated with SCH-23390 (0 or 1 mg/L bath concentration), a D1-R antagonist, and subsequently exposed to ethanol (0, 0.25, 0.5, 1.0 % v/v). To explore potential underlying mechanisms, we quantified levels of dopamine, DOPAC, serotonin, and 5-HIAA from whole-brain tissue using high-precision liquid chromatography. RESULTS: We found pre-treatment with the D1-R antagonist to attenuate locomotor activity independent of ethanol concentration. Furthermore, unlike ethanol, D1-R antagonism did not alter behavioral responses associated with fear and anxiety. Pre-treatment with SCH-23390 decreased levels of dopamine and DOPAC, but this effect was also independent of ethanol concentration. The D1-R antagonist also reduced serotonin and 5-hydroxyindole acetic acid (5-HIAA) levels. CONCLUSION: These results suggest a multifaceted and at least partially independent role of dopamine D1 receptors in ethanol-induced locomotor activity and anxiety-related responses as well as in the functioning of the dopaminergic and serotoninergic neurotransmitter systems in zebrafish.

Serotonin antagonists induce anxiolytic and anxiogenic-like behavior in zebrafish in a receptor-subtype dependent manner.

Motor function and anxiety-like responses are easily quantifiable in zebrafish, a novel model organism for behavioral pharmacology. Activation of serotonin receptors through the use of selective agonists has been shown to alter anxiety-like behaviors in zebrafish. However, few studies have examined the effect of blockade of specific serotonin receptors. In the current study, we examine the effect of 4 serotonin receptor antagonists selective for 5-HT1A, 5-HT1B/D, 5-HT2, and 5-HT3 receptors on zebrafish motor and anxiety-like responses. Exposure to the receptor antagonists did not change baseline motor responses. However, when placed in a novel environment, zebrafish previously exposed to GR 55562 (5-HT1B/D antagonist) exhibited reduced anxiety-like behavior, whereas zebrafish previously exposed to p-MPPF (5-HT1A antagonist), Ketanserin (5-HT2 antagonist), or Ondasetron (5-HT3 antagonist) exhibited increased anxiety-like behaviors. These results show that drugs developed for mammalian serotonin receptors are efficacious in the zebrafish too, a finding that demonstrates evolutionary conservation of the serotoninergic system. The results also imply that zebrafish may be an appropriate animal model for examining the serotonergic neurotransmitter system in vertebrates.