A3 adenosine receptors modulate behavior and serotonergic system in zebrafish: Mediation by the nitric oxide pathway

<p>Extracellular serotonin (5-HT) levels in the brain are thought to mediate many different behavioral functions, including anxiety and stress; the main mechanism to regulate extracellular 5-HT levels is through reuptake, which are regulated by a plethora of mechanisms. Here, we demonstrate that activation of A3 adenosine receptors by IB-MECA decreases scototaxis, geotaxis, neophobia and arousal in zebrafish in vivo, increases extracellular brain 5-HT levels ex vivo, and increases 5-HT uptake in vitro. The effects of IB-MECA on 5-HT uptake, on extracellular 5-HT concentrations and on scototaxis are dependent on L-type calcium channels, nitric oxide synthase, and serotonin transporters; the effects of IB-MECA on geotaxis, on the other hand, are dependent on nitric oxide synthase, but not on serotonin transporters or calcium channels. These results underline a potential target to control 5-HT uptake and its behavioral consequences.</p

Effects of (A) buspirone, (B) diazepam and (C) caffeine on time on white (upper left), risk assessment (upper right), thigmotaxis (lower left), and freezing (lower right).

<p>Bars represent standard error of the mean, and whiskers represent the 2.5 and 97.5 percentile. *, p<0.05; **, p<0.01; ***, p<0.001.</p

Behavioral fingerprint of selected drugs on the scototaxis test.

<p>Pharmacological manipulations were hierarchically clustered to link compounds to behaviors. In the clustergram, each cell represents the Maximum Predictive Value (red – higher than controls; green – lower than controls).</p

Effects of (A) caffeine, (B) ethanol, (C) bupropion and (D) verapamil on time on white (top) and entries on white (bottom).

<p>Bars represent standard error of the mean, and whiskers represent the 2.5 and 97.5 percentile. *, p<0.05; **, p<0.01; ***, p<0.001.</p

Drugs which cluster on the 'anxiolytic' group decrease 5-HT turnover in the brain.

<p>(A) Turnover rates, as measured by 5-HIAA:5-HT ratios, normalized to the values of vehicle-treated animals, for the following drugs: fluoxetine (FLX; chronic treatment with 10 mg/kg); chlordiazepoxide (CDZ; 0.02 mg/kg); clonazepam (CLZ; 0.05 mg/kg); diazepam (DZP; 1.25 mg/kg); buspirone (BUS; 50 mg/kg); ethanol (EtOH, 2.5%); dizocilpine (MK; 0.005 mg/kg); verapamil (VER; 5 mg/kg); WAY 100,635 (WAY; 0.03 mg/kg); and SB 224,289 (SB; 2.5 mg/kg). Asterisks mark statistically significant differences in relation to vehicle-treated animals (F_{10, 43} = 45.99, p<0.0001, one-way ANOVA followed by Dunnett's Multiple Comparison test). Bars represent mean (B) Correlation between turnover rates (<i>Y</i>-axis) and time spent in the white compartment (<i>X</i>-axis) for vehicle- and drug-treated animals (<i>n</i> = 4 for each point). Points represent means and error bars represent standard errors. A negative correlation is found between the decrease in serotonin turnover and the increase in time on white produced by a drug (r² = 0.5688, p = 0.0073).</p