The role of alpha7 and alpha4beta2 nicotinic receptors in the nicotine-induced anxiolytic effect in zebrafish

Abstract

Zebrafish (Danio rerio) have been widely used to study the molecular mechanisms of development including neurodevelopment. More recently, they have begun to be used to study neuropharmacology and neurotoxicology. Critical for this line of research are methods to study behavioral function in zebrafish. Previous studies have compared zebrafish with mammalian models to determine similarities and differences in locomotor behavior, learning and memory. Relatively little research has been conducted on stress response and anxiety behavior as well as the pharmacologic response in zebrafish. We have developed a test for zebrafish to assess stress response and anxiety: the novel tank diving test. In this short test normally zebrafish dive to the bottom of a novel tank and then gradually over the 5-min test begin exploring higher levels of the tank. Nicotine, which has anxiolytic effects in rodents and humans was found to diminish this novel tank diving response in zebrafish. The current study examined the nicotinic receptor subtype selectivity involved in the actions of nicotine. Two nicotinic receptor subtype selective antagonists were used: MLA (an alpha7 antagonist) and DHbetaE (an alpha4beta2 antagonist). We replicated our previous finding of the anxiolytic effect of nicotine with significantly less bottom dwelling by the fish after nicotine treatment. This nicotine-induced anxiolytic effect was reversed by both MLA and DHbetaE, indicating that both nicotinic alpha7 and alpha4beta2 receptors are involved in the nicotinic effect on anxiety in zebrafish.

Figure 1. Nicotinic Antagonist Dose-Effect Study: Swimming Speed

Swim activity (cm/min) after 50, 100 and 200 mg/l concentrations of MLA and DHβE and swim activity (mean±sem).

Figure 2. Nicotinic Antagonist Dose-Effect Study: Bottom-Dwelling

Time spent (sec/min) in bottom third of novel tank after 50, 100 and 200 mg/l concentrations of MLA and DHβE (mean±sem).

Figure 3. Nicotine and Nicotinic Antagonist Effects on Swimming Speed

Swim activity (cm/min) affected by nicotine and nicotinic antagonists as measured by total distance moved per minute averaged over the 5-minute session (mean±sem). Nicotine significantly increased swimming activity. MLA significantly decreased swim activity, while DHβE at the dose used had no significant effect on its own. Both MLA and DHβE completely blocked the nicotine-induced increase in swim activity.

Figure 4. Nicotine and Nicotinic Antagonist Effects on Swimming Speed over Minutes

Swim activity (cm/min) affected by nicotine and nicotinic antagonists as measured by total distance moved per minute (mean±sem).

Figure 5. Nicotine and Nicotinic Antagonist Effects on Bottom-Dwelling

Time spent (sec/min) in bottom third of novel tank (mean±sem). Total nicotine effects on dwelling of zebrafish in the bottom third of a new tank over a 5-min trial. There was a significant decrease in bottom dwelling of nicotine-dosed fish relative to controls. Those effects were reversed by both nicotinic antagonists, MLA and DHβE.

Figure 6. Nicotine and Nicotinic Antagonist Effects on Bottom Dwelling over Minutes

Time spent (sec/min) in bottom third of novel tank affected by nicotine and nicotinic antagonists as measured by total distance moved per minute (mean±sem).