Anxiolytic effects of nicotine in zebrafish.

Anxiolytic effects of nicotine have been documented in studies with rodents and humans. Understanding the neural basis of nicotine-induced anxiolysis can help both with developing better aids for smoking cessation as well as with the potential development of novel nicotinic ligands for treating anxiety. Complementary non-mammalian models may be useful for determining the molecular bases of nicotine effects on neurobehavioral function. The current project examined whether a zebrafish model of anxiety would be sensitive to nicotine. When zebrafish are placed in a novel environment, they dive to the bottom of the tank. In the wild, diving could help to escape predation. We tested the anxiolytic effect of nicotine on the novelty-elicited diving response and subsequent habituation. Zebrafish placed in a novel tank spent the majority of time at the bottom third of the tank during the first minute of a 5-min session and then show a gradual decrease in time spent at the tank bottom. Nicotine treatment at 100 mg/l for 3 min by immersion before testing caused a significant decrease in diving throughout the session, while 50 mg/l was effective during the first minute when the greatest bottom dwelling was seen in controls. Nicotine effects were reversed by the nicotinic antagonist mecamylamine given together with nicotine, but not when administered shortly before the test session after prior nicotine dosing. This implies that the effect of nicotine on diving was due to net stimulation at nicotinic receptors, an effect that is blocked by mecamylamine; and that once invoked, this effect is no longer dependent on continuing activation of nicotinic receptors. The effect of nicotine on diving did not seem to be the result of a general disorientation of the fish. The 100 mg/ml nicotine dose was shown in our earlier study to significantly improve spatial-discrimination learning in zebrafish. Nicotine-induced anxiolytic effects can be modeled in the zebrafish. This preparation will help in the investigation of the molecular bases of this effect.

Time-sharing in pigeons: Independent effects of gap duration, position and discriminability from the timed signal.

Previous data suggest that in a peak-interval procedure with gaps, memory for the pre-gap interval varies with the discriminability of the gap from the to-be-timed signal. Here we extend this finding by manipulating the pre-gap and gap intervals as well as the visual contrast between the gap and the to-be-timed signal. The delay in response function after the gap was found to vary with the duration and position of the gap. However, for each gap duration and position, the delay in response increased with the gap-signal contrast: at 60% gap-signal contrast pigeons continued to accumulate time during the gap, at 80% gap-signal contrast pigeons stopped timing during the gap, and at 100% gap-signal contrast pigeons reset their timing after the gap. Data are accounted for by a time-sharing model assuming two concurrent processes during the gap--time accumulation and memory decay controlled by the salience of the gap--whose interplay results in a continuum of responses in the gap procedure.

Timing in choice experiments.

In Experiment 1, pigeons chose between variable- and fixed-interval schedules. The timer for 1 schedule was reset by a reinforcement on that schedule or on either schedule. In both cases, the pigeons timed reinforcement on each schedule from trial onset. The data further suggest that their behavior reflects 2 independent processes: 1 deciding when a response should be emitted and responsible for the timing of the overall activity, and the other determining what this response should be and responsible for the allocation of behavior between the 2 response keys. Results from Experiment 2, which studied choice between 2 fixed-interval schedules, support those 2 conclusions. These results have implications for the study of operant choice in general.

Brief embryonic strychnine exposure in zebrafish causes long-term adult behavioral impairment with indications of embryonic synaptic changes.

Zebrafish provide a powerful model of the impacts of embryonic toxicant exposure on neural development that may result in long-term behavioral dysfunction. In this study, zebrafish embryos were treated with 1.5mM strychnine for short embryonic time windows to induce transient changes in inhibitory neural signaling, and were subsequently raised in untreated water until adulthood. PCR analysis showed indications that strychnine exposure altered expression of some genes related to glycinergic, GABAergic and glutamatergic neuronal synapses during embryonic development. In adulthood, treated fish showed significant changes in swimming speed and tank diving behavior compared to controls. Taken together, these data show that a short embryonic exposure to a neurotoxicant can alter development of neural synapses and lead to changes in adult behavior.

Zebrafish provide a sensitive model of persisting neurobehavioral effects of developmental chlorpyrifos exposure: comparison with nicotine and pilocarpine effects and relationship to dopamine deficits.

Chlorpyrifos (CPF) an organophosphate pesticide causes persisting behavioral dysfunction in rat models when exposure is during early development. In earlier work zebrafish were used as a complementary model to study mechanisms of CPF-induced neurotoxicity induced during early development. We found that developmental (first five days after fertilization) chlorpyrifos exposure significantly impaired learning in zebrafish. However, this testing was time and labor intensive. In the current study we tested the hypothesis that persisting effects of developmental chlorpyrifos could be detected with a brief automated assessment of startle response and that this behavioral index could be used to help determine the neurobehavioral mechanisms for persisting CPF effects. The swimming activity of adult zebrafish was assessed by a computerized video-tracking device after a sudden tap to the test arena. Ten consecutive trials (1/min) were run to determine startle response and its habituation. Additionally, habituation recovery trials were run at 8, 32 and 128 min after the end of the initial trial set. CPF-exposed fish showed a significantly (p<0.025) greater overall startle response during the 10-trial session compared to controls (group sizes: Control N=40, CPF N=24). During the initial recovery period (8 min) CPF-exposed fish showed a significantly (p<0.01) greater startle response compared to controls. To elucidate the contributions of nicotinic and muscarinic acetylcholine receptors to developmental CPF-mediated effects, the effects of developmental nicotine and pilocarpine exposure throughout the first five days after fertilization were determined. Developmental nicotine and pilocarpine exposure significantly increased startle response, though nicotine (group sizes: Control N=32, 15 mM N=12, 25 mM N=20) was much more potent than pilocarpine (group sizes: Control N=20, 100 microM N=16, 1000 microM N=12). Neither was as potent as CPF for developmental exposure increasing startle response in adulthood. Lastly, developmental CPF exposure decreased dopamine and serotonin levels and increased transmitter turnover in developing zebrafish larvae (N=4 batches of 50 embryos/treatment). Only the decline in dopamine concentrations persisted into adulthood (group sizes: Control N=14, CPF N=13). This study shows that a quick automated test of startle can detect persisting neurobehavioral impairments caused by developmental exposure to CPF. This may be helpful in screening for persisting neurobehavioral defects from a variety of toxicants.

Nicotine effects on learning in zebrafish: the role of dopaminergic systems.

RATIONALE: Nicotine improves cognitive function in a number of animal models including rats, mice, monkeys, and recently, zebrafish. The zebrafish model allows higher throughput and ease in discovering mechanisms of cognitive improvement. MATERIALS AND METHODS: To further characterize the neural bases of nicotine effects on learning in zebrafish, we determined changes in dopaminergic systems that accompany nicotine-enhanced learning. RESULTS: Nicotine improved learning and increased brain levels of dihydroxyphenylacetic acid (DOPAC), the primary dopamine metabolite. There was a significant correlation between choice accuracy and DOPAC levels. The nicotinic antagonist mecamylamine blocked the nicotine-induced increase in DOPAC concentrations, in line with our previous finding that mecamylamine reversed nicotine-induced learning improvement. CONCLUSIONS: Dopamine systems are related to learning in zebrafish; nicotine exposure increases both learning rates and DOPAC levels; and nicotinic antagonist administration blocks nicotine-induced rises in DOPAC concentrations. Rapid cognitive assessment of drugs with zebrafish could serve as a useful screening tool for the development of new therapeutics for cognitive dysfunction.