A startle response is an essential primal function exhibited by virtually all organisms in reaction to external stimuli. These responses — fight, flight, avoiding discomfort — underline natural behaviors and mechanisms, informing us about the fundamental biology of model organisms, and ultimately humans. These responses can be triggered by a range of external stimuli, and can be studied in the context of diseases, drug development or genetic screenings.
Types of Stimuli used in zebrafish assays
The stimuli used in startle response assays are based on the mechanisms being studied in zebrafish. Touch, visual, and audio stimuli can all be triggered in a controlled environment. A researcher might manually poke individual fish with a pipette, or use an integrated vibrating device to trigger a touch startle reaction in an entire well plate of fish. For visual stimuli, light flashes or moving bars of light across an array of LEDs can be used to instigate the startle. Audio stimuli might be given via vibration startle in early life, since sound is detected physically by hair cells along the lateral line — or in later life, when their otoliths (little ear stones that contribute to balance) develop, speakers might be used to play a tone. The distance traveled during a startle, the reaction to multiple pulses of the same type of stimuli, and latency at which the fish react are just a few examples of metrics researchers will consider in these studies to classify types of reactions and deduce the effectiveness of certain drugs.
Application Space of the Startle Response Assay:
Epilepsy
Visual stimuli, such as flashing lights or moving bars, can trigger seizures in fish lines expressing epileptic markers. Researchers must ensure a potential drug is stopping the neuronal misfiring that results in a seizure. It can be difficult to know if the anti-seizure drug is merely paralyzing the animal or treating the symptoms. A startle response can be used to tease apart the drug effect.
Cardiac
Genetic conditions and drugs affecting the cardiac function can be studied in conjunction with this assay. Any alteration of the heart rate could possibly result in a different startle response. A number of types of stimuli could be used to initiate this response, followed by observation of the results in vivo. Juvenile zebrafish are transparent, and genetic lines expressing fluorescent markers in the cardiovascular system provide an insight of how drugs and stimuli affect the heart.
Anesthetics
Startle responses are also used to study how the organism is being slowed or otherwise affected by drugs. Light flashes and vibrations are used to test the effectiveness of the anesthetic — how quickly the organism reacts to the stimuli, and, similar to epilepsy studies, at what concentration is the drug effective without harming the organism.
Conclusion
Multiple core functions and motor mechanisms are tied to the reaction to sudden external stimuli, and the number of studies implementing the startle response in the research into both disease and therapies continue to grow.
Zebrafish have numerous qualities which make them a particularly ideal candidate in this research. All of these aforementioned reactions can be observed in real time due to their transparent nature in their early life, and their many genetic similarities to humans makes them an effective alternative to mammal model testing.
