Animals appropriately respond to their constantly shifting environment through the critical process of decision-making: the selection of one behavioral response from a set of alternatives, each expected to produce different outcomes. While decision-making can involve complex cognitive processing, even simple responses can be dynamically biased and modulated, representing a more tractable system to study the underlying genetic and cellular decision-making mechanisms.
We have developed a simple decision-making paradigm in larval zebrafish using the evolutionarily conserved acoustic startle response. Larvae respond to acoustic stimuli with one of 2 kinematically, neuronally, and genetically distinct behaviors: a Short-Latency C-bend (SLC) initiated 4-15 ms post-stimulus, or a less vigorous Long-Latency C-bend (LLC) initiated 20-80 ms post-stimulus. Individual larvae can respond to acoustic stimuli with either behavior, yet bias their responses toward SLCs following intense stimuli and toward LLCs following weak stimuli. Importantly, individuals incorporate prior experience in selecting their behavioral output, shifting their response bias from SLCs to LLCs following repeated identical strong stimuli. Thus, the basic dynamic aspects of complex cognitive decision-making are present in this simple SLC/LLC decision-making paradigm.
To identify genes and pathways critical for the development and function of startle decision circuits, we performed a small molecule screen and a forward genetic screen. The small molecule screen results demonstrate that as in more complex cognitive assays, serotonergic modulation through the 5-HT1A receptor is critical for acoustic decision-making. Through our forward genetic screen we identified 10 mutants with specific defects in SLC/LLC bias, the first vertebrate mutants specifically isolated based solely on decision-making deficits. Using whole-genome sequencing we have identified mutations in both the calcium-sensing receptor (CaSR) gene and a regulator of CaSR endocytosis that both modify decision-making behavior. Furthermore, we have demonstrated an acute role for CaSR during decision-making and have identified the G-protein signaling pathways regulating appropriate bias through specific signaling agonists and antagonists. Finally, we have identified changes in the activity of key circuit components underlying these decision-making alterations through functional neural imaging to elucidate how serotonergic signaling and CaSR gene function influence simple decision-making.