SOCIAL REGULATION OF THE ENDOCANNABINOID SYSTEM IN ZEBRAFISH MOTOR CIRCUITS

Abstract

Social status-dependent modulation of neural circuits has been investigated extensively in vertebrate and invertebrate systems. However, the effect of social status on shifting the balance in activation between competing neural circuits is poorly understood. Zebrafish (Danio rerio) form stable social relationships that consist of socially dominant and subordinate animals. Once the social hierarchy is formed, social status-dependent differences in behavior patterns emerge. Subordinate animals startle more readily in response to auditory stimuli, while dominants swim at a higher frequency than subordinates. Here, we investigated the role of the endocannabinoid system (ECS) in regulating the activation of the swim and escape circuits based on social status. Our aim was to investigate how the ECS facilitates the transition between swim and escape circuits in socially dominant and subordinate animals. Endocannabinoids act as retrograde signaling molecules between neurons and are implicated in inhibition of both excitatory and inhibitory neurotransmission via retrograde binding of the cannabinoid 1 (CB1) or cannabinoid 2 (CB2) receptor. A previous study revealed a novel role for the endocannabinoid 2-Arachidonoylglycerol (2-AG) in modulating the switch in activation between the swim and startle circuits in zebrafish. The ECS can be up- or down-regulated by altering levels of 2-AG or targeting CB1 receptor function. To better understand how social status regulates the ECS and its effects on circuit activation, we studied the effects of two drugs, AM-251 and JZL184, on the regulation of status-dependent differences in swim and escape behavior. AM-251 competitively blocks endocannabinoid signaling by binding to CB1 receptor, while JZL184 increases 2-AG concentration by inhibiting monoacylglycerol lipase (MAGL), the degradative enzyme for 2-AG. First, we show that increasing ECS activity via intramuscular injection of JZL184 differentially affects swim and escape behavior according to social status. Secondly, we show that block of CB1 function with AM-251 reduces startle sensitivity and swimming frequency, and that its effects are concentration dependent. Thirdly, we utilize a dopamine receptor 1 knockout fish (D1KO) to demonstrate that the effects of ECS modulation on startle involves the dopamine D1 receptor system. Collectively, these findings support the notion that the ECS, as reflected by changes in swimming and escape behavior in response to treatment with JZL184 and AM-251, is socially regulated and involved in the social status-dependent shift in the balance of motor circuit activation, and that these effects are mediated in part via dopaminergic pathways. Our results represent an important step forward in the field of social neuroscience and better define the path toward a comprehensive understanding of the molecular factors that control social behavior.