Investigating the Leptinergic Regulation of Weakly Electric Fish Communication Signals

Abstract

Animals expend energy to gain information from their environments and communicate information to conspecifics. Understanding these energy-information tradeoffs is critical for understanding the constraints that shape animal sensory and communication systems. My dissertation focuses on understanding how animals manage the metabolic costs of active sensory and communication signals. Specifically, we study how the hormone leptin regulates energy-information tradeoffs in animals. These tradeoffs are pronounced in weakly electric fish (WEF) that generate and detect weak electric fields called electric organ discharges (EODs) to sense their environments and communicate. The EODs are produced by the thousands of electrogenic cells (electrocytes) in the electric organ in their tail. The combination of continuous high-frequency signals to enhance sensory sampling in fast-changing environments and large currents in the WEF’s electrocytes creates significant metabolic costs directly tied to EOD amplitude (EODa). Thus, management of the energy consumption for these sensory signals through behavioral and physiological adaptations is critical for the survival and reproductive fitness of these fish, especially when their aquatic habitats are impacted by anthropogenic changes that disturb their food availability—it then becomes critical to explore anthropogenic implications on the WEF population. These fish respond to energy shortfalls during food restriction by reducing EODa to conserve energy, but at the cost of degrading sensory and communication performance. EODa recovers to normal levels when fish return to normal feeding. These changes in EODa are mediated by the hormone leptin, a protein hormone that typically plays multiple roles in energy balance, through unknown mechanisms. The primary goal of my dissertation is to investigate the mechanisms through which leptin regulates electrical signal amplitude in WEF. My dissertation research consists of three chapters that analyzed the role of leptin in regulating animal communication signals, then compared the effects of leptin in two species of weakly electric fish that respond differently to energy shortfalls: Eigenmannia virescens and Brachyhypopomus gauderio. These research projects addressed three related questions: i) How will selective environmental pressures favor leptinergic regulation of communication signaling among different taxa? ii) What are the cellular mechanisms that leptin targets to regulate EODa? iii) How does leptinergic regulation of EODa vary across WEF species with different life histories and reproductive strategies? Chapter 1– Assessing the importance of leptinergic regulation of vertebrate communication signals. Animal communication signals are regulated by multiple hormonal axes that ensure appropriate signal targeting, timing, and information content. Although the regulatory roles of many peptide hormones are well understood and documented across a wide range of vertebrate taxa, the role of leptin in regulating animal communication signals remains largely unexplored. Only two recent studies have reported a novel function for leptin in regulating communication signals of weakly electric fish and singing mice. With only limited evidence available to date, in the first chapter of my dissertation, we assessed how widespread leptinergic regulation of communication signals is within and across taxa, and what features of communication signals are subject to leptinergic regulation. In this paper, we proposed that direct costs arise from metabolic investment in signal production, while indirect costs arise from the predation and social conflict consequences of the signal’s information content. Thus, we proposed a schematic framework for a number of directly testable predictions within and across taxa. Experimental tests of these predictions will lead to a better understanding of when and how selective pressures will favor leptinergic regulation of communication signaling effort and/or signal information content. Chapter 2 – Investigating the mechanisms through which leptin regulates electric signal amplitude regulation in E. virescens. I used integrative approaches to determine the cellular and molecular mechanisms associated with leptin-induced regulation of signal amplitude. This chapter describes several different projects directed to answering how leptin regulates the EOD amplitude in E. virescens. First, we conducted assays to clone and sequence genes encoding the E. virescens leptin receptor (LepR) from the E. virescens’ electrocyte. Then we localized expression of leptin receptors on E. viresens’ electrocyte membranes to understand what the potential cellular targets of leptin are, such as ion channels or receptors that work downstream of leptin receptor to regulate EOD amplitude in E. virescens. The results of LepR localization showed that LepR is co-localized with the electrocyte’s voltage-gated sodium channels and the acetylcholine receptors on the posterior side of electrocyte membrane. Therefore, LepR may regulate the function of voltage gated sodium channel or/and acetylcholine receptors to regulate EOD amplitude in E. virescens. We evaluated these possibilities with computational analyses to determine whether leptin-induced increases in EOD amplitude are caused by changes in the magnitude of the electrocyte sodium current and\or its cholinergic synaptic current. We performed a parallel analysis to analyze the contributions of electrocyte sodium current and cholinergic synaptic current to increased EOD amplitude in response to the melanocortin hormone ACTH. These analyses provided a better understanding regarding leptin’s effects on EOD amplitude and whether it uses the same pathway as ACTH in making relative change in the contributions of the synaptic current and the sodium current to EOD amplitude when EOD amplitude changes in response to food deprivation, leptin injections, or ACTH injection. Chapter 3 – Assessing the peripheral effects of leptin on electric signal characteristics in B. gauderio. This chapter used a comparative approach to determine whether peripheral leptinergic regulation of electric signal amplitude varies across species with different life histories and reproductive strategies. Eigenmannia virescens belongs to the Gymnotiform order of South American weakly electric fish that includes more than 200 species with a broad range of electric signal characteristics, ecologies, and life histories. Brachyhypopomus gauderio is another gymnotiform species with different life history and reproductive strategy than E. virescens. Contrary to E. virescens, B. gauderio continues reproductive behaviors even when energy reserves are depleted, and engages in terminal investments in reproduction during stressful periods. Previous findings from these two species of WEF showed that in contrast to E. virescens, food deprivation produces large transient increases in EODa during social challenges in male B. gauderio. The situationally dependent increase in EODa under food restriction in B. gauderio suggests that different life history and reproductive strategies may drive leptin’s regulatory effect on EODa in opposite directions under different circumstances in E. virescens vs. B. gauderio. However, the effect of leptin on EOD characteristics in B. gauderio has not yet been investigated. For the third chapter of my dissertation, we assessed the peripheral effects of leptin on B. gauderio’s EOD characteristics using similar experimental approaches as Sinnett and Markham (2015) used in E. virescens, aiming to understand whether leptinergic regulation of EODa varies across species with different life histories and reproductive strategies. This study gives us a better understanding of leptin’s full effects on regulating communication signal costs. Overall, this Ph.D. dissertation contributes to the field of animal behavior by providing critical resources to facilitate comparative studies on mechanisms that balance energy-information tradeoffs in active sensory systems.