Distribution and Female Reproductive State Differences in Orexigenic and Anorexigenic Neurons in the Brain of the Mouthbrooding African Cichlid Fish, Astatotilapia burtoni

Abstract

The integration of reproduction and metabolism is necessary for the survival and continuation of a species. While the neural circuits controlling energy homeostasis have been well-characterized, the signals controlling the relay of nutritional information to the reproductive axis are not conclusively defined. The cichlid fish Astatotilapia burtoni is ideal for studying the neural regulation of feeding and reproduction because during their parental care phase, females undergo a two-week period of forced starvation while holding developing young in their buccal cavity. To test the hypothesis that candidate neuropeptides known to be involved in feeding and energy homeostasis in mammals show conserved distribution patterns, we performed immunohistochemistry or in situ hybridization to localize appetite-stimulating (neuropeptide Y, NPY; agouti-related protein, AgRP) and appetite-inhibiting peptides (cocaine and amphetamine-regulated transcript, CART; pro-opiomelanocortin, POMC) in the cichlid fish brain. NPY, AgRP, CART, and pomc somata were localized to the lateral tuberal nucleus (NLT), the putative homolog of the arcuate nucleus, as well as other brain regions, and fiber distributions were similar to other teleosts as well as to mammals. To test whether conserved neuropeptide-containing neurons varied with reproductive state, we also quantified neuron somata size in the NLT as a proxy for their involvement in regulating changes in energy status and reproductive condition. Our results show that gravid females had larger NPY and AgRP neurons in the NLT compared to brooding females, but brooding females had larger POMC neurons compared to gravid females. CART neuron size did not differ between the two reproductive states. Thus, larger appetite-stimulating neurons (NPY, AgRP) likely promote feeding while females are gravid, while larger POMC neurons may act as a satiety signal to inhibit food intake during mouthbrooding. Hypothalamic mRNA levels for npy, agrp, pomc-α, cart 2 and cart 4 were also measured, and while AgRP mRNA levels were higher in gravid compared to brooding females, the remaining gene products did not differ between reproductive states. Collectively, however, our data suggest a potential role for NPY, AgRP, POMC and CART in regulating food intake in A. burtoni females during varying reproductive states.