Electrical synapses connect a network of gonadotropin releasing hormone neurons in a cichlid fish

Generation of the First Transgenic Line of the Iconic Coral Reef Fish Amphiprion ocellaris

The common clownfish, Amphiprion ocellaris, is an iconic coral reef fish, ubiquitous in the marine aquarium hobby and useful for studying a variety of biological processes (e.g., mutual symbiosis, ultraviolet vision, and protandrous sex change). Recently, CRISPR/Cas9 methods were developed for knocking out specific genes for mechanistic studies. Here, we expand the genetic toolkit for A. ocellaris by creating the first transgenic line using the Tol2 transposon system. Fertilized eggs were co-injected with Tol2 transposase mRNA and a plasmid encoding an elongation factor-1α (Ef1α): green fluorescent protein (GFP) cassette at various concentrations, needle tip dimensions, and timepoints post-fertilization. We compared various injection parameters and sterilization methods to maximize the survival of injected eggs. F0s (n = 10) that were genotyped GFP + were then raised to 6 months of age and crossed with wild-type (WT) females to confirm germline transmission. F1 offspring were also raised and crossed in the same manner. The highly efficient Tol2 transposon system resulted in a 37% rate of transgenesis for surviving eggs amounting to a 2.7% yield of all injected eggs surviving and being GFP + (n = 160). Of these, 10 were raised to adulthood, 8 spawned, and 5/8 (62.5%) produced GFP + offspring. Further, two F1s crossed with WT females produced 54.2% and 44.6% GFP + offspring respectively, confirming the creation of a stable line. This is, to our knowledge, the first generation of a transgenic line in any coral reef fish. The ability to express transgenes of interest in the iconic anemonefish opens the door to a new era of exploration into their fascinating biology.

The Transcriptome Characterization of the Hypothalamus and the Identification of Key Genes during Sexual Maturation in Goats

Sexual maturation in goats is a dynamic process regulated precisely by the hypothalamic-pituitary-gonadal axis and is essential for reproduction. The hypothalamus plays a crucial role in this process and is the control center of the reproductive activity. It is significant to study the molecular mechanisms in the hypothalamus regulating sexual maturation in goats. We analyzed the serum hormone profiles and hypothalamic mRNA expression profiles of female goats during sexual development (1 day old (neonatal, D1, n = 5), 2 months old (prepuberty, M2, n = 5), 4 months old (sexual maturity, M4, n = 5), and 6 months old (breeding period, M6, n = 5)). The results indicated that from D1 to M6, serum hormone levels, including FSH, LH, progesterone, estradiol, IGF1, and leptin, exhibited an initial increase followed by a decline, peaking at M4. Furthermore, we identified a total of 508 differentially expressed genes in the hypothalamus, with a total of four distinct expression patterns. Nuclear receptor subfamily 1, group D, member 1 (NR1D1), glucagon-like peptide 1 receptor (GLP1R), and gonadotropin-releasing hormone 1 (GnRH-1) may contribute to hormone secretion, energy metabolism, and signal transduction during goat sexual maturation via circadian rhythm regulation, ECM receptor interactions, neuroactive ligand-receptor interactions, and Wnt signaling pathways. This investigation offers novel insights into the molecular mechanisms governing the hypothalamic regulation of goat sexual maturation.

Expression of novel androgen receptors in three GnRH neuron subtypes in the cichlid brain

In teleosts, GnRH1 neurons stand at the apex of the Hypothalamo-Pituitary-Gonadal (HPG) axis, which is responsible for the production of sex steroids by the gonads (notably, androgens). To exert their actions, androgens need to bind to their specific receptors, called androgen receptors (ARs). Due to a teleost-specific whole genome duplication, A. burtoni possess two AR paralogs (ARα and ARβ) that are encoded by two different genes, ar1 and ar2, respectively. In A. burtoni, males stratify along dominance hierarchies, in which an individuals' social status determines its physiology and behavior. GnRH1 neurons have been strongly linked with dominance and circulating androgen levels. Similarly, GnRH3 neurons are implicated in the display of male specific behaviors. Some studies have shown that these GnRH neurons are responsive to fluctuations in circulating androgens levels, suggesting a link between GnRH neurons and ARs. While female A. burtoni do not naturally form a social hierarchy, their reproductive state is positively correlated to androgen levels and GnRH1 neuron size. Although there are reports related to the expression of ar genes in GnRH neurons in cichlid species, the expression of each ar gene remains inconclusive due to technical limitations. Here, we used immunohistochemistry, in situ hybridization chain reaction (HCR), and spatial transcriptomics to investigate ar1 and ar2 expression specifically in GnRH neurons. We find that all GnRH1 neurons intensely express ar1 but only a few of them express ar2, suggesting the presence of genetically-distinct GnRH1 subtypes. Very few ar1 and ar2 transcripts were found in GnRH2 neurons. GnRH3 neurons were found to express both ar genes. The presence of distinct ar genes within GnRH neuron subtypes, most clearly observed for GnRH1 neurons, suggests differential control of these neurons by androgenic signaling. These findings provide valuable insight for future studies aimed at disentangling the androgenic control of GnRH neuron plasticity and reproductive plasticity across teleosts.

Expression of novel androgen receptors in three GnRH neuron subtypes in the cichlid brain

Within a social hierarchy, an individuals' social status determines its physiology and behavior. In A. burtoni, subordinate males can rise in rank to become dominant, which is accompanied by the upregulation of the entire HPG axis, including activation of GnRH1 neurons, a rise in circulating androgen levels and the display of specific aggressive and reproductive behaviors. Cichlids possess two other GnRH subtypes, GnRH2 and GnRH3, the latter being implicated in the display of male specific behaviors. Interestingly, some studies showed that these GnRH neurons are responsive to fluctuations in circulating androgen levels, suggesting a link between GnRH neurons and androgen receptors (ARs). Due to a teleost-specific whole genome duplication, A. burtoni possess two AR paralogs (ARα and ARβ) that are encoded by two different genes, ar1 and ar2, respectively. Even though social status has been strongly linked to androgens, whether ARα and/or ARβ are present in GnRH neurons remains unclear. Here, we used immunohistochemistry and in situ hybridization chain reaction (HCR) to investigate ar1 and ar2 expression specifically in GnRH neurons. We find that all GnRH1 neurons intensely express ar1 but only a few of them express ar2, suggesting the presence of genetically-distinct GnRH1 subtypes. Very few ar1 and ar2 transcripts were found in GnRH2 neurons. GnRH3 neurons were found to express both ar genes. The presence of distinct ar genes within GnRH neuron subtypes, most clearly observed for GnRH1 neurons, suggests differential control of these neurons by androgenic signaling. These findings provide valuable insight for future studies aimed at disentangling the androgenic control of GnRH neuron plasticity and reproductive plasticity across teleosts.

Genome editing in East African cichlids and tilapias: state-of-the-art and future directions

African cichlid fishes of the Cichlidae family are a group of teleosts important for aquaculture and research. A thriving research community is particularly interested in the cichlid radiations of the East African Great Lakes. One key goal is to pinpoint genetic variation underlying phenotypic diversification, but the lack of genetic tools has precluded thorough dissection of the genetic basis of relevant traits in cichlids. Genome editing technologies are well established in teleost models like zebrafish and medaka. However, this is not the case for emerging model organisms, such as East African cichlids, where these technologies remain inaccessible to most laboratories, due in part to limited exchange of knowledge and expertise. The Cichlid Science 2022 meeting (Cambridge, UK) hosted for the first time a Genome Editing Workshop, where the community discussed recent advances in genome editing, with an emphasis on CRISPR/Cas9 technologies. Based on the workshop findings and discussions, in this review we define the state-of-the-art of cichlid genome editing, share resources and protocols, and propose new possible avenues to further expand the cichlid genome editing toolkit.

Gene duplication of C-type natriuretic peptide-4 (CNP4) in teleost lineage elicits subfunctionalization of ancestral CNP

The diversified natriuretic peptide (NP) family, consisting of four CNPs (CNP1-4), ANP, BNP, and VNP, has been identified in the eel. Here, we successfully cloned additional cnp genes from the brain of eel (a basal teleost) and zebrafish (a later branching teleost). The genes were identified as paralogues of cnp4 generated by the third round of whole genome duplication (3R) in the teleost lineage, thereby being named eel cnp4b and zebrafish cnp4-like, respectively. To examine the histological patterns of their expressions, we employed a newly developed in situ hybridization (ISH) chain reaction using short hairpin DNAs, in addition to conventional ISH. Eel cnp4b was expressed in the medulla oblongata, while mRNAs of eel cnp4a (former cnp4) were localized in the preoptic area. In the zebrafish brain, cnp4-like mRNA was undetectable, while the known cnp4 was expressed in both the preoptic area and medulla oblongata. Together with the different mRNA distribution of cnp4a and cnp4b in eel peripheral tissues determined by RT-PCR and ISH, it is suggested that subfunctionalization by duplicated cnp4s in ancestral teleosts has been retained only in basal teleosts. Intriguingly, cnp4b-expressing neurons in the glossopharyngeal-vagal motor complex of the medulla oblongata were co-localized with choline acetyltransferase, suggesting an involvement of Cnp4b in swallowing and respiration functions that are modulated by the vagus. Since teleost Cnp4 is an ortholog of mammalian CNP, the identified localization of teleost Cnp4 will contribute to future studies aimed at deciphering the physiological functions of CNP.

Multiple gonadotropin-releasing hormone systems in non-mammalian vertebrates: Ontogeny, anatomy, and physiology

Three paralogous genes for gonadotropin-releasing hormone (GnRH; gnrh1, gnrh2, and gnrh3) and GnRH receptors exist in non-mammalian vertebrates. However, there are some vertebrate species in which one or two of these paralogous genes have become non-functional during evolution. The developmental migration of GnRH neurons in the brain is evolutionarily conserved in mammals, reptiles, birds, amphibians, and jawed teleost fish. The three GnRH paralogs have specific expression patterns in the brain and originate from multiple sites. In acanthopterygian teleosts (medaka, cichlid, etc.), the preoptic area (POA)-GnRH1 and terminal nerve (TN)-GnRH3 neuronal types originate from the olfactory regions. In other fish species (zebrafish, goldfish and salmon) with only two GnRH paralogs (GnRH2 and GnRH3), the TN- and POA-GnRH3 neuronal types share the same olfactory origin. However, the developmental origin of midbrain (MB)-GnRH2 neurons is debatable between mesencephalic or neural crest site. Each GnRH system has distinctive anatomical and physiological characteristics, and functions differently. The POA-GnRH1 neurons are hypophysiotropic in nature and function in the neuroendocrine control of reproduction. The non-hypophysiotropic GnRH2/GnRH3 neurons probably play neuromodulatory roles in metabolism (MB-GnRH2) and the control of motivational state for sexual behavior (TN-GnRH3).

A half century of fish gonadotropin-releasing hormones: Breaking paradigms

The field of fish gonadotropin-releasing hormones (GnRHs) is also celebrating its 50th anniversary this year. This review provides a chronological history of fish GnRH biology over the past five decades. It demonstrates how discoveries in fish regarding GnRH and GnRH receptor multiplicity, dynamic interactions between GnRH neurons, and additional neuroendocrine factors acting alongside GnRH, amongst others, have driven a paradigm shift in our understanding of GnRH systems and functions in vertebrates, including mammals. The role of technological innovations in enabling scientific discoveries is portrayed, as well as how fundamental research in fish GnRH led to translational outcomes in aquaculture. The interchange between fish and mammalian GnRH research is discussed, as is the value and utility of using fish models for advancing GnRH biology. Current challenges and future perspectives are presented, with the hope of expanding the dialogue and collaborations within the neuroendocrinology scientific community at large, capitalizing on diversifying model animals and the use of comparative strategies.

Manipulation of the Tyrosinase gene permits improved CRISPR/Cas editing and neural imaging in cichlid fish

Direct tests of gene function have historically been performed in a limited number of model organisms. The CRISPR/Cas system is species-agnostic, offering the ability to manipulate genes in a range of models, enabling insights into evolution, development, and physiology. Astatotilapia burtoni, a cichlid fish from the rivers and shoreline around Lake Tanganyika, has been extensively studied in the laboratory to understand evolution and the neural control of behavior. Here we develop protocols for the creation of CRISPR-edited cichlids and create a broadly useful mutant line. By manipulating the Tyrosinase gene, which is necessary for eumelanin pigment production, we describe a fast and reliable approach to quantify and optimize gene editing efficiency. Tyrosinase mutants also remove a major obstruction to imaging, enabling visualization of subdermal structures and fluorophores in situ. These protocols will facilitate broad application of CRISPR/Cas9 to studies of cichlids as well as other non-traditional model aquatic species.

The gonadotropin-releasing hormones: Lessons from fish

Fish have been of paramount importance to our understanding of vertebrate comparative neuroendocrinology and the mechanisms underlying the physiology and evolution of gonadotropin-releasing hormones (GnRH) and their genes. This review integrates past and recent knowledge on the Gnrh system in the fish model. Multiple Gnrh isoforms (two or three forms) are present in all teleosts, as well as multiple Gnrh receptors (up to five types), which differ in neuroanatomical localization, pattern of projections, ontogeny and functions. The role of the different Gnrh forms in reproduction seems to also differ in teleost models possessing two versus three Gnrh forms, Gnrh3 being the main hypophysiotropic hormone in the former and Gnrh1 in the latter. Functions of the non-hypothalamic Gnrh isoforms are still unclear, although under suboptimal physiological conditions (e.g. fasting), Gnrh2 may increase in the pituitary to ensure the integrity of reproduction under these conditions. Recent developments in transgenesis and mutagenesis in fish models have permitted the generation of fish lines expressing fluorophores in Gnrh neurons and to elucidate the dynamics of the elaborate innervations of the different neuronal populations, thus enabling a more accurate delineation of their reproductive roles and regulations. Moreover, in combination with neuronal electrophysiology, these lines have clarified the Gnrh mode of actions in modulating Lh and Fsh activities. While loss of function and genome editing studies had the premise to elucidate the exact roles of the multiple Gnrhs in reproduction and other processes, they have instead evoked an ongoing debate about these roles and opened new avenues of research that will no doubt lead to new discoveries regarding the not-yet-fully-understood Gnrh system.

Morphological Evidence for Functional Crosstalk Between Multiple GnRH Systems in the Male Tilapia, Oreochromis niloticus

Gonadotropin-releasing hormone (GnRH) is a reproductive neuropeptide, which controls vertebrate reproduction. In most vertebrates, there are more than two GnRH orthologs in the brain. In cichlid fish, the Nile tilapia (Oreochromis niloticus), GnRH1 is the primary hypophysiotropic hormone, while GnRH2 and GnRH3 are non-hypophysiotropic but neuromodulatory in function. Hypophysiotropic GnRH neurons are thought to inter-communicate, while it remains unknown if hypophysiotropic and non-hypophysiotropic GnRH systems communicate with each other. In the present study, we examined interrelationship between three GnRH types using specific antibodies raised against their respective GnRH associated peptide (GAP) sequence. Double-immunofluorescence labeling coupled with confocal microscopy revealed that in sexually mature males, GnRH-GAP1-immunoreactive (-ir) processes are in proximities of GnRH-GAP3-ir cell somata in the terminal nerve, while GnRH-GAP1-ir cell somata were also accompanied by GnRH-GAP3-ir processes in the preoptic area. However, such interaction was not seen in immature males. Further, there was no interaction between GnRH-GAP2 and GnRH-GAP1 or GnRH-GAP3 neurons. Single cell gene expression analysis revealed co-expression of multiple GnRH receptor genes (gnrhr1 and gnrhr2) in three GnRH-GAP cell types. In mature males, high levels of gnrhr2 mRNA were expressed in GnRH-GAP1-ir cells. In immature males, gnrhr1 and gnrhr2 mRNAs are highly expressed in GnRH-GAP3-ir cells. These results suggest heterologous interactions between the three GnRH-GAP cell types and their potential functional interaction during different reproductive stages.

Mechanistic target of rapamycin (mTOR) implicated in plasticity of the reproductive axis during social status transitions

The highly conserved brain-pituitary-gonadal (BPG) axis controls reproduction in all vertebrates, so analyzing the regulation of this signaling cascade is important for understanding reproductive competence. The protein kinase mechanistic target of rapamycin (mTOR) functions as a conserved regulator of cellular growth and metabolism in all eukaryotes, and also regulates the reproductive axis in mammals. However, whether mTOR might also regulate the BPG axis in non-mammalian vertebrates remains unexplored. We used complementary experimental approaches in an African cichlid fish, Astatotilapia burtoni, to demonstrate that mTOR is involved in regulation of the brain, pituitary, and testes when males rise in rank to social dominance. mTOR or downstream components of its signaling pathway (p-p70S6K) were detected in gonadotropin-releasing hormone (GnRH1) neurons, the pituitary, and testes. Transcript levels of mtor in the pituitary and testes also varied when reproductively-suppressed subordinate males rose in social rank to become dominant reproductively-active males, a transition similar to puberty in mammals. Intracerebroventricular injection of the mTORC1 inhibitor, rapamycin, revealed a role for mTOR in the socially-induced hypertrophy of GnRH1 neurons. Rapamycin treatment also had effects at the pituitary and testes, suggesting involvement of the mTORC1 complex at multiple levels of the reproductive axis. Thus, we show that mTOR regulation of BPG function is conserved to fishes, likely playing important roles in regulating reproduction and fertility across all male vertebrates.

Genome-wide effects of social status on DNA methylation in the brain of a cichlid fish, Astatotilapia burtoni

BACKGROUND

Successful social behavior requires real-time integration of information about the environment, internal physiology, and past experience. The molecular substrates of this integration are poorly understood, but likely modulate neural plasticity and gene regulation. In the cichlid fish species Astatotilapia burtoni, male social status can shift rapidly depending on the environment, causing fast behavioral modifications and a cascade of changes in gene transcription, the brain, and the reproductive system. These changes can be permanent but are also reversible, implying the involvement of a robust but flexible mechanism that regulates plasticity based on internal and external conditions. One candidate mechanism is DNA methylation, which has been linked to social behavior in many species, including A. burtoni. But, the extent of its effects after A. burtoni social change were previously unknown.

RESULTS

We performed the first genome-wide search for DNA methylation patterns associated with social status in the brains of male A. burtoni, identifying hundreds of Differentially Methylated genomic Regions (DMRs) in dominant versus non-dominant fish. Most DMRs were inside genes supporting neural development, synapse function, and other processes relevant to neural plasticity, and DMRs could affect gene expression in multiple ways. DMR genes were more likely to be transcription factors, have a duplicate elsewhere in the genome, have an anti-sense lncRNA, and have more splice variants than other genes. Dozens of genes had multiple DMRs that were often seemingly positioned to regulate specific splice variants.

CONCLUSIONS

Our results revealed genome-wide effects of A. burtoni social status on DNA methylation in the brain and strongly suggest a role for methylation in modulating plasticity across multiple biological levels. They also suggest many novel hypotheses to address in mechanistic follow-up studies, and will be a rich resource for identifying the relationships between behavioral, neural, and transcriptional plasticity in the context of social status.

The Future of Gene-Guided Neuroscience Research in Non-Traditional Model Organisms

Natural variations across animals in form, function, and behavior have long been sources of inspiration to scientists. Despite this, experimentalists focusing on the neural bases of behavior have increasingly focused on a select few model species. This consolidation is motivated primarily by the availability of resources and technologies for manipulation in these species. Recent years have witnessed a proliferation of experimental approaches that were developed primarily in traditional model species, but that may in principle be readily applied to any species. High-throughput sequencing, CRISPR gene editing, transgenesis, and other technologies have enabled new insights through their deployment in non-traditional model species. The availability of such approaches changes the calculation of which species to study, particularly when a trait of interest is most readily observed in a non-traditional model organism. If these technologies are widely adopted in many new species, it promises to revolutionize the field of neuroethology.

Fgf3 is crucial for the generation of monoaminergic cerebrospinal fluid contacting cells in zebrafish

In most vertebrates, including zebrafish, the hypothalamic serotonergic cerebrospinal fluid-contacting (CSF-c) cells constitute a prominent population. In contrast to the hindbrain serotonergic neurons, little is known about the development and function of these cells. Here, we identify fibroblast growth factor (Fgf)3 as the main Fgf ligand controlling the ontogeny of serotonergic CSF-c cells. We show that fgf3 positively regulates the number of serotonergic CSF-c cells, as well as a subset of dopaminergic and neuroendocrine cells in the posterior hypothalamus via control of proliferation and cell survival. Further, expression of the ETS-domain transcription factor etv5b is downregulated after fgf3 impairment. Previous findings identified etv5b as critical for the proliferation of serotonergic progenitors in the hypothalamus, and therefore we now suggest that Fgf3 acts via etv5b during early development to ultimately control the number of mature serotonergic CSF-c cells. Moreover, our analysis of the developing hypothalamic transcriptome shows that the expression of fgf3 is upregulated upon fgf3 loss-of-function, suggesting activation of a self-compensatory mechanism. Together, these results highlight Fgf3 in a novel context as part of a signalling pathway of critical importance for hypothalamic development.

Summary: This study highlights Fgf3 in a novel context where it is part of a signalling pathway of critical importance for development of hypothalamic monoaminergic cells in zebrafish.

Social plasticity in the fish brain: Neuroscientific and ethological aspects

Social plasticity, defined as the ability to adaptively change the expression of social behavior according to previous experience and to social context, is a key ecological performance trait that should be viewed as crucial for Darwinian fitness. The neural mechanisms for social plasticity are poorly understood, in part due to skewed reliance on rodent models. Fish model organisms are relevant in the field of social plasticity for at least two reasons: first, the diversity of social organization among fish species is staggering, increasing the breadth of evolutionary relevant questions that can be asked. Second, that diversity also suggests translational relevance, since it is more likely that "core" mechanisms of social plasticity are discovered by analyzing a wider variety of social arrangements than relying on a single species. We analyze examples of social plasticity across fish species with different social organizations, concluding that a "core" mechanism is the initiation of behavioral shifts through the modulation of a conserved "social decision-making network", along with other relevant brain regions, by monoamines, neuropeptides, and steroid hormones. The consolidation of these shifts may be mediated via neurogenomic adjustments and regulation of the expression of plasticity-related molecules (transcription factors, cell cycle regulators, and plasticity products).

Expression of tachykinin3 and related reproductive markers in the brain of the African cichlid fish Astatotilapia burtoni

Neurokinin B, encoded by the tachykinin3 gene, plays a crucial role in regulating reproduction in mammals via KNDy neurons and interaction with GnRH. Previous work in teleost fishes has focused on hypothalamic tac3 expression for its role in reproduction, but detailed studies on extra-hypothalamic tac3 expression are limited. Here, we identified two tac3 genes in the social African cichlid fish Astatotilapia burtoni, only one of which produces a functional protein containing the signature tachykinin motif. In situ hybridization for tac3a mRNA identified cell populations throughout the brain. Numerous tac3a cells lie in several thalamic and hypothalamic nuclei, including periventricular nucleus of posterior tuberculum, lateral tuberal nucleus (NLT), and nucleus of the lateral recess (NRL). Scattered tac3-expressing cells are also present in telencephalic parts, such as ventral (Vv) and supracomissural (Vs) part of ventral telencephalon. In contrast to other teleosts, tac3 expression was absent from the pituitary. Using double-fluorescent staining, we localized tac3a-expressing cells in relation to GnRH and kisspeptin cells. Although no GnRH-tac3a colabeled cells were observed, dense GnRH fibers surround and potentially synapse with tac3a cells in the preoptic area. Only minimal (<5%) colabeling of tac3a was observed in kiss2 cells. Despite tac3a expression in many nodes of the mesolimbic reward system, it was absent from tyrosine hydroxylase (TH)-expressing cells, but tac3a cells were located in areas with dense TH fibers. The presence of tac3a-expressing cells throughout the brain, including in socially relevant brain regions, suggest more diverse functions beyond regulation of reproductive physiology that may be conserved across vertebrates.

Astatotilapia burtoni: A Model System for Analyzing the Neurobiology of Behavior

Most biomedical research is performed using a very limited number of "model" species. In part, this has resulted from a combination of full genomes, manipulation of genes, and short generation times in these species. However, the advent of low-cost sequencing and gene editing in any organism has increased the use of nontraditional organisms. Many scientists have paraphrased the adage by Krogh [ Krogh , A. ( 2018 ) Science 70 , 200 - 204 ] that for many biological problems some species will prove to be most convenient and useful to study. In particular, using organisms most suited to the specific research question can lead to novel insights about fundamental physiological, neurobiological, immunological, and neuroendocrine systems that can advance our understanding of the well-being and health of humans. In addition, such studies have led to new ideas about the evolution and mechanisms that control social behavior. Fishes constitute about 50% of all vertebrate species and are the most diverse vertebrate radiation. Here we review behavioral and neurobiological discoveries of plasticity in social behavior resulting from analysis of an African cichlid fish, showing how its unique behavioral system has facilitated a broad range of discoveries. For many future questions, Astatotilapia burtoni and other cichlid fishes may be ideally suited to study as we advance our understanding of the neural basis of social decisions.

Protein Kinase C Enhances Electrical Synaptic Transmission by Acting on Junctional and Postsynaptic Ca2+ Currents

By synchronizing neuronal activity, electrical transmission influences the coordination, pattern, and/or frequency of firing. In the hemaphroditic marine-snail, Aplysia calfornica, the neuroendocrine bag cell neurons use electrical synapses to synchronize a 30 min afterdischarge of action potentials for the release of reproductive hormone. During the afterdischarge, protein kinase C (PKC) is activated, although its impact on bag cell neuron electrical transmission is unknown. This was investigated here by monitoring electrical synapses between paired cultured bag cell neurons using dual whole-cell recording. Voltage clamp revealed a largely voltage-independent junctional current, which was enhanced by treating with a PKC activator, PMA, before recording. We also examined the transfer of presynaptic action potential-like waveforms (generated in voltage clamp) to the postsynaptic cell (measured in current clamp). For control pairs, the presynaptic spike-like waveforms mainly evoked electrotonic potentials; however, when PKC was triggered, these stimuli consistently produced postsynaptic action potentials. To assess whether this involved changes to postsynaptic responsiveness, single bag cell neurons were injected with junctional-like current mimicking that evoked by a presynaptic action potential. Unlike control neurons, which were less likely to spike, cells in PMA always fired action potentials to the junctional-like current. Furthermore, PKC activation increased a postsynaptic voltage-gated Ca²⁺ current, which was recruited even by modest depolarization associated with an electrotonic potential. Whereas PKC inhibits gap junctions in most systems, bag cell neurons are rather unique, as the kinase potentiates the electrical synapse; in turn, this synergizes with augmented postsynaptic Ca²⁺ current to promote synchronous firing.SIGNIFICANCE STATEMENT Electrical coupling is a fundamental form of communication. For the bag cell neurons of Aplysia, electrical synapses coordinate a prolonged burst of action potentials known as the afterdischarge. We looked at how protein kinase C, which is upregulated with the afterdischarge, influences information transfer across the synapse. The kinase activation increased junctional current, a remarkable finding given that this enzyme is largely considered inhibitory for gap junctions. There was also an augmentation in the ability of a presynaptic neuron to provoke postsynaptic action potentials. This increased excitability was, in part, due to enhanced postsynaptic voltage-dependent Ca²⁺ current. Thus, protein kinase C improves the fidelity of electrotonic transmission and promotes synchronous firing by modulating both junctional and membrane conductances.

The Value of Comparative Animal Research: Krogh's Principle Facilitates Scientific Discoveries

Biomedical research is dominated by relatively few nonhuman animals to investigate healthy and disease conditions. Research has overrelied on these models due to their well-described genomes, the capability to control specific genes, and the high rate of reproduction. However, recent advances in large-scale molecular sequencing experiments have revealed, in some cases, the limited similarities in experimental outcomes observed in common rodents (i.e., mice) compared with humans. The value of more varied comparative animal models includes examples such as long-term body weight regulation in seasonally breeding hamsters as a means to help understand the obesity epidemic, vocal learning in songbirds to illuminate language acquisition and maintenance, and reproduction in cichlid fish to discover novel genes conserved in humans. Studying brain genes in prairie voles and cichlids advanced knowledge about social behavior. Taken together, experiments on diverse animal species highlight nontraditional systems for advancing our understanding of human health and well-being.

High-Frequency Firing Activity of GnRH1 Neurons in Female Medaka Induces the Release of GnRH1 Peptide From Their Nerve Terminals in the Pituitary

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons play an important role in promoting secretion of pituitary luteinizing hormone (LH) and ovulation by releasing GnRH peptide. The release of GnRH peptide is generally assumed to be mainly modulated according to the firing activity of GnRH neurons. However, the relationship between the firing activity and the release of GnRH peptide has been elusive. We analyzed the relationship using two lines of transgenic medaka (gnrh1:enhanced green fluorescent protein and lhb:inverse-pericam) for the combined electrophysiological and Ca2+ imaging analyses. We show that a high-frequency firing activity induced by an excitatory neurotransmitter, glutamate, strongly increases [Ca2+]i in the cell bodies of GnRH1 neurons, which should lead to stimulation of GnRH release. We examined whether this high-frequency firing actually leads to the release of endogenous GnRH1 peptide from the nerve terminals projecting to the pituitary LH cells using a whole brain-pituitary preparation of a fish generated by crossing the two types of transgenic fish. Ca2+ imaging analyses showed that local glutamate activation of GnRH1 cell bodies, but not their nerve terminals in the pituitary, induced a substantial Ca2+ response in LH cells that was abolished in the presence of a GnRH receptor antagonist, Analog M. These results suggest that such an evoked high-frequency firing activity of GnRH1 cell body stimulates the release of endogenous GnRH1 peptide from the axon terminals to the pituitary LH cells. Thus, the findings of the present study have clearly demonstrated the relationship between the firing activity of hypothalamic GnRH neurons and the release of GnRH peptide.

Cognitive skills and the evolution of social systems

How do animal social skills influence evolution? Complex animal social behaviors require many cognitive skills including individual recognition and observational learning. For social systems to evolve, these abilities need to be transmitted genetically or culturally and supported by the evolution of underlying neural systems. Because animal skill sets are so varied, it seems best to describe animal cognitive behaviors as being a social calculus that can change with experience, which has evolved to match and facilitate the complexity of the social system where it arose. That is, acquiring and using social information in response to a rapidly changing complex world leads to social competence enabling success in essential behavioral interactions. Here, we describe the remarkable suite of social skills discovered in the African cichlid fish Astatotilapia burtoni, including an attention hierarchy, male deception, transitive inference, the mechanistic bases of social dominance, female mate choice and the neural control of female reproductive behavior. The social calculus of this species is presented as an example of a potential causal factor in the evolution of sophisticated social behavior necessary for the evolutionary success of their social system.

Dopaminergic inhibition of gonadotropin-releasing hormone neurons in the cichlid fish Astatotilapia burtoni

Dopamine regulates reproduction in part by modulating neuronal activity within the hypothalamic-pituitary-gonadal (HPG) axis. Previous studies suggested numerous mechanisms by which dopamine exerts inhibitory control over the HPG axis, ultimately changing the levels of sex steroids that regulate reproductive behaviors. However, it is not known whether these mechanisms are conserved across vertebrate species. In particular, it is unknown whether mechanisms underlying dopaminergic control of reproduction are shared between mammals and teleost fish. In mammals, dopamine directly inhibits gonadotropin-releasing hormone (GnRH1) hypothalamic neurons, the gatekeepers for activation of the HPG axis. Here, we demonstrate, for the first time in teleost fish, dopaminergic control of GnRH1 neurons via direct dopamine type-2-like receptor (D2R)-mediated inhibition within the hypothalamus. These results suggest that direct dopaminergic control of GnRH1 neurons via interactions in the hypothalamus is not exclusive to tetrapod reproductive control, but is likely conserved across vertebrate species.

Changes of Sexual Behaviors in Rapamycin-injected Cichlid Fish Astatotilapia burtoni Males

Cichlid fish species exhibit characteristic sexual behaviors according to not only reproductive stages but also social status. In a reproductive season, Astatotilapia burtoni males compete for females and a small number of dominant winners finally obtain the chance of spermiation. In addition to the characteristic behaviors, the dominant males have relatively bigger gonadotropin-releasing hormone 1 (GnRH1) neurons in the preoptic area (POA) of brain compared to those of subordinate males. Although the stimulatory effect of GnRH1 in vertebrate reproduction is well established, little is known about the triggering signal pathway to control GnRH1 neurons and GnRH1-mediated sexual behavior. In the present study, we evaluated the potential effect of TOR inhibitor rapamycin in relation to the cichlid male behaviors and GnRH1 neuron. After 14 h and 26 h of intraventricular injection of rapamycin, behavior patterns of chasing and courtship display did not show significant changes between rapamycin- and DMSO-injected males. Behaviors of spawning site entry increased in rapamycininjected fish at 26 h post-injection than at 14 h post-injection significantly (P<0.05). Meanwhile, there was a tendency that GnRH1 neurons' soma size in the POA shrank by rapamycin injection, whereas the testes did not show notable changes. Taken together, these results suggest the possible role of TOR signal on GnRH1-mediated sexual behavior in cichlid dominant males, although further biological characterization of the TOR signaling pathway will be required to clarify this matter.

The Severity of Acute Stress Is Represented by Increased Synchronous Activity and Recruitment of Hypothalamic CRH Neurons

The hypothalamo-pituitary-adrenocortical (HPA) axis regulates stress physiology and behavior. To achieve an optimally tuned adaptive response, it is critical that the magnitude of the stress response matches the severity of the threat. Corticotropin-releasing hormone (CRH) released from the paraventricular nucleus of the hypothalamus is a major regulator of the HPA axis. However, how CRH-producing neurons in an intact animal respond to different stressor intensities is currently not known. Using two-photon calcium imaging on intact larval zebrafish, we recorded the activity of CRH cells, while the larvae were exposed to stressors of varying intensity. By combining behavioral and physiological measures, we first determined how sudden alterations in environmental conditions lead to different levels of stress axis activation. Then, we measured changes in the frequency and amplitude of Ca(2+) transients in individual CRH neurons in response to such stressors. The response magnitude of individual CRH cells covaried with stressor intensity. Furthermore, stressors caused the recruitment of previously inactive CRH neurons in an intensity-dependent manner, thus increasing the pool of responsive CRH cells. Strikingly, stressor-induced activity appeared highly synchronized among CRH neurons, and also across hemispheres. Thus, the stressor strength-dependent output of CRH neurons emerges by a dual mechanism that involves both the increased activity of individual cells and the recruitment of a larger pool of responsive cells. The synchronicity of CRH neurons within and across hemispheres ensures that the overall output of the HPA axis matches the severity of the threat.

SIGNIFICANCE STATEMENT

Stressors trigger adaptive responses in the body that are essential for survival. How the brain responds to acute stressors of varying intensity in an intact animal, however, is not well understood. We address this question using two-photon Ca(2+) imaging in larval zebrafish with transgenically labeled corticotropin-releasing hormone (CRH) cells, which represent a major regulator of the stress axis. We show that stressor strength-dependent responses of CRH neurons emerge via an intensity-dependent increase in the activity of individual CRH cells, and by an increase in the pool of responsive CRH cells at the population level. Furthermore, we report striking synchronicity among CRH neurons even across hemispheres, which suggests tight intrahypothalamic and interhypothalamic coordination. Thus, our work reveals how CRH neurons respond to different levels of acute stress in vivo.

A Neural Basis for Control of Cichlid Female Reproductive Behavior by Prostaglandin F2α

In most species, females time reproduction to coincide with fertility. Thus, identifying factors that signal fertility to the brain can provide access to neural circuits that control sexual behaviors. In vertebrates, levels of key signaling molecules rise at the time of fertility to prime the brain for reproductive behavior [1-11], but how and where they regulate neural circuits is not known [12, 13]. Specifically, 17α,20β-dihydroxyprogesterone (DHP) and prostaglandin F2α (PGF2α) levels rise in teleost fish around the time of ovulation [10, 14, 15]. In an African cichlid fish, Astatotilapia burtoni, fertile females select a mate and perform a stereotyped spawning routine, offering quantifiable behavioral outputs of neural circuits. We show that, within minutes, PGF2α injection activates a naturalistic pattern of sexual behavior in female A. burtoni. We also identify cells in the brain that transduce the prostaglandin signal to mate and show that the gonadal steroid DHP modulates mRNA levels of the putative receptor for PGF2α (Ptgfr). We use CRISPR/Cas9 to generate the first targeted gene mutation in A. burtoni and show that Ptgfr is necessary for the initiation of sexual behavior, uncoupling sexual behavior from reproductive status. Our findings are consistent with a model in which PGF2α communicates fertility status via Ptgfr to circuits in the brain that drive female sexual behavior. Our targeted genome modification in a cichlid fish shows that dissection of gene function can reveal basic control mechanisms for behaviors in this large family of species with diverse and fascinating social systems [16, 17].

Timing reproduction in teleost fish: cues and mechanisms

Fish comprise half of extant vertebrate species and use a rich variety of reproductive strategies that have yielded insights into the basic mechanisms that evolved for sex. To maximize the chances of fertilization and survival of offspring, fish species time reproduction to occur at optimal times. For years, ethologists have performed painstaking experiments to identify sensory inputs and behavioral outputs of the brain during mating. Here we review known mechanisms that generate sexual behavior, focusing on the factors that govern the timing of these displays. The development of new technologies, including high-throughput sequencing and genome engineering, has the potential to provide novel insights into how the vertebrate brain consummates mating at the appropriate time.

Epigenetic DNA Methylation Linked to Social Dominance

Social status hierarchies are ubiquitous in vertebrate social systems, including humans. It is well known that social rank can influence quality of life dramatically among members of social groups. For example, high-ranking individuals have greater access to resources, including food and mating prerogatives that, in turn, have a positive impact on their reproductive success and health. In contrast low ranking individuals typically have limited reproductive success and may experience lasting social and physiological costs. Ultimately, social rank and behavior are regulated by changes in gene expression. However, little is known about mechanisms that transduce social cues into transcriptional changes. Since social behavior is a dynamic process, we hypothesized that a molecular mechanism such as DNA methylation might play a role these changes. To test this hypothesis, we used an African cichlid fish, Astatotilapia burtoni, in which social rank dictates reproductive access. We show that manipulating global DNA methylation state strongly biases the outcomes of social encounters. Injecting DNA methylating and de-methylating agents in low status animals competing for status, we found that animals with chemically increased methylation states were statistically highly likely to ascend in rank. In contrast, those with inhibited methylation processes and thus lower methylation levels were statistically highly unlikely to ascend in rank. This suggests that among its many roles, DNA methylation may be linked to social status and more generally to social behavior.

A new measure for the strength of electrical synapses

Electrical synapses, like chemical synapses, mediate intraneuronal communication. Electrical synapses are typically quantified by subthreshold measurements of coupling, which fall short in describing their impact on spiking activity in coupled neighbors. Here, we describe a novel measurement for electrical synapse strength that directly evaluates the effect of synaptically transmitted activity on spike timing. This method, also applicable to neurotransmitter-based synapses, communicates the considerable strength of electrical synapses. For electrical synapses measured in rodent slices of the thalamic reticular nucleus and in simple model neurons, spike timing is modulated by tens of ms by activity in a coupled neighbor.