Neuroanatomical Evidence for Catecholamines as Modulators of Audition and Acoustic Behavior in a Vocal Teleost

Organization of the Catecholaminergic System in the Short-Lived Fish Nothobranchius furzeri

The catecholaminergic system has received much attention based on its regulatory role in a wide range of brain functions and its relevance in aging and neurodegenerative diseases. In the present study, we analyzed the neuroanatomical distribution of catecholaminergic neurons based on tyrosine hydroxylase (TH) immunoreactivity in the brain of adult Nothobranchius furzeri. In the telencephalon, numerous TH+ neurons were observed in the olfactory bulbs and the ventral telencephalic area, arranged as strips extending through the rostrocaudal axis. We found the largest TH+ groups in the diencephalon at the preoptic region level, the ventral thalamus, the pretectal region, the posterior tuberculum, and the caudal hypothalamus. In the dorsal mesencephalic tegmentum, we identified a particular catecholaminergic group. The rostral rhombencephalon housed TH+ cells in the locus coeruleus and the medulla oblongata, distributing in a region dorsal to the inferior reticular formation, the vagal lobe, and the area postrema. Finally, scattered TH+ neurons were present in the ventral spinal cord and the retina. From a comparative perspective, the overall organization of catecholaminergic neurons is consistent with the general pattern reported for other teleosts. However, N. furzeri shows some particular features, including the presence of catecholaminergic cells in the midbrain. This work provides a detailed neuroanatomical map of the catecholaminergic system of N. furzeri, a powerful aging model, also contributing to the phylogenetic understanding of one of the most ancient neurochemical systems.

Postweaning Isolation Alters the Responses of Auditory Neurons to Serotonergic Modulation

Juvenile social experience, such as social isolation, has profound effects on communicative behavior, including signal production and reception. In the current study, we explored responsiveness to the neuromodulator serotonin as a potential mechanistic link between early life social isolation and auditory processing. The serotonergic system is sensitive to social isolation in many brain regions including the inferior colliculus (IC), an auditory midbrain nucleus. We investigated the effects of social experience on serotonergic responsiveness by measuring cFos, an immediate early gene product, in the IC of female mice. Serotonin was manipulated pharmacologically by administering fenfluramine, pCPA, or saline to mice that had undergone an extreme dichotomy in social experience after weaning: being housed in social groups versus individually. These mice were exposed to a 60-min recording of vocalizations from an opposite-sex interaction and perfused. Using immunohistochemistry, we measured the density of cFos-positive (cFos+) nuclei in the major subdivisions of the IC. Housing condition, drug treatment, and IC subregion all had a significant effect on cFos+ density. The central IC showed the highest density of cFos+ cells and also the most pronounced effects of housing condition and drug treatment. In the central IC, cFos+ density was higher following fenfluramine treatment than saline, and lower following pCPA treatment than fenfluramine. Individually housed mice showed a higher cFos+ density than socially housed mice in both of the pharmacological treatment groups, but not in the saline group. Drug treatment but not housing condition had strong effects on the behaviors of grooming, digging, rearing, and movement. Once the effects of drug condition were controlled, there were no across-individual correlations between cFos+ densities and behaviors. These findings suggest that the responses of auditory neurons to neuromodulation by serotonin are influenced by early life experience.

Testosterone Treatment Mimics Seasonal Downregulation of Dopamine Innervation in the Auditory System of Female Midshipman Fish

In seasonally breeding vertebrates, hormones coordinate changes in nervous system structure and function to facilitate reproductive readiness and success. Steroid hormones often exert their effects indirectly via regulation of neuromodulators, which in turn can coordinate the modulation of sensory input with appropriate motor output. Female plainfin midshipman fish (Porichthys notatus) undergo increased peripheral auditory sensitivity in time for the summer breeding season, improving their ability to detect mates, which is regulated by steroid hormones. Reproductive females also show differences in catecholaminergic innervation of auditory circuitry compared with winter, non-reproductive females as measured by tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholaminergic synthesis. Importantly, catecholaminergic input to the inner ear from a dopaminergic-specific forebrain nucleus is decreased in the summer and dopamine inhibits the sensitivity of the inner ear, suggesting that gonadal steroids may alter auditory sensitivity by regulating dopamine innervation. In this study, we gonadectomized non-reproductive females, implanted them with estradiol (E2) or testosterone (T), and measured TH immunoreactive (TH-ir) fibers in auditory nuclei where catecholaminergic innervation was previously shown to be seasonally plastic. We found that treatment with T, but not E2, reduced TH-ir innervation in the auditory hindbrain. T-treatment also reduced TH-ir fibers in the forebrain dopaminergic cell group that projects to the inner ear, and likely to the auditory hindbrain. Higher T plasma in the treatment group was correlated with reduced-ir TH terminals in the inner ear. These T-treatment induced changes in TH-ir fibers mimic the seasonal downregulation of dopamine in the midshipman inner ear and provide evidence that steroid hormone regulation of peripheral auditory sensitivity is mediated, in part, by dopamine.

Comparative Analysis of the Organization of the Catecholaminergic Systems in the Brain of Holostean Fishes (Actinopterygii/Neopterygii)

Living holosteans, comprising 8 species of bowfins and gars, form a small monophyletic group of actinopterygian fishes, which are currently considered as the sister group to the enormously numerous teleosts and have largely been neglected in neuroanatomical studies. We have studied the catecholaminergic (CAergic) systems by means of antibodies against tyrosine hydroxylase (TH) and dopamine (DA) in the brain of representative species of the 3 genera included in the 2 orders of holostean fishes: Amia calva (Amiiformes) and Lepisosteus platyrhincus, Lepisosteus oculatus, and Atractosteus spatula (Lepisosteiformes). Different groups of TH/DA-immunoreactive (ir) cells were observed in the olfactory bulb, subpallium, and preoptic area of the telencephalon. Hypothalamic groups were labeled in the suprachiasmatic nucleus, tuberal (only in A. calva), retrotuberal, and retromamillary areas; specifically, the paraventricular organ showed only DA immunoreactivity. In the diencephalon, TH/DA-ir groups were detected in the prethalamus, posterior tubercle, and pretectum. In the caudal hindbrain, the solitary tract nucleus and area postrema presented TH/DA-ir cell groups, and also the spinal cord and the retina. Only in A. calva, particular CAergic cell groups were observed in the habenula, the mesencephalic tegmentum, and in the locus coeruleus. Following a neuromeric analysis, the comparison of these results with those obtained in other classes of fishes and tetrapods shows many common traits of CAergic systems shared by most vertebrates and in addition highlights unique features of actinopterygian fishes.

Dopaminergic neurons are preferentially responsive to advertisement calls and co-active with social behavior network nuclei in sneaker male midshipman fish

Vocal species use acoustic signals to facilitate diverse behaviors such as mate attraction and territorial defense. However, little is known regarding the neural substrates that interpret such divergent conspecific signals. Using the plainfin midshipman fish model, we tested whether specific catecholaminergic (i.e., dopaminergic and noradrenergic) nuclei and nodes of the social behavior network (SBN) are differentially responsive following exposure to playbacks of divergent social signals in sneaker males. We chose sneaker (type II) males since they attempt to steal fertilizations from territorial type I males who use an advertisement call (hum) to attract females yet are also subjected to vocal agonistic behavior (grunts) by type I males. We demonstrate that induction of cFos (an immediate early gene product and proxy for neural activation) in two forebrain dopaminergic nuclei is greater in sneaker males exposed to hums but not grunts compared to ambient noise, suggesting hums preferentially activate these nuclei, further asserting dopamine as an important regulator of social-acoustic behaviors. Moreover, acoustic exposure to social signals with divergent salience engendered contrasting shifts in functional connectivity between dopaminergic nuclei and nodes of the SBN, supporting the idea that interactions between these two circuits may underlie adaptive decision-making related to intraspecific male competition.

Understanding Female Receiver Psychology in Reproductive Contexts

Mate choice decision-making requires four components: sensory, cognitive, motivation, and salience. During the breeding season, the neural mechanisms underlying these components act in concert to radically transform the way a female perceives the social cues around her as well as the way in which cognitive and motivational processes influence her decision to respond to courting males. The role of each of these four components in mate choice responses will be discussed here as well as the brain regions involved in regulating each component. These components are not independent, modular systems. Instead, they are dependent on one another. This review will discuss the many ways in which these components interact and affect one another. The interaction of these components, however, ultimately leads back to a few key neuromodulators that thread motivation, sensory, salience, and cognitive components into a set of inter-dependent processes. These neuromodulators are estrogens and catecholamines. This review will highlight the need to understand estrogens in reproductive contexts not just as simply a 'sexual motivation modulator' or catecholamines as 'cognitive regulators' but as neuromodulators that work together to fully transform a non-breeding female into a completely reproductive female displaying: heightened sexual interest in courting males, greater arousal and selective attention toward courtship signals, improved signal detection and discrimination abilities, enhanced contextual signal memory, and increased motivation to respond to signals assigned incentive salience. The aim of this review is to build a foundation in which to understand the brain regions associated with cognitive, sensory, motivational, and signal salience not as independently acting systems but as a set of interacting processes that function together in a context-appropriate manner.

Attention and Motivated Response to Simulated Male Advertisement Call Activates Forebrain Dopaminergic and Social Decision-Making Network Nuclei in Female Midshipman Fish

Little is known regarding the coordination of audition with decision-making and subsequent motor responses that initiate social behavior including mate localization during courtship. Using the midshipman fish model, we tested the hypothesis that the time spent by females attending and responding to the advertisement call is correlated with the activation of a specific subset of catecholaminergic (CA) and social decision-making network (SDM) nuclei underlying auditory- driven sexual motivation. In addition, we quantified the relationship of neural activation between CA and SDM nuclei in all responders with the goal of providing a map of functional connectivity of the circuitry underlying a motivated state responsive to acoustic cues during mate localization. In order to make a baseline qualitative comparison of this functional brain map to unmotivated females, we made a similar correlative comparison of brain activation in females who were unresponsive to the advertisement call playback. Our results support an important role for dopaminergic neurons in the periventricular posterior tuberculum and ventral thalamus, putative A11 and A13 tetrapod homologues, respectively, as well as the posterior parvocellular preoptic area and dorsomedial telencephalon, (laterobasal amygdala homologue) in auditory attention and appetitive sexual behavior in fishes. These findings may also offer insights into the function of these highly conserved nuclei in the context of auditory-driven reproductive social behavior across vertebrates.

Projections of the Diencephalospinal Dopaminergic System to Peripheral Sense Organs in Larval Zebrafish (Danio rerio)

Dopaminergic neurons of the descending diencephalospinal system are located in the posterior tuberculum (PT) in zebrafish (Danio rerio), and correspond in mammals to the A11 group in hypothalamus and thalamus. In the larval zebrafish, they are likely the only source of central dopaminergic projections to the periphery. Here, we characterized posterior tubercular dopaminergic fibers projecting to peripheral sense organs, with a focus on the lateral line neuromasts. We labeled and identified catecholaminergic neurons and their projections by combining two immunofluorescence techniques, (i) using an antibody against Tyrosine hydroxylase, and (ii) using an antibody against GFP in transgenic zebrafish expressing in catecholaminergic neurons either membrane-anchored GFP to track fibers, or a Synaptophysin-GFP fusion to visualize putative synapses. We applied the CLARITY method to 6 days old whole zebrafish larvae to stain and analyze dopaminergic projections by confocal microscopy. We found that all lateral line neuromasts receive direct innervation by posterior tubercular dopaminergic neurons, and tracked these projections in detail. In addition, we found dopaminergic fibers projecting to the anterior and posterior lateral line ganglia, and extensive central dopaminergic arborizations around the terminal projection field of the lateral line afferent neurons in the hindbrain medial octavolateralis nucleus (MON). Therefore, dopaminergic innervation may affect lateral line sense information at different processing stages. Additional dopaminergic fibers innervate the trigeminal ganglion, and we observed fine catecholaminergic fibers in the skin with arborization patterns similar to free sensory nerve endings. We also detected potentially dopaminergic fibers innervating inner ear sensory epithelia. Therefore, the diencephalospinal A11-type dopaminergic system may broadly modulate peripheral senses. We also briefly report peripheral sympathetic catecholaminergic projections labeled in our experiments, and their innervation of the developing intestine, swim bladder and abdominal organs.

Connectivity and ultrastructure of dopaminergic innervation of the inner ear and auditory efferent system of a vocal fish

Dopamine (DA) is a conserved modulator of vertebrate neural circuitry, yet our knowledge of its role in peripheral auditory processing is limited to mammals. The present study combines immunohistochemistry, neural tract tracing, and electron microscopy to investigate the origin and synaptic characteristics of DA fibers innervating the inner ear and the hindbrain auditory efferent nucleus in the plainfin midshipman, a vocal fish that relies upon the detection of mate calls for reproductive success. We identify a DA cell group in the diencephalon as a common source for innervation of both the hindbrain auditory efferent nucleus and saccule, the main hearing endorgan of the inner ear. We show that DA terminals in the saccule contain vesicles but transmitter release appears paracrine in nature, due to the apparent lack of synaptic contacts. In contrast, in the hindbrain, DA terminals form traditional synaptic contacts with auditory efferent neuronal cell bodies and dendrites, as well as unlabeled axon terminals, which, in turn, form inhibitory-like synapses on auditory efferent somata. Our results suggest a distinct functional role for brain-derived DA in the direct and indirect modulation of the peripheral auditory system of a vocal nonmammalian vertebrate.