Vasotocin actions on electric behavior: interspecific, seasonal, and social context-dependent differences

Intraperitoneal administration of arginine vasotocin (AVT) induces anorexigenic and anxiogenic actions via the brain V1a receptor-signaling pathway in the tiger puffer, Takifugu rubripes

Arginine vasotocin (AVT) is produced mainly in the hypothalamus and as a neurohypophyseal hormone peripherally regulates water-mineral balance in sub-mammals. In addition, AVT-containing neurons innervate several areas of the brain, and AVT also acts centrally as both an anorexigenic and anxiogenic factor in goldfish. However, it is unclear whether these central effects operate in fish in general. In the present study, therefore, we investigated AVT-like immunoreactivity in the brain of the tiger puffer, a cultured fish with a high market value in Japan and also a representative marine teleost species, focusing particularly on whether AVT affects food intake and psychomotor activity. AVT-like immunoreactivity was distributed higher in the ventral region of the telencephalon, the hypothalamus and midbrain. Intraperitoneal (IP) administration of AVT at 100 pmol g-1 body weight (BW) increased the immunoreactivity of phosphorylated ribosomal proteinS6 (RPS6), a neuronal activation marker, in the telencephalon and diencephalon, decreased food consumption and enhanced thigmotaxis. AVT-induced anorexigenic and anxiogenic actions were blocked by IP co-injection of a V1a receptor (V1aR) antagonist, Manning compound (MC) at 300 pmol g-1 BW. These results suggest that AVT acts as an anorexigenic and anxiogenic factor via the V1aR-signaling pathway in the tiger puffer brain.

Immunohistochemical description of isotocin neurons and the anatomo-functional comparative analysis between isotocin and vasotocin systems in the weakly electric fish, Gymnotus omaroum

The vasopressin-vasotocin (AVP-AVT) and oxytocin-mesotocin-isotocin (OT-MT-IT) families of nonapeptides are of great importance in shaping context-dependent modulations of a conserved and yet highly plastic network of brain areas involved in social behavior: the social behavior network. The nonapeptide systems of teleost fish are highly conserved and share a common general organization. In this study, we first describe the presence of IT cells and projections in the brain of an electric fish, Gymnotus omarorum. Second, we confirm that IT neuron types and distribution in the preoptic area (POA) follow the same general pattern previously described in other teleost species. Third, we show that although IT and AVT neurons occur intermingled within the POA of G. omarorum and can be classified into the same subgroups, they present subtle but remarkable differences in size, number, and location. Finally, we show that unlike AVT, IT has no effect on basal electric signaling, reinforcing the specificity in the actions that each one of these nonapeptides has on social behavior and communication.

Vocal and Electric Fish: Revisiting a Comparison of Two Teleost Models in the Neuroethology of Social Behavior

The communication behaviors of vocal fish and electric fish are among the vertebrate social behaviors best understood at the level of neural circuits. Both forms of signaling rely on midbrain inputs to hindbrain pattern generators that activate peripheral effectors (sonic muscles and electrocytes) to produce pulsatile signals that are modulated by frequency/repetition rate, amplitude and call duration. To generate signals that vary by sex, male phenotype, and social context, these circuits are responsive to a wide range of hormones and neuromodulators acting on different timescales at multiple loci. Bass and Zakon (2005) reviewed the behavioral neuroendocrinology of these two teleost groups, comparing how the regulation of their communication systems have both converged and diverged during their parallel evolution. Here, we revisit this comparison and review the complementary developments over the past 16 years. We (a) summarize recent work that expands our knowledge of the neural circuits underlying these two communication systems, (b) review parallel studies on the action of neuromodulators (e.g., serotonin, AVT, melatonin), brain steroidogenesis (via aromatase), and social stimuli on the output of these circuits, (c) highlight recent transcriptomic studies that illustrate how contemporary molecular methods have elucidated the genetic regulation of social behavior in these fish, and (d) describe recent studies of mochokid catfish, which use both vocal and electric communication, and that use both vocal and electric communication and consider how these two systems are spliced together in the same species. Finally, we offer avenues for future research to further probe how similarities and differences between these two communication systems emerge over ontogeny and evolution.

Preoptic Area Activation and Vasotocin Involvement in the Reproductive Behavior of a Weakly Pulse-Type Electric Fish, Brachyhypopomus gauderio

Social behavior exhibits a wide diversity among vertebrates though it is controlled by a conserved neural network, the social behavior network (SBN). The activity of the SBN is shaped by hypothalamic nonapeptides of the vasopressin-oxytocin family. The weakly electric fish Brachyhypopomus gauderio emits social electrical signals during courtship. Three types of vasotocin (AVT) cells occur in the preoptic area (POA), one of the SBN nodes. In this study, we aimed to test if POA neurons of the nucleus preopticus ventricularis anterior (PPa) and posterior (PPp), and in particular AVT+ cells, were activated by social stimuli using a 2-day behavioral protocol. During the first night, male-female dyads were recorded to identify courting males. During the second night, these males were divided in two experimental conditions: isolated and social (male with a female). Both AVT cells and the cellular activation of the POA neurons (measured by FOS) were identified. We found that the PPa of social males showed more FOS+ cells than the PPa of isolated males, and that the PPa had more AVT+ cells in social males than in isolated males. The double-immunolabeling for AVT and FOS indicated the activation of AVT+ neurons. No significant differences in the activation of AVT+ cells were found between conditions, but a clear association was observed between the number of AVT+ cells and certain behavioral traits. In addition, a different activation of AVT+ cell-types was observed for social vs. isolated males. We conclude that the POA of B. gauderio exhibits changes induced by social stimuli in reproductive context, involving an increase in AVT production and a different profile activation among AVT+ cell populations.

Distinctive mechanisms underlie the emission of social electric signals of submission in Gymnotus omarorum

South American weakly electric fish (order Gymnotiformes) rely on a highly conserved and relatively fixed electromotor circuit to produce species-specific electric organ discharges (EOD) and a variety of meaningful adaptive EOD modulations. The command for each EOD arises from a medullary pacemaker nucleus composed by electrotonically coupled intrinsic pacemaker and bulbospinal projecting relay cells. During agonistic encounters Gymnotus omarorum signals submission by interrupting its EOD (offs) and by emitting transient high rate barrages of low amplitude discharges (chirps). Previous studies in gymnotiformes have shown that electric signal diversity is based on the segregation of descending synaptic inputs to pacemaker or relay cells and differential activation of the neurotransmitter receptors -for glutamate or γ-aminobutyric acid (GABA)- of these cells. Therefore, we tested whether GABAergic and glutamatergic inputs to pacemaker nucleus neurons are involved in the emission of submissive electric signals in G. omarorum. We found that GABA applied to pacemaker cells evokes EOD interruptions that closely resembled natural offs. Although in other species chirping is likely due to glutamatergic suprathreshold depolarization of relay cells, here, application of glutamate to these cells was unable to replicate the emission of this submissive signal. Nevertheless, chirp-like discharges were emitted after the enhancement of excitability of relay cells by blocking an IA-type potassium current and, in some cases, by application of vasotocin, a status-dependent modulator peptide of G. omarorum agonistic behavior. Modulation of electrophysiological properties of pacemaker nucleus neurons in gymnotiformes emerges as a novel putative mechanism, endowing electromotor networks with higher functional versatility.

Status-Dependent Vasotocin Modulation of Dominance and Subordination in the Weakly Electric Fish Gymnotus omarorum

Dominant-subordinate status emerges from agonistic encounters. The weakly electric fish, Gymnotus omarorum, displays a clear-cut example of non-breeding territorial aggression. The asymmetry in the behavior of dominants and subordinates is outstanding. Dominants are highly aggressive and subordinates signal submission in a precise sequence of locomotor and electric traits: retreating, decreasing their electric organ discharge rate, and emitting transient electric signals. The hypothalamic neuropeptide arginine-vasotocin (AVT) and its mammalian homolog arginine-vasopressin, are key modulators of social behavior, known to adapt their actions to different contexts. By analyzing the effects of pharmacological manipulations of the AVT system in both dominants and subordinates, we show evidence of distinct status-dependent actions of AVT. We demonstrate an endogenous effect of AVT on dominants' aggression levels: blocking the V1a AVT receptor induced a significant decrease in dominants' attack rate. AVT administered to subordinates enhanced the expression of the electric signals of submission, without affecting subordinates' locomotor displays. This study contributes a clear example of status-dependent AVT modulation of agonistic behavior in teleosts, and reveals distinctive activation patterns of the AVT system between dominants and subordinates.

Genes linked to species diversity in a sexually dimorphic communication signal in electric fish

Sexually dimorphic behaviors are often regulated by androgens and estrogens. Steroid receptors and metabolism are control points for evolutionary changes in sexual dimorphism. Electric communication signals of South American knifefishes are a model for understanding the evolution and physiology of sexually dimorphic behavior. These signals are regulated by gonadal steroids and controlled by a simple neural circuit. Sexual dimorphism of the signals varies across species. We used transcriptomics to examine mechanisms for sex differences in electric organ discharges (EODs) of two closely related species, Apteronotus leptorhynchus and Apteronotus albifrons, with reversed sexual dimorphism in their EODs. The pacemaker nucleus (Pn), which controls EOD frequency (EODf), expressed transcripts for steroid receptors and metabolizing enzymes, including androgen receptors, estrogen receptors, aromatase, and 5α-reductase. The Pn expressed mRNA for ion channels likely to regulate the high-frequency activity of Pn neurons and for neuromodulator and neurotransmitter receptors that may regulate EOD modulations used in aggression and courtship. Expression of several ion channel genes, including those for Kir3.1 inward-rectifying potassium channels and sodium channel β1 subunits, was sex-biased or correlated with EODf in ways consistent with EODf sex differences. Our findings provide a basis for future studies to characterize neurogenomic mechanisms by which sex differences evolve.

Melatonin Regulates Daily Variations in Electric Behavior Arousal in Two Species of Weakly Electric Fish with Different Social Structures

Timing is crucial for social interactions. Animal behavior is synchronized with biotic and abiotic environment variables ensuring that the activity phase of conspecifics occurs during the same period of the day. As biological rhythms are embedded in the complex integrative control of the brain, it is fundamental to explore its interaction with environmental and social factors. This approach will unravel the link between external stimuli carrying information on environmental cycles and the neural commands for behavior, including social behavior, associated with precise phases of those cycles. Arousal in the solitary Gymnotus omarorum and in the gregarious Brachyhypopomus gauderio is characterized by a nocturnal increase in the basal discharge rate of electric behavior, which is mild and transient in G. omarorum and large and persistent in B. gauderio. In this study, we show that the major integrator of social behavior, AVT (arginine vasotocin), is not involved in the nocturnal increase of electric behavior basal rate in isolated animals of either species. On the other hand, endogenous melatonin, the major modulator of the circadian system, is responsible for the nocturnal increase in electric behavior in isolated individuals of both species.