Autoradiographic localization of dihydrotestosterone and testosterone concentrating neurons in the brain of the oyster toadfish

Fish as a model in social neuroscience: conservation and diversity in the social brain network

Mechanisms for fish social behaviours involve a social brain network (SBN) which is evolutionarily conserved among vertebrates. However, considerable diversity is observed in the actual behaviour patterns amongst nearly 30000 fish species. The huge variation found in socio-sexual behaviours and strategies is likely generated by a morphologically and genetically well-conserved small forebrain system. Hence, teleost fish provide a useful model to study the fundamental mechanisms underlying social brain functions. Herein we review the foundations underlying fish social behaviours including sensory, hormonal, molecular and neuroanatomical features. Gonadotropin-releasing hormone neurons clearly play important roles, but the participation of vasotocin and isotocin is also highlighted. Genetic investigations of developing fish brain have revealed the molecular complexity of neural development of the SBN. In addition to straightforward social behaviours such as sex and aggression, new experiments have revealed higher order and unique phenomena such as social eavesdropping and social buffering in fish. Finally, observations interpreted as 'collective cognition' in fish can likely be explained by careful observation of sensory determinants and analyses using the dynamics of quantitative scaling. Understanding of the functions of the SBN in fish provide clues for understanding the origin and evolution of higher social functions in vertebrates.

Agonistic sounds signal male quality in the Lusitanian toadfish

Acoustic communication during agonistic behaviour is widespread in fishes. Yet, compared to other taxa, little is known on the information content of fish agonistic calls and their effect on territorial defence. Lusitanian toadfish males (Halobatrachus didactylus) are highly territorial during the breeding season and use sounds (boatwhistles, BW) to defend nests from intruders. BW present most energy in either the fundamental frequency, set by the contraction rate of the sonic muscles attached to the swimbladder, or in the harmonics, which are multiples of the fundamental frequency. Here we investigated if temporal and spectral features of BW produced during territorial defence reflect aspects of male quality that may be important in resolving disputes. We found that higher mean pulse period (i.e. lower fundamental frequency) reflected higher levels of 11-ketotestosterone (11KT), the main teleost androgen which, in turn, was significantly related with male condition (relative body mass and glycogen content). BW dominant harmonic mean and variability decreased with sonic muscle lipid content. We found no association between BW duration and male quality. Taken together, these results suggest that the spectral content of fish agonistic sounds may signal male features that are key in fight outcome.

Reprint of "Current perspectives on the androgen 5 alpha-dihydrotestosterone (DHT) and 5 alpha-reductases in teleost fishes and amphibians"

The androgen 5 alpha-dihydrotestosterone (DHT) is a steroidogenic metabolite that has received little attention in non-mammalian species. DHT is produced by the reduction of the double-bond of testosterone by a group of enzymes called 5 alpha-reductases of which there can be multiple isoforms (i.e., srd5a1, srd5a2, and srd5a3). Data from amphibians suggest that the expression of the srd5a genes occurs in early development, and continues until adulthood; however insufficient data exist in fish species, where DHT is thought to be relatively biologically inactive. Here, we demonstrate that fathead minnow (FHM; Pimephales promelas) developing embryos and adults express srd5a enzyme isoforms. During FHM embryogenesis, both srd5a1 and srd5a3 mRNA levels were significantly correlated in expression levels while srd5a2 showed a more unique pattern of expression. In adult FHMs, males had significantly higher levels of srd5a2 in the liver and gonad compared to females. In the male and female liver, transcript levels for srd5a2 were more abundant compared to srd5a1 and srd5a3, suggesting a prominent role for srd5a2 in this tissue. Interestingly, the ovary expressed higher mRNA levels of srd5a3 than the testis. Thus, data suggest that srd5a isoforms can show sexually dimorphic expression patterns in fish. We also conducted a literature review of the biological effects observed in embryonic and adult fish and amphibians after treatments with DHT and DHT-related compounds. Treatments with DHT in teleost fishes and amphibians have resulted in unexpected biological responses that are characteristic of both androgens and anti-androgens. For example, in fish DHT can induce vitellogenin in vitro from male and female hepatocytes and can increase 17β-estradiol production from the teleost ovary. We propose, that to generate further understanding of the roles of DHT in non-mammals, studies are needed that (1) address how DHT is synthesized within tissues of fish and amphibians; (2) examine the full range of biological responses to endogenous DHT, and its interactions with other signaling pathways; and (3) investigate how DHT production varies with reproductive stage. Lastly, we suggest that the Srd5a enzymes can be targets of endocrine disruptors in fish and frogs, which may result in disruptions in the estrogen:androgen balance in aquatic organisms.

Current perspectives on the androgen 5 alpha-dihydrotestosterone (DHT) and 5 alpha-reductases in teleost fishes and amphibians

The androgen 5 alpha-dihydrotestosterone (DHT) is a steroidogenic metabolite that has received little attention in non-mammalian species. DHT is produced by the reduction of the double-bond of testosterone by a group of enzymes called 5 alpha-reductases of which there can be multiple isoforms (i.e., srd5a1, srd5a2, and srd5a3). Data from amphibians suggest that the expression of the srd5a genes occurs in early development, and continues until adulthood; however insufficient data exist in fish species, where DHT is thought to be relatively biologically inactive. Here, we demonstrate that fathead minnow (FHM; Pimephales promelas) developing embryos and adults express srd5a enzyme isoforms. During FHM embryogenesis, both srd5a1 and srd5a3 mRNA levels were significantly correlated in expression levels while srd5a2 showed a more unique pattern of expression. In adult FHMs, males had significantly higher levels of srd5a2 in the liver and gonad compared to females. In the male and female liver, transcript levels for srd5a2 were more abundant compared to srd5a1 and srd5a3, suggesting a prominent role for srd5a2 in this tissue. Interestingly, the ovary expressed higher mRNA levels of srd5a3 than the testis. Thus, data suggest that srd5a isoforms can show sexually dimorphic expression patterns in fish. We also conducted a literature review of the biological effects observed in embryonic and adult fish and amphibians after treatments with DHT and DHT-related compounds. Treatments with DHT in teleost fishes and amphibians have resulted in unexpected biological responses that are characteristic of both androgens and anti-androgens. For example, in fish DHT can induce vitellogenin in vitro from male and female hepatocytes and can increase 17β-estradiol production from the teleost ovary. We propose, that to generate further understanding of the roles of DHT in non-mammals, studies are needed that (1) address how DHT is synthesized within tissues of fish and amphibians; (2) examine the full range of biological responses to endogenous DHT, and its interactions with other signaling pathways; and (3) investigate how DHT production varies with reproductive stage. Lastly, we suggest that the Srd5a enzymes can be targets of endocrine disruptors in fish and frogs, which may result in disruptions in the estrogen:androgen balance in aquatic organisms.

Anatomical distribution of sex steroid hormone receptors in the brain of female medaka

Estrogen and androgen play crucial roles in coordinating reproductive functions through estrogen receptors (ERs) and androgen receptors (ARs), respectively. These receptors are considered important for regulation of the hypothalamo-pituitary-gonadal (HPG) axis. Despite their biological importance, the distribution of sex steroid receptors has not been fully analyzed anatomically in the teleost brain. The teleosts have many characteristic features, which allow unique approaches toward an understanding of the regulatory mechanisms of reproductive functions. Medaka serves as a good model system for studying the mechanisms by which steroid receptor-mediated systems are regulated, because (1) their breeding conditions can be easily manipulated; (2) we can take advantage of the genome database; and 3) molecular genetic tools, such as transgenic techniques, are applicable. We analyzed the distribution of ERα, ERβ1, ERβ2, ARα, and ARβ mRNA by in situ hybridization in the brain of female medaka. We found that all subtypes of ERs and ARs were expressed in the following nuclei: the dorsal part of the ventral telencephalic area (Vd), supracommissural part of the ventral telencephalic area (Vs), postcommissural part of the ventral telencephalic area (Vp), preoptic area (POA), and nucleus ventralis tuberis (NVT). These regions are known to be involved in the regulation of sexual behavior (Vd, Vs, Vp, POA) or the HPG axis (NVT). These ER- and/or AR-expressing neurons may regulate sexual behavior or the HPG axis according to their axonal projections. Future analysis should be targeted to the neurons described in the present study to extend our understanding of the central regulatory mechanisms of reproduction.

Neural and hormonal mechanisms of reproductive-related arousal in fishes

The major classes of chemicals and brain pathways involved in sexual arousal in mammals are well studied and are thought to be of an ancient, evolutionarily conserved origin. Here we discuss what is known of these neurochemicals and brain circuits in fishes, the oldest and most species-rich group of vertebrates from which tetrapods arose over 350 million years ago. Highlighted are case studies in vocal species where well-delineated sensory and motor pathways underlying reproductive-related behaviors illustrate the diversity and evolution of brain mechanisms driving sexual motivation between (and within) sexes. Also discussed are evolutionary insights from the neurobiology and reproductive behavior of elasmobranch fishes, the most ancient lineage of jawed vertebrates, which are remarkably similar in their reproductive biology to terrestrial mammals.

Distribution of sex steroid hormone receptors in the brain of an African cichlid fish, Astatotilapia burtoni

Sex steroid hormones released from the gonads play an important role in mediating social behavior across all vertebrates. Many effects of these gonadal hormones are mediated by nuclear steroid hormone receptors, which are crucial for integration in the brain of external (e.g., social) signals with internal physiological cues to produce an appropriate behavioral output. The African cichlid fish Astatotilapia burtoni presents an attractive model system for the study of how internal cues and external social signals are integrated in the brain as males display robust plasticity in the form of two distinct, yet reversible, behavioral and physiological phenotypes depending on the social environment. In order to better understand where sex steroid hormones act to regulate social behavior in this species, we have determined the distribution of the androgen receptor, estrogen receptor alpha, estrogen receptor beta, and progesterone receptor mRNA and protein throughout the telencephalon and diencephalon and some mesencephalic structures of A. burtoni. All steroid hormone receptors were found in key brain regions known to modulate social behavior in other vertebrates including the proposed teleost homologs of the mammalian amygdalar complex, hippocampus, striatum, preoptic area, anterior hypothalamus, ventromedial hypothalamus, and ventral tegmental area. Overall, there is high concordance of mRNA and protein labeling. Our results significantly extend our understanding of sex steroid pathways in the cichlid brain and support the important role of nuclear sex steroid hormone receptors in modulating social behaviors in teleosts and across vertebrates.

Distribution of androgen receptor mRNA expression in vocal, auditory, and neuroendocrine circuits in a teleost fish

Across all major vertebrate groups, androgen receptors (ARs) have been identified in neural circuits that shape reproductive-related behaviors, including vocalization. The vocal control network of teleost fishes presents an archetypal example of how a vertebrate nervous system produces social, context-dependent sounds. We cloned a partial cDNA of AR that was used to generate specific probes to localize AR expression throughout the central nervous system of the vocal plainfin midshipman fish (Porichthys notatus). In the forebrain, AR mRNA is abundant in proposed homologs of the mammalian striatum and amygdala, and in anterior and posterior parvocellular and magnocellular nuclei of the preoptic area, nucleus preglomerulosus, and posterior, ventral and anterior tuberal nuclei of the hypothalamus. Many of these nuclei are part of the known vocal and auditory circuitry in midshipman. The midbrain periaqueductal gray, an essential link between forebrain and hindbrain vocal circuitry, and the lateral line recipient nucleus medialis in the rostral hindbrain also express abundant AR mRNA. In the caudal hindbrain-spinal vocal circuit, high AR mRNA is found in the vocal prepacemaker nucleus and along the dorsal periphery of the vocal motor nucleus congruent with the known pattern of expression of aromatase-containing glial cells. Additionally, abundant AR mRNA expression is shown for the first time in the inner ear of a vertebrate. The distribution of AR mRNA strongly supports the role of androgens as modulators of behaviorally defined vocal, auditory, and neuroendocrine circuits in teleost fish and vertebrates in general.

Dynamic limbic networks and social diversity in vertebrates: from neural context to neuromodulatory patterning

Vertebrate animals exhibit a spectacular diversity of social behaviors, yet a variety of basic social behavior processes are essential to all species. These include social signaling; discrimination of conspecifics and sexual partners; appetitive and consummatory sexual behaviors; aggression and dominance behaviors; and parental behaviors (the latter with rare exceptions). These behaviors are of fundamental importance and are regulated by an evolutionarily conserved, core social behavior network (SBN) of the limbic forebrain and midbrain. The SBN encodes social information in a highly dynamic, distributed manner, such that behavior is most strongly linked to the pattern of neural activity across the SBN, not the activity of single loci. Thus, shifts in the relative weighting of activity across SBN nodes can conceivably produce almost limitless variation in behavior, including diversity across species (as weighting is modified through evolution), across behavioral contexts (as weights change temporally) and across behavioral phenotypes (as weighting is specified through heritable and developmental processes). Individual neural loci may also express diverse relationships to behavior, depending upon temporal variations in their functional connectivity to other brain regions ("neural context"). We here review the basic properties of the SBN and show how behavioral variation relates to functional connectivity of the network, and discuss ways in which neuroendocrine factors adjust network activity to produce behavioral diversity. In addition to the actions of steroid hormones on SBN state, we examine the temporally plastic and evolutionarily labile properties of the nonapeptides (the vasopressin- and oxytocin-like neuropeptides), and show how variations in nonapeptide signaling within the SBN serve to promote behavioral diversity across social contexts, seasons, phenotypes and species. Although this diversity is daunting in its complexity, the search for common "organizing principles" has become increasingly fruitful. We focus on multiple aspects of behavior, including sexual behavior, aggression and affiliation, and in each of these areas, we show how broadly relevant insights have been obtained through the examination of behavioral diversity in a wide range of vertebrate taxa.

Association of cocaine- and amphetamine-regulated transcript and neuropeptide Y in the forebrain and pituitary of the catfish, Clarias batrachus: a double immunofluorescent labeling study

Cocaine- and amphetamine-regulated transcript (CART) and neuropeptide Y (NPY) are involved in the regulation of food intake, body weight, pituitary hormones, and reproduction. While CART and NPY occupy overlapping fields in the brain of mammals, little is known about the interaction between these peptide-containing systems in other vertebrates. We explored neuroanatomical associations between CART and NPY in the olfactory system, forebrain and pituitary of the catfish, Clarias batrachus, using double immunofluorescence method. NPY-containing fascicles from olfactory receptor neurons innervated the olfactory glomeruli and mitral cell layer in close association with CART-containing terminal fields. Distinct CART- or NPY-containing fibers were seen in the medial olfactory tract. In the dorsal telencephalon, CART- and NPY-immunoreactive axons were closely associated in area dorsalis telencephali/pars lateralis dorsalis (Dld), and posterioris (Dlp). In the ventral telencephalon, while most of the cells of nucleus entopeduncularis (NE) showed the presence of CART as well as NPY, a few cells with only NPY-immunoreactivity were observed. Similarly, a CART and NPY colocalized cell population was prominent in the preoptic area (POA); and a small population of cells with NPY-immunoreactivity was also evident. Other areas where CART and NPY were colocalized included fibers in the tuberal area, inferior lobe, neurohypophysis, proximal pars distalis and pars intermedia of the pituitary. No association between CART and NPY was observed in the thalamus and habenular ganglion. These results suggest that CART- and NPY-peptidergic systems may interact in NE, POA, tuberal area, certain telencephalic areas and pituitary and jointly process information relating to reproduction, feeding and neuroendocrine regulation.

Central pattern generators for social vocalization: androgen-dependent neurophysiological mechanisms

Historically, most studies of vertebrate central pattern generators (CPGs) have focused on mechanisms for locomotion and respiration. Here, we highlight new results for ectothermic vertebrates, namely teleost fish and amphibians, showing how androgenic steroids can influence the temporal patterning of CPGs for social vocalization. Investigations of vocalizing teleosts show how androgens can rapidly (within minutes) modulate the neurophysiological output of the vocal CPG (fictive vocalizations that mimic the temporal properties of natural vocalizations) inclusive of their divergent actions between species, as well as intraspecific differences between male reproductive morphs. Studies of anuran amphibians (frogs) demonstrate that long-term steroid treatments (wks) can masculinize the fictive vocalizations of females, inclusive of its sensitivity to rapid modulation by serotonin. Given the conserved organization of vocal control systems across vertebrate groups, the vocal CPGs of fish and amphibians provide tractable models for identifying androgen-dependent events that are fundamental to the mechanisms of vocal motor patterning. These basic mechanisms can also inform our understanding of the more complex CPGs for vocalization, and social behaviors in general, that have evolved among birds and mammals.

Sex differences in adult cell proliferation within the zebrafish (Danio rerio) cerebellum

It has been reported that neurons generated in the adult brain show sex-specific differences in several brain regions of lower vertebrates and mammals. The present study questioned whether cell proliferation and survival in the adult zebrafish (Danio rerio) cerebellum, the most mitotically active area of adult teleost brain, is sexually differentiated. Adult zebrafish were treated with the thymidine analogue 5'-bromo-2'-deoxyuridine (BrdU) and allowed to survive for 24 h (short-term) and for 21 days (long-term). BrdU immunohistochemistry allowed visualization of cells incorporating BrdU at the S phase of mitosis. At short-term survival, male zebrafish had a higher number of labelled cells at proliferation sites of the molecular layer of corpus cerebelli (CCe) and the granular layer of the caudal lobe of the cerebellum (LCa) than did females. In long-term survival, BrdU-positive cells were found at their final destination, but only the granular layer of the medial division of the valvula cerebelli showed sex-specific differences in the number of labelled cells. This higher mitotic activity in male cerebellum might be related to sex-specific motor behaviour observed in male zebrafish. To investigate the role of programmed cell death, the terminal deoxynucleotidyl-mediated dUTP nick-end-labelling (TUNEL) method was applied. The vast majority of apoptotic figures occurred in the granular cell layer of valvula and CCe, only in a few cases within the BrdU-retaining cells. Apoptosis was found specifically at the sites of the final destination of proliferating cells, indicating that the close relation of cell birth and death might represent a possible plasticity mechanism in the adult zebrafish cerebellum.

Immunohistochemical localization of cocaine- and amphetamine-regulated transcript peptide in the brain of the catfish,Clarias batrachus (Linn.)

The organization of cocaine- and amphetamine-regulated transcript peptide (CARTp, 54-102) immunoreactivity was investigated in the brain of the catfish, Clarias batrachus. CARTp-immunoreactivity was observed in several granule cells of the olfactory bulbs, in dot-like terminals around mitral cells, and in the fibers of the medial olfactory tracts. While several groups of discrete cells in the telencephalon showed CARTp-immunoreactivity, the immunostained fibers were widely distributed in the area dorsalis and ventralis telencephali. Immunoreactivity was seen in several periventricular and a few magnocellular neurons, and in a dense fiber network throughout the preoptic area. Varying degrees of immunoreactive fibers were seen in the periventricular region in the thalamus, hypothalamus, and pituitary. Some neurons in the nucleus preglomerulosus medialis and lateralis, central nucleus of the inferior lobes, nucleus lobobulbaris of the posterior tuberculum, and nucleus recessus posterioris showed distinct CARTp-immunoreactivity. Considerable immunoreactivity was seen in the optic tectum, rostral torus semicircularis, central pretectal area, and granule cells of the cerebellum. While only isolated immunoreactive cells were seen at three distinct sites in the metencephalon, a fiber network was seen in the facial and vagal lobes and periventricular and ventral regions of the medulla oblongata. The pattern of the CARTp distribution in the brain of C. batrachus suggests that it may play an important role in the processing of sensory information, the regulation of hormone secretion by hypophysial cell types, and motor and vegetative function. Finally, as in other animal species, CARTp seems to play a role in the processing of gustatory information.

Reproduction phase-related expression of beta-endorphin-like immunoreactivity in the nucleus lateralis tuberis of the female Indian major carp Cirrhinus mrigala: correlation with the luteinising hormone cells-ovary axis

The present study aimed to determine whether beta-endorphin immunoreactivity (bEP-ir) in the neurones of the nucleus lateralis tuberis (NLT) is linked to the seasonal cycle and shows correlation with the number of luteinising hormone (LH) cells in the pituitary gland and ovaries in the teleost, Cirrhinus mrigala. Although LH cells were moderately immunostained during the resting phase (December to January), the morphological profile suggested increased synthetic and secretory activity during the preparatory (February to April) and prespawning (May to June) phases. However, LH immunoreactivity was greatly reduced (P < 0.001) in the spawning (July to August) phase, suggesting massive discharge of the hormone; this pool was partly replenished in the postspawning (September to November) phase. The ovaries grew rapidly in the preparatory and prespawning phases; maximal size was attained during spawning, when ovulation occurred. Thereafter, the ovaries regressed. The NLT of C. mrigala is divisible into the pars lateralis (NLTl) and medialis (NLTm). During the postspawning and resting phases, bEP-ir was readily detectable in the NLTm as well as NLTl neurones. However, a steady reduction in the immunoreactivity was observed in the NLTm neurones during the preparatory through spawning phases (P < 0.001), suggesting a negative correlation with the LH cells-ovary axis. Thus, the inhibitory influence of beta-endorphin on the gonadotrophin-releasing hormone (GnRH)-LH axis appears to be attenuated during the preparatory through spawning phases. This may be necessary for the rapid stimulation of the axis culminating in spawning. Neurones of the NLTl also showed a gradual reduction in bEP-ir during the preparatory and prespawning phases (P < 0.01) and may therefore play a similar role. However, significant augmentation of the immunoreactivity was noticed in these neurones during the spawning phase (P < 0.001), the physiological significance of which is unknown. Although the present study demonstrated a temporal correlation between the beta-endorphin in the NLT, LH cells and the ovary, we suggest that the peptide in the NLTl and NLTm may show functional duality during the spawning phase.

Sonic and electric fish: at the crossroads of neuroethology and behavioral neuroendocrinology

Field and laboratory studies of weakly electric and sound-producing teleost fishes demonstrate how steroidal and non-steroidal hormones mediate the translation of neural events into behavior. The development of this research program has depended upon an interdisciplinary neuroethological approach that has characterized the neurophysiological properties of the motor and sensory pathways that lead to the production and detection of easily quantified highly stereotyped behaviors, namely, electric organ discharges (EODs) and vocalizations. Neuroethological studies of these teleosts have now integrated a behavioral neuroendocrinology approach that has provided several examples of how hormone-sensitive neurobiological traits contribute to adaptive behavioral plasticity in natural habitats. As such, these studies provide guideposts for comparable studies in other groups of teleosts and vertebrates in general.

The vertebrate social behavior network: evolutionary themes and variations

Based on a wide variety of data, it is now clear that birds and teleost (bony) fish possess a core "social behavior network" within the basal forebrain and midbrain that is homologous to the social behavior network of mammals. The nodes of this network are reciprocally connected, contain receptors for sex steroid hormones, and are involved in multiple forms of social behavior. Other hodological features and neuropeptide distributions are likewise very similar across taxa. This evolutionary conservation represents a boon for experiments on phenotypic behavioral variation, as the extraordinary social diversity of teleost fish and songbirds can now be used to generate broadly relevant insights into issues of brain function that are not particularly tractable in other vertebrate groups. Two such lines of research are presented here, each of which addresses functional variation within the network as it relates to divergent patterns of social behavior. In the first set of experiments, we have used a sexually polymorphic fish to demonstrate that natural selection can operate independently on hypothalamic neuroendocrine functions that are relevant for (1) gonadal regulation and (2) sex-typical behavioral modulation. In the second set of experiments, we have exploited the diversity of avian social organizations and ecologies to isolate species-typical group size as a quasi-independent variable. These experiments have shown that specific areas and peptidergic components of the social behavior network possess functional properties that evolve in parallel with divergence and convergence in sociality.

Neuropeptide Y in the forebrain of the adult male cichlid fishOreochromis mossambicus: Distribution, effects of castration and testosterone replacement

We studied the organization of the neuropeptide Y (NPY)-immunoreactive system in the forebrain of adult male cichlid fish Oreochromis mossambicus and its response to castration and testosterone replacement by using morphometric methods. Immunoreactivity for NPY was widely distributed in the forebrain, and the pattern generally resembled that in other teleosts. Whereas immunoreactivity was conspicuous in the ganglia of nervus terminalis (NT; or nucleus olfactoretinalis), a weak reaction was detected in some granule cells in the olfactory bulb and in the cells of area ventralis telencephali pars lateralis (Vl). Moderately to intensely immunoreactive cells were distinctly seen in the nucleus entopeduncularis (NE), nucleus preopticus (NPO), nucleus lateralis tuberis (NLT), paraventricular organ (PVO), and midbrain tegmentum (MT). NPY fibers were widely distributed in the forebrain. Castration for 10/15 days resulted in a drastic loss of immunoreactivity in the cells of NE (P<0.001) and a significant decrease (P<0.01) in their cell nuclear size. However, cell nuclei of the NT neurons showed a significant increase in size. A highly significant reduction in the NPY-immunoreactive fiber density (P<0.001) was observed in several areas of the forebrain. Although testosterone replacement reversed these changes, fibers in some areas showed supranormal responses. Immunoreactive cells in Vl, NPO, NLT, PVO, and MT and fiber density in some other areas did not respond to castration. We suggest that the NPY-immunoreactive elements that respond to castration and testosterone replacement may serve as the substrate for processing the positive feedback action of the steroid hormone.

Hormonal control of swimbladder sonic muscle dimorphism in the Lusitanian toadfish Halobatrachus didactylus

The swimbladder and associated sonic muscle of the Lusitanian toadfish Halobatrachus didactylus increase in size throughout life and are, respectively, 25% and 30% larger in type I (nest-holder) males than females, which may generate sexual differences in sound production. Sexual dimorphism in swimbladder is also evident in the morphological features of sonic muscle fibers. During the breeding season, type I males have smaller myofibril contracting zones surrounded by larger sarcoplasm areas compared with females, possibly an adaptation to speed and fatigue resistance for the production of long mating calls. Type II (floater) males show characteristics that are intermediate, but statistically not significantly different, between type I males and females. Six weeks after castration and androgen (testosterone and 11-ketotestosterone) replacement in type I and type II males there were no alterations either in swimbladder mass or fiber morphology. However, 17beta-estradiol induced a significant decrease in swimbladder mass and sarcoplasm area/myofibril area ratio. Six months after castration there was a clear reduction in the seasonal swimbladder hypertrophy in males and induction of sonic fiber morphological characteristics that resemble those occurring in females (low sarcoplasm area/myofibril area ratio). These results suggest that testicular factors are required to initiate sonic muscle hypertrophy and type I sonic fiber phenotype in H. didactylus, but a specific involvement of androgens has not been completely clarified.

Aromatase activity in hindbrain vocal control region of a teleost fish: divergence amoung males with alternative reproductive tactics

Type I male midshipman fish acoustically court females, whereas type II males do not but instead sneak or satellite spawn to compete with type I males for fertilizations. 'Singing' type I males diverge from type II males and females in the organization of an expansive hindbrain pacemaker–motoneuron circuit that establishes the physical attributes of vocalizations. Here, levels of aromatase activity were determined in homogenates of brain by measuring the conversion of 3H-androstenedione (AE) to 3H-oestrone (E1) and 3H-oestradiol (E2). Levels were highest in the telencephalon–preoptic area and similar for all morphs. Lower levels were in a region including the diencephalon, midbrain and cerebellum, although levels were significantly higher in females compared with type I males. In the vocal hindbrain region, aromatase levels were three- to five-fold higher in type II males and females than in type I males, and in castrated type II males than in castrated type I males. Conversion of testosterone to oestrogen in type II males and females may effectively prevent testosterone-induced maturation of the vocal system that characterizes type I males. Aromatase may thus be a key enzyme regulating the expression of individual-specific brain circuitry and behaviours among members of one sex.

Sound production evoked by electrical stimulation of the forebrain in the oyster toadfish

In mammals, birds and amphibians the neural pathways controlling sound production descend from higher centers in the forebrain, whereas in fishes only brainstem and spinal centers have been explicitly implicated in sound production. We now report that electrical stimulation of the forebrain of the oyster toadfish (Opsanus tau) readily evokes both the agonistic grunt and the courtship boatwhistle. Boatwhistles are more realistic than ones previously evoked from lower centers. Positive stimulation sites are localized in the preoptic area (nucleus preopticus parvocellularis anterior) and the supra-commissural nucleus of the ventral telencephalon, a likely homologue of the amygdala. Both sites contain gonadal steroid-concentrating neurons and play a central role in fish courtship behavior. Evoked sounds form a continuum from knock grunts, burst grunts, transition boatwhistles to complete boatwhistles; sound pressure level (SPL), fundamental frequency and duration increase consistently within the continuum. For all sound types, SPLs exhibit the smallest variation (coefficients of variation of 2.7 to 5.7%), fundamental frequency is intermediate (5 to 13%) and durations vary most widely (18 to 60%). Boatwhistles, with the smallest variation and greatest amplitude, are likely generated by a maximal output of the CNS and sonic muscles. Grunt SPLs however, vary over a range of 26 dB for all fish and by as much as 18 dB in an individual; suggesting recruitment of variable numbers of motor units despite electrical coupling within the sonic motor nucleus.