Neuroanatomical distribution of vasotocin in a urodele amphibian (Taricha granulosa) revealed by immunohistochemical and in situ hybridization techniques

Modulation of social behavior by distinct vasopressin sources

The neuropeptide arginine-vasopressin (AVP) is well known for its peripheral effects on blood pressure and antidiuresis. However, AVP also modulates various social and anxiety-related behaviors by its actions in the brain, often sex-specifically, with effects typically being stronger in males than in females. AVP in the nervous system originates from several distinct sources which are, in turn, regulated by different inputs and regulatory factors. Based on both direct and indirect evidence, we can begin to define the specific role of AVP cell populations in social behavior, such as, social recognition, affiliation, pair bonding, parental behavior, mate competition, aggression, and social stress. Sex differences in function may be apparent in both sexually-dimorphic structures as well as ones without prominent structural differences within the hypothalamus. The understanding of how AVP systems are organized and function may ultimately lead to better therapeutic interventions for psychiatric disorders characterized by social deficits.

Distribution of Vasopressin and Oxytocin Neurons in the Basal Forebrain and Midbrain of Spiny Mice (Acomys cahirinus)

The nonapeptides vasopressin (VP) and oxytocin (OT) are present in some form in most vertebrates. VP and OT play critical roles in modulating physiology and are well-studied for their influences on a variety of social behaviors, ranging from affiliation to aggression. Their anatomical distributions have been mapped for numerous species across taxa, demonstrating relatively strong evolutionary conservation in distributions throughout the basal forebrain and midbrain. Here we examined the distribution of VP-immunoreactive (-ir) and OT-ir neurons in a gregarious, cooperatively breeding rodent species, the spiny mouse (Acomys cahirinus), for which nonapeptide mapping does not yet exist. Immunohistochemical techniques revealed VP-ir and OT-ir neuronal populations throughout the hypothalamus and amygdala of males and females that are consistent with those of other rodents. However, a novel population of OT-ir neurons was observed in the median preoptic nucleus of both sexes, located dorsally to the anterior commissure. Furthermore, we found widespread sex differences in OT neuronal populations, with males having significantly more OT-ir neurons than females. However, we observed a sex difference in only one VP cell group - that of the bed nucleus of the stria terminalis (BST), a VP neuronal population that exhibits a phylogenetically widespread sexual dimorphism. These findings provide mapping distributions of VP and OT neurons in Acomys cahirinus. Spiny mice lend themselves to the study of mammalian cooperation and sociality, and the nonapeptide neuronal mapping presented here can serve as a basic foundation for the study of nonapeptide-mediated behavior in a group of highly social rodents.

Homeostatic and regenerative neurogenesis in salamanders

Large-scale regeneration in the adult central nervous system is a unique capacity of salamanders among tetrapods. Salamanders can replace neuronal populations, repair damaged nerve fibers and restore tissue architecture in retina, brain and spinal cord, leading to functional recovery. The underlying mechanisms have long been difficult to study due to the paucity of available genomic tools. Recent technological progress, such as genome sequencing, transgenesis and genome editing provide new momentum for systematic interrogation of regenerative processes in the salamander central nervous system. Understanding central nervous system regeneration also entails designing the appropriate molecular, cellular, and behavioral assays. Here we outline the organization of salamander brain structures. With special focus on ependymoglial cells, we integrate cellular and molecular processes of neurogenesis during developmental and adult homeostasis as well as in various injury models. Wherever possible, we correlate developmental and regenerative neurogenesis to the acquisition and recovery of behaviors. Throughout the review we place the findings into an evolutionary context for inter-species comparisons.

Sensitive Periods, Vasotocin-Family Peptides, and the Evolution and Development of Social Behavior

Nonapeptides, by modulating the activity of neural circuits in specific social contexts, provide an important mechanism underlying the evolution of diverse behavioral phenotypes across vertebrate taxa. Vasotocin-family nonapeptides, in particular, have been found to be involved in behavioral plasticity and diversity in social behavior, including seasonal variation, sexual dimorphism, and species differences. Although nonapeptides have been the focus of a great deal of research over the last several decades, the vast majority of this work has focused on adults. However, behavioral diversity may also be explained by the ways in which these peptides shape neural circuits and influence social processes during development. In this review, I synthesize comparative work on vasotocin-family peptides during development and classic work on early forms of social learning in developmental psychobiology. I also summarize recent work demonstrating that early life manipulations of the nonapeptide system alter attachment, affiliation, and vocal learning in zebra finches. I thus hypothesize that vasotocin-family peptides are involved in the evolution of social behaviors through their influence on learning during sensitive periods in social development.

Structural and functional diversity of nonapeptide hormones from an evolutionary perspective: A review

The article presents an overview of the comparative distribution, structure and functions of the nonapeptide hormones in chordates and non chordates. The review begins with a historical preview of the advent of the concept of neurosecretion and birth of neuroendocrine science, pioneered by the works of E. Scharrer and W. Bargmann. The sections which follow discuss different vertebrate nonapeptides, their distribution, comparison, precursor gene structures and processing, highlighting the major differences in these aspects amidst the conserved features across vertebrates. The vast literature on the anatomical characteristics of the nonapeptide secreting nuclei in the brain and their projections was briefly reviewed in a comparative framework. Recent knowledge on the nonapeptide hormone receptors and their intracellular signaling pathways is discussed and few grey areas which require deeper studies are identified. The sections on the functions and regulation of nonapeptides summarize the huge and ever increasing literature that is available in these areas. The nonapeptides emerge as key homeostatic molecules with complex regulation and several synergistic partners. Lastly, an update of the nonapeptides in non chordates with respect to distribution, site of synthesis, functions and receptors, dealt separately for each phylum, is presented. The non chordate nonapeptides share many similarities with their counterparts in vertebrates, pointing the system to have an ancient origin and to be an important substrate for changes during adaptive evolution. The article concludes projecting the nonapeptides as one of the very first common molecules of the primitive nervous and endocrine systems, which have been retained to maintain homeostatic functions in metazoans; some of which are conserved across the animal kingdom and some are specialized in a group/lineage-specific manner.

Arginine Vasotocin, the Social Neuropeptide of Amphibians and Reptiles

Arginine vasotocin (AVT) is the non-mammalian homolog of arginine vasopressin (AVP) and, like vasopressin, serves as an important modulator of social behavior in addition to its peripheral functions related to osmoregulation, reproductive physiology, and stress hormone release. In amphibians and reptiles, the neuroanatomical organization of brain AVT cells and fibers broadly resembles that seen in mammals and other taxa. Both parvocellular and magnocellular AVT-containing neurons are present in multiple populations located mainly in the basal forebrain from the accumbens-amygdala area to the preoptic area and hypothalamus, from which originate widespread fiber connections spanning the brain with a particularly heavy innervation of areas associated with social behavior and decision-making. As for mammalian AVP, AVT is present in greater amounts in males in many brain areas, and its presence varies seasonally, with hormonal state, and in males with differing social status. AVT's social influence is also conserved across herpetological taxa, with significant effects on social signaling and aggression, and, based on the very small number of studies investigating more complex social behaviors in amphibians and reptiles, AVT may also modulate parental care and social bonding when it is present in these vertebrates. Within this conserved pattern, however, both AVT anatomy and social behavior effects vary significantly across species. Accounting for this diversity represents a challenge to understanding the mechanisms by which AVT exerts its behavioral effects, as well are a potential tool for discerning the structure-function relationships underlying AVT's many effects on behavior.

Arginine Vasotocin Preprohormone Is Expressed in Surprising Regions of the Teleost Forebrain

Nonapeptides play a fundamental role in the regulation of social behavior, among numerous other functions. In particular, arginine vasopressin and its non-mammalian homolog, arginine vasotocin (AVT), have been implicated in regulating affiliative, reproductive, and aggressive behavior in many vertebrate species. Where these nonapeptides are synthesized in the brain has been studied extensively in most vertebrate lineages. While several hypothalamic and forebrain populations of vasopressinergic neurons have been described in amniotes, the consensus suggests that the expression of AVT in the brain of teleost fish is limited to the hypothalamus, specifically the preoptic area (POA) and the anterior tuberal nucleus (putative homolog of the mammalian ventromedial hypothalamus). However, as most studies in teleosts have focused on the POA, there may be an ascertainment bias. Here, we revisit the distribution of AVT preprohormone mRNA across the dorsal and ventral telencephalon of a highly social African cichlid fish. We first use in situ hybridization to map the distribution of AVT preprohormone mRNA across the telencephalon. We then use quantitative real-time polymerase chain reaction to assay AVT expression in the dorsomedial telencephalon, the putative homolog of the mammalian basolateral amygdala. We find evidence for AVT preprohormone mRNA in regions previously not associated with the expression of this nonapeptide, including the putative homologs of the mammalian extended amygdala, hippocampus, striatum, and septum. In addition, AVT preprohormone mRNA expression within the basolateral amygdala homolog differs across social contexts, suggesting a possible role in behavioral regulation. We conclude that the surprising presence of AVT preprohormone mRNA within dorsal and medial telencephalic regions warrants a closer examination of possible AVT synthesis locations in teleost fish, and that these may be more similar to what is observed in mammals and birds.

Social functions of individual vasopressin-oxytocin cell groups in vertebrates: what do we really know?

Vasopressin-oxytocin (VP-OT) nonapeptides modulate numerous social and stress-related behaviors, yet these peptides are made in multiple nuclei and brain regions (e.g., >20 in some mammals), and VP-OT cells in these areas often exhibit overlapping axonal projections. Furthermore, the magnocellular cell groups release peptide volumetrically from dendrites and soma, which gives rise to paracrine modulation in distal brain areas. Nonapeptide receptors also tend to be promiscuous. Hence, behavioral effects that are mediated by any given receptor type (e.g., the OT receptor) in a target brain region cannot be conclusively attributed to either VP or OT, nor to a specific cell group. We here review what is actually known about the social behavior functions of nonapeptide cell groups, with a focus on aggression, affiliation, bonding, social stress, and parental behavior, and discuss recent studies that demonstrate a diversity of sex-specific contributions of VP-OT cell groups to gregariousness and pair bonding.

Roles of arginine vasotocin receptors in the brain and pituitary of submammalian vertebrates

This chapter reviews the functions of arginine vasotocin (AVT) and its receptors in the central nervous system (CNS) of primarily submammalian vertebrates. The V1a-type receptor, which is widely distributed in the CNS of birds, amphibians, and fish, is one of the most important receptors involved in the expression of social and reproductive behaviors. In mammals, the V1b receptor of arginine vasopressin, an AVT ortholog, is assumed to be involved in aggression, social memory, and stress responses. The distribution of the V1b-type receptor in the brain of submammalian vertebrates has only been reported in an amphibian species, and its putative functions are discussed in this review. The functions of V2-type receptor in the CNS are still unclear. Recent phylogenetical and pharmacological analyses have revealed that the avian VT1 receptor can be categorized as a V2b-type receptor. The distribution of this newly categorized VT1 receptor in the brain of avian species should contribute to our knowledge of the possible roles of the V2b-type receptor in the CNS of other nonmammalian vertebrates. The functions of AVT in the amphibian and avian pituitaries are also discussed, focusing on the V1b- and V1a-type receptors.

Evolution of a vertebrate social decision-making network

Animals evaluate and respond to their social environment with adaptive decisions. Revealing the neural mechanisms of such decisions is a major goal in biology. We analyzed expression profiles for 10 neurochemical genes across 12 brain regions important for decision-making in 88 species representing five vertebrate lineages. We found that behaviorally relevant brain regions are remarkably conserved over 450 million years of evolution. We also find evidence that different brain regions have experienced different selection pressures, because spatial distribution of neuroendocrine ligands are more flexible than their receptors across vertebrates. Our analysis suggests that the diversity of social behavior in vertebrates can be explained, in part, by variations on a theme of conserved neural and gene expression networks.

Evolving nonapeptide mechanisms of gregariousness and social diversity in birds

Of the major vertebrate taxa, Class Aves is the most extensively studied in relation to the evolution of social systems and behavior, largely because birds exhibit an incomparable balance of tractability, diversity, and cognitive complexity. In addition, like humans, most bird species are socially monogamous, exhibit biparental care, and conduct most of their social interactions through auditory and visual modalities. These qualities make birds attractive as research subjects, and also make them valuable for comparative studies of neuroendocrine mechanisms. This value has become increasingly apparent as more and more evidence shows that social behavior circuits of the basal forebrain and midbrain are deeply conserved (from an evolutionary perspective), and particularly similar in birds and mammals. Among the strongest similarities are the basic structures and functions of avian and mammalian nonapeptide systems, which include mesotocin (MT) and arginine vasotocin (VT) systems in birds, and the homologous oxytocin (OT) and vasopressin (VP) systems, respectively, in mammals. We here summarize these basic properties, and then describe a research program that has leveraged the social diversity of estrildid finches to gain insights into the nonapeptide mechanisms of grouping, a behavioral dimension that is not experimentally tractable in most other taxa. These studies have used five monogamous, biparental finch species that exhibit group sizes ranging from territorial male-female pairs to large flocks containing hundreds or thousands of birds. The results provide novel insights into the history of nonapeptide functions in amniote vertebrates, and yield remarkable clarity on the nonapeptide biology of dinosaurs and ancient mammals. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Identification and characterization of mesotocin and V1a-like vasotocin receptors in a urodele amphibian, Taricha granulosa

The cDNA sequences encoding the mesotocin receptor (MTR) and vasotocin 1a receptor (VTR-1a) were identified in a urodele amphibian, the rough-skinned newt, Taricha granulosa. Saturation binding of [(3)H]oxytocin (OT) to the Taricha MTR (tMTR) was best fit by a two-state model; a high affinity-low abundance site and a lower affinity-high abundance site. Competition-binding studies found the following rank-order affinities for the tMTR: mesotocin (MT)>OT≈vasotocin (VT)>vasopressin (VP)>isotocin (IT). Inositol phosphate (IP) accumulation studies demonstrated functional activity of both the tMTR and Taricha VTR-1a (tVTR-1a) in a heterologous cell culture system. The rank-order potencies for the tMTR were MT>OT>VT≈VP>IT. The combined binding and IP results indicate that VT may act as a partial agonist of the tMTR. Rank-order potencies for the tVTR-1a were VT>VP>MT≈OT>IT. For both receptors, stimulation of IP accumulation was blocked by d(CH(2))(5)[Tyr(Me)(2)]AVP (Manning compound) and d(CH(2))(5)[Tyr(Me)(2),Thr(4),Tyr-NH(2)]OVT (OTA). OTA was a more potent antagonist for the transiently expressed tMTR while Manning compound was relatively more potent at inhibiting IP accumulation in tVTR-1a expressing cells. In contradiction to earlier assumptions, the absolute IC(50) of Manning compound was lower for the tMTR (27nM±13) than the tVTR-1a (586nM±166) indicating its potential higher affinity for the tMTR, a finding with special relevance to interpretation of comparative studies investigating the behavioral and physiological actions of neurohypophysial peptides in non-mammalian species.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Pheromones enhance somatosensory processing in newt brains through a vasotocin-dependent mechanism

We tested whether the sex pheromones that stimulate courtship clasping in male roughskin newts do so, at least in part, by amplifying the somatosensory signals that directly trigger the motor pattern associated with clasping and, if so, whether that amplification is dependent on endogenous vasotocin (VT). Female olfactory stimuli increased the number of action potentials recorded in the medulla of males in response to tactile stimulation of the cloaca, which triggers the clasp motor reflex, as well as to tactile stimulation of the snout and hindlimb. That enhancement was blocked by exposing the medulla to a V1a receptor antagonist before pheromone exposure. However, the antagonist did not affect medullary responses to tactile stimuli in the absence of pheromone exposure, suggesting that pheromones amplify somatosensory signals by inducing endogenous VT release. The ability of VT to couple sensory systems together in response to social stimulation could allow this peptide to induce variable behavioural outcomes, depending on the immediate context of the social interaction and thus on the nature of the associated stimuli that are amplified. If widespread in vertebrates, this mechanism could account for some of the behavioural variability associated with this and related peptides both within and across species.

Testosterone stimulates mounting behavior and arginine vasotocin expression in the brain of both sexual and unisexual whiptail lizards

In nonmammalian vertebrates the abundance of arginine vasotocin (AVT) neurons in the brain is sexually dimorphic, a pattern that is modulated by testicular androgen. This peptide is thought to be involved in the control of male-typical mounting behaviors. The all-female desert-grasslands whiptail (Cnemidophorus uniparens) reproduces by obligate parthenogenesis and in nature no males exist, but eggs treated with aromatase inhibitor hatch into individuals (called virago C. uniparens) having testes, accessory sex structures, high circulating concentrations of androgens, and exhibiting only male-like copulatory behavior. To examine the 'sexual' dimorphism of AVT-containing neurons in these animals, we compared AVT immunoreactivity in gonadectomized control and virago C. uniparens, with that of gonadectomized male and female Cnemidophorus inornatus, a sexual species that is the maternal ancestor to the parthenogenetic species. Mounting behavior is elicited in both species and both sexes by testosterone, and it was predicted that the distribution and abundance of AVT cell bodies and fibers would reflect the propensity of males and females of the two species to display male-typical copulatory behavior. Since both this propensity and AVT abundance are controlled by androgens, we compared testosterone-implanted and control animals within each group. Testosterone treatment generally increased AVT abundance, except in lab-reared parthenoforms, in which testosterone treatment was the least effective in inducing male-like copulatory behavior.

Neuroanatomical Distribution of Cannabinoid Receptor Gene Expression in the Brain of the Rough-Skinned Newt, Taricha granulosa

Type I cannabinoid receptor (CB1) is a G-protein coupled receptor with a widespread distribution in the central nervous system in mammals. In a urodele amphibian, the rough-skinned newt (Taricha granulosa), recent evidence indicates that endogenous cannabinoids (endocannabinoids) mediate behavioral responses to acute stress and electrophysiological responses to corticosterone. To identify possible sites of action for endocannabinoids, in situ hybridization using a gene and species specific cRNA probe was used to label CB1 mRNA in brains of male T. granulosa. Labeling of CB1 mRNA in the telencephalon was observed in the olfactory bulb and all areas of the pallium, as well as the bed nucleus of the stria terminalis and nucleus amygdalae dorsolateralis. The labeling of CB1 mRNA was also found in regions of the preoptic area, thalamus, midbrain tegmentum and tectum, cerebellum, and the stratum griseum of the hindbrain. A notable difference in CB1 labeling between this amphibian and mammals is the abundance of labeling in areas associated with olfaction (anterior olfactory nuclei, nucleus amygdalae dorsolateralis, and lateral pallium), which hints that endocannabinoids might modulate responses to odors as well as pheromones. This widespread distribution of CB1 labeling, particularly in sensory and motor control centers, fits with prior results showing that endocannabinoids modulate sensorimotor processing and behavioral output in this species. The distribution of CB1 in the brain of T. granulosa was in many of the same sites previously observed in the brain of the anuran amphibian, Xenopus laevis, as well as those of different species of mammals, suggesting that endocannabinoid signaling pathways are conserved.

Identification of roughskin newt medullary vasotocin target neurons with a fluorescent vasotocin conjugate

Arginine8 vasotocin (AVT), a neurohypophyseal peptide in nonmammalian vertebrates, plays a key role in the regulation of social behaviors related to reproduction. In male roughskin newts (Taricha granulosa), AVT is an important facilitator of several reproductive behaviors, including courtship clasping of females. Although AVT is known to act in certain brain regions and AVT receptors have been localized to some extent, specific target neurons for this peptide have not been identified in any species. Internalization of a receptor-specific conjugate of AVT and the fluorescent dye Oregon green was used to identify AVT target cells in the medulla of male roughskin newts. Medullary neurons are of interest because they appear to mediate facilitation of clasping by AVT. Direct application of AVT-Oregon green to the fourth ventricular surface of the medulla in vivo resulted in conjugate internalization by a widespread population of medullary neurons, particularly in the medial reticular formation and nuclei of cranial nerves V, VII, VIII, IX, and X. Some fourth-ventricle ependymal cells were also labeled. Reticulospinal neurons, which play an important role in clasping, were identified by retrograde labeling with tetramethylrhodamine dextran amine. AVT-Oregon green was internalized by 72% of these neurons. These results show that AVT can directly affect a very large and diverse medullary neuronal population, which may underlie the peptide's role in multiple neuroendocrinological processes, including autonomic and behavioral regulation. Selectivity of the AVT action may arise through interactions between AVT and steroids such as corticosterone.

Sexual differentiation of central vasopressin and vasotocin systems in vertebrates: different mechanisms, similar endpoints

Vasopressin neurons in the bed nucleus of the stria terminalis and amygdala and vasotocin neurons in homologous areas in non-mammalian vertebrates show some of the most consistently found neural sex differences, with males having more cells and denser projections than females. These projections have been implicated in social and reproductive behaviors but also in autonomic functions. The sex differences in these projections may cause as well as prevent sex differences in these functions. This paper discusses the anatomy, steroid dependency, and sexual differentiation of these neurons. Although the final steps in sexual differentiation of vasopressin/vasotocin expression may be similar across vertebrate species, what triggers differentiation may vary dramatically. For example, during development, estrogen masculinizes vasopressin expression in rats but feminizes its counterpart in Japanese quail. Apparently, nature consistently finds a way of maintaining sex differences in vasopressin and vasotocin pathways, suggesting that the function of these differences is important enough that it was conserved during evolution.

Historical perspective: Hormonal regulation of behaviors in amphibians

This review focuses on research into the hormonal control of behaviors in amphibians that was conducted prior to the 21st century. Most advances in this field come from studies of a limited number of species and investigations into the hormonal mechanisms that regulate reproductive behaviors in male frogs and salamanders. From this earlier research, we highlight five main generalizations or conclusions. (1) Based on studies of vocalization behaviors in anurans, testicular androgens induce developmental changes in cartilage and muscles fibers in the larynx and thereby masculinize peripheral structures that influence the properties of advertisement calls by males. (2) Gonadal steroid hormones act to enhance reproductive behaviors in adult amphibians, but causal relationships are not as well established in amphibians as in birds and mammals. Research into the relationships between testicular androgens and male behaviors, mainly using castration/steroid treatment studies, generally supports the conclusion that androgens are necessary but not sufficient to enhance male behaviors. (3) Prolactin acts synergistically with androgens and induces reproductive development, sexual behaviors, and pheromone production. This interaction between prolactin and gonadal steroids helps to explain why androgens alone sometimes fail to stimulate amphibian behaviors. (4) Vasotocin also plays an important role and enhances specific types of behaviors in amphibians (frog calling, receptivity in female frogs, amplectic clasping in newts, and non-clasping courtship behaviors). Gonadal steroids typically act to maintain behavioral responses to vasotocin. Vasotocin modulates behavioral responses, at least in part, by acting within the brain on sensory pathways that detect sexual stimuli and on motor pathways that control behavioral responses. (5) Corticosterone acts as a potent and rapid suppressor of reproductive behaviors during periods of acute stress. These rapid stress-induced changes in behaviors use non-genomic mechanisms and membrane-associated corticosterone receptors.

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.

Neuroanatomical distribution of vasotocin and mesotocin in two urodele amphibians (Plethodon shermani and Taricha granulosa) based on in situ hybridization histochemistry

Previous research suggests that considerable species-specific variation exists in the neuroanatomical distributions of arginine vasotocin (AVT) and mesotocin (MST), non-mammalian homologues of vasopressin and oxytocin. An earlier study in rough-skinned newts (Taricha granulosa) indicated that the neuroanatomical distribution of cells labeled for AVT-immunoreactivity (ir) was greater in this urodele amphibian than in any other species. It was unknown whether the widespread distribution of AVT-ir is unique to T. granulosa or a feature common among salamanders. Using in situ hybridization (ISH) histochemistry and gene-specific riboprobes, the current study labeled AVT and MST mRNA in T. granulosa and the red-legged salamander (Plethodon shermani). In T. granulosa, AVT ISH-labeled cells were found to be widespread and localized in brain areas including the dorsal and medial pallium, lateral and medial septum, bed nucleus of the stria terminalis, amygdala, preoptic area, ventral hypothalamus, nucleus isthmus, tectum mesencephali, inferior colliculus, and hindbrain. In P. shermani, the distribution of AVT ISH-labeled neurons matched that of T. granulosa, except in the lateral septum, ventral hypothalamus, and inferior colliculus, but did however include labeled cell bodies in the lateral pallium. The distribution of MST ISH-labeled cells was more restricted than AVT ISH labeling and was limited to regions of the preoptic area and ventral thalamus, which is consistent with the limited distribution of MST/OXY in other vertebrates. These findings support the conclusion that urodele amphibians possess a well-developed vasotocin system, perhaps more extensive than other vertebrate taxa.

Chemoarchitectonic subdivisions of the songbird septum and a comparative overview of septum chemical anatomy in jawed vertebrates

Available data demonstrate that the avian septal region shares a number of social behavior functions and neurochemical features in common with mammals. However, the structural and functional subdivisions of the avian septum remain largely unexplored. In order to delineate chemoarchitectural zones of the avian septum, we prepared a large dataset of double-, triple-, and quadruple-labeled material in a variety of songbird species (finches and waxbills of the family Estrildidae and a limited number of emberizid sparrows) using antibodies against 10 neuropeptides and enzymes. Ten septal zones were identified that were placed into lateral, medial, caudocentral, and septohippocampal divisions, with the lateral and medial divisions each containing multiple zones. The distributions of numerous immunoreactive substances in the lateral septum closely match those of mammals (i.e., distributions of met-enkephalin, vasotocin, galanin, calcitonin gene-related peptide, tyrosine hydroxylase, vasoactive intestinal polypeptide, substance P, corticotropin-releasing factor, and neuropeptide Y), enabling detailed comparisons with numerous chemoarchitectonic zones of the mammalian lateral septum. Our septohippocampal and caudocentral divisions are topographically comparable to the mammalian septohippocampal and septofimbrial nuclei, respectively, although additional data will be required to establish homology. The present data also demonstrate the presence of a medial septal nucleus that is histochemically comparable to the medial septum of mammals. The avian medial septum is clearly defined by peptidergic markers and choline acetyltransferase immunoreactivity. These findings should provide a useful framework for functional and comparative studies, as they suggest that many features of the septum are highly conserved across vertebrate taxa.

Connectivity and cytoarchitecture of the ventral telencephalon in the salamanderPlethodon shermani

The cytoarchitecture and axonal connection pattern of centers in the ventral telencephalon of the salamander Plethodon shermani were studied using biocytin for anterograde and retrograde labeling of cell groups, as well as by intracellular injections. Application of biocytin to the main and accessory olfactory bulbs identified the olfactory pallial regions and the vomeronasal portion of the amygdala, respectively. According to our results, the amygdala of Plethodon is divided into (1) a rostral part projecting to visceral and limbic centers and receiving afferents from the dorsal thalamus, and (2) a caudal part receiving accessory olfactory input. The striatopallial transition area (SPTA) lies rostrodorsally to the caudal (vomeronasal) amygdala and is similar in connections and possibly in function. The rostral striatum has few descending projections to the medulla, whereas the intermediate striatum sends strong projections to the tegmentum and medulla. The caudal striatum has strong ascending projections to the striatum and descending projections to the ventral hypothalamus. The dendritic trees of neurons labeled below the striatum and in the SPTA spread laterally from the soma, whereas dendrites of striatal neurons converge into the laterally situated striatal neuropil. In the caudal amygdala, three distinct types of neurons are found differing in dendritic arborization. It is concluded that, hodologically, the rostral part of the urodele amygdala corresponds to the central and basolateral amygdala and the caudal part to the cortical/medial amygdala of mammals. The urodele striatum is divided into a rostral striatum proper, an intermediate dorsal pallidum, and a caudal part, with distinct connections described here for the first time in a vertebrate.

The effects of sex steroids and vasotocin on behavioral responses to visual and olfactory sexual stimuli in ovariectomized female roughskin newts

Previous studies have found that vasotocin (AVT) administration to male roughskin newts (Taricha granulosa) enhances courtship clasping as well as appetitive responses to specific sexual stimuli and that treating female newts with androgens plus AVT induces the expression of male-typical courtship clasping (the selective clasping of females). However, the unique and/or interactive effects of sex steroids and AVT on appetitive responses to specific sexual stimuli have not yet been determined. To first identify male-typical, sexually dimorphic appetitive responses to female sexual stimuli, we tested intact newts during the breeding season and found that males, but not females, are attracted to female visual and pheromonal sexual stimuli. We then used ovariectomized (ovx) females implanted with empty silastic capsules (Blk) or with capsules containing testosterone (T), dihydrotestosterone (DHT), or estradiol (E2) and then injected with either saline or AVT to determine the effects of steroids and AVT, alone or in combination with each other, on male-typical behavioral responses to those stimuli. E2 treatment depressed responses toward female visual stimuli independently of AVT. On the other hand, only T-implanted, AVT-injected females displayed male-typical behavioral responses toward female olfactory stimuli, preferring to spend more time in proximity to female-scented than unscented newt models and selectively clasping the female-scented models. Together, these results support the conclusion that sex steroids and AVT influence behavioral responses to sexual stimuli via sensory-specific mechanisms. Furthermore, they suggest that T and AVT interact within the brain to influence sensorimotor processing in the pathways that integrate olfactory sexual stimuli into male-typical courtship behaviors.

Differential distribution of melatonin receptors in the pituitary gland of Xenopus laevis

A major target site for melatonin action is thought to be the pituitary gland. We have detected differential expression and co-localization of the Mel(1a) and Mel(1c) receptors in cells of the Xenopus laevis pituitary gland. Sections of Xenopus pituitary glands were labeled with Mel(1a) and/or Mel(1c) antibodies, in combination with antibodies to arginine vasotocin (AVT), alpha-melanocyte stimulating hormone (alpha-MSH), prolactin (PRL), and luteinizing hormone (LH). Mel(1a) immunoreactivity was localized to cells of the pars intermedia and to elements within the pars nervosa. Mel(1c) immunoreactivity was also localized to the pars nervosa, and significant labeling was also observed in discrete clusters of cells in the pars distalis. Mel(1a) was absent from the pars distalis, while Mel(1c) was absent from the pars intermedia. Mel(1a) and Mel(1c) were co-localized in the pars nervosa. AVT was present in the pars nervosa, and appeared to be localized to the cell clusters of the pars distalis in which the Mel(1c) receptor was localized. alpha-MSH co-localized with the Mel(1a) receptor in the pars intermedia. LH appeared to localize to many of the cells in the pars distalis, with the notable exception of the Mel(1c) receptor-positive clusters of cells. PRL did not appear to co-localize with either melatonin receptor. The pattern of differential expression of the Mel(1a) and Mel(1c) receptors suggests that the receptors specifically mediate the cellular response to melatonin binding in the specific cell populations.

Behavioral neuroendocrinology of vasotocin and vasopressin and the sensorimotor processing hypothesis

Vasotocin (AVT) and vasopressin (AVP) are potent modulators of social behaviors in diverse species of vertebrates. This review addresses questions about how and where AVT and AVP act to modulate social behaviors, focusing on research with an amphibian model (Taricha granulosa). In general, the behaviorally important AVT and AVP neurons occur in the forebrain and project to sites throughout the brain. Social behaviors are modulated by AVT and AVP acting at multiple sites in the brain and at multiple levels in the behavioral sequence. This review proposes that AVT and AVP can act on sensory pathways to modulate the responsiveness of neurons to behaviorally relevant sensory stimuli and also can act on motor pathways in the brainstem and spinal cord to modulate the neuronal output to behavior-specific pattern generators. This neurobehavioral model, in which AVT and AVP are thought to modulate social behaviors by affecting sensorimotor processing, warrants further research.

The distribution of vasotocin and mesotocin immunoreactivity in the brain of the snake, Bothrops jararaca

Polyclonal antibodies against vasotocin (AVT) and mesotocin (MST) were used to explore the distribution of these peptides in the brain of the snake Bothrops jararaca. Magnocellular AVT- and MST-immunoreactive (ir) perikarya were observed in the supraoptic nucleus (SON), being AVT-ir neurons more numerous. A portion of the SON, in the lateroventral margin of the diencephalon ventrally to optic tract, showed only AVT-ir perikarya and fibers. However, the caudal most portion displayed only mesotocinergic perikarya. Parvocellular and magnocellular AVT- and MST-ir perikarya were present in the paraventricular nucleus (PVN) being AVT-ir fibers more abundant than MST-ir. Vasotocinergic perikarya were also found in a dorsolateral aggregation (DLA) far from the PVN. Mesotocinergic perikarya were also present in the recessus infundibular nucleus and ependyma near to paraventricular organ. Nerve fibers emerging from supraoptic and paraventricular nuclei run along the diencephalic floor, internal zone of the median eminence (ME) to end in the neural lobe. Also a dense network of AVT- and MST-ir fibers was present in the external zone of the ME, close to the vessels of the hypophysial portal system. As a rule, all regions having vasotocinergic and mesotocinergic perikarya also showed immunoreactive fibers. Vasotocinergic and mesotocinergic fibers but not perikarya were found in the lamina terminalis (LT). Moreover AVT-ir fibers were present in the nucleus accumbens and MST-ir fibers in the septum. In mesencephalon and rhombencephalon MST-ir fibers were more numerous than AVT-ir fibers. Vasotocinergic and mesotocinergic fibers in extrahypothalamic areas suggest that these peptides could function as neurotransmitters and/or neuromodulators in the snake B. jararaca.

Neuronal organization of the melanin-concentrating hormone system in primitive actinopterygians: evolutionary changes leading to teleosts

Hypothalamic melanin-concentrating hormone (MCH) neurones occur in all vertebrates and have an apparent neuromodulatory role. In teleost fish, however, MCH is used also as a neurohypophysial hormone, controlling skin color, and as a hypophysiotrophic peptide. This work describes the central location of immunoreactive MCH perikarya and their projections to the pituitary in a range of ancestral fish to determine the phylogenetic stage when the peptide adopted these roles. In all actinopterygians examined, including polypteriformes, chondrosteans, holosteans, and teleosts, immunoreactive fibers were abundant in the median eminence or, in the case of teleosts, within the pars distalis itself, suggesting MCH acquired a hypophysial regulatory role early in vertebrate evolution. MCH fibers appeared to be absent from the posterior neurohypophysis of the polypteriform Calamoichthys but were evident in this region in the chondrostean Acipensor, the holosteans Lepisosteus and Amia, and all teleosts, suggesting its use as a neurohypophysial hormone. The ability of MCH to induce skin pallor seems to have arisen at a later stage, probably in the preholosteans. This hormonal role coincides with the migration of MCH perikarya away from the ventricular surface and their enlargement into magnocellular neurones. In the higher teleosts, magnocellular hypothalamo-neurohypophysial neurones predominate in size and number, whereas smaller periventricular MCH neurones associated with the paraventricular organ, that are prominent in lampreys, early actinopterygians and tetrapods, are reduced in teleosts. The data suggest that, in teleost fish, earlier functions of the peptide may have become subordinate to its novel pigmentary role.

Vasotocin and mesotocin in the brains of amphibians: state of the art

Immunohistochemical studies during the last decade have revealed elaborate systems of vasotocinergic (AVT) and mesotocinergic (MST) neuronal elements in the brain of a variety of amphibians including anurans, urodeles, and gymnophionans. Apart from a well-developed hypothalamo-hypophysial system, the antibodies demonstrated the existence of extrahypothalamic AVT- and MST-immunoreactive cell groups as well as extensive extrahypothalamic networks of immunoreactive fibers. The wide distribution of AVT- and MST-immunoreactive fibers throughout the brains of amphibians suggests that the two neuropeptidergic systems are involved not only in hypothalamo-hypophysial interactions, but also in a variety of other brain functions. Moreover, there is now evidence that sex-related differences occur in amphibians as previously shown for amniotes. It should be noted, however, that substantial variation occurs in the relative densities of AVT- and MST-immunoreactive fibers and number of cells between species, even within a single order of amphibians. Similar observations have been made in other classes of vertebrates and prompt us, therefore, to critically evaluate conclusions with respect to specific functions of AVT and MST in the central nervous system of vertebrates.

Comparative analysis of adrenomedullin-like immunoreactivity in the hypothalamus of amphibians

Adrenomedullin (AM) is a novel neuropeptide with special significance in the mammalian hypothalamo-hypophysial axis. By using an antiserum specific for human AM, we have studied the localization of AM-like immunoreactive (AMi) cell bodies and fibers in the hypothalamus and hypophysis of the amphibians Rana perezi (anuran), Pleurodeles waltl (urodele), and Dermophis mexicanus (gymnophionan). Distinct AMi cell groups were found for each species. In the anuran, six cell groups were localized in the preoptic and infundibular regions, whereas only three and one were found in the urodele and gymnophionan, respectively. A comparative analysis of AMi cells and cells expressing arginine vasotocin (AVT), neuropeptide Y (NPY), and tyrosine hydroxylase (TH) revealed strong differences between species. Thus, colocalization of AVT/AM is most likely to occur in the preoptic magnocellular nucleus of urodeles and it is reflected by the intense AM immunoreactivity in the neural lobe of the hypophysis. Colocalization of NPY/AM seems to be possible in the suprachiasmatic nucleus of anurans. In the gymnophionan, cells containing AVT and NPY are distinct from AMi cells. Only in anurans, the ventral aspect of the suprachiasmatic nucleus possesses a small population of AMi cells that express also TH immunoreactivity and most likely also express NPY. The results strongly suggest that AM in amphibians plays an important regulatory role in the hypothalamo-hypophysial system, as has been demonstrated in mammals. On the other hand, substantial differences have been found between species with respect to the degree of colocalization with other chemical substances.

Social behavior functions and related anatomical characteristics of vasotocin/vasopressin systems in vertebrates

The neuropeptide arginine vasotocin (AVT; non-mammals) and its mammalian homologue, arginine vasopressin (AVP) influence a variety of sex-typical and species-specific behaviors, and provide an integrational neural substrate for the dynamic modulation of those behaviors by endocrine and sensory stimuli. Although AVT/AVP behavioral functions and related anatomical features are increasingly well-known for individual species, ubiquitous species-specificity presents ever increasing challenges for identifying consistent structure-function patterns that are broadly meaningful. Towards this end, we provide a comprehensive review of the available literature on social behavior functions of AVT/AVP and related anatomical characteristics, inclusive of seasonal plasticity, sexual dimorphism, and steroid sensitivity. Based on this foundation, we then advance three major questions which are fundamental to a broad conceptualization of AVT/AVP social behavior functions: (1) Are there sufficient data to suggest that certain peptide functions or anatomical characteristics (neuron, fiber, and receptor distributions) are conserved across the vertebrate classes? (2) Are independently-evolved but similar behavior patterns (e.g. similar social structures) supported by convergent modifications of neuropeptide mechanisms, and if so, what mechanisms? (3) How does AVT/AVP influence behavior - by modulation of sensorimotor processes, motivational processes, or both? Hypotheses based upon these questions, rather than those based on individual organisms, should generate comparative data that will foster cross-class comparisons which are at present underrepresented in the available literature.

Vasotocin stimulates appetitive responses to the visual and pheromonal stimuli used by male roughskin newts during courtship

It is now well established that vasotocin (AVT) and its mammalian homologue vasopressin influence various social behaviors in vertebrates, but less is known about the mechanisms through which these peptides modulate behavior. In male roughskin newts, Taricha granulosa, AVT stimulates a courtship behavior, amplectic clasping. Three general explanations for how AVT affects male courtship behavior have been considered: by enhancing a central state of sexual motivation, by affecting sensorimotor integration mechanisms in individual sensory modalities, or by influencing a nonspecific state of attention, arousal, or anxiety. AVT administration enhanced appetitive responses to visual and olfactory sexual stimuli, as would be expected if AVT affects a state of sexual motivation that affects behavioral responses to sexual stimuli regardless of the sensory modality in which they are processed. However, AVT selectively enhanced responses to female olfactory stimuli (sex pheromones), but similarly enhanced responses to female and food-related visual stimuli (worms), thus questioning the utility of such a motivational mechanism, as responses to female stimuli were not selectively enhanced in all sensory modalities. We therefore propose that exogenous AVT independently influences olfactory processes associated with orientation/attraction toward a female sex pheromone and visual processes associated with orientation/attraction toward a visual feature common to females and worms. In further experiments AVT administration failed to stimulate feeding behavior but did decrease locomotor activity. Thus, AVT does not stimulate courtship behavior in this species by enhancing the animals' general state of attention or by decreasing general anxiety, as responses to nonsexual, attractive stimuli were not uniformly enhanced, nor by stimulating general arousal, as activity levels did not increase. Rather, the data support the conclusion that AVT affects courtship by influencing specific sensorimotor processes associated with behavioral responses to individual releasing stimuli, which suggests a mechanistic framework for understanding socially motivated behavior is this species.

Vasotocin innervation and modulation of vocal-acoustic circuitry in the teleost Porichthys notatus

Arginine vasotocin (AVT) and its mammalian homologue arginine vasopressin (AVP) modulate reproduction-related and other social behaviors in a broad range of vertebrate species. These functions of AVT/AVP may be in part achieved through the modulation of sensorimotor integration, although experimental evidence supporting this hypothesis remains limited. In the present experiments, we demonstrate (1) AVT innervation of candidate vocal-acoustic brain regions, and (2) AVT modulation of vocal-motor physiology in the plainfin midshipman fish (Porichthys notatus), which uses vocalizations in both mate attraction and agonistic contexts. AVT distribution was compared with known vocally active brain regions and to central auditory and vocal pathways. AVT-immunoreactive fibers and putative terminals descend almost exclusively from the preoptic area and are found in two primary candidate sites for vocal-acoustic integration - the anterior tuberal hypothalamus and paralemniscal midbrain tegmentum. AVT immunoreactivity is also located in several other vocally active regions, including the ventral tuberal nucleus, periaqueductal gray, and paraventricular regions of the isthmus and rostral hindbrain. The parvocellular preoptic area itself is also vocally active, although thresholds are substantially higher than for other regions. The functional significance of AVT input to vocal-acoustic regions was demonstrated in the paralemniscal midbrain where local delivery of AVT modulated electrically evoked, rhythmic vocal-motor output, which precisely mimicked natural vocalizations. AVT produced dose-dependent inhibitions of parameters associated with call initiation (burst latency and number of vocal-motor bursts elicited) but not of vocal-motor patterning (fundamental frequency and burst duration). Together, these findings provide support for the proposal that AVT modulates sensorimotor processes underlying social/acoustic communication.

Sexual dimorphism in numbers of vasotocin-immunoreactive neurons in brain areas associated with reproductive behaviors in the roughskin newt

Vasotocin (VT) and vasopressin control many endocrine and neuroendocrine functions, including the regulation of reproductive behaviors. In the roughskin newt (Taricha granulosa), VT administration can enhance courtship behaviors in males and egg-laying behaviors in females. This study used immunohistochemistry to investigate whether there are sex differences in VT in specific brain areas, and whether these differences persist in nonbreeding animals. Numbers of VT immunoreactive (ir) cell bodies were counted in males and females collected in February, April, June, and August. Radioimmunoassay of plasma samples confirmed that testosterone and 5alpha-dihydrotestosterone concentrations were higher in males than females, and that 17beta-estradiol concentrations were higher in females than males. In 11 brain areas, no sexual or seasonal differences in the number of VTir cells were found. But in 3 brain regions-the bed nucleus of the stria terminalis (BNST), the nucleus amygdalae dorsolateralis (AMYG), and the anterior preoptic area (aPOA)-there were significantly greater numbers of VTir cells in males than in females, and these differences did not change seasonally. In the aPOA, an area important to male sex behaviors, the sexual dimorphism in VTir was particularly pronounced. In four brain regions, there were significantly greater numbers of VTir cells in females than males, but only in specific seasons. In April-collected (breeding) animals, more VTir cells were found in females than in males in the populations of VT cells within the pars dorsalis hypothalami and ventromedial hypothalamus, brain regions frequently associated with stress responses and female mating behaviors. In August-collected (nonbreeding) animals, more VTir cells were found in females than in males, in the region of the bed nucleus of the decussation of the fasciculus lateralis telencephali and in the nucleus visceralis superior, nucleus isthmi region. Significantly greater numbers of VTir cells were observed in the magnocellular preoptic area of males and females collected in February. These results indicate that the functional interactions between gonadal steroid hormones and VT are complex and appear to involve site-, sex-, and season-specific regulatory mechanisms. Furthermore, it seems likely that populations of VT neurons in the BNST, AMYG, and aPOA are involved in regulating male-specific behaviors, and that the VT neurons in the pars dorsalis hypothalami/ventromedial hypothalamus may be involved in female-specific behaviors.

Preoptic AVT immunoreactive neurons of a teleost fish with alternative reproductive tactics

Recent reports have implicated an important role for arginine vasotocin (AVT) in the socially mediated sexual differentiation of fishes. This study focuses on the plainfin midshipman (Porichthys notatus) which exhibits two male morphs, type I and type II, differing in a suite of behavioral, neurobiological, and endocrine traits. Immunocytochemical techniques were used to detect neurons containing AVT-like peptide in the forebrain of juveniles, adult females, and type I and type II males. AVT immunoreactive (ir) somata were localized to three regions: the terminal nerve ganglion, the preoptic area (POA), and the pineal stalk. The profile area, or size, of AVT-ir POA neurons differed across the four classes of midshipman and was strongly correlated to differences in body size among the groups. By contrast, the number of AVT-ir cells in the POA exhibited no difference across the classes of midshipman. The number of POA cells containing AVT is therefore likely to be set early in development and not to change with the growth of the animal. An analysis of AVT-ir cell number normalized by body mass revealed that the larger morphs, type I males and females, have fewer cells per gram body mass than type II males and juveniles. Therefore, type II males have a juvenile-like AVT POA phenotype with smaller cells and more numerous cells per unit body mass than type I males. Type II males also exhibit more variability in the number of AVT-ir cells found in the POA compared to type I males.

Comparative neuroanatomy of vasotocin and vasopressin in amphibians and other vertebrates

This review focuses on the neuroanatomical distribution of vasotocin (VT) and vasopressin (VP) and presents a comparative analysis of brain areas in which VT and VP cell bodies have been reported in fish, amphibians, reptiles, birds and mammals. A comparison of information from previous neuroanatomical studies of VT and VP with findings from a recent study of VT in an amphibian (Taricha granulosa) supports the conclusions that the VT/VP system can be subdivided into identifiable groups of cell bodies, based on neuroanatomical and cell morphology characteristics, and that these cell groups are not necessarily delimited by classical neuroanatomical boundaries. The comparative neuroanatomy of the distribution of VT and VP cell bodies also indicates that the neuroanatomy of the VT/VP system is fairly conserved among vertebrates. The review uses comparative data to present a series of tentative hypotheses about the homology of the VT cell groups and VP cell groups in the different vertebrate taxa.