Midbrain acoustic circuitry in a vocalizing fish

Interruption of pacemaker signals by a diencephalic nucleus in the African electric fish, Gymnarchus niloticus

The African electric fish Gymnarchus niloticus rhythmically emits electric organ discharges (EODs) for communication and navigation. The EODs are generated by the electric organ in the tail in response to the command signals from the medullary pacemaker complex, which consists of a pacemaker nucleus (PN), two lateral relay nuclei (LRN) and a medial relay nucleus (MRN). The premotor structure and its modulatory influences on the pacemaker complex have been investigated in this paper. A bilateral prepacemaker nucleus (PPn) was found in the area of the dorsal posterior nucleus (DP) of the thalamus by retrograde labeling from the PN. No retrogradely labeled neurons outside the pacemaker complex were found after tracer injection into the LRN or MRN. Accordingly, anterogradely labeled terminal fibers from PPn neurons were found only in the PN. Iontophoresis of L-glutamate into the region of the PPn induced EOD interruptions. Despite the exclusive projection of the PPn neurons to the PN, extracellular and intracellular recordings showed that PN neurons continue their firing while MRN neurons ceased their firing during EOD interruption. This mode of EOD interruption differs from those found in any other weakly electric fishes in which EOD cessation mechanisms have been known.

Vasotocin/Isotocin-immunoreactive Neurons in the Medaka Fish Brain Are Sexually Dimorphic and Their Numbers Decrease After Spawning in the Female

In teleosts, the distribution of neurons in the preoptic-hypothalamic region and their associated neurohypophysial hormones, such as vasotocin (VT), appears to be different among species. This differential distribution is thought to reflect the social and/or sexual status of individuals within a species. In the present study, we analyzed the number, size and the distribution of vasotocin/isotocin (VT/IT) neurons in the brains of both male and female medaka (Oryzias latipes) using immunohistochemistry. VT/IT neurons were similarly located in an inverted L-shape in the nucleus preopticus in both gender, as has been already reported in salmonids. However, computer-assisted image analysis revealed sexual dimorphism in the number of VT/IT-immunoreactive (ir) neurons, with greater numbers found in males as compared to females. Further, in the female brain, the number of VT/IT-ir neurons decreased significantly after spawning. In pre-spawning compared to post-spawning females, the small-sized VT/IT-ir neurons dominated. Sexual differentiation of the medaka is fully dependent upon the steroid status during the early developmental stages and steroids are also known to trigger gender-specific behavior in the adult medaka. Our findings strongly suggest that VT and/or IT neurons may be functionally related to ovulation and/or the reproductive axes through connections to their steroidal status.

Sharpening of directional responses along the auditory pathway of the oyster toadfish, Opsanus tau

Our previous studies have shown that the peripheral auditory system of the toadfish encodes the direction of a sound source. Here, we compare directional responses of peripheral saccular afferents, cells in the descending octaval nucleus (DON) of the medulla, and the torus semicircularis (TS) of the midbrain. Recording locations in the brain were labeled with neurobiotin to confirm the site. To compare directional responses among cells, we calculated an index [sharpening ratio (SR)] that weights the relative strength of responses to the best direction for that cell and to the adjacent stimulus angles tested. Unsharpened saccular afferents tend to have a cosinusoidal directional response pattern (DRP) with an expected SR of 0.87. In DON, more than 60% of the cells exhibited directional sharpening (defined as SR <0.8). In TS, more than 80% of the cells exhibited directional sharpening. We conclude that directional auditory sharpening first occurs in DON and some additional sharpening occurs in the ascending pathway to the midbrain, particularly in azimuth. The sharpening of directional selectivity is likely to be an important component of the neural computations underlying directional hearing.

Projections to Bimodal Sites in the Torus semicircularis of the Toadfish, Opsanus tau

Bimodal cells in the torus semicircularis of the toadfish respond to both directional acoustic stimuli and hydrodynamic stimuli. Our previous physiological work indicated that bimodal cells may be distributed throughout the torus semicircularis. In this study, neurobiotin was used to compare the distribution of auditory-only and bimodal sites and to assess the inputs to those sites. A brief neurobiotin injection with short survival time was used to document the recording location. In other fish, a longer injection and survival time was used at an auditory-only or a bimodal site to fill the axons of the medullary inputs. Auditory-only sites were located in the most dorsal and medial sites in nucleus centralis. Bimodal sites were identified within both nucleus centralis and nucleus ventrolateralis. The greatest number of retrogradely filled cell bodies was found in the descending octaval nucleus following injection at auditory-only recording sites in nucleus centralis. In contrast, retrogradely filled cell bodies were found in both the descending octaval nucleus and the lateral line nucleus medialis following injection at bimodal sites in nucleus centralis or nucleus ventrolateralis. Retrogradely filled cell bodies were located in the dorsal and ventral divisions of the secondary octaval population from injections at either bimodal or auditory-only sites. The secondary octaval population has been implicated in auditory processing based on previous studies of both auditory specialist and auditory generalist fishes; however, this study is the first to reveal the potential role of the secondary octaval population in directional hearing in a fish. Relatively large numbers of retrogradely filled cells around the lateral lemniscus at consistent locations in the medulla indicate that a perilemniscal cell group also might be a component of the directional hearing circuit.

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.

Fiber connections of the central nucleus of semicircular torus in cyprinids

Fiber connections of the presumed auditory portion of the semicircular torus or the central nucleus (TSc) as well as other central and peripheral auditory structures were studied by tract-tracing methods in carp and goldfish. Major ascending projections to the TSc originated from the descending octaval nucleus (DO) and medullary secondary octaval population (SO). Toropetal neurons of the DO were within the saccular terminal zone. A small number of toropetal DO neurons might receive inputs from other inner ear and lateral line endorgans as well. Fibers from the DO terminated in a deep zone of the TSc, while those from the SO in both deep and superficial zones. Afferents to the TSc also arose from the rhombencephalic reticular formation, anterior octaval nucleus, isthmic reticular nucleus, perilemniscular nucleus, medial pretoral nucleus, anterior tuberal nucleus, and central posterior thalamic nucleus. The TSc projected ascending fibers to the medial pretoral nucleus, anterior tuberal nucleus, central posterior thalamic nucleus, and the preglomerular complex (anterior preglomerular nucleus, the caudomedial region of lateral preglomerular nucleus, and a medial zone of medial preglomerular nucleus). These parts of preglomerular complex appear structurally continuous sharing common hodological and cytoarchitectonic features, and thus might be regarded as a single neuronal population. Abundant descending pathways were also noted in the present study. Of particular note is the medial pretoral nucleus, which received fibers from diencephalic auditory nuclei. The nucleus gave rise to indirect descending pathways to a medial zone of the cerebellar crest where dendrites of crest cells in the SO ramify.

Fiber connections of the anterior preglomerular nucleus in cyprinids with notes on telencephalic connections of the preglomerular complex

Fiber connections of the anterior preglomerular nucleus (PGa) were studied in carp and goldfish by tracer injection experiments to the nucleus and telencephalon. The PGa received fibers from the central nucleus of semicircular torus, perilemniscular nucleus, anterior tuberal nucleus, and medial pretoral nucleus, all of which are presumed auditory structures. The PGa projected to the dorsal (dDm) and ventral (vDm) regions of medial part of dorsal telencephalic area. The caudomedial region of lateral preglomerular nucleus (PGl) and medial zone of medial preglomerular nucleus (PGm), which also receive auditory toral fibers, projected to the same telencephalic regions as did PGa. These preglomerular structures and the PGa also received in common descending fibers from a rostromedial portion of dDm. The PGa also received fibers from the parvocellular and magnocellular preoptic nuclei, and suprachiasmatic nucleus and projected to the anterior tuberal nucleus and medial inferior lobe, suggesting neurohormonal and circadian control on the PGa and auditory influences on hypothalamic functions. Of other diencephalic nuclei that receive auditory toral fibers, only small numbers of neurons were labeled in the central posterior thalamic nucleus and anterior tuberal nucleus even after large injections to the dorsal telencephalic area. Thus, the PGa and closely related preglomerular regions, not the dorsal thalamus, appear to constitute the major auditory relay station to the dorsal telencephalic area. The rostrolateral region of PGl, rostral and lateral zones of PGm, commissural preglomerular nucleus, and preglomerular tertiary gustatory nucleus, which do not receive auditory toral fibers, also projected to the dorsal telencephalic area.

Distribution of estrogen receptor alpha mRNA in the brain and inner ear of a vocal fish with comparisons to sites of aromatase expression

Among vertebrates, teleost fish have the greatest capacity for estrogen production in the brain. Previously, we characterized the distribution of the estrogen-synthesizing enzyme aromatase in the brain of the midshipman fish. Here, we investigated the distribution of estrogen receptor alpha (ERalpha). A partial cDNA of ERalpha was cloned and used to generate midshipman-specific primers for RT and real-time PCR which identified transcripts in liver and ovary, the CNS, and the sensory epithelium of the main auditory endorgan (sacculus). In situ hybridization revealed abundant expression throughout the preoptic area, a vocal-acoustic site in the hypothalamus, amygdala homologs of the dorsal pallium, the pineal organ, the inner ear, the pituitary, and the ovary. Weaker expression was found in the midbrain's nucleus of the medial longitudinal fasciculus and in the dimorphic vocal motor nucleus. ERalpha expression in the pineal, gonad, and pituitary axis may function to time seasonal abiotic cues to reproductive state, while expression in the vocal motor and auditory systems support neurophysiological evidence for estrogen as a modulator of vocal motor and auditory encoding mechanisms in midshipman fish. While ERalpha is restricted to specific nuclei, aromatase expression is abundant in glial cells throughout the entire forebrain, and high in midbrain and hindbrain - spinal vocal regions. The only site of aromatase-containing neurons is in the peripheral auditory system, where it is localized to ganglion cells in the auditory nerve. Estrogen production proximal to ERalpha-positive neurons may provide for focal sites of estrogen effects on reproductive-, vocal-, and auditory-related neurons.

An experimental study of the connections of the telencephalon in the rainbow trout (Oncorhynchus mykiss). I: Olfactory bulb and ventral area

The fluorescent carbocyanine dye 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was used in fixed tissue to comprehensively analyze the connections of the olfactory bulbs and the different regions of the ventral (V) area of the telencephalic lobes (subpallium) of the rainbow trout. With this goal, DiI was applied to the different telencephalic nuclei and zones, as well as to the olfactory nerve, the olfactory bulb, the retina, and to several structures in the diencephalon and brainstem of juvenile trout. The olfactory bulbs maintain reciprocal connections with several regions of the telencephalon [ventral nucleus of V (Vv), supracommissural nucleus (Vs), posterior zone of D (Dp), preoptic nucleus], and also project to the diencephalon (posterior tuberal nucleus, posterior hypothalamic lobe). Vv receives afferents from Vs, the dorsal nucleus of V (Vd), the preoptic nucleus, and from several nuclei in the diencephalon and brainstem (suprachiasmatic nucleus, anterior and lateral tuberal nuclei, preglomerular complex, tertiary gustatory nucleus, posterior tubercle, inferior hypothalamic lobes, thalamus, torus semicircularis, secondary gustatory nucleus, locus coeruleus, superior raphe nucleus, central gray, and reticular formation), and projects to dorsal (pallial) regions and most of the nuclei afferent to Vv. The dorsal nucleus of V (Vd) and Vs mainly project to the dorsal area. In an accompanying article (Folgueira et al., 2004), we present the results of application of DiI to dorsal (pallial) telencephalic regions, as well as of several experiments of tracer application to extratelencephalic regions. The results presented here, together with those of the accompanying article, reveal a complex connectional pattern of the rainbow trout ventral telencephalon, most of these connections having not been described previously in salmonids.

Reciprocal neural connections between the central posterior/prepacemaker nucleus and nucleus G in the gymnotiform fish, Apteronotus leptorhynchus

The central posterior nucleus of teleost fish is a cluster of neurons in the dorsal thalamus that plays an important role in controlling social behaviors. In the weakly electric gymnotiform fish, Apteronotus leptorhynchus, this nucleus forms a larger complex together with the prepacemaker nucleus, hence called central posterior/prepacemaker nucleus (CP/PPn). This complex is crucially involved in neural control of transient modulations of the electric organ discharge, which occur both spontaneously and in the context of social interactions. This control function is intimately linked to its pattern of connectivity with other brain regions. By employing an in vitro neuronal tract-tracing technique, we have, in the present study, identified a novel reciprocal connection between the CP/PPn and a cell group situated in the region between the ventral thalamus and the inferior lobe. Despite the previous interpretation by other authors of this cell group as the glomerular nucleus, the lack of a projection of this nucleus to the hypothalamus, as also demonstrated in the present investigation, makes such a homology unlikely. We, therefore, interpret this nucleus as a brain structure of unknown homology in other teleosts and suggest 'nucleus G' to identify it.

The adult central nervous cholinergic system of a neurogenetic model animal, the zebrafish Danio rerio

The central nervous cholinergic system of the zebrafish (Danio rerio), a model animal for neurogenetics, is documented here using immunohistochemical methods for visualizing choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme. Neuronal cell bodies containing ChAT are present in the telencephalon (lateral nucleus of ventral telencephalic area), preoptic region (anterior/posterior parvocellular and magnocellular preoptic nuclei), diencephalon (habenula, dorsal thalamus, posterior tuberculum), mesencephalon (Edinger-Westphal (EW) nucleus, oculomotor nerve nucleus, rostral tegmental nucleus, tectal type XIV neurons), isthmic region (nucleus lateralis valvulae, secondary gustatory-viscerosensory nucleus, nucleus isthmi (NI), perilemniscal nucleus, superior reticular nucleus (SRN)) and rhombencephalon (trochlear, trigeminal, abducens, facial, glossopharyngeal-vagal motor nerve nuclei, rostral and caudal populations of octavolateralis efferent neurons). In addition, some ChAT positive neurons are present in the rhombencephalic reticular formation, the central gray, and in cells accompanying the descending trigeminal tract. Obvious ChAT positive terminal fields are present in the supracommissural nucleus of area ventralis telencephali and the medial zone of area dorsalis telencephali, parvocellular superficial pretectal nucleus, torus semicircularis, medial octavolateralis nucleus, facial, glossopharyngeal, and vagal lobes, and in the inferior lobe (around the periventricular nucleus of the lateral recess and in the diffuse nucleus). The identification of all central nervous cholinergic systems provided here in this model system is pivotal for future detailed studies of their development and maintenance, e.g., with regard to the zebrafish ventral telencephalic and isthmic superior reticular neuronal populations, likely representing the homologues of at least part of the cholinergic basal forebrain and pedunculopontine/laterodorsal tegmental ascending activating systems of mammals, respectively.

Fiber connections of the lateral valvular nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

Fiber connections of the lateral valvular nucleus were investigated in a percomorph teleost, the tilapia (Oreochromis niloticus), by tract-tracing methods. Following tracer injections into the lateral valvular nucleus, neurons were labeled in the ipsilateral dorsal part of dorsal telencephalic area, corpus glomerulosum pars anterior, dorsomedial thalamic nucleus, central nucleus of the inferior lobe, mammillary body, semicircular torus, valvular and cerebellar corpus, in the bilateral rostral regions of the central part of dorsal telencephalic area, dorsal region of the medial part of dorsal telencephalic area, habenula, anterior tuberal nucleus, posterior tuberal nucleus, and spinal cord, and in the contralateral lateral funicular nucleus. Labeled fibers and terminals were found in the ipsilateral cerebellar corpus and bilateral valvula of the cerebellum. Tracers were injected into portions of the telencephalon, pretectum, inferior lobe, and cerebellum to confirm reciprocally connections with the lateral valvular nucleus and to determine afferent terminal morphology in the lateral valvular nucleus. Telencephalic fibers terminated mainly in a dorsolateral portion of the lateral valvular nucleus. Terminals from the corpus glomerulosum pars anterior, central nucleus of the inferior lobe, and mammillary body showed more diffuse distributions and were not confined to particular portions of the lateral valvular nucleus. Labeled terminals in the lateral valvular nucleus were cup-shaped or of beaded morphology. These results indicate that the lateral valvular nucleus receives projections from various sources including the telencephalon, pretectum, and inferior lobe to relay information to the valvular and cerebellar corpus. In addition, the corpus glomerulosum pars anterior in tilapia is considered to be homologous to the magnocellular part of superficial pretectal nucleus in cyprinids.

Octavolateral projections and organization in the medulla of a teleost fish, the sleeper goby (Dormitator latifrons)

This study is the first to employ simultaneous labeling with different colored fluorescent dyes and confocal microscopy to investigate the central projections of the octavolateral nerves in any fish. Three-dimensional reconstructions of the hindbrain octavolateral nuclei were made and overlap of octavolateral projections was assessed in a teleost, the sleeper goby (Dormitator latifrons). The octavolateral nerves, which innervate the otolithic organs, semicircular canals, and lateral lines, project to seven hindbrain nuclei in diverse, complex patterns. The medulla is generally organized with auditory regions dorsal to vestibular regions. The intermediate subdivision of the descending octaval nucleus (DON) receives interdigitating projections from the otolithic organs, and the dorsomedial DON likely integrates multiple auditory inputs. Afferents from the three otolithic organs (the utricle, saccule, and lagena) project to the intermediate DON in approximately equal proportion, supporting physiological evidence that suggests auditory roles for all three otolithic organs in the sleeper goby. The anterior octaval nucleus receives partially segregated inputs from the octavolateral organs. The dorsal division of the magnocellular octaval nucleus (MgON) receives highly overlapping otolithic organ and semicircular canal input, and we propose that this region is a major octaval integration center. Regions in the ventral medulla (the tangential octaval nucleus, ventral DON, and ventral MgON) receive mainly utricular and semicircular canal inputs, suggesting vestibular roles. Each semicircular canal nerve projects to distinct regions of the hindbrain, with little overlap in most octaval nuclei. Efferent neurons receive bilateral input and project unilaterally to the octavolateral organs.

Putative isotocin distributions in sonic fish: relation to vasotocin and vocal-acoustic circuitry

Recent neurophysiological evidence in the plainfin midshipman fish (Porichthys notatus) demonstrated that isotocin (IT) and arginine vasotocin (AVT) modulate fictive vocalizations divergently between three reproductive morphs. To provide an anatomical framework for the modulation of vocalization by IT and to foster comparisons with the distributions of the IT homologues mesotocin (MT) and oxytocin (OT) in other vertebrate groups, we describe putative IT distributions in the midshipman and the closely related gulf toadfish, Opsanus beta. Double-label fluorescent histochemistry was used for IT and AVT (by using antibodies for MT, OT, and the mammalian AVT homologue, arginine vasopressin [AVP]). MT/OT-like immunoreactive (MT/OT-lir) cell groups were found in the anterior parvocellular, posterior parvocellular, and magnocellular preoptic nuclei. MT/OT-lir fibers and putative terminals densely innervated the ventral telencephalon and numerous areas in the hypothalamus and brainstem. These distributions included all sites of vocal-acoustic integration recently identified for the forebrain and midbrain and diencephalic components of the ascending auditory pathway. Results were qualitatively comparable across morphs, species, and seasons. In contrast to the widespread distribution of MT/OT-lir, AVP-lir somata, fibers, and putative terminals were almost completely restricted to vocal-acoustic regions. These data parallel previous descriptions of AVT immunoreactivity in these species, although the present methods showed a previously undescribed, seasonally variable AVP-lir cell group in the anterior tuberal hypothalamus, a vocally active site and a component of the ascending auditory pathway. These findings provided anatomic support for the role of IT and AVT in the modulation of vocal behavior at multiple levels of the central vocal-acoustic circuitry.

Directional selectivity and frequency tuning of midbrain cells in the oyster toadfish, Opsanus tau

Single-unit recordings were made from areas in the midbrain (torus semicircularis) of the oyster toadfish. We evaluated frequency tuning and directional responses using whole-body oscillation to simulate auditory stimulation by particle motion along axes in the horizontal and mid-sagittal planes. We also tested for bimodality in responses to auditory and hydrodynamic stimuli. One recording location in each animal was marked by a neurobiotin injection to confirm the recording site. Recordings were made in nucleus centralis, nucleus ventrolateralis, and the deep cell layer. Most units were frequency-selective with best frequencies between 50 and 141 Hz. Suppression of activity was apparent in 10% of the cells. Bimodality was common, including inhibition and suppression of background activity by auditory or hydrodynamic stimulation. The majority of the cells were directionally selective with directional response patterns that were sharpened compared with those of primary saccular afferents. The best directional axes were arrayed widely in spherical space, covering most azimuths and elevations. This representation is adequate for the computation of the motional axis of an auditory stimulus for sound source localization.

Neural mechanisms and behaviors for acoustic communication in teleost fish

Sound communication is not unique to humans but rather is a trait shared with most non-mammalian vertebrates. A practical way to address questions of vocal signal encoding has been to identify mechanisms in non-mammalian model systems that use acoustic communication signals in their social behavior. Teleost fishes, the largest group of living vertebrates, include both vocal and non-vocal species that exploit a wide range of acoustic niches. Here, we focus on those vocal species where combined behavioral and neurobiological studies have recently begun to elucidate a suite of adaptations for both the production and the perception of acoustic signals essential to their reproductive success and survival. Studies of these model systems show that teleost fish have the vocal-acoustic behaviors and neural systems both necessary and sufficient to solve acoustic problems common to all vertebrates. In particular, behavioral studies demonstrate that temporal features within a call, including pulse duration, rate and number, can all be important to a call's communicative value. Neurobiological studies have begun to show how these features are produced by a vocal motor system extending from forebrain to hindbrain levels and are encoded by peripheral and central auditory neurons. The abundance and variety of vocal fish present unique opportunities for parallel investigations of neural encoding, perception, and communication across a diversity of natural, acoustic habitats. As such, investigations in teleosts contribute to our delineating the evolution of the vocal and auditory systems of both non-mammalian and mammalian species, including humans.

Neuroanatomy of the mormyrid electromotor control system

Mormyrid fish produce a diverse range of electric signals that are under the control of a central electromotor network. The anatomical organization of this network was delineated by injecting biotinylated compounds into neurophysiologically identified nuclei. Previous work using retrograde labeling with horseradish peroxidase indicated that the medullary command nucleus (CN) receives inputs from the precommand nucleus (PCN) at the mesencephalic-diencephalic border and the ventroposterior nucleus (VP) in the torus semicircularis. This study confirms these projections and identifies the dorsal posterior nucleus (DP) in the thalamus as an additional input to CN. DP and PCN form a bilateral column of cells extending ventrolaterally and caudally from the dorsal thalamus. The primary input to DP/PCN is from VP, which is identified as having two distinct subdivisions. A small group of large, multipolar cells along the ventral edge projects to DP/PCN and to CN, whereas a dorsal group of small, ovoid cells projects to DP/PCN but not to CN. VP receives input from the tectum mesencephali and the mesencephalic command-associated nucleus (MCA). As in all vertebrates, the tectum mesencephali receives input from several sources and likely provides multimodal sensory input to the electromotor system. MCA is part of the electromotor corollary discharge pathway, and its projection to VP suggests a feedback loop. These results, combined with recent physiological studies and comparisons with other taxa, suggest that modifiable feedback to DP/PCN plays a critical role in electromotor control and that the different inputs to CN may each be responsible for generating distinct electric signals.

Frequency response properties of lateral line superficial neuromasts in a vocal fish, with evidence for acoustic sensitivity

The mechanosensory lateral line of fish is a hair cell based sensory system that detects water motion using canal and superficial neuromasts. The trunk lateral line of the plainfin midshipman fish, Porichthys notatus, only has superficial neuromasts. The posterior lateral line nerve (PLLn) therefore innervates trunk superficial neuromasts exclusively and provides the opportunity to investigate the physiological responses of these receptors without the confounding influence of canal organs. We recorded single-unit activity from PLLn primary afferents in response to a vibrating sphere stimulus calibrated to produce an equal velocity across frequencies. Threshold tuning, isovelocity, and input/output curves were constructed using spike rate and vector strength, a measure of phase locking of spike times to the stimulus waveform. All units responded maximally to frequencies of 20-50 Hz. Units were classified as low-pass, band-pass, broadly tuned, or complex based on the shapes of tuning and isovelocity curves between 20 and 100 Hz. A 100 Hz stimulus caused an increase in spike rate in almost 50%, and significant synchronization in >80%, of all units. Midshipman vocalizations contain significant energy at and below 100 Hz, so these results demonstrate that the midshipman peripheral lateral line system can encode these acoustic signals. These results provide the first direct demonstration that units innervating superficial neuromasts in a teleost fish have heterogeneous frequency response properties, including an upper range of sensitivity that overlaps spectral peaks of behaviorally relevant acoustic stimuli.

Conserved and divergent patterns of Reelin expression in the zebrafish central nervous system

The protein Reelin is suggested to function in cell-cell interactions and in mediating neuronal migrations in layered central nervous system structures. With the aim of shedding light on the development of the teleost telencephalon, which forms through the process of eversion and results in the formation of a nonlaminar pallium, we isolated a zebrafish ortholog of the reelin gene and studied its expression in developing and adult brain. The pattern of expression is highly dynamic during the first 24-72 hours of development. By 5 days postfertilization, high amounts of reelin mRNA are found in the dorsal telencephalon, thalamic and hypothalamic regions, pretectal nuclei, optic tectum, cerebellum, hindbrain, reticular formation, and spinal cord, primarily confined to postmitotic neurons. This pattern persists in 1- to 3-month-old zebrafish. This study, together with reports on reelin expression in other vertebrates, shows that reelin mRNA distribution is conserved in many regions of the vertebrate brain. A major exception is that reelin is expressed in the majority of the cells of the dorsal regions of the everted telencephalon in zebrafish embryos, whereas it is restricted to specific neuronal populations in the developing telencephalon of amniotes. To better understand the origin of these differences, we analyzed reelin expression in the telencephalon of an amphibian. Telencephalic reelin expression in Xenopus laevis shows more similarities with the sauropsidian than with the teleostean pattern. Thus, the differences in the telencephalic expression of reelin between teleosts and tetrapods are likely to be due to different roles for Reelin during eversion, a process that is specific for the teleost telencephalon.

Vocal-acoustic circuitry and descending vocal pathways in teleost fish: convergence with terrestrial vertebrates reveals conserved traits

Vocal behavior is multifaceted and requires that vocal-motor patterning be integrated at multiple brain levels with auditory, neuroendocrine, and other social behavior processes (e.g., courtship and aggression). We now provide anatomical evidence for an extensive vocal network in teleost fishes (Batrachoididae: Porichthys notatus; Opsanus beta) that is strongly integrated with neuroendocrine and auditory pathways and that exhibits striking similarities to the vocal-acoustic circuitry known for mammals. Biotin compound injections into neurophysiologically identified vocal regions of the forebrain (preoptic area and anterior hypothalamus) and of the midbrain (periaqueductal gray and paralemniscal tegmentum) reveal extensive connectivity within and between these regions, as well as reciprocal relationships with the auditory thalamus and/or auditory midbrain (torus semicircularis). Thus, specific components of the basal forebrain and midbrain are here designated as the forebrain vocal-acoustic complex (fVAC) and midbrain vocal-acoustic complex (mVAC), respectively. Biotin injections into the mVAC and a previously identified hindbrain vocal pattern generator likewise provide anatomical evidence for a distributed network of descending projections to the vocal pacemaker-motoneuron circuitry. Together, the present experiments establish a vocal-auditory-neuroendocrine network in teleost fish that links the forebrain and midbrain to the hindbrain vocal pattern generator (i.e., fVAC --> mVAC --> pattern generator) and provides an anatomical framework for the previously identified neuropeptide modulation of vocal activity elicited from the forebrain and midbrain, which contributes to the expression of sex- and male morph-specific behavior. We conclude with a broad comparison of these findings with those for other vertebrate taxa and suggest that the present findings provide novel insights into the structure of conserved behavioral regulatory circuits that have led to evolutionary convergence in vocal-acoustic systems.

Acoustic competition in the gulf toadfish Opsanus beta: acoustic tagging

Nesting male gulf toadfish Opsanus beta produce a boatwhistle advertisement call used in male-male competition and to attract females and an agonistic grunt call. The grunt is a short-duration pulsatile call, and the boatwhistle is a complex call typically consisting of zero to three introductory grunts, a long tonal boop note, and zero to three shorter boops. The beginning of the boop note is also gruntlike. Anomalous boatwhistles contain a short-duration grunt embedded in the tonal portion of the boop or between an introductory grunt and the boop. Embedded grunts have sound-pressure levels and frequency spectra that correspond with those of recognized neighbors, suggesting that one fish is grunting during another's call, a phenomenon here termed acoustic tagging. Snaps of nearby pistol shrimp may also be tagged, and chains of tags involving more than two fish occur. The stimulus to tag is a relatively intense sound with a rapid rise time, and tags are generally produced within 100 ms of a trigger stimulus. Time between the trigger and the tag decreases with increased trigger amplitude. Tagging is distinct from increased calling in response to natural calls or stimulatory playbacks since calls rarely overlap other calls or playbacks. Tagging is not generally reciprocal between fish, suggesting parallels to dominance displays.

Processing of auditory midbrain interspike intervals by model neurons

A question central to sensory processing is how signal information is encoded and processed by single neurons. Stimulus features can be represented through rate coding (via firing rate), temporal coding (via firing synchronization to temporal periodicities), or temporal encoding (via intricate patterns of spike trains). Of the three, examples of temporal encoding are the least documented. One region in which temporal encoding is currently being explored is the auditory midbrain. Midbrain neurons in the plainfin midshipman generate different interspike interval (ISI) distributions depending on the frequencies of the concurrent vocal signals. However, these distributions differ only along certain lengths of ISIs, so that any neurons trying to distinguish the distributions would have to respond selectively to specific ISI ranges. We used this empirical observation as a realistic challenge with which to explore the plausibility of ISI-tuned neurons that could validate this form of temporal encoding. The resulting modeled cells--point neurons optimized through multidimensional searching--were successfully tuned to discriminate patterns in specific ranges of ISIs. Achieving this task, particularly with simplified neurons, strengthens the credibility of ISI coding in the brain and lends credence to its role in auditory processing.

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.

Brainstem acoustic areas in the marine catfish, Arius felis

The marine catfish Arius felis produces low frequency sounds for communication and obstacle detection. It was hypothesized that the utriculus of the inner ear might play an important role in these behaviors. In the current study, brainstem acoustic areas were studied to reveal possible neuroanatomical specializations in utricular processing areas. The first-order octaval nuclei in Arius were identical in number, anatomical characteristics, and organization of saccular, lagenar, and utricular inputs to previous reports of these features in Ictalurus, a closely related species of catfish that does not exhibit the specialized acoustic behaviors present in Arius. Similarly, injections of neural tracer in the acoustic midbrain (nucleus centralis) of Arius revealed afferent and retrograde pathways almost identical to those previously reported in Ictalurus. It is suggested that areas within the primary and higher-order octaval nuclei that utilize utricular input in acoustic processing are likely identical in Arius and Ictalurus. Two sets of higher-order connections in Arius differ from those in Ictalurus. First, Arius apparently lacks the direct input from the anterior octaval nucleus to nucleus centralis reported in Ictalurus. Second, in Arius nucleus centralis projects bilaterally to a strip of neurons positioned ventral to the ventral boundary of the torus semicircularis. This projection is apparently absent in Ictalurus and in the related species Carassius (goldfish), but has been previously reported in Porichthyes, a sound-producing species belonging to a different teleost taxon.

Coding of concurrent vocal signals by the auditory midbrain: effects of stimulus level and depth of modulation

The segregation of concurrent vocal signals is an auditory processing task faced by all vocal species. To segregate concurrent signals, the auditory system must encode the spectral and temporal features of the fused waveforms such that at least one signal can be individually detected. In the plainfin midshipman fish (Porichthys notatus), the overlapping mate calls of neighboring males produce acoustic beats with amplitude and phase modulations at the difference frequencies (dF) between spectral components. Prior studies in midshipman have shown that midbrain neurons provide a combinatorial code of the temporal and spectral characteristics of beats via synchronization of spike bursts to dF and changes in spike rate and interspike intervals with changes in spectral composition. In the present study we examine the effects of changes in signal parameters of beats (overall intensity level and depth of modulation) on the spike train outputs of midbrain neurons. The observed changes in spike train parameters further support the hypothesis that midbrain neurons provide a combinatorial code of the spectral and temporal features of concurrent vocal signals.

Acoustic nuclei in the medulla and midbrain of the vocalizing Gulf toadfish (Opsanus beta)

The organization of the descending and secondary octaval nuclei in the hindbrain of the Gulf toadfish, Opsanus beta, was revealed following the injection of biotin compounds into a physiologically identified auditory region of the torus semicircularis. The results show retrogradely-filled neurons mainly in a dorsomedial division of the descending octaval nucleus, and dorsal and ventral divisions of a secondary octaval nucleus; minor labeling also appeared in dorsolateral and rostromedial intermediate divisions of the descending nucleus. The pattern identified is consistent with that reported in other teleosts, including both vocal and non-vocal species, and clarifies earlier reports of the organization of hindbrain octaval nuclei in toadfish and the closely related midshipman fish.

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.

Sonic/vocal motor pathways in squirrelfish (Teleostei, Holocentridae)

Similar to many teleost fish, squirrelfish (family Holocentridae) produce vocalizations by the contraction of muscles that lead to vibration of the swimbladder. We used biotinylated compounds to identify the position and extent of vocal motor neurons in comparison to additional motor neuron groups, namely those of red and white dorsal epaxial muscle and opercular muscle that are located adjacent to or near the sonic muscle. The sonic motor nucleus (SMN) was located in the caudal medulla and rostral spinal cord in a ventrolateral position with dendrites extending dorsally in a dense bundle along the lateral edge of the medulla and axons exiting via ventral occipital nerve roots. Transneuronal transport of biocytin identified premotor neurons within the SMN and in the medially adjacent reticular formation that projected to the contralateral SMN and more rostrally to the octavolateralis efferent nucleus and nucleus praeeminentialis, suggesting interactions between vocal and octavolateralis systems as seen in other teleosts. Motor neurons innervating the red and white dorsal muscle formed a loose aggregate in the dorsal motor column, adjacent to the medial longitudinal fasciculus, sending fibers bilaterally throughout the spinal cord with axons exiting via ventral spinal nerve roots. Opercular motor neurons were located within the facial motor nucleus. The anatomical characteristics of the SMN of squirrelfish, a representative member of the order Beryciformes, are similar to those of representative members of the closely related order Scorpaeniformes, but diverge from the SMN of more distantly related orders of paracanthopterygiian and ostariophysan teleosts. These results therefore suggest a possible homology among the SMNs of acanthopterygiian fishes.

Rhythmic midbrain-evoked vocalization is inhibited by vasoactive intestinal polypeptide in the teleost Porichthys notatus

Vasoactive intestinal polypeptide (VIP) is distributed in vocal midbrain areas of multiple vertebrate taxa, suggesting that VIP may modulate midbrain-evoked vocalization. To test this hypothesis, neurophysiological experiments were conducted in the teleost Porichthys notatus which generates vocalizations in mating and agonistic contexts. Electrical stimulation of the paralemniscal midbrain and local delivery of VIP were conducted in conjunction with occipital nerve recordings that reflect the patterned output of hindbrain vocal circuitry. Consistent with our hypothesis, VIP significantly reduced the duration and number of rhythmic vocal-motor bursts obtained in a dose-dependent manner; vocalization latency was concomitantly increased. These results provide the first evidence for VIP modulation of midbrain vocal function.