Torus Semicircularis Research Articles

Cell's intrinsic biophysical properties play a role in the systematic decrease in time-locking ability of central auditory neurons

Studies in the vertebrates have shown that the time-locking ability of central auditory neurons decreases progressively along the ascending auditory pathway. This decrease is presumably attributed to a progressive reduction in the fidelity of synaptic transmission and an increase in the influence of synaptic inhibition along the cascade. The extent to which neurons' intrinsic biophysical properties contribute to the change in time-locking ability is unclear. We carried out whole-cell patch clamp recordings from the auditory thalamus of leopard frogs and compared their biophysical properties and time-locking abilities (determined by cell's responses to depolarizing pulse trains applied intracellularly) with those of lower auditory brainstem neurons. We found that frog thalamic neurons were homogeneous, exhibiting uniformly sustained, regular firing patterns, but not having low-threshold transient Ca2+ current which mammal thalamic neurons generally possess. Furthermore, intrinsic biophysical properties of the thalamic neurons are such that the time-locking ability of these neurons was very poor. The homogeneity of thalamic auditory neurons is in contrast to the heterogeneity of lower auditory brainstem neurons, with different phenotypes exhibiting different time-locking abilities and with sustained-regular phenotype consistently showing the worst time-locking ability among all biophysical phenotypes. Auditory nuclei along the ascending auditory pathway showed a progressive increase in the population of sustained-regular phenotype—this corresponded to a systematic decrease in the overall time-locking ability, with neurons in the dorsal medullary nucleus showing the best, and thalamic neurons exhibiting the poorest time-locking ability, whereas neurons in the torus semicircularis displayed intermediate time-locking ability. These results suggest that the biophysical characteristics of single neurons also likely play a role in the change in temporal coding ability along the ascending auditory pathway.

Are FoxP2, Otx1 and FoxO3 candidates for deep homology in weakly electric fish?

Event Abstract Back to Event Are FoxP2, Otx1 and FoxO3 candidates for deep homology in weakly electric fish? Erik Harvey-Girard1*, Anna Catarina Giassi1 and Leonard Maler1 1 University of Ottawa, Cellular and Molecular Medecine, Canada During embryological development the teleost telencephalon undergoes eversion while tetrapod telencephalon goes through evagination. This difference in development has caused major problems to establish any homology between teleost and tetrapod telencephalon. We recently determined the extrinsic and intrinsic neuronal connections of weakly electric fish (Apteronotus leptorhynchus) telencephalon. In particular, we observed that the dorsocentral division of the pallium (DC) contains glutamatergic spiny neurons that receive inputs from the dorsolateral (DL) pallium (homologous to the tetrapod hippocampus) and project to midbrain sensory nuclei (optic tectum, torus semicircularis) involved in visual and electrosensory signal processing. This pattern of connectivity is similar to that of the spiny glutamatergic pyramidal cells within the deep neocortical layers (V/VI). We wondered if there was a deep homology between teleost DC neurons and tetrapod neocortical neurons. To test that hypothesis, we investigated some molecular markers from deep neocortical layers (FoxP2 and Otx1) and hippocampus (FoxO3). First, we have cloned the apteronotid homologs of FoxP2, Otx1 and FoxO3. There was, in the case of all three genes, good similarity between the apteronotid and human amino acid sequences: FoxP2 – 78%, Otx1 – 54%, FoxO3 – 71%. The functional domains of these genes were conserved to a far greater extent – on average: FoxP2 – 89%, Otx1 – 76%, FoxO3 – 82%. This led us to hypothesize that the cellular functions of these genes may also be conserved. We then used in situ hybridization to examine the distribution of the mRNA transcripts of these genes in the apteronotid telencephalon. We found that AptFoxP2 and AptOtx1 transcripts were expressed predominantly in DC, with only a few weakly labeled neurons in DL. In contrast, we found that most neurons in DL strongly expressed AptFoxO3 mRNA, while there was only weak expression in a small number of cells within DC. Together with our previous hodological and anatomical data, these data suggest a deep homology exists between teleost DC and tetrapod neocortical deep layers. Furthermore, in accordance with previous studies in other teleosts, our data suggest that there is also a direct homology between teleost DL and mammal hippocampus. Acknowledgements CIHR Keywords: Apteronotus leptorhynchus, Deep homology, dorsal telencephalon, pallium, FOXO3, FoxP2, Hippocampus, Neocortex, Otx1 Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Poster Presentation (see alternatives below as well) Topic: Evolution Citation: Harvey-Girard E, Giassi A and Maler L (2012). Are FoxP2, Otx1 and FoxO3 candidates for deep homology in weakly electric fish?. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00089 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 18 Apr 2012; Published Online: 07 Jul 2012. * Correspondence: Dr. Erik Harvey-Girard, University of Ottawa, Cellular and Molecular Medecine, Ottawa, Ontario, K1H 8M5, Canada, erikhg@uottawa.ca Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Erik Harvey-Girard Anna Catarina Giassi Leonard Maler Google Erik Harvey-Girard Anna Catarina Giassi Leonard Maler Google Scholar Erik Harvey-Girard Anna Catarina Giassi Leonard Maler PubMed Erik Harvey-Girard Anna Catarina Giassi Leonard Maler Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Adaptation in phasic auditory units of the frog midbrain is sensitive to changes in stimulus frequency and ear of input

Event Abstract Back to Event Adaptation in phasic auditory units of the frog midbrain is sensitive to changes in stimulus frequency and ear of input Abhilash Ponnath1, Kim Hoke2 and Hamilton Farris1* 1 Louisiana St. University Health Sciences Center, Neuroscience, United States 2 Colorado State University, Biology, United States Neural adaptation, a reduction in response to a constant or maintained stimulus, is an important mechanism for detecting stimulus change. Contributing to stimulus change detection is the fact that neural adaptation is often stimulus specific: adaptation to a particular stimulus reduces excitability to only a subset of stimuli, while the ability to respond to other stimuli is unaffected. Phasic cells (e.g., responding to stimulus onset), with their fast adaptation, are considered good candidates for detecting the most rapid changes in natural auditory scenes, as they adapt quickly to the initial presentation of a stimulus and do not require adaptation buildup from many repeated stimulus presentations. Our study uses in vivo, single unit recordings to show that the fast and complete adaptation of phasic auditory units in the frog torus semicircularis is specific to stimulus frequency and ear of input. In response to an instantaneous frequency step in a 200 ms tone, 28% of cells exhibit frequency specific adaptation (FSA) based on relative frequency change (delta-f = +/-16%). FSA is not limited to frequency steps, however, as adaptation is also overcome during continuous frequency modulated stimuli. When temporal gaps in tones produce frequency transients, FSA cells outperform non-FSA cells in gap detection. When gaps in noise produce only temporal cues, however, gap detection is identical in the two cell classes. The results suggest that adaptation is separated for peripheral (e.g., frequency) channels, which we tested directly using dichotic stimuli. In 45% of binaural phasic units, adaptation was ear specific: adaptation to stimulation of one ear does not affect responses to stimulation of the other ear. Thus, adaptation exhibits specificity during processing of frequency and lateralization at the level of the midbrain. This mechanism could be employed to detect stimulus change within and between acoustic sources in rapidly changing complex acoustic environments. Acknowledgements KLH and HEF were supported in part by a Grass Faculty Fellowship at the Marine Biological Laboratory, Woods Hole, MA (2009). AP and HEF were supported by NIH grant P20RR016816 (to N. Bazan). Keywords: adaptation, auditory scene, chorus, critical band Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Poster Presentation (see alternatives below as well) Topic: Sensory: Audition Citation: Ponnath A, Hoke K and Farris H (2012). Adaptation in phasic auditory units of the frog midbrain is sensitive to changes in stimulus frequency and ear of input. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00157 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Apr 2012; Published Online: 07 Jul 2012. * Correspondence: Dr. Hamilton Farris, Louisiana St. University Health Sciences Center, Neuroscience, New Orleans, Louisiana, 70112, United States, hfarri@lsuhsc.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Abhilash Ponnath Kim Hoke Hamilton Farris Google Abhilash Ponnath Kim Hoke Hamilton Farris Google Scholar Abhilash Ponnath Kim Hoke Hamilton Farris PubMed Abhilash Ponnath Kim Hoke Hamilton Farris Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Two modes of information processing in the electrosensory system of the paddlefish (Polyodon spathula)

Paddlefish are uniquely adapted for the detection of their prey, small water fleas, by primarily using their passive electrosensory system. In a recent anatomical study, we found two populations of secondary neurons in the electrosensory hind brain area (dorsal octavolateral nucleus, DON). Cells in the anterior DON project to the contralateral tectum, whereas cells in the posterior DON project bilaterally to the torus semicircularis and lateral mesencephalic nucleus. In this study, we investigated the properties of both populations and found that they form two physiologically different populations. Cells in the posterior DON are about one order of magnitude more sensitive and respond better to stimuli with lower frequency content than anterior cells. The posterior cells are, therefore, better suited to detect distant prey represented by low-amplitude signals at the receptors, along with a lower frequency spectrum, whereas cells in the anterior DON may only be able to sense nearby prey. This suggests the existence of two distinct channels for electrosensory information processing: one for proximal signals via the anterior DON and one for distant stimuli via the posterior DON with the sensory input fed into the appropriate ascending channels based on the relative sensitivity of both cell populations.

Sparse and dense coding of natural stimuli by distinct midbrain neuron subpopulations in weakly electric fish

While peripheral sensory neurons respond to natural stimuli with a broad range of spatiotemporal frequencies, central neurons instead respond sparsely to specific features in general. The nonlinear transformations leading to this emergent selectivity are not well understood. Here we characterized how the neural representation of stimuli changes across successive brain areas, using the electrosensory system of weakly electric fish as a model system. We found that midbrain torus semicircularis (TS) neurons were on average more selective in their responses than hindbrain electrosensory lateral line lobe (ELL) neurons. Further analysis revealed two categories of TS neurons: dense coding TS neurons that were ELL-like and sparse coding TS neurons that displayed selective responses. These neurons in general responded to preferred stimuli with few spikes and were mostly silent for other stimuli. We further investigated whether information about stimulus attributes was contained in the activities of ELL and TS neurons. To do so, we used a spike train metric to quantify how well stimuli could be discriminated based on spiking responses. We found that sparse coding TS neurons performed poorly even when their activities were combined compared with ELL and dense coding TS neurons. In contrast, combining the activities of as few as 12 dense coding TS neurons could lead to optimal discrimination. On the other hand, sparse coding TS neurons were better detectors of whether their preferred stimulus occurred compared with either dense coding TS or ELL neurons. Our results therefore suggest that the TS implements parallel detection and estimation of sensory input.

Ltor PROCESSES MALE TÚNGARA MATING CALLS

It's a warm Panamanian evening and male Physalaemus frogs are out crooning to their females. But with many other species serenading in the night's air, how do túngara females pick out their mates from the rest of the hullabaloo? Lisa Mangiamele and Sabrina Burmeister from the University of North Carolina at Chapel Hill explain that, `Sensory systems respond selectively to information in the environment – extracting some stimulus features while discarding others.' According to Burmeister, females respond to the drop in frequency as the male's whine glissandos from 900 to 400 Hz. They also respond equally well to a synthesised two-tone call that drops from 800 Hz to 500 Hz tone; the underlying two-tone signature is all that a túngara female needs to correctly select a mate of her own species. Knowing that a transcription factor, egr-1, is produced in brain tissue in response to stimulation, the duo decided to identify the regions of the female brain that are involved in processing the males' serenades by looking for evidence that the egr-1 gene had been activated in the brain after listening to natural and simulated mating calls (p. 2911).The duo found that one tiny region of the female's brain, the laminar nucleus of the torus semicircularis (Ltor), responded to both the males' whining glissando and the signature two-tone simulation, suggesting that Ltor is the `neural analyser for call recognition'. Other regions of the auditory brainstem and thalamus also responded to the males' full-spectrum call, but they failed to activate egr-1 in response to the two-tone signature, leading the duo to conclude, `Ltor activation is sufficient to explain species recognition decisions for female túngara frogs.'

Auditory selectivity for acoustic features that confer species recognition in the túngara frog

In anurans, recognition of species-specific acoustic signals is essential to finding a mate. In many species, behavioral tests have elucidated which acoustic features contribute to species recognition, but the mechanisms by which the brain encodes these species-specific signal components are less well understood. The túngara frog produces a `whine' mating call that is characterized by a descending frequency sweep. However, much of the signal is unnecessary for recognition, as recognition behavior can be triggered by a descending two-tone step that mimics the frequency change in a portion of the whine. To identify the brain regions that contribute to species recognition in the túngara frog, we exposed females to a full-spectrum whine, a descending two-tone step that elicits recognition, the reversed two-tone step that does not elicit recognition, or no sound, and we measured expression of the neural activity-dependent gene egr-1 in the auditory brainstem and thalamus. We found that the behavioral relevance of the stimuli was the best predictor of egr-1 expression in the laminar nucleus of the torus semicircularis but not elsewhere. That is, the laminar nucleus responded more to the whine and the two-tone step that elicits recognition than to the reversed two-tone step. In contrast, in other brainstem and thalamic nuclei, whines induced egr-1 expression but tones did not. These data demonstrate that neural responses in the laminar nucleus correspond to behavioral responses of females and they suggest that the laminar nucleus may act as a feature detector for the descending frequencies characteristic of conspecific calls.

Sexual Phenotype Differences in zic2 mRNA Abundance in the Preoptic Area of a Protogynous Teleost, Thalassoma bifasciatum

The highly conserved members of the zic family of zinc-finger transcription factors are primarily known for their roles in embryonic signaling pathways and regulation of cellular proliferation and differentiation. This study describes sexual phenotype differences in abundances of zic2 mRNA in the preoptic area of the hypothalamus, a region strongly implicated in sexual behavior and function, in an adult teleost, Thalassoma bifasciatum. The bluehead wrasse (Thalassoma bifasciatum) is a valuable model for studying neuroendocrine processes because it displays two discrete male phenotypes, initial phase (IP) males and territorial, terminal phase (TP) males, and undergoes socially-controlled protogynous sex change. Previously generated microarray-based comparisons suggested that zic2 was upregulated in the brains of terminal phase males relative to initial phase males. To further explore this difference, we cloned a 727 bp sequence for neural zic2 from field-collected animals. Riboprobe-based in situ hybridization was employed to localize zic2 signal in adult bluehead brains and assess the relative abundance of brain zic2 mRNA across sexual phenotypes. We found zic2 mRNA expression was extremely abundant in the granular cells of the cerebellum and widespread in other brain regions including in the thalamus, hypothalamus, habenula, torus semicircularis, torus longitudinalis, medial longitudinal fascicle and telencephalic areas. Quantitative autoradiography and phosphorimaging showed zic2 mRNA hybridization signal in the preoptic area of the hypothalamus was significantly higher in terminal phase males relative to both initial phase males and females, and silver grain analysis confirmed this relationship between phenotypes. No significant difference in abundance was found in zic2 signal across phenotypes in the habenula, a brain region not implicated in the control of sexual behavior, or cerebellum.

Sites of origin and developmental dynamics of the neurons in the core and shell regions of torus semicircularis in the chinese softshell turtle (Pelodiscus sinensis)

To know the embryogenesis of the core and shell regions of the midbrain auditory nucleus, a single dose of [(3)H]-thymidine was injected into the turtle embryos at peak stages of neurogenesis in the shell and core of the torus semicircularis. Following sequential survival times, labeled neurons and the dynamics of cell proliferation were examined. The expression of vimentin (VM), reelin, calbindin, parvalbumin, and substance P were also studied. The results showed that: 1) progenitor cells for the core and shell regions were generated in different sites of the ventricular zone; 2) the length of the cell cycle or S-phase for the shell region were both longer than those for the core region (4.7 and 3.2 hours longer, respectively), suggesting that mitotic activity in the core region is higher than it is in the shell region; 3) the elongated cell bodies of the labeled core and shell cells had close apposition to VM fibers, suggesting that the migration of these cells is guided by VM fibers; 4) the germinal sites of the core and shell constructed by projecting the orientation of radial VM fibers back to the ventricular zone was consistent with those obtained by short and sequential survival [(3)H]-thymidine radiography; and 5) the beginning of positive staining for parvalbumin in the core region was interposed between those for calbindin and substance P in the shell regions. This study contributes to the understanding of how auditory nuclei are organized and how their components developed and evolved.

Monoamine innervation of vocal and auditory circuits in the plainfin midshipman fish.

Monoamines, which include catecholamines (e.g., dopamine and noradrenaline) and serotonin, are important regulators of reward, motivation and reproductive-related behaviors across vertebrates. Studies in tetrapods have demonstrated that monoamines can also serve a modulatory function for vocalization and audition. Antibodies directed against serotonin (5-HT) and tyrosine hydroxylase (TH) (the rate limiting enzyme in catecholamine synthesis) were employed to investigate monoamine innervation of known central vocal and auditory nuclei and the sensory epithelium of the saccule, the main endorgan of hearing, in the plainfin midshipman fish, Porichthys notatus. Both 5-HT and TH-immunoreactive (-ir) terminals are abundant in hindbrain (dorsal descending octaval nucleus), midbrain (torus semicircularis), thalamic (central posterior nucleus), and ventral telencephalic auditory nuclei. In addition, very robust TH-ir is found in the octavolateralis efferent nucleus, and prominent TH-ir projections travel along the eighth nerve and terminate within the hair cell layer of the saccular epithelium. Aside from the midbrain periaqueductal gray, TH-ir is much more pronounced than 5-HT-ir throughout the descending vocal motor system, particularly in the tuberal hypothalamus and the hindbrain-spinal vocal motor nucleus, which directly innervates vocal musculature on the swimbladder. These data provide anatomical evidence that monoamines are modulators of vocal and auditory-driven behaviors in fishes.

Ultrasound-evoked immediate early gene expression in the brainstem of the Chinese torrent frog, Odorrana tormota

The concave-eared torrent frog, Odorrana tormota, has evolved the extraordinary ability to communicate ultrasonically (i.e., using frequencies > 20 kHz), and electrophysiological experiments have demonstrated that neurons in the frog’s midbrain (torus semicircularis) respond to frequencies up to 34 kHz. However, at this time, it is unclear which region(s) of the torus and what other brainstem nuclei are involved in the detection of ultrasound. To gain insight into the anatomical substrate of ultrasound detection, we mapped expression of the activity-dependent gene, egr-1, in the brain in response to a full-spectrum mating call, a filtered, ultrasound-only call, and no sound. We found that the ultrasound-only call elicited egr-1 expression in the superior olivary and principal nucleus of the torus semicircularis. In sampled areas of the principal nucleus, the ultrasound-only call tended to evoke higher egr-1 expression than the full-spectrum call and, in the center of the nucleus, induced significantly higher egr-1 levels than the no-sound control. In the superior olivary nucleus, the full-spectrum and ultrasound-only calls evoked similar levels of expression that were significantly greater than the control, and egr-1 induction in the laminar nucleus showed no evidence of acoustic modulation. These data suggest that the sampled areas of the principal nucleus are among the regions sensitive to ultrasound in this species.

Temporally selective processing of communication signals by auditory midbrain neurons

Perception of the temporal structure of acoustic signals contributes critically to vocal signaling. In the aquatic clawed frog Xenopus laevis, calls differ primarily in the temporal parameter of click rate, which conveys sexual identity and reproductive state. We show here that an ensemble of auditory neurons in the laminar nucleus of the torus semicircularis (TS) of X. laevis specializes in encoding vocalization click rates. We recorded single TS units while pure tones, natural calls, and synthetic clicks were presented directly to the tympanum via a vibration-stimulation probe. Synthesized click rates ranged from 4 to 50 Hz, the rate at which the clicks begin to overlap. Frequency selectivity and temporal processing were characterized using response-intensity curves, temporal-discharge patterns, and autocorrelations of reduplicated responses to click trains. Characteristic frequencies ranged from 140 to 3,250 Hz, with minimum thresholds of -90 dB re 1 mm/s at 500 Hz and -76 dB at 1,100 Hz near the dominant frequency of female clicks. Unlike units in the auditory nerve and dorsal medullary nucleus, most toral units respond selectively to the behaviorally relevant temporal feature of the rate of clicks in calls. The majority of neurons (85%) were selective for click rates, and this selectivity remained unchanged over sound levels 10 to 20 dB above threshold. Selective neurons give phasic, tonic, or adapting responses to tone bursts and click trains. Some algorithms that could compute temporally selective receptive fields are described.

Gonadotropin-releasing hormone (GnRH) modulates auditory processing in the fish brain

Gonadotropin-releasing hormone 1 (GnRH1) neurons control reproductive activity, but GnRH2 and GnRH3 neurons have widespread projections and function as neuromodulators in the vertebrate brain. While these extra-hypothalamic GnRH forms function as olfactory and visual neuromodulators, their potential effect on processing of auditory information is unknown. To test the hypothesis that GnRH modulates the processing of auditory information in the brain, we used immunohistochemistry to determine seasonal variations in these neuropeptide systems, and in vivo single-neuron recordings to identify neuromodulation in the midbrain torus semicircularis of the soniferous damselfish Abudefduf abdominalis. Our results show abundant GnRH-immunoreactive (-ir) axons in auditory processing regions of the midbrain and hindbrain. The number of extra-hypothalamic GnRH somata and the density of GnRH-ir axons within the auditory torus semicircularis also varied across the year, suggesting seasonal changes in GnRH influence of auditory processing. Exogenous application of GnRH (sGnRH and cGnRHII) caused a primarily inhibitory effect on auditory-evoked single neuron responses in the torus semicircularis. In the majority of neurons, GnRH caused a long-lasting decrease in spike rate in response to both tone bursts and playbacks of complex natural sounds. GnRH also decreased response latency and increased auditory thresholds in a frequency and stimulus type-dependent manner. To our knowledge, these results show for the first time in any vertebrate that GnRH can influence context-specific auditory processing in vivo in the brain, and may function to modulate seasonal auditory-mediated social behaviors.

Characterization of Genetically Targeted Neuron Types in the Zebrafish Optic Tectum

The optically transparent larval zebrafish is ideally suited for in vivo analyses of neural circuitry controlling visually guided behaviors. However, there is a lack of information regarding specific cell types in the major retinorecipient brain region of the fish, the optic tectum. Here we report the characterization of three previously unidentified tectal cell types that are specifically labeled by dlx5/6 enhancer elements. In vivo laser-scanning microscopy in conjunction with ex vivo array tomography revealed that these neurons differ in their morphologies, synaptic connectivity, and neurotransmitter phenotypes. The first type is an excitatory bistratified periventricular interneuron that forms a dendritic arbor in the retinorecipient stratum fibrosum et griseum superficiale (SFGS) and an axonal arbor in the stratum griseum centrale (SGC). The second type, a GABAergic non-stratified periventricular interneuron, extends a bushy arbor containing both dendrites and axons into the SGC and the deepest sublayers of the SFGS. The third type is a GABAergic periventricular projection neuron that extends a dendritic arbor into the SGC and a long axon to the torus semicircularis, medulla oblongata, and anterior hindbrain. Interestingly, the same axons form en passant synapses within the deepest neuropil layer of the tectum, the stratum album centrale. This approach revealed several novel aspects of tectal circuitry, including: (1) a glutamatergic mode of transmission from the superficial, retinorecipient neuropil layers to the deeper, output layers, (2) the presence of interneurons with mixed dendrite/axon arbors likely involved in local processing, and (3) a heretofore unknown GABAergic tectofugal projection to midbrain and hindbrain. These observations establish a framework for studying the morphological and functional differentiation of neural circuits in the zebrafish visual system.

Germinal sites and migrating routes of cells in the mesencephalic and diencephalic auditory areas in the African clawed frog (Xenopus laevis)

There is a clear core–shell organization in the auditory nuclei of amniotes. However, such organization only exists in the mesencephalic, but not in the diencephalic auditory regions of amphibians. To gain insights into how this core–shell organization developed and evolved, we injected a small dose of [3H]-thymidine into tadpoles of Xenopus laevis at peak stages of neurogenesis in the mesencephalic and diencephalic auditory areas. Following different survival times, the germinal sites and migrating routes of cells were examined in the shell (laminar nucleus, Tl; magnocellular nucleus, Tmc) and core (principal nucleus, Tp) regions of the mesencephalic auditory nucleus, torus semicircularis (Ts), as well as in the diencephalic auditory areas (posterior thalamic nucleus, P; central thalamic nucleus, C). Double labeling for [3H]-thymidine autoradiography and immunohistochemistry for vimentin was also performed to help determine the routes of cell migration. We found three major results. First, the germinal sites of Tp were intercalated between Tl and Tmc, arising from those of the shell regions. Second, although the germinal sites of Tl, Tmc, and Tp were located in the same brain levels (at rostromedial or caudomedial levels of Ts), neurogenesis in Tl or Tmc started earlier than that in Tp. Finally, the P and C were also generated in different ventricle sites. However, unlike Ts their neurogenesis showed no obvious temporal differences. These data demonstrate that a highly differentiated auditory region, such as Tp in Ts, is lacking in the diencephalon of amphibian. Our data are discussed from the view of the constitution and evolutionary origins of auditory nuclei in vertebrates.

Immunohistochemical localization of DARPP-32 in the brain and spinal cord of anuran amphibians and its relation with the catecholaminergic system

The relationship between dopaminergic neuronal structures and dopaminoceptive structures in the amphibian brain and spinal cord are assessed by means of single and double immunohistochemical techniques with antibodies directed against DARPP-32 (a phosphoprotein related to the dopamine D 1-receptor) and tyrosine hydroxylase (TH) applied to the brain of the anurans Rana perezi and Xenopus laevis. The DARPP-32 antibody yielded a well-differentiated pattern of staining in the brain of these anurans. In general, areas that are densely innervated by TH-immunoreactive fibers such as the nucleus accumbens, striatum, amygdaloid complex, thalamus, optic tectum, torus semicircularis and spinal cord display a remarkable immunoreactivity for DARPP-32 in cell bodies and neuropil. Distinct cellular DARPP-32 immunoreactivity was also found in the septum, preoptic area, suprachiasmatic nucleus, tuberal hypothalamic region, habenula, retina, midbrain tegmentum, rhombencephalic reticular formation and solitary tract nucleus. Hodological data supported that striatal projection neurons were DARPP-32 immunoreactive. Double immunohistofluorescence staining revealed that catecholaminergic cells generally do not stain for DARPP-32, except for some cells in the ventral mesencephalic tegmentum of Xenopus and cells in the nucleus of the solitary tract of Rana. Several interspecies differences were noted for the DARPP-32 distribution in the brain of the two anurans, namely in the habenula, the thalamus and prethalamus, the cerebellum and octavolateral area and the structures with DARPP-32/TH colocalization. However, in general, the distribution of DARPP-32 in the brain of the anuran amphibians resembles in many aspects the pattern observed in amniotes, especially in reptiles.

Calcium-binding protein immunoreactivity characterizes the auditory system of Gekko gecko.

Geckos use vocalizations for intraspecific communication, but little is known about the organization of their central auditory system. We therefore used antibodies against the calcium-binding proteins calretinin (CR), parvalbumin (PV), and calbindin-D28k (CB) to characterize the gecko auditory system. We also examined expression of both glutamic acid decarboxlase (GAD) and synaptic vesicle protein (SV2). Western blots showed that these antibodies are specific to gecko brain. All three calcium-binding proteins were expressed in the auditory nerve, and CR immunoreactivity labeled the first-order nuclei and delineated the terminal fields associated with the ascending projections from the first-order auditory nuclei. PV expression characterized the superior olivary nuclei, whereas GAD immunoreactivity characterized many neurons in the nucleus of the lateral lemniscus and some neurons in the torus semicircularis. In the auditory midbrain, the distribution of CR, PV, and CB characterized divisions within the central nucleus of the torus semicircularis. All three calcium-binding proteins were expressed in nucleus medialis of the thalamus. These expression patterns are similar to those described for other vertebrates.

Sexually dimorphic androgen and estrogen receptor mRNA expression in the brain of túngara frogs

Sex steroid hormones are potent regulators of behavior and they exert their effects through influences on sensory, motor, and motivational systems. To elucidate where androgens and estrogens can act to regulate sex-typical behaviors in the túngara frog ( Physalaemus pustulosus), we quantified expression of the androgen receptor ( AR), estrogen receptor alpha ( ERα), and estrogen receptor beta ( ERβ) genes in the brains of male and females. To do so, we cloned túngara-specific sequences for AR, ERα, and ERβ, determined their distribution in the brain, and then quantified their expression in areas that are important in sexual communication. We found that AR, ERα, and ERβ were expressed in the pallium, limbic forebrain (preoptic area, hypothalamus, nucleus accumbens, amygdala, septum, striatum), parts of the thalamus, and the auditory midbrain (torus semicircularis). Males and females had a similar distribution of AR and ER expression, but expression levels differed in some brain regions. In the auditory midbrain, females had higher ERα and ERβ expression than males, whereas males had higher AR expression than females. In the forebrain, females had higher AR expression than males in the ventral hypothalamus and medial pallium (homolog to hippocampus), whereas males had higher ERα expression in the medial pallium. In the preoptic area, striatum, and septum, males and females had similar levels of AR and ER expression. Our results suggest that sex steroid hormones have sexually dimorphic effects on auditory processing, sexual motivation, and possibly memory and, therefore, have important implications for sexual communication in this system.

Temporal Selectivity in Midbrain Electrosensory Neurons Identified by Modal Variation in Active Sensing

Mormyrid weakly electric fish actively sense their surroundings by continuously emitting discrete pulses of electricity separated by varying intervals of silence. The temporal pattern of this pulsing behavior is related to context. While resting in the absence of an overt stimulus, baseline interpulse intervals (IPIs) mostly range 200-450 ms, and sequential variation is relatively high. Spontaneously, or following the presentation of a novel stimulus, IPIs transiently shorten during the performance of an electromotor "burst" display. We made intracellular whole cell recordings in vivo from neurons in the lateral nucleus of the torus semicircularis while the fish's dynamic pulsing behavior modified the temporal pattern of stimulation. Stimulation was designed to simulate the spatial patterns of AM that occur during the electrolocation of a resistive object. We discovered that toral neurons selectively respond to stimulation during a particular mode of electromotor activity. Two types of temporally selective neurons were discovered: baseline-selective neurons that displayed significantly higher postsynaptic potential (PSP) amplitude and spike count per electric organ discharge (EOD) during baseline electromotor activity and burst-selective neurons that displayed significantly higher PSP amplitude and spike count per EOD during electromotor burst displays. Interval-dependent changes in the strength of excitation and inhibition contributed to their selectivity.

Core-and-belt organisation of the mesencephalic and forebrain auditory centres in turtles: Expression of calcium-binding proteins and metabolic activity

The distribution of immunoreactivity to the calcium-binding proteins parvalbumin, calbindin and calretinin and of cytochrome oxidase activity was studied in the mesencephalic (torus semicircularis), thalamic (nucleus reuniens) and telencephalic (ventromedial part of the anterior dorsal ventricular ridge) auditory centres of two chelonian species Emys orbicularis and Testudo horsfieldi. In the torus semicircularis, the central nucleus (core) showed intense parvalbumin immunoreactivity and high cytochrome oxidase activity, whereas the laminar nucleus (belt) showed low cytochrome oxidase activity and dense calbindin/calretinin immunoreactivity. Within the central nucleus, the central and peripheral areas could be distinguished by a higher density of parvalbumin immunoreactivity and cytochrome oxidase activity in the core than in the peripheral area. In the nucleus reuniens, the dorsal and ventromedial (core) regions showed high cytochrome oxidase activity and immunoreactivity to all three calcium-binding proteins, while its ventrolateral part (belt) was weakly immunoreactive and showed lower cytochrome oxidase activity. In the telencephalic auditory centre, on the other hand, no particular region differed in either immunoreactivity or cytochrome oxidase activity. Our findings provide additional arguments in favour of the hypothesis of a core-and-belt organisation of the auditory sensory centres in non-mammalian amniotes though this organisation is less evident in higher order centres. The data are discussed in terms of the evolution of the auditory system in amniotes.