Torus Semicircularis Research Articles

Alpha-melanocyte stimulating hormone immunoreactivity in the brain of the cichlid fish Oreochromis mossambicus

This study reports the distribution of a pro-opiomelanocortin-derived neuropeptide α–MSH in the brain of the cichlid fish Oreochromis mossambicus. α–MSH-ir fibres were found in the granule cell layer of the olfactory bulb, the medial olfactory tract, the pallium and the subpallium, whereas in the preoptic area of the telencephalon, few large α–MSH-ir perikarya along with extensively labeled fibres were observed close to the ventricular border. Dense network of α–MSH-ir fibres were seen in the hypothalamic areas such as the nucleus preopticus pars magnocellularis, the nucleus preopticus pars parvocellularis, the suprachiasmatic nucleus, the nucleus anterior tuberis, the paraventricular organ, the subdivisions of the nucleus recessus lateralis and the nucleus recessus posterioris. In the nucleus lateralis pars medialis, some α–MSH-ir perikarya and fibres were found along the ventricular margin. In the diencephalon, numerous α–MSH-ir fibres were detected in the nucleus posterior tuberis, the nucleus of the fasciculus longitudinalis medialis and the nucleus preglomerulosus medialis, whereas in the mesencephalon, α–MSH-ir fibres were located in the optic tectum, the torus semicircularis and the tegmentum. In the rhombencephalon, α–MSH-ir fibres were confined to the medial octavolateralis nucleus and the descending octaval nucleus. In the pituitary gland, densely packed α–MSH-ir cells were observed in the pars intermedia region. The widespread distribution of α–MSH-immunoreactivity throughout the brain and the pituitary gland suggests a role for α–MSH peptide in regulation of several neuroendocrine and sensorimotor functions as well as darkening of pigmentation in the tilapia.

Subdivisions of the mesencephalon and isthmus in the lizard Gekko gecko as revealed by ChAT immunohistochemistry.

The distribution of cholinergic cell bodies and fibers was examined in the mesencephalon and isthmus of Gekko gecko. Distinct groups with prominent labeled cells were observed in the cranial nerve motor nuclei and isthmic nuclei, and weak labeled cell bodies and fibers were observed in the mesencephalic nucleus of the trigeminal nerve and the central nucleus of the torus semicircularis. After discussing the topological relationships within the tectum and isthmus, we unify the nomenclature of the caudal deep mesencephalic nucleus in lizards and the rostral magnocellular nucleus isthmi in turtles that is similar in terms of the preisthmic position, nontopographic connections with the tectum, and the same midbrain origin to the magnocellular preisthmic nucleus in birds, and may be homologous to the superficial cuneiform nucleus in mammals. None of them belong to the cholinergic nucleus isthmi, as the latter has isthmus origin and topographic reciprocal connections with the tectum. We also discuss the origin and intrinsic function of the inner longitudinal tract of the thick ChAT-ir fibers that course through the mesencephalon and diencephalon. We review the subdivisions of the mesencephalon and isthmus of Gekko gecko as revealed by ChAT immunohistochemistry, as well as the limits of the diencephalo-mesencephalic, mesencephalic-isthmo, and isthmo-rhombocephalic by the ChAT-ir cell- and fiber-poor distribution, and discuss the caudal limit of the isthmus. Our research on the subdivisions of the mesencephalon and isthmus in G. gecko as revealed by ChAT immunohistochemistry will serve as the neuroanatomical basis for subsequent relevant studies of Gekko gecko.

Differential expression of somatostatin genes in the central nervous system of the sea lamprey.

The identification of three somatostatin (SST) genes (SSTa, SSTb, and SSTc) in lampreys (Tostivint et al. Gen Comp Endocrinol 237:89-97 https://doi.org/10.1016/j.ygcen.2016.08.006 , 2016) prompted us to study their expression in the brain and spinal cord of the sea lamprey by in situ hybridization. These three genes were only expressed in equivalent neuronal populations in the hypothalamus. In other regions, SST transcripts showed clear differential expression. In the telencephalon, SSTc-positive cells were observed in the medial pallium, ventral part of the lateral pallium, striatum, subhippocampal lobe, and preoptic region. In the diencephalon, SSTa-positive cells were observed in the thalamus and SSTc-positive cells in the prethalamus, posterior tubercle, pretectal area, and nucleus of the medial longitudinal fascicle. In the midbrain, SSTc-positive cells were observed in the torus semicircularis, lateral reticular area, and perioculomotor tegmentum. Different SSTa- and SSTc-positive populations were observed in the isthmus. SSTc neurons were also observed in the rostral octavolateralis area and caudal rhombencephalon. In the spinal cord, SSTa was expressed in cerebrospinal-fluid-contacting (CSF-c) neurons and SSTc in non-CSF-c interneurons. Comparison with previous immunohistochemical studies using anti-SST-14 antibodies strongly suggests that SST-14-like neurons correspond with the SSTa populations. Thus, the SSTc populations were not reported previously in immunohistochemical studies. Cluster-based analyses and alignments of mature peptides suggested that SSTa is an ortholog of SST1 and that SSTb is closely related to SST2 and SST6. These results provide important new insights into the evolution of the somatostatinergic system in vertebrates.

Altered brain-wide auditory networks in a zebrafish model of fragile X syndrome

BackgroundLoss or disrupted expression of the FMR1 gene causes fragile X syndrome (FXS), the most common monogenetic form of autism in humans. Although disruptions in sensory processing are core traits of FXS and autism, the neural underpinnings of these phenotypes are poorly understood. Using calcium imaging to record from the entire brain at cellular resolution, we investigated neuronal responses to visual and auditory stimuli in larval zebrafish, using fmr1 mutants to model FXS. The purpose of this study was to model the alterations of sensory networks, brain-wide and at cellular resolution, that underlie the sensory aspects of FXS and autism.ResultsCombining functional analyses with the neurons’ anatomical positions, we found that fmr1−/− animals have normal responses to visual motion. However, there were several alterations in the auditory processing of fmr1−/− animals. Auditory responses were more plentiful in hindbrain structures and in the thalamus. The thalamus, torus semicircularis, and tegmentum had clusters of neurons that responded more strongly to auditory stimuli in fmr1−/− animals. Functional connectivity networks showed more inter-regional connectivity at lower sound intensities (a − 3 to − 6 dB shift) in fmr1−/− larvae compared to wild type. Finally, the decoding capacities of specific components of the ascending auditory pathway were altered: the octavolateralis nucleus within the hindbrain had significantly stronger decoding of auditory amplitude while the telencephalon had weaker decoding in fmr1−/− mutants.ConclusionsWe demonstrated that fmr1−/− larvae are hypersensitive to sound, with a 3–6 dB shift in sensitivity, and identified four sub-cortical brain regions with more plentiful responses and/or greater response strengths to auditory stimuli. We also constructed an experimentally supported model of how auditory information may be processed brain-wide in fmr1−/− larvae. Our model suggests that the early ascending auditory pathway transmits more auditory information, with less filtering and modulation, in this model of FXS.

A visual lamina in the medulla oblongata of the frog, Rana pipiens

We have discovered a lamina of visually responsive units in the medulla oblongata of the frog. It spans the entire medial aspect of the rostrocaudal length of the medulla and extends dorsoventrally from the cell-dense dorsal zone into the cell-sparse ventral zone. Most visual units within this lamina have large receptive fields, with the majority extending bilaterally in the frontal visual field. Most of these neurons are binocular, have no apparent directional preference, respond equally well to stimuli of a variety of shapes and sizes, and exhibit strong habituation. More medial locations in the visual lamina represent ipsilateral visual space while more lateral locations within the lamina represent contralateral visual space. Many units in the caudal aspect of the visual lamina are bimodal, responding to both visual and somatosensory stimuli. HRP tracing reveals inputs to the lamina from many primary and secondary visual areas in the midbrain and diencephalon. There is no area-by-area segregation of the projections to the visual lamina. For example, most parts of the tectum project across the visual lamina. The only spatial order in the visual lamina is that at more medial sites there tends to be more input from contralateral tectum; and at more lateral sites there tends to be more input from ipsilateral tectum. There is bilateral input to the visual lamina from tectum, tegmentum, posterior nucleus of the thalamus, posterior tuberculum, and ventromedial thalamic nucleus. There is ipsilateral input to the visual lamina from torus semicircularis, pretectum, nucleus of Bellonci, and ventrolateral thalamic nucleus. There is contralateral input to the visual lamina from basal optic complex. Collectively, these results show the presence of visual influences in regions of the medulla that likely represent an important step in sensorimotor transformation.

Chronic Social Defeat Stress Up-Regulates Spexin in the Brain of Nile Tilapia (Oreochromis niloticus)

Spexin (SPX), a neuropeptide evolutionarily conserved from fish to mammals, is widely distributed in the brain and peripheral tissues and associated with various physiological functions. Recently SPX has been suggested to be involved in neurological mechanism of stress. The current study investigates the involvement of SPX in chronic social defeat stress, using male teleost, the Nile tilapia (Oreochromis niloticus) as an animal model due to its distinct social hierarchy of dominant and subordinate relationship. The tilapia genome has SPX1a and SPX1b but has no SPX2. In the Nile tilapia, we localized SPX1a and SPX1b in the brain using in-situ hybridization. Next, using qPCR we examined gene expression of SPX1a and SPX1b in chronically stress (socially defeated) fish. SPX1a expressing cells were localized in the semicircular torus of the midbrain region and SPX1b expressing cells in the telencephalon. Chronically stress fish showed elevated plasma cortisol levels; with an upregulation of SPX1a and SPX1b gene expression in the brain compared to non-stress (control) fish. Since social defeat is a source of stress, the upregulated SPX mRNA levels during social defeat suggests SPX as a potentially inhibitory neuropeptide capable of causing detrimental changes in behaviour and physiology.

Tyrosine hydroxylase-immunoreactive neurons in the brain of tadpole of the narrow mouthed frog Microhyla ornata

Catecholamines serve as a neuromodulators of many behavioral and endocrine responses in different vertebrates including amphibians. However, the neuroanatomical studies on catecholamines, especially in the tadpole brain are limited. In this study, we report the distribution of catecholaminergic neurons in different areas of the brain in the tadpole of Microhyla ornata at metamorphic climax stage. Application of antisera against tyrosine hydroxylase (TH) revealed the presence of catecholaminergic cells and fibres in the olfactory bulb, the telencephalon, the diencephalon, the mesencephalon, the spinal cord and the pituitary gland. Whereas densest aggregations of TH-immunoreactive (TH-ir) fibres were noticed in the nucleus accumbens and the amygdala pars medialis regions of the telencephalon, highest population of TH-ir cells with dorsolaterally and rostrocaudally oriented fibres was observed in the preoptic area. Larger and distinct TH-ir cell bodies along with few dorsolaterally oriented TH-ir fibres were scattered throughout the suprachiasmatic nucleus. While moderate to intensely stained clusters of TH-ir cells were observed in dorsal and ventral hypothalamic regions, conspicuous TH-ir cells and fibres were seen in the pars distalis of the pituitary gland. In the nucleus tuberculi posterioris, numerous moderate sized TH-ir cells were found along the margin of the third ventricle and the fibres from this region were oriented dorsolaterally towards the torus semicircularis and tectal regions, whereas well organized largest TH-ir cells and fibres were seen in the tegmentum. In the spinal cord, medium sized TH-ir cells along with numerous laterally running fibres were encountered. Overall, widespread distribution of the TH-ir cells and fibres in the brain and the pituitary gland of the tadpole suggest diverse roles for the catecholamines in regulation of locomotion, olfaction, skin pigmentation and endocrine responses during final stages of metamorphosis in M. ornata.

MEMRI for visualizing brain activity after auditory stimulation in frogs.

Anuran amphibians are common model organisms in bioacoustics and neurobiology. To date, however, most available methods for studying auditory processing in frogs are highly invasive and thus do not allow for longitudinal study designs, nor do they provide a global view of the brain, which substantially limits the questions that can be addressed. The goal of this study was to identify areas in the frog brain that are responsible for auditory processing using in vivo manganese-enhanced MRI (MEMRI). We were interested in determining if the neural processing of socially relevant acoustic stimuli (e.g., species-specific calls) engages a specific pattern of brain activation that differs from patterns elicited by less- or nonrelevant acoustic signals. We thus designed an experiment, in which we presented three different types of acoustic stimuli (species-specific calls, band-limited noise, or silence) to fully awake northern leopard frogs (Rana pipiens) and then conducted MEMRI T1-weighted imaging to investigate differences in signal intensity due to manganese uptake as an indication of brain activity across all three conditions. We found the greatest change in signal intensity within the torus semicircularis (the principal central auditory region), the habenula, and the paraphysis of frogs that had been exposed to conspecific calls compared with noise or silence conditions. Stimulation with noise did not result in the same activation patterns, indicating that signals with contrasting social relevance are differentially processed in these areas of the amphibian brain. MEMRI provides a powerful approach to studying brain activity with high spatial resolution in frogs. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Central connections of the trigeminal motor command system in the weakly electric Elephantnose fish (Gnathonemus petersii).

The highly mobile chin appendage of Gnathonemus petersii, the Schnauzenorgan, is used to actively probe the environment and is known to be a fovea of the electrosensory system. It receives an important innervation from both the trigeminal sensory and motor systems. However, little is known about the premotor control pathways that coordinate the movements of the Schnauzenorgan, or about central pathways originating from the trigeminal motor nucleus. The present study focuses on the central connections of the trigeminal motor system to elucidate premotor centers controlling Schnauzenorgan movements, with particular interest in the possible connections between the electrosensory and trigeminal systems. Neurotracer injections into the trigeminal motor nucleus revealed bilateral, reciprocal connections between the two trigeminal motor nuclei and between the trigeminal sensory and motor nuclei by bilateral labeling of cells and terminals. Prominent afferent input to the trigeminal motor nucleus originates from the nucleus lateralis valvulae, the nucleus dorsalis mesencephali, the cerebellar corpus C1, the reticular formation, and the Raphe nuclei. Retrogradely labeled cells were also observed in the central pretectal nucleus, the dorsal anterior pretectal nucleus, the tectum, the ventroposterior nucleus of the torus semicircularis, the gustatory sensory and motor nuclei, and in the hypothalamus. Labeled terminals, but not cell bodies, were observed in the nucleus lateralis valvulae and the reticular formation. No direct connections were found between the electrosensory system and the V motor nucleus but the central connections identified would provide several multisynaptic pathways linking these two systems, including possible efference copy and corollary discharge mechanisms.

The Fractal Features of the Background Activity of Neurons in the Auditory Center of the Frog Midbrain

Abstract—Statistics were obtained for background firing activity recorded from several neurons of the auditory center of the midbrain (semicircular torus) in the common frog. The methods were those used to analyze the temporal flow of fractal point processes. Most neurons showed a power-law increase in the variance-to-mean ratio (Fano factor) of pulse number with the time window increasing from hundreds of milliseconds to several tens of seconds, although cells whose background firing activity was close to random were also observed. A method based on the registration of local features (the Allan factor as a function of the same time window) confirmed the fractal properties for many cells. A power-law growth of the Allan factor usually began at large time windows. The third method of fractal analysis (the Hurst index) also argued in favor of the randomness and fractality of background firing activity. The relationships with other statistics of random point processes were analyzed for the fractal characteristics. A comparison with data obtained for neurons of the medulla oblongata indicated that the variability of firing activity slight increases along the auditory pathway.

Pattern of nitrergic cells and fibers organization in the central nervous system of the Australian lungfish, Neoceratodus forsteri (Sarcopterygii: Dipnoi).

The Australian lungfish Neoceratodus forsteri is the only extant species of the order Ceratodontiformes, which retained most of the primitive features of ancient lobe finned-fishes. Lungfishes are the closest living relatives of land vertebrates and their study is important for deducing the neural traits that were conserved, modified, or lost with the transition from fishes to land vertebrates. We have investigated the nitrergic system with neural nitric oxide synthase (NOS) immunohistochemistry and NADPH-diaphorase (NADPH-d) histochemistry, which yielded almost identical results except for the primary olfactory projections and the terminal and preoptic nerve fibers labeled only for NADPH-d. Combined immunohistochemistry was used for simultaneous detection of NOS with catecholaminergic, cholinergic, and serotonergic structures, aiming to establish accurately the localization of the nitrergic elements and to assess possible interactions between these neurotransmitter systems. The results demonstrated abundant nitrergic cells in the basal ganglia, amygdaloid complex, preoptic area, basal hypothalamus, mesencephalic tectum and tegmentum, laterodorsal tegmental nucleus, reticular formation, spinal cord, and retina. In addition, low numbers of nitrergic cells were observed in the olfactory bulb, all pallial divisions, lateral septum, suprachiasmatic nucleus, prethalamic and thalamic areas, posterior tubercle, pretectum, torus semicircularis, cerebellar nucleus, interpeduncular nucleus, the medial octavolateral nucleus, nucleus of the solitary tract, and the dorsal column nucleus. Colocalization of NOS and tyrosine hydroxylase was observed in numerous cells of the ventral tegmental area/substantia nigra complex. Comparison with other vertebrates, using a neuromeric analysis, reveals that the nitrergic system of Neoceratodus shares many neuroanatomical features with tetrapods and particularly with amphibians.

Melatonin receptor expression in vocal, auditory, and neuroendocrine centers of a highly vocal fish, the plainfin midshipman (Porichthys notatus).

Melatonin plays a central role in entraining activity to the day-night cycle in vertebrates. Here, we investigate neuroanatomical substrates of melatonin-dependent vocal-acoustic behavior in the nocturnal and highly vocal teleost fish, the plainfin midshipman (Porichthys notatus). Using in situ hybridization (ISH) and quantitative real-time PCR (qPCR), we assess the mRNA distribution and transcript abundance of melatonin receptor subtype 1B (mel1b), shown to be important for vocalization in midshipman fish and songbirds. ISH shows robust mel1b expression in major nodes of the central vocal and auditory networks in the subpallium, preoptic area (POA), anterior hypothalamus, dorsal thalamus, posterior tuberculum, midbrain torus semicircularis and periaqueductal gray, and hindbrain. Mel1b label is also abundant in secondary targets of the olfactory, visual, and lateral line systems, as well as telencephalic regions that have been compared to the amygdala, extended amygdala, striatum, septum, and hippocampus of tetrapods. Q-PCR corroborates mel1b abundance throughout the brain and shows significant increases in the morning compared with nighttime in tissue samples inclusive of the telencephalon and POA, but remains stable in other brain regions. Plasma melatonin levels show expected increase at night. Our findings support the hypothesis that melatonin's stimulatory effects on vocal-acoustic mechanisms in midshipman is mediated, in part, by melatonin binding in vocal, auditory, and neuroendocrine centers. Together with robust mel1b expression in multiple telencephalic nuclei and sensory systems, the results further indicate an expression pattern comparable to that in birds and mammals that is indicative of melatonin's broad involvement in the modulation of physiology and behavior.

Brain anatomy and Histology in Teleosts

he anatomical and histological observations of the brain in fishes differ among species, but they resemble each others in the number of brain compartments, and it is necessary to characterize well the anatomical and histological observations in the brain of each particular kind of fishes for doing further subjects in the brain. Five brain divisions usually observed which are from cranial to caudal; telencephalon or forebrain (contain 2 olfactory lobes and cerebrum), diencephalon (contain epithalamus, thalamus and hypothalamus), mesencephalon or midbrain (contain 2 optic lobes which are connected internally with torus longitudinalis and medially with the torus semi-circularis, and optic tegmentum), metencephalon or hindbrain (cerebellum) and myelencephalon or brain stem (medulla oblongata). The ventricular organization composed of the olfactory, lateral, the third, the tectal and the fourth ventricles. The adult neurogenesis process is usually observed in the fish brain, unlike mammal’s brain. The adult neurogenesis usually detected in cerebellum, optic lobe, and telencephalon.

The Mormyrid Optic Tectum Is a Topographic Interface for Active Electrolocation and Visual Sensing.

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

Statistical Characteristics of the Spike Activity of Neurons in the Midbrain Auditory Center in Frogs on Exposure to Tones Modulated by Low-Frequency Noise

The time parameters of the spike activity of single neurons in the brainstem part of the auditory system carry basic information on the properties of perceived signals. To characterize these parameters, the activity of single neurons in the midbrain center (semicircular torus) of the common frog (Rana temporaria) was studied on exposure to prolonged tones amplitude-modulated with repeating bursts of low-frequency noise. Time-shuffled autocorrelation functions were calculated, these corresponding to the sum of the correlation functions between responses to all bursts presented with the exception of responses evoked by one of the bursts. This approach excludes the effects of the refractory properties of the neuron being studied, allowing the responses of a single neuron to be used to evaluate the nature of the spike activity in the local population of cells with presumptively identical properties but which are statistically independent of each other. Significant variation in the statistical characteristics of output spike activity in different cells in the semicircular toroid was found, such that signals could be characterized in terms of different time properties. Neurons with highly specific reactions to the time characteristics of the envelope were observed.

Brain Activation Patterns in Response to Conspecific and Heterospecific Social Acoustic Signals in Female Plainfin Midshipman Fish, Porichthys notatus

While the peripheral auditory system of fish has been well studied, less is known about how the fish’s brain and central auditory system process complex social acoustic signals. The plainfin midshipman fish, Porichthys notatus, has become a good species for investigating the neural basis of acoustic communication because the production and reception of acoustic signals is paramount for this species’ reproductive success. Nesting males produce long-duration advertisement calls that females detect and localize among the noise in the intertidal zone to successfully find mates and spawn. How female midshipman are able to discriminate male advertisement calls from environmental noise and other acoustic stimuli is unknown. Using the immediate early gene product cFos as a marker for neural activity, we quantified neural activation of the ascending auditory pathway in female midshipman exposed to conspecific advertisement calls, heterospecific white seabass calls, or ambient environment noise. We hypothesized that auditory hindbrain nuclei would be activated by general acoustic stimuli (ambient noise and other biotic acoustic stimuli) whereas auditory neurons in the midbrain and forebrain would be selectively activated by conspecific advertisement calls. We show that neural activation in two regions of the auditory hindbrain, i.e., the rostral intermediate division of the descending octaval nucleus and the ventral division of the secondary octaval nucleus, did not differ via cFos immunoreactive (cFos-ir) activity when exposed to different acoustic stimuli. In contrast, female midshipman exposed to conspecific advertisement calls showed greater cFos-ir in the nucleus centralis of the midbrain torus semicircularis compared to fish exposed only to ambient noise. No difference in cFos-ir was observed in the torus semicircularis of animals exposed to conspecific versus heterospecific calls. However, cFos-ir was greater in two forebrain structures that receive auditory input, i.e., the central posterior nucleus of the thalamus and the anterior tuberal hypothalamus, when exposed to conspecific calls versus either ambient noise or heterospecific calls. Our results suggest that higher-order neurons in the female midshipman midbrain torus semicircularis, thalamic central posterior nucleus, and hypothalamic anterior tuberal nucleus may be necessary for the discrimination of complex social acoustic signals. Furthermore, neurons in the central posterior and anterior tuberal nuclei are differentially activated by exposure to conspecific versus other acoustic stimuli.

Distribution of NADPH-diaphorase reactivity in the central nervous system of the common toad (Bufo bufo)

We examined the distribution of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-reactive elements in the central nervous system (CNS) of the common toad, Bufo bufo. The investigation involved adult male and female toads collected during the breeding season. Labeled neurons of different morphological appearances (weakly or darkly stained, unipolar, bipolar, and multipolar) and fibers were observed across all subdivisions of the amphibian brain. Overall, a similar distribution of NADPH-d-labeled neurons was observed in the brain of male and female toads. In the secondary prosencephalon NADPH-d-labeled neurons were observed in the olfactory bulbs, pallial regions, nucleus accumbens, diagonal band of Broca, septum, striatum, amygdala, suprachiasmatic and magnocellular preoptic nuclei, dorsal and ventral hypothalamus. In the diencephalon, NADPH-d-positive neurons were seen in the anterior thalamic nuclei, ventromedial and ventrolateral nuclei, central and lateral thalamic nuclei, posterior tubercle, posterodorsal division of the lateral thalamic nucleus, and in the pretectal and pretoral gray. In the mesencephalon, heavily stained neurons were present in the anterodorsal and anteroventral tegmental nuclei, magnocellular, principal and laminar nuclei of the torus semicircularis, and nucleus profundus mesencephali. In the isthmus, stained cells were observed medially and ventrally in the posterodorsal and posteroventral tegmental nuclei. In the rhombencephalon, numerous NADPH-d-stained neurons were distributed in the cerebellar nucleus, sensory and descending trigeminal nuclei, motor nuclei of the glossopharyngeal and vagus nerves, the nucleus of the solitary tract, nuclei of the hypoglossal and octaval nerves, dorsal column nucleus, central gray region, and in reticular formation. However, the complete absence of NADPH-d-stained neurons in the cerebellar cortex was an unusual feature observed in this study. The widespread distribution of NADPH-d staining in diverse cell types, belonging to a variety of neuronal systems suggests a widespread role for NADPH-d in modulating diverse functions, including sensory coding in the amphibian nervous system.

Immunohistochemical Localization of DARPP-32 in the Brain of Two Lungfishes: Further Assessment of Its Relationship with the Dopaminergic System

The distribution of DARPP-32 (a phosphoprotein related to the dopamine D₁ receptor) has been widely used as a means to clarify the brain regions with dopaminoceptive cells, primarily in representative species of tetrapods. The relationship between dopaminergic and dopaminoceptive elements is frequently analyzed using the catecholamine marker tyrosine hydroxylase (TH). In the present study, by means of combined immunohistochemistry, we have analyzed these relationships in lungfishes, the only group of sarcopterygian fishes represented by 6 extant species that are the phylogenetically closest living relatives of tetrapods. We used the Australian lungfish Neoceratodus forsteri and the African lungfish Protopterus dolloi. The DARPP-32 antibody yields a distinct and consistent pattern of neuronal staining in brain areas that, in general, coincide with areas that are densely innervated by TH-immunoreactive fibers. The striatum, thalamus, optic tectum, and torus semicircularis contain intensely DARPP-32-immunoreactive cell bodies and fibers. Cells are also located in the olfactory bulbs, amygdaloid complex, lateral septum, pallidum, preoptic area, suprachiasmatic nucleus, tuberal hypothalamic region, rostral rhombencephalic reticular formation, superior raphe nucleus, octavolateral area, solitary tract nucleus, and spinal cord. Remarkably, DARPP-32-immunoreactive fibers originating in the striatum reach the region of the dopaminergic cells in the mesencephalic tegmentum and represent a well-established striatonigral pathway in lungfishes. Double immunolabeling reveals that DARPP-32 is present in neurons that most likely receive TH input, but it is absent from the catecholaminergic neurons themselves, with the only exception of a few cells in the suprachiasmatic nucleus of Neoceratodus and the solitary tract nucleus of Protopterus. In addition, some species differences exist in the localization of DARPP-32 cells in the pallium, lateral amygdala, thalamus, prethalamus, and octavolateral area. In general, the present study demonstrates that the distribution pattern of DARPP-32, and its relationship with TH, is largely comparable to those reported for tetrapods, highlighting a shared situation among all sarcopterygians.

Evaluation of Postspike Changes in Neuronal Excitability by Comparing Ordinary and Shuffled Autocorrelation Functions

Responses of single neurons to tonal signals amplitude-modulated by repeating segments of lowfrequency noise were studied in the dorsal (cochlear) medullary nucleus and midbrain auditory center (torus semicircularis) of the grass frog Rana temporaria. An autocorrelation function of the response to a total presentation and a shuffled autocorrelation function were derived. The latter was obtained by correlating the impulse response to each segment of the modulated signal with responses to all other segments with the exception of the initial one. After the necessary normalization, the function differed from the initial autocorrelation only in lacking postspike changes in excitability. A delay dependence of the ratio of the two functions directly demonstrated the time course of the postspike change in excitability of the studied cell. The majority of second-order neurons, which are in the dorsal nucleus of the medulla oblongata, were characterized only by brief intervals of absolute and relative refractoriness. However, cells with excitability that was markedly facilitated immediately after the refractory period were observed even in this nucleus. Neurons with a complex pattern of postspike changes in excitability were detected in the torus semicircularis. In these cells, a comparatively long postspike decrease in excitability was usually interrupted by intervals in which the neuron sensitivity was significantly higher than normal. The results demonstrate that spike generation has a marked effect on subsequent activity in brainstem auditory units. The effects may play an important role in the formation of the temporal pattern of neuronal responses to auditory signals.

Cell Proliferation, Migration, and Neurogenesis in the Adult Brain of the Pulse Type Weakly Electric Fish, Gymnotus omarorum.

Adult neurogenesis, an essential mechanism of brain plasticity, enables brain development along postnatal life, constant addition of new neurons, neuronal turnover, and/or regeneration. It is amply distributed but negatively modulated during development and along evolution. Widespread cell proliferation, high neurogenic, and regenerative capacities are considered characteristics of teleost brains during adulthood. These anamniotes are promising models to depict factors that modulate cell proliferation, migration, and neurogenesis, and might be intervened to promote brain plasticity in mammals. Nevertheless, the migration path of derived cells to their final destination was not studied in various teleosts, including most weakly electric fish. In this group adult brain morphology is attributed to sensory specialization, involving the concerted evolution of peripheral electroreceptors and electric organs, encompassed by the evolution of neural networks involved in electrosensory information processing. In wave type gymnotids adult brain morphology is proposed to result from lifelong region specific cell proliferation and neurogenesis. Consistently, pulse type weakly electric gymnotids and mormyrids show widespread distribution of proliferation zones that persists in adulthood, but their neurogenic potential is still unknown. Here we studied the migration process and differentiation of newborn cells into the neuronal phenotype in the pulse type gymnotid Gymnotus omarorum. Pulse labeling of S-phase cells with 5-Chloro-2′-deoxyuridine thymidine followed by 1 to 180 day survivals evidenced long distance migration of newborn cells from the rostralmost telencephalic ventricle to the olfactory bulb, and between layers of all cerebellar divisions. Shorter migration appeared in the tectum opticum and torus semicircularis. In many brain regions, derived cells expressed early neuronal markers doublecortin (chase: 1–30 days) and HuC/HuD (chase: 7–180 days). Some newborn cells expressed the mature neuronal marker tyrosine hydroxylase in the subpallium (chase: 90 days) and olfactory bulb (chase: 180 days), indicating the acquisition of a mature neuronal phenotype. Long term CldU labeled newborn cells of the granular layer of the corpus cerebelli were also retrogradely labeled “in vivo,” suggesting their insertion into the neural networks. These findings evidence the neurogenic capacity of telencephalic, mesencephalic, and rhombencephalic brain proliferation zones in G. omarorum, supporting the phylogenetic conserved feature of adult neurogenesis and its functional significance.