Development of brainstem-evoked responses in congenital auditory deprivation.

To compare the development of the auditory system in hearing and completely acoustically deprived animals, naive congenitally deaf white cats (CDCs) and hearing controls (HCs) were investigated at different developmental stages from birth till adulthood. The CDCs had no hearing experience before the acute experiment. In both groups of animals, responses to cochlear implant stimulation were acutely assessed. Electrically evoked auditory brainstem responses (E-ABRs) were recorded with monopolar stimulation at different current levels. CDCs demonstrated extensive development of E-ABRs, from first signs of responses at postnatal (p.n.) day 3 through appearance of all waves of brainstem response at day 8 p.n. to mature responses around day 90 p.n.. Wave I of E-ABRs could not be distinguished from the artifact in majority of CDCs, whereas in HCs, it was clearly separated from the stimulus artifact. Waves II, III, and IV demonstrated higher thresholds in CDCs, whereas this difference was not found for wave V. Amplitudes of wave III were significantly higher in HCs, whereas wave V amplitudes were significantly higher in CDCs. No differences in latencies were observed between the animal groups. These data demonstrate significant postnatal subcortical development in absence of hearing, and also divergent effects of deafness on early waves II-IV and wave V of the E-ABR.

Recruitment of the auditory cortex in congenitally deaf cats by long-term cochlear electrostimulation.

In congenitally deaf cats, the central auditory system is deprived of acoustic input because of degeneration of the organ of Corti before the onset of hearing. Primary auditory afferents survive and can be stimulated electrically. By means of an intracochlear implant and an accompanying sound processor, congenitally deaf kittens were exposed to sounds and conditioned to respond to tones. After months of exposure to meaningful stimuli, the cortical activity in chronically implanted cats produced field potentials of higher amplitudes, expanded in area, developed long latency responses indicative of intracortical information processing, and showed more synaptic efficacy than in naïve, unstimulated deaf cats. The activity established by auditory experience resembles activity in hearing animals.

Correlation of cardiotoxicity mediated by halogenated aromatic hydrocarbons to aryl hydrocarbon receptor activation.

In mammals, the toxicity of halogenated aromatic hydrocarbons (HAH) correlates with their ability to activate the aryl hydrocarbon receptor (AHR). To test this correlation in an avian model, we selected six HAHs based on their affinity for the mammalian AHR, including: 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD); 2,3,7,8-tetrachlorodibenzofuran (TCDF); 2,3,4,7,8-pentachlorodibenzofuran (PCDF); 3,3',4,4'-tetrachlorobiphenyl (PCB 77); and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153). We determined the ability of these compounds to induce cardiotoxicity, as measured by an increase in heart wet weight on incubation day 10 in the chick embryo (Gallus gallus) and formation of the avian AHR/ARNT/DNA binding complex in chicken hepatoma cells. Relative potency values (RPs) were calculated by dividing the TCDD EC(50) (AHR/ARNT/DNA binding) or ED(50) (15% increase in day-10 heart wet weight) by the HAH congeners EC(50) or ED(50), respectively. The rank order of potencies for inducing cardiotoxicity were TCDD > PCDD = PCDF = TCDF > PCDF > PCB77, PCB 153, no effect. The RP values for inducing AHR/ARNT DNA binding were then correlated with those for inducing cardiotoxicity (the RP values of PCDD were determined to be statistical outliers). This correlation was found to be highly significant (r = 0.94, p = 0.017). The ability of PCDD to act as an AHR agonist was verified using luciferase reporter assays and analysis of cytochrome P4501A1 protein levels. These results indicate that the ability of HAHs to activate the avian AHR signaling pathway, in general, correlates with their ability to mediate cardiotoxicity in the chick embryo.

A model for prelingual deafness, the congenitally deaf white cat--population statistics and degenerative changes.

Cochlear implantation in congenitally deaf children leads to electrical stimulation of an entirely naive central auditory system. In this case, processes of central auditory maturation are induced by the electric stimuli. For the study of these processes the deaf white cat (DWC) appears to be an appropriate model. However, a knowledge of the basic data of these animals is necessary before such a model may be used. This paper presents these data and is one of a series of publications concerning congenital deafness in children and cochlear implantation. In our strain 72% of the animals are totally deaf as judged by the absence of any brain stem evoked potentials at click intensities up to 120 dB SPL peak equivalent. Primarily, there is a degeneration of the entire organ of Corti during the first postnatal weeks. An absence of acoustically evoked brain stem responses in the early postnatal weeks shows that DWCs probably never have any hearing experience. Months after the degeneration of the organ of Corti, the spiral ganglion starts to degenerate from the midportion of the cochlea. However, even in adult cats (2 years), a sufficient number of functionally intact auditory afferents remain, which are suitable for electrical cochlear stimulation.

Response of the primary auditory cortex to electrical stimulation of the auditory nerve in the congenitally deaf white cat.

Neural activity plays an important role in the development and maintenance of sensory pathways. However, while there is considerable experience using cochlear implants in both congenitally deaf adults and children, little is known of the effects of a hearing loss on the development of the auditory cortex. In the present study, cortical evoked potentials, field potentials, and multi- and single-unit activity evoked by electrical stimulation of the auditory nerve were used to study the functional organisation of the auditory cortex in the adult congenitally deaf white cat. The absence of click-evoked auditory brainstem responses during the first weeks of life demonstrated that these animals had no auditory experience. Under barbiturate anaesthesia, cortical potentials could be recorded from the contralateral auditory cortex in response to bipolar electrical stimulation of the cochlea in spite of total auditory deprivation. Threshold, morphology and latency of the evoked potentials varied with the location of the recording electrode, with response latency varying from 10 to 20 ms. There was evidence of threshold shifts with site of the cochlear stimulation in accordance with the known cochleotopic organisation of AI. Thresholds also varied with the configuration of the stimulating electrodes in accordance with changes previously observed in normal hearing animals. Single-unit recordings exhibited properties similar to the evoked potentials. Increasing stimulus intensity resulted in an increase in spike rate and a decrease in latency to a minimum of approximately 8 ms, consistent with latencies recorded in AI of previously normal animals (Raggio and Schreiner, 1994). Single-unit thresholds also varied with the configuration of the stimulating electrodes. Strongly driven responses were followed by a suppression of spontaneous activity. Even at saturation intensities the degree of synchronisation was less than observed when recording from auditory brainstem nuclei. Taken together, in these auditory deprived animals basic response properties of the auditory cortex of the congenitally deaf white cat appear similar to those reported in normal hearing animals in response to electrical stimulation of the auditory nerve. In addition, it seems that the auditory cortex retains at least some rudimentary level of cochleotopic organisation.

Afferent projection patterns in the auditory brainstem in normal and congenitally deaf white cats.

Cochlear implantation in congenitally deaf children is developing to a successful medical tool. Little is known, however, on morphology and pathophysiology of the central auditory system in these auditory deprived children. One form of congenital hearing loss, that seen in the deaf white cat, was investigated to see if there are differences in the afferent pathways from the cochlear nuclei to the inferior colliculus. The retrogradely transported fluorescent tracer diamidino yellow (DY) was injected into different parts of the central nucleus of the inferior colliculus (ICC) of normal cats and deaf white cats. It was found that the main afferent projection patterns in deaf white cats were unchanged in spite of congenital auditory deprivation; minor differences were seen.

Postnatal cortical development in congenital auditory deprivation.

The study investigates early postnatal development of local field potentials (LFPs) in the primary auditory cortex of hearing and congenitally deaf cats. In hearing cats, LFPs elicited by electrical intracochlear stimulation demonstrated developmental changes in mid-latency range, including reductions in peak and onset latencies of individual waves and a maturation of their shape and latencies during the first 2 months of life. In long latency range (>80 ms), the P(1)/N(1) response appeared after the fourth week of life and further increased in amplitude and decreased in latency, reaching mature shapes between the fourth and sixth months after birth (p.n.). Cortical activated areas became increasingly smaller during the first 3 months of life, reaching mature values at the fourth month p.n. The layer-specific pattern of synaptic activity matured 4 months p.n. In congenitally deaf cats, the developmental pattern was different. The lowest cortical LFP thresholds were significantly smaller than in hearing controls, demonstrating a "hypersensitivity" to sensory inputs. The development of N(b) waves was delayed and altered and the long latency responses became smaller than in controls at the second and third months. The activated areas remained smaller than in controls until the third month, then they increased rapidly and exceeded the activated areas of age-matched controls. From the fourth month on, the activated areas decreased again and smaller synaptic currents were found in deaf cats than in controls. The presented data demonstrate that functional development of the auditory cortex critically depends on auditory experience.

Role of heat shock protein 90 dissociation in mediating agonist-induced activation of the aryl hydrocarbon receptor.

The aryl hydrocarbon receptor (AhR) is a cytosolic basic helix-loop-helix protein that associates with a chaperone complex that includes two molecules of heat shock protein 90 (HSP90). It has been hypothesized that after ligand binding, the AhR dissociates from its chaperone complex and translocates into the nucleus, where it heterodimerizes with its DNA binding partner, the AhR nuclear translocator (ARNT), and activates specific genes. However, it remains unclear whether nuclear translocation of the AhR occurs before or after dissociation of the HSP90/chaperone complex. Because sodium molybdate stabilizes the AhR-HSP90 interaction and inhibits the gene activation of a number of steroid receptors, we reasoned that molybdate would be a useful tool in delineating the role of HSP90 dissociation in AhR nuclear translocation. In this study, we demonstrate that molybdate inhibits AhR gene activation in both HepG2 and Hepa-1 cells in a concentration-dependent manner and protects the AhR against agonist-induced proteolysis. In addition, we demonstrate that AhR/ARNT dimerization, but not nuclear translocation of the AhR, is inhibited by molybdate. This indicates that 1) HSP90 dissociation is not required for nuclear translocation of the AhR, 2) HSP90 dissociation is essential for formation of the AhR/ARNT heterodimer, and 3) an additional undefined regulatory step is required for AhR/ARNT dimerization in the nucleus.

Delayed maturation and sensitive periods in the auditory cortex.

Behavioral data indicate the existence of sensitive periods in the development of audition and language. Neurophysiological data demonstrate deficits in the cerebral cortex of auditory-deprived animals, mainly in reduced cochleotopy and deficits in corticocortical and corticothalamic loops. In addition to current spread in the cochlea, reduced cochleotopy leads to channel interactions after cochlear implantation. Deficits in corticocortical and corticothalamic loops interfere with normal processing of auditory activity in cortical areas. Thus, the deprived auditory cortex cannot mature normally in congenital deafness. This maturation can be achieved using auditory experience through cochlear implants. However, implantation is necessary within the sensitive period of the auditory system. The functional role of long-term potentiation and long-term depression, inhibition, cholinergic modulation and neurotrophins in auditory development and sensitive periods are discussed.

Hearing after congenital deafness: central auditory plasticity and sensory deprivation.

The congenitally deaf cat suffers from a degeneration of the inner ear. The organ of Corti bears no hair cells, yet the auditory afferents are preserved. Since these animals have no auditory experience, they were used as a model for congenital deafness. Kittens were equipped with a cochlear implant at different ages and electro-stimulated over a period of 2.0-5.5 months using a monopolar single-channel compressed analogue stimulation strategy (VIENNA-type signal processor). Following a period of auditory experience, we investigated cortical field potentials in response to electrical biphasic pulses applied by means of the cochlear implant. In comparison to naive unstimulated deaf cats and normal hearing cats, the chronically stimulated animals showed larger cortical regions producing middle-latency responses at or above 300 microV amplitude at the contralateral as well as the ipsilateral auditory cortex. The cortex ipsilateral to the chronically stimulated ear did not show any signs of reduced responsiveness when stimulating the 'untrained' ear through a second cochlear implant inserted in the final experiment. With comparable duration of auditory training, the activated cortical area was substantially smaller if implantation had been performed at an older age of 5-6 months. The data emphasize that young sensory systems in cats have a higher capacity for plasticity than older ones and that there is a sensitive period for the cat's auditory system.

Congenital auditory deprivation reduces synaptic activity within the auditory cortex in a layer-specific manner.

The present study investigates the functional deficits of naive auditory cortices in adult congenitally deaf cats. For this purpose, their auditory system was stimulated electrically using cochlear implants. Synaptic currents in cortical layers were revealed using current source density analyses. They were compared with synaptic currents found in electrically stimulated hearing cats. The naive auditory cortex showed significant deficits in synaptic activity in infragranular cortical layers. Furthermore, there was also a deficit of synaptic activities at longer latencies (>30 ms). The 'cortical column' was not activated in the well-defined sequence found in normal hearing cats. These results demonstrate functional deficits as a consequence of congenital auditory deprivation. Similar deficits are likely in congenitally deaf children.

Molecular characterization and developmental expression of the aryl hydrocarbon receptor from the chick embryo.

The aryl hydrocarbon receptor (AhR) was cloned from the chick embryo and its function and developmental expression characterized. Chicken AhR cDNA coded for 858 amino acid protein and 396 bp of 3' UTR. The basic helix loop helix domain exhibited 87-100% amino acid identity to avian, mammalian, and amphibian AhR, and 69-74% to piscine AhR. The PAS (Per-ARNT-Sim) region was slightly less well conserved with (a) 97% identity to other avian sequences, (b) 81-86% to amphibian and mammalian AhR, and (c) 64-69% with piscine AhR. The carboxy terminus diverged the most among species with less than 53% amino acid identity between chicken and any available mammalian and piscine AhR sequences. The chicken AhR RNA and protein were 6.1 kb and 103 kDa, respectively. Chicken AhR dimerized with human AhR nuclear translocator and bound the mammalian dioxin-response element in a ligand-dependent manner. AhR protein was detected in neural ganglia; smooth, cardiac, and skeletal muscle; and epithelium involved in epithelial-to-mesenchymal transformations, such as pituitary, gastrointestinal tract, limb apical-ectodermal ridge, and kidney collecting ducts. AhR mRNA was detected in all tissues expressing protein, except myocardium. Cytochrome P4501A4 mRNA was highly induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a subset of tissues expressing AhR, including small intestine, liver, kidney, blood vessels, and outflow tract myocardium. In conclusion, the AhR sequence and function is highly conserved between birds and mammals, and although many tissues express AhR during chick embryo development, only a subset are responsive to TCDD induction of CYP1A4.

Plastic changes in the auditory cortex of congenitally deaf cats following cochlear implantation.

Congenitally deaf cats were used as a model for human inborn deafness and auditory deprivation. The deaf cats were supplied with a cochlear implant, chronically exposed to an acoustic environment and conditioned to acoustic stimuli. In case of early implantation the cats learned to make use of the newly gained auditory channel behaviourally. Neurophysiological and fMRI data showed that the central auditory system was recruited, if implantation took place within a sensitive period of <6 months.

The central auditory system and auditory deprivation: experience with cochlear implants in the congenitally deaf.

In the present paper we briefly review the response of the central auditory system to auditory deprivation and describe recent experimental and clinical experience with cochlear implants. While the central auditory system undergoes marked changes in response to auditory deprivation, it would appear that at least a rudimentary cochleotopic organisation is maintained at the level of the brainstem and auditory cortex in animals deafened from birth. Moreover, recent studies have demonstrated the ability of the central auditory system to undergo functional reorganisation in response to changes in the pattern of afferent activity. Clinical experience has shown that deaf children with little or no prior auditory experience can obtain significant benefit from cochlear implants, provided the device is fitted at a young age. Furthermore, factors predicting successful clinical outcomes with these devices reflect the importance of auditory experience, either prior to an acquired loss or with the use of a cochlear implant. These findings suggest that functional reorganisation within the central auditory pathway can at least partially account for improvements in clinical performance over time.

On the reproduction of brazilian fishes: XIII. Scanning electron microscopic study of the rhythin of development in oocyte of the lambari (Astyanax bimaculatus) Reinhardt 1874 (Pisces, Characidae)

A oogênese do lambari Astyanax bimaculatus foi dividida em três grandes fases: 1) fase primária de crescimento - caracterizada pelo aumento mitótico do número de oogônias, sendo constituída pelas sub-fases cromatina-nucléolo, perinucleolar inicial e perinucleolar avançada; 2) fase vitelogênica - que envolve duas sub-fases: a de vesícula vitelínica contendo vesícula de glicoproteínas e a de vitelogênese contendo grânulos de glicoproteínas; 3) fase de maturaçäo - caracterizada pela formaçäo de oócitos maduros