Lifelong corticosterone level determines age-related decline in neurogenesis and memory.

Ageing is accompanied by an alteration of spatial memory, a decline in hippocampal neurogenesis and a dysregulation of the hypothalamic-pituitary axis (HPA) leading to elevated levels of circulating corticosterone. However, the role of the HPA axis in age-related decline in cognitive functions and in neurogenesis decline remains unclear. We found that suppression of glucocorticoids secretion from midlife to the rest of the animals' life increases neurogenesis in old animals and prevents the emergence of age-related memory disorders. Reciprocally, aged rats with a chronic upregulation of the HPA axis exhibit not only spatial memory impairments but also very low levels of hippocampal cell proliferation and survival. Altogether, these results indicate that the extent of lifetime exposure to glucocorticoids determines the extent of age-related decline in hippocampal neurogenesis and consequently age-related cognitive dysfunctions.

Behavioural recovery after unilateral lesion of the dopaminergic mesotelencephalic pathway: effect of repeated testing.

Functional recovery following a complete unilateral lesion of the nigrostriatal pathway in adult rats was studied. We examined the effect of training on the spontaneous or induced postural bias following the lesion. Two tasks measuring lateralization were used to assess the lesion-induced postural bias: spontaneous asymmetry was evaluated in the Y-maze, whereas induced body bias was measured by hanging the rat by its tail. Recovery was assessed at three different times following the lesion. The effects of lesion in adult rats in the short, medium and long term were evaluated and compared with the effects of dopaminergic transplants. In adult lesioned rats, destruction of dopaminergic innervation of the neostriatum induced initially an ipsilateral bias as measured in the "tail hang test" and the Y-maze. Recovery of function was observed in the tail hang test as ipsilateral bias declined on repeated testing. Apart from this effect, there was a post-lesion interval effect, since the postural bias disappeared more rapidly on repeated testing in the long-term lesioned rats. This spontaneous recovery was impaired by intrastriatal dopaminergic grafts. Furthermore, no spontaneous recovery was observed in the Y-maze test. These observations show that repeated testing can influence the long-term effects of damage to the nigrostriatal dopamine system.

Immunoreactivity for taurine in the cochlea: its abundance in supporting cells.

Taurine is the second most abundant free amino acid in the brain where its osmoregulatory function is well established. Taurine-deprived kittens show retinal pathology leading to blindness. In the inner ear, taurine has been reported to be the most abundant free amino acid although its role in inner ear function is not known. Immunohistochemistry was employed here to investigate the localisation of taurine in normal cochleae of the guinea pig compared with two different conditions: experimentally induced endolymphatic hydrops and after oral administration of glycerol. In normal cochleae, by light microscopy, taurine-like immunoreaction was never observed in the sensory outer hair cells and appeared absent from the inner hair cells. In contrast taurine-like immunolabeling was found to be present in all supporting tissue with the striking exception of the tectorial membrane and the outer pillar cell which had no or little taurine immunoreactivity respectively. In early experimental endolymphatic hydrops, the distribution of taurine-like immunoreactivity appeared similar to that observed for normal cochleae. In long-term hydrops, degenerated outer hair cells were replaced by the swelling of the phalangeal process of the Deiters' cells which became highly immunoreactive to taurine. After glycerol administration, the tectorial membrane became more tightly bound to the apical surface of the sensory hair cells and distinctly immunoreactive to taurine. The localisation of taurine in the organ of Corti shown here is consistent with taurine being involved in the maintenance of osmotic equilibrium in the normal and perhaps also in the restructuration of the pathological organ of Corti.

Eighth nerve auditory evoked responses recorded at the base of the vestibular nucleus in the guinea pig.

Anatomical and physiological studies of brainstem acoustic nuclei involving the classical ascending auditory pathway or the cerebellar and reticular pathways imply that all afferents from the cochlea terminate in the cochlear nucleus. In the experimental pathology of complete and selective destruction of the cochlea in the guinea pig acoustic responses still evoked at mid and high intensities, demonstrated to come from the saccule, show a pattern of far field evoked brainstem potentials quite different from that of normal animals. Intracranial electrophysiological investigations of the brainstem were undertaken in such pathological animals and in normal guinea pigs for comparison. In both cases acoustic responses were recorded at the base of the vestibular nucleus, showing a first peak corresponding to an eighth nerve projection and after a synaptic delay a second peak of local activation. In normal animals acoustic responses from the vestibular nucleus showing normal threshold and tuning curves may represent a direct projection from the cochlea.

Acute effects of noradrenalin related vasoactive agents on the ototoxicity of aspirin: an experimental study in the guinea pig.

Aspirin is known to be ototoxic when administered at high doses. Its mode of action is unknown but an alteration of the vascular function has been suspected. To further document this hypothesis, acute effects of some vasoactive agents on the ototoxicity of aspirin were tested in experiments on the guinea pig using sensori-neural electrophysiological responses and morphometry of the vessels of the stria and the spiral lamina. Electrophysiological measures showed no modification of sensory responses but neural responses revealed clear changes after administration of noradrenalin related agents, limited modifications after a drug acting partly as a serotonin antagonist, and no change after a dopaminergic agent. Morphometric studies showed no modification of the strial but some effect on the spiral vessels. The results are compatible with the hypothesis of a vascular involvement in the ototoxicity of aspirin and they point toward an interaction with the noradrenergic sympathetic cochlear system in the spiral lamina.

Uptake of amikacin by hair cells of the guinea pig cochlea and vestibule and ototoxicity: comparison with gentamicin.

The distribution of amikacin (AK), an exclusive cochleo-toxic aminoglycosidic antibiotic (AA), and of gentamicin (GM), which is both cochleo- and vestibulo-toxic, has been studied in cochlear and vestibular hair cells. Guinea pigs were treated during six days with one daily injection of AK (450 mg/kg/day) or GM (60 mg/kg/day). AAs were detected, using immunocytochemical technique with scanning laser confocal microscopy, in isolated cells from guinea pigs sacrificed from 2 to 30 days after the end of the treatments. Results demonstrate a rapid uptake (as soon as after 2-day treatment) of both AAs by cochlear and vestibular hair cells and a very slow clearance. Particularly GM and AK are detected in type I and type II hair cells of the utricles and cristae ampullaris. The presence of these two molecules with different toxic potentialities towards cochlear and vestibular hair cells indicates that the selective ototoxicity of aminoglycosides cannot be explained simply on the basis of particular uptake and accumulation in the different sensory hair cells.

Brainstem connections of the anterior and posterior parts of the saccule in the guinea pig.

Studies on specific brainstem connections of the saccule are scanty, present notable divergences, have never concerned the guinea pig, and have not considered separately the two branches of its innervation. In this study, the central connections of the entire saccule and of anterior or posterior parts of the epithelium were investigated in the guinea pig, using horseradish peroxidase (HRP). HRP was deposited on the saccular epithelium and associated with surgical section of the inferior or superior division of the VIII nerve in order to study the connections of specific parts of the saccule. The whole organ was found to project to the ipsilateral side, principally to the lateral part of the lateral vestibular nucleus (LVN) and to the gamma group. The posterior part of the saccule projects also to the gamma group and to a lesser extent to the LVN, the anterior part projects to the LVN but not to the gamma group. In all cases, labelled efferent cells were seen located bilaterally between the genu of the VII nerve and the medial vestibular nucleus (MVN).

Relationship between the nephrotoxicity and ototoxicity induced by gentamicin in the guinea pig.

Guinea pigs were treated with daily single subcutaneous injections of 60 mg gentamicin per kg for 3 weeks. Renal, cochlear and vestibular functions were monitored before, during and after treatment. The degree and onset of gentamicin oto- and nephrotoxocity differed during the treatment period. Alterations to the kidney functions were observed from the first week while the onset of ototoxicity occurred later, at the third and fourth week for the cochlear and vestibular functions respectively. Moreover, when treatment ended, renal function demonstrated signs of recovery, while auditory and vestibular function continued to worsen. Deficits in cochlear function and structural changes (missing outer hair cells) correlated with gentamicin serum concentrations, while vestibular alterations (loss in nystagmic reactions) did not. No distinct relationship could be established between auditory and vestibular loss and the renal parameters monitored. The results suggest that gentamicin-induced nephro- and oto-toxicity are dissociated phenomena and that cochleotoxicity was dependent on aminoglycoside serum level.

Long-term treatment with chlorthalidone reduces experimental hydrops but does not prevent the hearing loss.

Long-term treatment of guinea pigs with the diuretics chlorthalidone and acetazolamide, following the experimental obstruction of the endolymphatic duct, was assessed using chronically implanted round window cochlear electrodes. The diuretic chlorthalidone appeared to curb the progressive low-frequency sensitivity loss during the first 4 weeks following surgery, as compared with animals receiving the diuretic acetazolamide or no treatment. However, this apparent beneficial effect decreased after 4 weeks and was not apparent at 14 weeks post-induction. On the other hand, morphological control at the end of 14 weeks confirmed a marked reduction in hydrops in the chlorthalidone-treated animals. The data clearly demonstrate a dissociation between hydrops and the development of hearing loss and suggest that the augmenting endolymph volume is only one of several contributing factors to the deteriorating auditory function in experimental hydrops.

Kinetics of gentamicin in cochlear hair cells after chronic treatment.

Presence of gentamicin (GM) in cochlear hair cells was detected by immunohistochemistry in guinea pigs (GPs) cochlea 1, 9 and 41 days after a 6-day treatment with GM at 60 mg/kg/day (s.c.). The number of GPs in each group was respectively 7, 12 and 6. Twelve other non-treated GPs served as controls. Cochlear function was measured, just before sacrifice, by VIIIth nerve compound action potential (CAP) audiograms. Functional and immunohistological evaluations were performed by two independent naïve observers respectively. Functional changes were minimal: only one out of the 25 treated GPs, from the 41-day group, showed significant threshold elevations on high frequencies. Meanwhile GM labelling was observed in most outer hair cells (OHCs) from the three rows of all the treated GPs, with radial and longitudinal gradients, and found similar in the 3 groups. These results 1) confirm that GM is significantly present in OHCs before the development of ototoxicity and 2) indicate that GM accumulates and is maintained inside the OHCs for very long periods of time, i.e. that its clearance from the hair cells, if any, would be very slow.

Vestibular acoustic reception in the guinea pig: a saccular function?

After complete destruction of cochlear but preservation of vestibular hair cells in the guinea pig acoustically evoked responses can still be recorded from the round window up to the auditory cortex. At all levels these responses differ from those observed in normal animals but their frequency sensitivity and selectivity make them akin to responses from auditory organs. In a series of experiments a complete cochlear destruction was combined with a total or partial destruction of the vestibule. After complete cochlear and vestibular hair cell destruction no acoustic response could be recorded. But in cases of total cochlear and drastic ampullar and utricular destruction together with an almost undamaged saccular sensory epithelium the same peculiar acoustic responses could be observed. These results support the hypothesis of a functional acoustic reception by the saccule in a mammal.

[Decalcified, lyophilized, sterile heterotopic porcine ossicular xenografts. Experimental evaluation in the guinea pig]. / Les xéno-implants ossiculaires hétérotopiques porcins décalcifiés, lyophilisés et stérilisés. Evaluation expérimentale chez le cobaye.

Using the guinea pig middle ear model, we assessed decalcified, lyophylized, sterile heterotopic porcine ossicular xeno-implants based on a histology (optic and electron scan microscope) and immunologic (immunofluorscence) methods. Implants were placed in the middle ear and others in the dorsal subcutaneous area. Allo-implants were compared as controls. Implants were placed in the middle ear in 54 animals and skin implants in 14. Under the influence of BMP, the implant ossified in all cases in the middle ear. Intense immune recruitment was not observed. Inversely, there was a mononuclear infiltration reaction to the skin implants with formation of a fibrous capsule, immunoglobulin and complement influx and consequently sequestration. The allo-implants were partially reossified. These findings confirm the value of decalcification with hydrochloric acid for BMP induction, independent of species and the failure of attempted immune despecification. Implant outcome is not dependent on its antigen load, which is high compared with its weight, but on the site of implantation. The middle ear appears to be a privileged site of implantation.

[Ototoxicity of aminosides: recent results on uptake and clearance of gentamycin by sensory cells of the cochlea]. / L'ototoxicité d'aminosides: résultats récents sur la captation et la clairance de la gentamicine par les cellules sensorielles du limaçon osseux.

Recent experiments using autoradiographic and immunohistochemical labelling of gentamicin have demonstrated that GM penetrates specifically into the sensory cells of the inner ear, with a good correlation between the intensity of the labelling and the respective degrees and localisations of ototoxic damages. In the sensory hair cells GM ils localised below the cuticular plate, in an area rich in lysosomes during the ototoxic treatment. This penetration precedes the development of ototoxicity and there seems to be a threshold of intracellular concentration of GM for development of intracellular ototoxic processes. Clearance is very slow since GM can still be observed 11 months after the end of a non toxic treatment (60 mg/kg/day for 6 consecutive days). This observation is of clinical interest, in view of the delayed development of ototoxicity often observed clinically, and with respect to other hazards, including new ototoxic treatments, to which the cells can be exposed while loaded with the aminoglycoside molecule.

Organic material concentration in auditory outer hair cells measured by laser interferometry.

Outer hair cells (OHC) of the mammalian cochlea are quasicylindrical cells of different length, which play a major role in hearing at threshold. Their particular shape allows the use of a noninvasive laser interferometric technique of isolated cells in vitro in order to measure the organic material concentration (OMC), hence the density of each cell body. In most (95%) of the OHCs isolated from the same guinea pig, when the cell diameter is normalized, the results show that the cell body OMC does not vary with cell length. In different animals, the respective normalized OMC mean values can vary between 70 kg/m3 and 103 kg/m3. A few OHCs with morphological particularities often possess cell body OMCs > 103 kg/m3. The results of the interferometric measurements in isolated OHCs confirm that density variations in the cell bodies are not involved in a sound frequency coding. The in vitro OMC variations of the OHCs could be related to the isolation procedure; however, they could also correlate with actual in vivo OMC variations.

Ototoxicity of teicoplanin in the guinea pig.

Experiments were performed on guinea pigs to assess the eventual ototoxic side effects of a new glycopeptide antibiotic called teicoplanin. For a better validation, in the same study other animals were treated with tobramycin, a well known ototoxic antibiotic of the aminoglycoside family, and a group of control animals was also constituted. Antibiotic treated animals were injected with the high dose of 90 mg/kg/day during 21 consecutive days. Electrophysiological measures of equilibrium and acoustic functions were taken and morphological evaluations of the cochleas were performed. The results indicate that in these animal experiments teicomycin appears to be clearly toxic for the vestibule and the cochlea.

Effects of amphetamine and cocaine treatment on c-Fos, Jun-B, and Krox-24 expression in rats with intrastriatal dopaminergic grafts.

Activation of dopaminergic (DA) transmission by psychostimulants increases c-fos expression. d-Amphetamine-induced c-fos activation is reduced in the neostriatum deprived of DA afferents. Dopaminergic grafts implanted into the denervated neostriatum induce a c-fos hyperexpression when challenged with d-amphetamine, which is correlated with the exaggerated compensation of d-amphetamine-induced rotation. The aim of the present study was to test the generality of this phenomenon and the effects of DA grafts on the expression of three immediate early gene-coded proteins (c-Fos, Jun-B, Krox-24) following a challenge with either d-amphetamine or cocaine. c-fos basal expression was low in the neostriatum and was increased by the administration of psychostimulants. These effects were blocked by the DA lesion and restored by the DA grafts. A c-fos hyperexpression was observed within the grafted neostriatum, which was correlated with the compensation of d-amphetamine- or cocaine-induced rotation. Basal levels of Jun-B- and Krox-24-LI nuclei were high within the neostriatum. Administration of d-amphetamine or cocaine did not influence the expression of these IEG-coded proteins. Jun-B expression was not affected by the surgical procedure. In contrast, lesion of DA afferents of neostriatum decreased Krox-24 basal expression, an effect reversed by the grafts. Thus, the expression of c-fos but not Jun-B or Krox-24 appeared to be a good marker for the rotational behavior exhibited by DA-grafted rats challenged with drugs that increased DA transmission. This generalized c-fos overshoot indicates an abnormal activation of postsynaptic neurons by dopamine and points to its value as an indicator of the deleterious effects of DA grafts.

Effects of Ginkgo biloba extract (EGb 761) on cochlear vasculature in the guinea pig: morphometric measurements and laser Doppler flowmetry.

Ginkgo biloba extract (EGb 761) was administered orally for 4 or 6 weeks to healthy adult guinea pigs. Animals were then decapitated under deep ketamine anesthesia. Post-mortem morphometric measurements of cochlear vessels in the spiral lamina revealed a vasodilating effect of the extract in four of ten animals following 6 weeks of treatment. In vivo testing of the effect of 4 or 6 weeks of treatment with EGb 761 was monitored with laser Doppler flowmetry of the cochlear blood flow under pathological conditions. Results demonstrated that EGb 761 partly counteracted sodium salicylate-induced decreases in cochlear blood flow (CBF) and enhanced CBF increases induced by hypoxia. These findings indicate that EGb 761 may help to improve oxygenation in cochleas with compromised blood flow.

Gentamicin uptake by cochlear hair cells precedes hearing impairment during chronic treatment.

Immunodetection of gentamicin (GM) was carried out on surface preparations of the whole organ of Corti from cochleas of guinea pigs treated daily with GM at a dose of 60 mg/kg/day and sacrificed at the end of different treatment periods. Cochlear function was determined just before sacrifice, 24 h after the last injection. Threshold elevations, mainly at high frequencies, were noted only after 10-14 days of treatment. However, the presence of GM was observed much earlier, as early as after the second injection, and specifically in the sensory hair cells. GM labelling was essentially observed in the outer hair cells (OHC) and increased from the apex to the base of the cochlea and from the third to the first row of OHC. GM labelling of inner hair cells was less pronounced and was observed only after the 8th day of treatment. These observations demonstrate that GM specifically enters and accumulates in the sensory hair cells and that the uptake precedes the development of functional and cellular damage which may result from a long-term intracellular cytotoxic action of the molecule.

Pharmacokinetics of gentamicin in the sensory hair cells of the organ of Corti: rapid uptake and long term persistence.

Confocal laser scanning microscopy and immunofluorescence were used to detect gentamicin at the single cell level in the sensory cells of the guinea-pig hearing organ after a chronic systemic treatment. Gentamicin uptake by the hair cells precedes largely the manifestation of hearing impairment. We demonstrate that for a non-ototoxic treatment the captured gentamicin is eliminated in two phases: one rapid and one slow with approximate half-life of 2 days and 5-6 months, respectively. These novel observations should be of primary importance for new rational approaches to both the mechanism of ototoxicity and therapeutic strategies for the treatment of infectious diseases requiring the aminoglycoside antibiotics.

Behavioural trait of reactivity to novelty is related to hippocampal neurogenesis.

The hippocampal formation is one of the brain areas where neurogenesis persists during adulthood, with new neurons being continuously added to the population of dentate granule cells. However, the functional implications of this neurogenesis are unknown. On the other hand, the hippocampal formation is particularly concerned with the detection of novelty, and there are indications that dentate granule cells play a significant role in this function. Recently, the existence of inter-individual differences in behavioural reactivity to novelty has been evidenced, related to differences in the reactivity of the hypothalamic-pituitary-adrenal axis (HPA). Rats that are highly reactive to novelty (HR) exhibit a prolonged corticosterone secretion in response to novelty and to stress when compared with low reactive rats (LR). Taking advantage of the existence of these inter-individual differences, we investigated whether neurogenesis in the dentate gyrus is correlated with the behavioural trait of reactivity to novelty. Rats were first selected according to their locomotor reactivity to a novel environment. Two weeks later, cell proliferation, evaluated by the incorporation of 5-bromo-2'-deoxyuridine (BrdU) in progenitors, was studied by immunohistochemistry. We found that cell proliferation in the dentate gyrus was negatively correlated with locomotor reactivity to novelty. Indeed, cell proliferation in LR rats was twice that observed in HR rats. In contrast, survival of nascent neurons was not influenced by the behavioural trait of reactivity to novelty. Using an unbiased stereology, we show that LR rats had more cells within the granule cell layer of the dentate gyrus than did HR rats. These results demonstrate the existence of inter-individual differences in neurogenesis and total granule cell number within the dentate gyrus. These differences in hippocampal plasticity can be predicted by the behavioural trait of reactivity to novelty.