Philippe Siaud

Publications (3)12.12 Total impact

• Article: Enhanced cochlear acoustic sensitivity and susceptibility to endotoxin are induced by adrenalectomy and reversed by corticosterone supplementation in rat.

  Philippe Siaud, Daniel Maurel, Michel Lucciano, Erzebet Kosa, Yves Cazals
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  ABSTRACT: Glucocorticoid receptors are widely distributed in the cochlea but their role remains poorly known. Previous studies provided contradictory reports on a possible cochlear acoustic hypersensitivity induced by adrenal insufficiency, while several experiments agree on a significant action of glucocorticoid receptors in adverse conditions such as acoustic trauma and restraint stress. The present experiments confirmed a cochlear acoustic hypersensitivity induced by adrenalectomy and reversed by corticosterone supplementation. These observations point to a significant role of corticosteroids in basal cochlear functioning. The glucocorticoids are known to be essential for limiting and resolving inflammatory processes. The endotoxin Escherichia coli lipopolysaccharide is widely used to induce inflammatory reactions. However, in various organs several toxic processes of this endotoxin are not influenced by glucocorticoids. From previous experiments on the cochlea there is no evidence that glucocorticoids are an essential factor against endotoxin cochlear toxicity. In the present experiments it was found that adrenalectomy greatly increased the cochlear susceptibility to endotoxin; the effect was reversed by providing corticosterone supplementation. This shows the essential role of corticosterone in this cochlear inflammation model. In previous studies local administration (at the cochlear base) of endotoxin was used and losses of cochlear acoustic sensitivity were found predominantly at high frequencies; in contrast, the systemic injection used in this study produced a cochlear loss of acoustic sensitivity at all frequencies, indicating a uniform cochlear sensitivity to the toxic effects of endotoxin.
  European Journal of Neuroscience 01/2007; 24(12):3365-71. · 3.63 Impact Factor
• Article: Hexachlorobenzene, a dioxin-like compound, disrupts auditory function in rat.

  Saida Hadjab, Daniel Maurel, Yves Cazals, Philippe Siaud
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  ABSTRACT: Hexachlorobenzene (HCB) is a dioxin-like compound widely distributed in the environment. In this study, we investigated the effects of HCB on the cochlea. Conscious free-moving rats were given HCB per os daily for 4 weeks at doses of 0.16, 4 or 16 mg/kg in olive oil, whereas the control group received olive oil only. The effects of HCB were evaluated at various time intervals, by measuring auditory nerve acoustic thresholds and plasma thyroid hormone concentration by radioimmunoassay. Histological evaluation involved surface preparation and scanning electron microscopy observations of cochlear hair cells. At a dose of 0.16 mg/kg, HCB induced no loss of acoustic sensitivity, whereas at 4 mg/kg, it induced cochlear sensitivity deficits at the mid-frequencies (2-16 kHz) with complete recovery once treatment was stopped. At a dose of 16 mg/kg, permanent threshold shifts were observed at all frequencies tested (from 1 to 32 kHz). Morphological studies showed no cochlear hair cell loss or alteration of stereocilia. HCB treatment reduced circulating thyroxine concentrations. Thyroidectomy had no effect on cochlear sensitivity in control animals. Thus, HCB is a potent oto-toxicant, and its ototoxicity may be independent of its thyroidal effects.
  Hearing Research 06/2004; 191(1-2):125-34. · 2.70 Impact Factor
• Article: Testicular activity is restored by melatonin replacement after suprachiasmatic nucleus lesion or superior cervical ganglionectomy in mink.

  Daniel L Maurel, Mohamed Moncef Ben Saad, Gisèle Roch, Philippe Siaud
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  ABSTRACT: Subcutaneous melatonin implants were inserted in mink subjected to natural (autumn) or experimental gonadostimulatory short-days (4L:20D), after lesion of the suprachiasmatic nucleus (SCNx) or after superior cervical ganglionectomy (SCGx). Gonad stimulation was assessed by measuring testicular volume and plasma testosterone level. In SCNx and SCGx animals, all measurements were indicative of sexual quiescence. In contrast, both SCNx and SCGx animals with melatonin, maintained in natural or experimental gonadostimulating short-days, showed an increase in testicular activity 2 months after melatonin implantation. Thus, melatonin (and pineal activity) is a prerequisite for the photoperiodic stimulation of reproductive activity, and the SCN is not necessarily the target site for melatonin action on the renewal of reproduction in the mink.
  Journal of Pineal Research 02/2002; 32(1):15-20. · 5.79 Impact Factor
• Article: of plasma interleukin-1f3to single and recurrent endotoxin challenges

  Mourad Mekaouche, Philippe Siaud, Laurent Givalois, Gerard Barbanel, Francis Malaval, Daniel Maurel, Ivan Assenmacher, Guy Ixart
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  ABSTRACT: In a parallel study in 10 individual rats, three time series of plasma concentrations of ACTH, corticosterone (CORT), and interleukin-1� (IL-1�) were measured before (time 0) and at intervals between 15 and 480 mm following intra-arterial (i.a.) infusions of 25�tg/kg lipopolysaccharide (LPS). All LPS injections were given at 9 AM. The first time series was performed on naive rats (day 1 ). A se- quence of six daily injections (days 3-8) of the same dose of LPS followed. The post-LPS time course of the plasma ACTH, CORT and IL-1�3 levels were studied on days 3 (second injection) and 8 (seventh injection). The first LPS injection induced a rapid (30 min) eightfold rise in plasma ACTH and CORT, culminating in concentrations 30 times the baseline at 60 nlin (ACTH) and 15 times baseline at 120 min (CORT). Both hormones receded back to the initial basal level at 480 miii. On the other hand, IL-1�3 increased slowly to peak at 13 times baseline 120 mm before declining to minimal seven- to ninefold basal levels, 480 mm and even 48 h post-LPS. During the second phase of the experiment starting 48 h after the initial LPS priming sequence, the ACTH and CORT responses to daily recurrent LPS injections again differed from those of IL-1�. The post-LPS time courses of the ACTH and CORT reaction dis- played a typical pattern of a progressive attenuation studied at days 3 and 8. The peak amplitudes at days 3 and 8 were reduced to 60 and 10%, respectively, for ACTH, and to 85 and 45% for CORT of those observed at the first LPS test. The duration of the response (both) was also shortened from 480 mm (first LPS test) to 300 mm at days 3 and 8. The post-LPS patterns of the IL- 1 � responses were char- acterized, first by basal levels seven to nine times higher than the initial baseline values (day 1), and by a rapid suppression of the post-LPS response, with only a slight (30%) increase at day 3 and no increase at day 8. Thus, after both acute and recurrent LPS administration, ACTH/CORT and IL-1� reacted dif- ferently to the endotoxin challenge. The two LPS reactive systems were not correlated. This is incon- sistent with the often proposed role of increased plasma IL- 1 � release as an intermediary factor in the LPS-induced recruitment of the corticotropic axis in general infections. J. Leukoc. Biol. 59: 341-346; 1996.