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This Article Full Text Full Text (PDF) All Versions of this Article: 70/4/1001    most recent biolreprod.103.020008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal My Folders Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ben Saad, M.M. Articles by Maurel, D.L. Search for Related Content PubMed PubMed Citation Articles by Ben Saad, M.M. Articles by Maurel, D.L. Agricola Articles by Ben Saad, M.M. Articles by Maurel, D.L.

Reciprocal Interaction Between Seasonal Testis and Thyroid Activity in Zembra Island Wild Rabbits (Oryctolagus cuniculus): Effects of Castration, Thyroidectomy, Temperature, and Photoperiod

Laboratoire de Physiologie Animale,² Faculté des Sciences de Tunis, Campus Universitaire, 1060 Tunis, Tunisia Pathologie de l'Oreille interne et Réhabilitation,³ EPI 9902 INSERM, Faculté de Médecine Nord, 13916 Marseille cedex 20, France

The reproductive cycle of wild rabbits (Oryctolagus cuniculus) living in Zembra Island (North Tunisia) is dependent on an external factor, the photoperiod: the gonads are inhibited by long days and stimulated by short days or melatonin implants. Here we studied the role of an internal factor, thyroid hormones and the possible thyroid-gonadal interrelationships, in animals captured on Zembra Island and maintained in natural conditions of photoperiod and temperature. We determined the seasonal profile of the thyroid and testis cycles and investigated the effects of castration and thyroidectomy on the seasonal testosterone and thyroxine cycles. Plasma thyroxine and testosterone levels followed similar, parallel seasonal patterns, with a peak in autumn (October) and low values from January to August. In thyroidectomized animals, plasma testosterone levels, although significantly higher than those in controls (P < 0.001), remained low throughout the 13 mo of the experiment, and no testicular reactivation was observed in the fall. In castrated animals, despite the increase in thyroxine concentration in the 3 mo following castration (P < 0.01), plasma thyroxine levels remained low during the 2 yr of the study. We then investigated the combined effects of long days (16L:8D) and moderately high temperature (25°C) on these two endocrine axes. In constant gonado-inhibiting conditions (16L:8D), whether the temperature was kept constantly high or allowed to fluctuate naturally, no reactivation of the thyroid and testicular axes was observed in the fall. In control animals, the peaks of testosterone and thyroxine concentrations observed in September were larger (P < 0.001) than those in animals subjected to the same natural photoperiod conditions but with constantly high temperature. The lower level of autumnal testis stimulation (P < 0.001) in animals maintained in conditions of constant high temperature (25°C) may be attributed to the low thyroxine levels induced by high temperature. These results clearly confirm that the thyroid and testicular cycles display similar seasonal variations and show that the thyroid and gonadal axes are strictly interdependent. This study provides the first demonstration, for a given species, that the seasonal reactivation of gonad activity is controlled by the thyroid, and thyroid activity is controlled by the gonads.

¹ Correspondence: Daniel Maurel, Pathologie de l'Oreille interne et Réhabilitation, EPI 9902 INSERM, Faculté de Médecine Nord, Boulevard Pierre-Dramard, 13916 Marseille cedex 20, France. FAX: 33 4 91 69 87 31; maurel.d{at}jean-roche.univ-mrs.fr