Surgical pinealectomy in ewes

Abstract

Chapitre 7The pineal gland once regarded as the “seat of the soul” by René Descartes is the structure attached to the caudal part of the roof of the third ventricle. In mammals, the pineal gland ensures the transduction of the photoperiodic information captured by the retina and transmitted to the pineal gland through a transsynaptic network and translated into a hormonal signal through the nocturnal secretion of melatonin. The circadian rhythm of melatonin synthesis (high levels at night, low levels during the day) is triggered by the circadian ‘clock’ located in the suprachiasmatic nuclei of the hypothalamus that projects to the pineal gland via a multi-synaptic pathway. In sheep, the duration of melatonin secretion is proportional to the length of the night and thus providing cues about the time of the year to the animals. Melatonin is involved in several physiological functions including the seasonal control of reproduction in mammals from temperate latitudes. The key role of the pineal gland in this function has been demonstrated by numerous experiments showing that pinealectomy strongly affected the effect of photoperiod on seasonal reproduction. In sheep, the pineal gland is hidden at the bottom of cerebral transverse fissure, in a medial position, close to the cerebral vein. This deep localization makes the surgical excision of the pineal gland very difficult and requires a specific training. In order to avoid pain and side effects both in pre and post surgical care, we designed a protocol of analgesic treatment made of morphine in combination with antiinflammatory compounds. Future improvements of our approach should arise from the use of scanner and magnetic resonance imaging techniques