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Bilateral Lesions of the Suprachiasmatic Nuclei Alter the Nocturnal Melatonin Secretion in Sheep
Author(s) -
Tessonneaud A.,
Locatelli A.,
Caldani M.,
ViguierMartinez M. C.
Publication year - 1995
Publication title -
journal of neuroendocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.062
H-Index - 116
eISSN - 1365-2826
pISSN - 0953-8194
DOI - 10.1111/j.1365-2826.1995.tb00677.x
Subject(s) - melatonin , medicine , endocrinology , circadian rhythm , pineal gland , biology , nocturnal , suprachiasmatic nucleus , lesion , photoperiodism , pars tuberalis , hypothalamus , hormone , pituitary gland , pathology
The hypothalamic suprachiasmatic nuclei (SCN) constitute both the biological clock of many circadian rhythms, and the first relay in the transmission of light cues from the retina to the pineal gland, which releases, via nocturnal melatonin secretion, an endocrine expression of the daylength. The aim of the present work was to investigate the precise role of the SCN in the entrainment of the nocturnal rhythm of melatonin (MEL) in sheep. Bilateral lesions of the SCN were performed via a transsinusal surgical approach in 10 adult rams submitted to a constant photoperiod (16L:8D). Lesioned rams were compared to 4 sham and 2 control animals. Blood samples were collected 8 days before, 8 days after, and one month after surgery. Plasma MEL levels were estimated using direct radioimmunoassay. At the end of the experiment, histology and immunohistochemistry of the suprachiasmatic area were performed, and the extent of lesions was evaluated using a computerized image analysis system. Six rams exhibited a complete lesion of the SCN, and in the four remaining animals, the lesions were restricted to the anterior part of the SCN. For all animals, the nocturnal melatonin secretion was altered, but depending on the extent of the lesion, two types of results were observed: Eight days after surgery, in 3 of the 4 rams bearing anterior lesions of the SCN (SCNJ, a nocturnal increase in melatonin secretion still occurred at dusk, but the duration of this secretion extended beyond the end of the night. One month after surgery, melatonin profiles were once again normal, as compared to sham animals. The fourth SCN, ram exhibited a cyclic MEL secretion not synchronized with the light cycle 8 days after surgery, and synchronized with dusk but not dawn one month after surgery. Eight days after surgery, all the rams bearing complete lesions of the SCN (SCNZ) showed strongly altered MEL secretion. This secretion was continuous during the period of sampling, without any characteristic nocturnal peak. Nevertheless, one month after surgery, a nocturnal increase in melatonin secretion synchronized with the dusk was again observed, but the duration of the melatonin secretion was longer than the duration of the night. These results confirm that the SCN mediates the nocturnal pineal gland activity, demonstrate that the anterior part of the sheep SCN is the main drive for the inhibition of the melatonin secretion at dawn and suggest that a positive input to the pineal gland may stimulate MEL secretion at dusk. The partial recovery of the cyclic secretion of melatonin after complete lesioning of the SCN suggests the existence of other structure(s) involved in this rhythmic function.