Lise Martinet

Entrainment of circadian rhythms by S-20098, a melatonin agonist, is dose and plasma concentration dependent

The present study determined first the dose-response (0.5 to 10 mg.kg-1) to daily oral administration of S-20098, a melatonin agonist, in entraining circadian rhythms of rats free-running in constant darkness; second, the relation between entrainment and the plasma concentration of S-20098. Finally, responses to 8 mg.kg-1 of S-20098 were compared with those obtained with the same dose of melatonin and ipsapirone. Responses were classified as negative, transient, or true entrainment. The data indicated a clear dose-dependent response from 2.5 to 10 mg.kg-1 of S-20098 with an ED50 of 5.7 mg.kg-1 for true entrainment and a clear relation between entrainment and the plasma concentration of S-20098. S-20098 was as effective as melatonin to entrain free-running rhythms. Ipsapirone was ineffective in our experimental conditions.

Neurons containing gastrin-releasing peptide and vasoactive intestinal polypeptide are involved in the reception of the photic signal in the suprachiasmatic nucleus of the Syrian hamster: an immunocytochemical ultrastructural study.

Abstract In mammals, the suprachiasmatic nuclei are involved in the generation of biological rhythms and are synchronized by light input coming from the retina. The targets of retinal afferents and the involvement of neurons containing gastrin-releasing and vasoactive intestinal peptides in photic reception were investigated in the suprachiasmatic nuclei of the Syrian hamster by using light- and electron-microscopic immunocytochemistry. Cholera toxin was used to trace retinal fibers and Fos immunoreactivity to visualize cellular response to light stimulation. Ultrastructural observations were made in the intermediate third of the nuclei, the area of highest overlap for the immunoreactivities investigated. Gastrin-releasing peptide and vasoactive intestinal peptide cell bodies were localized in the ventral part of the nuclei; their dense immunoreactive fiber network often displayed synaptic contacts. Both neuropeptides were colocalized in elongated cells observed near the optic chiasm. Following a light pulse in the middle of the subjective night, Fos protein was expressed in most gastrin-releasing peptide perikarya and in some vasoactive intestinal peptide cells. Retinal terminals mostly occurred in the midline zone between the suprachiasmatic nuclei. Symmetrical or asymmetrical retinal synapses were observed on gastrin-releasing peptide-immunoreactive dendrites and somata, but never on vasoactive intestinal peptide neurons. These results are discussed in relation to the photic entrainment of the circadian clock.

Estrogen receptor and progesterone receptor-immunoreactive cells are not co-localized with gonadotropin-releasing hormone in the brain of the female mink (Mustela vison)

Abstract The distribution of gonadal steroid (estrogen, progesterone) receptors in the brain of the adult female mink was mapped by immunocytochemistry. Using a monoclonal rat antibody raised against human estrogen receptor (ER), the most dense collections of ER-immunoreactive (IR) cells were found in the preoptic/anterior hypothalamic area, the mediobasal hypothalamus (arcuate and ventromedial nuclei), and the limbic nuclei (amygdala, bed nucleus of the stria terminalis, lateral septum). Immunoreactivity was mainly observed in the cell nucleus and a marked heterogeneity of staining appeared from one region to another. A monoclonal mouse antibody raised against rabbit uterine progesterone receptor (PR) was used to identify the PR-IR cells in the preoptic/anterior hypothalamic area and the mediobasal hypothalamus (arcuate and ventromedial nuclei). This study also focused on the relationship between cells containing sex-steroid receptors and gonadotropin-releasing hormone (GnRH) neurons on the same sections of the mink brain using a sequential double-staining immunocytochemistry procedure. Although preoptic and hypothalamic GnRH neurons were frequently in close proximity to perikarya containing ER or PR, they did not themselves possess receptor immunoreactivity. The present study provides neuroanatomical evidence that GnRH cells are not the major direct targets for gonadal steroids and confirms for the first time in mustelids the results previously obtained in other mammalian species.

Seasonal variations in mink (Mustela vison) plasma prolactin measured by heterologous radioimmunoassay

Abstract Monthly prolactin levels were measured in mink by a heterologous radioimmunoassay using porcine prolactin SP162C as the labeled hormone and an antiovine prolactin guinea-pig serum. The yearly variation of plasma prolactin concentration closely followed the day-length ratio throughout the year in males as in females.

Melatonin and the circadian clock in mink: effects of daily injections of melatonin on circadian rhythm of locomotor activity and autoradiographic localization of melatonin binding sites.

The present study examines a putative effect of exogenous melatonin on the Circadian organization of the mink. Two approaches were used to determine first whether entrainment of free‐running rhythms of locomotor activity in constant darkness can be obtained by daily melatonin injections, thus demonstrating a control of melatonin on the clock generating Circadian rhythms, the suprachiasmatic nucleus of the hypothalamus. Entrainment was never obtained in the 8 vehicle‐injected females and 7 out of the 8 melatonin injected‐ones. In 3 females free‐running in constant darkness, a phase advance followed by a few days of transient effect was observed when melatonin injections coincided with the onset of activity. However, the comparison of the regression of the daily activity onset related to successive days by covariance analysis revealed that true entrainment was effective in only 1 female. Second, we examined the distribution of melatonin binding sites within the brain of juvenile and adult mink using an in vitro autoradiographic procedure with [125l]2‐iodomelatonin. No binding sites were observed in the suprachiasmatic nucleus of any of the animals. However, all animals displayed a high density of melatonin binding sites in the pars tuberalis of the pituitary. The relation between a modulatory control of melatonin on the Circadian clock and the presence and density of melatonin binding sites in the clock is discussed. In mink, melatonin does not seem to act as an internal Zeitgeber.

Effects of Timed Melatonin Infusions and Lesions of the Suprachiasmatic Nuclei on Prolactin and Progesterone Secretions in Pregnant or Pseudopregnant Mink (Mustela vison)

To test the hypothesis that the duration of melatonin secretion may be a critical parameter in the transduction of photoperiodic signals on prolactin and progesterone secretions, timed intravenous melatonin infusions were carried out in intact and ganglionectomized pregnant and pseudopregnant mink. To localize the target sites of melatonin, electrolytic lesions of hypothalamic nuclei were performed in females receiving melatonin infusions. As a control, the first experiment was designed to confirm that pineal denervation by bilateral ablation of the superior cervical ganglion rendered the pregnant mink totally unresponsive to the inhibitory effects of short days on progesterone secretion. In the following experiments, timed intravenous melatonin infusions were carried out in intact and ganglionectomized females from Day 12 to 32 of pregnancy or pseudopregnancy. Daily infusions of melatonin for 16 h in intact females or for 11 or 13 h in ganglionectomized females suppressed the rise in plasma prolactin and progesterone levels. In intact as in ganglionectomized females, daily infusions of melatonin for 9 h delayed the rise in plasma prolactin concentrations without affecting the secretion of progesterone. In ganglionectomized females, saline infusions for 13 h or melatonin infusions for 7h did not modify the secretions of prolactin and progesterone. In ganglionectomized females bearing lesions of the Suprachiasmatic nuclei or the retrochiasmatic area, melatonin infusions for 13 h were still able to inhibit prolactin and progesterone secretions. These results are consistent with the hypothesis postulating that the peak duration of melatonin secretion is a critical parameter for transducing photoperiodic responses in pregnant or pseudopregnant mink. Secondly, they suggest that the Suprachiasmatic nuclei and the retrochiasmatic area are not essential for the action of melatonin in the photoperiodic control of prolactin and progesterone secretions during pregnancy or pseudopregnancy in the mink.

Diurnal Variations of Plasma Melatonin Concentrations in Pregnant or Pseudopregnant Mink (Mustela vison) Maintained Under Different Photoperiods

Removal of the superior cervical ganglia suppresses the inhibitory role of short days in prolactin secretion and luteal activity in pregnant or pseudopregnant mink. Alternatively, timed injections of melatonin replicate the inhibitory role of short days in females maintained under long days. To understand if the diurnal variation of melatonin secretion is part of the mechanism by which the mink measure day length, diurnal variations in plasma melatonin concentrations have been measured in intact and ganglionectomized females maintained under long or short days after mating. Melatonin was measured by radioimmunoassay according to Brun et al. [Adv. Biosci. 53: 41–45]. In intact females, plasma concentrations ranged from nondetectable levels to 40 pg/ml during the day, increased shortly after the onset of the dark phase, and reached peak values during the middle of the night. The duration of the elevated levels was roughly proportional to the length of the night. No diurnal variations could be detected in ganglionectomized females; melatonin levels never exceeded the day values observed in intact females.

Innervation of the mink pineal gland with neuropeptide Y (NPY)-containing nerve fibers

SummaryAn immunohistochemical investigation of the mink pineal gland was performed by use of antibodies raised in rabbits against neuropeptide Y (NPY) and Cys-NPY (32–36)-amide recognizing neuropeptide Y with an amidation at position 36 (NPYamide). NPY-immunoreactive nerve fibers were located predominantly in the rostral part of the pineal gland and in the pineal stalk. Immunoreactive nerve fibers were found throughout the pineal gland, but the number of fibers in the caudal part of the gland was low. The fibers were present both in the perivascular spaces and between the pinealocytes. Many NPY-immunoreactive fibers were also located in the posterior and habenular commissures; some of these fibers were connected with the fibers in the rostral part of the mink pineal gland, indicating that at least some of the NPY-immunoreactive nerve fibers are of central origin. The nerve fibers immunoreactive to amidated NPY were distributed in a similar manner. However, the number of fibers immunoreactive to NPYamide was lower than the number of fibers immunoreactive to NPY itself. After removal of the superior cervical ganglia bilaterally 22 days or 12 months before sacrifice, NPY-immunoreactive nerve fibers remained in the gland. This immunohistochemical study of the mink pineal gland therefore shows that the NPY/NPYamide-immunoreactive nerve fibers innervating the pineal gland in this spegcies are a component of the central innervation or originnate from extracerebral parasympathetic ganglia.

Effects of melatonin treatment on the gonadotropin‐releasing hormone neuronal system and on gonadotropin secretion in male mink, Mustela vison, in the presence or absence of testosterone feedback

NToumi F, Martinet L, Mondain‐Monval M. Effects of melatonin treatment on the gonadotropin‐releasing hormone neuronal system and on gonadotropin secretion in male mink, Mustela vison, in the presence or absence of testosterone feedback. J. Pineal Res. 1994: 16: 18–25.

Effect of temperature on the growth and fertility of the field-vole, Microtus arvalis, raised in different daylength and feeding conditions

This report studies the effects of 3 temperatures (5 OC, 22 °C or 33 °C) and their interaction with daily daylength (15 h or 10 h per 24 h) and the date of alfalfa harvest (april or june) on growth and fertility of the field-vole from birth to 45 days. Body growth and fertility decrease inversely with the temperature. When added to a short daylight ratio (10 h), hot ambient temperature has an unfavorable effect on fertility ; clfalfa harvested late in the year plus the short daylength also adversely affect body growth and fertility. At 35 days in females and at 45 days in males, the largest and most fertile animals are those raised at 5°C under a long daylight ratio and fed alfalfa harvested in april. On the other hand, the smallest and least fertile animals are raised at 33 OC under a short daylength and fed alfalfa harvested in june. The synergic effects of the three factors studied can therefore explain the seasonal cycles of growth and fertility in the field-vole.