Plasma progesterone (P4) concentrations during pregnancy and lactation in Mongolian gerbils were measured by enzymeimmunoassay (EIA). The P4 concentration was found to increase rapidly after mating and reach a peak on day 6 of pregnancy. The maximum level (166.0 to 184.8 ng/ml) was maintained to day 12 and then decreased to a plateau at a moderate level until day 21. The P4 concentration declined to its lowest level on day 24 of pregnancy, one day before parturition. The P4 concentration on the first day of lactation was similar to that on the last day of pregnancy. The concentration then increased to a significantly higher level on day 4 of lactation, remained relatively stable until day 19, and then significantly dropped on day 22. The present study in gerbils showed that the plasma P4 concentration during pregnancy was higher in the first half than in the second half of gestation and that the P4 level during lactation was fairly constant. The pattern of changes in the P4 level during pregnancy and lactation is therefore quite different from that of other rodent species, such as rats, mice and hamsters.
A total of 31 female Mongolian gerbils (19-54 weeks of age at mating) were used for this maternal age did not affect to litter size and gestation length, litter size did significantly to gestation length. Plasma progesterone (P4) was measured in serial blood samples collected every 3 days throughout gestation period. The P4 concentration increased rapidly after mating, reaching a peak level on day 6 of gestation, maintained the high level to day 12 of gestation, then decreased to a moderate level and sustained at the level until day 21. The P4 concentration decreased to a lowest level on day 24 of gestation (mean gestation length: 25.2 days). Females having a large number of litter size tended to reveal higher level of P4 during gestation, comparing with that females having a fewer litter size, and significant differences between them were found on days 6 and 9 of gestation. There were significant differences in P4 levels among 3 groups according to gestation length, resulting in lower P4 level from females with longer gestation length. Therefore, difference of plasam P4 level during the gestation is due to litter size. Animal Science Journal 70 (1): 18-23, 1999 Key words: Mongolian gerbil, Litter size, Gestation length, Plasma P4 level In a previous paper13), we reported plasma concentration of progesterone (P4) during gestation and lactation in Mongolian gerbils. The pattern of the P4 level in pregnant gerbils was quite different from those in mice1, 8, 15) and rats3, 10). In these animals, blood P4 level increased rapidly during early gestation and maintained a plateau which was followed by a higher peak, starting at midgestation. The P4 level declined rapidly in the 2 days before parturition.Bridges3) reported in rats that serum P4 rose from 5-10ng/ml on days 1 and 2 of gestation to about 70ng/ml on days 4-10. Then, a second increase in the level occurred between days 10 and 15 of gestation. Average P4 level ranged from 100 to 152ng/ml between days 12 and 20 of gestation. The P4 level declined significantly on days 20 and 21, and dropped further to about 10ng/ml on day 22 of gestation3). In gerbils, however, plasma P4 level showed a peak in early gestation but not in second half of gestation13) A sharp increase in the P4 level was found in early gestation and it reached a peak on day 6. The higher concentration was maintained to day 12, and then decreased to a moderate level on day 15 and continued at that level until day 21. The P4 level declined to the lowest level on day 24 of gestation, one day before parturition (mean gestation length: 25.2 days). However the result was obtained with limited number of animals (n=7)13), effects of the litter size at birth to the gestation length and the plasma P4 level in the gestation were possible to be in gerbils, as found as some species of polytocous animals. In pregnant rats with 0, 1 or 10 and more conceptuses experimentally adjusted on day 7 of gestation, a direct relation has been found between the number of conceptuses and
Changes in prolactin (PRL)-, growth hormone (GH)- and PRL/GH-containing cells in the anterior pituitary of pre- and postnatal male and female rats were determined using immunocytochemistry with double fluorescent antibodies. The pituitary glands from a fetus on Day 20 of gestation and pups on Days 0, 1, 4, 7, 12, and 20 of the postnatal period with sex distinction were monodispersed and subjected to immunocytochemistry. Following immunostaining, the three types of cells described above were counted (i.e., only PRL-, only GH-, and both PRL and GH-positive cells in a visual field of microscope). Anterior pituitaries and blood were obtained from pups on Days 0, 1, 4, 7, 12, and 20 after birth and pooled with sex distinction, and bioactive PRL levels in the anterior pituitary and serum were measured using an Nb2 rat lymphoma cell bioassay. Double fluorescent-labeled immunocytochemistry was able to distinguish reliably between PRL-, GH-, and PRL/GH-containing cells. The PRL cells increased rapidly after parturition to 4 days of age and after 12 days of age in both genders. The latter increase was involved in the remarkable increase of PRL/GH-containing cells, which were first found at term and remained at low levels until 12 days of age. The weight and PRL level of the anterior pituitary increased in both sexes throughout the lactational stage, but the levels in female rats at 20 days of age were significantly greater than those of male rats. Serum PRL concentration remained at low levels except on Day 0. The present results demonstrate that the development of PRL-containing cells in postnatal rats during the lactational period is activated with two distinct stages, at term to Day 4 and after Day 12, and the development in the latter stage is involved with sexual dimorphism.
The present study was conducted to investigate how the maternal behavior and emotional status of mothers in lactating rats alter with lactational stage. On day 3 and 12 of lactation, mothers were allowed to nurse their litters for 50min following 4h isolation and the behavioral responses during the nursing period were recorded. Latency to pup-retrieval and duration of retrieval behavior by mothers were significantly longer on day 3 than on day 12 of lactation, and the time that mothers spent on the nest with pups and the duration of licking by mothers increased significantly on day 12 as compared with day 3. The frequency of self-grooming and scratching behavior in mothers, an indirect index with which to evaluate emotional status, was significantly higher on day 3 of lactation than day 12. Moreover, the frequency of self-grooming showed a significant negative correlation with the time that mothers spent on the nest with pups and that mothers spent crouching over pups on day 3 of lactation, whereas there was no correlation between self-grooming behavior and pup-directed activities on day 12 of lactation. These results suggest that the attenuation of non pup-directed activities concerned with emotional status of mothers as well as the reinforcement of pup-directed activities is important for the establishment of lactational performance.
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