Observations on the Suckling Stimulus and Lactation in the Rat

Oestrus and Ovarian Histology in the Lactating Vole (Microtus Agrestis)

“…In lactating rats and mice there is a prolonged dioestrus (Long & Evans, 1922;Parkes, 1926) which is, in rats, at least partly due to suckling (McKeown & Zuckerman, 1938). There is a general correlation between the number of young suckled and the length of the dioestrous period and gestation (Enzmann, Saphir & Pincus, 1932;Weichert, 1940;Bruce, 1961). Administration of oestrogen enables implantation to take place at the normal time (Weichert, 1942;Whitten, 1955).…”

Induction by young of prolonged dioestrus in virgin rats behaving maternally

“…Since induc¬ tion of maternal behaviour in virgin rats by exposure to young requires about 7 days, during which oestrous cyclicity is maintained, it is possible that the virgins in the study of Selye & McKeown (1934a) continued to cycle regularly because the time with young was too short to induce maternal behaviour. Bruce (1961), however, recorded prolonged dioestrus in 2 virgin rats kept with young for 12 weeks. Therefore, one might expect that virgin rats would enter pro¬ longed dioestrus only after becoming maternal.…”

“…After the post-partum oestrus, lactating unmated rats and mice experience a prolonged dioestrous phase which ends at the time of weaning (about 3 weeks post partum) with resumption of oestrous cycles (Parkes, 1926;Rothchild, 1960;Bruce, 1961). When suckling is maintained for an extended period, resulting in prolonged lactation, this post-partum dioestrus is followed by cycles with a prolonged dioestrous phase (Parkes, 1926;Crew & Mirskaia, 1930;Selye & McKeown, 1934c;Bruce, 1961;Nagasawa & Yanai, 1976; Tomogane, Ota, Unno & Yokoyama, 1976).…”

“…When suckling is maintained for an extended period, resulting in prolonged lactation, this post-partum dioestrus is followed by cycles with a prolonged dioestrous phase (Parkes, 1926;Crew & Mirskaia, 1930;Selye & McKeown, 1934c;Bruce, 1961;Nagasawa & Yanai, 1976; Tomogane, Ota, Unno & Yokoyama, 1976). Removal of young during the 3-week post-partum dioestrus leads to a rapid resumption of regular oestrous cycles (Crew & Mirskaia, 1930;Ota & Yokoyama, 1965;Moltz, Levin & Leon, 1969;Zarrow, Johnson, Denenberg & Bryant, 1973;Maneckjee & Moudgal, 1975).…”

Milk Yield During Prolonged Lactation in Mice; Effect of Ovariectomy

“…

Prolongation of lactation beyond its natural duration has been induced in rats (Bruce, 1958(Bruce, , 1961Nicoll & Meites, 1959) and mice (Parkes, 1926;Selye & McKeown, 1934;Fekete, 1940; M\l=u"\hlbock & Tengbergen, 1961;Zeilmaker, 1968) by regular replacement of the suckling litter.Measurement of milk yield of mice bearing pituitary grafts and subjected to prolonged lactation, suggested that milk production occurs in a cyclic fashion (Zeilmaker, 1968).In the present study, this phenomenon has been re-investigated using mice with or

…”