Elevated prolactin secretion during proestrus in mice: Absence of a defined surge

Abstract

Throughout the reproductive cycle in rodents, prolactin levels are generally low. In some species, including rats, a prolactin surge occurs on proestrus with peak concentrations coinciding with the preovulatory luteinizing hormone (LH) surge. In mice, however, there are conflicting reports relating to the occurrence and timing of a proestrous prolactin surge. To gain further insight into the incidence and characteristics of this surge in mice, we have used serial tail tip blood sampling and trunk blood collection from both C57BL/6J (inbred) and Swiss Webster (outbred) mouse strains to build a profile of prolactin secretion during proestrus in individual mice. A clearly defined LH surge was detected in most animals, suggesting the blood sampling approach was suitable for detecting patterns of hormone secretion on proestrus. Despite this, levels of prolactin were quite variable between individuals. Overall both mouse strains showed a generalized rise in prolactin levels on the day of proestrus compared with levels seen in diestrus. This pattern is quite distinct from the discreet, circadian-entrained surge observed in rats.

Keywords: luteinizing hormone; proestrus; prolactin.

FIGURE 1

Examples of estrous cycles in C57BL/6J and Swiss Webster Mice. A, C, and E show regular 4–5 day estrous cycles in C57BL/6J mice. B and F show examples of regular cycling Swiss Webster mice. D shows the estrous cycle in a Swiss Webster mouse that has undergone a prolonged period in diestrus (8 days, pseudopregnancy) then undergone two regular estrous cycles. Red arrows and dots represent days during the cycle in which blood samples for measurements of prolactin and LH were taken. A–D, and F show tail tip blood sampling occurring on either diestrus or proestrus across subsequent cycles and E shows when tail tip blood samples were taken continuously through diestrus and proestrus

FIGURE 2

Examples of levels of circulating prolactin and LH in whole blood samples collected by the tail tip method in individual C57BL/6J and Swiss Webster mice during diestrus and proestrus of the reproductive cycle. Repeated tail blood sampling in individual C57BL/6J (A–F) and Swiss Webster (G–L) mice during either diestrus or proestrus in a single cycle. (M–O) Prolactin and LH secretion profiles from single C57BL/6J mice in which blood samples were collected over two consecutive days of a single estrous cycle. Red arrow indicates maximum LH levels recorded during proestrus

FIGURE 3

Mean prolactin and LH levels obtained via tail tip blood sampling during diestrus and proestrus. Levels of prolactin show a significantly different pattern of change across the day of proestrus as compared to diestrus in both C57BL/6J (A, n = 8, p = .0122, F(1.000 (DFn), 7.000 (DFd)) = 11.24) and Swiss Webster mice, (B, n = 10, p = .0125, F(1.000 (DFn), 105.0 (DFd)) = 6.459). A mixed effects analysis of repeated measures was used for analysis and results presented as mean ± SEM, *Significant (p < .05) with respect to prolactin profile during diestrus levels. Red arrow indicates mean maximum LH levels

FIGURE 4

Circulating prolactin during the light phase of diestrus and proestrus stages of the mouse estrous cycle. The overall level of prolactin mice are exposed to during the light phase is higher in proestrus as compared to diestrus in both C57BL/6J (A, n = 8, p = .0065, unpaired t test) and Swiss Webster (B, n = 10, p = .0134, unpaired t test) mice. Results presented as mean ± SEM. *p < .05, **p < .01. Black dots represent AUC for individual mice

FIGURE 5

Prolactin (A) and LH (B) levels obtained via trunk blood collection following decapitation during proestrus. (A) Mice are exposed to higher prolactin levels in the evening of proestrus as compared to the morning (n = 8, p = .0047, Student's t test (t = 3.354, df = 14). (B) Shows a significant difference in LH levels (n = 8, p = .0002, Mann Whitney U test) on the evening of proestrus as compared to the morning. Results presented as mean ± SEM. ** P< .01, *** P< .001. Black dots indicate levels of LH and prolactin in individual mice. (C) Percentage increase in prolactin (ng/ml) levels taken from mean levels measured by the different sampling methods (tail tip sampling and trunk blood collection) at 0900 h (morning) and 1900 h (evening) on proestrus