Attenuated neuroendocrine responses to emotional and physical stressors in pregnant rats involve adenohypophysial changes

Abstract

1. The responsiveness of the rat hypothalamo-pituitary-adrenal (HPA) axis and hypothalamo-neurohypophysial system (HNS) to emotional (elevated plus-maze) and physical (forced swimming) stressors and to administration of synthetic corticotrophin-releasing hormone (CRH) was investigated during pregnancy and lactation. In addition to pregnancy-related adaptations at the adenohypophysial level, behavioural responses accompanying the neuroendocrine changes were studied. 2. Whereas basal (a.m.) plasma corticosterone, but not corticotrophin (adrenocorticotrophic hormone; ACTH), levels were increased on the last day (i.e. on day 22) of pregnancy, the stress-induced rise in both plasma hormone concentrations was increasingly attenuated with the progression of pregnancy beginning on day 15 and reaching a minimum on day 21 compared with virgin control rats. A similar attenuation of responses to both emotional and physical stressors was found in lactating rats. 3. Although the basal plasma oxytocin concentration was elevated in late pregnancy, the stress-induced rise in oxytocin secretion was slightly lower in day 21 pregnant rats. In contrast to vasopressin, oxytocin secretion was increased by forced swimming in virgin and early pregnant rats indicating a differential stress response of these neurohypophysial hormones. 4. The blunted HPA response to stressful stimuli is partly due to alterations at the level of corticotrophs in the adenohypophysis, as ACTH secretion in response to CRH in vivo (40 ng kg-1, i.v.) was reduced with the progression of pregnancy and during lactation. In vitro measurement of cAMP levels in pituitary segments demonstrated reduced basal levels of cAMP and a lower increase after CRH stimulation (10 nM, 10 min) in day 21 pregnant compared with virgin rats, further indicating reduced corticotroph responsiveness to CRH in pregnancy. 5. The reduced pituitary response to CRH in late pregnancy is likely to be a consequence of a reduction in CRH receptor binding as revealed by receptor autoradiography. [125I] CRH binding in the anterior pituitary was significantly reduced in day 11, 17 and 22 pregnant rats compared with virgin controls. 6. Anxiety-related behaviour of the animals as revealed by the time on and entries into the open arms of the elevated plus-maze was different between virgin and pregnant rats with decreased number of entries indicating increased anxiety with the progression of pregnancy (except on pregnancy day 18). The emotional behaviour, however, was not correlated with the neuroendocrine responses. 7. The results indicate that the reduced response of the HPA axis to stressors described previously during lactation is already manifested around day 15 of pregnancy in the rat and involves physiological adaptations at the adenohypophysial level. However, alterations in stressor perception at higher brain levels with the progression of pregnancy may also be involved.

Figures

Figure 1

ACTH (A) and corresponding corticosterone (B) concentrations in plasma collected from the jugular vein of conscious virgin and day 10, 15, 18 and 21 pregnant rats on experimental day 1 under basal conditions (at 09.30 and 10.00 h) and 10 min after exposure to the elevated plus-maze (EPM; 5 min), as well as 5, 15 and 25 min after forced swimming (FS; 90 s, 19 °C). Five days before the experiment, rats were fitted with a jugular vein catheter under halothane anaesthesia and caged singly afterwards. Data are presented as means ±s.e.m.; numbers in parentheses indicate group size. Two-way ANOVA for repeated measures for ACTH (corticosterone); factor: time × group, P < 0.0001 (P < 0.0083). *P < 0.01, virgin and day 10 vs. day 18 and 21; †P < 0.01, day 15 vs. day 21; ‡P < 0.01, day 10 vs. day 18 and 21.

Figure 2

Plasma ACTH (A) and corticosterone (B) concentrations in virgin rats and between days 9 and 12 of lactation under basal conditions and in response to exposure to the elevated plus-maze (EPM) followed by forced swimming (FS) according to the protocol described in the legend to Fig. 1. Data are means ±s.e.m.; numbers in parentheses indicate group size. Two-way ANOVA for repeated measures for ACTH and corticosterone; factor: time × group, P < 0.0001; *P < 0.01, vs. lactating rats. Except for the ACTH level 10 min after EPM in lactating rats, ACTH and corticosterone concentrations were significantly increased at each time point after stimulation (P < 0.01).

Figure 3

Plasma oxytocin concentration in conscious virgin and pregnant rats under basal conditions and 15 min after exposure to forced swimming (FS + 15 min) (A) as described in the legend to Fig. 1 and the calculated ratio between stimulated and basal neuropeptide concentrations for both oxytocin and vasopressin (B). Data are means ±s.e.m.; numbers in parentheses indicate group size. Two-way ANOVA for repeated measures; factor: time, P < 0.0001; factor: group, P < 0.503, factor: time × group, P < 0.7134. *P < 0.05, vs. corresponding basal values; †P < 0.045, vs. virgin, day 10 (Kruskal-Wallis test, P < 0.0027).

Figure 4

Anxiety-related behaviour of virgin and day 10, 15, 18 and 21 pregnant rats on the elevated plus-maze on day 1 of the experiment as indicated by the percentage of time spent on the open arms (□) and by the percentage of entries into the open arms (▪). For details see legend to Fig. 1. One-way ANOVA; percentage time: P < 0.021; †P < 0.01, vs. day 15 and 21. One-way ANOVA; percentage entries: P < 0.027; ‡P < 0.05, vs. day 10, *P < 0.01, vs. virgin, day 10 and day 18. Data are means ±s.e.m.

Figure 5

ACTH (A) and corresponding corticosterone (B) concentrations in plasma collected from the jugular vein of conscious virgin and day 11, 16, 19 and 22 pregnant rats on experimental day 2 in response to i.v. CRH (40 ng kg−1, arrow). For details see legend to Fig. 1. Insets show the area under the curve corrected for baseline (AUC; in arbitrary units (a.u.)). Data are means ±s.e.m.; numbers in parentheses indicate group size. *P < 0.01, virgin and pregnancy day 11 vs. days 19 and 22; †P < 0.05, pregnancy day 11 vs. days 16 and 19; ‡P < 0.01, pregnancy day 22 vs. all other groups; §P < 0.01, **P < 0.05, vs. virgin and day 11.

Figure 6

Specific [125I] CRH binding in the anterior and intermediate lobe of pituitaries collected from virgin, day 11, day 17 and day 22 pregnant rats (n = 6 each). The film was quantified by image analysis to measure silver grain density (arbitrary units; a.u.). Data are means ±s.e.m.*P < 0.05 and **P < 0.01, vs. virgin; †P < 0.05, vs. day 11 and day 17 pregnant groups.