Brainβ-Endorphin during Pregnancy, Parturition, and the Postpartum Period*

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Abstract
Brain .beta.-endorphin (.beta.-EP) was measured in the rat during pregnancy, parturition and the postpartum period. .beta.-EP increased in the hypothalamus, midbrain and amygdala during gestation and remained elevated through delivery until 1-2 days postpartum. The concentration of .beta.-EP increased in the hypothalamus from 31.8 .+-. 1.4 (.+-. SE) ng/mg protein in nonpregnant controls to 41.4 .+-. 1.8 and 39.2 .+-. 1.9 during early (8-10 days) and late (18-20 days) pregnancy, respectively, and in the midbrain from 3.20 .+-. 0.17 to 5.21 .+-. 0.30 and 5.25 .+-. 0.64 ng/mg protein (P < 0.01). In another experiment, the brain content of .beta.-EP expressed as nanograms per region, increased from 12.6 .+-. 0.29 to 14.7 .+-. 0.33 in the hypothalamus, from 4.09 .+-. 0.44 to 6.03 .+-. 0.34 in the midbrain, and from 0.93 .+-. 0.11 to 1.32 .+-. 0.06 ng in the amygdala at 16-17 days of gestation compared with that in nonpregnant controls (P < 0.01). When hypothalamic .beta.-EP was measured 1 wk postpartum in lactating and nonlactating rats, a significant decline in the .beta.-EP concentration of both groups was noted compared with that measured during pregnancy; .beta.-EP levels were similar in the lactating and nonlactating rats. Pregnancy and parturition are associated with significant changes in brain .beta.-EP, and apparently .beta.-EP of central rather than peripheral origin may mediate changes in pain perception and maternal behavior during pregnancy.

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