Changes in uterine expression of leukemia inhibitory factor during pregnancy in the Western spotted skunk.

Mutation of the leukemia inhibitory factor (LIF) gene results in reproductive failure in LIF -/- mice due to an inability to implant their blastocysts. This condition is reversed by infusion of LIF or by transferral of embryos to pseudopregnant, wild-type mice. This led us to hypothesize that embryonic diapause in the spotted skunk is due to insufficient uterine expression of LIF whereas resumption of development and implantation are associated with increased LIF expression. We also investigated the hormonal control of LIF expression. Uterine concentrations of LIF mRNA were determined by quantitative reverse transcription-polymerase chain reaction. Changes in cell-specific localization of LIF mRNA and protein were determined by in situ hybridization and immunocytochemistry. LIF mRNA was detected but was not abundant during embryonic diapause; it then increased when blastocysts resumed development and remained elevated prior to implantation. LIF mRNA and protein could not be localized in the uterus during embryonic diapause but were quite apparent in luminal and glandular epithelium during blastocyst activation. Prolactin, progesterone, and estradiol failed to increase uterine concentrations of LIF mRNA above those in ovariectomized controls. These data are consistent with the initial hypothesis and suggest that LIF may somehow be involved in preparing the uterus for implantation in the spotted skunk.

Social relationships and the management of stress.

Two different types of social relationships exist in mammalian social systems: dominance relationships and social bondings. This article shows that both are crucial for the management of stress. The following general conclusions are derived: (1) In stable social systems, established dominance relationships result in predictable behaviour. As a consequence, low positions in the hierarchy do not necessarily lead to enhanced endocrine stress responses. Under conditions of instability, however, distinct increases in the activities of the pituitary-adrenocortical- and the sympathetic-adrenomedullary systems are found; (2) The ability to establish and to respect dominance relationships is a prerequisite to build up stable social systems. Whether this ability is realized, however, depends on social experiences made during behavioural development. The time around puberty seems to be essential for the acquisition of those social skills needed to adapt to unfamiliar conspecifics in a non-stressful and non-aggressive way; (3) Stress responses can be ameliorated by the presence of members of the same species. This phenomenon is called social support. In general, social support cannot be provided by any conspecific, but the ability to give social support is restricted to bonding partners. In most mammalian species mothers are important bonding partners for their infants. In some species bondings also occur between adult individuals; and (4) On a physiological level the bonding partner reduces the activities of the pituitary-adrenocortical- and the sympathetic-adrenomedullary systems. On a psychological level he/she can be regarded as a 'security-giving and arousal-reducing structure'. This is true irrespective of whether the bonding partner is the mother, in the case of an infant, or a male or a female in the case of an adult individual.