Impaired hypothalamic leptin sensitivity in pseudopregnant rats treated with chronic prolactin to mimic pregnancy

Abstract

Pregnancy in rodents is associated with hyperphagia, increased fat deposition, elevated leptin concentrations and insensitivity to the satiety action of leptin. To investigate the hormonal mechanisms involved in the development of this state of pregnancy‐induced leptin resistance, we have used a pseudopregnancy rat model. We have previously demonstrated that pseudopregnant rats have a normal feeding response to leptin, but if pseudopregnancy is extended using chronic i.c.v. ovine prolactin infusion along with progesterone implants, then leptin no longer suppresses food intake. In this study, we aimed to investigate the effect of chronically high lactogen levels, as seen in mid pregnancy, on leptin‐induced activation of hypothalamic JAK/STAT signal transduction and mRNA expression of leptin (LepR‐B) and prolactin (Prlr‐L) receptors, using pseudopregnant rats chronically infused with ovine prolactin. Groups of virgin (diestrous) and pseudopregnant rats were treated with chronic i.c.v. infusion of either prolactin (2.5 μg/μl/hr for 5 days) or vehicle (aCSF) via a minipump connected to a cannula surgically implanted into the lateral ventricle. Rats were fasted overnight and then received an i.c.v. injection of leptin (400 ng) or vehicle (aCSF), and perfused 30 minutes later. In chronic vehicle‐infused pseudopregnant rats, i.c.v. leptin increased the number of phosphorylated STAT3 positive cells in the arcuate nucleus and ventromedial nucleus (VMH) of the hypothalamus, similar to all acute‐leptin treated virgin groups. This effect of leptin, however, was not observed in the pseudopregnant rats that were chronically infused with prolactin. Quantitative PCR analysis also showed decreased expression of LepR‐B in the arcuate and VMH nuclei, and decreased Prlr‐L in the arcuate nucleus of prolactin‐infused EPSP rats. These data suggest that the attenuation of the leptin‐induced suppression of food intake caused by chronically high lactogen levels in pseudopregnant rats is associated with impaired leptin‐induced activation of the JAK/STAT pathway in specific hypothalamic nuclei.

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