Prolactin acts on the hypothalamic–pituitary axis to modulate follicle-stimulating hormone gene expression in the female brushtail possum (Trichosurus vulpecula)

Brushtail possums exhibit a distinct preovulatory pattern of prolactin (Prl) secretion suggesting that Prl is involved in normal reproductive function. In some mammals, Prl is essential for corpus luteum (CL) function and/or modulation of steroidal effects on hypothalamic–pituitary activity. The aim of this study was to test the effects of biologically active recombinant possum Prl (recPosPrl) on both pituitary gland and CL function in possums. To confirm biological activity, administration of recPosPrl-N2C1 (10 μg) resulted in an 18-fold stimulation (P < 0.05) of progesterone (P4) production by possum granulosa cells in vitro. Based on these findings, minipumps containing either recPosPrl-N2C1 (n = 10) or saline (n = 8) were inserted into lactating female possums. The expression levels of pituitary-derived PRL, LHB, FSHB and GNRHR and CL-derived LHR mRNA were quantified. Following a resumption of reproductive activity, no differences in ovulation incidence or plasma Prl concentrations were observed. Plasma Prl levels were less variable (P < 0.001) in Prl-treated possums, confirming a self-regulatory role for Prl in this species. There was a marked down-regulation (P < 0.001) of FSHB mRNA at the mid-luteal stage in Prl-treated possums, whereas mean PRL, LHB, GNRHR and LHR mRNA expression levels were not different between experimental groups. Plasma P4 concentrations were not different (P = 0.05) in Prl-treated possums, although tended to be higher in the peri-ovulatory and early-luteal phase. We conclude in the brushtail possum that Prl is self-regulated via a short-feedback loop common to all mammals studied and is able to modulate FSHB expression probably at the level of the hypothalamus and/or pituitary gland.

Research highlights
► Prl regulation in possums is via a self-regulatory short feedback loop common to mammals.
► Prl acts on the hypothalamic–pituitary axis to modulate FSH gene expression.
► Despite the presence of receptors, Prl does not appear to directly modulate ovarian function.