Impaired prolactin transport into the brain and functional responses to prolactin in aged male mice

Ageing is related to changes in a number of endocrine systems that impact on the central actions of hormones. The anterior pituitary hormone prolactin is present in the circulation in both males and females, with widespread expression of the prolactin receptor throughout the forebrain. We aimed to investigate prolactin transport into the brain, as well as circulating levels of prolactin and functional responses to prolactin, in aged male mice (23 months). Transport of ¹²⁵I‐labelled prolactin (¹²⁵I‐prolactin) from the peripheral circulation into the brain was suppressed in aged compared to young adult (4 months) male mice, with no significant transport into the brain occurring in aged males. We subsequently investigated changes in the negative‐feedback regulation of prolactin secretion and prolactin‐induced suppression of luteinising hormone (LH) pulsatile secretion in aged male mice. Feedback regulation of prolactin secretion appeared to be unaffected in aged males, with no change in levels of circulating prolactin, and normal prolactin‐induced phosphorylated signal transducer and activator of transcription 5(pSTAT5) immunoreactivity in tuberoinfundibular dopaminergic (TIDA) neurones in the arcuate nucleus. There were, however, significant impairments in the ability of prolactin to suppress LH pulsatile secretion in aged males. In young adult males, acute prolactin administration significantly decreased LH pulses from 1.5 ± 0.19 pulses of LH in 4 hours to 0.5 ± 0.27 pulses. In contrast, prolactin did not suppress LH pulse frequency in aged males, with prolactin leading to an increase in mean LH concentration. These data demonstrate the emergence of impairments in prolactin transport into the brain and deficits in specific functional responses to prolactin with ageing.