Evidence that thyrotropin-releasing hormone is not a major prolactin-releasing factor during suckling in the rat

The purpose of this study was to investigate whether TRH could be an important PRL-releasing factor during suckling in the rat. Plasma PRL, TSH, beta-endorphin-like immunoreactivity, and GH responses in serial blood samples from unanesthetized suckled rats were determined. The resulting hormonal profile was compared with that obtained when TRH (500 ng/kg BW, iv) was injected at the onset of suckling. Suckling evoked a rise in plasma levels of PRL, beta-endorphin-like immunoreactivity, and GH, but not in TSH. In contrast, exogenous TRH caused a 9-fold increase in plasma TSH levels during suckling without further increasing the PRL response. Since plasma PRL responses are reportedly enhanced by previous suckling, we also determined plasma PRL and TSH levels when TRH (25 ng/rat, iv) was given 30 min after a brief suckling episode. TRH caused a 2.5-fold increase in plasma TSH, but did not significantly increase plasma PRL levels. Since suckling increases plasma PRL without increasing plasma TSH, and TRH increases TSH but not PRL levels, we conclude that TRH is not a major PRL-releasing factor during suckling.