Opiate Modulation of the Anterior Pituitary Hormone Response during Suckling in the Rat*

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Abstract
The influence of endogenous opiates on hormone responses was investigated during suckling in the rat. In complementary experiments, opiate receptors were blocked by naloxone (NAL) or endogenous opiate release from the pituitary was inhibited by dexamethasone (DEX). Serial blood samples from unanesthetized suckled rats were then assayed for plasma PRL [prolactin], .beta.-endorphin-like immunoreactivity (.beta.-END-LI), TSH and GH [growth hormone] levels. Identical studies were also done in saline-treated (control) suckled rats and in unsuckled rats exposed to control objects. Whereas suckling caused a rise in plasma PRL, .beta.-END-LI and GH, introduction of plastic control objects did not elevate hormone levels. NAL blocked the GH rise and depressed TSH levels, but did not significantly inhibit the PRL or .beta.-END-LI response. DEX prevented the .beta.-END-LI rise and blocked the GH rise, but did not inhibit TSH, DEX enhanced PRL release during suckling. The responses of .beta.-END-LI, PRL and GH are not an artifact of the sampling procedure; PRL release during suckling is independent of .beta.-END-LI release by the pituitary; and suckling stimulates the release of ACTH, .beta.-END, and .beta.-lipotropin from the anterior pituitary. The results are consistent with both a role of pituitary .beta.-END in the control of GH and a role of corticosterone in the control of PRL during suckling.