Opiates act centrally on GH and PRL release.

This study was conducted in order to further understand whether the effect of opiates on growth hormone and prolactin release was exerted centrally or at least in part peripherally. Male rats were treated with morphine-HCl after pretreatment with saline with either the opiate antagonist naloxone-HCl or the quaternary derivative naloxone methyl bromide (Naloxone-Br), the latter of which does not cross the blood brain barrier. Morphine-HCl elicited a clear cut increase in prolactin and growth hormone release after pretreatment with naloxone-Br, but not after pretreatment with naloxone-HCl. Naloxone-Br, however, was able to inhibit the effect of morphine when administered directly in the brain ventricles. To further confirm these results, we administered the quaternary derivative morphine-methyl-iodide (morphine-I), which unlike morphine-HCl, does not cross the blood brain barrier. Morphine-I was ineffective in eliciting growth hormone and prolactin release when administered peripherally, but was effective when administered intraventricularly.