Changes in the levels of several endogenous opioid peptides in dog cerebrospinal fluid following morphine administration.

Abstract

The cisterna magna of dogs anesthetized with sodium Surital was fitted with a cannula, and cerebrospinal fluid (CSF) was withdrawn before (control) and one hour after the s.c. injection of 10 mg/kg of morphine sulfate (morphine). The CSF from control and morphine-treated dogs was purified initially by gel filtration. Each fraction was submitted to opiate bioassay procedures, followed by high performance liquid chromatography (HPLC) purification on a mu-Bondapak C18 column. Two of the CSF fractions from HPLC purification showed greater opiate-like activity after morphine treatment than that in controls. One fraction contained morphine, the other an unknown peptide. This latter fraction produced a dose-dependent effect in the mouse tail-flick test. This fraction did not show radioimmunoreactivity to methionine (met)- or leucine (leu)-enkephalins, but showed a small amount of reactivity to beta-endorphin and dynorphin (1-13). Further purification of this fraction by HPLC yielded a fraction with five peaks, which upon amino acid analysis were found to contain small peptides. Met- and Leu-enkephalins, beta-endorphin and dynorphin (1-13)-like immunoreactivity in the fraction in which the respective standard was eluted by HPLC was significantly increased after a single administration of morphine. Based on these results, it is suggested that morphine at an antinociceptive dose causes the release of endogenous opioid peptides and may also stimulate the biosynthesis of their precursor molecules, pre-pro-opiomelanocortin, pre-pro-enkephalin A and pre-pro-enkephalin B.