Central changes of β-endorphin-like immunoreactivity during rat tonic pain differ from those of purified β-endorphin

Several studies have described changes in β-endorphin-like immunoreactivity (β-ELI) in the rat brain in response to pain and stress stimuli. In order to ascertain the components of β-ELI, brain samples of rats experiencing acute prolonged (tonic) pain were evaluated for their β-ELI and later submitted to a Chromatographie purification allowing the measurement of β-endorphin (β-EP) and acetyl β-EP. The Chromatographie analysis of both ventromedial hypothalamus (VMH) and periaqueductal grey (PAG) homogenates indicates that β-ELI is distributed in several fractions including shortened forms of β-EP and their respective acetylated compounds. Quantitatively, while β-ELI in formalin-injected animals was increased by 48% in VMH and 45% in PAG in respect to controls, the net increase of purified β-EP was 1100% and 470%, respectively, for VMH and PAG. Moreover, the maximal increase of β-ELI was evident at 120 min, in both tissues. In contrast, the β-EP peak was reached at 30 min in VMH and at 60 min in PAG. Acetyl β-EP was unchanged by treatment in both central areas. No correlation of β-ELI and β-EP was found in VMH. These data demonstrate that the evaluation of β-ELI gives a poor estimate of β-EP changes, due to several components of the endorphin family.