N^α-acetyl-γ-endorphin is an endogenous non-opioid neuropeptide with biological activity

N^α-acetyl-γ-endorphin (AcγE) was identified in the rat neurointermediate pituitary, based on its immunological properties, comigration with synthetic AcγE on HPLC and resistance to aminopeptidase-M degradation. The peptide appeared to be the main form of γ-endophin (γE) in this tissue and in brain areas remote from the hypothalamus (hippocampus, septum, amygdala). The anterior pituitary, the hypothalamus and the thalamus contained almost exclusively the non-acetylated form of γE. In contrast to γE, AcγE was completely devoid of specific affinity for brain opiate binding sites. Yet, the peptide mimicked γE in that if potently attenuated passive avoidance behaviour in rats, when injected topically into the nucleus accumbens. It is concluded that AcγE is an endogenous neuropeptide with non-opioid biological activity. N^α-acetylation may not merely represent a mechanism for the inactivation of opioid activities of endorphins, but rather allow the organism to select specific sets of biological activities that reside in the endorphin structure.