Changes in methionine-enkephalin levels in specific rat brain regions following repeated treatment with selective dopaminergic agonists and antagonists

Abstract

The present study was conducted to investigate the effects of repeated treatment with selective dopaminergic agents on the level of methionine-enkephalin (Met-Enk) in rat brain cortex (CTX), hypothalamus, (HYPO), hippocampus (HIPP) and midbrain (MID). Male Sprague-Dawley rats were kept under controlled conditions for at least one week. After adaptation period, rats were randomly assigned into nine groups (7–9 rats per group) for intraperitoneal treatment with dopaminergic agents. Group 1 served as control, while, groups 2, 3 and 4 were treated with either SKF-81297, SCH 23390 or their combination, respectively. Groups 5, 6 and 7 received either LY 171555, (-)-sulpiride or their combination, whereas, groups 8 and 9 were treated with nomifensine or selegiline, respectively. One hour after the last injection, rats were sacrificed, brains were removed and dissected into different regions, then extracted and their Met-Enk levels determined by radioimmunoassay (RIA). Administration of SKF-81297 or SCH 23390 significantly elevated Met-Enk levels in all brain regions examined, while their combination elevated Met-Enk levels in HYPO and HIPP only. On the other hand, treatment with LY 171555 or (-)-sulpiride, but not their combination, markedly increased Met-Enk content in all brain regions examined. Treatment with nomifensine increased Met-Enk levels in all brain regions investigated, whereas, treatment with selegiline significantly elevated Met-Enk in HYPO, HIPP and MID but not in CTX. These findings clearly indicate that dopaminergic agonists and antagonists alter Met-Enk levels in specific rat brain regions.