Central monoaminergic changes induced by morphine in hypoalgesic and hyperalgesic strains of domestic fowl

The present research examined morphine dose-response effects on both the formalin test and on CNS monoamine (MA) levels and the metabolites dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) in hypoalgesic (76) and hyperalgesic (SN) strains of domestic fowl. Morphine produced a significant hypoalgesic response in the 76 strain at 15-45 mg/kg and a significant hyperalgesic response in the SN strain at 5-10 mg/kg. In subsequent experiments, analyses of whole brain (minus tectum), brainstem, and spinal cord MA, DOPAC, and 5-HIAA via high-performance liquid chromatography with electrochemical detection (HPLC-EC) were performed following morphine administration in both the 76 and SN strains. Morphine produced a significant elevation of brain dopamine (DA) and a significant elevation of brain, brainstem, and spinal cord serotonin (5-HT) in both the 76 and SN strains. Morphine elevated brain norepinephrine (NE) in the 76 strain. However, morphine failed to affect brain NE in the SN strain. This distinct morphine effect on brain NE differentiates strain-dependent hypoalgesia and hyperalgesia in domestic fowl.