Effect of minute amounts of [ d-Ala 2,MePhe 4,Gly(ol) 5]enkephalin injected into the tuberomammillary nucleus of rats on histamine release from the cerebral cortex

Abstract

[ d-Ala 2,MePhe 4,Gly(ol) 5]enkephalin (DAGO) (10 ng/0.5 μ1 saline solution) injected into the tuberomammillary nucleus (TM) of rats in minute amounts decreased the amount of histamine released to approximately 50% of the basal value on measurements taken 20–40 min after administration. This effect of DAGO was inhibited by the simultaneous microinjection of naloxone (320 ng). These results may be explained in two ways. The first is that the stimulation of μ-receptors results in the inhibition of histaminergic cell bodies. The second is that the somatodendritic release of histamine was increased by the stimulation of μ-receptors and as a result of increased histamine concentration in TM, many histaminergic neurons may be inhibited through the stimulation of H 3-receptors. Further studies are necessary regarding the influence of μ-agonists on various cellular sites of histaminergic neurons.