Met-enkephalin induces fast synaptic plasticity of magnocellular neurons in the rat supraoptic nucleus.

A morphometric-ultrastructural study was made of the supraoptic nucleus of rats of both sexes following central administration of met-enkephalin. Ten minutes after met-enkephalin treatment the number of axo-somatic synapses was significantly increased. This effect was more pronounced in female rats than in males and could be prevented by preceding administration of naloxone. Animals that received naloxone followed by met-enkephalin showed a dilation of the rough endoplasmic reticulum into a vesicular shape. Our results provide preliminary evidence for a fast remodeling of synaptic input to magnocellular hypothalamic neurons. It is likely that the known inhibitory action of opioids on the hypothalamo-neurohypophysial system is partly mediated by this plasticity.