[Met5]enkephalin concentrations in rat pituitary are maintained under opioid inhibition

[Met5]Enkephalin concentrations in rat pituitary were examined following the administration of the opiate antagonist naloxone 5 mg/kg per day as single daily s.c. injections for 9 days. Compared to vehicle administration, there was a significant increase of [Met5]enkephalin concentrations in both pituitary lobes, ranging from 50-90% above control values. Similar treatment with the opioid agonist [D-Ala2,MePhe4,Met(o)5ol]enkephalin in doses up to 500 micrograms/kg per day did not result in any alteration of [Met5]enkephalin levels. These data suggest that opioid receptor mechanisms play a role in maintaining [Met5]enkephalin concentrations in both neurointermediate and anterior pituitary.