Pituitary somatotrophs contain [Met]enkephalin-like immunoreactivity

Serial sections from epoxy resin-embedded rat anterior pituitaries were sequentially immunostained for endorphin, [Met]enkephalin, and growth hormone, respectively. We found that [Met]enkephalin immunoreactivity was confined to the growth hormone producing cells. Corticotropin/endorphin cells in the anterior pituitary from both normal and adrenalectomized rats did not contain any [Met]enkephalin immunoreactivity. When anterior pituitary cells were maintained in monolayer culture for 10 days, [Met]enkephalin immunoreactivity was still located in the growth hormone-producing cells although the staining was weaker than in the somatotrophs in pituitary tissue fixed immediately after death of the animals. This suggested that somatotrophs synthesize [Met]enkephalin. However, this cannot be proved conclusively until biosynthesis experiments have been performed. The following conclusions were drawn from these findings. (i) Anterior pituitary [Met]enkephalin is not an extraction artifact derived from beta-endorphin with which it shares the NH2-terminal pentapeptide sequence. (ii) In the anterior pituitary, beta-endorphin is not the precursor to [Met]enkephalin. [Met]Enkephalin in somatotrophs may be of brain origin and in the somatotrophs may be bound to intracellllar receptors as has been shown for luteotropin releasing hormone in gonadotropic cells.