The failure of L-prolyl-L-leucylglycinamide to inhibit the release of alpha-melanocyte-stimulating hormone in the rat.

We have studied the effect of L-prolyl-L-leucyglycinamide (PLG) on the release of melanocyte-stimulating hormone (MSH) from the rat pituitary gland both in vitro and in vivo. Superfusion of neurointermediate lobes with medium containing PLG or PLG-acetate (dose range 1-1000 ng/ml) did not affect the spontaneous release of MSH as determined by radioimmunoassay or by bioassay. Also during static incubations of rat hemipituitaries, PLG (6.6-50 ng/ml) was without effect on alpha-MSH release. Administration of PLG (1-250 ng iv) to intact male and female rats and rats being lesions in the medio-basal hypothalamus did not induce a fall in serum alpha-MSH levels. It is concluded that PLG does not affect the release of alpha-MSH from the rat intermediate lobe. It is unlikely therefore that PLG mediates the inhibitory influence of the CNS on MSH release.