Species-specific processing of prodynorphin in the posterior pituitary of mammals

The steady state levels of the prodynorphin-derived opioid peptides, dynorphin A, dynorphin B, and alpha-neo-endorphin, have been extensively studied in the magnocellular/posterior pituitary system of the rat. To determine whether the rat system serves as a general model for prodynorphin processing in the mammalian posterior pituitary, we examined the steady state levels of prodynorphin-derived opioid peptides in the posterior pituitary systems of representatives of three diverse orders of mammals: guinea pig (order Rodentia), pig (order Artiodactyla), and rhesus monkey (order Primates). In each species studied there was evidence for species-specific deviations from the rat model. The most pronounced differences were observed with respect to the conversion of dynorphin A-(1-17) to dynorphin A-(1-8). In rodents, rats, and guinea pigs, under steady state conditions, the molar ratios of these forms are approximately 1:2 and 2.5:1, respectively. However, in the pig and rhesus monkey, the molar ratios of these forms are 10:1 and 14:1, respectively. Thus, under steady state conditions, the conversion of dynorphin A-(1-17) to dynorphin A-(1-8) appears to be a minor event in porcine and rhesus monkey posterior pituitary. Species-specific variations were also observed with respect to the steady state levels of alpha-neo-endorphin and dynorphin B-(1-13). In addition, the results of these studies suggest that the conversion of prodynorphin-derived opioids to leu-enkephalin probably represents a minor event in the species studied.