Evidence for enkephalin- and endorphin-immunoreactive cells in the anterior pituitary of the axolotl Ambystoma mexicanum.

An immunohistochemical study of opioid peptides in the hypophysis of the axolotl, Ambystoma mexicanum, was carried out with antisera against leu-enkephalin, beta-endorphin, met-enkephalin, and dynorphin A (1-8). We found leu-enkephalin immunoreactivity in some fibers of the neural lobe and the median eminence. In contrast to previous reports on mammals and other vertebrates, we found leu-enkephalin immunoreactivity in many cells scattered throughout the anterior lobe. As in other vertebrates, the beta-endorphin immunoreactivity was present in all the cells of the intermediate lobe and in a few cells of the anterior lobe. Met-enkephalin and dynorphin A (1-8) immunoreactivities were only present in the neural lobe and the median eminence. The presence of leu-enkephalin and beta-endorphin in the anterior lobe suggests that these peptides could be acting as hormones released from the hypophysis of the unmetamorphosed amphibian.