Stereological analysis of the effects of 6-hydroxydopamine on the ultrastructure of the melanocyte-stimulating hormone cell of the pars intermedia of the pituitary of Xenopus laevis.

It has been confirmed that intracisternal administration of the sympatholytic compound 6-hydroxydopamine (6-OHDA) to the toad Xenopus brings about degenerative changes in the appearance of the catecholaminergic innervation of the pars intermedia of the pituitary. This observation has been extended by stereological analysis of the ultrastructural composition of the presumed melanocyte-stimulating hormone (MSH) cells of the pars intermedia, which reveals that following the administration of this compound there is an increase in the amount of rough endoplasmic reticulum and a decrease in the numbers of secretory granules. These changes are consistent with increased secretory activity and are considered to reflect the removal of an inhibitory catecholaminergic influence from the MSH cells. 6-OHDA also brings about a prolonged darkening of the animals, but it is suggested that this cannot be due entirely to increased circulating levels of MSH in view of the finding that 6-OHDA has a pituitary-independent effect on pigmentation, as demonstrated by the administration of 6-OHDA to hypophysectomised animals when a transitory darkening occurs.