Developmental studies for identification of the inhibitory center of melanotropes in the toad, Bufo japonicus

Two series of experiments were performed to identify the inhibitory center of the melanotropes in the intermediate lobe of hypophysis of the toad, Buf o japonicus. First, developmental changes in the distribution of dopaminerg ic neurons were examined from hatching stage to postmetamorphosis using an antiserum against dopamine synthase (tyrosine hydroxylase, TH). in the post metamorphic toads, TH-positive cell bodies were localized in three clusters . One was the preoptic recess organ (PRO) in the prechiasmatic area, the ot her two were the paraventricular organ (PVO) and infundibular nucleus (IN) in the postchiasmatic area. Each of them exhibited different ontogenetic ch anges. During larval development, TH-positive cell bodies were first detect ed in the PVO and IN at a premetamorphic stage. The number of immunoreactiv e cells increased rapidly in both loci as metamorphosis proceeded, although the two nuclei showed different growth profiles. By contrast, in the PRO, a very small number of immunoreactive cells were observed before the onset of the prometamorphic period. Although the number of immunoreactive neurons increased as metamorphosis progressed, early neurons were confined to the caudal area of the PRO (cPRO), the rostral area of the PRO (rPRO) being dev oid of TH-positive cells. Immunoreactive TH neurons appeared in the rPRO fo r the first time at the end of metamorphic climax, This timing coincided we ll with the development of TH-positive nerve endings in the pars intermedia (Pi) and median eminence. In the second series of experiments, the embryon ic primordium of the PRO was surgically extirpated from open neurulae to ex amine the effects of PRO-ectomy. In 75% of the operated animals, background adaptation was not observed, their dermal melanophores remained permanentl y dispersed even on the white background. Dopaminergic neurons in the rPRO and the immunoreactive nerve endings in the PI and median eminence were sca rcely observed in these animals. it was concluded that the present data str ongly support the hypothesis that rPRO is the center of white-background ad aptation.