Development and distribution of FMRFamide-like immunoreactivity in the toad (Bufo bufo) brain

By using immunohistochemistry, we studied the development and distribution of the FMRFamide-like immunoreactive (ir) neuronal system in the toad brain during the ontogeny. In addition to this, experimental evidence was provid ed to show that the rostral forebrain-located FMRFamide neurons originate i n the olfactory placode and then migrate into the brain along the olfactory pathway. During early development, within the brain, FMRFamide-ir perikary a first appeared in the periventricular hypothalamus. Later in development, FMRFamide-ir cells were visualized in the rostralmost forebrain simultaneo usly with similar ir cells in the developing olfactory mucosa. Selective ab lation of the olfactory placode(s), prior to the appearance of the first FM RFamide-ir cells in the brain, resulted in the total absence of ir cells in the telencephalon (medial septum and mediobasal telencephalon) of the oper ated sides(s). The preoptic-suprachiasmatic-infundibular hypothalamus-locat ed FMRFamide-ir neurons were not affected by olfactory placodectomy, arguin g that they do not originate in the placode. This result points to the plac ode as the sole source of such neurons in the rostral forebrain. (C) 2001 E lsevier Science B.V. All rights reserved.