FMRFamide in the amphibian brain: A comprehensive survey

Mapping of FMRFamidergic neural circuitry in the amphibian brain has been d one by immunohistochemical methods. Comparative evidence suggests that ther e are similarities and differences in the overall pattern of distribution o f FMRFamide-ir elements in the brain among the three amphibian orders and w ithin each order. FMRFamide is expressed in neurons in some circumscribed a reas of the brain. A part of these neurons is concentrated in classical neu rosecretory areas of the hypothalamus in a bilaterally symmetrical fashion. Similar neurons occur occasionally in the midbrain, but are virtually abse nt from the hindbrain. Anurans are unique among amphibians to show FMRFamid e neurons in the medial septum and diagonal band of Broca. A viviparous gym nophione is known to possess a small population of such neurons in the dors al thalamus. Together, the FMRFamide neurons contribute to an extensive fib er network throughout the amphibian brain. Descriptive developmental studie s suggest that the rostral forebrain-located FMRFamide neurons originate in the olfactory placode and then migrate into the brain along the route of t he vomeronasal-olfactory-terminal nerve complex. Olfactory placodal ablatio n in an anuran and a urodele provide experimental support to this contentio n. Other FMRFamide neuronal cell groups, in the hypothalamus and dorsal tha lamus, are supposed to arise fi-om non-placodal precursors. The neuroanatom ical distribution (projection of immunoreactive processes to areas of the f ore-, mid-, and hindbrain as well as to cerebrospinal fluid, co-localizatio n with other neuropeptides, and presence in the median eminence) has furnis hed morphological correlates of possible functions of FMRFamide in the amph ibian CNS. While amphibian FMRFamide-like or structurally related peptides remain to be isolated and characterized, the sum of the distribution patter n of FMRFamide-like immunoreactivity suggests that it may act as a neurotra nsmitter or a neuromodulator, and also may have endocrine regulatory functi ons. (C) 2001 Wiley-Liss, Inc.