Ontogeny of pituitary adenylate cyclase-activating polypeptide (PACAP) in the frog (Rana ridibunda) tadpole brain: Immunohistochemical localization and biochemical characterization

The anatomic distribution and biochemical characteristics of the neuropepti de pituitary adenylate cyclase-activating polypeptide (PACAP) were investig ated in the central nervous system of the frog, Rana ridibunda, during deve lopment. Three to four days after hatching, at stages TV-VII, PACAP-immunor eactive perikarya were detected in the dorsal thalamus within the anterior ventral area, and a few fibers were found in the medial pallium. Positive c ell bodies were first observed in the hypothalamus at stages VIII-M, at the level of the dorsal and ventral infundibular nuclei. In these regions, the number of positive perikarya increased during ontogeny. In tadpoles, durin g the mid- and late premetamorphosis, a more complex organization of the PA CAP-immunoreactive system was found in the thalamus with the appearance, at stages M-XII, of two additional groups of positive neurons in the ventrola teral area and posterocentral nucleus. At stages XIII-XVIII of larval devel opment and subsequent larval stages, PACAP-immunoreactive fibers were found in the median eminence. In newly metamorphosed animals, several additional groups of positive perikarya appeared in the medial pallium, the preoptic nucleus, the torus semicircularis, the tegmentum of the mesencephalon, and the cerebellum. The immunoreactive peptide contained in the tadpole brain w as characterized by high performance liquid chromatography analysis combine d with radioimmunoassay quantification. At all stages investigated, the pre dominant form of PACAP-immunoreactive material coeluted with synthetic frog PACAP38. The occurrence of PACAP soon after hatching indicates that the pe ptide may exert neurotrophic activities. The existence of immunoreactive el ements in several thalamic regions at mid- and late premetamorphic stages s uggests that PACAP may act as a neurotransmitter, neuromodulator, or both, during ontogenesis. Finally, the presence of PACAP immunoreactive perikarya in hypothalamic nuclei and nerve fibers in the median eminence supports th e view that PACAP may play a role in the control of pituitary hormone secre tion during larval development. J. Comp. Neurol. 431:11-27, 2001. (C) 2001 Wiley-Liss, Inc.