Immunohistochemical distribution of enkephalin, substance P, and somatostatin in the brainstem of the leopard frog, Rana pipiens

The brainstems of frogs contain many of the neurochemicals that are found i n mammals. However, the clustering of nuclei near the ventricles makes it d ifficult to distinguish individual cell groups. We addressed this problem b y combining immunohistochemistry with tract tracing and an analysis of cell morphology to localize neuropeptides within the brainstem of Rana pipiens. We injected a retrograde tracer, Fluoro-Gold, into the spinal cord, and, i n the same frog, processed adjacent sections for immunohistochemical locati on of antibodies to the neuropeptides enkephalin (ENK), substance P (SP), a nd somatostatin (SOM). SOM+ cells were more widespread than cells containin g immunoreactivity (ir) to the other substances. Most reticular nuclei in f rog brainstem contained ir to at least one of these chemicals. Cells with S OM ir were found in nucleus (n.) reticularis pontis oralis, n. reticularis magnocellularis, n. reticularis paragigantocellularis, n. reticularis dorsa lis, the optic tectum, n. interpeduncularis, and n. solitarius. ENK-contain ing cell bodies were found in n. reticularis pontis oralis, n. reticularis dorsalis, the nucleus of the solitary tract, and the tectum. The midbrain c ontained most of the SP+ cells. Six nonreticular nuclei (griseum centrale r hombencephali, n. isthmi, n. profundus mesencephali, n. interpeduncularis, torus semicircularis laminaris, and the tectum) contained ir to one or more of the substances but did not project to the spinal cord. The descending t ract of V, and the rubrospinal, reticulospinal, and solitary tracts contain ed all three peptides as did the n. profundus mesencephali, n. isthmi, and specific tectal layers. Because the distribution of neurochemicals within t he frog brainstem is similar to that of amniotes, our results emphasize the large amount of conservation of structure, biochemistry, and possibly func tion that has occurred in the brainstem, and especially in the phylogenetic ally old reticular formation. (C) 2001 Wiley-Liss, Inc.