Using the indirect antibody peroxidase-antiperoxidase method of Sternberger, we localized substance P (SP), somatostatin (SOM), enkephalin (ENK), and serotonin (5HT, 5-hydroxytryptamine) in the spinal cord of Rana pipiens. This is the first study to demonstrate all four substances in adjacent sections of frog spinal cord. The distribution patterns of ENK, SP, SOM, and 5HT in our study differ from that described for laminae I and II in amniotes. A high density of ENK, SP, and SOM fibers is present in a band ventral to the dorsal terminal field of cutaneous primary afferent fibers and slightly overlapping the ventral terminal field of muscle primary afferent fibers. However, a high density of 5HT fibers is present in the dorsal terminal field.

Dorsal roots were transected and filled with horseradish peroxidase to identify the primary afferent axons in Lissauer's tract (LT) of the spinal cord in a ranid frog with light and electron microscopy. Axons in LT could be traced at least 2–3 segments rostral and caudal to the level of the filled root. Some axons in LT entered a nucleus in the dorsal lateral funiculus at brachial and lumbar levels, while other axons terminated in widespread regions of the dorsal horn. Electron microscopy revealed unlabeled terminals with numerous large, dense-cored vesicles in LT. However, large vesicles were rarely observed and were never abundant in labelled primary afferent axons. S type synaptic contacts between labelled axons and unlabelled profiles were observed. The presynaptic axons in these synapses contained many small, spherical vesicles. The cross-sectional area of LT was related to the spinal cord level, with the largest area at the brachial level, an intermediate area at the lumbar level, and the smallest area at the thoracic level. No difference in number of labelled axons was observed in the medial and lateral parts of LT.

A lateral and a medial motoneuron in the brachial spinal cord of the leopard frog, Rana pipiens, were labeled by horseradish peroxidase applied to the ventral root. Their dendritic trees were traced, analyzed, and plotted using a computer-microscope system. Some dendrites of the medial motoneuron crossed the midline of the spinal cord, but no dendrites of the lateral motoneuron crossed the midline. Nevertheless, the total dendritic length of the lateral motoneuron exceeded that of the medial motoneuron. The peak number of dendritic branch segments of the medial motoneuron was located at a greater distance from its soma than that of the lateral motoneuron. Three-dimensional reconstruction and rotation of the dendritic trees revealed that the dendrites of the medial motoneuron had a greater rostrocaudal extent than those of the lateral motoneuron. When compared to reports of Golgi-impregnated motoneurons our results suggest that the HRP technique labels dendrites more completely. However, use of the HRP technique may introduce greater errors in the subsequent measurement of dendritic segments due to nonuniform tissue shrinkage.

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