Distribution of125I-galanin binding sites, immunoreactive galanin, and its coexistence with 5-hydroxytryptamine in the cat spinal cord: Biochemical, histochemical, and experimental studies at the light and electron microscopic level

Abstract

The distribution of galanin‐like immunoreactivity (GAL‐LI) in the spinal cord of the cat was studied by use of indirect histochemistry and the peroxidase‐antiperoxidase (PAP) technique. In the ventral horn GAL‐immunoreactive (IR) axonal fibers and terminals were most frequent in the ventral part of the motor nucleus. The GAL‐IR axons also contained 5‐hydroxytryptamine (5‐HT)‐LI, and they disappeared after spinal cord transection. It was concluded that these GAL‐IR fibers belong to the serotoninergic bulbospinal pathway. In the medulla oblongata from normal cats, scattered GAL‐IR cell bodies were encountered within the nucleus raphe obscurus and nucleus raphe pallidus. Electron microscopic observations revealed that the fine structure of the GAL‐IR axonal boutons in the motor nucleus was similar to that of 5‐HT‐IR boutons with a varying number of immunoreactive large dense core vesicles. The postsynaptic element in all cases studied was a dendrite. A dense GAL‐IR axonal plexus was found in the superficial laminae I‐II of the dorsal horn. Coexistence was found between the GAL‐ and substance P‐LI in fibers within the dorsal horn plexus. Spinal cord transection did not alter the pattern of GAL‐LI in the dorsal horn, while the vast majority of GAL‐IR axonal swellings disappeared following dorsal root sectioning. Electron microscopic observations in lamina II (substantia gelatinosa) revealed that the GAL‐IR axonal terminals could be divided into two main groups. One with small to medium‐sized axonal buttons formed synaptic contacts with both dendritic and axonal profiles. The other formed the central axon terminals of glomeruli, suggesting that GAL‐LI may be present in C‐type primary afferents. Numerous small GAL‐IR cell bodies were encountered in laminae II and III. GAL‐IR cell bodies were also observed in lamina X. The dorsal root ganglia contained a low but consistent number of small to medium‐sized GAL‐IR cell bodies, which all contained immunoreactive calcitonin gene‐related peptide (CGRP). Following peripheral sciatic nerve transection, the number and the labeling intensity of GAL‐IR cell bodies in the corresponding dorsal root ganglia showed a moderate increase. Radioimmunoassay revealed that the concentration of GAL‐LI increased along the rostrocaudal axis of the normal spinal cord, and was about three times higher in the dorsal than in the ventral regions. The concentration in the dorsal root ganglia was intermediate to those seen in the corresponding dorsal and ventral cord regions. Autoradiography showed a high density of ¹²⁵I‐GAL binding sites in the superficial laminae of the dorsal horn throughout the spinal cord.