Serotonergic terminals express a growth associated protein (GAP-43) in the adult rat spinal cord

Dual colour immunofluorescence has been used to compare the distribution of serotonin (5-hydroxytryptamine, 5-HT) and GAP-43 in the adult rat. GAP-43 immunostaining was observed in all spinal cord regions containing 5-HT immunoreactivity. 5-HT and GAP-43 double labelled fibres and varicosities were present and were most evident around motoneurones, in lamina X, and in the intermediolateral cell column. Single labelled GAP-43 fibres and varicosities were also observed and were the dominant population in the dorsal horn and in certain fibre tracts. We conclude that the 5-HT system is one of a small number of spinal cord systems that express high levels of GAP-43 in the adult.