Expression of GAP-43 mRNA in the Adult Carp Central Nervous System

Abstract

The distribution of neurons which express the gene for the growth-associated protein, GAP-43, in the adult carp central nervous system (CNS) was studied by in situ hybridization using newly formed RNA probes for carp GAP-43 mRNA. A great number of neurons heavily labeled by the 35 S-labeled antisense probe were found in the telencephalon, diencephalon, mesencephalon, optic tectum, pontine area, medulla oblongata and spinal cord. Motoneurons of the cranial nerves, i.e., the oculomotor, trochlear, trigeminal and spinal motor nerves, also strongly expressed GAP-43 mRNA, in contrast to the low level of GAP-43 signals in the motoneurons in the adult mammalian CNS. These results suggested that synaptogenesis and continuous synaptic reorganization might normally occur in the adult carp nervous system, since GAP-43 protein is generally accepted to be essential for the dynamic growth of axonal processes which leads to synaptogenesis.In the mature skeletal muscle of the adult carp, a number of small-sized neuromuscular junctions (NMJs), which were visualized with acetylcholinesterase (AchE) histochemistry, were detected on each muscle fiber. This polyinnervation pattern was similar to that of the immature muscle of mammalian embryos. These findings indicate that, unlike mammalian muscles, maturation of carp muscles is not accompanied by the synapse elimination which is thought to be coupled with the down-regulation of motoneuron GAP-43. NMJs of the adult carp muscle are supposed to be continuously reorganized, keeping the motoneurons expressing GAP-43.The expression of GAP-43 under physiological conditions in the adult carp CNS may facilitate axonal regeneration in various kinds of carp CNS neurons.