Different localizations of growth-associated protein (GAP-43) in mechanoreceptors and free nerve endings of adult rat periodontal ligament, dental pulp and skin.

Distributions of growth-associated protein-43 (GAP-43) in the periodontal ligament and dental pulp of adult rats were studied by light and electron microscopy. The mature periodontal ligament and dental pulp contained numerous GAP-43-positive neural elements, comprising periodontal Ruffini endings and thin nerve fibers, but expression patterns differed among the kinds of nerves. In the periodontal ligament of rat molars, immunoelectron microscopy revealed that GAP-43 like immunoreactivity in the Ruffini ending, an essential mechanoreceptor, was confined to the Schwann sheaths around the axon terminals and was not in the axon terminals themselves, unlike free endings that revealed axonal GAP-43. However, the lamellar Schwann cells associated with the cutaneous receptors did not exhibit any GAP-43 like immunoreactivity though they were intensely reactive for low affinity nerve growth factor receptor (p75-NGFR), a marker for lamellar Schwann cells in mechanoreceptors. The characteristically uniform expression of GAP-43 in the Schwann lamellae that surround the Ruffini mechanoreceptors of rat molar ligament suggests that Schwann cells are involved in the GAP-43 mediated plasticity of these receptors. On the other hand, the pulpal nerves were filled with the reaction products in their axonal spaces, suggesting the potential for neuronal plasticity during normal function and after tooth injury.