Tactile allodynia is characterized by normally non-noxious cutaneous stimuli that become abnormally very painful, and can often occur in association with peripheral nerve injury. It has been attributed partly to loss of nociceptive C-fiber synapses with concommitant sprouting and synaptogenesis of mechanoreceptive Ag-fibers into spinal lamina II. Our long-term goal is to characterize molecular mechanisms of axon sprouting, target cell recognition and synapse formation in mature spinal cord following injury. The classic cadherins are a family of synaptically enriched cell adhesion molecules which have been strongly linked to synapse formation, targeting and plasticity in other parts of the CNS during development. Here, we examine the hypothesis that regulation of cadherin localization and function is a critical molecular component of intraspinal sprouting and synaptic reorganization induced by sciatic nerve injury in adult rats. Initial studies demonstrate multiple classic cadherins are expressed in spinal cord; are differentially distributed across spinal layers and cell types; and are mostly synaptically localized. To extend this, in Aim 1, the normal cellular and synaptic distribution of classic cadherins will be characterized by a combination of in situ hybridization, immunocytochemistry and combined immunofluorescent localization of cadherins to selectively labeled synaptic terminations of AB- and C-fibers in dorsal horn. Confocal microscopy will be used to determine which classic cadherin (s) associate with AB-fiber and C-fiber synapses.
Aim 2 will test the hypothesis that changes in cadherin mRNA expression and recruitment of cadherin protein to newly formed synapses is a component of the molecular mechanisms of injury-induced plasticity of dorsal horn circuitry. Adult rats will be subjected to sciatic nerve crush, and a combination of semi-quantitative RT-PCR and immunoblotting, in situ hybridization, immunocytochemistry, and direct labeling of Ag-fibers will be used to elucidate changes in cadherin mRNA and protein levels of expression by spinal cord and DRG neurons in response to injury, the time-course of such changes, and the identity of the cadherin(s) present at the aberrant synapses formed by sprouted AB axons. These studies will reveal how cadherin adhesion molecules participate in the molecular events that underlie axon sprouting and new synapse formation in the adult spinal cord induced by peripheral nerve injury. The work here will contribute to an understanding of how to prevent such maladaptive plasticity of the kind, which may underlie tactile allodynia.
