RESUMEN
It has been suggested that altered retrograde neurotrophin support contributes to the phenotypic switch observed in BDNF expression in injured sensory neurons. Thus, modulatory influences of NGF and NT-3 on BDNF expression in injured adult rat DRG neurons were examined using in situ hybridization and immunohistochemical approaches. Quantitative analysis reveals a biphasic response to sciatic nerve injury, whereby in the first day following injury, BDNF expression is up-regulated in approximately 83% of injured neurons including all small neurons, and a larger pool of trkB expressing neurons than in intact. By 1 week and up to 3 weeks later expression is still seen in approximately 66% of injured neurons, but the characteristic phenotypic switch in the subpopulations expressing BDNF occurs, whereby expression in the trkA population is reduced and expression in trkB- and in trkC-positive neurons is elevated. NGF infusion results in elevated levels of BDNF expression in both intact and injured trkA-positive neurons, accompanied by reduced trkB expression. NT-3 acts in an opposite fashion effecting a down-regulation in BDNF expression in intact neurons and preventing/reducing the injury-associated increases in BDNF expression in both trkC- and nontrkC-expressing subpopulations of injured neurons. These effects suggest NGF can regulate BDNF expression in trkA-expressing neurons regardless of the axonal state and that elevated levels of BDNF may contribute to the down-regulation in trkB expression associated with these states. Furthermore, the findings demonstrate that NT-3 can act in an antagonistic fashion to NGF in the regulation of BDNF expression in intact neurons, and mitigate BDNF's expression in injured neurons.
Asunto(s)
Factor Neurotrófico Derivado del Encéfalo/metabolismo , Ganglios Espinales/lesiones , Factor de Crecimiento Nervioso/metabolismo , Neuronas Aferentes/metabolismo , Neurotrofina 3/metabolismo , Nervio Ciático/lesiones , Animales , Factor Neurotrófico Derivado del Encéfalo/genética , Tamaño de la Célula/fisiología , Ganglios Espinales/metabolismo , Ganglios Espinales/fisiopatología , Expresión Génica/efectos de los fármacos , Expresión Génica/fisiología , Inmunohistoquímica , Masculino , Factor de Crecimiento Nervioso/farmacología , Regeneración Nerviosa/efectos de los fármacos , Regeneración Nerviosa/fisiología , Neuronas Aferentes/efectos de los fármacos , Neuronas Aferentes/patología , Neurotrofina 3/farmacología , ARN Mensajero/metabolismo , Ratas , Ratas Wistar , Receptor trkA/genética , Receptor trkA/metabolismo , Receptor trkB/genética , Receptor trkB/metabolismo , Receptor trkC/genética , Receptor trkC/metabolismo , Nervio Ciático/metabolismo , Nervio Ciático/fisiopatología , Factores de TiempoRESUMEN
Expression of pituitary adenylate cyclase-activating polypeptide in sensory neurons varies with injury or inflammation. The neurotrophins NGF and NT-3 are profound regulators of neuronal peptidergic phenotype in intact and injured sensory neurons. This study examined their potential for modulation of PACAP expression in adult rat with intact and injured L4-L6 spinal nerves with or without immediate or delayed intrathecal infusion of NT-3 or NGF. Results indicate that in L5 DRG, few trkC neurons express high levels of PACAP mRNA in the intact state, but many do following injury. The elevated expression in injured neurons is mitigated by NT-3 infusion, suggesting a role for NT-3 in returning the 'injured phenotype' back towards an 'intact phenotype'. NGF dramatically up-regulated PACAP expression in trkA-positive neurons in both intact and injured DRGs, implicating NGF as a positive regulator of PACAP expression in nociceptive neurons. Surprisingly, NT-3 modulates PACAP expression in an antagonistic fashion to NGF in intact neurons, an effect most evident in the trkA neurons not expressing trkC. Both NT-3 and NGF infusion results in decreased detection of PACAP protein in the region of the gracile nuclei, where central axons of the peripherally axotomized large sensory fibers terminate. NGF infusion also greatly increased the amount of PACAP protein detected in the portion of the dorsal horn innervated by small-medium size DRG neurons, while both neurotrophins appear able to prevent the decrease in PACAP expression observed in these afferents with injury. These results provide the first insights into the potential molecules implicated in the complex regulation of PACAP expression in sensory neurons.
Asunto(s)
Ganglios Espinales/metabolismo , Inflamación/metabolismo , Factor de Crecimiento Nervioso/metabolismo , Neuronas Aferentes/metabolismo , Neuropéptidos/metabolismo , Neurotrofina 3/metabolismo , Traumatismos de los Nervios Periféricos , Vías Aferentes/citología , Vías Aferentes/efectos de los fármacos , Vías Aferentes/metabolismo , Animales , Axotomía , Ganglios Espinales/efectos de los fármacos , Ganglios Espinales/fisiopatología , Expresión Génica/efectos de los fármacos , Expresión Génica/fisiología , Inmunohistoquímica , Inflamación/fisiopatología , Masculino , Neuroglía/citología , Neuroglía/efectos de los fármacos , Neuroglía/metabolismo , Plasticidad Neuronal/efectos de los fármacos , Plasticidad Neuronal/fisiología , Neuronas Aferentes/efectos de los fármacos , Neuropéptidos/efectos de los fármacos , Nervios Periféricos/metabolismo , Nervios Periféricos/fisiopatología , Enfermedades del Sistema Nervioso Periférico/metabolismo , Enfermedades del Sistema Nervioso Periférico/fisiopatología , Polipéptido Hipofisario Activador de la Adenilato-Ciclasa , Células del Asta Posterior/citología , Células del Asta Posterior/efectos de los fármacos , Células del Asta Posterior/metabolismo , ARN Mensajero/efectos de los fármacos , ARN Mensajero/metabolismo , Ratas , Ratas Wistar , Receptor trkA/genética , Receptor trkC/genética , Nervio Ciático/lesiones , Nervio Ciático/fisiopatología , Nervio Ciático/cirugíaRESUMEN
A role for neurotrophins in mature primary sensory neurons persists, extending beyond that of promoting survival during development, to one of maintaining phenotypic and functional properties. Many adaptive changes that occur after peripheral axotomy and in axonal repair are believed to be influenced by altered availability of neurotrophic molecules to the neuron in this state. Indeed, administration of exogenous nerve growth factor counteracts many degenerative changes observed in the subpopulation of axotomized neurons which are nerve growth factor-responsive. Current efforts focus on defining actions of other neurotrophins (brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5) in nerve injury and repair, and the intracellular pathways involved. Knowledge gained from work focusing on nerve growth factor and neurotrophin-3 in supporting maintenance or modulation of aspects of the differentiated state of adult primary sensory neurons is discussed.
