RESUMO
Primary afferent fibers are divided into three main subgroups: Abeta-, Adelta-, and C-fibers. Morphological studies have demonstrated that neonatal capsaicin treatment (NCT) depletes C-fiber inputs in the spinal dorsal horn; the electrophysiological features of the NCT-induced changes, however, remain unclear. This issue was addressed by performing whole-cell voltage-clamp recordings from substantia gelatinosa (SG) neurons in dorsal root-attached spinal cord slices. When estimated from excitatory postsynaptic currents (EPSCs) evoked by stimulating primary afferent fibers, 24 (49%) of 49 neurons examined exhibited C-fiber EPSCs that were either monosynaptic (n = 15) or polysynaptic (n = 9) in origin; only two of the neurons had Abeta-fiber responses. In NCT rats, however, SG neurons exhibiting C-fiber-mediated EPSCs decreased to 7% (3 of 41 neurons tested), whereas Abeta-fiber EPSCs were observed in 21 (51%) of the neurons, and 14 (67%) of them exhibited monosynaptic ones. There was no change in the cell proportion having Adelta-fiber innervation after NCT. Our electrophysiological data suggest that NCT diminishes primary afferent C-fiber inputs while enhancing Abeta-fiber direct innervation in the SG in adulthood.
Assuntos
Capsaicina/farmacologia , Potenciais Pós-Sinápticos Excitadores/efeitos dos fármacos , Substância Gelatinosa/efeitos dos fármacos , Potenciais de Ação/efeitos dos fármacos , Potenciais de Ação/fisiologia , Vias Aferentes/efeitos dos fármacos , Vias Aferentes/fisiologia , Animais , Animais Recém-Nascidos , Potenciais Pós-Sinápticos Excitadores/fisiologia , Técnicas In Vitro , Ratos , Medula Espinal/efeitos dos fármacos , Medula Espinal/crescimento & desenvolvimento , Substância Gelatinosa/crescimento & desenvolvimentoRESUMO
The development of spinal cord sensory pathways has been investigated in postnatal day (P) 21 rat pups following neonatal capsaicin treatment. Capsaicin-induced destruction of C fibres was confirmed by 62% loss of Isolectin B4 (IB4)-binding and an 86% loss of calcitonin gene-related peptide (CGRP)-immunoreactive small diameter dorsal root ganglion cells. Neonatal capsaicin treatment prevented the normal withdrawal of choleragenoid-horseradish peroxidase (B-HRP)-labelled A fibres from lamina II (substantia gelatinosa) to deeper laminae postnatally. A fibre terminals projected more dorsally, extending into 43% of lamina II compared to vehicle-treated littermates. A small cell loss in, and/or shrinkage of, substantia gelatinosa cannot account for this. These support the concept of a competitive interaction between A and C fibre afferents to establish final terminal fields. However the continued exuberant A fibre termination in capsaicin-treated rats did not lead to continued c-fos induction in the superficial dorsal horn by innocuous stimulation. In normal development, exuberant A fibre terminals coincide with c-fos activation in lamina II by innocuous skin stimulation [23]. Despite the continued presence of exuberant A fibre terminals, c-fos was not induced by innocuous peripheral stimulation in P21 capsaicin-treated rats implying that these superficial terminals do not activate lamina II neurons in the same way as in the neonate.