RESUMO
In cats anaesthetized with alpha-chloralose or sodium pentobarbitone a study was made of the effects of supraspinal electrical stimulation on the excitation of dorsal horn neurones by noxious and non-noxious cutaneous stimuli. Stimulation near the dorsal raphe of the midbrain non-selectively reduced the responses of neurones to both noxious and non-noxious stimuli. Intravenous naloxone (0.3-0.6 mg/kg) had no effect on this inhibition. Electrical stimulation near the medullary raphe selectively reduced the excitation of dorsal horn neurones by noxious cutaneous stimuli. Excitation of neurones by deflection of hairs was unaffected. The inhibition of nociceptive responses outlasted the period of raphe stimulation by up to 6 min. Intravenous naloxone (0.6-1.0 mg/kg) also failed to affect this selective inhibition.
Assuntos
Estimulação Elétrica , Mesencéfalo/fisiologia , Inibição Neural , Nociceptores/fisiologia , Animais , Gatos , Vias Neurais , Medula Espinal/fisiologia , Transmissão SinápticaRESUMO
The present study investigated whether mechanical allodynia following contusive spinal cord injury (SCI) of the thoracic segments 12 and 13 of the rat was associated with a reduction in gamma-aminobutyric acid (GABA)ergic inhibition adjacent to the site of injury. Five to 7 days following SCI, extracellular recordings were obtained from dorsal horn neurones located 1-2 segments caudal to the injury, in non-allodynic and allodynic halothane anaesthetised rats and from comparable neurones in normal rats. To assess spinal GABAergic inhibition in the three groups of animals, spontaneous and evoked cell firing rates were recorded before, during and after microiontophoretic application of the GABA(A) receptor antagonist bicuculline. Administration of bicuculline to normal animals resulted in significant and reversible increases in the receptive field size, spontaneous firing rate, response to brushing and pinching the skin and afterdischarge activity of dorsal horn neurones, as well as decreasing paired-pulse depression of responses evoked by transcutaneous electrical stimulation. In non-allodynic SCI animals, bicuculline ejection led to significant changes in receptive field size, paired-pulse depression and responses to brush and pinch stimulation that were comparable to those observed in normal animals. By contrast, in allodynic SCI animals, bicuculline ejection had little or no effect on dorsal horn neurone responses to mechanical skin stimuli and paired-pulse depression despite reliably blocking the inhibition of cell firing produced by similarly applied GABA. The demonstration of reduced GABAergic inhibition predominantly in the allodynic SCI rats suggests that such a deficiency contributed to this pain-related behaviour acutely following SCI.
Assuntos
Hiperalgesia/etiologia , Inibição Neural , Dor/etiologia , Células do Corno Posterior/fisiopatologia , Traumatismos da Medula Espinal/complicações , Ácido gama-Aminobutírico/metabolismo , Potenciais de Ação/efeitos dos fármacos , Potenciais de Ação/fisiologia , Animais , Bicuculina/farmacologia , Modelos Animais de Doenças , Estimulação Elétrica , Feminino , Antagonistas GABAérgicos/farmacologia , Antagonistas de Receptores de GABA-A , Hiperalgesia/patologia , Hiperalgesia/fisiopatologia , Vias Neurais/metabolismo , Vias Neurais/fisiopatologia , Dor/patologia , Dor/fisiopatologia , Estimulação Física , Células do Corno Posterior/metabolismo , Ratos , Ratos Wistar , Tempo de Reação/efeitos dos fármacos , Tempo de Reação/fisiologia , Receptores de GABA-A/metabolismo , Traumatismos da Medula Espinal/patologia , Traumatismos da Medula Espinal/fisiopatologia , Tato/fisiologia , Ferimentos não PenetrantesRESUMO
Long-term potentiation (LTP) of transmission of impulses in unmyelinated (C-fibre) primary afferents by prior tetanic conditioning stimulation has been demonstrated in the dorsal horn of the spinal cord. Since this potentiation has been proposed to be relevant to the increased responsiveness of spinal neurones associated with peripheral inflammation (central sensitisation), the present experiments compared the induction of LTP in normal rats and rats with monoarthritis. Monoarthritis was induced by injection of complete Freund's adjuvant (CFA) into the left ankle joint of 12 rats. All animals showed behavioural signs of thermal hyperalgesia and were used for electrophysiological experiments after 4-8 days. In each animal, extracellular recordings were obtained from a single, wide dynamic range (WDR) dorsal horn neurone. High frequency tetanic conditioning stimulation of the sciatic nerve gave varying effects on the C-fibre-evoked responses of neurones in the normal rats, with potentiation in two, no change in five and a depression in five. By contrast, conditioning stimulation in rats with inflammation produced a long-lasting potentiation of C-fibre-evoked responses in 11 out of 12 neurones, with no effect in one. The ease with which LTP was induced in animals with inflammation supports the proposal that the underlying mechanisms of LTP are similar to those of the central sensitisation associated with peripheral inflammation.
Assuntos
Artrite Experimental/fisiopatologia , Potenciação de Longa Duração/fisiologia , Células do Corno Posterior/fisiologia , Animais , Comportamento Animal/fisiologia , Eletrofisiologia , Adjuvante de Freund , Masculino , Microeletrodos , Fibras Nervosas Amielínicas/fisiologia , Nociceptores/fisiologia , Medição da Dor , Ratos , Ratos Wistar , Medula Espinal/fisiopatologia , Técnicas Estereotáxicas , Transmissão Sináptica/fisiologiaRESUMO
Antibody-coated microprobes have been demonstrated to be useful for detecting the release of neuropeptide transmitters from discrete sites in the central nervous system (CNS). This technique uses glass micropipettes taken through a series of chemical coatings, starting with a γ-aminopropyltriethoxysilane solution and ending with the antibody specific to the peptide transmitter of interest. The key to the reliability and repeatability of the technique is a uniform, even coating of the siloxane polymer to the glass micropipette. The microprobes, as they are called following the completion of the coating process, are inserted stereotaxically into a specific area of the CNS and the physiological intervention is performed. Tip diameters are around 5-10 µm and, depending on the length of the pipette inserted into the CNS, diameters of the pipette shaft will approach 40-50 µm. Once removed, the microprobe is then incubated with the radiolabeled peptide. Binding of the radiolabeled peptide will occur to the antibody sites not occupied by the endogenously released peptide. The images of the microprobes on sensitive autoradiographic film are analyzed for differences in the optical density along a specified length of probe. Areas of lighter density signify sites along the microprobe where endogenous peptide was biologically released during the physiological intervention. Knowing the exact location of the probe tip in vivo in the CNS permits identification of neurophysiological sites corresponding along the length of the microprobe where the peptide was released.
Assuntos
Anticorpos/metabolismo , Neuropeptídeos/metabolismo , Sistema Nervoso Central/metabolismoRESUMO
The proinflammatory cytokine interferon-gamma (IFN-gamma), which can be present in elevated levels in the central nervous system during pathological conditions, may be involved in the generation of persistent pain states by inducing neuronal hyperexcitability. The aim of the present study was to examine whether loss of dorsal horn GABAergic inhibition may underlie this IFN-gamma-mediated neuronal hyperexcitability. Repetitive intrathecal injections of recombinant rat IFN-gamma (1000 U) or control buffer were administered to rats every second day for eight days. Electrophysiological recordings from lumbar dorsal horn neurons (n=46) were performed under halothane anaesthesia. Cellular responses were recorded before, during and after microiontophoretic application of the GABA antagonist bicuculline. In control animals, all cellular responses studied were significantly enhanced in the presence of bicuculline, including increased spontaneous activity, enhanced responses to innocuous and noxious mechanical stimulation and reduced paired-pulse depression. In contrast, in IFN-gamma-treated animals, bicuculline ejection had little or no facilitating effect on neuronal responses and instead a significant proportion of neurons displayed reduced responses. Seventy-four percent of cells from IFN-gamma treated animals showed a reduction in the response to noxious stimulation and 47% of the cells showed increased rather than reduced paired-pulse depression in the presence of bicuculline, thus suggesting IFN-gamma-induced excitatory actions by GABA. These findings show that the prolonged presence of increased levels of IFN-gamma in the central nervous system may contribute to the generation of central sensitization and persistent pain by reducing inhibitory tone in the dorsal horn. This implies a potential link between disinhibition and cytokine action in the spinal cord.