Administration of copper reduced the hyper-excitability of neurons in CA1 hippocampal slices from epileptic rats.

Copper as a trace metal is involved in several neurodegenerative illnesses, such as Menkes, Wilson's, Alzheimer's, amyotrophic lateral sclerosis (ALS), and Creutzfeldt-Jakob. Electrophysiological evidence indicates that acute perfusion of copper can inhibit long-term synaptic potentiation in hippocampal slices. The objective of this work is to determine whether Cu perfusion can perturb synaptic transmission in hippocampal slices derived from pilocarpine treated epileptic rats. Field potential (FP) recordings of the CA1 neurons of rats with chronic epilepsy showed voltage and response duration decrease following copper sulfate perfusion. However, voltage and response duration were higher after removing copper by washing. The discharge frequency of the CA1 neurons of hippocampal slices from non-epileptic control rats was increased after acute perfusion of 10 µM of pilocarpine. This increase was blocked by administering copper sulphate 10 µM. Krebs-Ringer solution washing re-established the discharges, with a higher frequency than that provoked by pilocarpine perfusion. We discuss the blocking effect of copper and the synaptic hyper-excitability generated by its removal.

Stage-dependent C-reflex, pain-like behavior and opioid analgesia during the induction of chronic arthritis in rats.

Chronic arthritis (CA) is a common clinical entity associated with persistent pain and limited response to opioid analgesic therapy. However, it is unknown whether these features of CA change depending on its stage of evolution. To address this, in a well-established animal model of CA we studied the time course of electromyographic responses to electrical stimulation of C fibers (C-reflex), pain-like behavior as a response to mechanical nociceptive stimulation, and the inhibition of both responses by a prototypic opioid analgesic, morphine. To induce CA, rats received a single injection of complete Freund's adjuvant into the ankle joint and the C-reflex responses to electrical stimuli or the nociceptive response to paw pressure test were studied 2, 4 or 6 weeks later. The C-reflexes evoked by threshold and supra-threshold electrical stimulation exhibited progressive increases together with enhancement of the nociceptive behavior to mechanical stimulation during induction of monoarthritis. Notably, while systemic morphine produced antinociceptive effects upon both experimental approaches, the effects were markedly reduced during the early stages of CA but enhanced at later stages. These data indicate that C-reflex and pain-like responses evolve in parallel, and are inhibited by morphine in a stage-dependent manner through the induction of CA. The present results may contribute to explain the enhanced pain response and variable analgesic efficacy of opioids that characterize arthritic pain in humans.

Involvement of spinal cord BDNF in the generation and maintenance of chronic neuropathic pain in rats.

Brain-derived neurotrophic factor (BDNF) is involved in neuronal survival and synaptic plasticity of the central and peripheral nervous system. In chronic pain, plastic changes in dorsal horn neurons contribute to a phenomenon of hypersensitivity to pain sensation that is maintained over time, known as central sensitization. This process is accompanied by BDNF overexpression, but the role of BDNF in the generation and maintenance of the hyperalgesic phenomenon is still unclear. The present study was aimed to investigate if exogenous BDNF administered to the rat spinal cord, in addition to trigger pain, participates in the maintenance of the central sensitization process (i.e., pain persistence) and to determine if the pain generated is comparable to that observed in a neuropathic pain model. Results showed that a single intrathecal injection of 0.003 ng of BDNF was able to decrease the nociceptive threshold (Randall-Selitto test) in normal rats, for at least a 42-day period. Furthermore, the hyperalgesia generated was comparable to that observed in rats with a 42-day history of mononeuropathy. Increasing the dose or administering additional doses of BDNF resulted neither in additional effectiveness in reducing the pain threshold nor in the prolongation of the hyperalgesic effect, thus showing that central sensitization induced by BDNF is a dose-independent, all-or-none process. It is concluded that BDNF alone is sufficient for generating a long-lasting neural excitability change in the spinal cord via tyrosine kinase B receptor signaling, similar to that observed in chronic pain models such as neuropathy.

Antinociceptive effect and interaction of uncompetitive and competitive NMDA receptor antagonists upon capsaicin and paw pressure testing in normal and monoarthritic rats.

We assessed whether intrathecal administration of the uncompetitive and competitive NMDA receptor antagonists ketamine and (+/-)CPP, respectively, could produce differential modulation on chemical and mechanical nociception in normal and monoarthritic rats. In addition, the antinociceptive interaction of ketamine and (+/-)CPP on monoarthritic pain was also studied using isobolographic analysis. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the tibio-tarsal joint. Four weeks later, the antinociceptive effect of intrathecal administration of the drugs alone or combined was evaluated by using the intraplantar capsaicin and the paw pressure tests. Ketamine (0.1, 1, 10, 30, 100, 300 and 1000 microg i.t.) and (+/-)CPP (0.125, 2.5, 7.5, 12.5, 25 and 50 microg i.t.) produced significantly greater dose-dependent antinociception in the capsaicin than in the paw pressure test. Irrespective of the nociceptive test employed, both antagonists showed greater antinociceptive activity in monoarthritic than in healthy rats. Combinations produced synergy of a supra-additive nature in the capsaicin test, but only additive antinociception in paw pressure testing. The efficacy of the drugs, alone or combined, is likely to depend on the differential sensitivity of tonic versus phasic pain and/or chemical versus mechanical pain to NMDA antagonists.

Expression of nitric oxide synthase isoforms in the dorsal horn of monoarthritic rats: effects of competitive and uncompetitive N-methyl-D-aspartate antagonists.

Chronic pain is associated with N-methyl-D-aspartate (NMDA) receptor activation and downstream production of nitric oxide, which has a pivotal role in multisynaptic local circuit nociceptive processing in the spinal cord. The formation of nitric oxide is catalyzed by three major nitric oxide synthase (NOS) isoforms (neuronal, nNOS; inducible, iNOS; endothelial, eNOS), which are increased in the spinal cord of rodents subjected to some tonic and chronic forms of experimental pain. Despite the important role of NOS in spinal cord nociceptive transmission, there have been no studies exploring the effect of NMDA receptor blockade on NOS expression in the dorsal horn during chronic pain. Furthermore, NOS isoforms have not been fully characterized in the dorsal horn of animals subjected to arthritic pain. The aim of this work was therefore to study the expression of nNOS, iNOS and eNOS in the dorsal horns of monoarthritic rats, and the modifications in NOS expression induced by pharmacological blockade of spinal cord NMDA receptors. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the right tibio-tarsal joint. At week 4, monoarthritic rats were given either the competitive NMDA antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or the uncompetitive NMDA antagonist ketamine. After 6 and 24 hours, animals were killed and posterior quadrants of the lumbar spinal cord were dissected. Sample tissues were homogenized and subjected to immunoblotting with anti-nNOS, anti-iNOS or anti-eNOS monoclonal antibodies. The nNOS isoform, but not the iNOS and eNOS isoforms, were detected in the dorsal horns of control rats. Monoarthritis increased the expression of nNOS, iNOS and eNOS in the dorsal horns ipsilateral and contralateral to the inflamed hindpaw. Intrathecal administration of CPP and ketamine reduced nNOS expression in monoarthritic rats but increased the expression of iNOS and eNOS. Results suggest that blockade of spinal cord NMDA receptors produces complex regulatory changes in the expression of NOS isoforms in monoarthritic rats that may be relevant for nitridergic neuronal/glial mechanisms involved in the pathophysiology of monoarthritis and in the pharmacological response to drugs interacting with NMDA receptors.

Papel del complejo pulvinar-lateral posterior en la respuesta de rotación: relación con otras estructuras cerebrales; sistemas farmacológicos implicados en esta conducta / Role of pulvinar lateralis posterior complex on rotation response: relation with other cerebral structures; pharmacological systems involved in this behavior

Se hace un análisis crítico de los resultados obtenidos en 4 series experimentales efectuadas en gatos. En estos experimentos se muestran los resultados siguientes: 1) la estimulación eléctrica del completo pulvinar-lateral posterior (P-LP) evoca ratación contralateral de la cabeza, acompañada de una rotación de ambos ojos en la misma dirección, la mayor parte de las veces de tipo sacádico. Con intensidades de corriente superior se produce rotación contralaterl de todo el cuerpo; 2) las eferencias desde el P-LP para producir estos efectos se hacen a través del colículo superior ipsilateral. La lesión electrolítica de esta última estructura suprime la conducta de rotación inducida por estimulación elétrica del P-LP. Este resultado no se obtiene cuando es lesionado el núcleo causado o el centralis later alis; 3) la ablación de la corteza cerebral ipsilateral, que tiene conexiones recíprocas importantes con el P-LP. produce la supresión de la respuesta de rotación en el 50 % de los gatos en la primeira semana que sigue a la resección cortical. Sin embargo, 30 días después de la ablación, el 86 % de los electrodos implantados en en P-LP son capaces de inducir, por estimulación elétrica, la respuesta de rotación, y 4) la inyección local en el P-LP a través de una cánula de carbacol o dioxolano, este último activador exclusivo de los receptores muscarínicos, produce también respuesta de rotación contralateral de cabeza-ojo-cuerpo. Los resultados expuestos nos permiten postular que el P-LP es otra estructura cerebral relacionada con rotación y diferente del sistema nigro-estriatal. Este último no es afectado por lesión del colículo superior, como sucede con el P-LP. Por otra parte, el neurotransmisor más importante implicado en la conducta de rotación en el sistema nigro-estriatal es la dopamina, en tanto que en el P-LP es la acetilcolina