ro-25-6981 and Chronic-Disease

The midbrain periaqueductal grey (PAG) is a structure known for its roles in pain transmission and modulation. Noxious stimuli potentiate the glutamate synaptic transmission and enhance glutamate NMDA receptor expression in the PAG. However, little is known about roles of NMDA receptor subunits in the PAG in processing the persistent inflammatory pain. The present study was undertaken to investigate NR2A- and NR2B-containing NMDA receptors in the PAG and their modulation to the peripheral painful inflammation. Noxious stimuli induced by hind-paw injection of complete Freund's adjuvant (CFA) caused up-regulation of NR2B-containing NMDA receptors in the PAG, while NR2A-containing NMDA receptors were not altered. Whole-cell patch-clamp recordings revealed that NMDA receptor mediated mEPSCs were increased significantly in the PAG synapse during the chronic phases of inflammatory pain in mice. PAG local infusion of Ro 25-6981, an NR2B antagonist, notably prolonged the paw withdrawal latency to thermal radian heat stimuli bilaterally in rats. Hyperoside (Hyp), one of the flavonoids compound isolated from Rhododendron ponticum L., significantly reversed up-regulation of NR2B-containing NMDA receptors in the PAG and exhibited analgesic activities against persistent inflammatory stimuli in mice. Our findings provide strong evidence that up-regulation of NR2B-containing NMDA receptors in the PAG involves in the modulation to the peripheral persistent inflammatory pain.

Topics: Afferent Pathways; Animals; Anti-Inflammatory Agents, Non-Steroidal; Chronic Disease; Disease Models, Animal; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Freund's Adjuvant; Glutamic Acid; Inflammation; Male; Mice; Mice, Inbred C57BL; Nociceptors; Organ Culture Techniques; Pain Measurement; Pain, Intractable; Patch-Clamp Techniques; Periaqueductal Gray; Phenols; Piperidines; Quercetin; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Synaptic Transmission; Up-Regulation

Clinically, inflammatory pain is far more persistent than that typically modelled pre-clinically, with the majority of animal models focussing on short-term effects of the inflammatory pain response. The large attrition rate of compounds in the clinic which show pre-clinical efficacy suggests the need for novel models of, or approaches to, chronic inflammatory pain if novel mechanisms are to make it to the market. A model in which a more chronic inflammatory hypersensitivity phenotype is profiled may allow for a more clinically predictive tool. The aims of these studies were to characterise and validate a chronic model of inflammatory pain. We have shown that injection of a large volume of adjuvant to the intra-articular space of the rat knee results in a prolonged inflammatory pain response, compared to the response in an acute adjuvant model. Additionally, this model also results in a hypersensitive state in the presence and absence of inflammation. A range of clinically effective analgesics demonstrate activity in this chronic model, including morphine (3mg/kg, t.i.d.), dexamethasone (1mg/kg, b.i.d.), ibuprofen (30mg/kg, t.i.d.), etoricoxib (5mg/kg, b.i.d.) and rofecoxib (0.3-10mg/kg, b.i.d.). A further aim was to exemplify the utility of this chronic model over the more acute intra-plantar adjuvant model using two novel therapeutic approaches; NR2B selective NMDA receptor antagonism and iNOS inhibition. Our data shows that different effects were observed with these therapies when comparing the acute model with the model of chronic inflammatory joint pain. These data suggest that the chronic model may be more relevant to identifying mechanisms for the treatment of chronic inflammatory pain states in the clinic.

Topics: Animals; Arthritis; Chronic Disease; Cyclooxygenase 2 Inhibitors; Disease Models, Animal; Freund's Adjuvant; Lactones; Male; Nitric Oxide Synthase Type II; Pain; Phenols; Piperidines; Rats; Receptors, N-Methyl-D-Aspartate; Stifle; Sulfides; Sulfones; Time Factors; Weight-Bearing

We have shown previously that when postsynaptic NMDA receptors are blocked, the frequency, but not amplitude, of spontaneous EPSCs (sEPSCs) at synapses in the entorhinal cortex is reduced by NMDA receptor antagonists, demonstrating that glutamate release is tonically facilitated by presynaptic NMDA autoreceptors. In the present study, we recorded sEPSCs using whole-cell voltage clamp in neurons in layer V in slices of the rat entorhinal cortex. Using specific antagonists for NR2A [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid] and NR2B [(alphaR, betaS)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidinepropanol hydrochloride (Ro 25-6981)] subunit-containing receptors, we confirmed that in slices from juvenile rats (4-6 weeks of age), the autoreceptor is predominantly of the NR1-NR2B subtype. In older (4-6 months of age) control animals, the effect of the NR2B antagonist was less marked, suggesting a decline in autoreceptor function with development. In slices from rats (aged 4-6 months) exhibiting spontaneous recurrent seizures induced with a lithium-pilocarpine protocol, Ro 25-6981 again robustly reduced sEPSC frequency. The effect was equal to or greater than that seen in the juvenile slices and much more pronounced than that seen in the age-matched control animals. In all three groups, the NR2A antagonist was without effect on sEPSCs. These results suggest that there is a developmental decrease in NMDA autoreceptor function, which is reversed in a chronic epileptic condition. The enhanced autoreceptor function may contribute to seizure susceptibility and epileptogenesis in temporal lobe structures.

Topics: Age Factors; Animals; Autoreceptors; Chronic Disease; Entorhinal Cortex; Epilepsy, Generalized; Excitatory Amino Acid Antagonists; Glutamic Acid; Male; Membrane Potentials; Neurons; Patch-Clamp Techniques; Phenols; Pilocarpine; Piperidines; Quinoxalines; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Status Epilepticus