Cooperative interaction among the various regulatory sites within the NMDA receptor-channel complex in modulating the evoked responses to noxious thermal stimuli of spinal dorsal horn neurons in the cat

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, β-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

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.

Effects of glutamate receptor antagonists on spinal dorsal horn neurons during zymosan-induced inflammation in rats

These experiments examined the effects of spinal administration of the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX), or the metabotropic glutamate receptor antagonist DL-2-amino-3-phosphonoproprionic acid (AP3) on responses of spinal dorsal horn neurons evoked by thermal and mechanical stimuli applied to the rat hindpaw in either an inflamed or noninflamed state. Administration of APV, DNQX, or AP3 decreased heat-evoked neuronal discharges of wide dynamic range (WDR) neurons that were previously augmented by zymosan-induced inflammation. APV and DNQX also decreased heat-evoked discharges of WDR neurons that were previously unaffected by saline injection. Administration of either APV or DNQX, but not AP3, decreased heat-evoked neuronal discharges of nociceptive-specific (NS) neurons in both zymosan- and saline-injected rats. These data suggest that NMDA and non-NMDA receptors contribute to spinal processing of thermal stimuli in both the inflamed and noninflamed state, whereas metabotropic glutamate receptors might serve a role that is unique to WDR neurons in the inflamed state. Only DNQX consistently increased mechanical response thresholds and decreased slopes of the mechanical stimulus response functions (SRFs) of NS and WDR neurons, but this effect was observed in both inflamed and noninflamed states. These data suggest that spinal processing of mechanical stimuli is preferentially mediated by glutamate acting at non-NMDA receptors in either the inflamed or noninflamed state.

Selective antihyperalgesic effect of [Ser1] histogranin on complete Freund's adjuvant-induced hyperalgesia in rats

The search for alternative pharmacotherapies that target abnormal pain has focused on N-methyl-D-aspartate glutamate receptor (NMDAR) antagonists, since they are efficacious in various chronic pain models. However, adverse effects of currently available agents limit their therapeutic usefulness. The naturally derived NMDAR antagonist peptide, histogranin, is thought to interact at a novel site on the NMDAR subunit. Previous studies in our laboratory have suggested the potential for histogranin analogs to attenuate neuropathic pain. The ability of this peptide derivative to reduce inflammatory pain was evaluated in the present study. The effect of intrathecal (i.t.) injection of the stable analog [Ser(1)] histogranin (SH) was evaluated in rats with a unilateral hind paw inflammation. Following injection of complete Freund's adjuvant into the hind paw, responsiveness to noxious thermal and mechanical stimuli were greatly enhanced (hyperalgesia). The i.t. injection of SH partially attenuated mechanical hyperalgesia for up to 2 h post-injection, with no effect on withdrawal thresholds of the non-inflamed paw. In contrast, SH had no effect on thermal hyperalgesia. No attendant motor abnormalities were noted. These results indicate that SH has selective and modest antinociceptive effects on inflammatory pain and suggests that novel histogranin analogs may be safe and useful adjuncts in the management of chronic pain.

Electrophysiological studies on the role of the NMDA receptor in nociception in the developing rat spinal cord

The present study investigated the effects of spinally applied N-methyl-D-aspartate (NMDA) antagonists 2-amino-5-phosphonovaleric acid (AP5) and ketamine on convergent neurones in the deep dorsal horn of rats, in vivo at different postnatal ages (P) 14, 21, 28, and 56 days. AP5 inhibited the primary afferent fibre input, the C fibre, post-discharge and windup evoked responses in a dose-dependent manner at each age, and was significantly more effective in the pups than adult rats (P<0.03 at 100-microg dose). AP5 100 microg abolished windup almost completely in the pups, whilst the adults required 10-fold higher doses. In contrast there was no difference in ketamine potency between age groups. Windup in the ketamine groups was reduced in a dose-dependent manner equally across all the age groups. The differential inhibitory effects of AP5 and ketamine may be due to postnatal changes in density, localisation and receptor subunit composition, altering receptor affinity and kinetics.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.