Characterization of 'non-N-methyl-D-Aspartate' binding sites for gacyclidine enantiomers in the rat cerebellar and telencephalic structures

Development of NMDAR antagonists with reduced neurotoxic side effects: a study on GK11

The NMDAR glutamate receptor subtype mediates various vital physiological neuronal functions. However, its excessive activation contributes to neuronal damage in a large variety of acute and chronic neurological disorders. NMDAR antagonists thus represent promising therapeutic tools that can counteract NMDARs' overactivation. Channel blockers are of special interest since they are use-dependent, thus being more potent at continuously activated NMDARs, as may be the case in pathological conditions. Nevertheless, it has been established that NMDAR antagonists, such as MK801, also have unacceptable neurotoxic effects. Presently only Memantine is considered a safe NMDAR antagonist and is used clinically. It has recently been speculated that antagonists that preferentially target extrasynaptic NMDARs would be less toxic. We previously demonstrated that the phencyclidine derivative GK11 preferentially inhibits extrasynaptic NMDARs. We thus anticipated that this compound would be safer than other known NMDAR antagonists. In this study we used whole-genome profiling of the rat cingulate cortex, a brain area that is particularly sensitive to NMDAR antagonists, to compare the potential adverse effects of GK11 and MK801. Our results showed that in contrast to GK11, the transcriptional profile of MK801 is characterized by a significant upregulation of inflammatory and stress-response genes, consistent with its high neurotoxicity. In addition, behavioural and immunohistochemical analyses confirmed marked inflammatory reactions (including astrogliosis and microglial activation) in MK801-treated, but not GK11-treated rats. Interestingly, we also showed that GK11 elicited less inflammation and neuronal damage, even when compared to Memantine, which like GK11, preferentially inhibits extrasynaptic NMDAR. As a whole, our study suggests that GK11 may be a more attractive therapeutic alternative in the treatment of CNS disorders characterized by the overactivation of glutamate receptors.

Strategies for spinal cord repair after injury: a review of the literature and information

INTRODUCTION

Thanks to the Internet, we can now have access to more information about spinal cord repair. Spinal cord injured (SCI) patients request more information and hospitals offer specific spinal cord repair medical consultations.

OBJECTIVE

Provide practical and relevant elements to physicians and other healthcare professionals involved in the care of SCI patients in order to provide adequate answers to their questions.

METHOD

Our literature review was based on English and French publications indexed in PubMed and the main Internet websites dedicated to spinal cord repair.

RESULTS

A wide array of research possibilities including notions of anatomy, physiology, biology, anatomopathology and spinal cord imaging is available for the global care of the SCI patient. Prevention and repair strategies (regeneration, transplant, stem cells, gene therapy, biomaterials, using sublesional uninjured spinal tissue, electrical stimulation, brain/computer interface, etc.) for the injured spinal cord are under development. It is necessary to detail the studies conducted and define the limits of these new strategies and benchmark them to the realistic medical and rehabilitation care available to these patients.

CONCLUSION

Research is quickly progressing and clinical trials will be developed in the near future. They will have to answer to strict methodological and ethical guidelines. They will first be designed for a small number of patients. The results will probably be fragmented and progress will be made through different successive steps.

Early functional outcomes and histological analysis after spinal cord compression injury in rats

Object

Neuroprotective and repair strategies in spinal cord injuries (SCIs) have been so far largely unsuccessful. One of the prerequisites is the use of appropriate preclinical models to decipher pathophysiological mechanisms; another is the identification of optimal time windows for therapeutic interventions. The authors undertook this study to characterize early motor, sensory, autonomic, and histological outcomes after balloon compression of the spinal cord at the T8–9 level in adult rats.

Methods

A total of 91 rats were used in this study. Spinal cord balloon compression was performed at T8–9 in adult rats by inflation of a 2 Fr Fogarty catheter into the epidural space. The authors first characterized early motor, sensory, and autonomic outcomes of 2 volumes of compression (10 and 15 μl) using behavioral tests and then examined histological outcomes in the spinal cord using Luxol fast blue staining. To further substantiate the characterization of the epidural balloon-compression model, they used a noncompetitive N-methyl-d-aspartate antagonist, GK11, and demonstrated the involvement of excitotoxicity in this model.

Results

Proportional and reproducible functional impairment resulted from compression caused by balloon inflation with either 10 or 15 μl of water and corresponded to the extent of the lesion. Indeed, during the early phase following SCI (1 week postinjury), recovery of locomotor function and bladder control correlated with the volume of inflation, whereas outcomes with respect to sensory function and reflexes were independent of compression severity. Treatment with GK11 significantly improved motor function in all groups of rats 1 week after injury and bladder voiding in the 10-μl injured rats compared to the 15-μl injured rats.

Conclusions

The results of this study demonstrate that spinal balloon-compression injury in the rat is a well-characterized, reproducible, and predictable model to analyze early events following SCI.

Characterization of hydroxyl radical generation in the striatum of free-moving rats due to carbon monoxide poisoning, as determined by in vivo microdialysis

Carbon monoxide (CO) poisoning caused by CO exposure at 3000 ppm for 40 min resulted in stimulation of hydroxyl radical (*OH) generation (estimated by measuring 2,3-dihydroxybenzoic acid (2,3-DHBA) production from salicylic acid) in the striatum of free-moving rats, as determined by means of brain microdialysis. Pretreatment with a voltage-dependent Na+ channel blocker, tetrodotoxin (TTX), lowered the basal level of 2,3-DHBA and strongly suppressed the increase in 2,3-DHBA induced by CO poisoning. CO poisoning significantly, though only slightly, increased extracellular glutamate in the striatum, and glutamate (Glu) receptor antagonists, such as MK-801 (dizocilpine) and NBQX, failed to suppress the CO-induced increase in 2,3-DHBA. These findings suggest that CO poisoning may induce Na+ influx via the voltage-dependent Na+ channels, resulting in stimulation of *OH generation in rat striatum. This effect may be independent of Glu receptor activation by increased extracellular Glu.

Strategies for regeneration and repair in spinal cord traumatic injury

Spinal cord injury is frequently followed by the loss of supraspinal control of sensory, autonomic and motor functions at the sublesional level. In order to enhance recovery in spinal cord-injured patients, we have developed three fundamental strategies in experimental models. These strategies define in turn three chronological levels of postlesional intervention in the spinal cord. Neuroprotection soon after injury using pharmacological tools to reduce the progressive secondary injury processes that follow during the first week after the initial lesion. This strategy was conducted up to clinical trials, showing that a pharmacological therapy can reduce the permanent neurological deficit that usually follows an acute injury of the central nervous system (CNS). A second strategy, which is initiated not long after the lesion, aims at promoting axonal regeneration by acting on the main barrier to regeneration of lesioned axons: the glial scar. Finally a mid-term substitutive strategy is the management of the sublesional spinal cord by sensorimotor stimulation and/or supply of missing key afferents, such as monoaminergic systems. These three strategies are reviewed. Only a combination of these different approaches will be able to provide an optimal basis for potential therapeutic interventions directed to functional recovery after spinal cord injury.

Re-evaluation of phencyclidine low-affinity or "non-NMDA" binding sites

TCP and its derivative gacyclidine (+/- GK11) are high-affinity non-competitive antagonists of N-methyl-D-aspartate (NMDA) receptors (NMDARs) and as such exhibit significant neuroprotective properties. These compounds also bind with a low affinity to binding sites whose pharmacological profiles are different from that of NMDARs. With the intention to develop new strategies of neuroprotection, we found it mandatory to investigate whether 1-[1-(2-thienyl)cyclohexyl]piperidine (TCP) and gacyclidine low-affinity sites are similar. The effects of several drugs selective for either NMDARs or the [(3)H]TCP low-affinity site (or PCP(3) site) on (+), (-)[(3)H]GK11 and [(3)H]TCP specific binding were investigated. Competition experiments on cerebellum homogenates revealed substantial differences between the pharmacological profiles of the PCP(3) site and that of gacyclidine's enantiomers low-affinity sites. Under experimental conditions preventing the interaction of the radioligands with NMDARs, the autoradiographic study showed, however, that the distributions of both [(3)H]TCP and (-)[(3)H]GK11 specific binding were similar. The specific labelling was low and uniform in telencephalic structures, whereas in the cerebellum it was higher in the molecular than in the granular layer. Finally, the analysis of competition experiments performed on tissues slices demonstrated that PCP(3) selective ligands were unable to prevent [(3)H]TCP or (-)[(3)H]GK11 binding to "non-NMDA" binding sites. As a whole, our data suggest that: (1) the different pharmacological profiles of [(3)H]TCP and [(3)H]gacyclidine enantiomers on low-affinity sites are due to their selectivity for specific NMDARs subpopulations; (2) the pharmacological isolation of TCP and gacyclidine "non-NMDA" binding sites is the most appropriate way to further study the low-affinity component of their specific binding. Obtaining reliable and specific pharmacological tools for those binding sites is of particular interest, since it is likely that they play a substantial role in the low neurotoxicity, and therefore tolerability, of gacyclidine, a new neuroprotective drug currently evaluated in clinical trials for the treatment of brain and spinal cord injuries.

Gacyclidine: a new neuroprotective agent acting at the N-methyl-D-aspartate receptor

Gacyclidine is a new phencyclidine derivative with neuroprotective properties. Tritiated gacyclidine and its enantiomers bind to NMDA receptors with binding parameters similar to those of other non-competitive NMDA receptor antagonists. The (-)enantiomer, (-)GK11, exhibits an affinity (2.5 nM) similar to that of dizocilpine (MK-801), while the (+)enantiomer, (+)GK11, has a 10 times lower affinity. When its interaction with NMDA receptors is prevented, gacyclidine binds also to "non-NMDA" binding sites which are mainly located in the molecular layer of the cerebellum on the dendritic tree of Purkinje cells. These binding sites do not appear to be related to any known neurotransmitters. In primary cortical cultures, gacyclidine and its enantiomers, at 0.1 to 5.0 microM, prevent glutamate-induced neuronal death. In rats, in vivo neurotoxicity of gacyclidine is far low than that of MK-801. No necrotic neurons were detected in animals sacrificed at 18 or 96 h after treatment with gacyclidine (1, 5, 10 or 20 mg/kg i.v.). At the highest (20 mg/kg) but not the lower doses (1-100 mg/kg) electron microscopy revealed the presence of few cytoplasmic or intramitochondrial vacuoles. In soman-treated monkeys gacyclidine enhanced neuroprotective activity of "three drugs cocktail" (atropine + diazepam + pralidoxime). Moreover, in rats, gacyclidine exerts a dose- and time-dependent neuroprotection in three models of spinal cord lesion. Beneficial effects of gacyclidine include reduction of lesion size and improvement of functional parameters after injury. In traumatic brain injury models gacyclidine improves also behavioral parameters and neuronal survival. Optimal protection is obtained when gacyclidine is administered at 0 to 30 min after injury. It is, therefore, concluded that gacyclidine exhibits neuroprotective effects similar to those of other NMDA receptor antagonists, with the advantage of being substantially less neurotoxic maybe due to its interaction with "non-NMDA" binding sites.

Gacyclidine: a new neuroprotective agent acting at the N-methyl-D-aspartate receptor

Gacyclidine is a new phencyclidine derivative with neuroprotective properties. Tritiated gacyclidine and its enantiomers bind to NMDA receptors with binding parameters similar to those of other non-competitive NMDA receptor antagonists. The (-)enantiomer, (-)GK11, exhibits an affinity (2.5 nM) similar to that of dizocilpine (MK-801), while the (+)enantiomer, (+)GK11, has a 10 times lower affinity. When its interaction with NMDA receptors is prevented, gacyclidine binds also to "non-NMDA" binding sites which are mainly located in the molecular layer of the cerebellum on the dendritic tree of Purkinje cells. These binding sites do not appear to be related to any known neurotransmitters. In primary cortical cultures, gacyclidine and its enantiomers, at 0.1 to 5.0 microM, prevent glutamate-induced neuronal death. In rats, in vivo neurotoxicity of gacyclidine is far low than that of MK-801. No necrotic neurons were detected in animals sacrificed at 18 or 96 h after treatment with gacyclidine (1, 5, 10 or 20 mg/kg i.v.). At the highest (20 mg/kg) but not the lower doses (1-100 mg/kg) electron microscopy revealed the presence of few cytoplasmic or intramitochondrial vacuoles. In soman-treated monkeys gacyclidine enhanced neuroprotective activity of "three drugs cocktail" (atropine + diazepam + pralidoxime). Moreover, in rats, gacyclidine exerts a dose- and time-dependent neuroprotection in three models of spinal cord lesion. Beneficial effects of gacyclidine include reduction of lesion size and improvement of functional parameters after injury. In traumatic brain injury models gacyclidine improves also behavioral parameters and neuronal survival. Optimal protection is obtained when gacyclidine is administered at 0 to 30 min after injury. It is, therefore, concluded that gacyclidine exhibits neuroprotective effects similar to those of other NMDA receptor antagonists, with the advantage of being substantially less neurotoxic maybe due to its interaction with "non-NMDA" binding sites.