Anti-allodynic interactions between NMDA receptor channel blockers and morphine or clonidine in neuropathic rats

Previous studies suggested that combining N-methyl-d-aspartate (NMDA) receptor antagonists with either mu-opioid agonist morphine or alpha2-adrenoreceptor agonist clonidine results in the significant synergistic enhancement of analgesic activity in the animal models of acute and neuropathic pain. When given alone, NMDA receptor antagonists, morphine and clonidine are capable of attenuating tactile allodynia associated with chronic nerve injury. The present study aimed to assess anti-allodynic effects of these compounds and to test additivity of these interactions using isobolographic analysis. Adult male Wistar rats with unilateral loose ligation of sciatic nerve developed significant tactile allodynia (between-paw difference of about 18-20 g). In separate groups of animals, dose-dependent anti-allodynic activity was confirmed for memantine (1.8-17.8 mg/kg), neramexane (1.8-17.8 mg/kg), morphine (1-10 mg/kg) and clonidine (0.01-0.1 mg/kg). In a subsequent series of experiments, memantine (or neramexane) and morphine (or clonidine) were co-administered at the fixed equi-effective dose ratios (six dose levels per drug combination). None of the tested combinations produced supra-additive, synergistic effects. In fact, memantine+clonidine, neramexane+clonidine and morphine+neramexane were producing simple additive effects, while morphine+memantine was characterized as the infra-additive combination. Thus, despite expectations based on previous studies, NMDA receptor channel blockers, memantine and neramexane, produce no synergistic interactions with either morphine or clonidine when administered acutely to rats with nerve injury-induced tactile allodynia.     

Excitatory amino acid neurotransmission

In recent years great progress has been made in understanding the function of ionotropic and metabotropic glutamate receptors; their pharmacology and potential therapeutic applications. It should be stressed that there are already N-methyl-D-aspartate (NMDA) antagonists in clinical use, such as memantine, which proves the feasibility of their therapeutic potential. It seems unlikely that competitive NMDA receptor antagonists and high-affinity channel blockers will find therapeutic use due to limiting side-effects, whereas agents acting at the glycineB site, NMDA receptor subtype-selective agents and moderate-affinity channel blockers are far more promising. This is supported by the fact that there are several glycineB antagonists, NMDA moderate-affinity channel blockers and NR2B-selective agents under development. Positive and negative modulators of AMPA receptors such as the AMPAkines and 2,3-benzodiazepines also show more promise than e.g. competitive antagonists. Great progress has also been made in the field of metabotropic glutamate receptors since the discovery of novel, allosteric modulatory sites for these receptors. Selective agents acting at these transmembrane sites have been developed that are more drug-like and have a much better access to the central nervous system than their competitive counterparts. The chapter will critically review preclinical and scarce clinical experience in the development of new ionotropic and metabotropic glutamate receptor modulators according to the following scheme: rational, preclinical findings in animal models and finally clinical experience, where available.     

Effects of the Positive Allosteric Modulator of Metabotropic Glutamate Receptor 5, VU-29, on Impairment of Novel Object Recognition Induced by Acute Ethanol and Ethanol Withdrawal in Rats

Glutamate is essential for learning and memory processes, and acute and chronic exposures to ethanol (or protracted abstinence) alter glutamatergic transmission. In the current study, we investigated the effects of VU-29, positive allosteric modulator of metabotropic glutamate 5 (mGlu5) receptor, on the acute ethanol- and ethanol withdrawal-induced impairment of novel object recognition (NOR) task in rats. The influence of VU-29 (30 mg/kg) on memory retrieval was measured (a) at 4-h delay after acute ethanol administration, as well as (b) after acute withdrawal (24 and 48 h) of repeated (2.0 g/kg, once daily for 7 days) ethanol administration. Additionally, the effects of VU-29 on expression of mGlu5 and mGlu2 receptor proteins in the hippocampus, prefrontal cortex, and striatum were determined 48 h after ethanol withdrawal. Our results indicated that VU-29, given before acute ethanol administration, prevented the ethanol-induced impairments in spatial memory retrieval. Furthermore, VU-29 given before the testing session on the first day of abstinence facilitated NOR performance in ethanol-withdrawn rats at 4- and 24-h delay after administration. Our ELISA results show that VU-29 normalized ethanol withdrawal induced increase in expression of mGlu5 receptor protein in the hippocampus, prefrontal cortex, and striatum, as well as expression of mGlu2 receptor protein in the hippocampus. Thus, results from our study indicate that positive modulation of mGlu5 receptor prevented and reversed ethanol-induced memory impairment. Moreover, mGlu5 (hippocampus, prefrontal cortex, and striatum) and mGlu2 (hippocampus) receptors play an important role in the ethanol-induced recognition memory impairment induced by ethanol withdrawal.     

5-Methoxy-N,N-dimethyltryptamine-induced analgesia is blocked by alpha-adrenoceptor antagonists in rats.

The effects of the alpha-adrenoceptor antagonists prazosin, phentolamine and yohimbine upon 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)-induced analgesia were tested in the hot-plate, tail-flick and shock-titration tests of nociception with rats. Intrathecally injected yohimbine and phentolamine blocked or attenuated the analgesia produced by systemic administration of 5-MeODMT in all three nociceptive tests. Intrathecally administered prazosin attenuated the analgesic effects of 5-MeODMT in the hot-plate and tail-flick tests, but not in the shock titration test. Intrathecal yohimbine showed a dose-related lowering of pain thresholds in saline and 5-MeODMT-treated animals. Phentolamine and prazosin produced normal dose-related curves in the hot-plate test and biphasic effects in the shock titration and tail-flick tests. These results demonstrate a functional interaction between alpha 2-adrenoceptors and 5-HT agonist-induced analgesia at a spinal level in rats.     

Down-regulation of the GABA-ergic system in selected brain areas of spontaneously hypertensive rats (SHR)

The relevance of GABA-ergic system in hypertensive state has been studied. GABA content, GAD activity and GABA-A receptor binding in various brain areas in age-matched spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY) was compared. Out of 9 brain areas studied the GABA content was significantly lower in the substantia nigra, hypothalamus, hypothalamus posterior and hippocampus of SHR rats. GAD activity was lowered in the hypothalamus and hippocampus of young SHR and adult SHR rats (4, 8, 14 weeks old). Scatchard analysis of the binding isotherms indicated a lower Bmax of the binding sites in the hypothalamus and hippocampus of 8 and 14 weeks old SHR rats. These results suggest that activity of GABA-ergic system differs substantially in SHR and WKY rats brain. Furthermore, these differences appear already in young prehypertensive SHR rats as well as in the early stages of hypertension.     

Metaphit fails to antagonize PCP-induced passive avoidance deficit.

Pretreatment with metaphit (1-[1-(3-isothiocyanotophenyl)cyclohexyl]piperidine), a putative irreversible antagonist of phencyclidine (PCP) receptors, did not antagonize PCP-induced passive avoidance deficit in rats, and did not decrease [3H]MK-801 (5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate) binding to PCP recognition sites coupled to NMDA receptors. The effectiveness of the metaphit treatment was evidenced by the occurrence of audiogenic seizures. These results suggest that previously reported antagonism in vivo actions of PCP by metaphit, is mediated by sites not involved in PCP-induced passive avoidance deficit, and not related to the NMDA receptor complex in brain structures studied (striatum, hippocampus, and cortex).     

Learning impairment in rats by N-methyl-D-aspartate receptor antagonists

2-Amino-5-phosphonovalerate (APV, icv) phencyclidine (PCP, ip) and scopolamine (sc) dose-dependently disrupted short term working memory in radial maze. These drugs injected before, but not after training attenuated retention of long term memory in passive avoidance task. A relation of PCP action to its antagonism at NMDA receptors may be suggested.     

Effects of the Uncompetitive NMDA Receptor Antagonist Memantine on Hippocampal Long-term Potentiation, Short-term Exploratory Modulation and Spatial Memory in Awake, Freely Moving Rats

Chronic treatment of adult male F-344 rats (9–12 months old) with therapeutically relevant doses of memantine (30 mg/kg/day in chow for <8 weeks) increased the maintenance of long-term potentiation of field excitatory postsynaptic potentials from perforant path—granule cell hippocampal synapses recorded in the fascia dentata in vivo. In contrast, there was no effect of memantine on baseline synaptic responses or population spikes. Likewise, short-term exploratory modulation of these hippocampal evoked responses was not different between memantine-treated and control rats. Both groups of rats were able to learn the spatial version of the Morris water task equally well, but the memantine-treated group showed a strong tendency to show more selective spatial search patterns in the training quadrant of the water pool during a final probe trial. As such, these studies provide the first electrophysiological evidence that memantine can increase the durability of synaptic plasticity and provide preclinical confirmation of the cognitive improvement seen with memantine in the treatment of demented patients.     

Pharmacological effects of Ukrain in rats and rabbits

The effect of Ukrain administered in various doses on mean blood pressure (MAP) and breathing rate in rats and rabbits was evaluated. It was found that MAP was reduced and breathing rate increased significantly in both animal species. Maximum tolerated dose (MTD) of Ukrain was 10-fold higher in rats than in rabbits, and it amounted to 3.5 mg x kg(-1) and 0.35 mg x kg(-1), respectively. Possible clinical implications of these findings were discussed.