Antagonism of kainic acid lesions in the mouse hippocampus by U-54494A and U-50488H

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Pharmacological and genetic manipulation of kappa opioid receptors: Effects on cocaine- and pentylenetetrazol-induced convulsions and seizure kindling

The present study used pharmacological and gene ablation techniques to examine the involvement of kappa opioid receptors (KOPr) in modulating the convulsant effects of two mechanistically different drugs: cocaine and pentylenetetrazol (PTZ; GABA-A receptor antagonist) in mice. Systemic administration of the selective KOPr-1 agonist, U69593 (0.16-0.6mg/kg; s.c.), failed to modify cocaine-evoked convulsions or cocaine kindling. Similarly, no alteration in responsiveness to cocaine was observed in wild-type mice that received the selective KOPr-1 antagonist, nor-binaltorphimine (nor-BNI; 5mg/kg) or in mice lacking the gene encoding KOPr-1. In contrast to cocaine, U69593 attenuated the seizures induced by acute or repeated PTZ administration. Nor-BNI decreased the threshold for PTZ-evoked seizures and increased seizure incidence during the initial induction of kindling relative to controls. Decreased thresholds for PTZ-induced seizures were also observed in KOPr-1 knock out mice. Together, these data demonstrate an involvement of endogenous KOPr systems in modulating vulnerability to the convulsant effects of PTZ but not cocaine. Furthermore, they demonstrate that KOPr-1 activation protects against acute and kindled seizures induced by this convulsant. Finally, the results of our study suggest that KOPr-1 antagonists will not have therapeutic utility against cocaine-induced seizures, while they may prove beneficial in attenuating several actions of cocaine that have been linked to its abuse.

Purines and neuroprotection

The activation of adenosine A1, A2 andA3 receptors can protect neurones against damage generated by mechanical or hypoxic/ischaemic insults as well as excitotoxins. A1 receptors are probably effective by suppressing transmitter release and producing neuronal hyperpolarisation. They are less likely to be of therapeutic importance due to the plethora of side effects resulting from A1 agonism, although the existence of receptor subtypes and recent synthetic chemistry efforts to increase ligand selectivity, may yet yield clinically viable compounds. Activation of A2A receptors can protect neurons, although there is much uncertainty as to whether agonists are acting centrally or via a peripheral mechanism such as altering blood flow or immune cell function. Selective antagonists at the A2A receptor, such as 4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5)triazin-5-yl-amino]ethyl)phenol (ZM 241385) and 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261), can also protect against neuronal death produced by ischaemia or excitotoxicity. In addition, A2A receptor antagonists can reduce damage produced by combinations of subthreshold doses of the endogenous excitotoxin quinolinic acid and free radicals. Since the A2A receptors do not seem to be activated by normal endogenous levels of adenosine, their blockade should not generate significant side effects, so that A2A receptor antagonists appear to be promising candidates as new drugs for the prevention of neuronal damage. Adenosine A3 receptors have received less attention to date, but agonists are clearly able to afford protection against damage when administered chronically. Given the disappointing lack of success of NMDA receptor antagonists in human stroke patients, despite their early promise in animal models, it is possible that A2A receptor antagonists could have a far greater clinical utility.

Opioid peptide pharmacology and immunocytochemistry in an animal model of self-sustaining status epilepticus

In a model of self-sustaining status epilepticus induced in rats by 30 min intermittent stimulation of the perforant path through chronically implanted electrodes, a decrease in dynorphin-like immunoreactivity in the dentate gyrus and CA3 was observed 3 h and 24 h after the induction of status epilepticus. Enkephalin-like immunoreactivity decreased 3 h but not 24 h after perforant path stimulation. Injection into the hilus of the dentate gyrus 10 min prior to stimulation of the kappa-receptor agonist dynorphin-A(1-13), the delta-receptor antagonists ICI-174864 and naltrindole, as well as i.p. injection of naloxone prevented the development of status epilepticus. Perihilar administration of the delta-agonist [D-Ser2]Leu-enkephalin-Thr6 or the kappa-antagonist nor-Binaltorphimine, but not of the mu-agonist [D-Ala2,N-Me-Phe4,Gly-ol5]-Enkephalin, facilitated the establishment of self-sustaining status epilepticus. Injection into the hilus of dynorphin-A(1-13) after the end of perforant path stimulation, stopped established status epilepticus, while administration of naloxone, naltrindole and ICI-174864 were ineffective. We conclude that kappa-opioids in the hippocampus counteract initiation and maintenance of status epilepticus, while delta-opioids promote initiation, but not maintenance of seizure activity. These data are important for the understanding the mechanisms which underlie initiation and maintenance of status epilepticus and for the development of new approaches for its effective management.

Protection by an adenosine analogue against kainate-induced extrahippocampal neuropathology

1. The glutamate analogue kainic acid produces neuronal damage in the central nervous system. We have reported that analogues of adenosine, such as R-N6-phenylisopropyladenosine (R-PIA) can, at doses as low as 10 microg/kg IP, prevent the hippocampal damage that follows the systemic administration of kainate. The present work was designed to examine purine protection against kainate in extrahippocampal regions by using histological methods. 2. The results show that R-PIA, at a dose of 25 microg/kg IP in rats, can protect against the neuronal damage caused by kainate in the basolateral amygdaloid nuclei, the pyriform cortex and around the rhinal fissure. This protection could be prevented by the simultaneous administration of the A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine, confirming that the protection involved adenosine A1 receptors. No protection was observed in the posterior amygdaloid nuclei or the entorhinal cortex, suggesting the absence of relevant adenosine receptors or a different mechanism of excitotoxicity.

Differential response of calmodulin genes in the mouse brain after systemic kainate administration

In the central nervous system, many of the effects resulting from an increase in the intracellular levels of calcium are mediated by calmodulin, a major calcium-binding protein in the mammalian brain. Calmodulin is expressed by three different genes, namely CaM I, CaM II and CaM III, all of which encode an identical protein. We studied the expression of calmodulin in the mouse brain at different times after the administration of a convulsant dose of kainate, a potent neuroexcitotoxic agent. We detected the presence of the different calmodulin messenger RNAs and of the protein itself in brain sections by in situ hybridization histochemistry and immunocytochemistry respectively. In addition, we determined the calmodulin content in brain regions by radioimmunoassay. Kainate-treated animals did not show areas of neuronal death at the different times following administration considered. An increase in the hybridization signal for CaM I messenger RNAs was observed from 5 h after kainate administration in the different brain regions tested. In contrast, the CaM II messenger RNA signal decreased gradually to a minimum 24 h after treatment in the hippocampus, while the CaM III messenger RNA signal was mostly unaffected. Calmodulin immunoreactivity also increased in the hippocampus. Nevertheless, we did not detect any significant difference in calmodulin content between brain regions of control and treated animals by radioimmunoassay. Kainate treatment induced modifications in the expression of calmodulin at the level of both messenger RNAs and protein. The results suggest a differential regulation of the three calmodulin genes in the adult mouse brain and a post-transcriptional or a post-translational regulation of calmodulin expression.