Current Medicinal Chemistry - Volume 10, Issue 20, 2003

The excitatory neurotransmitter glutamate plays a central role in the development, plasticity, and repair of the CNS. The last decade known as the 'Decade of the Brain' has been a fruitful epoch in the field of glutamate receptor biology. However, many questions still remain about the true in vivo function of glutamate receptor subtype mediated signaling. The first review by G.N. Barnes, M.D. / PhD. and J.T. Slevin, M.D. is designed to familiarize the reader with recent developments, especially those involved in neurological disease. First, the history and development of glutamate receptor biology up to the beginning of the 1990's is reviewed. Ionotropic and metabotropic glutamate receptors are discussed with particular emphasis on the NMDA, AMPA, and kainic acid ionotropic receptors. The cellular and subcellular localization, signaling cascades linked to each receptor subtype, and role in normal synaptic physiology are reviewed. The last segment of the article covers recent advances in our understanding of glutamate receptor signaling that induces changes in synaptic connectivity and function in animal models of epilepsy and stroke. The second review by Tsung-Ping Su, Ph.D., an eminent authority on sigma receptors, along with Teruo Hayashi, Ph. D., brings attention to the biochemical and physiological roles of sigma receptors particularly sigma-1 receptors that have just begun to unveil. Sigma-1 receptors exist mainly in the central nervous system. Sigma-1 receptor ligands include cocaine, (+) benzomorphans like (+) pentazocine and (+) N-allyl-normetazocine [or (+) - (SKF-10047] and endogenous neurosteroids like progesterone and pregnenolone sulfate. Many pharmacological and physiological actions have been attributed to sigma-1 receptors. These include the regulation of IP3 receptors and calcium signaling at the endoplasmic reticulum, mobilization of cytoskeletal adaptor proteins, modulation of nerve growth factor-induced neurite sprouting, modulation of neurotransmitter release and neuronal firing, modulation of potassium channels as a regulatory subunit, alteration of psychostimulant-induced gene expression, and blockade of spreading depression. Behaviorally, sigma-1 receptors are involved in learning and memory, psychostimulant-induced sensitization, cocaine-induced conditioned place preference, and pain perception. Notably, in almost all the aforementioned biochemical and behavioral tests, sigma-1 agonists, while having no effects by themselves, caused the amplification of signal transductions incurred upon the stimulation of the glutamatergic, dopaminergic, IP3-related metabotropic, or nerve growth factor-related systems. Thus, it is hypothesized that sigma-1 receptors, at least in part, are intracellular amplifiers creating a supersensitized state for signal transduction in the biological system. The third review by Pankaja K. Kadaba, Ph.D., the guest editor, conducts a comprehensive examination of the Δ2-1,2,3- triazoline heterocycles, a unique class of anticonvulsants and antiischemic agents. The triazolines are hypothesized to act as a group of “built in” heterocyclic prodrugs and their potential metabolites (β-amino alcohols V and VA, and α-amino acid VI) are postulated from a knowledge of their chemistry. Studies on their metabolism along with their pharmacology, have proved the prodrug hypothesis and the β-amino alcohol V has been identified as the active species; there is no proof for the formation of the α-amino acid VI. Radio-ligand binding studies indicate that the active β-amino alcohol metabolite acts as an MK- 801 / NMDA antagonist and that the triazolines act as excitatory amino acid (EAA) antagonists. The parent triazolines inhibit presynaptic glutamate release. Some triazolines show an augmentation of 50-63%, in the C1- channel activity, a useful membrane action that reduces the excessive L-Glu release that occurs during epileptic seizures. The high anticonvulsant activity of TRs in a variety of seizure models including their effectiveness in the kindling model of complex partial seizures may be due to their unique dual-action mechanism whereby the TR and V together effectively impair both pre- and postsynaptic aspects of EAA neurotransmission; thus the TRs have clinical potential in the treatment of complex partial epilepsy which is refractory to currently available drugs. Since there is strong evidence that L-Glu plays an important role in human epilepsy as well as in brain ischemia / stroke, and since the TRs act by inhibiting EAA neurotransmission, it was logical to expect that the anticonvulsant TRs may evince beneficial therapeutic potential in cerebral ischemia resulting from stroke as well. And indeed, several TRs, when tested in the standard gerbil model of global ischemia did evince remarkable ability to prevent neuronal death. The fourth review section, also written by Pankaja K. Kadaba, Ph.D. along with Trupti Dixit, Ph. D., elucidates the metabolism and pharmacology of the 1,2,3-triazolines (TRs) and the identification of the triazoline pharmacophore and the subsequent evolution of the aminoalkylpyridines (AAPs). The AAPs have no activity in the scMet test but are highly effective in the MES seizure test by the oral route. The AAPs bind to the σ1 receptor with low affinity, but high selectivity. They impair Glu release to the same extent as the triazolines and afforded a high degree of protection in the kindled rat. They show no affinity for the NMDA / PCP receptor sites; thus the toxic side effects of NMDA antagonists are absent in the σ selective AAPs. Variations of the heterocyclic unit, the alkyl chain and the amino group in the AAP molecules indicated that the 4- pyridyl substituent along with a methyl (alkyl) group, and a 4-C1, 3-C1 or 3,4-C12 substitution on the N-phenyl group, afforded the most active compounds. Although the AAPs are very effective in the MES and the kindling models of epilepsy, they showed only low to moderate activity in protecting neuronal cells in stroke-induced cerebral ischemia. In the case of the TR compounds, even the least effective TR afforded 47% protection from neuronal injury. It is not known at this point, whether activity in both the MES and scMet test, which would imply a role for both Glu and GABA is a prerequisite for antiischemic activity. Finally, I wish to take this opportunity to thank Professor Atta-ur Rahman for his gracious invitation requesting me to be guest editor for a special topics issue that will cover mostly my research efforts on central nervous system (CNS) drugs over a span of more that 35 years, particularly anticonvulsant sand antiischemic agents. I also want to thank all the authors who so kindly agreed to contribute to this special topics issue. I also thank Dr. Atif Hussain, the Manager of publications, for his continuing patience on several occasions when unexpected delays in article submission and refereeing had occurred.

Glutamate receptor signaling is essential to normal synaptic function in the central nervous system. The major ionotropic glutamate receptors (AMPA, Kainic, and NMDA) have different synaptic functions depending upon cellular and subcellular localization, subunit composition, and second messenger systems linked to the receptors. In this review, we examine major advances in glutamate receptor biology whose physiology plays a central role in neurologic disease such as epilepsy and stroke. A key feature of glutamate receptor activation in neurologic disease is the downstream effects on cell survival, genetic expression of axon guidance cues, synaptic connectivity / formation of networks, and neuronal excitability. Identification of therapeutic pharmacologic targets and development of antagonists specific to the disease process remain central themes in epilepsy and stroke research.

Although sigma receptors were discovered in 1982, the biochemical and physiological roles of sigma receptors have just begun to unveil. Sigma receptors are non-opioid, non-phencyclidine receptors that contain two subtypes: sigma-1 and sigma-2 receptors. The sigma-1 receptor has been cloned and its sequence does not resemble that of any mammalian protein. Sigma-2 receptors have not been cloned. The focus of this review will be on sigma-1 receptors. Sigma-1 receptors contain 223 amino acids and reside primarily at the endoplasmic reticulum. Sigma-1 receptors exist mainly in the central nervous system, but also in the periphery. Sigma-1 receptor ligands include cocaine, (+)-benzomorphans like (+)-pentazocine and (+)N-allyl-normetazocine (or (+)- SKF-10047), and endogenous neurosteroids like progesterone and pregnenolone sulfate. Many pharmacological and physiological actions have been attributed to sigma-1 receptors. These include the regulation of IP3 receptors and calcium signaling at the endoplasmic reticulum, mobilization of cytoskeletal adaptor proteins, modulation of nerve growth factor-induced neurite sprouting, modulation of neurotransmitter release and neuronal firing, modulation of potassium channels as a regulatory subunit, alteration of psychostimulant-induced gene expression, and blockade of spreading depression. Behaviorally, sigma-1 receptors are involved in learning and memory, psychostimulant-induced sensitization, cocaine-induced conditioned place preference, and pain perception. Notably, in almost all the aforementioned biochemical and behavioral tests, sigma-1 agonists, while having no effects by themselves, caused the amplification of signal transductions incurred upon the stimulation of the glutamatergic, dopaminergic, IP3-related metabotropic, or nerve growth factor-related systems. Thus, it is hypothesized that sigma-1 receptors, at least in part, are intracellular amplifiers creating a supersensitized state for signal transduction in the biological system.

The Δ2- 1,2,3- triazoline anticonvulsants (TRs) may be considered as representing a unique class of “built-in” heterocyclic prodrugs where the active “structure element” is an integral part of the ring system and can be identified only by a knowledge of their chemical reactivity and metabolism. Investigations on the metabolism and pharmacology of a lead triazoline, ADD17014 suggest that the triazolines function as “prodrugs” and exert their anticonvulsant activity by impairing excitatory amino acid (EAA) L-Glutamate (L-Glu) neurotransmission via a unique “dualaction” mechanism. While an active primary β-amino alcohol metabolite from the parent prodrug acts as an N-methyl-D-aspartate (NMDA) / MK -801 receptor antagonist, the parent triazoline impairs the presynaptic release of L-Glu. Various pieces of theoretical reasoning and experimental evidence have led to the clucidation of the dual-action mechanism. Based on the unique chemistry of the triazolines, and their metabolic pathways, biotransformation products of TRs were predicted to be the β-amino alcohols V and VA, the α-amino acid VI, the triazole VII, the aziridine VIII and the ketimine IX. In vivo and in vitro pharmacological studies of the TR and potential metabolites, along with a full quantitative urinary metabolic profiling of TR indicated the primary β-amino alcohol V as the active species. It was the only compound that inhibited the specific binding of [3H]MK-801 to the MK-801 site, 56% at 10 μM drug concentration, but itself had no anticonvulsant activity, suggesting TR acted as a prodrug. Three metabolites were identified; V was the most predominant (45.7 ± 7.6) % of administered drug, with lesser amounts of VA, (17.3 ± 5.1) % and very minor amounts of aziridine VIII (4.0 ± 0.02)%. Since only VIII can yield VA, its formation indicated that the biotransformation of TR occurred, at least in part, through aziridine. No amino acid metabolite was detected, which implied that no in vivo oxidation of V or oxidative biotransformation of TR or aziridine by hydroxylation at the methylene group occurred. While triazoline significantly decreased Ca2+ -dependent, k+ -evoked L-Glu release (83% at 100 μM drug concentration ), some triazolines showed an augmentation of 50-63%, in the Cl- channel activity, a useful membrane action that reduces the excessive LGlu release that occurs during epileptic seizures. The high anticonvulsant activity of TRs in a variety of seizure models including their effectiveness in the kindling model of complex partial seizures may be due to their unique dual-action mechanism whereby the TR and V together effectively impair both pre- and postsynaptic aspects of EAA neurotransmission; thus the TRs have clinical potential in the treatment of complex partial epilepsy which is refractory to currently available drugs. Since there is strong evidence that L-Glu plays an important role in human epilepsy as well as in brain ischemia / stroke, and since the TRs act by inhibiting EAA neurotransmission, it was logical to expect that the anticonvulsant TRs may evince beneficial therapeutic potential in cerebral ischemia resulting from stroke as well. And indeed, several TRs, when tested in the standard gerbil model of global ischemia did evince remarkable ability to prevent neuronal death.

Elucidation of the metabolism and pharmacology of 1,2,3-triazolines (TRs) led to the identification of the triazoline pharmacophore and the evolution of the aminoalkylpyridines (AAPs). The AAPs have no activity in the scMet test but are highly effective in the MES seizure test by the oral route. The AAPs bind to the σ1 receptor with low affinity, but high selectivity. They impair Glu release to the same extent as the triazolines and afforded a high degree of protection in the kindled rat. They show no affinity for the NMDA / PCP receptor sites; thus the toxic side effects of NMDA antagonists are absent in the σ selective AAPs. Variations of the heterocyclic unit, the alkyl chain and the amino group in the AAP leads, indicated that the 4-pyridyl substituent along with a methyl (alkyl) group, and a 4-C1, 3-C1 or 3,4-C12 substitution on the N-phenyl group, afforded the most active compounds. Amino group modification by acylation did not improve activity. The hydrazone compounds were the most active. Although the AAPs are very effective in the MES and the kindling models of epilepsy, they showed only low to moderate activity in protecting neuronal cells in stroke-induced cerebral ischemia. In the case of the TR compounds, even the least effective TR afforded 47% protection from neuronal injury. It is not known at this point, whether activity in both the MES and scMet tests, which would imply a role for both Glu and GABA, is a prerequisite for antiischemic activity.

Inflammatory joint diseases are of considerable socio-economic significance. However, mechanisms of cartilage destruction are so far only poorly understood. This review is dedicated to reactive oxygen species (ROS) like superoxide anion radicals, hydrogen peroxide, singlet oxygen, hypochlorous acid, hydroxyl radicals and nitric oxide that are generated under inflammatory conditions and also to their potential contribution to cartilage degradation. First, the relevance of rheumatic diseases and potential mechanisms of cartilage degradation are discussed in this review, followed by the description of the chemical constituents and the molecular architecture of articular cartilage as well as the different cell types that play a role in inflammation and cartilage destruction. Methods of the assessment of cartilage degeneration are also shortly discussed. In the main chapter of this review the characteristics of individual ROS, their generation under in vivo conditions as well as their reactivities with individual cartilage components are discussed. Because of the low selectivity of ROS, useful “markers” of cartilage degradation allowing the differentiation of effects induced by individual ROS are also discussed. In the last chapter current therapeutic concepts of the treatment of rheumatic diseases are reviewed. The recently developed “anti-TNF-α” therapy that is primarily directed against neutrophilic granulocytes that are powerful sources of ROS and, therefore, important mediators of joint degeneration are emphasised.

Nitric oxide (NO) has been established as an important messenger molecule in various steps of brain physiology, from development to synaptic plasticity, learning and memory. However, NO has also been viewed as a major agent of neuropathology when, escaping controlled production it may directly or indirectly promote oxidative and nitrosative stress. The exact borderline between physiological, and therefore neuroprotective, and pathological, and therefore neurodegenerative, actions of NO is a matter of controversy among researchers in the field. This is reflected in the present status of drug research, that is focused on finding ways to block NO production, and therefore limit neuropathology, as well as on finding ways to increase NO availability and therefore elicit neuroprotection. As an unavoidable consequence, both classes of drugs are reported to have neurodegenerative or neuroprotective effects, depending on the models in which they are tested. Aim of the present paper is to provide the reader with a survey, as much complete as possible, on the main aspects of NO biology, from biochemistry and chemical reactivity to the molecular signals elicited in neural cells target of its neurodegenerative or neuroprotective action. In doing that, many controversial aspects related to basic biology and to neuropathology of NO are taken into account. In the final sections, main classes of drugs able to interfere with NO physiopathology are examined, in order to try to devise possible directions for future NO-based therapeutical strategies.

Regulation of expression in gene therapy is considered to be a very desirable goal, preventing toxic effects and improving biological efficacy. A variety of systems have been reported in an ever widening range of applications, this paper describes these systems with specific reference to cancer gene therapy.

Central nervous system (CNS) neural stem cells (NSCs), which are mostly defined by their ability to self-renew and to generate the three main cell lineages of the CNS, were isolated from discrete regions of the adult mammalian CNS including the subventricular zone (SVZ) of the lateral ventricle and the dentate gyrus in the hippocampus. At early stages of CNS cell fate determination, NSCs give rise to progenitors that express the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). PSA-NCAM+ cells persist in adult brain regions where neuronal plasticity and sustained formation of new neurons occur. PSA-NCAM has been shown to be involved in the regulation of CNS myelination as well as in changes of cell morphology that are necessary for motility, axonal guidance, synapse formation, and functional plasticity in the CNS. Although being preferentially committed to a restricted either glial or neuronal fate, cultured PSA-NCAM+ progenitors do preserve a relative degree of multipotentiality. Considering that PSA-NCAM+ cells can be neatly used for brain repair purposes, there is much interest for studying signaling factors regulating their development. With this regard, it is noteworthy that neurotransmitters, which belong to the micro-environment of neural cells in vivo, regulate morphogenetic events preceding synaptogenesis such as cell proliferation, migration, differentiation and death. Consistently, several ionotropic but also G-protein-coupled neurotransmitter receptors were found to be expressed in CNS embryonic and postnatal progenitors. In the present review, we outlined the ins and outs of PSA-NCAM+ cells addressing to what extent our understanding of extrinsic and in particular neurotransmitter-mediated signaling in these CNS precursor cells might represent a new leading track to develop alternative strategies to stimulate brain repair.