Current Medicinal Chemistry - Central Nervous System Agents - Volume 4, Issue 1, 2004

Nucleic acid based technology (antisense, ribozyme, DNA enzyme, RNAi) has made huge advances in the last few years and generated considerable expectations especially in the neurosciences. As a frontier method, these new technologies may be used to combat diseases, as well as to study gene functions by interfering in the genetic flow of information from DNA to RNA to protein. In the following article I will describe antisense techniques, starting with the chemical nature of the antisense oligodeoxynucleotides and some of the most commonly used derivatives. I will then give some guidelines on antisense, ribozyme, DNA enzyme and RNAi construction, modifications and application. This will be followed by discussion of the possible mode of action as well as expansion of the oligo-based technique to ribozyme, DNA enzyme morpholino and anti-RNA mediated gene inhibition. Finally, I will describe some common problems that may be encountered during these applications and some experimental examples.

Aim of this review is to outline the relevance of the neuronal histaminergic system in cognition and to suggest potential therapeutic uses of histaminergic ligands for the treatment of cognitive disfunctions that accompany several neural disorders. Recent elegant data have shown that histaminergic neurones are one of the major excitatory sources of cortical activation, and maintain the brain awake allowing it to perform diverse functions such as emotional and cognitive activities. Indeed, both physiological and morphological characteristics of histaminergic neurones are coherent with such a role, as they project profusely to the whole brain and spinal cord and fire according to the behavioural state of the animal. Activation of histaminergic hetero- and / or autoreceptor subtypes modulate neurotransmitter release and neuronal firing frequency, presumably to different degrees and at different times during the elaboration of disparate behavioural responses. Therefore, the histaminergic system may provide a crucial mechanism to fine tuning brain activation for an adequate behavioural response. There is strong evidence of more direct effects of histamine on cognitive processes: one of the mechanisms implicated seems to rest on the modulation of the cholinergic function, as systemic or intracerebral administrations of histaminergic compounds modulate acetylcholine release and affect memory. Furthermore, recent evidence suggests that activation of histaminergic receptors subtypes regulate neuronal intracellular signalling pathways such as MAPK cascade, and modulate plastic changes, which are two early cellular mechanisms likely involved in memory consolidation. Not surprisingly, both facilitatory and inhibitory effects of histamine on learning and memory have been described, as it is likely that the memory modulating action of histamine affects several brain regions differently. Studies of the physiology and function of the histaminergic system would not be possible without the tremendous effort of several investigators to synthesise ever more selective and more potent ligands for the histaminergic receptor subtypes. The recent discovery of constitutively active histaminergic receptors adds a further degree of complexity to the scenario, but at the same time broadens the expectations for new therapeutic approaches for the treatment of CNS dysfunctions.

Non-immunosuppressive immunophilin ligands (NI-IPLs) are attracting attention as new candidate drugs for neuroprotection and / or neurorestoration, particularly, since they do not have the adverse effects of immunosuppressants. However, it is not yet enough to understand that NI-IPLs are useful drugs for treating neurological disorders. In particular, the molecular mechanism of NI-IPL activity in target cells in the brain remains obscure. In this review, we focused on the molecular basis of the neuroprotective properties of FK506 binding protein (FKBP)-binding IPLs. Our findings suggest that IPLs have neuroprotective effects mediated by multiple beneficial properties such as a GSH-activating effect, a NTF-activating effect, and an anti-apoptotic effect, but not by an immunosuppressive effect, both in cell cultures and in vivo. In particular, the GSH-activating effect and the NTF-activating effect of NI-IPLs may be essential to the expression of their neuroprotective properties. Thus, NI-IPLs might have a potentially beneficial effect by ameliorating neurological disorders, since they do not cause serious side effects such as immune deficiency.

Adenosine is an endogenous modulator that regulates many Central Nervous System functions, and whose effects are mediated by four G-protein coupled receptors (A1 , A2A , A2B , A3 ). Adenosine A2A receptors have been recently regarded as promising targets for the development of neuroprotective strategies. In particular, since an abnormal glutamate outflow is thought to play a crucial role in triggering the cellular events leading to excitotoxic neuronal death, and since A2A receptors positively modulate glutamate outflow, it has been supposed that their blockade could represent a suitable approach to the treatment of neurodegenerative diseases. In agreement with this hypothesis, both the genetic inactivation and the pharmacological blockade of A2A receptors proved effective in several models of brain injury. In some studies, the neuroprotective effects of A 2A receptor antagonists correlated well with their ability to prevent injury- or toxin- stimulated glutamate outflow. The “beneficial” effects of A2A receptor antagonists at the pre-synaptic sites (namely their ability to attenuate glutamate release), however, seem to occur only at very low doses and at certain time points after the insult is given. Moreover, A2A receptor antagonists seem unable to prevent the toxic effects elicited by direct stimulation of post-synaptic NMDA receptors. It is concluded that, although A2A antagonists show clear neuroprotective effects in models of brain injury, their actual therapeutic potential needs to be confirmed in a wider range of doses, at different stages of brain injury as well as in models of neurodegenerative diseases, in which pre- and post-synaptic effects play different relative roles.

At the present time, there are no drugs are approved in the U.S. for the treatment of stimulant abuse. Based on a diverse body of preclinical data, sometimes termed the “dopamine hypothesis” of drug addiction, one approach to the treatment and prevention of stimulant abuse is the development of high affinity inhibitors of the dopamine transporter. This approach awaits clinical validation. As part of our program on the development of novel probes to study the structure and function of central nervous system, we have focused on the high affinity dopamine transport inhibitor GBR12909 (1- {2-[bis-(4-fluorophenyl)methoxy]ethyl}-4-(3-phenylpropyl)piperazine) as a tool to test the dopamine hypothesis in humans. Various analogues of GBR12909 have been evaluated both in vitro and in vivo. In particular, behavioral studies have shown that GBR12909 and several of its analogues decrease cocaine-maintained responding without affecting food-maintained responding and also appear to have reduced abuse liability as compared to cocaine in the monkey. Initial clinical studies in humans demonstrated that orally administered GBR12909 is safe, well-tolerated and achieves moderate occupancy of the dopamine transporter. This article will review the structure-activity relationships of GBR-type agents at the biogenic amine transporters, their biological activity and their potential as tools to test the dopamine hypothesis of cocaine addiction in humans.

Reactive oxygen species (ROS) are considered to be the cause of oxidative stress, and consequently, ROS significantly contribute to aging and various diseases. Oxidative stress involves an imbalance (oxidants vs antioxidants) inclining toward oxidation, and is significant especially in the brain, because much oxygen is consumed and concentrations of antioxidants and related enzymes are relatively low. Therefore, it is considered that oxidative stress is closely related to the mechanisms of brain aging and neurodegenerative diseases. This paper reviews in vivo and in vitro molecular imaging techniques, with the results of imaging used to assess the molecular mechanism of oxidative stress, originating from the mitochondria of cerebral tissue. The mitochondrial electron transfer function, which can be inhibited by in vitro simulated ischemia, causes glycolysis to switch from an aerobic (36 ATP molecules formed from one glucose molecule) to anaerobic process (2 ATP molecules formed from one glucose molecule). Under anaerobic conditions, glucose uptake into cells is enhanced because of the low efficiency of ATP formation. This causes the stimulation of ROS formation when aerobic conditions return. The content of glutathione (GSH), a major antioxidant in the brain, and mitochondrial electron transport function decrease with age, whereas glucose transport and metabolism are maintained throughout the aging process. It is thought that glucose metabolism remains constant with increasing age to compensate for the decreased efficiency of electron transfer in the mitochondria; a phenomenon that is also observed in cerebral ischemia. Mitochondria are one of the main sources of ROS, because 95% of molecular oxygen is metabolized within mitochondria, and 2% of metabolized oxygen is converted to ROS as a byproduct, even under normal conditions. The mitochondrial GSH concentration is maintained at a high level by the transport system located on the inner membrane of the mitochondria. Manganese-superoxide dismutase (Mn-SOD) decomposes . O2 - in mitochondria. However, ROS may form at a rate that exceeds the mitochondrial antioxidants and antioxidative enzymes. This causes an accumulation of oxidative damage at the molecular level. Such damage leads to delayed neuronal death through the release of apoptogenic factors such as cytochrome c, from the mitochondrial membrane. To limit oxidative stress in mitochondria, the development of antioxidant related drugs and effective methods of delivering these drugs into mitochondria are required to prevent and / or cure brain aging and neurodegenerative diseases.