Current Drug Targets-CNS & Neurological Disorders - Volume 1, Issue 3, 2002

The basal ganglia are implicated in a number of disorders including neurodegenerative motor diseases such as Huntington's and Parkinson's disease, as well as psychiatric disorders such as schizophrenia and obsessive compulsive disorder. In recent years, a great deal of effort has been focused on determining the basal ganglia circuitry that underlies normal behavior, as well as many of these syndromes. This has led to a detailed understanding of both the normal and pathophysiological flow of information through the basal ganglia, and has provided the opportunity to begin developing novel pharmacological methods of intervention by targeting neuromodulatory receptors with in the basal ganglia circuit. One group of receptors that holds much promise for several basal ganglia disorders is the metabotropic glutamate receptors. Data from behavioral, neurochemical, neuroanatomical and electrophysiological studies has begun to reveal the functional roles that the metabotropic glutamate receptors play in modulating the basal ganglia circuit, and suggests that compounds selectively targeting these receptors may provide novel therapies for a variety of disorders including Parkinson's disease, addiction, and epilepsy.

Epilepsy is a disorder that afflicts more than 50 million people worldwide. Current antiepileptic drugs (AEDs), although effective in controlling seizures for the majority of individuals, remain far from ideal as therapeutics. There is a need for new drugs that act at different molecular targets than currently available AEDs and for new therapies designed to block the process of epileptogenesis. Because of their central role in modulating numerous physiological processes in the central nervous system, metabotropic glutamate receptors (mGluRs) have been implicated in the pathophysiology of a variety of neurological conditions including epilepsy. mGluRs represent attractive new targets for therapeutic control of seizures and interruption of the epileptogenic process. We review the involvement of mGluRs in the induction and expression of epileptic seizures, their potential roles in the process of epileptogenesis, and their altered expression and function in the epileptic human brain.

Schizophrenia is a debilitating chronic psychiatric illness affecting 1% of the population. The cardinal features of schizophrenia are positive symptoms (thought disorder, hallucinations, catatonic behavior), negative symptoms (social withdrawal, anhedonia, apathy) and cognitive impairment. Although progress in elucidating the aetiology of schizophrenia has been slow, new insights on the neurochemical and neurophysiological mechanisms underlying the pathophysiology of this illness are beginning to emerge. The glutamate / N-methyl-D-aspartate (NMDA) hypofunction hypothesis of schizophrenia is supported by observations that administration of NMDA glutamate receptor antagonists such as phencyclidine (PCP) or ketamine induces psychosis in humans moreover, decreased levels of glutamate and changes in several markers of glutamatergic function occur in schizophrenic brain. Administration of PCP or ketamine to rodents elicits an increase in locomotion and stereotypy accompanied by an increase in glutamate efflux in several brain regions. Systemic administration of group II metabotropic glutamate (mGlu) receptor agonists suppresses PCP-induced behavioral effects and the increase in glutamate efflux. Activation of group II mGlu receptors (mGlu2 and mGlu3) decreases glutamate release from presynaptic nerve terminals, suggesting that group II mGlu receptor agonists may be beneficial in the treatment of schizophrenia. In addition, pharmacological manipulations that enhance NMDA function may be efficacious antipsychotics. Selective activation of mGlu5 receptors significantly potentiates NMDAinduced responses, supporting this novel approach for the treatment of schizophrenia.The glutamate hypothesis of schizophrenia predicts that agents that restore the balance in glutamatergic neurotransmission will ameliorate the symptomatology associated with this illness. Development of potent, efficacious, systemically active drugs will help to address the antipsychotic potential of these novel therapeutics.This review will discuss recent progress in elucidating the pharmacology and function of group II mGlu and mGlu5 receptors in the context of current hypotheses on the pathophysiology of schizophrenia and the need for new and better antipsychotics.

The excitatory amino acid glutamate plays a major role in nociceptive processing. Ionotropic and metabotropic glutamate receptors are expressed in relevant areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Activation of mGlu receptors along the pain neuraxis can result in either pronociceptive or antinociceptive behaviors depending on the subtype of mGluR and its location. The data published to date most strongly support the idea that mGlu1 antagonists might act as broad-spectrum analgesics. Several studies pointing to a functional upregulation of mGlu2 / 3 in chronic pain models suggest that agonists of these receptors might also be effective analgesics in certain conditions, most notably inflammation-induced hyperalgesia and allodynia. The expression of mGluRs throughout the pain neuraxis and the differing roles of the mGluRs in each of these regions makes it difficult to predict the efficacy of mGluR ligands based on in vitro or local administration studies. Potent, systemically active compounds that show mGluR subtype selectivity will be critical to undertake more detailed analyses in animal models of pain.

The metabotropic glutamate receptors are G-protein coupled receptors (GPCR) involved in the regulation of many synapses, including most glutamatergic fast excitatory synapses. Eight subtypes have been identified that can be classified into three groups. The molecular characterization of these receptors revealed proteins much more complex than any other GPCRs. They are composed of a Venus Flytrap (VFT) module where glutamate binds, connected to a heptahelical domain responsible for G-protein coupling. Recent data including the structure of the VFT module determined with and without glutamate, indicate that these receptors function as dimers. Moreover a number of intracellular proteins can regulate their targeting and transduction mechanism. Such structural features of mGlu receptors offer multiple possibilities for synthetic compounds to modulate their activity. In addition to agonists and competitive antagonists acting at the glutamate binding site, a number of noncompetitive antagonists with inverse agonist activity, and positive allosteric modulators have been discovered. These later compounds share specific properties that make them good candidates for therapeutic applications. First, their non-amino acid structure makes them pass more easily the blood brain barrier. Second, they are much more selective than any other compound identified so far, being the first subtype selective molecules. Third, for the negative modulators, their non competitive mechanism of action makes them relatively unaffected by high concentrations of glutamate that may be present in disease states (e.g. stroke, epilepsy, neuropathic pain, etc.). Fourth, like the benzodiazepines acting at the GABA A receptors, the positive modulators offer a new way to increase the activity of these receptors in vivo, with a low risk of inducing their desensitization. The present review article focuses on the specific structural features of these receptors and highlights the various possibilities these offer for drug development.