The mGluRs group II agonist (2S,3S,4S)-alpha-carboxycyclopropyl-glycine induces catalepsy in the rat, which is pronouncedly antagonised by dizocilpine and D,L-amphetamine

Activation of metabotropic glutamate receptor 5 has direct excitatory effects and potentiates NMDA receptor currents in neurons of the subthalamic nucleus

The subthalamic nucleus (STN) is a key nucleus in the basal ganglia motor circuit that provides the major glutamatergic excitatory input to the basal ganglia output nuclei. The STN plays an important role in normal motor function, as well as in pathological conditions such as Parkinson's disease (PD) and related disorders. Development of a complete understanding of the roles of the STN in motor control and the pathophysiological changes in STN that underlie PD will require a detailed understanding of the mechanisms involved in regulation of excitability of STN neurons. Here, we report that activation of group I metabotropic glutamate receptors (mGluRs) induces a direct excitation of STN neurons that is characterized by depolarization, increased firing frequency, and increased burst-firing activity. In addition, activation of group I mGluRs induces a selective potentiation of NMDA-evoked currents. Immunohistochemical studies at the light and electron microscopic levels indicate that both subtypes of group I mGluRs (mGluR1a and mGluR5) are localized postsynaptically in the dendrites of STN neurons. Interestingly, pharmacological studies suggest that each of the mGluR-mediated effects is attributable to activation of mGluR5, not mGluR1, despite the presence of both subtypes in STN neurons. These results suggest that mGluR5 may play an important role in the net excitatory drive to the STN from glutamatergic afferents. Furthermore, these studies raise the exciting possibility that selective ligands for mGluR5 may provide a novel approach for the treatment of a variety of movement disorders that involve changes in STN activity.

The neuroprotectant properties of glutamate antagonists and antiglutamatergic drugs

In the slowly progressive neurodegenerative disorders like Parkinson's disease and Alzheimer's disease very different neuronal populations undergo degenerative processes, although the cascades of cellular events leading to death are supposed to be similar. We suggest that the complex pattern of degeneration in Parkinson's disease depends on two processes, a 'primary neurodegeneration' that takes place in the striato-nigral dopamine neurons and a 'secondary degeneration', occurring in distant structures of the basal ganglia network. For the purpose of explaining the regionally different expression of 'primary neurodegeneration' in different diseases, we postulate that the origin of neurodegeneration is associated with the local release of a neurotransmitter. For Parkinson's disease this would mean that the metabolism of dopamine in the striatum, nucleus accumbens and presumably the pedunculopontine tegmental nucleus, together with one or more pathological factors contribute to the initial neurodegeneration. There are recent studies indicating that a transneuronal retrograde degeneration of the substantia nigra pars compacta neurons might be induced by a loss of function of dopaminergic synapses in the striatum. We have recently established an animal model of retrograde striato-nigral degeneration, where the assessment of markers for cellular stress is possible. In Parkinson's disease, several structures distal from the substantia nigra pars compacta undergo neuropathological changes, characterizing the 'secondary neurodegeneration. Our recent studies provide experimental evidence for a chronic cellular stress in these structures because of a relative or absolute glutamatergic overactivity due to the initial loss of dopaminergic innervation. Thus, a loss of dopamine transforms the basal ganglia to a 'destructive network'. Both processes, the 'primary' and 'secondary neurodegeneration', affecting each other, characterize the progress of chronic neurodegeneration. From this point of view, we would further like to develop strategies for symptomatic amendment. Excitatory amino acids seem to be involved not only in the secondary processes of neurodegeneration, but also in initiation of the 'primary degeneration' of the substantia nigra pars compacta. Therefore, a reduction of glutamatergic overactivity constitutes a promising neuroprotective strategy. Especially the new antagonists of the NMDA-receptors with high affinity to the NR2B subunit of the receptor are in focus of our interest, since they reveal a favourable profile of side effects, therefore providing a promising tool for neuroprotection.