Paolo Calabresi

Receptor subtypes involved in the presynaptic and postsynaptic actions of dopamine on striatal interneurons

By stimulating distinct receptor subtypes, dopamine (DA) exerts presynaptic and postsynaptic actions on both large aspiny (LA) cholinergic and fast-spiking (FS) parvalbumin-positive interneurons of the striatum. Lack of receptor- and isoform-specific pharmacological agents, however, has hampered the progress toward a detailed identification of the specific DA receptors involved in these actions. To overcome this issue, in the present study we used four different mutant mice in which the expression of specific DA receptors was ablated. In D1 receptor null mice, D1R-/-, DA dose-dependently depolarized both LA and FS interneurons. Interestingly, SCH 233390 (10 μm), a D1-like (D1 and D5) receptor antagonist, but not l-sulpiride (3–10 μm), a D2-like (D2, D3, D4) receptor blocker, prevented this effect, implying D5 receptors in this action. Accordingly, immunohistochemical analyses in both wild-type and D1R-/- mice confirmed the expression of D5 receptors in both cholinergic and parvalbumin-positive interneurons of the striatum. In mice lacking D2 receptors, D2R-/-, the DA-dependent inhibition of GABA transmission was lost in both interneuron populations. Both isoforms of D2 receptor, D2L and D2S, were very likely involved in this inhibitory action, as revealed by the electrophysiological analysis of the effect of the DA D2-like receptor agonist quinpirole in two distinct mutants lacking D2L receptors and expressing variable contents of D2S receptors. The identification of the receptor subtypes involved in the actions of DA on different populations of striatal cells is essential to understand the circuitry of the basal ganglia and to develop pharmacological strategies able to interfere selectively with specific neuronal functions.

LONG-TERM CHANGES OF CORTICOSTRIATAL SYNAPTIC TRANSMISSION: POSSIBLE IMPLICATION FOR MOTOR MEMORY

Intracellular and extracellular recordings from striatal neurons maintained in brain slices have shown that the activation of corticostriatal terminals produces excitatory postsynaptic potentials (EPSPs) mediated by the release of excitatory amino acids. Tetanic stimulation of cortical fibers induces long-term depression (LTD) of corticostriatal transmission. Membrane depolarization during the tetanus was required to produce LTD. LTD was not blocked by AVP indicating that the activation of NMDA receptors is not required for this event. LTD was blocked either by intracellular application of calcium (Ca2+)-chelators or by bath application of the Ca2+ channel blocker nifedipine suggesting that a rise in intracellular Ca2+ levels is necessary for the generation of striatal LTD. LTD was also blocked by inhibitors of Ca2+-dependent protein kinases. The role of metabotropic glutamate receptors (mGluRs) and of dopamine (DA) receptors in the formation of this form of synaptic plasticity was studied by utilizing different pharmacological and physiological approaches. When NMDA receptors were deinactivated by removing magnesium (Mg2+) from the external medium, the same tetanic stimulation which in control condition produced LTD, under this condition caused long-term potentiation (LTP) of synaptic transmission. LTP was fully blocked by NMDA-receptor antagonists. Our findings show that in the striatum it is possible to induce both LTD and LTP of excitatory synaptic transmission. These forms of synaptic plasticity may play a role in motor memory.

Basic Electrophysiology and Possible New Therapeutic Approaches to Movement Disorders

What the basal ganglia do, is it the on-going question? New models have reevaluated the input/output ratio of single structures as inserted in parallel, functional systems (Alexander and Crutcher, 1990). These models have reinforced the assumption that the basal ganglia are a key station for the execution of organized movements (DeLong, 1990; Goldman-Rakic and Selemon, 1990). At the molecular level, new families of receptors are explored. The cloning of glutamate metabotropic receptors is heading the surprising multiplicity of the neurobiology of excitatory transmission (Gasic, 1992). The definition of new subclasses of dopamine receptors is an invitation to reconsider the pharmacology of the amine (Surmeier et al., 1992). Radical changes, however, in the therapy of movement disorders have barely taken place, being the introduction of levo-dopa still a “cornerstone” of the therapy of the parkinsonian patient (Hornykiewicz, 1966). Whatever are the fundamental functions of the basal ganglia, a striking dichotomy risks to develop between basic research acquisition and the daily urgency of patient’s quality of life. In presenting our recent findings, we aim to highlight those aspects of mesencephalic, neostriatal and pallidal physiology whose clinical impact could be relevant.

Glutamate Transmission in the Pathogenesis of Parkinson’s Disease

Parkinson’s disease (PO), a progressive neurodegenerative disorder, is a common cause of disability. The pathological hallmarks are the presence of Lewy bodies and massive loss of dopaminergic neurons in the pars compacta of the substantia nigra. The current pathophysiological concept of PD postulates a multifactorial origin, where alterations in neurotransmitter content are combined with genetic and environmental factors. With nigrostriatal dopamine depletion, a complex set of changes occurs in the functional anatomy of the basal ganglia circuitry. As a result, the firing pattern of certain glutamatergic pathways has been shown to change significantly, and to play a central role in the pathogenesis of parkinsonian symptoms. Advances in genetics have led to the discovery of gene mutations underlying some forms of PD. The mutated genes encode proteins of unknown function, such as alpha-synuclein and parkin. Moreover, compelling evidence supports the involvement of mitochondrial metabolism failure as an essential cofactor in the pathogenesis of PD. Interestingly, some environmental toxins are thought to be able to act as mitochondrial toxins. The comprehension of the pathways leading to PD requires an intense effort in order to identify and establish a plausible connection between genetic causes, altered neurotransmission and metabolic impairment.

Dopamine-Dependent Long-Term Potentiation Induced by 3-Nitropropionic Acid in Striatal Medium Spiny Neurons

The classical clinical symptoms of Huntington’s disease (HD) include abnormal involuntary movements (chorea) and cognitive impairment. This genetically determined disorder selectively involves degeneration of striatal spiny neurons while sparing striatal large cholinergic interneurons.1 HD is caused by an expansion of CAG repeats near the 5′ end of the IT15 gene. IT15 encodes an ubiquitously expressed protein called huntingtin. Moreover, a remarkable decrease in the activity of mitochondrial complex II (succinate dehydrogenase, SD) has been found in brains of HD patients.2 Indeed, the link between bioenergetic defects and excitotoxic mechanisms, two pathological events which seems to play a major role in HD3,4 to the mutated huntingtin, remains unknown. The corticostriatal projection represents one of the major glutamatergic pathways in the brain and an abnormal release of glutamate from this pathway seems to play a pathogenic role in HD. The complex II inhibitors 3-nitropropionic acid (3-NP) and methylmalonic acid (MMA) mimic the pathology of HD.5,6 Thus, enhanced glutamatergic transmission may trigger neurodegeneration in neurons, the energy metabolism of which is compromised due to impaired SD activity. We studied the electrophysiological effects of the pharmacological blockade of SD by either 3-NP or MMA on glutamatergic excitatory postsynaptic potentials (EPSPs), in order to investigate the link between metabolism impairment and glutamatergic transmission both in striatal spiny neurons and cholinergic interneurons. The t-LTP might play a key role in the regional and cell-type specific neuronal death observed in HD.

Role of D1 and D2 Dopamine Receptors in the Mammalian Striatum: Electrophysiological Studies and Functional Implications

Pharmacological, biochemical, and anatomical studies have indicated that at least two receptors for dopamine (D1 and D2) are present in mammalian CNS (Creese et al., 1983; Stoof and Kebabian, 1984). D1 receptors are associated with the stimulation of adenylate cyclase, while D2 receptors mediate either the inhibition of adenylate cyclase or are unlinked with this enzyme. Although the pharmacological evidence in favor of this receptor heterogeneity is strong, the physiological and clinical implications of these receptor classifications are not fully clarified.