Signaling in the basal ganglia: postsynaptic and presynaptic mechanisms.

The selection and execution of appropriate motor behavior result in large part from the ability of the basal ganglia to collect, integrate and feedback information coming from the cerebral cortex. The GABAergic medium spiny neurons (MSNs) of the striatum represent the main receiving station of the basal ganglia. These cells are innervated by excitatory glutamatergic fibers from cortex and thalamus, and modulatory dopaminergic fibers from the midbrain. MSNs comprise two populations of projection neurons, which give rise to the direct, striatonigral pathway, and indirect, striatopallidal pathway. Changes in transmission at the level MSNs affect the activity of thalamocortical projection neurons, thereby influencing motor behavior. For instance, the cardinal symptoms of Parkinson's disease, such as tremor, rigidity and bradykinesia, are caused by the selective degeneration of dopaminergic neurons originating in the substantia nigra pars compacta, which modulate the activity of MSNs in the dorsal striatum. The therapy for Parkinson's disease relies on the use of levodopa, but is hampered by neuroadaptive changes affecting dopaminergic and glutamatergic transmission in striatonigral neurons. MSNs are also the target of many psychoactive drugs. For example, caffeine affects motor activity by blocking adenosine receptors in the basal ganglia, thereby affecting neurotransmission in striatopallidal neurons. The present review focuses on studies performed in our laboratory, which provide a molecular framework to understand the effects on motor activity of adenosine and caffeine.