a to-do-list for the striatum

There is abundant information about striatal circuits described in reviews and textbooks. Some believe that the cellular details of the basal ganglia circuits and the role of interneurones have been already described in textbooks. So, one might get the wrong impression that there is nothing further to investigate.

Although much is known about the basal ganglia, some of the synaptic interactions between components are not known. In the classical scheme of the direct and indirect patways, the thalamo-striatal component is not shown. What is the function of this pathway? does it regulate principal neurones or interneurones? There is input from the cerebral cortex directly to the nucleus subthalamicus, skiping the striatum. What is the significance of this pathway, and what information does it carries? The tail of the nucleus caudatus is continuous with the nucleus accumbens, is there any functional significance?

Recently, Ann Graybiel (2005) facilitated our task in finding things to be investigated. In a review published in the December issue of Currents Opinions in Neurobiology, she identify six challenges to the basal ganglia anatomy and function. These are the following:

• Do the direct and indirect pathway project exclusively to different target nuclei?
• Is the pallido-thalamic pathway only inhibitory?
• Is dopamine the only neurotransmitter substance released by the dopamine-containing neurons of the midbrain?
• Do the direct and indirect pathway receives equivalent information from cortical afferents?
• Do the basal ganglia and cerebellum have fully separated functions and pathways to the neocortex?
• Do striosome code reinforcement-related signals?

This list gives us some idea of where to start asking questions. It can be added to the list whether all GABAergic interactions in the striatum are excitatory. In addition to the mechanisms commented in a previous post, GABAergic cell can respond to hyperpolarisation with a rebound spike. Is this spike physiologically significant? i.e. can it lead to excitation of a postsynaptic cell?

Happily for researchers, there is still work to do.