Synaptic transmission and modulation in the neostriatum.
The neostriatum is the entryway into the basal ganglia and is the site of many of the neurological defects involving basal ganglia function. Thus, it is important to understand the regulation of synaptic transmission at afferent synapses innervating the neostriatum. Cortical glutamatergic and nigral dopaminergic afferent input impinge on neurons in the neostriatum, providing the most significant afferent inputs to this structure. Our understanding of the mechanisms involved in transmission and modulation of transmission at these synapses has greatly increased. It is now apparent that the corticostriatal glutamatergic inputs produce rapid depolarization of striatal neurons via activation of ionotropic AMPA-type glutamate receptors. In addition, transmission is modulated by a number of presynaptic, G-protein-coupled receptors but, surprisingly, relatively little evidence of postsynaptic modulation has been observed. Corticostriatal synapses also express certain forms of plasticity, most notably short- and long- term synaptic depression (STI) and LTD, respectively). It appears that LTD may involve convergent actions of glutamate and dopamine. Striatal LTD may have important roles in information storage and motor set selection in the striatum. However, some aspects of synaptic transmission in the striatum remain unclear. In particular, the exact physiological roles of dopaminergic nigrostriatal input and the role of NMDA-type glutamate receptors are not well understood. In addition, intrastriatal synaptic connections have received relatively little attention as compared with extrinsic input to the neostriatum. Future studies will need to focus on elucidating these aspects of neostriatal function.