This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.During the past year we completed a series of electrophysiological studies of the thalamostriatal system in awake monkeys that aim at: 1) Determine the effects of stimulation of the caudal intralaminar thalamic nuclei centre median and parafascicular (CM/Pf) nuclei on the activity of striatal projection neurons and interneurons and 2) Assess the effects of CM/Pf stimulation on the release of GABA and acetylcholine in the sensorimotor territory of the monkey striatum. The responses of 99 striatal cells to burst CM/Pf stimulation have been studied. The cells were categorized as phasically active medium spiny neurons (PANs) if their discharge rate was below 3 spikes/s, and their discharge pattern was irregular. Cells with a higher firing rate and a more regular pattern of firing were considered as tonically active interneurons (TANs). The CM/Pf stimulation with trains of stimuli at high frequency (100 Hz, 100 pulses/train) had an effect in the majority of recorded cells, which responded with either excitatory or inhibitory responses. In brief, an increase in firing occurred in 14/28 (50%) PANs, while 3 cells (10.7%) showed a decrease and 11/28 cells (39.3%) did not respond. In TANs, increases were less common (9/46 cells, 19.5%), and decreases occurred more often (16/46 cells, 34.8%). No response was seen in 21/46 cells (45.6%). Overall, the results suggest that the majority of putamenal projection cells respond to thalamic stimulation by increasing their activity while interneurons more frequently decrease their activity during CM/Pf stimulation. These observations are consistent with the hypothesis that the striatum receives glutamatergic projections from the CM/Pf that contact directly medium spiny striatal neurons, while the predominant inhibitory effects on interneurons may be mediated indirectly via intrastriatal GABAergic transmission. Surprisingly, CM stimulation did not induce significant change in striatal GABA level, while it reduced significantly striatal acetylcholine. Together, findings of this study provide direct evidence for powerful and complex physiological effects of thalamic inputs upon striatal neurons. These data are highly significant and help understand better the role of the thalamostriatal system in the functional circuitry of the basal ganglia.
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