Acute and Chronic Dopaminergic Depletion Differently Affect Motor Thalamic Function

The motor thalamus (MTh) plays a crucial role in the basal ganglia (BG)-cortical loopin motor information codification. Despite this, there is limited evidence of MTh functionalityin normal and Parkinsonian conditions. To shed light on the functional properties of the MTh,we examined the effects of acute and chronic dopamine (DA) depletion on the neuronal firing ofMTh neurons, cortical/MTh interplay and MTh extracellular concentrations of glutamate (GLU) andgamma-aminobutyric acid (GABA) in two states of DA depletion: acute depletion induced by thetetrodotoxin (TTX) and chronic denervation obtained by 6-hydroxydopamine (6-OHDA), both infusedinto the medial forebrain bundle (MFB) in anesthetized rats. The acute TTX DA depletion caused aclear-cut reduction in MTh neuronal activity without changes in burst content, whereas the chronic6-OHDA depletion did not modify the firing rate but increased the burst firing. The phase correlationanalysis underscored that the 6-OHDA chronic DA depletion affected the MTh-cortical activitycoupling compared to the acute TTX-induced DA depletion state. The TTX acute DA depletioncaused a clear-cut increase of the MTh GABA concentration and no change of GLU levels. On theother hand, the 6-OHDA-induced chronic DA depletion led to a significant reduction of local GABAand an increase of GLU levels in the MTh. These data show that MTh is affected by DA depletionand support the hypothesis that a rebalancing of MTh in the chronic condition counterbalances theprofound alteration arising after acute DA depletion state.