Preservation of Extracellular and Tissue Dopamine During Tyrosine Hydroxylase Loss in Rat 6-OHDA Parkinson's Model: Selective Compensation Restricted to Substantia Nigra.

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Compensatory mechanisms are thought to maintain sufficient dopamine (DA) signaling to mitigate locomotor impairment during progressive nigrostriatal neuron loss in Parkinson's disease (PD). Recent evidence indicated augmented DA tissue content in the substantia nigra (SN), not striatum, compensates for tyrosine hydroxylase (TH) and neuronal loss, and alleviates the severity of hypokinesia during neuronal loss. Here, we determined if increased extracellular DA in the SN may also be a compensatory mechanism to augment DA signaling. Following unilateral 6-hydroxydopamine (6-OHDA) lesion or sham-operation, we contemporaneously evaluated extracellular DA against both DA tissue and TH levels in striatum and SN at 7 and 28 days. At 7 days post-lesion, TH loss exceeded ~90% in striatum, and ~70% in the SN. The severity of DA tissue loss coincided with TH protein loss only in striatum (>90%) on both days after lesion, whereas in the SN, DA loss was absent on day 7 and significantly less than TH loss by day 28. Whereas there was a robust increase in extracellular DA in striatum in our sham-operation group, the severe TH and DA tissue loss in striatum practically abolished KCl (K+)-stimulated extracellular DA by day 7. In contrast, whereas striatal K+-stimulation had no effect on extracellular DA in the SN in sham-operation group, extracellular DA levels increased in the SN 7 days after nigrostriatal lesion: an increase no longer apparent by day 28. Thus, despite significant loss of TH protein loss in the SN, extracellular and tissue DA tissue levels were augmented during neuronal loss. These results build upon evidence that compensatory mechanisms to augment DA signaling are not engaged in striatum, and point to the SN as the locus of augmented DA signaling to offset loss of TH during nigrostriatal neuron loss.

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