Identifying disease-modifying therapies to slow nigrostriatal neuron loss is an unmet need in Parkinson's disease (PD) therapy (AlDakheel et al., 2014; Barker et al., 2020; Björklund et al., 2021; Manfredsson et al., 2020; Paul and Sullivan, 2019). This challenge may be further compounded by the complexities of PD etiopathology (Kalia and Lang, 2015), detecting PD pathology prior to onset of motor impairment (Berg et al., 2021; Blesa et al., 2022; Heinzel et al., 2019; Salvatore et al., 2021), and still limited knowledge of the mechanisms underlying the neurodegenerative process in nigrostriatal neurons (Chmielarz and Saarma, 2020; Espay et al., 2020; Kalia et al., 2015; Zeng et al., 2018).

Neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) have been investigated as potential neurodegenerative therapies (Airaksinen and Saarma, 2002; Burke, 2006; Chmielarz and Saarma, 2020; Ferreira et al., 2018; Palasz et al., 2020; Tome et al., 2017). This is primarily due to their ability to support the survival of neurons and increase function and connectivity (Chmielarz and Saarma, 2020). BDNF promotes the survival of dopaminergic neurons in animal models of PD, however, unlike GDNF, BDNF efficacy may be limited in early-stages of nigrostriatal neuron loss PD (Chmielarz and Saarma, 2020). GDNF on the other hand, has been widely investigated (Emborg et al., 2009; Gash et al., 1996; Grondin et al., 2019; Kasanga et al., 2019; Kordower et al., 2000; Manfredsson et al., 2009, Manfredsson et al., 2020; Salvatore et al., 2009, Salvatore et al., 2004) since its discovery in the early 90s (Lin et al., 1993). Unfortunately, clinical trials, so far, have produced inconsistent results. Whereas initial phase I trials achieved their primary outcomes, with improvements in UPDRS scores (Gill et al., 2003; Slevin et al., 2005), subsequent trials did not meet primary endpoints (Lang et al., 2006; Whone et al., 2019). Some of the reasons for failure in later trials include insufficient GDNF dose, duration of delivery, and delivery method of GDNF (Ai et al., 2003; Bartus and Johnson, 2017; Gash et al., 2005; Kasanga et al., 2019; Luz et al., 2018; Polinski et al., 2016; Salvatore et al., 2006; Sherer et al., 2006). A common theme for all trials was that degree of nigrostriatal terminal loss at the time of GDNF delivery was likely beyond complete. The mean time of GDNF delivery from PD diagnosis was ∼8 years, and terminal loss is virtually complete by 5 years diagnosis (Kordower et al., 2013). Thus, the responsivity of nigrostriatal terminals to GDNF, namely via the availability of GDNF receptors, at that degree of loss could be severely compromised and reduce potential for plasticity in dopamine signaling in residual nigrostriatal neurons (Chu and Kordower, 2021; Espay et al., 2020; Quintino et al., 2019).

GDNF is a potent dopaminergic neurotrophic factor which is normally expressed at low levels in the adult brain (Liberatore et al., 1997; Strömberg et al., 1993). However, the role of the GDNF family receptor, GFR-α1, in mediating neuroprotection in PD has been sparsely studied. Global knockout of GFR-α1 leads to death of mice soon after birth (Enomoto et al., 1998), however, heterozygous GFR-α1 knockout mice develop decreased motor function concurrent with a reduction in nigral dopaminergic neurons with aging (Zaman et al., 2008). RET, the GDNF/GFR-α1 activated receptor tyrosine kinase, has been more widely investigated for a role in nigrostriatal function. Small molecules targeting RET receptors stimulate neuronal signaling and dopamine release indicating their therapeutic potential (Conway and Kramer, 2021; Kramer and Liss, 2015; Mahato and Sidorova, 2020). Post-mortem analysis of putamen samples from PD subjects revealed no significant changes in GFR-α1 and RET mRNA expression with a modest increase in GDNF mRNA levels and significant loss of nigral neurons (Bäckman et al., 2006). In the substantia nigra (SN), decreased RET expression suggests a key role in nigrostriatal neuron viability (Chu and Kordower, 2021). Moreover, RET activation differentially protects dopaminergic cell bodies instead of axon terminals after neurotoxic insult, indicating RET signaling in the SN is critical for neuroprotection (Mijatovic et al., 2011).

Dopamine marker loss occurs at a greater rate and with more severity in striatal regions than in SN during PD progression (Kordower et al., 2013). Given the physiological impact of GDNF on nigrostriatal function (Ibáñez and Andressoo, 2017), we evaluated the impact of nigrostriatal neuron loss on expression of GDNF and BDNF and their receptors, early and late after 6-hydroxydopamine (6-OHDA) lesion in rats. Relative differences in expression of these receptors in nigrostriatal terminals vs cell bodies revealed critical time windows and nigrostriatal compartments of when, and where, the efficacy of GDNF to recover or protect DA function during progressive nigrostriatal pathway loss is feasible or limited.