Introduction

Parkinson’s disease (PD) is the second most common neurodegenerative disorder affecting an estimated 6.1 million people worldwide. The disease is characterised by age-related loss of dopamine-containing neurones in the substantia nigra pars compacta (SNpc) region of the adult brain. Alpha-synuclein immunopositive aggregates, termed Lewy bodies, are a neuropathological hallmark of PD that form in neurones (Irwin et al. 2013). This neuroanatomical pathology results in the loss of motor (bradykinesia, resting tremor) and non-motor (cognitive decline, sleep disturbance, constipation) function. Current therapies are available to control symptoms of disease, however, a large proportion of patients do not respond to treatment. In addition, the prolonged use of the dopamine replacement therapy, l-DOPA, often results in debilitating dyskinesias (Jenner 2008). Therefore, there is a major un-met clinical need for new protective and restorative treatments for PD.

Recently identified mutations in familial PD (LRRK2, GLB) and neurotoxins that induce Parkinsonian characteristics in animals converge on the cells’ energy producing organelles, the mitochondria. Parallel studies have identified a clear association between dopamine neurone dysfunction within the SNpc of PD brain and metabolic dysfunction (Burbulla et al. 2017). This knowledge has led to the search for molecules that improve mitochondrial function and biogenesis with the aim of enhancing cellular resilience to periods of bioenergetics stress. One such environmental factor that enhances both mitochondrial function and resilience against neurodegeneration is calorie restriction (CR; Morgan et al. 2018), but until recently, the underlying mechanism was not well understood. The stomach hormone, acyl-ghrelin, which is elevated by CR (Lutter et al. 2008) and increases mitochondrial activity in neurones (Andrews et al. 2008), is essential for mediating the beneficial effects of CR in the MPTP-model of PD (Bayliss et al. 2016). Acyl-ghrelin signalling induces phosphorylation of the cellular energy sensor, adenosine monophosphate-activated protein kinase (AMPK), to promote cell survival. The selective ablation of AMPK from dopaminergic neurones prevented acyl-ghrelin from protecting against MPTP-induced neurotoxicity. These findings are consistent with reports that treatment with exogenous acyl-ghrelin or synthetic ligands for the ghrelin receptor (GHS-R) protect against MPTP-induced SNpc lesions in mice (Andrews et al. 2009; Jiang et al. 2016) and the rat intra-striatal 6-OHDA model of PD (He et al. 2021). The major advantage of using this pre-clinical model is the quantifiable motor deficit induced by unilateral 6-OHDA lesion, and to the best of our knowledge, this is the first demonstration that acyl-ghrelin administration is efficacious in this rat model of PD.

Next, we performed histological analysis of dopamine (TH+) containing cells within the SNpc. The number of dopamine cells in the SNpc of the lesioned hemisphere was significantly reduced (58%) by 6-OHDA, relative to the non-lesioned hemisphere (P < 0.0001, Fig. 2E, F). In contrast, acyl-ghrelin pre-treatment attenuated this loss, resulting in the preservation of significantly more dopamine cells in the lesioned SNpc, relative to the 6-OHDA lesion group (P < 0.01, Fig. 2E, F). Indeed, there was no significant difference in the number of dopamine-containing cells in the SNpc of the acyl-ghrelin treated and sham groups (P > 0.05; Fig. 2E, F). Consistent with the expectation of this pre-clinical model, the number of dopamine cells in the SNpc was inversely correlated with amphetamine-induced ipsilateral rotations (Spearman r2 = 0.2442, P < 0.0001; Fig. 2G). Our results are consistent with previously published findings that acyl-ghrelin prevents dopamine cell loss in pre-clinical toxin-based models of PD (Andrews et al. 2009; Bayliss et al. 2016; Jiang et al. 2008; Moon et al. 2009) and extend the repertoire of pre-clinical models used to validate the therapeutic potential of acyl-ghrelin signalling in PD.

In support of our hypothesis that acyl-ghrelin signalling may be a valid therapeutic target for neuroprotection in PD, evidence from human PD patients suggests that post-prandial plasma total-ghrelin is reduced in PD individuals diagnosed with REM-sleep disorder, considered a pro-drome of PD (Unger et al. 2011). Furthermore, fasting levels of total- and acyl-ghrelin were reduced in male and female subjects diagnosed with PD (Song et al. 2017). Importantly, induced-pluripotent stem cell (iPSC)-derived dopaminergic neurones generated from patients carrying parkin gene (PARK2) mutations had significantly decreased GHS-R expression. Similarly, dopamine neurones of isogenic PARK2-iPSC lines mimicking PARK2 loss of function via CRISPR Cas9 technology also had reduced expression of GHS-R (Suda et al. 2018). More recently, we reported a reduction in the ratio of plasma acyl-ghrelin to unacyl-ghrelin specifically in individuals diagnosed with PD dementia when compared with cognitively intact PD and healthy control subjects (Hornsby et al. 2020). Given the limitations of administering putative therapeutic agents before the introduction of acute SNpc lesion, we suggest that studies using pre-clinical models linked with sporadic and familial PD are warranted to provide further insights into the role of ghrelin signalling in PD pathogenesis and progression. With growing interest in the role of systemic, particularly gastro-intestinal, factors in PD pathology (Arotcarena et al. 2020), our findings provide further evidence that the stomach hormone, acyl-ghrelin, protects dopamine neurones in a pre-clinical model of PD. Whilst the mechanism for the transport of acyl-ghrelin across the blood–brain barrier is not fully understood, the efficacy of acyl-ghrelin in promoting dopaminergic neurone survival and function in the absence of the widely reported metabolic effects of hyperghrelinaemia, suggest that human studies are warranted to test the efficacy of acyl-ghrelin peptide administration in slowing the progression of this debilitating disease.