Exendin-4 reverts behavioural and neurochemical dysfunction in a pre-motor rodent model of Parkinson's disease with noradrenergic deficit

Abstract

BACKGROUND AND PURPOSE Parkinson's disease (PD) is characterized by progressive dopaminergic cell loss; however, the noradrenergic system exhibits degeneration as well. Noradrenergic deficit in PD may be responsible for certain non-motor symptoms of the pathology, including psychiatric disorders and cognitive decline. The aim of this study was to generate a pre-motor rodent model of PD with noradrenergic denervation, and to assess whether treatment with exendin-4 (EX-4), a glucagon-like peptide 1 receptor agonist, could reverse impairment exhibited by our model. EXPERIMENTAL APPROACH We generated a model of PD utilizing N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine and 6-hydroxydopamine to create partial lesions of both the noradrenergic and dopaminergic systems respectively. We then assessed the validity of our model using an array of behavioural paradigms and biochemical techniques. Finally, we administered EX-4 over a 1 week period to determine therapeutic efficacy. KEY RESULTS Our model exhibits anhedonia and decreased object recognition as indicated by a decrease in sucrose preference, increased immobility in the forced swim test and reduced novel object exploration. Tissue and extracellular dopamine and noradrenaline were reduced in the frontal cortex and striatum. TH+ cell counts decreased in the locus coeruleus and substantia nigra. Treatment with EX-4 reversed behavioural impairment and restored extracellular/tissue levels of both dopamine and noradrenaline and TH+ cell counts. CONCLUSION AND IMPLICATIONS We conclude that early treatment with EX-4 may reverse certain neuropsychiatric dysfunction and restore dopamine and noradrenaline content.

Figure 1

Behavioural assessments. This figure illustrates the various behavioural paradigms utilized in this study. (A) Sucrose preference test, expressed as preference percentage. (B) Forced swim test, measured as percentage of time spent immobile. (C) Novel object recognition, shown as percentage of time exploring the novel object. (D) Open field test, depicted as number of squares crossed. All results were analysed using one-way anova and a post hoc Bonferroni's test to compare differences between groups (n= 10). *Indicates different from sham, P < 0.05. **Indicates different from sham P < 0.001. +Indicates different from 6-OHDA only, P < 0.05.

Figure 2

Noradrenaline (NA) and dopamine (DA) tissue levels in the frontal cortex and striatum. All values are expressed as mean ± SEM denoted in ng·g⁻¹ tissue. All results were analysed using one-way anova and a post hoc Bonferroni's test to compare differences between groups (n= 10). (A) NA in the FC; (B) DA in the FC; (C) NA in the FC; and (D) DA in the STR. **Indicates different from sham P < 0.001; +Indicates different from 6-OHDA only, P < 0.05. FC, frontal cortex; STR, striatum.

Figure 3

Extracellular noradrenaline (NA) and dopamine (DA) concentrations in the frontal cortex and striatum. All values are expressed as mean ± SEM denoted in fmol 10 µL⁻¹. All results were analysed using two-way anova and a post hoc Bonferroni's test to compare differences between groups (n= 10). (A) NA in the FC; (B) DA in the FC; (C) NA in the STR; and (D) DA in the STR. * Indicates different from sham, P < 0.05. ** Indicates different from sham P < 0.001. + Indicates different from 6-OHDA only, P < 0.05. FC, frontal cortex; STR, striatum.

Figure 4

TH immunostaining in the locus coeruleus – each slice is 12 µm in thickness and the scale bar represents a length of 100 µm. Noradrenergic cell bodies appear dark brown in colour visualized using DAB staining. (A) Shams; (B) DSP-4; (C) DSP-4 + 6-OHDA; and (D) DSP-4 + 6-OHDA + EX-4. Bar graph on the right-hand side shows TH+ cell counts. ** Indicates P < 0.001 compared with sham value. Cell counts were an average of stained neuronal bodies across three adjacent tissue sections.

Figure 5

TH immunostaining in the substantia nigra – each slice is 12 µm in thickness and the scale bar represents a length of 100 µm. Dopaminergic cell bodies appear dark brown in colour visualized using DAB staining. (A) Shams; (B) 6-OHDA; (C) DSP-4 + 6-OHDA; and (D) DSP-4 + 6-OHDA + EX-4. Bar graph on the right-hand side shows TH+ cell counts. ** Indicates P < 0.001 compared with sham value. Cell counts were an average of stained neuronal bodies across three adjacent tissue sections.