The rat rotenone model reproduces the abnormal pattern of central catecholamine metabolism found in Parkinson's disease

Abstract

Recent reports indicate that Parkinson's disease (PD) involves specific functional abnormalities in residual neurons - decreased vesicular sequestration of cytoplasmic catecholamines via the vesicular monoamine transporter (VMAT) and decreased aldehyde dehydrogenase (ALDH) activity. This double hit builds up the autotoxic metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL), the focus of the catecholaldehyde hypothesis for the pathogenesis of PD. An animal model is needed that reproduces this abnormal catecholamine neurochemical pattern. Adult rats received subcutaneous vehicle or the mitochondrial complex 1 inhibitor rotenone (2 mg/kg/day via a minipump) for 10 days. Locomotor activity was recorded, and striatal tissue sampled for catechol contents and catechol ratios that indicate the above abnormalities. Compared to vehicle, rotenone reduced locomotor activity (P=0.002), decreased tissue dopamine concentrations (P=0.00001), reduced indices of vesicular sequestration (3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine) and ALDH activity (DOPAC/DOPAL) (P=0.0025, P=0.036), and increased DOPAL levels (P=0.04). The rat rotenone model involves functional abnormalities in catecholaminergic neurons that replicate the pattern found in PD putamen. These include a vesicular storage defect, decreased ALDH activity and DOPAL build-up. The rat rotenone model provides a suitable in vivo platform for studying the catecholaldehyde hypothesis.

Keywords: Aldehyde dehydrogenase; Catechol; Catecholamine; DOPAL; Dopamine; Lewy body diseases; Norepinephrine; Parkinson's disease; Rotenone; Vesicular uptake.

Fig. 1.

Concept diagram showing enzymatic steps in the synthesis, vesicular storage, release, reuptake and metabolism of dopamine (DA) and norepinephrine (NE). The six endogenous catechols (in white rectangles) were measured simultaneously. DA is synthesized in the neuronal cytoplasmic via tyrosine hydroxylase (TH) acting on tyrosine to form 3,4-dihydroxyphenylalanine (DOPA) and then L-aromatic-amino-acid decarboxylase (LAAAD) acting on DOPA. Most of cytoplasmic DA is taken up into vesicles via the vesicular monoamine transporter (VMAT) but a minority undergoes enzymatic oxidation catalyzed by monoamine oxidase (MAO) to form 3,4-dihydroxyphenylacetaldehyde (DOPAL). DOPAL is metabolized by aldehyde dehydrogenase (ALDH) to form 3,4-dihydroxyphenylacetic acid (DOPAC), which exits the cell. DA in the vesicles undergoes enzymatic hydroxylation by DA-beta-hydroxylase (DBH) to form NE. Catecholamines released into the extracellular fluid is taken back up into the cytoplasm via the cell membrane DA transporter (DAT) or NE transporter (NET). NE in the cytoplasm can undergo vesicular uptake or MAO-catalyzed oxidative deamination to form 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL), which is reduced by aldehyde/aldose reductase (AR) to form 3,4-dihydroxyphenylglycol (DHPG). DHPG rapidly exits the neuron. Font sizes correspond roughly to tissue concentrations of the analytes in rat striatum.

Fig. 2.

Comparison of rearing behavior (counts/2 min) between days 1 and 9 in rotenone- and vehicle-treated rats. Representation of individual animal data (dotted lines) with mean values per group shown as solid lines. There is a significant difference (t-test) in rearing count between days in the rotenone group (P=0.0015; n=8) but not in the vehicle group (P=0.41; n=11).

Fig. 3.

Activity difference (activity in seconds on day 0 minus activity on day 1 and day 9) in the rotenone- and vehicle-treated groups. Representation of individual animal data (dotted lines) with mean values per group shown as solid lines. There is a significant difference (t-test) in activity, i.e. activity (in s) on day 0 minus activity on days 1 and 9, between days in the rotenone group (P=0.0001; n=8) but not in the vehicle group (P=0.41; n=11).