Neurodegeneration in the substantia nigra, a critical area of the brain involved in Parkinson’s, leads to memory deficits, according to a study that explored the effects of levodopa on a rat model of the disease.
Findings of the study, titled “Implication of nigral dopaminergic lesion and repeated L-dopa exposure in neuropsychiatric symptoms of Parkinson’s disease,” also illustrated the key role dopamine plays in memory. It was published in the journal Behavioral Brain Research.
Similar to motor symptoms, loss of dopamine-producing neurons in the substantia nigra — a brain area implicated in motor function — has been proposed as the cause of neuropsychiatric manifestations in Parkinson’s disease. This has been shown in animal models, in which bilateral lesion of the substantia nigra (which selectively affects this region only) resulted in anxiety- and depression-like behaviors, as well as motivational, memory and social interaction deficits. A study in rats also reported psychosis-like behavior in animals subjected to bilateral lesion.
Prior research has shown that chronic dopamine replacement therapy (DRT) — which compensates for the lack of dopamine and represents the standard treatment for different Parkinson’s motor symptoms — leads to impaired ability of neurons to fine-tune their responses in animals and is associated with Parkinson’s-related neuropsychiatric disorders in patients.
After lesion of the nigrostriatal pathway — including the substantia nigra and the dorsal striatum — DRT led to compulsive behavior in animals, which was associated with cellular alterations in brain regions involved in cognitive/affective information processing. Progressive degeneration of the brain’s nigrostriatal pathway — one of the four major dopamine pathways in the brain, involved in production of movement — is a characteristic event in Parkinson’s.
While chronic administration of levodopa worsened lesion-induced anxiety and depression in rats, lesioned primates showed psychotic-like behavior only after dopaminergic treatment.
Biotrial Pharmacology researchers have now evaluated the impact of substantia nigra neurodegeneration, repeated exposure to dopaminergic medication, and the combination of both on the development of neuropsychiatric symptoms of Parkinson’s. The team hypothesized that repeated exposure to levodopa could promote brain remodeling and avoid cognitive-like and affective-like deficits.
The team used a rat model based on bilateral substantia nigra injection of a toxin called 6-hydroxydopamine, which induces selective lesions of specific dopaminergic neurons that do not provoke motor deficits.
A subgroup of animals was repeatedly administered with levodopa (20 mg/kg per day) and benserazide (5 mg/kg daily) via under-the-skin injection over 10 consecutive days (chronic exposure), starting 10 days after partial substantia nigra lesion. Benserazide is an inhibitor of an enzyme called DOPA decarboxylase, which converts levodopa into dopamine.
Behavioral tests were started three weeks after the lesions. These included assessments of spontaneous locomotor activity; forelimb voluntary movement with the stepping test; anxiety-like behavior with the elevated plus maze, in which increased time spent on an open arm correlates with greater anxiety; social interaction, as indicated by the amount of time spent in active non-aggressive social behavior with another rat; memory through novel object recognition; and amphetamine-induced hyperlocomotion (AIH), used to assess responses to psychostimulants.
All animals (both with and without chronic levodopa exposure) received single acute injections of levodopa (12.5 mg/kg)/benserazide (15 mg/kg) before the elevated plus maze and novel object recognition tests.
Results revealed that, in contrast to preserved motor function, lesioned rats showed a significant memory deficit as well as anxiety-like behavior. Social interaction and AIH were unchanged. These animals also demonstrated a 48% decrease in the number of dopaminergic cells within the substantia nigra, whose extent did not correlate with the memory and anxiety results.
Researchers found that a single injection of levodopa/benserazide reversed the memory deficit, but not the anxiety-like behavior. According to the scientists, this suggests that dopaminergic pathways “were less directly involved in lesion-induced anxiety-like behavior.”
Chronic administration of levodopa did not change the results seen in lesioned rats (not chronically exposed), which was contrary to the team’s hypothesis. The investigators attributed this finding to having induced a partial lesion. Studies in more severely injured animals and with longer administrations of levodopa would be of interest, they believe.
The results in the memory test indicate “a not previously clearly identified critical role in cognition for the [substantia nigra],” and illustrate “the critical role of [dopamine] in this behavioural outcome,” the researchers wrote.
According to the team, the findings are in accordance “with clinical data suggesting that a panel of non-motor impairment including cognitive deficit and anxiety may be used as prodromal (early) markers of early stages of [Parkinson’s].”