Serotonergic neurotransmission in early Parkinson's disease: a pilot study to assess implications for depression in this disorder.

OBJECTIVES: Depression, a disease usually accompanied by a serotonergic deficit, has been observed in about 40% of patients suffering from Parkinson's disease (PD). Thus, a serotonergic dysfunction in PD can be assumed. We aimed to investigate the interaction between serotonergic (5-HT) and dopaminergic activity in early PD. We hypothesized a serotonergic as well as a dopaminergic deficit in PD patients. We also assumed a correlation between these neurotransmitters indicating a relationship between dopaminergic and serotonergic function in PD.
METHODS: Nine unmedicated PD patients before and 12 weeks after L-dopa treatment and nine healthy subjects were examined using the loudness dependence of auditory evoked potentials (LDAEP), a promising indicator of central serotonergic function. Dopaminergic transporters (DAT) were collected using (123)I-FP-CIT and single photon emission computer tomography (SPECT). LDAEP values were correlated with (123)I-FP-CIT SPECT data.
RESULTS: A significant difference between LDAEP of controls and patients (P= 0.05) suggested lower serotonergic activity in PD. Twelve weeks after initiation of L-dopa treatment this difference was lost between patients and controls (P= 0.20). There was a trend towards a correlation between LDAEP and DAT (r= 0.65; P = 0.057) of the unmedicated patients, suggesting a low serotonergic activity may be related to a dopamine deficit in PD.
CONCLUSIONS: Our results support the hypothesis that serotonergic neurotransmission is decreased in untreated PD and suggest that a low serotonergic activity may be related to the dopamine pathology in PD. This could be related to the high prevalence of depression in PD.