No direct projection is observed from the substantia nigra to the dorsal vagus complex in the rat

Parkinson's disease (PD) is a neurodegenerative disorder that is characterized by degeneration of dopaminergic neurons in the substantia nigra (SN). Destruction of the SN can lead to gastric dyskinesis accompanied by decreased expression of choline acetyltransferase (ChAT) and increased expression of tyrosine hydroxylase (TH) in the dorsal vagus complex (DVC), which includes the dorsal motor nucleus of the vagus (DMV) and nucleus tractus solitarius (NTS). However, it is unclear if the SN and DVC are directly connected.

To investigate the neural projection from the SN to the DVC in rats.

Retrograde and anterograde tracing techniques combined with double-labeling immunofluorescence technique were used.

Destruction of the SN significantly decreases ChAT immunoreactivity (IR) and increases TH-IR in the DVC. After injection of the retrograde tracer fluoro-gold (FG) into the DVC, FG-labeled neurons were observed in the hypothalamic paraventricular nucleus (PVN), lateral hypothalamus (LH), inferior olive (IO), and locus coeruleus (LC). No FG-positive cells were observed in the SN or striatum. Furthermore, after injection of anterograde tracer biotinylated dextran amine (BDA) into the SN, BDA-positive fibers were observed in the caudate putamen (Cpu), globus pallidus (GP), LC, and LH but not in the DVC.

This study is the first to demonstrate that neurons in the SN do not directly innervate the DVC in rats. The DVC might be indirectly innervated by the SN through the hypothalamus and/or the LC. These data provide important morphological insights into the potential mechanism underlying the gastroparesis observed in PD patients.