Ultrastructural evidence for GABAergic brain stem projections to spinal motoneurons in the rat

In the present ultrastructural study in the rat, it was determined whether GABA was present in projections descending from the ventromedial reticular formation of the lower brain stem to motoneuronal cell groups in the lumbar spinal cord. For this purpose, the anterograde transport of WGA-HRP was combined with the postembedding immunogold technique for GABA, with the advantage that both markers could be visualized simultaneously in a single terminal. In 4 rats, WGA-HRP was injected in the ventromedial part of the brain stem reticular formation at levels between the rostral inferior olive and the caudal part of the facial nucleus. Vibratome sections were cut from the lumbar spinal cord, reacted for WGA-HRP, and processed for electron microscopy. Ultrathin sections containing the lateral motoneuronal cell groups were cut and treated following the immunogold technique using a polyclonal antibody directed against GABA. It was found that nearly 40% of the terminal profiles that were labeled with WGA-HRP reaction products from the ventromedial brain stem were also labeled for GABA (double labeled). Most of the double-labeled terminals (81%) were F-type (containing many flattened vesicles), 12% were G-type (containing many granular vesicles), and 7% were S-type (containing many spherical vesicles). The majority of the double-labeled terminals contacted proximal dendrites. It is argued that the descending GABAergic projection produces a general inhibitory effect on spinal motoneurons, counteracting the general facilitation produced by the serotonergic projection derived from the same brain stem area.(ABSTRACT TRUNCATED AT 250 WORDS)