The zona incerta (ZI) of macaque monkeys contains two morphological cell types, local circuit and projection neurons. Physiological study has demonstrated that these cells pause for saccades. As the ZI has a large projection to the superior colliculus (SC) where saccades are generated we hypothesized that the ZI provides a disinhibitory signal which permits saccades to be generated by the SC. To test this hypothesis, we examined whether the ZI contained GABA-immunoreactive neurons. Seven cynomolgus monkeys (Macaca fascicularis) were perfused with mixed aldehyde fixatives. Fifty μm frozen sections were cut and incubated with primary antisera to a variety of neurotransmitters. Subsequently, labeling was visualized using the ABC technique and the sections were counterstained with thionin. Approximately 200 each of immunoreactive and non-immunoreactive cells from the same sections, representing the full range of somatic dimensions within the ZI, were-drawn under high power and somatic dimensions were calculated with a computerized morphometric program (SigmaScan). GABA-immunoreactive cells ranged from 9.5-16 μm long axis and Nissl-stained non-immunoreactive cells ranged from 9.8-20 μm long axis, Glutamate-immunoreactive neurons ranged from 9.7-21.8 μm and non-immunoreactive neurons ranged from 8.2-18.7 μ. From these data we conclude that some ZI projection neurons are probably GABAergic. However, the largest projection neurons in the ZI are not GABAergic and are likely to be glutaminergic. Thus it is unlikely that the ZI disinhibits the SC only through a GABAergic projection. Rather, the ZI is likely to provide both a direct inhibition through a GABAergic projection onto tectoefferent cells and an indirect inhibition through a glutaminergic projection onto tectal local circuit neurons.
|Original language||English (US)|
|Publication status||Published - Dec 1 1997|
ASJC Scopus subject areas
- Molecular Biology