The macaque midbrain reticular formation sends side-specific feedback to the superior colliculus.

Area of Science:

• Neuroscience
• Ophthalmology
• Gaze Control Mechanisms

Background:

• The central mesencephalic reticular formation (cMRF) receives input from the superior colliculus (SC).
• The cMRF projects back to the SC, suggesting a role in gaze control.
• The nature of this feedback (excitatory or inhibitory) was previously unclear.

Purpose of the Study:

• To determine if the reticulotectal feedback from the cMRF to the SC is excitatory or inhibitory.
• To investigate the role of cMRF projections in gaze control and SC activity.

Main Methods:

• Utilized biotinylated dextran amine (BDA) injections into the cMRF of M. fascicularis monkeys.
• Employed electron microscopy to examine BDA-labeled reticulotectal terminals in the SC.
• Conducted postembedding GABA immunochemistry to identify inhibitory neuronal profiles.

Main Results:

• Nearly all ipsilateral BDA-labeled terminals in the SC were GABA-positive, indicating inhibitory feedback.
• Ipsilateral feedback involved a monosynaptic loop connecting the cMRF and SC.
• Contralateral SC showed a mix of GABA-positive and GABA-negative BDA-labeled terminals, suggesting complex effects.

Conclusions:

• The cMRF exerts inhibitory feedback onto the ipsilateral SC, potentially tuning saccade generation.
• Complex, mixed effects on the contralateral SC suggest a role in inter-collicular coordination.
• Findings elucidate the cMRF's role in precise gaze control and coordinated eye movements.