Omnipause neurons (OPNs) within the nucleus raphe interpositus (RIP) help gate the transition between fixation and saccadic eye movements by monosynaptically suppressing activity in premotor burst neurons during fixation, and releasing them during saccades. Premotor neuron activity is initiated by excitatory input from the superior colliculus (SC), but how the tectum's saccade-related activity turns off OPNs is not known. Since the central mesencephalic reticular formation (cMRF) is a major SC target, we explored whether this nucleus has the appropriate connections to support tectal gating of OPN activity. In dual-tracer experiments undertaken in macaque monkeys (Macaca fascicularis), cMRF neurons labeled retrogradely from injections into RIP had numerous anterogradely labeled terminals closely associated with them following SC injections. This suggested the presence of an SC-cMRF-RIP pathway. Furthermore, anterograde tracers injected into the cMRF of other macaques labeled axonal terminals in RIP, confirming this cMRF projection. To determine whether the cMRF projections gate OPN activity, postembedding electron microscopic immunochemistry was performed on anterogradely labeled cMRF terminals with antibody to GABA or glycine. Of the terminals analyzed, 51.4% were GABA positive, 35.5% were GABA negative, and most contacted glycinergic cells. In summary, a trans-cMRF pathway connecting the SC to the RIP is present. This pathway contains inhibitory elements that could help gate omnipause activity and allow other tectal drives to induce the bursts of firing in premotor neurons that are necessary for saccades. The non-GABAergic cMRF terminals may derive from fixation units in the cMRF.

The central mesencephalic reticular formation (cMRF) likely plays a role in gaze control, as cMRF neurons receive tectal input and provide a bilateral projection back to the superior colliculus (SC). We examined the important question of whether this feedback is excitatory or inhibitory. Biotinylated dextran amine (BDA) was injected into the cMRF of M. fascicularis monkeys to anterogradely label reticulotectal terminals and retrogradely label tectoreticular neurons. BDA labeled profiles in the ipsi- and contralateral intermediate gray layer (SGI) were examined electron microscopically. Postembedding GABA immunochemistry was used to identify putative inhibitory profiles. Nearly all (94.7%) of the ipsilateral BDA labeled terminals were GABA positive, but profiles postsynaptic to these labeled terminals were exclusively GABA negative. In addition, BDA labeled terminals were observed to contact BDA labeled dendrites, indicating the presence of a monosynaptic feedback loop connecting the cMRF and ipsilateral SC. In contrast, within the contralateral SGI, half of the BDA labeled terminals were GABA positive, while more than a third were GABA negative. All the postsynaptic profiles were GABA negative. These results indicate the cMRF provides inhibitory feedback to the ipsilateral side of the SC, but it has more complex effects on the contralateral side. The ipsilateral projection may help tune the “winner-take-all” mechanism that produces a unified saccade signal, while the contralateral projections may contribute to the coordination of activity between the two colliculi.

The mesencephalic trigeminal nucleus (MesV) contains the somata of primary afferent neurons that innervate muscle spindles in masticatory muscles and mechanoreceptors in the periodontal ligaments. There are conflicting reports about additional peripheral targets of MesV, such as the extraocular muscles, as well as about its central targets. In addition, only limited primate data are available. Consequently, we examined MesV projections in macaque monkeys. The retrograde tracer wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected into masticatory or extraocular muscles to define the peripheral targets of the primate MesV. Numerous labeled neurons were found in ipsilateral MesV after masticatory muscle injections. The scattered distribution of labeled cells, and their presence among clusters of unlabeled cells, suggests the muscle representations overlap. Just a few MesV neurons were labeled after extraocular muscle injections. This correlates with the small number of muscle spindles present in macaque extraocular muscles, suggesting MesV cells supplying extraocular muscle spindles may contribute a minor component to oculomotor proprioception. To examine the central connections of MesV, biotinylated dextran amine (BDA) was injected into the spinal trigeminal nucleus (Vs). The presence of retrogradely labeled MesV cells indicated a projection to Vs from MesV. These injections also anterogradely labeled terminals that lay in close association with MesV cells, suggesting an ascending projection from Vs to MesV. Finally, a small number of MesV neurons were labeled after WGA-HRP injections into the upper cervical spinal cord. This pattern of central connections indicates MesV and Vs information is combined to guide mastication. Anat Rec, 291:974-987, 2008. 2008 Wiley-Liss, Inc.