Existence in the nucleus incertus of the cat of horizontal-eye-movement-related neurons projecting to the cerebellar flocculus

Abstract

1. Properties of nucleus incertus (NIC) neurons projecting to the cerebellar flocculus were studied in alert cats by using chronic unit and eye movement recording and antidromic activation. Projection of these neurons onto the flocculus was verified with retrograde transport of horseradish peroxidase after injections in the flocculus. 2. Bipolar stimulation electrodes were implanted into the "middle" zone of each flocculus because this zone is known to be involved in the control of horizontal eye movements. The dorsomedial aspect of the pontine tegmentum was explored with microelectrodes during stimulation of both flocculi. The majority of neurons antidromically activated from the flocculus were found in the caudal part of the NIC. 3. Of the 69 neurons activated from the flocculus, 44 were classified as burst-tonic (BT) neurons; 34 discharged in relation with horizontal movements of the eye, 10 in relation with vertical movements. Of the 14 remaining neurons, 6 were not related to eye movements and 8 were classified as burst neurons. The BT neurons of the NIC displayed a great sensitivity to both horizontal eye position and horizontal eye velocity. 4. This study demonstrates the presence of a new group of horizontal eye movement related BT neurons situated in the NIC. The fact that they project to the horizontal floccular zone emphasizes the importance of the functional specialization of the different Purkinje cell zones.

Connections of the nucleus incertus

“…Finally, a projection from the caudal NI to the cerebellar flocculus, which is known to receive inputs from the nucleus prepositus and dorsal cap of Kooy, has been reported in the cat (Cheron et al, 1995). We did not identify such a projection with PHA-L, and the reason for this discrepancy is not known, although it may be due to a difference between species.…”

“…In this context, it is important to point out that the present anterograde tracer findings indicate that the NI projects substantially to neural sites implicated in oculomotor control, including the "paralamellar region" of the lateral part of the mediodorsal thalamic nucleus, the nucleus prepositus, and the medial accessory inferior olive (McCrea and Baker, 1985;Groenewegen, 1988). Furthermore, it has been reported that the region of the NI in cats has burst-tonic neurons that display great sensitivity to both horizontal eye position and horizontal eye velocity, and their spiking behavior is quite similar to burst-tonic neurons in the nucleus prepositus (Cheron et al, 1995).…”

Translational Approach to Behavioral Learning: Lessons from Cerebellar Plasticity

“…By such electrical stimulation, it has been possible to identify by antidromic invasion and collision technique the brainstem neurons projecting to the cerebellum. The cerebellar input signals of the median vestibular, prepositus and incertus nuclei were thus identified [22, 23, 25]. Pure head velocity and eye movement neurons (velocity plus position signals of the eye, so-called burst-tonic neurons) were recruited in the median vestibular nucleus [23], while burst neurons (eye-velocity) were identified in the prepositus and burst-tonic neurons in the prepositus [25] and incertus nuclei [22].…”

“…The IO is considered a crucial pathway in VOR adaptation [72–74] and also in the motor plasticity that compensates for vestibular damages [75]. As the majority of the mossy fibers (MFs) reaching the horizontal zone of the flocculus originated in the contralateral vestibular [23], the prepositus [25], the incertus nuclei [22], and the paramedian tract region [76], the section of the vestibular commissure in the cat resulted in the absence of VOR adaptation [77], and that in spite of that fact, the right and left flocculus were preserved. This demonstrates that VOR adaptation by the cerebellum necessitated the preservation of the contralateral MF inputs, contrasting with the classical wiring diagram of the VOR adaptation where only the ipsilateral vestibular input was represented.…”

Localization of relaxin‐3 in brain of Macaca fascicularis: Identification of a nucleus incertus in primate

“…Electrophysiological studies in conscious cats revealed that NI neurons were highly responsive to changes in eye movement and horizontal head movement (Cheron et al, 1995(Cheron et al, , 1996, suggesting that the NI is involved in oculomotor and cerebellar function. Tanaka et al (2005) demonstrated that in the rat almost all RLN3 neurons in the NI express corticotropin-releasing factor receptor-1 (CRF-R1) and a high proportion of RLN3-positive neurons display elevated nuclear Fos levels, after intracerebroventricular injection of 1 g CRF and prolonged (2 to 6 hours) water immersion-restraint stress, suggesting that there is a functional interaction between these two signaling systems.…”