N. M. Gerrits

Secondary vestibulocerebellar projections to the flocculus and uvulo-nodular lobule of the rabbit: a study using HRP and double fluorescent tracer techniques

SummaryThe distribution of vestibular neurons projecting to the flocculus and the nodulus and uvula of the caudal vermis (Larsells lobules X and IX) was investigated with retrograde axonal transport of horseradish peroxidase and the fluorescent tracers Fast Blue, Nuclear Yellow and Diamidino Yellow. The presence of collateral axons innervating the flocculus on one hand and the nodulus and uvula on the other was studied with simultaneous injection of the different fluorescent tracers. The distribution of vestibular neurons projecting to either flocculus or caudal vermis is rather similar and has a bilateral symmetry. The projection from the magnocellular medial vestibular nucleus is very sparse, while that from the lateral vestibular nucleus is absent. The majority of labeled neurons was found in the medial, superior, and descending vestibular nuclei, in that order. Double labeled neurons were distributed in a similar way as the single labeled ones. Labeled neurons project to the nodulus and uvula, the flocculus, and to both parts of the cerebellum simultaneously in a ratio of 12:4:1. Five different populations of vestibulocerebellar neurons can be distinguished on the basis of their projection to the: (1) ipsilateral flocculus, (2) contralateral flocculus, (3) ipsilateral flocculus and nodulus/uvula, (4) contralateral flocculus and nodulus/uvula, and (5) nodulus/uvula.

INHIBITORY SYNAPTIC INPUTS TO THE OCULOMOTOR NUCLEUS FROM VESTIBULO-OCULAR -REFLEX-RELATED NUCLEI IN THE RABBIT

Studies of the pathways involved in the vestibulo-ocular reflex have suggested that the projection from the superior vestibular nucleus to the ipsilateral oculomotor nucleus is inhibitory, whereas the medial vestibular nucleus, the abducens nucleus and the contralateral superior vestibular nucleus most likely exert excitatory effects on oculomotor neurons. In order to determine directly the termination pattern and the neurotransmitter of these afferents, we studied their input to the oculomotor nucleus in the rabbit at the light microscopic level with the use of anterograde tracing of Phaseolus vulgaris-leucoagglutinin combined with retrograde tracing of horseradish peroxidase from the extraocular muscles, and at the ultrastructural level with the use of anterograde tracing of wheatgerm-agglutinated horseradish peroxidase combined with GABA and glycine postembedding immunocytochemistry. The general ultrastructural characteristics of the neuropil and the types of boutons observed in the rabbit oculomotor nuclei are in general agreement with the descriptions for the oculomotor complex of other mammals. The superior vestibular nucleus projected bilaterally to the superior rectus and inferior oblique subdivisions, and ipsilaterally to the inferior rectus and medial rectus subdivision; the medial vestibular nucleus projected bilaterally to the medial rectus, inferior oblique, inferior rectus and superior rectus subdivisions with a strong contralateral predominance. The abducens nucleus projected contralaterally to the medial rectus subdivision. More than 90% of all the anterogradely labeled terminals from the ipsilateral superior vestibular nucleus were GABAergic. These terminals were characterized by flattened vesicles and symmetric synapses, and they contacted somata, as well as proximal and distal dendrites of motoneurons. All terminals derived from the medial vestibular nucleus the abducens nucleus and the contralateral superior vestibular nucleus were non-GABAergic. These non-GABAergic terminals showed spherical vesicles and asymmetric synapses, and they contacted predominantly distal dendrites. None of the anterogradely labeled terminals from the studied vestibular nuclei or abducens nucleus were glycinergic. The present study provides the first direct anatomical evidence that most, if not all, of the synaptic input from the superior vestibular nucleus to the ipsilateral oculomotor nucleus is GABAergic, and that the medial rectus subdivision is included in the termination area. Furthermore, it confirms that the projections from the medial vestibular nucleus, the abducens nucleus and the contralateral superior vestibular nucleus are exclusively non-GABAergic.

Chapter 20 Behavioural analysis of Purkinje cell output from the horizontal zone of the cat flocculus

Publisher Summary This chapter describes the properties of the simple spike discharges of Purkinje cells (P-cells) located in the horizontal zone of the cat flocculus and compares the output signals with those of the flocculus projecting neurons (FPNs) in the medial vestibular nucleus (MVN), nucleus prepositus hypoglossi (NPH), and nucleus incertus (NIC). Recently, floccular functions have been analyzed by an approach based on recording in the vestibular nuclei of the floccular target neuron (FTN) and on the comparison of these FTNs with vestibular neurons that do not receive direct floccular inhibition. Another approach to the floccular function consists of the recording of the FPNs identified in different nuclei of the brainstem by antidromic activation from the flocculus. The behavior of the P-cells of the horizontal zone of the flocculus differs significantly from those of the FPNs located in the MVN, NPH, and NIC. The convergence into the horizontal zone of the cat flocculus of an input signal only related to head velocity and of other input signals combining eye velocity and eye position is one of the main factors for the emergence of the pure position P-cell behavior and the more complex head velocity plus position P-cells.

GABAergic and glycinergic inputs to the rabbit oculomotor nucleus with special emphasis on the medial rectus subdivision

Contradictory results have been reported about the inhibitory input to the medial rectus subdivision of the oculomotor nucleus of the cat. In the present ultrastructural study, we quantified the GABAergic and glycinergic terminals in the various subdivisions of the rabbit oculomotor nucleus with the use of post-embedding immunocytochemistry combined with retrograde tracing of horseradish peroxidase. The density of the GABAergic input to the medial rectus subdivision was as substantial as that to the other subdivisions and the postsynaptic distribution of the GABAergic and glycinergic innervation did not differ among the different oculomotor subdivisions.