Ascending and Descending Projections of the Lateral Vestibular Nucleus in the Rat*

Acta Biologica Hungarica 53 (1–2), pp. 7–21 (2002) ASCENDING AND DESCENDING PROJECTIONS OF THE LATERAL VESTIBULAR NUCLEUS IN THE RAT* TIMEA BÁCSKAI, G. SZÉKELY, CLARA MATESZ** Department of Anatomy, Histology and Embryology, University of Debrecen, Medical and Health Science Center, Nagyerdei krt. 98, H-4012 Debrecen, Hungary (Received: September 10, 2001; accepted: November 17, 2001) The tracer neurobiotin was injected into the lateral vestibular nucleus in rat and the efferent fiber connections of the nucleus were studied. The labeled fibers reached the diencephalon rostrally and the sacral segments of the spinal cord caudally. In the diencephalon, the ventral posteromedial and the gustatory nuclei received the most numerous labeled fibers. In the mesencephalon, the inferior colliculus, the interstitial nucleus of Cajal, the nucleus of Darkschewitch, the periaqueductal gray matter and the red nucleus received large numbers of labeled fibers. In the rhombencephalon, commissural and internuclear connections originated from the lateral vestibular nucleus to all other vestibular nuclei. The medioventral (motor) part of the reticular formation was richly supplied, whereas fewer fibers were seen in the lateral (vegetative) part. In the spinal cord, the descending fibers were densely packed in the anterior funiculus and in the ventral part of the lateral funiculus. Collaterals invaded the entire gray matter from lamina IX up to lamina III; the fibers and terminals were most numerous in laminae VII and VIII. Collateral projections were rich in the cervical and lumbosacral segments, whereas they were relatively poor in the tho- racic segments of the spinal cord. It was concluded that the fiber projection in the rostral direction was primarily aimed at sensory-motor centers; in the rhombencephalon and spinal cord, fibers projected onto structures subserving various motor functions. Keywords: Lateral vestibular nucleus – rat – neurobiotin labeling INTRODUCTION Primary afferent fibers originating in the vestibular sense organs terminate on second-order vestibular neurons of the superior (SVN), medial (MVN), lateral or Deiters (LVN) and descending (DVN) vestibular nuclei. Numerous experiments have been performed to study the ascending and descending fiber connections, which con- vey the most versatile functions of these nuclei, but unfortunately the different functions of the individual nuclei, depending on their different fiber connections, are known to only a limited extent. There is general agreement that the most rostral terminals of secondary vestibular fibers are found in different thalamic nuclei. The lateral nuclear group [6, 21, 38, 41], *Dedicated to Professor József Hámori on the occasion of his 70th birthday. **Corresponding author; e-mail: [email protected] 0236-5383/2002/$ 5.00 © 2002 Akadémiai Kiadó, Budapest 8 TIMEA BÁCSKAI et al. the medial geniculate body [18, 33] and the intralaminar nuclei [11, 18] are the main targets of ascending secondary vestibular fibers. These thalamic nuclei relay vestibular information to wide areas of the prosencephalon, including the motor cortex [16], the limbic system [3, 42], the insula [2], the striatum [18], the auditory cortex [32] and above all the vestibular cortex in the posterior part of the superior temporal gyrus [14]. With these radiations, the vestibular system contributes greatly to the organiza- tion of various motor functions. Vestibular inputs to the hippocampus are presumed to generate important cues in orientation in space [39]. In the brainstem region, fiber connections to the mesencephalic tectal and tegmen- tal nuclei are well known in a variety of experimental animals [10, 40], as are the pre- cise interconnections with cranial motor nuclei innervating the external eye muscles [25]. In addition to these, there are extensive connections between vestibular nuclei on either side of the rhombencephalon [9, 13]. The commissural pathways are thought to be of special importance in mediating the neural mechanisms for vestibular compensation, which can be experienced at various times following hemi- labyrinthectomy in different animal species. Despite much work along this line, it is still not clear which of these commissural interactions have specific roles in the com- pensatory mechanisms, and few data are available on intrinsic synaptic connections that can be demonstrated between neurons of the same nucleus in some species [9, 13]. The vestibulo-hippocampal projection is suggested to be an important factor in the mechanism of vestibular compensation [44]. As the vestibular nuclei are virtually embedded into the rhombencephalic reticular formation (RF), many collaterals and direct fiber terminals are found in various parts of this structure [43]. The contributions of the individual nuclei to the RF are not known; nevertheless, vestibular afferents can be shown to terminate in the dorsal vagal nucleus and in the nucleus of the solitary tract [4]. It has been suggested that movements and cardiorespiratory activity are coordinated by a linkage between the control mechanisms of somatomotor and visceromotor activities. The somatic sensory signals required in this regulation are in part conveyed through the vestibular system, indicating the body position in space. Various vestibular inputs to the RF may be the source of dizziness and vomiting reflexes. Unfortunately, the anatomy and physiology of vestibulo-autonomic interactions have not been fully explored. There is a substantial descending projection from the vestibular nuclei onto different parts of the spinal cord [5]. At variance with the classical description that vestibulospinal fibers and the medial longitudinal fascicle travel in the anterior funiculus of the spinal cord, we have located descending fibers in the lateral and dorsal funiculi as well [26, 27]. Another classical thesis that spinal vestibular fibers orig- inate in the LVN was contradicted by the multiple origin of the descending tracts in our experiments. In view of the general situation that very few studies made a point as to the nuclear origin of the different vestibular projections, we have decided to start a series of investigations on the fiber connections of individual vestibular nuclei. In an earlier work we described the projections of the LVN in the frog [19]. In the present paper we give an account of the fiber connections of the LVN in the rat, using the tracer dye neurobiotin. Acta Biologica Hungarica 53, 2002 Projections of lateral vestibular nucleus in the rat 9 MATERIALS AND METHODS The experiments were performed on 9 adult Wistar rats, in accordance with state reg- ulations and with the approval of the University Animal Care Committee. Under ure- thane anesthesia, the head of the animal was fixed in a stereotactic holder, and the cranial cavity and subsequently the dura mater were opened. A glass micropipette with a tip diameter of 10–15 μm was filled with neurobiotin and introduced into the rostral part of the LVN according to the coordinates of Paxinos and Watson [35]. A positive direct current of 5 μA was used for the injection, with a pulse duration of 7 s, followed by 3–5 s intervals for a period of 15–20 min. After a survival period of 8–14 days, the animals were perfused transcardially with isotonic saline for 2–3 min, followed by a fixative containing 1.25% glutaraldahyde and 2% paraformaldahyde in 0.1M phosphate buffer (pH 7.4). The diencephalon, the brainstem and the spinal cord were removed and fixed by immersion in the same fixative overnight at 4 °C. The blocks of tissue were then washed in 0.1M phosphate buffer, and sections of 60 μm were made with a vibrotome. For the histochemical detection of neurobiotin, the sections were incubated in avidin-biotin-peroxidase complex for 90 min at room temperature and the reaction was visualized with DAB. The sections were mounted on glass slides coated with gelatine, counterstained with 1% toluidine blue, air-dried and coverslipped. Labeled boutons and fibers were drawn through a camera lucida from three con- secutive sections. The photographs were taken with a Nikon microscope. RESULTS The neurobiotin injection was restricted to the rostral part of the LVN, 1.2 mm caudally to the interaural line (Fig. 3A). Three animals in which the injection spread into the surrounding structures were excluded from the analysis. Labeled fibers were followed bilaterally in the rostral and the caudal directions from the level of the diencephalon rostrally and as far as the lumbosacral spinal cord caudally. Rostral projections Diencephalon Labeled fibers reach the diencephalon by way of the medial longitudinal fascicle (MLF). The fascicle loosens up at the mesencephalic and diencephalic border and the fibers spread predominantly to the caudal one-third of the thalamus (Fig. 2A). The densest termination area is seen in the ventral posteromedial (VPM) (Figs 1A and 2A) and in the gustatory nuclei. Terminals are less numerous in the parafascicular nucleus and in the nucleus of Forel. In the subthalamus, the ventral and dorsal sub- Acta Biologica Hungarica 53, 2002 10 TIMEA BÁCSKAI et al. Fig. 1. Microphotographs of labeled terminals in the ventral posteromedial nucleus of the thalamus (A) and in the magnocellular part of the red nucleus (B). Bar: 100 μm divisions of the zona incerta are evenly supplied by the fibers of LVN origin. The medial geniculate body and the pretectal nucleus receive vestibular fibers on both sides. Mesencephalon The projection of the LVN is equally strong on both sides at this level (Fig. 2B). The inferior colliculus is invaded by appreciable numbers of labeled fibers, probably by more numerous fibers on the side contralateral to than on the side ipsilateral to the injection. A few fibers can also be found in the superior colliculus. The periaqueductal gray matter (PAG) is richly supplied on both sides. A strong projection is detected within the interstitial nucleus of Cajal and the nucleus of Darkschewitch, but the oculomotor and trochlear nuclei receive relatively few fibers. Labeled fibers can be seen all over the mesencephalic RF.

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