Nucleus Prepositus Research Articles

Vestibular imbalance associated with a lesion in the nucleus prepositus hypoglossi area.

The nucleus prepositus hypoglossi (NPH) is known to be a neural integrator of horizontal eye movements. Although the role of the human NPH is not well known, it may also function in postural balance, in view of its anatomic connections with the vestibular nuclei and vestibulocerebellum and of lesion studies in experimental animals. To show that the human NPH contributes to vestibular function in addition to eye movement control. Case series. University hospital. Patients Six patients with small and discrete brainstem infarctions that predominantly involved the NPH region.Main Outcome Measure Findings on magnetic resonance images. The NPH was affected at the lower pontine level in 2 patients and at the upper medullary level in 4. In addition to gaze-evoked nystagmus, all patients had vertigo, vomiting, and postural ataxia, suggesting vestibular dysfunction. The patients typically fell contralaterally or bilaterally to the lesion side. The NPH serves a vestibular function in addition to its oculomotor control function.

Central distribution of vestibular afferents that innervate the anterior or lateral semicircular canal in the mongolian gerbil

The central distribution of afferents that innervate the crista ampullaris of the anterior or lateral semicircular canals was determined in gerbils following the direct injection of tracers into one sensory neuroepithelia. Labeled somata were scattered throughout the superior ganglion. The central distribution of fibers demonstrated extensive overlap. The central branch of afferents innervating either canal was located in the rostral part of the nerve. Nerve fibers divided into ascending and descending branches. Ascending branch ramifications terminated in the superior vestibular nucleus, the magnocellular and parvicellular medial vestibular nuclei, and the cerebellum. Cerebellar terminal areas include the flocculus, nodulus and uvula. Descending branch ramifications terminated in the caudal medial, parvicellular medial and descending vestibular nuclei, and the nucleus prepositus hypoglossi. Lateral canal afferents terminated sparsely in nucleus cuneatus. The anterior canal had sparse innervation in the paratrigeminal and gigantocellular reticular formation. This study has shown many similarities in the central distribution of fibers that innervate the anterior and lateral canals and a few areas of segregated input. Projections outside the vestibular nuclei are more extensive than previously determined, including afferents to prepositus hypoglossi, cochlear nuclei, and reticular formation. Projections to the flocculus appear as numerous as those to the vermis.

Expression of Trk receptors in the oculomotor system of the adult cat.

We examined the expression of the three Trk receptors for neurotrophins (TrkA, TrkB, and TrkC) in the extraocular motor nuclei of the adult cat by using antibodies directed against the full-Trk proteins in combination with horseradish peroxidase retrograde tracing. The three receptors were present in all neuronal populations investigated, including abducens motoneurons and internuclear neurons, medial rectus motoneurons of the oculomotor nucleus, and trochlear motoneurons. They were also present in the vestibular and prepositus hypoglossi nuclei. TrkA, TrkB, and TrkC immunopositive cells were found in similar percentages in the oculomotor and in the trochlear nuclei. In the abducens nucleus, however, a significantly higher percentage of cells expressed TrkB than the other two receptors, among both motoneurons (81.8%) and internuclear neurons (88.4%). The percentages obtained for the three Trk receptors in identified neuronal populations pointed to the colocalization of two or three receptors in a large number of cells. We used confocal microscopy to elucidate the subcellular location of Trk receptors. In this case, abducens motoneurons and internuclear neurons were identified with antibodies against choline acetyltransferase and calretinin, respectively. We found a different pattern of staining for each neurotrophin receptor, suggesting the possibility that each receptor and its cognate ligand may use a different route for cellular signaling. Therefore, the expression of Trk receptors in oculomotor, trochlear, and abducens motoneurons, as well as abducens internuclear neurons, suggests that their associated neurotrophins may exert an influence on the normal operation of the oculomotor circuitry. The presence of multiple Trk receptors on individual cells indicates that they likely act in concert with each other to regulate distinct functions.

An immunocytochemical mapping of methionine-enkephalin-arg 6-gly 7-leu 8 in the human brainstem

Using an indirect immunoperoxidase technique, we studied the distribution of immunoreactive fibers and cell bodies containing methionine-enkephalin-Arg 6-Gly 7-Leu 8 in the adult human brainstem. Immunoreactive cell bodies were found in the reticular formation of the medulla oblongata (in which we observed the highest density of immunoreactive cell bodies) and the pons, the solitary nucleus, the hypoglossal nucleus, the medial and spinal vestibular nuclei, the lateral cuneate nucleus, the nucleus prepositus, the central gray of the pons and mesencephalon, the central and pericentral nuclei of the inferior colliculus, the superior colliculus, ventral to the superior olive and in the midline region of the pons and mesencephalon. The highest density of immunoreactive fibers containing methionine-enkephalin-Arg 6-Gly 7-Leu 8 was found in the spinal trigeminal nucleus, the central gray and the reticular formation of the medulla oblongata, pons and mesencephalon, the solitary nucleus, the spinal vestibular nucleus, the dorsal accessory olivary nucleus, the raphe obscurus, the substantia nigra and in the interpeduncular nucleus. The widespread distribution of immunoreactive structures containing methionine-enkephalin-Arg 6-Gly 7-Leu 8 in the human brainstem indicates that this neuropeptide might be involved in several physiological mechanisms, acting as a neurotransmitter and/or neuromodulator.

Distribution of androgen receptor immunoreactivity in the brainstem of male rats

Gonadal steroids such as testosterone and estrogen are necessary for the normal activation of male rat sexual behavior. The medial preoptic area (MPOA), an important neural substrate regulating mating, accumulates steroids and also expresses functional androgen receptors (AR). The MPOA is intimately connected with other regions implicated in copulation, such as the bed nucleus of the stria terminalis and medial amygdala. Inputs to the MPOA arise from several areas within the brainstem, synapsing preferentially onto steroid sensitive MPOA cells which are activated during sexual activity. Given that little is known about the distribution of AR protein in the brainstem of male rats, we mapped the distribution of AR expressing cells in the pons and medulla using immunocytochemistry. In agreement with previous reports, AR immunoreactivity (AR-ir) was detected in ventral spinal motoneurons and interneurons. In addition, AR-ir was detected in areas corresponding to the solitary tract, lateral paragigantocellular and α and ventral divisions of the gigantocellular reticular nuclei, area postrema, raphe pallidus, ambiguus nucleus, and intermediate reticular nucleus. Several regions within the pons contained AR-ir, such as the tegmental and central gray, parabrachial nucleus, locus coeruleus, Barrington's nucleus, periaqueductal gray, and dorsal raphe. In contrast with in situ hybridization studies, auditory and somatosensory areas were AR-ir negative, and, except for very light staining in the prepositus nucleus, areas carrying vestibular information did not display AR-ir. Additionally, cranial nerve motoneurons of the hypoglossal, facial, dorsal vagus, and spinal trigeminal did not display AR-ir in contrast to previous reports. The data presented here indicate that androgens may influence numerous cell groups within the brainstem. Some of these probably constitute a steroid sensitive circuit linking the MPOA to motoneurons in the spinal cord via androgen responsive cells in the caudal ventral medulla.

Discharge dynamics of oculomotor neural integrator neurons during conjugate and disjunctive saccades and fixation.

Burst-tonic (BT) neurons in the prepositus hypoglossi and adjacent medial vestibular nuclei are important elements of the neural integrator for horizontal eye movements. While the metrics of their discharges have been studied during conjugate saccades (where the eyes rotate with similar dynamics), their role during disjunctive saccades (where the eyes rotate with markedly different dynamics to account for differences in depths between saccadic targets) remains completely unexplored. In this report, we provide the first detailed quantification of the discharge dynamics of BT neurons during conjugate saccades, disjunctive saccades, and disjunctive fixation. We show that these neurons carry both significant eye position and eye velocity-related signals during conjugate saccades as well as smaller, yet important, "slide" and eye acceleration terms. Further, we demonstrate that a majority of BT neurons, during disjunctive fixation and disjunctive saccades, preferentially encode the position and the velocity of a single eye; only few BT neurons equally encode the movements of both eyes (i.e., have conjugate sensitivities). We argue that BT neurons in the nucleus prepositus hypoglossi/medial vestibular nucleus play an important role in the generation of unequal eye movements during disjunctive saccades, and carry appropriate information to shape the saccadic discharges of the abducens nucleus neurons to which they project.

Retrograde-labeling of pretecto-vestibular pathways in cats

The projections of the nucleus of the optic tract (NOT) were studied in cats using three kinds of retrograde tracers (horseradish peroxidase wheat germ agglutinin (WGA-HRP), dextran fluorescein (DF) and dextran tetramethylrhodamine (DTR)). First, in the cases that WGA-HRP were injected into the rostral part of medial vestibular nucleus (VN), a significant number of retrograde-labeled neurons, which concentrated between 600 and 800 μm from the rostral part of the NOT, were observed in the NOT ipsilateral to the injection sites. Second, no double-labeled neurons were found following injections of DF in the medial VN and DTR in the nucleus prepositus hypoglossi (NPH) at the same time. In this experiment, it was made clear that two different kinds of neurons in the NOT projected to the medial VN and the NPH, respectively.

Mapping of neurokinin-like immunoreactivity in the human brainstem

BackgroundUsing an indirect immunoperoxidase technique, we have studied the distribution of immunoreactive fibers and cell bodies containing neurokinin in the adult human brainstem with no prior history of neurological or psychiatric disease.ResultsClusters of immunoreactive cell bodies and high densities of neurokinin-immunoreactive fibers were located in the periaqueductal gray, the dorsal motor nucleus of the vagus and in the reticular formation of the medulla, pons and mesencephalon. Moreover, immunoreactive cell bodies were found in the inferior colliculus, the raphe obscurus, the nucleus prepositus hypoglossi, and in the midline of the anterior medulla oblongata. In general, immunoreactive fibers containing neurokinin were observed throughout the whole brainstem. In addition to the nuclei mentioned above, the highest densities of such immunoreactive fibers were located in the spinal trigeminal nucleus, the lateral reticular nucleus, the nucleus of the solitary tract, the superior colliculus, the substantia nigra, the nucleus ambiguus, the gracile nucleus, the cuneate nucleus, the motor hypoglossal nucleus, the medial and superior vestibular nuclei, the nucleus prepositus hypoglossi and the interpeduncular nucleus.ConclusionThe widespread distribution of immunoreactive structures containing neurokinin in the human brainstem indicates that neurokinin might be involved in several physiological mechanisms, acting as a neurotransmitter and/or neuromodulator.

Zonal organization of the vestibulocerebellum in pigeons (Columba livia): II. Projections of the rotation zones of the flocculus.

Previous neurophysiologic research in birds and mammals has shown that there are two types of Purkinje cells in the flocculus. The first type shows maximal modulation in response to rotational optokinetic stimulation about the vertical axis (rVA neurons). The second type shows maximal modulation in response to rotational optokinetic stimulation about a horizontal axis oriented 45 degrees to contralateral azimuth (rH45c neurons). In pigeons, the rVA and rH45c are organized into four alternating parasagittal zones. In this study we investigated the projections of Purkinje cells in the rVA and rH45c zones by using the anterograde tracers biotinylated dextran amine and cholera toxin subunit B. After iontophoretic injections of tracers into the rH45c zones, heavy anterograde labeling was found in the infracerebellar nucleus and the medial margin of the superior vestibular nucleus. Some labeling was also consistently observed in the lateral cerebellar nucleus and the dorsolateral vestibular nucleus. After injections into the rVA zones, heavy anterograde labeling was found in the medial and descending vestibular nuclei, the nucleus prepositus hypoglossi, and the central region of the superior vestibular nucleus. Less labeling was seen in the tangential nucleus, the dorsolateral vestibular nucleus, and the lateral vestibular nucleus, pars ventralis. These results are compared and contrasted with findings in mammalian species.

Linear regression of eye velocity on eye position and head velocity suggests a common oculomotor neural integrator.

The oculomotor system produces eye-position signals during fixations and head movements by integrating velocity-coded saccadic and vestibular inputs. A previous analysis of nucleus prepositus hypoglossi (nph) lesions in monkeys found that the integration time constant for maintaining fixations decreased, while that for the vestibulo-ocular reflex (VOR) did not. On this basis, it was concluded that saccadic inputs are integrated by the nph, but that the vestibular inputs are integrated elsewhere. We re-analyze the data from which this conclusion was drawn by performing a linear regression of eye velocity on eye position and head velocity to derive the time constant and velocity bias of an imperfect oculomotor neural integrator. The velocity-position regression procedure reveals that the integration time constants for both VOR and saccades decrease in tandem with consecutive nph lesions, consistent with the hypothesis of a single common integrator. The previous evaluation of the integrator time constant relied upon fitting methods that are prone to error in the presence of velocity bias and saccades. The algorithm used to evaluate imperfect fixations in the dark did not account for the nonzero null position of the eyes associated with velocity bias. The phase-shift analysis used in evaluating the response to sinusoidal vestibular input neglects the effect of saccadic resets of eye position on intersaccadic eye velocity, resulting in gross underestimates of the imperfections in integration during VOR. The linear regression method presented here is valid for both fixation and low head velocity VOR data and is easy to implement.

Estimation of premotor synaptic drives to simulated abducens motoneurons for control of eye position.

The firing rate of an abducens motoneuron (AbMN) is linearly related to eye position with slope K, above recruitment threshold theta. Within the AbMN population K increases as theta increases. It is possible that these properties depend on the synaptic drives generated by the major premotor inputs to AbMNs, namely position-vestibular-pause (PVP) cells and eye and head velocity (EHV) cells in the medial vestibular nucleus, and eye-position and burst-position cells in the nucleus prepositus hypoglossi (NPH). Premotor inputs to AbMNs were therefore modelled by a two-layer net, in which the output nodes represented the AbMNs (with fixed intrinsic properties) and the input nodes the three classes of premotor units ( n=20/class). Conjugate eye-position commands were used to generate the firing rates in premotor units found experimentally. The output of the net was compared with observed AbMN firing rates, and the resultant error used to adjust the magnitude and sign of the connection weights between premotor units and AbMNs. To provide additional constraints on permitted weights, the net was also trained under simulated smooth pursuit, cancellation of the vestibular-ocular reflex, and the vestibulo-ocular reflex itself (all at 0.5 Hz). Since the projections of EHV cells have not been clearly characterized, two versions of the model were trained, corresponding to different assumptions about these projections. In both versions, position-related AbMN firing rates were derived mainly from an excitatory drive from PVP cells and an inhibitory drive from NPH cells with the opposite ON direction. Variation in AbMN threshold theta and position sensitivity K depended on the strength of the drive from the NPH: the stronger the drive, the higher both K and theta. This arrangement was observed in six variants of the basic model with different parameter values, and in a simplified form (constant PVP drive and varying NPH drive) was able to generate qualitatively the observed relationship between K and theta even in the absence of input from EHV cells. It appears to be a robust mechanism for producing the experimentally observed variation in position-related firing of AbMNs, even without a contribution from their intrinsic properties, and predicts that local blocking of the inhibitory drive from cells in the NPH should lower both the position threshold and sensitivity of an individual AbMN. The model also indicates that if EHV cells have ipsilateral inhibitory projections, as has been proposed on the basis of their similarity with cells receiving input from the flocculus, then their role in eye-position control would reinforce that of cells in the NPH.

Distribution of apelin-synthesizing neurons in the adult rat brain

The peptide apelin originating from a larger precursor preproapelin molecule has been recently isolated and identified as the endogenous ligand of the human orphan G protein-coupled receptor, APJ (putative receptor protein related to the angiotensin receptor AT 1). We have shown recently that apelin and apelin receptor mRNA are expressed in brain and that the centrally injected apelin fragment K17F (Lys 1-Phe-Arg-Arg-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe 17) decreased vasopressin release and altered drinking behavior. Using a specific polyclonal antiserum against K17F for immunohistochemistry, the aim of the present study was to establish the precise topographical distribution of apelin immunoreactivity in colchicine-treated adult rat brain. Immunoreactivity was essentially detected in neuronal cell bodies and fibers throughout the entire neuroaxis in different densities. Cells bodies have been visualized in the preoptic region, the hypothalamic supraoptic and paraventricular nuclei and in the highest density, in the arcuate nucleus. Apelin immunoreactive cell bodies were also seen in the pons and the medulla oblongata. Apelin nerve fibers appear more widely distributed than neuronal apelin cell bodies. The hypothalamus represented, by far, the major site of apelin-positive nerve fibers which were found in the suprachiasmatic, periventricular, dorsomedial, ventromedial nuclei and in the retrochiasmatic area, with the highest density in the internal layer of the median eminence. Fibers were also found innervating other circumventricular organs such as the vascular organ of the lamina terminalis, the subfornical and the subcommissural organs and the area postrema. Apelin was also detected in the septum and the amygdala and in high density in the paraventricular thalamic nucleus, the periaqueductal central gray matter and dorsal raphe nucleus, the parabrachial and Barrington nuclei in the pons and in the nucleus of the solitary tract, lateral reticular, prepositus hypoglossal and spinal trigeminal nuclei. The topographical distribution of apelinergic neurons in the brain suggests multiple roles for apelin especially in the central control of ingestive behaviors, pituitary hormone release and circadian rhythms.

Ascending and descending projections of the lateral vestibular nucleus in the frog Rana esculenta.

The lectin Phaseolus vulgaris leucoagglutinin was injected into the frog lateral vestibular nucleus (LVN) to study its antero- and retrograde projections. The following new observations were made. 1) In the diencephalon, vestibular efferents innervate the thalamus in a manner similar to that of mammalian species. The projections show a preference for the anterior, central, and ventromedial thalamic nuclei. 2) In the mesencephalon, vestibular fibers terminate in the tegmental nuclei and the nucleus of medial longitudinal fascicle. 3) In the rhombencephalon, commissural and internuclear projections interconnect the vestibular nuclei. Some of the termination areas in the reticular formation can be homologized with the mammalian inferior olive and the nucleus prepositus hypoglossi. Another part of the vestibuloreticular projection may transmit vestibular impulses toward the vegetative centers of the brainstem. A relatively weak projection is detected in the spinal nucleus of the trigeminal nerve, dorsal column nuclei, and nucleus of the solitary tract. 4) In the spinal cord, vestibular terminals are most numerous in the ipsilateral ventral horn and in the triangular area of the dorsal horn. 5) The coincidence of retrogradely labeled cells with vestibular receptive areas suggests reciprocal interconnections between these structures and the LVN. 6) In seven places, the LVN projections overlap the receptive areas of proprioceptive fibers, suggesting a convergence of sensory modalities involved in the sense of balance.

Localization of P2X3 receptors and coexpression with P2X2 receptors during rat embryonic neurogenesis.

It is well known that extracellular ATP mediates rapid excitatory signaling by means of the ionotropic P2X receptors. One of its subunits, the P2X(3) receptor, is well documented to be associated with sensory innervation in adult animals. It is speculated that the P2X(3) receptor may have already been present in the early sensory system. The aim of this study was to investigate the distribution of the P2X(3) receptor during neurogenesis by using immunohistochemistry on rat embryos from embryonic day (E)9.5-18.5. The P2X(3) receptor was first identified in the hindbrain neural tube and the sensory ganglia in E11-11.5 embryos. At E14.5, the optic tract and retina, nucleus tractus solitarius, mesencephalic trigeminal nucleus, and sensory nerves in both respiratory and digestive tract showed positive staining. The facial nucleus, the prepositus hypoglossal nucleus, and the sympathetic ganglia also showed P2X(3) immunoreactivity, even though these are not sensory associated. P2X(3) immunoreactivity was detected in the vestibular nucleus, the nerves in mesentery, bladder, and kidney in E16.5 and in nerves in vibrissae in E18.5. P2X(3) immunoreactivity in the facial nucleus, spinal trigeminal tract, the mesencephalic trigeminal nucleus, and the vestibular nucleus were undetectable in postnatal day 16 rat brainstem. The P2X(3) receptor was coexpressed with the P2X(2) receptor in nucleus tractus solitarius, dorsal root ganglion, nodose ganglion, and the taste bud in E16.5 embryo, which was 5 days later than the first appearance of the native P2X(3) receptor. In summary, we present a detailed expression pattern of the P2X(3) receptor during neurogenesis and report that P2X(3) immunoreactivity is down-regulated in early postnatal brainstems.

一側内耳破壊後のマウス脳幹内ｃ‐Ｆｏｓ発現様式

Vestibular compensation is attributed to functional and structural reorganization of neural networks in the central vestibular system, but its precise mechanism is still not clear. Immediate early gene c-Fos is used as a marker of neuronal activation, because of its very limited expression in the normal state and quick appearance after stimulation. Previous reports, investigating c-Fos expression after unilateral labyrinthectomy were made in rats and guinea pigs, but not in the mouse brainstem. For future application to the gene knockout mouse, we examined c-Fos expression in the mouse after unilateral labyrinthectomy. Twenty-four hours after surgery, a highest number of c-Fos positive cells were observed in the bilateral medial vestibular nucleus (MVe), spinal vestibular nucleus (SpVe) and in the contra lateral prepositus hypoglossal nucleus (PrH). In the inferior olive nucleus (IO), c-Fos positive cells were significantly higher in the intact side.

Transtympanic tetrodotoxin alters the VOR and Fos labeling in the vestibular complex

The sodium channel blocker tetrodotoxin (TTX) is an effective tool for blockade of action potentials. Unilateral transtympanic administration of 3 mM TTX produced behavioral symptoms similar to those following unilateral peripheral vestibular ablation. Complete resolution of visible symptoms occurred between 48 and 72 h post-TTX. Eye-coil recordings indicated a spontaneous nystagmus and a decrease in the VOR in TTX-treated animals. Neuronal activity in the central vestibular complex (VC), as monitored with Fos immunocytochemistry, revealed an asymmetric pattern of Fos labeling in the medial, inferior and superior vestibular nuclei and the prepositus hypoglossal nucleus. Although the spatio-temporal pattern of Fos labeling was consistent and reproducible at each time-point, changes were noted among time-points. Transient blockade with TTX may be useful for studying the central vestibular response to recurrent or episodic vestibular disruption in the intact system.

Electroacupuncture-related changes of NADPH-diaphorase and neuronal nitric oxide synthase in the brainstem of spontaneously hypertensive rats

The aim of this study was to investigate the electroacupuncture-related changes of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and neuronal nitric oxide synthase (nNOS) in the brainstem of spontaneously hypertensive rats (SHR). We evaluated the changes of NADPH-d-positive neurons using a histochemical method and the changes of nNOS-positive neurons using an immunohistochemical method. The staining intensities of NADPH-d-positive neurons and nNOS-positive neurons were assessed in a quantitative fashion using a microdensitometrical method based on optical density by means of an image analyzer. The optical density of NADPH-d-positive neurons and nNOS-positive neurons of the Shinsu (BL23) and Choksamni (ST36) electroacupuncture groups were significantly decreased in most brainstem areas as compared to the normal and arbitrary groups, with the exception of the optical density of NADPH-d positive neurons in the prepositus nucleus as compared to the arbitrary group. The present results demonstrated that electroacupuncture changes the activity in the NO system in the brainstem of SHR and the site where electroacupuncture is administered is of importance for this effect.

Tyrosine hydroxylase-immunoreactive projections from the caudal ventrolateral medulla to the subfornical organ in the rat

The subfornical organ (SFO) is known to be innervated by noradrenergic fibers. One possible origin of these fibers, which carry peripheral baroreceptor information to enhance the activity of SFO neurons, is the nucleus tractus solitarius (NTS). To investigate possible sites of origin of the catecholaminergic projections to the SFO, a retrograde tracing method was combined with immunohistochemistry in the rat. Stereotaxical injection of a retrograde tracer, wheat germ agglutinin-conjugated horseradish peroxidase–colloidal gold complex, into the SFO from the dorsal aspect revealed retrogradely labeled neurons in several catecholaminergic cell groups. A substantial number of retrogradely labeled neurons showing tyrosine hydroxylase (TH) immunoreactivity were found in the NTS and ventrolateral medulla (VLM) at levels caudal to the obex and in the locus coeruleus, while retrogradely labeled neurons without TH immunoreactivity were found in the VLM at levels rostral to the obex and in the nucleus prepositus hypoglossi. When the tracer was injected into the structures dorsal to the SFO, including the triangular septal nucleus, the frequency of retrogradely labeled neurons in the NTS and VLM at the caudal level was very low. These findings indicate the existence of catecholaminergic projections from the VLM (probably A1) to the SFO, in addition to the noradrenergic projections from the NTS previously reported.

Morphological identification of nitric oxide sources and targets in the cat oculomotor system

Nitric oxide (NO) production by specific neurons in the prepositus hypoglossi (PH) nucleus is necessary for the correct performance of eye movements in alert cats. In an attempt to characterize the morphological substrate of this NO function, the distribution of nitrergic neurons and NO-responding neurons has been investigated in different brainstem structures related to eye movements. Nitrergic neurons were stained by either immunohistochemistry for NO synthase I or histochemistry for reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase. The NO targets were identified by cyclic guanosine monophosphate (cGMP) immunohistochemistry in animals treated with a NO donor immediately before fixation of the brain. Connectivity between cells of the NO-cGMP pathway was analyzed by injections of the retrograde tracers horseradish peroxidase or fast blue in different structures. The motor nuclei commanding extraocular muscles did not contain elements of the NO-cGMP pathway, except for some scattered nitrergic neurons in the most caudal part of the abducens nucleus. The PH nucleus contained the largest number of nitrergic cell bodies and a rich neuropil, distributed in two groups in medial and lateral positions in the caudal part, and one central group in the rostral part of the nucleus. An abundant cGMP positive neuropil was the only NO-sensitive element in the PH nucleus, where no cGMP-producing neuronal cell bodies were observed. The opposite disposition was found in the marginal zone between the PH and the medial vestibular nuclei, with a large number of NO-sensitive cGMP-producing neurons and almost no nitrergic cells. Both nitrergic and NO-sensitive cell bodies were found in the medial and inferior vestibular nuclei and in the superior colliculus, whereas the lateral geniculate nucleus contained nitrergic neuropil and a large number of NO-sensitive cell bodies. Some of the cGMP-positive neurons in the marginal zone and medial vestibular nucleus projected to the PH nucleus, predominantly to the ipsilateral side. These morphological findings may help to explain the mechanism of action of NO in the oculomotor system.

Gamma amino butyric acid (GABA) immunoreactivity in the vestibular nuclei of normal and unilateral vestibular neurectomized cats.

Recent neurochemical investigations of the central vestibular pathways have demonstrated that several neurotransmitters are involved in various operations required for stabilizing posture and gaze. Neurons of the vestibular nuclei (VN) receive GABAergic inhibitory afferents, and GABAergic neurons distributed throughout the vestibular complex are implicated in inhibitory vestibulo-ocular and vestibulo-spinal pathways. The aim of this study was to analyse the modifications of GABA immunoreactivity (GABA-ir) in the cat VN after unilateral vestibular neurectomy (UVN). Indeed, compensation of vestibular deficits is a good model for studying adult central nervous system (CNS) plasticity and the GABAergic system is involved in CNS plasticity. We studied GABA-ir by using a purified polyclonal antibody raised against GABA. Light microscopic preparations of thin (20 microm) sections of cat VN were used to quantify GABA-ir by an image analysing system measuring GABA-positive punctate structures and the number of GABA-positive neurons. Both the lesioned and intact sides were analysed in three populations of UVN cats killed at different times after injury (1 week, 3 weeks and 1 year). These data were compared to those collected in normal unlesioned and sham-operated cats. Results showed a spatial distribution of GABA-ir in the control cats that confirmed previous studies. GABA-ir neurons, fibres and nerve terminals were scattered in all parts of the VN. A higher concentration of GABA-positive neurons (small cells) was detected in the medial and inferior VN (MVN and IVN) and in the dorsal part of the lateral VN (LVNd). A higher level of GABA-positive punctate structures was observed in the MVN and in the prepositus hypoglossi (PH) nucleus. Lesion-induced changes were found at each survival time. One week after injury the number of GABA-positive neurons was significantly increased in the MVN, the IVN and the dorsal part of the LVN on the lesioned side and in the ventral part of the LVN on the intact side. One year later a bilateral increase in GABA-positive neurons was detected in the MVN whilst a bilateral decrease was observed in both the SVN and the ventral part of the LVN. Changes in the GABA-staining varicosities did not strictly coincide with the distribution of GABA-ir cells, suggesting that GABA-ir fibres and nerve terminals were also modified. One week and later after injury, higher GABA-staining varicosities were seen unilaterally in the ipsilateral MVN. In contrast, bilateral increases (in PH) and bilateral decreases (in SVN and the ventral part of the LVN) were recorded in the nearly (3 weeks) or fully (1 year) compensated cats. At this stage GABA-staining varicosities were significantly increased in the lesioned side of the MVN. These findings demonstrate the reorganization of the GABAergic system in the VN and its possible role in recovery process after UVN in the cat. The changes seen during the acute stage could be causally related to the VN neuron deafferentation, contributing to the static vestibular deficits. Those found in the compensated cats would be more functionally implicated in the dynamic aspects of vestibular compensation.