Comparisons of the immunocytochemical localization of choline acetyltransferase in the vestibular nuclei of the monkey and rat

Vestibular Function Impairment in Alzheimer's Disease

BACKGROUND

Falls and fractures due to impaired balance in patients with Alzheimer's disease (AD) have an adverse effect on the clinical course of the disease.

OBJECTIVE

To evaluate balance impairment in AD from the viewpoint of vestibular functional impairment.

METHODS

The subjects were 12 patients with AD, 12 dementia-free elderly adults, and 12 younger adults. Vestibular function was assessed using a stepping test, caloric nystagmus, and a visual suppression (VS) test.

RESULTS

The stepping test was abnormal in 9 of the 12 patients in the AD group. An abnormal stepping test was not associated with self-reported dizziness or tendency to fall. Significant VS abnormalities were present in the AD group. The suppression rate of VS was lower in AD patients with either a tendency to fall or constructional apraxia than in AD patients without either. The velocity of the rapid phase of caloric nystagmus before the VS test was similar in the AD group and the elderly control group. Significant abnormalities of both caloric nystagmus and VS were not present in either the elderly or the younger control groups.

CONCLUSION

AD could involve impairments in the vestibular control of balance. The VS test is useful for assessing the tendency to fall in AD. Impairment of VS in AD might arise from cerebral vestibular cortex impairment rather than comorbid peripheral vestibular disorders.

Anatomical and pharmacological relationship between acetylcholine and nitric oxide in the prepositus hypoglossi nucleus of the cat: Functional implications for eye-movement control

The prepositus hypoglossi (PH) nucleus has been proposed as a pivotal structure for horizontal eye-position generation in the oculomotor system. Recent studies have revealed that acetylcholine (ACh) in the PH nucleus could mediate the persistent activity necessary for this process, although the origin of this ACh remains unknown. It is also known that nitric oxide (NO) in the PH nucleus plays an important role in the control of velocity balance, being involved in a negative feedback control of tonic signals arriving at the PH nucleus. As it could be expected that neurons taking part in eye-position generation must control their tonic background inputs, the existence of a relationship between nitrergic and cholinergic neurons is hypothesized. In the present study we analyzed the distribution, size, and morphology of choline acetyltransferase-positive neurons, and their relationship with neuronal nitric oxide synthase in the PH nucleus of the cat. As presumed, some 96% of cholinergic neurons were also nitrergic in the PH nucleus, suggesting that NO is regulating the level of ACh released by cholinergic PH neurons. Furthermore, we studied the alterations induced by muscarinic-receptor agonists and antagonists on spontaneous and vestibularly induced eye movements in the alert cat and compared them with those induced in previous studies by modification of NO levels in the same animal preparation. The results suggest that ACh is necessary for the generation of saccadic and vestibular eye-position signals, whereas the NO is stabilizing the eye-position generator by controlling background activity reaching cholinergic neurons in the PH nucleus.

Cooperative glutamatergic and cholinergic mechanisms generate short-term modifications of synaptic effectiveness in prepositus hypoglossi neurons

To maintain horizontal eye position on a visual target after a saccade, extraocular motoneurons need a persistent (tonic) neural activity, called "eye-position signal," generated by prepositus hypoglossi (PH) neurons. We have shown previously in vitro and in vivo that this neural activity depends, among others mechanisms, on the interplay of glutamatergic transmission and cholinergic synaptically triggered depolarization. Here, we used rat sagittal brainstem slices, including PH nucleus and paramedian pontine reticular formation (PPRF). We made intracellular recordings of PH neurons and studied their synaptic activation from PPRF neurons. Train stimulation of the PPRF area evoked a cholinergic-sustained depolarization of PH neurons that outlasted the stimulus. EPSPs evoked in PH neurons by single pulses applied to the PPRF presented a short-term potentiation (STP) after train stimulation. APV (an NMDA-receptor blocker) or chelerythrine (a protein kinase-C inhibitor) had no effect on the sustained depolarization, but they did block the evoked STP, whereas pirenzepine (an M1 muscarinic antagonist) blocked both the sustained depolarization and the STP of PH neurons. Thus, electrical stimulation of the PPRF area activates both glutamatergic and cholinergic axons terminating in the PH nucleus, the latter producing a sustained depolarization probably involved in the genesis of the persistent neural activity required for eye fixation. M1-receptor activation seems to evoke a STP of PH neurons via NMDA receptors. Such STP could be needed for the stabilization of the neural network involved in the generation of position signals necessary for eye fixation after a saccade.

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

A cholinergic synaptically triggered event participates in the generation of persistent activity necessary for eye fixation

An exciting topic regarding integrative properties of the nervous system is how transient motor commands or brief sensory stimuli are able to evoke persistent neuronal changes, mainly as a sustained, tonic action potential firing. A persisting firing seems to be necessary for postural maintenance after a previous movement. We have studied in vitro and in vivo the generation of the persistent neuronal activity responsible for eye fixation after spontaneous eye movements. Rat sagittal brainstem slices were used for the intracellular recording of prepositus hypoglossi (PH) neurons and their synaptic activation from nearby paramedian pontine reticular formation (PPRF) neurons. Single electrical pulses applied to the PPRF showed a monosynaptic glutamatergic projection on PH neurons, acting on AMPA-kainate receptors. Train stimulation of the PPRF area evoked a sustained depolarization of PH neurons exceeding (by hundreds of milliseconds) stimulus duration. Both duration and amplitude of this sustained depolarization were linearly related to train frequency. The train-evoked sustained depolarization was the result of interaction between glutamatergic excitatory burst neurons and cholinergic mesopontine reticular fibers projecting onto PH neurons, because it was prevented by slice superfusion with cholinergic antagonists and mimicked by cholinergic agonists. As expected, microinjections of cholinergic antagonists in the PH nucleus of alert behaving cats evoked a gaze-holding deficit consisting of a re-centering drift of the eye after each saccade. These findings suggest that a slow, cholinergic, synaptically triggered event participates in the generation of persistent activity characteristic of PH neurons carrying eye position signals.

Cholinergic elements in the zebrafish central nervous system: Histochemical and immunohistochemical analysis

Recently, the zebrafish has been extensively used for studying the development of the central nervous system (CNS). However, the zebrafish CNS has been poorly analyzed in the adult. The cholinergic/cholinoceptive system of the zebrafish CNS was analyzed by using choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) histochemistry in the brain, retina, and spinal cord. AChE labeling was more abundant and more widely distributed than ChAT immunoreactivity. In the telencephalon, ChAT-immunoreactive (ChAT-ir) cells were absent, whereas AChE-positive neurons were observed in both the olfactory bulb and the telencephalic hemispheres. The diencephalon was the region with the lowest density of AChE-positive cells, mainly located in the pretectum, whereas ChAT-ir cells were exclusively located in the preoptic region. ChAT-ir cells were restricted to the periventricular stratum of the optic tectum, but AChE-positive neurons were observed throughout the whole extension of the lamination except in the marginal stratum. Although ChAT immunoreactivity was restricted to the rostral tegmental, oculomotor, and trochlear nuclei within the mesencephalic tegmentum, a widespread distribution of AChE reactivity was observed in this region. The isthmic region showed abundant AChE-positive and ChAT-ir cells in the isthmic, secondary gustatory and superior reticular nucleus and in the nucleus lateralis valvulae. ChAT immunoreactivity was absent in the cerebellum, although AChE staining was observed in Purkinje and granule cells. The medulla oblongata showed a widespread distribution of AChE-positive cells in all main subdivisions, including the octavolateral area, reticular formation, and motor nuclei of the cranial nerves. ChAT-ir elements in this area were restricted to the descending octaval nucleus, the octaval efferent nucleus and the motor nuclei of the cranial nerves. Additionally, spinal cord motoneurons appeared positive to both markers. Substantial differences in the ChAT and AChE distribution between zebrafish and other fish species were observed, which could be important because zebrafish is widely used as a genetic or developmental animal model.

Spontaneous activity in rat vestibular nuclei in brain slices and effects of acetylcholine agonists and antagonists

Extracellular recording was used to investigate spontaneously active neurons in all four major nuclei of the rat vestibular nuclear complex (VNC) in brainstem slices. The density of spontaneously active neurons was highest in the medial vestibular nucleus (MVN), slightly lower in the superior (SuVN) and spinal (SpVN) nuclei, and lowest in the lateral vestibular nucleus (LVN). We compared the effects of acetylcholine agonists and antagonists on spontaneously discharging neurons in MVN, SuVN, and SpVN with those in the nearby dorsal cochlear nucleus (DCN). The proportion of neurons responding to carbachol was greatest in DCN and smallest in SpVN. Unlike in DCN, some neurons in MVN, SuVN, and SpVN showed decreased firing during carbachol or muscarine. Magnitudes of responses to carbachol and muscarine were closely correlated (P<0.01). MVN neurons possessed nicotinic as well as muscarinic receptors. Activation of either type was unaffected by blocking synaptic transmission. The IC(50) values for the muscarinic subtype-preferential antagonists were compared, and tropicamide, preferential for M(4), was the most potent. Our results suggest that: (1) the relative numbers of spontaneously active neurons in rat VNC differ among nuclei; (2) acetylcholine agonists elicit changes in mean firing rates of neurons in MVN, SuVN and SpVN, but fewer neurons respond, and responses are smaller than in DCN; (3) both muscarinic and nicotinic acetylcholine receptors are present on MVN neurons, but muscarinic receptors may be more prominent.

Ultrastructural evaluation of calcitonin gene-related peptide immunoreactivity in the human cochlea and vestibular endorgans

Calcitonin gene-related peptide (CGRP) is a neuropeptide widely distributed in the peripheral and central nervous system. Demonstrated in the efferent systems of the mammalian cochlea and vestibule, immunoreactive patterns of CGRP may vary by species. There is, however, no information in the literature investigating CGRP localization in the human cochlea. In the present study, the ultrastructural localization of CGRP immunoreactivity was evaluated in the human inner ear with immunoelectron microscopy. It was found that, in human cochlea, CGRP immunoreactivity was located in unmyelinated nerve fibres of the spiral lamina, inner spiral fibres beneath inner hair cells, tunnel spiral fibres, tunnel crossing fibres and outer radial fibres. In endorgans of human vestibule, CGRP immunoreactivity was located in vesiculated nerve fibres and bouton-type nerve terminals which were seen to contact afferent nerve chalices surrounding type I sensory cells and afferent nerve fibres, or to form an en passant contact with afferent dendrites. CGRP immunoreactivity appeared to be confined to efferent systems in all cases. This study presents evidence that CGRP could serve a role in neurotransmission or neuroregulation in both cochlear and vestibular efferent systems of human.

Primary neurotransmitters and regulatory substances onto vestibular nucleus neurons

This review article focused on the primary neurotransmitters involved in transmission from the otolith to the vestibular nucleus (VN), especially in relation to the neurotransmission to the VN neurons (gravity-sensitive neurons) activated by tilt stimulation. The medial vestibular nucleus (MVN) neurons were classified in 8 types (alpha-theta) according to the patterns in response to the clockwise and counterclockwise tilt-stimulations. The tilt-induced firing was inhibited by GDEE (a non-selective glutamate receptor antagonist) and/or atropine (a muscarinic receptor antagonist). Thus, glutamate and/or acetylcholine may serve as the primary neurotransmitters. This conclusion is supported by the previous findings that glutamate exists in the vestibular nerve and is released from the nerve besides the presence of glutamate receptor subtypes in the VN. In addition, acetylcholine induced atropine-reversible firing of MVN neurons, and the enzymes involved in acetylcholine synthesis/metabolism are also found in the VN. Furthermore, serotonin was found to inhibit the MVN neuronal activities via the 5-HT1A receptors. As such, the 5-HT1A agonist, tandospirone, may be effective in preventing and/or treating motion sickness and/or space sickness.

Distribution of choline acetyltransferase immunoreactivity in the brain of an elasmobranch, the lesser spotted dogfish (Scyliorhinus canicula)

Although the distribution of cholinergic cells is remarkably similar across the vertebrate species, no data are available on more primitive species, such as cartilaginous fishes. To extend the evolutionary analysis of the cholinergic systems, we studied the distribution of cholinergic neurons in the brain and rostral spinal cord of Scyliorhinus canicula by immunocytochemistry using an antibody against the enzyme choline acetyltransferase (ChAT). Western blot analysis of brain extracts of dogfish, sturgeon, trout, and rat showed that this antibody recognized similar bands in the four species. Putative cholinergic neurons were observed in most brain regions, including the telencephalon, diencephalon, cerebellum, and brainstem. In the retrobulbar region and superficial dorsal pallium of the telencephalon, numerous small pallial cells were ChAT-like immunoreactive. In addition, tufted cells of the olfactory bulb and some cells in the lateral pallium showed faint immunoreactivity. In the preoptic-hypothalamic region, ChAT-immunoreactive (ChAT-ir) cells were found in the preoptic nucleus, the vascular organ of the terminal lamina, and a small population in the caudal tuber. In the epithalamus, the pineal photoreceptors were intensely positive. Many cells of the habenula were faintly ChAT-ir, but the neuropil of the interpeduncular nucleus showed intense ChAT immunoreactivity. In the pretectal region, ChAT-ir cells were observed only in the superficial pretectal nucleus. In the brainstem, the somatomotor and branchiomotor nuclei, the octavolateral efferent nucleus, and a cell group just rostral to the Edinger-Westphal (EW) nucleus contained ChAT-ir neurons. In addition, the trigeminal mesencephalic nucleus, the nucleus G of the isthmus, some locus coeruleus cells, and some cell populations of the vestibular nuclei and of the electroreceptive nucleus of the octavolateral region exhibited ChAT immunoreactivity. In the reticular areas of the brainstem, the nucleus of the medial longitudinal fascicle, many reticular neurons of the rhombencephalon, and cells of the nucleus of the lateral funiculus were immunoreactive to this antibody. In the cerebellum, Golgi cells of the granule cell layer and some cells of the cerebellar nucleus were also ChAT-ir. In the rostral spinal cord, ChAT immunoreactivity was observed in cells of the motor column, the dorsal horn, the marginal nucleus (a putative stretch-receptor organ), and in interstitial cells of the ventral funiculus. These results demonstrate for the first time that cholinergic neurons are distributed widely in the central nervous system of elasmobranchs and that their cholinergic systems have evolved several characteristics that are unique to this group.

Cholinergic and glutamatergic transmission in medial vestibular nucleus neurons responding to lateral roll tilt in rats

The responses of the medial vestibular nucleus (MVN) neurons to lateral tilt and the neurotransmitters mediating otolith information to MVN neurons were investigated using rats. A computer-operated goniometer was tilted 20 degrees clockwise and counterclockwise at an angular speed of 5 degrees /s and paused in the inclined positions for 10 s to record neuronal responses in the static phase. The 185 MVN neurons recorded were classified into eight types according to their responses to tilt (alpha, beta, gamma, delta, epsilon, zeta, eta and theta). A majority showed increased firing in response to ipsilateral tilting and decreased firing in response to contralateral tilting (alpha type: 31.4%) or exhibited the reverse pattern (beta type: 36.8%). Further, other groups of neurons increased (gamma type) or decreased (delta type) firing rates to either side tilting and increased (epsilon and zeta type) or decreased (eta and theta type) firing only on one side. Atropine or L-glutamic acid diethyl ester hydrochloride (GDEE) applied microiontophoretically antagonized tilt-induced firing of alpha type neurons in 58.8% or 60.0%, respectively, and of beta type neurons in 66.7% or 58.3%, respectively. When the effects of atropine and GDEE were examined in the same neurons, antagonizing effects of both drugs on tilt-induced firing were obtained in 28.6% and 40.0% of alpha and beta type neurons, respectively. These results suggest that both acetylcholine and glutamate act as neurotransmitters in the transmission of otolith information to most MVN neurons.

Organization of the coeruleo-vestibular pathway in rats, rabbits, and monkeys

Inputs from locus coeruleus (LC) appear to be important for altering sensorimotor responses in situations requiring increase vigilance or alertness. This study documents the organization of coeruleo-vestibular pathways in rats, rabbits and monkeys. A lateral descending noradrenergic bundle (LDB) projects from LC to the superior vestibular nucleus (SVN) and rostral lateral vestibular nucleus (LVN). A medial descending noradrenergic bundle (MDB) projects from LC to LVN, the medial vestibular nucleus (MVN), group y and rostral nucleus prepositus hypoglossi (rNPH). There is a characteristic, specific pattern of innervation of vestibular nuclear regions across the three species. A quantitative analysis revealed four distinct innervation density levels (minimal, low, intermediate and high) across the vestibular nuclei. The densest plexuses of noradrenergic fibers were observed in the SVN and LVN. Less dense innervation was observed in the MVN, and minimal innervation was observed in the inferior vestibular nucleus (IVN). In monkeys and rabbits, rostral MVN contained a higher innervation density than the rat MVN. In monkeys, the rNPH also contained a dense plexus of fibers. Selective destruction of terminal LC projections (distal axons and terminals) by the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) resulted in a dramatic reduction of immunoreactive fibers within the vestibular nuclear complex of rats, suggesting that the source of these immunoreactive fibers is LC. Retrograde tracer injections into the vestibular nuclei resulted in labeled cells in the ipsilateral, caudal LC and adjacent nucleus subcoeruleus. It is hypothesized that the regional differences in noradrenergic innervation are a substrate for differentially altering vestibulo-ocular and vestibulo-spinal responses during changes in alertness or vigilance.

Distribution and cerebellar projections of cholinergic and corticotropin-releasing factor-containing neurons in the caudal vestibular nuclear complex and adjacent brainstem structures

By using immunohistochemistry combined with lesioning and retrograde neuronal labeling techniques, cholinergic neurons and corticotropin-releasing factor-immunoreactive neurons were examined for their distribution, coincidence and cerebellar projections in feline vestibular nuclear complex and adjacent brainstem structures. Cholinergic neurons as revealed here with choline acetyltransferase immunoreactivity were found massively in the abducens and hypoglossal nuclei, dorsal motor nucleus of the vagus nerve and nucleus of Roller; less numerously in the medial vestibular, prepositus hypoglossi and solitary nuclei and the caudal two-thirds of descending vestibular nucleus; and only occasionally in the intercalated and supravestibular nuclei and cell groups f, x and z. Corticotropin-releasing factor-immunoreactive neurons were found clustered in the prepositus hypoglossi nucleus and also in cell groups f and x and the rostral two-thirds of descending vestibular nucleus, less numerously in the medial vestibular, intercalated and solitary nuclei and nucleus of Roller, and only occasionally in the caudal one-third of descending vestibular nucleus, the dorsal motor nucleus of the vagus nerve, supravestibular nucleus and cell group z. The lateral and superior vestibular nuclei did not contain either type of neuron. The two types of immunopositive neurons observed in most of the brainstem nuclei differed in cell size, distribution-pattern and rostrocaudal level of occurrence. While there were many regions which exhibited both types of immunopositive neurons, perikarya colocalizing the cholinergic and peptide markers were not detected in the brainstem. Following unilateral, partial lesioning of the vestibular nuclear complex, corticotropin-releasing factor-immunoreactive mossy fiber terminals (rosettes) disappeared from the ipsilateral flocculus. However, such lesions did not produce clear-cut changes of cholinergic terminals in the vermis. Following retrograde neuronal labeling combined with immunohistochemistry, the two types of immunopositive neurons observed in most of the brainstem sites were found to project to the vermal lobules I-III, IX and X. On comparison of these immunopositive projection neurons with non-immunoreactive, retrogradely labeled neurons, the cholinergic neurons and the peptide-immunoreactive neurons were found to constitute a major part of the total vestibulocerebellar neuronal population. The results indicate chemical heterogeneity in vestibular nuclear complex and cerebellar afferents.

Cholinergic innervation of the cerebellum of the rat by secondary vestibular afferents

The cholinergic innervation of the cerebellar cortex of the rat was studied by immunohistochemical localization of choline acetyltransferase, radiochemical measurement of ChAT activity, and double labeling of ChAT-positive neurons with HRP injected into the cerebellum. ChAT immunohistochemistry revealed large mossy fiber rosettes as well as finely beaded terminals with different morphological characterization, laminar distribution within the cerebellar cortex, and regional differences within the cerebellum. Large "grapelike" ChAT-positive mossy fiber rosettes that were distributed primarily in the granule cell layer were concentrated, but not exclusively located, in three separate regions of the cerebellum: (1) the uvula-nodulus (lobules 9 and 10); (2) the flocculus, and (3) the anterior lobe vermis (lobules 1 and 2). Regional differences in ChAT-positive afferent terminations in the cerebellar cortex demonstrated by immunohistochemistry were confirmed by regional biochemical measurements of ChAT activity. Using ChAT immunohistochemistry in combination with HRP injections into the uvula-nodulus, we have studied the origin of the cholinergic projection. The caudal medial vestibular nucleus and to a lesser extent the nucleus prepositus hypglossus contain ChAT-positive neurons that were double labeled following HRP injections into the uvula-nodulus. We conclude that (1) there is a prominent cholinergic mossy fiber pathway to the vestibulocerebellum, (2) this pathway originates primarily in the caudal third of the medial vestibular nucleus, and (3) this cholinergic pathway likely mediates secondary vestibular information related to postural adjustment.

Direct muscarinic and nicotinic receptor-mediated excitation of rat medial vestibular nucleus neurons in vitro

We have utilized intracellular recording techniques to investigate the cholinoceptivity of rat medial vestibular nucleus (MVN) neurons in a submerged brain slice preparation. Exogenous application of the mixed cholinergic agonists, acetylcholine (ACh) or carbachol (CCh), produced predominantly membrane depolarization, induction of action potential firing, and decreased input resistance. Application of the selective muscarinic receptor agonist muscarine (MUSC), or the selective nicotinic receptor agonists nicotine (NIC) or 1,1-dimethyl-4-phenylpiperazinium (DMPP) also produced membrane depolarizations. The MUSC-induced depolarization was accompanied by decreased conductance, while an increase in conductance appeared to underlie the NIC- and DMPP-induced depolarizations. The muscarinic and nicotinic receptor mediated depolarizations persisted in tetrodotoxin and/or low Ca2+/high Mg2+ containing media, suggesting direct postsynaptic receptor activation. The MUSC-induced depolarization could be reversibly blocked by the selective muscarinic-receptor antagonist, atropine, while the DMPP-induced depolarization could be reversibly suppressed by the selective ganglionic nicotinic-receptor antagonist, mecamylamine. Some neurons exhibited a transient membrane hyperpolarization during the depolarizing response to CCh or MUSC application. This transient inhibition could be reversibly blocked by the gamma-aminobutyric acid (GABA) antagonist, bicuculline, suggesting that the underlying hyperpolarization results indirectly from the endogenous release of GABA acting at GABA receptors. This study confirms the cholinoceptivity of MVN neurons and establishes that individual MVN cells possess muscarinic as well as nicotinic receptors. The data provide support for a prominent role of cholinergic mechanisms in the direct and indirect regulation of the excitability of MVN neurons.

Secondary vestibular cholinergic projection to the cerebellum of rabbit and rat as revealed by choline acetyltransferase immunohistochemistry, retrograde and orthograde tracers

Previously we have shown that four regions of the cerebellum, the uvula-nodulus, flocculus, ventral paraflocculus, and anterior lobe 1, receive extensive, but not exclusive, cholinergic mossy fiber projections. In the present experiment we have studied the origin of three of these projections in the rat and rabbit (uvula-nodulus, flocculus, ventral paraflocculus), using choline acetyltransferase (ChAT) immunohistochemistry in combination with a double label, retrogradely transported horseradish peroxidase (HRP). We have demonstrated that in both the rat and rabbit the caudal medial vestibular nucleus (MVN) and to a lesser extent the nucleus prepositus hypoglossus (NPH) contain ChAT-positive neurons. Neurons of the caudal MVN are double-labeled following HRP injections into the uvula-nodulus. HRP injections into the uvula-nodulus also labeled less than 5% of the neurons in the cholinergic vestibular efferent complex. Fewer ChAT-positive neurons in the MVN and some ChAT-positive neurons in the NPH are double-labeled following HRP injections into the flocculus. Almost no ChAT-positive neurons in the MVN and some ChAT-positive neurons in the NPH are double-labeled following HRP injections into the ventral paraflocculus. Injections of Phaseolus leucoagglutinin (PHA-L) into the caudal MVN of both the rat and rabbit demonstrated projection patterns to the uvula-nodulus and flocculus that were qualitatively similar to those observed using ChAT immunohistochemistry. We conclude that the cholinergic mossy fiber pathway to the cerebellum in general and the uvula-nodulus in particular is likely to mediate secondary vestibular information related to postural adjustments.

Distribution of choline acetyltransferase immunoreactive axons and terminals in the rat and ferret brainstem

A survey was made of the density of the cholinergic innervation of different parts of the brainstem of the rat and ferret. Sections of rat and ferret brainstems were stained for choline acetyltransferase (ChAT) immunoreactivity by using a sensitive immunocytochemical method. Adjacent sections were stained for acetylcholinesterase activity or Nissl substance. The density of the distribution of fine calibre, varicose ChAT-positive axons, assumed to represent cholinergic terminals, was categorised arbitrarily into high, medium, or low. A high density of ChAT-positive terminals was found in all or parts of these structures: interpeduncular nucleus, superficial grey layer of the superior colliculus (ferret), intermediate layers of the superior colliculus, lateral part of the central grey (rat), an area medial to the parabigeminal nucleus (rat), pontine nuclei, ventral tegmental nucleus (rat), midline pontine reticular formation, and an area ventral to the exit point of the 5th nerve (ferret). A medium density of ChAT-positive terminals was observed in all or parts of: the substantia nigra zona compacta (ferret), ventral tegmental area (ferret), superficial grey layer of the superior colliculus, intermediate and deep layers of the superior colliculus, lateral central grey, area medial to the parabigeminal nucleus, inferior colliculus, dorsal tegmental nucleus, ventral tegmental nucleus (ferret), pontine nuclei, ventral nucleus of the lateral lemniscus (ferret), midline pontine reticular formation, ventral cochlear nucleus, dorsal cochlear nucleus, lateral superior olive, spinal trigeminal nuclei, prepositus hypoglossal nucleus, lateral reticular nucleus, paragigantocellular nucleus, and the dorsal column nuclei including the cuneate, external cuneate, and gracile nuclei. A low density of ChAT-positive terminals was seen throughout the remainder of the brainstem of the rat and ferret, but these terminals were absent from the medial superior olive, substantia nigra zona reticulata (rat), and the central part of the ferret lateral superior olive. A pericellular-like distribution of ChAT-positive terminals was observed in the ventral cochlear nucleus and in association with some of the cells of the nucleus of the mesencephalic tract of the trigeminal nerve. A climbing fibre type arrangement of ChAT-positive terminals was found in the substantia nigra zona compacta (ferret) and medial reticular formation. In general, the distribution of staining for AChE activity reflected that of the distribution of ChAT immunoreactivity in the brainstem, except in a few regions where there were also species differences in the distribution of ChAT-positive terminals, e.g., in the superficial grey layer of the superior colliculus and in the substantia nigra.(ABSTRACT TRUNCATED AT 400 WORDS)

Further evidence for the absence of a descending cholinergic projection from the brainstem to the spinal cord in the rat

Serotonergic and catecholaminergic neurons are known to project from the brainstem to the spinal cord. However, evidence for a bulbo-spinal projection that is cholinergic is sparse despite immunocytochemical and physiological evidence for a cholinergic influence on the cord. In this study we examined the possibility of a direct cholinergic bulbo-spinal projection in the rat using a combination of retrograde axonal tracing techniques and choline acetyltransferase immunocytochemistry. Although many cells were found to project to the cord from the brainstem, none were identified as being cholinergic, confirming previous evidence that the cholinergic innervation of the cord is intrinsic.

Aspartate-like immunoreactivity in vestibulospinal neurons in gerbils

In an effort to further characterize vestibulospinal pathways in the gerbil, immunocytochemistry was combined with retrograde identification of neurons. Vestibulospinal neurons were retrogradely labeled following injections of horseradish peroxidase into the cervical cord of anesthetized gerbils. Sections were reacted with nickel acetate-diaminobenzidine for horseradish peroxidase, giving a black reaction product. Sections were incubated in polyclonal antisera to aspartate, incubated in an avidin-biotin-peroxidase procedure, and reacted to give a brown reaction product. Alternatively, fluoro-gold was used as a retrograde tracer and aspartate-like immunoreactivity was demonstrated with avidin conjugated to Texas red. Cells stained with aspartate-like immunoreactivity, were located in all vestibular nuclei. Double-labeled cells were located in the medial nucleus and in the lateral vestibular nucleus where many of the large cells were double labeled.

Choline acetyltransferase immunoreactivity in the cat cerebellum

Choline acetyltransferase immunoreactivity was demonstrated in particular projection systems in cat cerebellum by combining immunohistochemistry, retrograde tracing and lesioning paradigms. The monoclonal antibody used in this study recognized a 68,000 mol. wt protein on immunoblots of cat cerebellum and striatum. Choline acetyltransferase immunoreactivity was localized to some neurons and varicose fibers in the cerebellar nuclei, and also to some mossy fibers and endings (rosettes), fiber plexuses around Purkinje cells, granule cells and parallel fibers in the cerebellar cortex. In addition, the presence of choline acetyltransferase-immunoreactive large cells, presumptive Golgi cells, in the granular layer was confirmed. In each cerebellar nucleus, choline acetyltransferase-immunoreactive neurons contained either large, medium-sized or small cell bodies and were distributed evenly in the entire nuclear domain. Large and medium-sized ones were frequently encountered. Choline acetyltransferase-immunoreactive mossy fibers and rosettes were most abundant in the vermal lobules I-III, VIII, IX and the simple lobule, moderately accumulated in the vermal lobules IV-VII, X, crus I and crus II, and less abundant in the paramedian lobule, paraflocculus and flocculus. Some granule cells with prominent dendritic claws and bifurcating parallel axons were immunolabeled in the entire vermis with infrequent occurrence in the remaining cortices. Following unilateral lesioning of the cerebellar nuclei with electrocoagulation or kainate injections, a reduction in number of choline acetyltransferase-immunoreactive fibers occurred ipsilaterally in the cerebellar cortex and contralaterally in the red nucleus, ventrolateral thalamic nucleus and ventroanterior thalamic nucleus. In addition, perikarya of some cerebellothalamic neurons were shown to contain choline acetyltransferase immunoreactivity. The results indicate that some nucleocortical, cerebellorubral and cerebellothalamic projections are cholinergic and that a subpopulation of cholinergic granule cell-parallel fibers exists.

Cholinergic innervation of the rat cerebellum: qualitative and quantitative analyses of elements immunoreactive to a monoclonal antibody against choline acetyltransferase

Cholinergic innervation of the rat cerebellum was investigated immunohistochemically by using a monoclonal antibody against choline acetyltransferase. Immunoreactive structures included: 1) a subpopulation of mossy fibers and glomerular rosettes; 2) thin varicose fibers, which were closely associated with the Purkinje cell layer and also found in the molecular layer; and 3) relatively dense networks of varicose fibers distributing in the cerebellar nuclei. Quantitative analysis indicated that a great many immunoreactive rosettes were localized in lobules IXc and X, although their density in lobule X was approximately four times that in the lobule IXc. A considerable number of immunoreactive structures were also present in all other lobules. In the hemispheres they were confined to a zone immediately beneath the Purkinje cell layer, whereas in the vermis they were scattered throughout the granular layer. Most of the immunoreactive fibers found in the molecular layer coursed toward the pial surface and were distributed within the inner half of the molecular layer. In the cerebellar nuclei, portions of the medial nucleus and magnocellular portion of the lateral nucleus had moderately dense networks of immunoreactive fibers, whereas loose networks of fibers were observed in the posterior interposed nucleus. Other parts of the cerebellar nuclei contained a smaller number of varicose fibers.

Muscarinic regulation of spontaneously active medial vestibular neurons in vitro

We examined the effects of cholinergic agonists and antagonists on spontaneously occurring action potentials extracellularly recorded from medial vestibular nucleus (MVN) neurons in rat brainstem slice preparation to elucidate the cholinergic mechanism involved in excitation. Addition of carbachol (10(-6)-10(-5) M) and muscarine (10(-6)-10(-5) M) into the bath dose-dependently increased the spontaneous firing rate, while nicotine (10(-5)-10(-4) M) had no effects. Acetylcholine (10(-6)-10(-5) M) in the presence of physostigmine (10(-7) M) also increased the firing rate in a dose-dependent manner. Conversely, atropine (10(-8)-3 x 10(-7) M) slightly decreased the firing and dose-dependently inhibited the carbachol-induced increase in the firing rate. These results suggest that the firing rate of spontaneously active MVN neurons are regulated by acetylcholine via muscarinic receptors.

Distribution of choline acetyltransferase immunopositive structures in the rat brainstem

The distribution of neurons, fibers and terminal fields in rat brainstem displaying positive immunoreactivity to a polyclonal antiserum to human placental choline acetyltransferase (ChAT) is described. The antiserum was used at the high dilution of 1:10,000 and was coupled with a sensitive detection system using the nickel ammonium sulfate intensification method. In addition to previously described ChAT immunopositive groups of large cells in the cranial motor nuclei, and the parabrachial and reticular complexes, many small or medium size, weakly immunopositive neurons were identified. Some of these appeared in structures in the region of the fourth ventricle, including the area postrema. Others were in structures associated with the superior olivary complex, including the lateral superior olive, and the medioventral, lateroventral and superior periolivary nuclei. Scattered, weakly positive cells were seen in numerous other structures, including the ventral tegmental area of Tsai, central gray, superior colliculus, spinal nucleus of nerve 5, dorsal cochlear nucleus and non-motor regions of the spinal cord. The prominent ascending fiber tract of the laterodorsal tegmental pathway was traceable from the parabrachial area to the subgeniculate region of the thalamus. Prominent terminal fields were seen in a number of brainstem structures, including the superior colliculus, pontine nuclei, anterior pretectal nucleus, interpeduncular nucleus and spinal nucleus of nerve 5. The association of small ChAT positive cells and terminal fields with many sensory structures suggests a significant cholinergic participation in the physiology of sensory function.

Electrophysiological evidence for cholinoceptive neurons in the medial vestibular nucleus: studies on rat brain stem in vitro

Electrophysiological studies were performed to determine whether or not cholinoceptive neurons are present in the rat medial vestibular nucleus (MVN) using brainstem slice preparations. Fifty-three MVN neurons, whose activities were extracellularly recorded, fired spikes spontaneously and regularly with an interspike interval of 180 +/- 27 ms (mean +/- S.E.M.) and a coefficient of variation of 0.11 +/- 0.02. Intracellularly recorded neurons also exhibited similar spontaneous and regular generation of action potentials. Carbachol dose-dependently increased the spontaneous firing, although the firing rate was decreased in a few neurons. The addition of atropine reduced the firing rate, and dose-dependently attenuated the carbachol-induced excitation of the neurons. In a low Ca2+ and high Mg2+ medium, carbachol also increased the firing rate. These results indicate that the MVN contains neurons with spontaneous and regular firing, and that the excitability of these neurons is regulated by a cholinergic muscarinic mechanism.