Ketamine as a pharmacological model for tongue dyskinesia.

The purpose of this article is to document that ketamine hydrochloride, administered at an anesthetic dosage of about 100 mg/kg, produces tongue contractile activity in the rat. The methods for monitoring and quantitating ketamine-induced tongue contractions (KITCs) are described. We also found that neuroleptic agents consistently and readily abolish KITCs. On the basis of these observations and other pharmacological properties of ketamine, we propose that KITCs may be a useful model for studying neuroleptic-induced oral dyskinesia, e.g., tardive dyskinesia. Additional findings in support of this model are presented.

Topographically organized projections from the nucleus subceruleus to the hypoglossal nucleus in the rat: a light and electron microscopic study with complementary axonal transport techniques.

Projections from the nucleus subceruleus (nSC) to the hypoglossal nucleus (XII) were investigated with complementary retrograde and anterograde axonal transport techniques at the light and electron microscopic level in the rat. Injections of WGA-HRP into XII resulted in labeling of neurons in and around the nSC. Labeled nSC neurons were few in number (less than 4 per 40-60 microns sections) and variable in size and shape. Most labeled nSC neurons were medium-sized (mean = 16.89 microns), fusiform, triangular, or oval, with 3-4 dendrites typically oriented dorsomedially and ventrolaterally. These neurons were found throughout the rostrocaudal extent of the nSC but were most numerous medial, dorsomedial, and ventromedial to the motor trigeminal nucleus. Others were observed rostral to the motor trigeminal nucleus and ventral to the parabrachial nuclear complex. Confirmation of retrograde results was obtained following injections of tritiated amino acids or WGA-HRP into the nSC. This resulted in labeling throughout the rostrocaudal extent of XII mainly ipsilaterally. Labeled fibers descended the brainstem in the dorsolateral and, to a lesser extent, in the ventromedial component of Probst's tract. Fibers entered XII mainly rostrally along the lateral border of the nucleus. All regions of XII were recipients of nSC afferents, but the caudoventromedial quadrant contained the greatest density of terminal labeling. Electron microscopic evaluation confirmed that nSC afferents synapsed on motoneurons in XII. Axon terminals containing WGA-HRP reaction product were found contacting dendrites and somata, but primarily the former (81.3% versus 10.6%). Axodendritic terminals synapsed mainly on medium-to-small sized dendrites (less than 3 microns in diameter). The majority of labeled axodendritic terminals (90.1%) contained small, round, and clear synaptic vesicles (S-type: 20-50 nm) and were associated with an asymmetric (60.6%), symmetric (11.4%), or no (18%) postsynaptic specialization. By contrast, most axosomatic terminals contained flattened vesicles (F-type) and formed a symmetric or no postsynaptic specialization (75%). Large dense core vesicles (55-90 nm) were observed within a small proportion of all labeled axon terminals (1.3%). The results from this study demonstrate that the nSC projects to XII, preferentially targets a specific subgrouping of protrusor motoneurons, and synapses on both somata and dendrites, although mainly on the latter. The implications of these data are discussed relative to tongue control.

Subcompartmental organization of the ventral (protrusor) compartment in the hypoglossal nucleus of the rat.

The extent and myotopic organization of the ventral (protrusor) compartment of the hypoglossal nucleus (nXII) in the rat is controversial. Of particular concern is the location of motoneurons that innervate the intrinsic (verticalis, transversus) as compared to extrinsic (genioglossus) tongue protrusor muscles. These issues were investigated with retrograde transport, lesion/degeneration/immunocytochemical, and classic Golgi staining techniques. Results from these experiments demonstrate the following: (1) the ventral compartment extends the entire rostrocaudal length of nXII and is organized into three longitudinally oriented subcompartments, one medial and one lateral within the boundaries of nXII, and one outside the confines of nXII, defined as the lateral accessory subcompartment; 2) the medial and lateral subcompartments contain motoneurons that innervate the intrinsic (verticalis, transversus) and extrinsic (genioglossus) tongue protrusor muscles, respectively, while the lateral accessory subcompartment innervates the geniohyoid muscle; (3) ventral subcompartments are unequal in size and vary along the rostrocaudal dimension of nXII. The medial subcompartment is largest caudally and smallest rostrally, while the converse is true for the lateral subcompartment. By contrast, the lateral accessory subcompartment is present only along the caudal one-half of nXII; (4) medial and lateral subcompartments are further organized into smaller subgroups. Medial and centromedial subgroups are discernible within the medial subcompartment, lateral and centrolateral subgroups within the lateral subcompartment. Both medial and lateral subgroups extend throughout the rostrocaudal length of nXII, whereas the centromedial and centrolateral subgroups are present only along the middle two-thirds of nXII where they form a central motoneuron band; (5) there is an inverse myotopic organization within the medial and lateral subcompartments such that proximal and distal portions of intrinsic and extrinsic protrusor muscles receive innervation from rostral and caudal motoneurons, respectively; and (6) there is a correlation between motoneuron morphology (size, shape and dendritic field domains), subcompartment localization, and myotopic specificity. Motoneurons in the medial subcompartment are small (mean = 23.08 microns), round to globular, with dendrites oriented medially, dorsomedially, dorsolaterally, and caudally, whereas lateral subcompartment motoneurons are large (mean = 29.49 microns), round to triangular, with dendrites directed mainly mediolaterally and dorsally. These data are relevant to understanding the functional organization of nXII and the motor control of the tongue. Results are further discussed relative to the convergence of multifunctional afferent systems in the ventromedial subcompartment of nXII.

Effects of cholinergic and anticholinergic drugs on ketamine-induced linguopharyngeal motor activity.

Benztropine mesylate (Cogentin) and physostigmine salicylate (Antilirium), were tested for changes in tongue protrusions, retrusions, and swallowing acts in rats anesthetized with a 100 mg/kg IM injection of ketamine hydrochloride. These ketamine-induced linguopharyngeal events were monitored by means of a force displacement transducer fed onto a polygraph. Benztropine (0.05-1 mg/kg) caused mild to moderate reductions in the rate of these events for a short period of time, up to about 30 min. With physostigmine (5-25 micrograms/kg), linguopharyngeal activity was markedly increased, up to 50-fold by the highest dose within 5 min and returned almost to the baseline within 60 min. With lower doses, more moderate responses were obtained. If methscopolamine (1.4, 3, 6 mg/kg IM) preceded physostigmine, the physostigmine enhancement was preserved.

The enkephalinergic innervation of the genioglossus musculature in the rat: implications for the respiratory control of the tongue.

This study sought to determine if the enkephalinergic (ENK) innervation of the hypoglossal nucleus (nXII) in the rat was organized differentially for the control of the genioglossus musculature whose activity is essential in maintaining the patency of the upper airway. Immunocytochemical results revealed that the genioglossus motoneuron pool, comprising the ventrolateral subcompartment of the nXII, was consistently and heavily labeled throughout its rostrocaudal dimension. Labeling was characterized by dense focal clustering throughout the neuropil, and by the appearance of numerous perisomatic-like profiles. Similarly, the ventromedial subcompartment mainly rostrally, and the dorsal compartment caudally, whose motoneurons control the caudal intrinsic protrusor and rostral retrusor muscles, respectively, were also consistently labeled. While these results demonstrate that the genioglossus musculature is targeted by ENK inputs, they also suggest that other selected musculature of the tongue is controlled by ENK. It is argued that the innervation pattern identified in the present study is consistent with a functional role for ENK in the respiratory control of the tongue.

Effects of clozapine on ketamine-induced linguopharyngeal events in rats.

We tested the effects of clozapine (0.02-20 mg/kg i.p.) on ketamine-induced linguopharyngeal events in rats anesthetized with i.m. injections of ketamine hydrochloride (100 mg/kg) and mounted on a stereotaxic with the tip of the tongue tied to a force displacement transducer monitoring tongue protrusions, retrusions and swallows. Reduction began at the 0.04 mg/kg dose. At 4.8 mg/kg there was total suppression of events. At 20 mg/kg, suppression lasted for 1 h. Notably clozapine doses causing total suppression of events in our model were much lower than those usually reported to alter dopamine turnover.

Pre- and postnatal development of the catecholamine innervation of the hypoglossal nucleus in the rat: an immunocytochemical study.

The pre- and postnatal development of the catecholamine (CA) innervation to the hypoglossal nucleus (nXII) in the rat was investigated immunocytochemically with antisera to tyrosine hydroxylase (TH). Immunoreactive profiles positive for TH were first identified in nXII on gestational day (GD) 16. By GD 18, the adult-like distribution pattern was evident, characterized by the preferential targeting of the ventromedial region of nXII, but this pattern was not consistently found in all fetuses until GD 19. From GD 19 to postnatal day (PD) 180, the overall density of TH immunoreactivity, particularly in the ventromedial region, increased with further growth and maturation of nXII. These results establish the early prenatal CA innervation of nXII and support the hypothesis that CA are important in regulating motor tongue behavior in the newborn. Moreover, because the ventral compartment of nXII contains motoneurons that innervate protrusor muscles of the tongue, and tongue protrusor mechanisms play an essential role in suckling, deglutition, and respiratory (maintaining a patent upper airway) behaviors, it is further proposed that the CA innervation of nXII is critical to the survival of the newborn.

Thalamic connectivity of rat somatic motor cortex.

The purpose of this study was to investigate certain organizational features regarding the connectional relationships between the somatic motor cortex (SMI) and dorsal thalamus in the rat. This was accomplished by applying microiontophoretically horseradish peroxidase and tritiated amino acids into low threshold stimulation sites within hindlimb, forelimb and face representations of the SMI as defined by intracortical microstimulation. Injections into the SMI produced labeling in distinct sets of specific and non-specific thalamic nuclei. The former included the ventrolateral (VL), ventromedial (VM), posteromedial (Pom) and posterior (Po) nuclei, and the latter included the centrolateral (CL), paracentral (PC), centromedial (CeM) and parafascicular (PF) nuclei. The densest labeling, both retrograde and anterograde, was found in the VL, and to a slightly lesser extent, in the Pom nuclei. Labeling of specific nuclei was more extensive than in non-specific groups. Thalamic-SMI projections were reciprocal in nature with the exception of the reticular thalamic nucleus (R). The ventrobasal (VB) nucleus was labeled only after injections into the SMI representations of the hindlimb and forelimb. A topographic organization between the SMI and dorsal thalamic nuclei was indicated by the varied position of label following injections into different cortical representations. Injections into the hindlimb representation resulted in labeling in the rostrodorsolateral VL, ventrolateral VM, rostrodorsomedial Pom, dorsal Po, rostrodorsolateral R, dorsolateral PF, CL and PC. In contrast, injections into the face representation resulted in labeling in the caudoventromedial VL, dorsolateral VM, caudoventrolateral Pom, ventral Po, caudoventromedial R, ventrolateral PF, the PC, CeM and ventral CL. The position of thalamic labeling following injections into the forelimb representation was intermediate between that for hindlimb and face representations. In coronal section, distinct patterns of labeling were identified in all specific thalamic nuclei following injections into each topographic region of the SMI. In the VL, labeling appeared as an obliquely-oriented, longitudinal strip or band (more C-shaped following injections of the hindlimb representation); in the VM as a horizontal strip; in the Pom as an irregular V-shaped band; and in the Po as a circular-to-ovoid cluster. No distinct patterns of labeling were discerned in non-specific nuclei. Labeling in the anterior grouping of non-specific nuclei overlapped to a large extent, but this was not the case in either the specific thalamic nuclei or the PF nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin-containing axon terminals in the hypoglossal nucleus of the rat. An immuno-electronmicroscopic study.

The morphology and distribution of serotonin-containing axon terminals in the rat hypoglossal nucleus (XII) was investigated immunocytochemically at the electron microscopic level. Serotonin-positive profiles were found throughout all regions of XII and included unmyelinated axons, varicosities and axon terminals. Most labeled profiles (68.1%) were nonsynaptic unmyelinated axons and varicosities, while synaptic profiles, ending on dendrites and somata, were seen less frequently (28.7%). The majority of labeled axon terminals (76.9%) ended on small-to-medium-sized dendrites. Most axodendritic terminals contained small, round agranular vesicles (20-55 microns), several large (60-100 microns) dense core vesicles, and were associated with a pronounced asymmetric postsynaptic specialization. By contrast, labeled axosomatic terminals were seen less often than those ending on dendrites (23.0%). Axosomatic terminals typically contained small, round, agranular and large dense core vesicles and were associated with a symmetric or no postsynaptic specialization. These results provide the structural substrates for elucidating the functional role of serotonin in tongue control.

Sources of noradrenergic afferents to the hypoglossal nucleus in the rat.

The sources of noradrenergic (NA) innervation to the hypoglossal nucleus (nXII) in the rat were investigated with double-labeling histochemical/immunocytochemical and lesion/degeneration techniques. Following injection of wheat germ-agglutinin conjugated to horseradish peroxidase into nXII, brain stem sections were reacted with tetramethylbenzidine, stabilized, and incubated in antiserum to tyrosine hydroxylase (TH). Double-labeled neurons were observed in three pontine sites bilaterally, although mainly ipsilaterally, that included the nucleus subceruleus (nSC; 68.75%) and the A7 (21.09%) and A5 (10.15%) cell groups. Confirmation of the above results and identification of the course taken by descending NA-nXII projections was accomplished by lesioning the rostral pons, the nSC, or the medullary catecholamine bundle (MB), the suspected route by which NA afferents reach nXII. Quantitative estimates of the reduction of TH immunoreactivity on the lesioned compared to nonlesioned side of nXII were made densitometrically. In each case, TH immunostaining was significantly decreased (75%) in the ipsilateral caudoventromedial district of nXII, the predominant target area of NA input. The results from this study establish that multiple NA sources in the pons project to nXII in the rat, the majority of NA-nXII afferents are derived from the nSC, and descending NA-nXII projections course in the MB. These data are discussed relative to tongue control.

Interconnections between substantia nigra reticulata and medullary reticular formation.

Injections of wheat germ agglutinin-HRP into the medullary reticular formation (MRf) or the substantia nigra reticulata (SNr) revealed the presence of reciprocating fiber connections between the two areas. Large injections in the MRf demonstrated the existence of labeled neurons in the lateral portions of the SNr. Isolated injections into the parvocellular nuclei of the MRf resulted in the presence of terminal fields in the SNr particularly its lateral portions. Injections in the SNr resulted in the presence of labeled cells in the parvocellular nuclei. The significance of these findings is discussed in terms of oro-facial dyskinesias.

Catecholamine innervation of the rat hypoglossal nucleus.

The catecholamine innervation of the hypoglossal nucleus (XII) was investigated immunocytochemically by comparing the distribution patterns of tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT) in the rat. Numerous TH- and DBH-positive profiles were found throughout XII, while only occasional PNMT immunoreactivity was observed. Significantly, the distribution patterns of TH and DBH immunoreactivity were coextensive with the most intense staining found ventromedially along the caudal half of XII. We conclude, therefore, that the catecholamine innervation of XII is largely noradrenergic, and that motoneurons innervating the genioglossi muscles, the principal protrusors of the tongue, are the primary targets of this input.

An electrographic characterization of ketamine-induced linguopharyngeal motor activity.

Ketamine-induced buccolinguopharyngeal motor activity was studied in rats visually and by means of force displacement transduction of tongue retrusions, electromyogram (EMG) of motor units of tongue muscles, and pressure transduction of swallowing acts. Each animal was anesthetized with an intramuscular injection of ketamine hydrochloride (100 mg/kg body weight). Through a tracheotomy the airway was intubated and the animal was mounted on a stereotaxic frame in a supine position for monitoring of the above parameters. Four varieties of events were demonstrated: (a) swallowing acts followed by tongue retrusion, (b) tongue retrusions in isolation, (c) tongue retrusions followed by swallowing events, and (d) swallowing events in isolation. All four types of events were vulnerable to intramuscular injection of haloperidol 0.75-2.5 mg/kg within 5 to 10 min and the suppression endured for at least several hours. We conclude that there is a parallel between ketamine-induced oral motor activity and neuroleptic-induced dyskinesia in that both are temporarily suppressed by neuroleptic drugs.

Does the interstitial nucleus of cajal project to the hypoglossal nucleus in the rat?

The present study sought to determine whether or not the hypoglossal nucleus receives direct afferent projections from the interstitial nucleus of Cajal. Results from anterograde and retrograde labeling experiments in the rat indicated that while projections from the interstitial nucleus of Cajal do not terminate within the hypoglossal nucleus, they do so among a small group of neurons located ventrolateral to the hypoglossal nucleus, the nucleus of Roller. These findings are discussed in relation to orolingual motor behavior.

The ultrastructure of two distinct neuron populations in the hypoglossal nucleus of the rat.

An ultrastructural study of the hypoglossal nucleus (XII) in the rat has revealed two distinct neuronal populations. Hypoglossal motoneurons comprised the largest population of neurons in XII and were identified following injection of horseradish (HRP) into the tongue. Motoneurons were large (25-50 mu), multipolar in shape and distributed throughout XII. The nucleus was large, round and centrally located, and the cytoplasm was characterized by dense lamellar arrays of rough endoplasmic reticulum. In contrast, a second population of small (10-18 mu), round to oval shaped neurons was found restricted to the ventral and dorsolateral regions of XII. The nucleus was markedly invaginated and eccentric, the cytoplasm scant and filled with free ribosomes, and the absence of lamellar arrays of rough endoplasmic reticulum was conspicuous. Neurons of this type were never found to contain HRP reaction product. These results demonstrate that the hypoglossal nucleus does not consist solely of motoneurons, but includes a distinctly separate, presumably non-motoneuronal pool. Arguments are presented in favor of this second neuron population being interneurons. The functional significance of these findings in relation to tongue control is discussed.

A combined flat-embedding, HRP histochemical method for correlative light and electron microscopic study of single neurons.

A simple and reproducible method is described for studying the morphology of the same neuron at the light and electron microscopic level. This method utilizes horseradish peroxidase histochemistry and a recently described flat-embedding procedure wherein thin, aldehyde-fixed sections are placed in resin between glass microscope slides pretreated with dimethyldichlorosilane. The significance of these combined methodologies for correlative light and electron microscopic studies of single neurons is discussed.

Synaptology of the hypoglossal nucleus in the rat.

The purpose of this study was to define the types and distribution of synaptic terminals in the hypoglossal nucleus (XII) of the rat. Based on differences in bouton and vesicle size and shape, synaptic specializations and association with postsynaptic organelles, five types of terminals were identified in XII. In order of decreasing frequency they were: 1) S-boutons (spherical vesicles with an asymmetrical synapse); 2) F-boutons (flattened vesicles with a symmetrical synapse); 3) P-boutons (pleomorphic admixture of flattened and spherical vesicles with a symmetrical synapse); 4) C-boutons (pleomorphic vesicles with a subsynaptic cistern); and 5) T-boutons (spherical vesicles with an asymmetrical synapse and subsynaptic dense bodies). S-boutons were the predominant type found on dendrites, while boutons containing flattened vesicles were more prevalent on motoneuron somata. C-boutons were restricted exclusively to cell bodies and large dendrites, and T-boutons were seen primarily on smaller dendritic profiles. These results are, in general, comparable to those previously described in the ventral horn and cranial nerve motor nuclei in several species. However, differences were noted. Specifically, large M-boutons and axo-axonic synapses were not observed in the present study. The functional significance of these findings are discussed in relation to oro-lingual behaviour.

Organization of projections from the principal sensory trigeminal nucleus to the hypoglossal nucleus in the rat: an experimental light and electron microscopic study with axonal tracer techniques.

The organization of projections from the principal sensory trigeminal nucleus (PSN) to the hypoglossal nucleus (XII) in the rat was investigated at the light and electron microscopic level with retrograde and anterograde axonal tracer techniques. Microiontophoretic injection of horseradish peroxidase (HRP) into XII resulted in retrograde labeling of neurons confined to the dorsal one-third of the PSN. Labeled neurons were found bilaterally, although a clear preponderance for ipsilateral distribution was evident. Most labeled neurons were found in the medial one-third and caudal two-thirds of the PSN. Labeled neurons were large (30-50 micron), round-to-pear shaped multipolar cells with dendrites oriented primarily in the mediolateral direction. At the electron microscopic level, HRP reaction product was found throughout the cytoplasm of soma and processes of PSN projection neurons. The ultrastructural characteristics of these cells included a round, centrally placed nucleus and invaginated nuclear envelope, sparse Nissl bodies, numerous free ribosomes, mitochondria, lysosomes and Golgi complexes. Three to four main stem dendrites gradually tapered from the cell body and numerous synaptic terminals impinged upon soma and dendrites of labeled PSN neurons. Microiontophoretic injection of tritiated amino acids or HRP into the dorsal one-third of the PSN resulted in moderately dense terminal labeling in XII bilaterally, although mainly ipsilaterally. Terminal labeling was found diffusely throughout all regions of XII. Fibers descended the brainstem in the dorsolateral reticular formation and entered XII ventrolaterally. At the electron microscopic level, boutons containing HRP reaction product were found to synapse on dendritic processes in XII. Labeled boutons were characterized by clear, spherical vesicles and an asymmetrical postsynaptic density. The significance of these results are discussed in relation to oro-lingual motor behavior.

Organization of the cerebellar tongue representation in the monkey.

The cerebellar nuclei of rhesus monkeys were stimulated electrically and evoked responses of the tongue recorded with force-displacement transducers and electromyographically. Stimulation of the dentate (DN) and fastigial (FN) nuclei produced different effects on the tongue musculature, the former evoking movements characterized by changes in both the lateral and anteroposterior (AP) dimensions of the tongue and the latter movements associated with a preferential change in the AP dimension. While all three deep nuclei influenced the genioglossus muscle, the influences varied across nuclei in the following manner: FN produced the largest amplitude genioglossus mediated AP excusions and DN the smallest, with the nucleus interpositus (NI) occupying an intermediate position; and, FN and medial NI stimulation evoked changes in genioglossus respiratory activity whereas DN stimulation did not. These data indicate that the cerebellar tongue representation is organized into different functional zones and that within an anatomically broad representation of a single target muscle, nuclear functional specificity exists.

Distribution of glutamic acid decarboxylase and gamma-aminobutyric acid in the hypoglossal nucleus in the rat.

Immunocytochemistry was used to investigate the distribution of glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in the hypoglossal nucleus (XII) of the adult rat. The distribution of GAD and GABA was found to be co-extensive throughout XII. Although immunoreactivity was moderately dense in all regions, the intensity of staining was greatest in the ventral district of XII particularly ventromedially in the caudal half of the nucleus. Immunoreactive terminal-like profiles were observed around motoneuron somata and dendrites. There also was evidence of sparse mediolaterally-oriented densities of immunoreactivity at the junction of XII and the dorsal vagal nucleus and between dorsal and ventral districts of XII. In addition, neurons staining positive for GABA were found scattered within XII laterally and immediately outside of XII in and around the Nucleus of Roller. These observations suggest a complex, differential distribution of GABA in XII and are discussed in relation to tongue motor behavior.