Neurons of the motor trigeminal nucleus project to the hypoglossal nucleus in the rat

Summary The hypoglossal nucleus (Mo12) contains motoneurons that innervate the tongue, while the motor trigeminal nucleus (Mo5) contains motoneurons that elevate or depress the mandible. Previous studies have revealed lateral and medial tegmental field neuronal afferents to the Mo12 adjacent to, but not within, the motor trigeminal nucleus (Mo5). The current studies demonstrate the presence of retrogradely labeled neuronal afférents to the Mo12 within the Mo5 produced by as little as 10 nl of Fast Blue (FB) injected into the Mo12. Retrograde labeling of Mo5 afferents to the Mo12 with injections of Diamidino Yellow (DY) combined with injections of FB into the lumbar spinal cord showed these neuronal afferents to the Mo12 are not part of the diffuse projections to motoneurons from the nucleus subcoeruleus. Retrograde labeling of Mo5 afferents to the Mo12 with DY combined with injections of FB into the masseter revealed these neuronal afferents to the Mo12 are not trigeminal motoneurons. These results indicate that Mo5 interneurons are part of the lateral tegmental field projections to the Mo12, and are likely to comprise part of the neural substrate coordinating the motor activity of the jaw and tongue.     

Synaptology of the hypoglossal nucleus in the rat

Summary 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) Tboutons (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.     

Neuronal activity in the nucleus accumbens is necessary for performance-related increases in cortical acetylcholine release

In vivo microdialysis was used to determine the necessity of neuronal activity in the nucleus accumbens (NAC) for task-induced increases in cortical acetylcholine (ACh) efflux. Rats were trained in a behavioral task in which they were required to perform a defined number of licks of a citric acid solution in order to gain access to a palatable, cheese-flavored food. Upon reaching a consistent level of performance, rats were implanted with microdialysis cannula in the medial prefrontal cortex (mPFC) and either the ipsilateral shell of the NAC or in the dorsal striatum (STR; control site). Dialysis samples from the mPFC were analyzed for ACh concentrations and samples from the NAC were analyzed for dopamine (DA) concentrations. Performance in the task was associated with increases in both ACh efflux in the cortex (150-200%) and DA efflux in the NAC (50-75%). These increases were blocked by administration of tetrodotoxin (TTX; 1.0 microM) via reverse dialysis into the NAC. Administration of TTX into the dorsal STR control site was ineffective in blocking performance-associated increases in cortical ACh. The D2 antagonist sulpiride (10 or 100 microM) administered into the NAC via reverse dialysis was ineffective in blocking increases in cortical ACh efflux. The present data reveal that neuronal activity in the NAC is necessary for behaviorally induced increases in cortical ACh efflux and that this activation does not require increases in D2 receptor activity.     

Quantitative electron microscopy on the injured hypoglossal nucleus in the rat

Summary The hypoglossal nuclei of adult male rats which had received left hypoglossal axotomies, were studied quantitatively by making area measurements of structures on twenty electron micrographs per hypoglossal nucleus. Boutons and dendrites were analysed in further detail by counting and measuring profiles, counting synaptic thickenings per profile, and counting degenerating profiles. Results of light microscopy were confirmed and extended. In injured nuclei, microglia increased from 2 days to 2 weeks after axotomy, wrapping around neuron perikarya and parts of the dendrites between boutons and their post-synaptic sites. As microglia diminished after 2 weeks, astrocyte processes increased and replaced them, reaching peak growth at 5 weeks after axotomy. From 7 days to 5 weeks after axotomy there were fewer synaptic junctions. Neurons became stripped of boutons, but dendrites retained some. Dendrites shrank and some degenerated. Boutons shrank and decreased in numbers. At 10 and 12 weeks, all parameters returned to normal, by which time the hypoglossal nerve had reconnected with the tongue.Possibly the chronological succession from boutons to microglial processes to astrocyte processes to boutons covering the neuron perikarya reflects changes in the neuronal glycocalyx during repair.     

Modulation of acetylcholine release by histamine in the nucleus accumbens

Inflammation Research -     

Differential distribution of biogenic amines in the hypoglossal nucleus of the rat

Summary The distribution of biogenic amines in the rat hypoglossal nucleus (nXII) was investigated with immunocytochemical methods using antibodies to tyrosine hydroxylase (TH) as a marker for catecholamines, and to 5-hydroxytryptamine (5-HT), the principal indoleamine, at the light microscopic level. TH and 5-HT immunoreactivity were found throughout all regions of nXII. Although the innervations overlapped, clearly differnt patterns of distribution were observed. TH immunoreactivity was localized primarily in the ventromedial quadrant of the caudal half of nXII and appeared largely as perisomatic-like profiles. In contrast, 5-HT immunoreactivity was greatest dorsally along the caudal half of nXII, although secondary foci of staining were evident ventrolaterally and, to a lesser extent, ventromedially. A perisomatic-like pattern of termination was observed for 5-HT in both dorsal and ventral regions of nXII. Since ventral and dorsal districts of nXII contain motoneurons that innervate protrusor and retrusor tongue muscles, respectively, we propose that the overlapping, yet differential distributions of catecholamines and indoleamines are important in controlling the relationships between functionally related groups of nXII motoneurons. These findings are discussed in relation to oro-lingual motor dysfunction.     

Mesopontine cholinergic projections to the hypoglossal motor nucleus

Mesopontine cholinergic (ACh) neurons have increased discharge during wakefulness, rapid eye movement (REM) sleep, or both. Hypoglossal (12) motoneurons, which play an important role in the control of upper airway patency, are postsynaptically excited by stimulation of nicotinic receptors, whereas muscarinic receptors presynaptically inhibit inputs to 12 motoneurons. These data suggest that ACh contributes to sleep/wake-related changes in the activity of 12 motoneurons by acting within the hypoglossal motor nucleus (Mo12), but the origins of ACh projections to Mo12 are not well established. We used retrograde tracers to assess the projections of ACh neurons of the mesopontine pedinculopontine tegmental (PPT) and laterodorsal tegmental (LDT) nuclei to the Mo12. In six Sprague-Dawley rats, Fluorogold or B subunit of cholera toxin, were pressure injected (5-20nl) into the Mo12. Retrogradely labeled neurons, identified as ACh using nitric oxide synthase (NOS) immunohistochemistry, were found bilaterally in discrete subregions of both PPT and LDT nuclei. Most retrogradely labeled PPT cells (96%) were located in the PPT pars compacta region adjacent to the ventrolateral tip of the superior cerebellar peduncle. In the LDT, retrogradely labeled neurons were located exclusively in its pars alpha region. Over twice as many ACh neurons projecting to the Mo12 were located in the PPT than LDT. The results demonstrate direct mesopontine ACh projections to the Mo12. These projections may contribute to the characteristic of wakefulness and REM sleep increases, as well as REM sleep-related decrements, of 12 motoneuronal activity.     

Microdialysis without acetylcholinesterase inhibition reveals an age-related attenuation in stimulated cortical acetylcholine release

Aging-related differences in the ability of cortical cholinergic inputs to respond to local depolarization was assessed in young (3-6 months) and old (26-33 months) awake rats using in vivo microdialysis in the absence of an inhibitor of acetylcholinesterase. Rats were perfused, using a within-subjects, repeated session design, with vehicle (aCSF) or K(+) (25, 50, 100 mM). Perfusion of K(+) resulted in a dose-dependent increase in cortical ACh efflux with comparable efflux seen between the two ages following 25 mM (50%) and 50 mM (100%) K(+). In contrast, aged rats exhibited a marked attenuation (330%) in ACh efflux relative to young adult rats (650%). These data reveal aging-related decreases in the responsiveness of cortical cholinergic afferents, tested under physiologically relevant conditions, to local depolarization and may provide a neuronal mechanism contributing to the cognitive deficits reported in normal aging- and age-related pathological conditions.     

Effect of unilateral decortication on acetylcholine release from the nucleus basalis

Acetylcholine (ACh) is released from the rat nucleus basalis magnocellularis (nbM) following tissue depolarization with 35 mM K+. In the present study, we report that cholinergic neurons within the nbM undergo retrograde changes resulting in a significant reduction in K+-evoked ACh release following cortical lesions.     

Application of histamine or serotonin to the hypoglossal nucleus increases genioglossus muscle activity across the wake–sleep cycle

The decrease in genioglossus (GG) muscle activity during sleep, especially rapid eye movement (REM) or paradoxical sleep, can lead to airway occlusion and obstructive sleep apnoea (OSA). The hypoglossal nucleus innervating the GG muscle is under the control of serotonergic, noradrenergic and histaminergic neurons that cease firing during paradoxical sleep. The objectives of this study were to determine the effect on GG muscle activity during different wake–sleep states of the microdialysis application of serotonin, histamine (HA) or noradrenaline (NE) to the hypoglossal nucleus in freely moving cats. Six adult cats were implanted with electroencephalogram, electro-oculogram and neck electromyogram electrodes to record wake–sleep states and with GG muscle and diaphragm electrodes to record respiratory muscle activity. Microdialysis probes were inserted into the hypoglossal nucleus for monoamine application. Changes in GG muscle activity were assessed by power spectrum analysis. In the baseline conditions, tonic GG muscle activity decreased progressively and significantly from wakefulness to slow-wave sleep and even further during slow-wave sleep with ponto-geniculo-occipital waves and paradoxical sleep. Application of serotonin or HA significantly increased GG muscle activity during the wake–sleep states when compared with controls. By contrast, NE had no excitatory effect. Our results indicate that both serotonin and HA have a potent excitatory action on GG muscle activity, suggesting multiple aminergic control of upper airway muscle activity during the wake–sleep cycle. These data might help in the development of pharmacological approaches for the treatment of OSA.     

Common synaptic input to the human hypoglossal motor nucleus

The tongue plays a key role in various volitional and automatic functions such as swallowing, maintenance of airway patency, and speech. Precisely how hypoglossal motor neurons, which control the tongue, receive and process their often concurrent input drives is a subject of ongoing research. We investigated common synaptic input to the hypoglossal motor nucleus by measuring the coordination of spike timing, firing rate, and oscillatory activity across motor units recorded from unilateral (i.e., within a belly) or bilateral (i.e., across both bellies) locations within the genioglossus (GG), the primary protruder muscle of the tongue. Simultaneously recorded pairs of motor units were obtained from 14 healthy adult volunteers using tungsten microelectrodes inserted percutaneously into the GG while the subjects were engaged in volitional tongue protrusion or rest breathing. Bilateral motor unit pairs showed concurrent low frequency alterations in firing rate (common drive) with no significant difference between tasks. Unilateral motor unit pairs showed significantly stronger common drive in the protrusion task compared with rest breathing, as well as higher indices of synchronous spiking (short-term synchrony). Common oscillatory input was assessed using coherence analysis and was observed in all conditions for frequencies up to ～ 5 Hz. Coherence at frequencies up to ～ 10 Hz was strongest in motor unit pairs recorded from the same GG belly in tongue protrusion. Taken together, our results suggest that cortical drive increases motor unit coordination within but not across GG bellies, while input drive during rest breathing is distributed uniformly to both bellies of the muscle.     

Food-elicited increases in cortical acetylcholine release require orexin transmission

The corticopetal basal forebrain cholinergic system (BFCS) is crucial for normal attentional function and cortical acetylcholine release is increased by stimuli with high motivational salience. Projections from the lateral hypothalamus to the basal forebrain have been previously described and have been hypothesized to relay interoceptive information to this area but little is known about the phenotypic and functional nature of hypothalamic modulation of the BFCS. We have previously shown that orexin (hypocretin) fibers from the hypothalamus distribute densely among basal forebrain choline acetyltransferase-positive neurons and that intrabasalis administration of orexin A increases cortical acetylcholine release. Here, we used in vivo microdialysis to test the hypothesis that the orexin system is necessary for activation of the BFCS in response to a food-related stimulus in food-restricted rats. Elimination of the majority of orexin neurons with the toxin orexin B-saporin significantly blunted the cholinergic response to presentation of palatable food in these animals. Similar effects were seen in animals acutely pretreated with the orexin 1 receptor antagonist, SB-334867, which also increased feeding latency. Collectively, these data suggest that orexin interactions with the BFCS may be a critical component of the neurobiological substrates by which interoceptive cues bias the allocation of attentional resources toward exteroceptive stimuli related to homeostatic challenges.     

Failure of acetylcholine to release histamine from rat mast cells

The action of various salts of acetylcholine on isolated mast cells from Wistar and F₁ hybrids of Wistar and August rats was investigated.None of the acetylcholine salts within the concentration range 10^–12 M to 10^–2 M was able to release histamine either from Wistar or from hybrid mast cells. Compound 48/80, used as a control, was active in both cases.The results obtained are in opposition to some recent reports. The possible reasons for these contradictions are discussed.This work was supported by the Polish Academy of Sciences (Grant No. 10.5).     

Muscarinic action of acetylcholine in the rat ventromedial thalamic nucleus

Summary Several lines of evidence suggest a role for ACh in the mediation of cerebello-thalamic transmission. The physiological, pharmacological and biochemical experiments described were designed to test this hypothesis for the rat cerebello-thalamic pathway. Unilateral electrolytic lesions of the superior cerebellar peduncle resulted in modest falls of CAT from both ventromedial thalamic nuclei (contralateral 35%, ipsilateral 15%). Iontophoretic application of ACh to relay cells evokes three types of response (i) excitation (ii) inhibition (iii) polyphasic combinations of (i) and (ii). The type of response evoked was directly related to the firing pattern of the cell. Thus, for example, excitatory responses were never recorded during high-frequency bursting but were easily evoked following a switch to tonic, single-spike activity. All responses to ACh and synaptic responses to cerebellar stimulation were sensitive to muscarinic but not to nicotinic cholinergic antagonists. The nicotinic antagonist mecamylamine was a potent blocker of excitant amino acid responses but had no effect on cerebellarevoked synaptic responses. Cholinergic and anticholinergic agents had a profound action on relay cell firing pattern. ACh promoted single-spike activity whereas atropine promoted high-frequency bursting. The actions of ACh are discussed with reference to recently discovered voltage-sensitive ionic conductances. Because of the modulatory action of ACh on relay cell firing pattern and excitability no firm conclusion can be reached concerning the hypothesis under test here. We tentatively suggest a dual role for ACh as both neurotransmitter and neuromodulator.