An immunocytochemical and autoradiographic investigation of the serotoninergic innervation of trigeminal mesencephalic and motor nuclei in the rabbit

Treatment with the essential amino acid L-tryptophan reduces masticatory impairments in experimental cerebral palsy

Purpose Children with cerebral palsy (CP) often exhibit difficulties in feeding resulting from deficits in chewing. This study investigates the therapeutic potential of L-tryptophan (TRI) to reduce deficits in chewing in rats subjected to an experimental model of CP.Methods A total of 80 Wistar albino rats were used. Pups were randomly assigned to 4 experimental groups: Control Saline, Control TRI, CP Saline, and CP TRI groups. The experimental model of CP was based on the combination of perinatal anoxia associated with postnatal sensorimotor restriction of the hind limbs. TRI was administered subcutaneously during the lactation period. Anatomical and behavioral parameters were evaluated during maturation, including body weight gain, food intake, chewing movements, relative weight and the distribution of the types of masseter muscle fibers.Results The induction of CP limited body weight gain, decreased food intake and led to impairment in the morphological and functional parameters of chewing. Moreover, for a comparable amount of food ingested, CP TRI animals grew the most. In addition, supplementation with TRI improved the number of chewing movements, and increased the weight and proportion of type IIB fibers of the masseter in rats subjected to CP.Conclusion These results demonstrate that experimental CP impaired the development of mastication and that TRI supplementation increased masticatory maturation in animals subjected to CP.

Serotonergic modulation of slow inward rectification in mesencephalic trigeminal neurons

Inward rectification in response to membrane hyperpolarization is a prominent feature of mesencephalic trigeminal (Mes V) neurons and the hyperpolarization-activated inward current (I_h), as the basis of this property, regulates the spike discharge characteristics and input frequency preference (resonance) in these neurons, suggesting that I_h modulation is an important regulator of oral motor activity. To examine a possible contribution of serotonin (5-HT) to the modulation of I_h activation characteristics, in the present study, we investigated the modulatory effects of 5-HT receptor activation on I_h in postnatal day (P) 2-12 rat Mes V neurons by whole-cell patch-clamp recording. Bath application of 5-HT suppressed the I_h-dependent voltage sag and I_h conductance, but induced only a modest shift in the voltage dependence of I_h activation. This 5-HT-induced suppression of I_h was greater in P10-12 than P2-4 neurons, and involved the cAMP/protein kinase A (PKA) signaling pathway but not the PKC pathway. Pharmacological activation of the 5-HT_1A receptor mimicked the effect of 5-HT, while modulation of other receptor subtypes, including 5-HT_1B,1D, 5-HT₂, and 5-HT₃, had little or no effect on I_h. Low-frequency (<10 Hz) resonance at membrane potentials below the resting potential were reduced by 5-HT, suggesting that serotonergic I_h modulation can substantially alter the frequency preference to synaptic inputs. These results suggest that changes in resonance properties through serotonergic modulation of I_h may tune the firing of Mes V neurons to different afferent input frequencies and alter motor outputs to the jaw, thereby regulating oral motor activity.

5-HT2A receptor activation enhances NMDA receptor-mediated glutamate responses through Src kinase in the dendrites of rat jaw-closing motoneurons

KEY POINTS

5-HT increases the excitability of brainstem and spinal motoneurons, including the jaw-closing motoneurons, by depolarizing the membrane potential and decreasing the medium-duration afterhyperpolarization. In this study, we focused on how 5-HT enhances postsynaptic glutamatergic responses in the dendrites of the jaw-closing motoneurons. We demonstrate that 5-HT augments glutamatergic signalling by enhancing the function of the GluN2A-containing NMDA receptor (NMDAR) through the activation of 5-HT_2A receptors (5-HT_2A Rs) and Src kinase. To enhance glutamatergic responses, activation of the 5-HT_2A Rs must occur within ∼60 μm of the location of the glutamate responses. 5-HT inputs to the jaw-closing motoneurons can significantly vary their input-output relationship, which may contribute to wide-range regulation of contractile forces of the jaw-closing muscles.

ABSTRACT

Various motor behaviours are modulated by 5-HT. Although the masseter (jaw-closing) motoneurons receive both glutamatergic and serotonergic inputs, it remains unclear how 5-HT affects the glutamatergic inputs to the motoneuronal dendrites. We examined the effects of 5-HT on postsynaptic responses evoked by single- or two-photon uncaging of caged glutamate (glutamate responses) to the dendrites of masseter motoneurons in postnatal day 2-5 rats of either sex. Application of 5-HT induced membrane depolarization and enhanced the glutamate-response amplitude. This enhancement was mimicked by the 5-HT_2A receptor (5-HT_2A R) agonist and was blocked by the 5-HT_2A/2C R antagonist. However, neither the 5-HT_2B R nor the 5-HT_2C R agonists altered glutamate responses. Blockade of the NMDA receptors (NMDARs), but not AMPA receptors, abolished the 5-HT-induced enhancement. Furthermore, the selective antagonist for the GluN2A subunit abolished the 5-HT-induced enhancement. 5-HT increased GluN2A phosphorylation, while the Src kinase inhibitor reduced the 5-HT-induced enhancement and GluN2A phosphorylation. When exposure to the 5-HT_2A R agonist was targeted to the dendrites, the enhancement of glutamate responses was restricted to the loci of the dendrites near the puff loci. Electron microscopic immunohistochemistry revealed that both the NMDARs and the 5-HT_2A Rs were close to each other in the same dendrite. These results suggest that activation of dendritic 5-HT_2A Rs enhances the function of local GluN2A-containing NMDARs through Src kinase. Such enhancement of the glutamate responses by 5-HT may contribute to wide-range regulation of contractile forces of the jaw-closing muscles.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Differential effect of clomipramine on habituation and prepulse inhibition in dominant versus subordinate rats

Many patients with depression have comorbidities associated with an impairment of sensorimotor gating, such as e.g. schizophrenia, Parkinson Disease, or Alzheimer disease. Anti-depressants like clomipramine that modulate serotonergic or norepinephrinergic neurotransmission have been shown to impact sensorimotor gating, it is therefore important to study potential effects of clomipramine in order to rule out an exacerbation of sensorimotor gating impairment. Prior studies in animals and humans have been inconclusive. Since serotonin and norepinephrine levels are closely related to anxiety and stress levels and therefore to the social status of an animal, we tested the hypothesis that acute and chronic effects of clomipramine on sensorimotor gating are different in dominant versus subordinate rats, which might be responsible for conflicting results in past animal studies. We used habituation and prepulse inhibition (PPI) of the acoustic startle response as operational measures of sensorimotor gating. After establishing the dominant animal in pair-housed male rats, we injected clomipramine for two weeks and measured acute effects on baseline startle, habituation and PPI after the first injection and chronic effects at the end of the two weeks. Chronic treatment with clomipramine significantly increased habituation in subordinate rats, but had no effect on habituation in dominant animals. Furthermore, PPI was slightly enhanced in subordinate rats upon chronic treatment while no changes occurred in dominant animals. We conclude that the social status of an animal, and therefore the basic anxiety/stress level determines whether or not clomipramine has a beneficial effect on sensorimotor gating and discuss possible underlying mechanisms.

Activation properties of trigeminal motoneurons in participants with and without bruxism

In animals, sodium- and calcium-mediated persistent inward currents (PICs), which produce long-lasting periods of depolarization under conditions of low synaptic drive, can be activated in trigeminal motoneurons following the application of the monoamine serotonin. Here we examined if PICs are activated in human trigeminal motoneurons during voluntary contractions and under physiological levels of monoaminergic drive (e.g., serotonin and norepinephrine) using a paired motor unit analysis technique. We also examined if PICs activated during voluntary contractions are larger in participants who demonstrate involuntary chewing during sleep (bruxism), which is accompanied by periods of high monoaminergic drive. In control participants, during a slowly increasing and then decreasing isometric contraction, the firing rate of an earlier-recruited masseter motor unit, which served as a measure of synaptic input to a later-recruited test unit, was consistently lower during derecruitment of the test unit compared with at recruitment (ΔF = 4.6 ± 1.5 imp/s). The ΔF, therefore, is a measure of the reduction in synaptic input needed to counteract the depolarization from the PIC to provide an indirect estimate of PIC amplitude. The range of ΔF values measured in the bruxer participants during similar voluntary contractions was the same as in controls, suggesting that abnormally high levels of monoaminergic drive are not continually present in the absence of involuntary motor activity. We also observed a consistent "onion skin effect" during the moderately sized contractions (<20% of maximal), whereby the firing rate of higher threshold motor units discharged at slower rates (by 4-7 imp/s) compared with motor units with relatively lower thresholds. The presence of lower firing rates in the more fatigue-prone, higher threshold trigeminal motoneurons, in addition to the activation of PICs, likely facilitates the activation of the masseter muscle during motor activities such as eating, nonnutritive chewing, clenching, and yawning.

Chapter 12--modulation of rhythmic movement: control of coordination

Three rhythmic movements, breathing, walking, and chewing, are considered from the perspective of the emerging factors that control their coordination. This takes us beyond the concept of a core excitatory kernel and into the common principles that govern the interaction between components of the neural networks that must be orchestrated properly to produce meaningful movement beyond the production of the basic rhythm. We focus on the role of neuromodulators, especially 5-hydroxytryptamine (5-HT), in the production of coordinated breathing, walking, and chewing, and we review the evidence that at least in the case of breathing and walking, 5-HT input to the CPGs acts through the selection of inhibitory interneurons that are essential for coordination. We review data from recently developed mouse models that offer insight into the contributions of inhibitory coordinating neurons, including the development of a new model that has allowed the revelation that there are glycinergic pacemaker neurons that likely contribute to the production of the respiratory rhythm. Perhaps walking and chewing will not be far behind.

SEROTONERGIC MECHANISMS IN AMYOTROPHIC LATERAL SCLEROSIS

Serotonin (5-HT) has been intimately linked with global regulation of motor behavior, local control of motoneuron excitability, functional recovery of spinal motoneurons as well as neuronal maturation and aging. Selective degeneration of motoneurons is the pathological hallmark of amyotrophic lateral sclerosis (ALS). Motoneurons that are preferentially affected in ALS are also densely innervated by 5-HT neurons (e.g., trigeminal, facial, ambiguus, and hypoglossal brainstem nuclei as well as ventral horn and motor cortex). Conversely, motoneuron groups that appear more resistant to the process of neurodegeneration in ALS (e.g., oculomotor, trochlear, and abducens nuclei) as well as the cerebellum receive only sparse 5-HT input. The glutamate excitotoxicity theory maintains that in ALS degeneration of motoneurons is caused by excessive glutamate neurotransmission, which is neurotoxic. Because of its facilitatory effects on glutaminergic motoneuron excitation, 5-HT may be pivotal to the pathogenesis and therapy of ALS. 5-HT levels as well as the concentrations 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of 5-HT, are reduced in postmortem spinal cord tissue of ALS patients indicating decreased 5-HT release. Furthermore, cerebrospinal fluid levels of tryptophan, a precursor of 5-HT, are decreased in patients with ALS and plasma concentrations of tryptophan are also decreased with the lowest levels found in the most severely affected patients. In ALS progressive degeneration of 5-HT neurons would result in a compensatory increase in glutamate excitation of motoneurons. Additionally, because 5-HT, acting through presynaptic 5-HT1B receptors, inhibits glutamatergic synaptic transmission, lowered 5-HT activity would lead to increased synaptic glutamate release. Furthermore, 5-HT is a precursor of melatonin, which inhibits glutamate release and glutamate-induced neurotoxicity. Thus, progressive degeneration of 5-HT neurons affecting motoneuron activity constitutes the prime mover of the disease and its progression and treatment of ALS needs to be focused primarily on boosting 5-HT functions (e.g., pharmacologically via its precursors, reuptake inhibitors, selective 5-HT1A receptor agonists/5-HT2 receptor antagonists, and electrically through transcranial administration of AC pulsed picotesla electromagnetic fields) to prevent excessive glutamate activity in the motoneurons. In fact, 5HT1A and 5HT2 receptor agonists have been shown to prevent glutamate-induced neurotoxicity in primary cortical cell cultures and the 5-HT precursor 5-hydroxytryptophan (5-HTP) improved locomotor function and survival of transgenic SOD1 G93A mice, an animal model of ALS.

Neurochemistry of identified motoneurons of the tensor tympani muscle in rat middle ear

The objective of the present study was to identify efferent and afferent transmitters of motoneurons of the tensor tympani muscle (MoTTM) to gain more insight into the neuronal regulation of the muscle. To identify MoTTM, we injected the fluorescent neuronal tracer Fluoro-Gold (FG) into the muscle after preparation of the middle ear in adult rats. Upon terminal uptake and retrograde neuronal transport, we observed FG in neurons located lateral and ventrolateral to the motor trigeminal nucleus ipsilateral to the injection site. Immunohistochemical studies of these motoneurons showed that apparently all contained choline acetyltransferase, demonstrating their motoneuronal character. Different portions of these cell bodies were immunoreactive to bombesin (33%), cholecystokinin (37%), endorphin (100%), leu-enkephalin (25%) or neuronal nitric oxide synthase (32%). MoTTM containing calcitonin gene-related peptide, tyrosine hydroxylase, substance P, neuropeptide Y or serotonin were not found. While calcitonin gene-related peptide was not detected in the region under study, nerve fibers immunoreactive to tyrosine hydroxylase, substance P, neuropeptide Y or serotonin were observed in close spatial relationship to MoTTM, suggesting that these neurons are under aminergic and neuropeptidergic influence. Our results demonstrating the neurochemistry of motoneuron input and output of the rat tensor tympany muscle may prove useful also for the general understanding of motoneuron function and regulation.

Serotonin and the orchestration of energy balance

The phylogenetically ancient signaling molecule serotonin is found in all species that possess nervous systems and orchestrates diverse behavioral and physiological processes in the service of energy balance. In some instances, the manner in which serotonin signaling influences these processes appears comparable among invertebrate and vertebrate species. Within mammalian species, central nervous system serotonergic signaling influences both behavioral and physiological determinants of energy balance. Within the gastrointestinal tract, serotonin mediates diverse sensory, motor, and secretory functions. Further examinations of serotonergic influences on peripheral organ systems are likely to uncover novel functions consistent with an apparently pervasive association between serotonergic signaling and physiological substrates of energy balance.

Neurobiology of orofacial proprioception

Primary sensory fibers innervating the head region derive from neurons of both the trigeminal ganglion (TG) and mesencephalic trigeminal nucleus (MTN). The trigeminal primary proprioceptors have their cell bodies in the MTN. Unlike the TG cells, MTN neuronal somata are centrally located within the brainstem and receive synaptic inputs that potentially modify their output. They are a crucial component of the neural circuitry responsible for the generation and control of oromotor activities. Gaining an insight into the chemical neuroanatomy of the MTN is, therefore, of fundamental importance for the understanding of neurobiology of the head proprioceptive system. This paper summarizes the recent advances in our knowledge of pre- and postsynaptic mechanisms related to orofacial proprioceptive signaling in mammals. It first briefly describes the neuroanatomy of the MTN, which is involved in the processing of proprioceptive information from the face and oral cavity, and then focuses on its neurochemistry. In order to solve the puzzle of the chemical coding of the mammalian MTN, we review the expression of classical neurotransmitters and their receptors in mesencephalic trigeminal neurons. Furthermore, we discuss the relationship of neuropeptides and their corresponding receptors in relaying of masticatory proprioception and also refer to the interactions with other atypical neuromessengers and neurotrophic factors. In extension of previous inferences, we provide conclusive evidence that the levels of transmitters vary according to the environmental conditions thus implying the neuroplasticity of mesencephalic trigeminal neurons. Finally, we have also tried to give an integrated functional account of the MTN neurochemical profiles.

Two Opposing Roles of 4-AP–Sensitive K+ Current in Initiation and Invasion of Spikes in Rat Mesencephalic Trigeminal Neurons

The axon initial segment plays important roles in spike initiation and invasion of axonal spikes into the soma. Among primary sensory neurons, those in the mesencephalic trigeminal nucleus (MTN) are exceptional in their ability to initiate soma spikes (S-spikes) in response to synaptic inputs, consequently displaying two kinds of S-spikes, one caused by invasion of an axonal spike arising from the sensory receptor and the other initiated by somatic inputs. We investigated where spikes are initiated in such MTN neurons and whether there are any differences between the two kinds of S-spikes. Simultaneous patch-clamp recordings from the soma and axon hillock revealed a spike-backpropagation from the spike-initiation site in the stem axon to the soma in response to 1-ms somatic current pulse, which disclosed the delayed emergence of S-spikes after the current-pulse offset. These initiated S-spikes were smaller in amplitude than S-spikes generated by stimulation of the stem axon; however, 4-AP (≤0.5 mM) eliminated the amplitude difference. Furthermore, 4-AP dramatically shortened the delay in spike initiation without affecting the spike-backpropagation time in the stem axon, whereas it substantially prolonged the refractory period of S-spikes arising from axonal-spike invasion without significantly affecting that of presumed axonal spikes. These observations suggest that 4-AP–sensitive K+ currents exert two opposing effects on S-spikes depending on their origins: suppression of spike initiation and facilitation of axonal-spike invasion at higher frequencies. Consistent with these findings, strong immunoreactivities for Kv1.1 and Kv1.6, among 4-AP–sensitive and low-voltage–activated Kv1 family examined, were detected in the soma but not in the stem axon of MTN neurons.

Neuro-modulation, aminergic neuro-disinhibition and neuro-degeneration. Draft of a comprehensive theory for Alzheimer disease

A comprehensive theory for Alzheimer disease (AD) which can provide a clue to the neuronal selective vulnerability (pathoklisis) is still missing. Based upon evidence from the current literature, the present work is aimed at proposing such a theory, namely the 'aminergic disinhibition theory' of AD. It includes data-based hypotheses as to the pathoklisis, mechanisms of neuro-degeneration and dementia as well as the aetiology of the disease. Alzheimer disease is regarded as a disorder of neural input modulation caused by the degeneration of four modulatory amine transmitter (MAT) systems, namely the serotoninergic, the noradrenergic, the histaminergic, and the cholinergic systems with ascending projections. MATs modulate cognitive processing including arousal, attention, and synaptic plasticity in learning and memory, not only through direct, mostly inhibitory impact on principal neurones but also partially through interaction with local networks of GABA-ergic inter-neurones. The distribution and magnitude of the pathology in AD roughly correlate with the distribution and magnitude of MAT modulation: Regions more densely innervated by ascending MAT projections are, as a rule, more severely affected than areas receiving less MAT innervation. Because the global effect of MATs in the forebrain is inhibition, the degeneration of four MAT systems, some related peptidergic systems and a secondary alleviation of the GABA-ergic transmission means a fundamental loss of inhibitory impact in the neuronal circuitry resulting in neuronal (aminergic) disinhibition. Clearly, the basic mechanism promoting neuronal death in AD is thought to be a chronic disturbance of the inhibition-excitation balance to the advantage of excitation. Chronic over-excitation is conceived to result in Ca2+ dependent cellular excito-toxicity leading to neuro-degeneration including amyloid-beta production and NFT formation. Disinhibited neurons will degenerate while less excited (relatively over-inhibited) neurones will survive. Because the decline of aminergic transmission in AD is likely to start at the receptor level, it is hypothesized that early impairment by a molecular 'hit' to an MAT receptor (or a group of receptors) initiates a pathogenetic cascade that develops in an avalanche-like manner. Based on experimental evidence from the literature, the 'hit' might be the attachment of a targeted pathogen like a small roaming amino acid sequence to the receptor(s), e.g., the serotoninergic 5-HT2A-R. Referential sequence analysis could be a means to identify such a small pathogen hidden in a large receptor molecule.

Voltage-dependent calcium currents in trigeminal motoneurons of early postnatal rats: modulation by 5-HT receptors

Trigeminal motoneurons relay the final output signals generated within the oral-motor pattern generating circuit(s) to muscles for execution of various motor patterns. In recent years, these motoneurons were shown to possess voltage dependent nonlinear membrane properties that allow them to actively participate in sculpting their final output. A complete understanding of the factors controlling trigeminal motoneuronal (TMN) discharge during oral-motor activity requires, at a minimum, a detailed understanding of the palette of ion channels responsible for membrane excitability and a determination of whether these ion channels are targets for modulation. Toward that end, we studied in detail the properties of calcium channels in TMNs and their susceptibility to modulation by 5-HT in rat brain slices. We found that based on pharmacological and voltage-dependent properties, high-voltage-activated (HVA) N-type [omega-conotoxin GVIA (omega-CgTX)]-sensitive, and to a lesser extent P/Q-type [omega-agatoxin IVA (omega-Aga IVA)]-sensitive, calcium channels make up the majority of the whole cell calcium current. 5-HT (5.0 microM) decreased HVA current by 31.3 +/- 2.2%, and the majority of this suppression resulted from reduction of current flow through N- and P/Q-type calcium channels. In contrast, 5-HT had no effect on low-voltage-activated (LVA) current amplitude in TMNs. HVA calcium current inhibition was mimicked by 5-CT, a 5-HT1 receptor agonist, and by R(+)-8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT), a specific 5-HT1A agonist. The effects of 5-HT were blocked by the 5-HT1A antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine hydrobromide (NAN-190) but not by ketanserin, a 5-HT(2/1C) antagonist. Under current clamp, omega-CgTX and 5-HT were most effective in suppressing the mAHP and both increased the spike frequency and input/output gain in response to current injection. Calcium current modulation by 5-HT1A receptors likely is an important mechanism to fine tune the input/output gain of TMNs in response to small incoming synaptic inputs and accounts for some of the previously reported effects of 5-HT on TMN excitability during tonic and burst activity during oral-motor behavior.

Bidirectional interactions between h-channels and Na+-K+ pumps in mesencephalic trigeminal neurons

The Na(+)-K(+) pump current (I(p)) and the h-current (I(h)) flowing through hyperpolarization-activated channels (h-channels) participate in generating the resting potential. These two currents are thought to be produced independently. We show here bidirectional interactions between Na(+)-K(+) pumps and h-channels in mesencephalic trigeminal neurons. Activation of I(h) leads to the generation of two types of ouabain-sensitive I(p) with temporal profiles similar to those of instantaneous and slow components of I(h), presumably reflecting Na(+) transients in a restricted cellular space. Moreover, the I(p) activated by instantaneous I(h) can facilitate the subsequent activation of slow I(h). Such counteractive and cooperative interactions were also disclosed by replacing extracellular Na(+) with Li(+), which is permeant through h-channels but does not stimulate the Na(+)-K(+) pump as strongly as Na(+) ions. These observations indicate that the interactions are bidirectional and mediated by Na(+) ions. Also after substitution of extracellular Na(+) with Li(+), the tail I(h) was reduced markedly despite an enhancement of I(h) itself, attributable to a negative shift of the reversal potential for I(h) presumably caused by intracellular accumulation of Li(+) ions. This suggests the presence of a microdomain where the interactions can take place. Thus, the bidirectional interactions between Na(+)-K(+) pumps and h-channels are likely to be mediated by Na(+) microdomain. Consistent with these findings, hyperpolarization-activated and cyclic nucleotide-modulated subunits (HCN1/2) and the Na(+)-K(+) pumpalpha3 isoform were colocalized in plasma membrane of mesencephalic trigeminal neurons having numerous spines.

Neurobiological mechanisms involved in sleep bruxism

Sleep bruxism (SB) is reported by 8% of the adult population and is mainly associated with rhythmic masticatory muscle activity (RMMA) characterized by repetitive jaw muscle contractions (3 bursts or more at a frequency of 1 Hz). The consequences of SB may include tooth destruction, jaw pain, headaches, or the limitation of mandibular movement, as well as tooth-grinding sounds that disrupt the sleep of bed partners. SB is probably an extreme manifestation of a masticatory muscle activity occurring during the sleep of most normal subjects, since RMMA is observed in 60% of normal sleepers in the absence of grinding sounds. The pathophysiology of SB is becoming clearer, and there is an abundance of evidence outlining the neurophysiology and neurochemistry of rhythmic jaw movements (RJM) in relation to chewing, swallowing, and breathing. The sleep literature provides much evidence describing the mechanisms involved in the reduction of muscle tone, from sleep onset to the atonia that characterizes rapid eye movement (REM) sleep. Several brainstem structures (e.g., reticular pontis oralis, pontis caudalis, parvocellularis) and neurochemicals (e.g., serotonin, dopamine, gamma aminobutyric acid [GABA], noradrenaline) are involved in both the genesis of RJM and the modulation of muscle tone during sleep. It remains unknown why a high percentage of normal subjects present RMMA during sleep and why this activity is three times more frequent and higher in amplitude in SB patients. It is also unclear why RMMA during sleep is characterized by co-activation of both jaw-opening and jaw-closing muscles instead of the alternating jaw-opening and jaw-closing muscle activity pattern typical of chewing. The final section of this review proposes that RMMA during sleep has a role in lubricating the upper alimentary tract and increasing airway patency. The review concludes with an outline of questions for future research.

Involvement of 5-HT7 receptors in serotonergic effects on spike afterpotentials in presumed jaw-closing motoneurons of rats

Intracellular recordings were obtained from rat presumed jaw-closing motoneurons in slice preparations to investigate the involvement of the serotonin(7) (5-HT(7)) receptors in serotonergic inhibition of the postspike medium-duration afterhyperpolarization (mAHP) and enhancement of the afterdepolarization (ADP). 5-HT-induced suppression of the mAHP and enhancement of the ADP were mimicked by application of the 5-HT(1A/7) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and antagonized by the 5-HT(2/6/7) receptor antagonist clozapine, whereas the 5-HT(2) receptor agonist alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT) did not affect the mAHP and ADP. 8-OH-DPAT-induced attenuation of the mAHP and enhancement of the ADP were also antagonized by clozapine and another 5-HT(2/6/7) receptor antagonist ritanserin, whereas the 5-HT(1A) receptor antagonist pindolol failed to block the 8-OH-DPAT-induced effects on the mAHP and ADP. 8-OH-DPAT-induced suppression of the mAHP and enhancement of the ADP were also antagonized by a protein kinase A (PKA) inhibitor H89, whereas 8-OH-DPAT could inhibit the mAHP and enhance the ADP in the presence of a protein kinase C (PKC) inhibitor chelerythrine. The 8-OH-DPAT-induced suppression of the mAHP was enhanced under raised [Ca(2+)](o) and this enhancement was reduced by chelerythrine. It is suggested that the 5-HT(7) receptors are involved in 5-HT-induced attenuation of the mAHP and enhancement of the ADP through activation of PKA, and the attenuation of mAHP through the 5-HT(7) receptors is enhanced under raised [Ca(2+)](o) by PKC activation.

Effect of serotonin (5-HT) on trigeminal rhythmic activities generated in in vitro brainstem block preparations

We used rat isolated brainstem block preparations to analyze the functional roles of serotonin receptors in the generation of trigeminal rhythmic activities. We previously reported that trigeminal rhythmic activities could be induced by some pharmacological applications in an isolated brainstem preparation with a rostral boundary at the border between the inferior and superior colliculus, and a caudal border at the level of the rostral facial nucleus. However, the same stimulation did not induce trigeminal rhythmic activities in a whole brainstem block preparation with the same rostral boundary and a caudal border at the obex level. In the present study, both the 5-HT(1A) phthalimido-butyl-piperazine, and the 5-HT(2C) agonist, 1-2,5-dimethoxy-4-iodophenyl-2-aminopropane, combined with N-methyl-D,L-aspartate and bicuculline, elicited trigeminal rhythmic activities in a whole brainstem block preparation. Our results suggest that serotonin has both facilitation and inhibition effects on the generation of trigeminal rhythmic activities in an isolated brainstem block preparation in vitro.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Migraine and tension-type headache reduction through pericranial muscular suppression: a preliminary report

Migraine and tension-type headaches have always plagued mankind. In spite of all the research dollars spent trying to determine the etiologies of these headaches, the neurology community still has not established a known cause of migraine and tension type headaches. This paper describes a study that was conducted for the U.S. Food and Drug Administration in which the efficacy of the Nociceptive Trigeminal Inhibition Tension Suppression System was evaluated and proved safe and efficacious in the reduction of medically diagnosed migraine and tension-type headache.

Effects of post-natal serotonin levels on craniofacial complex

Although the role of serotonin (5-hydroxytryptamine or 5-HT) in pre-natal craniofacial growth and development has been studied, no research has been done on the effects of serotonin on post-natal craniofacial growth and development. The following experimental question was tested: What effect does increasing in vivo serotonin levels adjacent to trigeminal motoneurons have on post-natal craniofacial structures in young, actively growing rats? Forty male Sprague-Dawley rats were divided into 4 experimental groups (10% serotonin microspheres, 15% serotonin microspheres, blank microspheres, sham surgeries) and underwent stereotactic neurosurgery at post-natal day 35; 5 rats of each group were killed at 14 and 21 post-surgical days for data collection. Statistical analyses by mixed-model, 4 x 2 repeated-measures ANOVA, and post hoc Fisher LSD tests revealed significant (P < or = 0.05, 0.01) differences between groups and sides for muscle weight, cranial dimension, and TMJ dimension data. Data described here indicate that significant alterations of post-natal craniofacial structures can be caused by altered in vivo levels of serotonin adjacent to trigeminal motoneurons.

Serotonergic innervation of mesencephalic trigeminal nucleus neurons: a light and electron microscopic study in the rat

Neurons of the mesencephalic trigeminal nucleus (MTN) are considered to be homologous to mechanosensitive neurons in the sensory ganglia. The sites of origin of serotonin (5HT)-immunoreactive axons on neuronal cell bodies in the MTN were studied in the rat by combining immunofluorescence histochemical techniques with retrograde tracing of Fluoro-Gold (FG) and anterograde tracing of biotin-conjugated dextran amine (BDA). The tracing studies, which were combined with multiple-labeling immunohistochemistry and confocal microscopy, indicated that 5HT-immunoreactive axon terminals on the cell bodies of MTN neurons originated from the medullary raphe nuclei, such as the nucleus raphes magmus (RMg), alpha part of the nucleus reticularis gigantocellularis (GiA) and nucleus raphes obscurus (ROb), as well as from the mesopontine raphe nuclei, such as the nucleus raphes dorsalis (DR), nucleus raphes pontis (PnR) and nucleus raphes medianus (MnR); mainly from the RMg, GiA and DR, and additionally from the ROb, PnR and MnR. The cell bodies in close apposition to 5HT-immunoreactive axon terminals were found through the whole rostrocaudal extent of the MTN. Electron microscopically a number of axon terminals that were labeled with BDA injected into the raphe nuclei were confirmed to be in asymmetric synaptic contact with the cell bodies of MTN neurons. It was also indicated that substance P existed in some of the 5HT-containing axosomatic terminals arising from the ROb, RMg and GiA. The present results indicated that proprioceptive sensory signals from the muscle spindles or periodontal ligament might be modulated at the level of the primary afferent cell bodies in the MTN by 5HT-containing axons from the mesopontine and medullary raphe nuclei including the GiA.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Evidence for functional compartmentalization of trigeminal muscle spindle afferents during fictive mastication in the rabbit

Primary afferent neurons innervating muscle spindles in jaw-closing muscles have cell bodies in the trigeminal mesencephalic nucleus (NVmes) that are electrically coupled and receive synapses. Each stem axon gives rise to a peripheral branch and a descending central branch. It was previously shown that some spikes generated by constant muscle stretch fail to enter the soma during fictive mastication. The present study examines whether the central axon is similarly controlled. These axons were functionally identified in anaesthetized and paralysed rabbits, and tonic afferent firing was elicited by muscle stretch. For the purpose of comparison, responses were recorded extracellularly both from the somatic region and from the central axon in the lateral brainstem. Two types of fictive masticatory movement patterns were induced by repetitive stimulation of the masticatory cortex and monitored from the trigeminal motor nucleus. Field potentials generated by spike-triggered averaging of action potentials from the spindle afferents were employed to determine their postsynaptic effects on jaw-closing motoneurons. Tonic firing of 32% NVmes units was inhibited during the jaw-opening phase, but spike frequency during closing was almost equal to the control rate during both types of fictive mastication. A similar inhibition occurred during opening in 83% of the units recorded along the central branch. However, firing frequency in these was significantly increased during closing in 94%, probably because of the addition of antidromic action potentials generated by presynaptic depolarization of terminals of the central branch. These additional spikes do not reach the soma, but do appear to excite motoneurons. The data also show that the duration and/or frequency of firing during the bursts varied from one pattern of fictive mastication to another. We conclude that the central axons of trigeminal muscle spindle afferents are functionally decoupled from their stem axons during the jaw-closing phase of mastication. During this phase, it appears that antidromic impulses in the central axons provide one of the inputs from the masticatory central pattern generator (CPG) to trigeminal motoneurons.

Serotonergic modulation of the hyperpolarizing spike afterpotential in rat jaw-closing motoneurons by PKA and PKC

Intracellular recordings were obtained from rat jaw-closing motoneurons (JCMNs) in slice preparations to investigate the effects of serotonin (5-HT) on the postspike medium-duration afterhyperpolarization (mAHP) and an involvement of protein kinases in the effects. Application of 50 microM 5-HT caused membrane depolarization and increased input resistance in the most cells without affecting the mAHP, whereas not only membrane depolarization and an increase in input resistance, but also the suppression of the mAHP amplitude was induced by higher dose of 5-HT (100 or 200 microM). On the other hand, when the mAHP amplitude was increased by raising [Ca(2+)](o) from 2 to 6 mM, 5-HT-induced attenuation of the mAHP amplitude was enhanced, and even 50 microM 5-HT reduced the mAHP amplitude. This 5-HT-induced suppression of the mAHP could be mimicked by application of membrane-permeable cAMP analogue 8-Bromo-cAMP, potentiated by the cAMP-specific phosphodiesterase inhibitor Ro 20-1724 and antagonized by protein kinase A (PKA) inhibitor H89. The enhancement of the mAHP attenuation induced by 50 microM 5-HT under raised [Ca(2+)](o) was blocked by a protein kinase C (PKC) inhibitor chelerythrine, suggesting an involvement of PKC in this enhancement. On the other hand, the attenuation of the mAHP induced by PKC activator phorbol 12-myristate 13-acetate was blocked almost completely by H89, suggesting that the PKC action on the mAHP requires PKA activation. Neither 5-HT(1A) antagonist NAN-190 or 5-HT(4) antagonist SB 203186 blocked 5-HT-induced attenuation of the mAHP. We conclude that 5-HT induces dose-dependent attenuation of the mAHP amplitude through cAMP-dependent activation of PKA and that PKC-dependent PKA activation is also likely to be involved in the enhancement of 5-HT-induced attenuation of the mAHP under raised [Ca(2+)](o). Because the slope of the linear relationship between firing frequency and injected current was increased only when the mAHP amplitude was decreased by 5-HT, it is suggested that the relation between incoming synaptic inputs and firing output in JCMNs varies according to serotonergic effects on JCMNs and calcium-dependent modulation of its effects.

Brainstem mechanisms underlying feeding behaviors

The essential elements controlling trigeminal motoneurons during feeding lie between the trigeminal and facial motor nuclei. These include populations of neurons in the medial reticular formation and pre-motoneurons in the lateral brainstem that reorganize to generate various patterns. Orofacial sensory feedback, antidromic firing in spindle afferents and intrinsic properties of motoneurons also contribute to the final masticatory motor output.

Ionic basis for serotonin-induced bistable membrane properties in guinea pig trigeminal motoneurons

Intracellular recordings and pharmacological manipulations were employed to investigate the ionic basis for serotonin-induced bistable membrane behaviors in guinea pig trigeminal motoneurons (TMNs). In voltage clamp, 10 microM serotonin (5-HT) induced a region of negative slope resistance (NSR) in the steady-state current-voltage (I-V) relationship at potentials less negative than -58 mV, creating the necessary conditions for membrane bistability. The contributions of sustained Na+ and Ca2+ currents to the generation of the NSR were investigated using specific ion channel antagonists and agonists. The NSR was eliminated by the L-type Ca2+ channel antagonist nifedipine (5-10 microM), indicating the contribution of L channels. In nifedipine, inward rectification was present in the I-V relationship in a similar voltage range (greater than -58 mV). This region was subsequently linearized by tetrodotoxin (TTX), indicating the presence of a persistent Na+ current. When the 5-HT-induced NSR was eliminated by perfusion in low Ca2+ solution (0.4 mM), it was restored by the Na+ channel agonist veratridine (10 microM). Commensurate with bistability, in current clamp during bath application of 5-HT, plateau potentials were elicited by transient depolarizing or hyperpolarizing stimuli. Plateau potentials evoked by depolarization were observed under control and TTX conditions, but were blocked by nifedipine, suggesting the participation of an L-type Ca2+ current. Plateau potentials initiated after release from hyperpolarization (anode break) were blocked by 300 microM Ni2+, suggesting the responses relied on deinactivation of a T-type Ca2+ current. Conditional bursting was also observed in 5-HT. Nifedipine or low Ca2+ solutions blocked bursting, and the L-channel agonist Bay K 8644 (10 microM) extended the duration of individual bursts, demonstrating the role of L-type Ca2+ currents. Interestingly, when bursting was blocked by nifedipine or low Ca2+, it could be restored by veratridine application via enhancement of the persistent Na+ current. We conclude that bistable membrane behaviors in TMNs are mediated by L-type Ca2+ and persistent Na+ currents. 5-HT is associated with enhancement of TMN activity during oral-motor activity; the induction of bistable membrane properties by 5-HT represents a cellular mechanism for this enhancement.

Serotonin receptor mRNA expression in the hypoglossal motor nucleus

Brainstem serotonin (5-HT)-containing cells are remarkable for their widespread axonal projections and having their highest activity during wakefulness and lowest during rapid eye movement sleep. One important site of action of 5-HT is on upper airway motoneurons. However, which of the 14 known 5-HT receptors mediate the effects is uncertain. We used the reverse transcriptase/polymerase chain reaction to detect mRNA for six distinct 5-HT receptors (1A, 1B, 2A, 2C, 3 and 7) in 50 nl micro-punches collected from the hypoglossal (XII) motor nucleus and, for comparison, from the viscerosensory nucleus of the solitary tract (NTS) in adult rats. The relative abundance of the distinct mRNAs was characterized by the minimal number of amplification cycles (25-40) necessary to detect a given mRNA. In the XII nucleus, mRNA for type 1B, 2A and 2C receptors was detectable after 29-31 cycles, detection of type 3 and 7 receptor mRNA required 33-35 cycles; and type 1A receptor mRNA was not detected. In the NTS, detection of mRNA for type 1B, 2C and 7 receptors required 31-33 cycles; type 1A receptor mRNA required 39 cycles; and type 2A receptor mRNA was not detected. The data from the XII nucleus demonstrate that not only the previously recognized type 1B, 2A and 2C receptors, but also type 3 and 7 receptors have the potential to mediate serotonergic effects in XII motoneurons.

Mesencephalic trigeminal nucleus neurons supplying the jaw closing muscles have no spinal projection: a fluorescent double-labeling study in birds and mammals

BACKGROUND

The present study deals with the possibility that the mesencephalic trigeminal nucleus (MeV) neurons that innervate the muscle spindles of the jaw closing muscles may also have collaterals projecting to the cervical spinal cord. At the same time, we reexamine the morphology of these cells and their location within the MeV.

METHODS

The fluorescent retrograde tracers Fast Blue (FB) and Diamidino Yellow dihydrochloride (DY) were injected into the jaw closing muscles and C2-C3 spinal cord segments, respectively, of duck, rat, and rabbit in one series of experiments. In a second series of animals, the targets of the tracers were reversed.

RESULTS

Retrogradely double-labeled cells (FB+DY) were not found in the MeV. On the contrary, the tracer injected into the muscles retrogradely labeled only large unipolar MeV cells, whereas the tracer injected into C2-C3 spinal cord segments labeled only small multipolar cells which were intermingled with the MeV somata of muscle spindle afferents.

CONCLUSIONS

These findings exclude the possibility of spinal projections via collaterals of MeV cells supplying muscle spindles of jaw closing muscles in duck, rat, and rabbit. Moreover, the retrograde double-labeling technique evidences two cellular populations within the MeV of the duck, rat, and rabbit: large unipolar neurons which are the cell bodies of primary afferents from jaw closing muscles and small multipolar cells projecting to the upper cervical spinal cord.

Multiple effects of serotonin on membrane properties of trigeminal motoneurons in vitro

Intracellular recordings from guinea pig trigeminal motoneurons (TMNs) in brain stem slices were used to determine the underlying ionic mechanisms responsible for our previously demonstrated enhancement of TMN excitability during jaw movements by serotonin (5-HT). 5-HT (0.5-100 microM) depolarized motoneurons and increased input resistance in the majority of neurons tested. Additionally, 5-HT reduced the amplitude of the postspike medium-duration afterhyperpolarization, decreased the current threshold for maintained spike discharge, and increased the maximum slope of the steady-state spike frequency-current relationship. Under voltage clamp, from holding potentials close to resting potential, 5-HT produced an inward current and a decrease in instantaneous slope conductance, suggesting a reduction in a resting K+ leak conductance (I(leak)). The instantaneous current-voltage (I-V) relationship for the inward 5-HT current (I(5-HT)) was linear throughout most of the voltage range tested. However, the steady-state I-V relationship showed some degree of inward rectification at potentials starting around -70 mV. The mean reversal potential for the instantaneous I(5-HT) was -86.2 +/- 4.5 (SE) mV (n = 9), a value slightly negative to the predicted potassium equilibrium potential of -82 mV in these neurons. In the presence of 2 mM Ba2+, 5-HT application did not produce a further reduction in input conductance, but did expose a Ba2+-insensitive residual inward current that was resistant to Cs+ application. The instantaneous I-V relationship during 5-HT application in the presence of Ba2+ was shifted downward and parallel to control, suggesting that Ba2+ and 5-HT block the same resting I(leak). The residual Ba2+- and Cs+-insensitive component of the total inward I(5-HT) was voltage independent and was blocked when the extracellular Na+ was replaced by choline, suggesting that the predominant charge carrier for this residual current is Na+. 5-HT enhanced a hyperpolarization-activated cationic current, I(h). In the presence of Ba2+, the time course of I(5-HT) resembled that of I(h) and showed a similar voltage dependence that was blocked by extracellular Cs+ (1-3 mM). The effects of 5-HT on membrane potential, input resistance, and I(h) were partially mimicked by 5-HT2 agonists and suppressed by 5-HT2 antagonists. It is concluded that 5-HT enhances TMN membrane excitability through modulation of multiple intrinsic membrane conductances. This provides for a mechanism(s) to fine tune the input-output discharge properties of these neurons, thus providing them with greater flexibility in output in response to time-varying synaptic inputs during various movements of the jaw.

Serotonergic neurons in the caudal raphe nuclei discharge in association with activity of masticatory muscles

There is a dense serotonergic projection from nucleus raphe pallidus and nucleus raphe obscurus to the trigeminal motors nucleus and serotonin exerts a strong facilitatory action on the trigeminal motoneurons. Some serotonergic neurons in these caudal raphe nuclei increase their discharged during feeding. The objective of the present study was to investigate the possibility that the activity of these serotonergic neurons is related to activity of masticatory muscles. Cats were implanted with microelectrodes and gross electrodes. Caudal raphe single neuron activity, electrocorticographic activity, and splenius, digastric and masseter electromyographic activities were recorded during active behaviors (feeding and grooming), during quiet waking and during sleep. Seven presumed serotonergic neurons were identified. These neurons showed a long duration action potential (> 2.0 msec) and discharged slowly (2-7 Hz) and very regularly (interspike interval coefficient of variation < 0.3) during quiet waking. The activity of these neurons decreased remarkably during fast wave sleep (78-100%). Six of these neurons showed tonic changes in their activity positively related to digastric and/or masseter muscles activity but not to splenius muscle activity during waking. These data are consistent with the hypothesis that serotonergic neurons in the caudal raphe nuclei play an important role in the control of jaw movements.

Peptidergic innervation of the mesencephalic trigeminal nucleus in the cat

BACKGROUND

The trigeminal processing of proprioceptive information is unique and very little is known about the neurochemical organization of trigeminal primary afferent neurons which mediate the sensory aspects of proprioception. In studies using immunocytochemicalretrograde tracing techniques, some classical neurotramsitters mediating the afferent modulation of the mesencephalic trigeminal nucleus (MTN) have been investigated. This paper summarizes our current understanding of the peptidergic innervation of the cat MTN.

METHODS

The distribution of immunoreactive substances was studied using specific antisera against 11 major neuropeptides. Light and electron microscopic peroxidase-antiperoxidase immunocytochemical staining techniques in colchicine-treated animals were used to clarify the distribution of peptide-identified fibers related to the MTN.

RESULTS

Immunoreactivity to any of the tested neuropeptides could not be detected in the MTN cell bodies. Numerous fibers containing various peptides such as substance P, bombesin, enkephalins, cholecystokinin, vasoactive intestinal polypeptide, vasopressin, and neuropeptide Y were present in the nucleus, however. These thin positive fibers covered the neuronal surface of the MTN cell bodies and some of the immunoreactive varicosities appeared to be in close proximity to profiles of MTN neurons. Electron microscopic observations revealed that perisomatic fibers were in direct apposition to perikarya of unstained large cells and some of them made synaptic contacts with their cell bodies and dendrites.

CONCLUSIONS

The present results demonstrate that the MTN neurons receive dense basket-like innervation from peptidergic neurons on somata and processes and have supported earlier evidence that the MTN of the cat is under influence of peptidergic input. Results of this study provide further evidence that the neuropeptides examined may play an important role in the integration and transmission of trigeminal proprioceptive information. Most likely they may co-exist with a classical but hitherto unknown neurotransmitter(s), that is unique for this region and whose release can be modulated by peptides.

Morphological evidence for recurrent jaw-muscle spindle afferent feedback within the mesencephalic trigeminal nucleus

Horseradish peroxidase was injected into the jaw-elevator muscles of rats. Jaw-muscle spindle afferent axons were then intracellularly stained with biotinamide. Eleven intracellularly stained somata were closely apposed by intracellularly labeled spindle afferent boutons. Spindle afferent boutons closely apposed another 10 mesencephalic trigeminal nucleus (Vme) perikarya labeled from the jaw muscles. These results indicate that an anatomical substrate exists for recurrent feedback between jaw-muscle spindle afferents within Vme.

Immunocytochemical localization of tyrosine hydroxylase and gamma-aminobutyric acid in the mesencephalic trigeminal nucleus of the cat: a light and electron microscopic study

BACKGROUND

Recent studies conducted on the rat have demonstrated that the mesencephalic trigeminal nucleus (MTN) neurons, involved in the proprioceptive transmission, contain some neuroactive substances, including classical and amino acid neurotransmitters. In addition, there is evidence that serotonin could not act as a neurotransmitter at the first synaptic relay in the cat MTN. In the present study, we aimed to examine two other possible neurotransmitter systems, i.e., catecholamines and gamma-aminobutyric acid (GABA), and the relationships between GABA-immunoreactive (IR) neurons and tyrosine hydroxylase (TH)-IR axonal varicosities in the MTN of the cat.

METHODS

To ensure the localization of immunoreactive structures, the experiments were carried out at the light and electron microscopic level using single immunostaining for TH and GABA alone. The correlation between GABA-IR cell bodies and TH-IR fibers was investigated by means of double-labeling immunogold and peroxidase technique for GABA and TH.

RESULTS

Light microscopically, a few GABA-IR neurons were observed in the cat MTN. These small-size, labeled cells, most likely interneurons, were apposed to unstained large mesencephalic trigeminal cells. Most of the large nonreactive MTN neurons were closely surrounded by fine TH-IR varicose or nonvaricose fibers and dot-like structures, presumably nerve terminals. Under the electron microscope, TH-IR fibers were not seen in synaptic contact and only rarely appeared to be in close proximity to neuronal profiles of small GABAergic cells, which contained gold particles.

CONCLUSIONS

Taken together with earlier studies from other laboratories, the present findings suggest that GABAergic system might play an indirect role in the proprioceptive information processing in the cat MTN by interactions of GABA-immunoreactive neurons with the systems that control the transmission of selected sensory information. In contrast, the presence of TH-IR fibers in direct apposition to the majority of MTN neurons provide further evidence that presumed GABAergic interneurons have extensive interactions with catecholamine varicosities and raise the possibility that catecholamines could modify the transmission on these neurons.

5-HT1A receptor localization on the axon hillock of cervical spinal motoneurons in primates

Serotonin (5-HT) has direct and specific effects on the activity of spinal cord motoneurons. The 5-HT1A receptor has been shown to mediate motoneuron responses in spinal reflex pathways using the highly selective 5-HT1A receptor agonist 8-OH-DPAT. We have developed an antipeptide antibody that recognizes a specific region (the second external loop) of the 5-HT1A receptor. This 5-HT1A receptor antibody labels populations of neurons and glia in the primate cervical spinal cord. The highest receptor density is present in the superficial lamina of the dorsal horn, around the central canal, and on the axon hillock of large ventral horn motoneurons. The cellular labeling pattern on motoneurons shows a single, densely stained, tapering process emanating from the perikaryon. A more diffuse label is also present throughout the soma. Dendritic labeling was not apparent. These results suggest that post-synaptic 5-HT1A receptors may be involved in modulating spinal motoneuron activity at the key site of action potential initiation, the axon hillock.

Serotonin-containing projections to the mesencephalic trigeminal nucleus of the cat

BACKGROUND

It is well known that the mesencephalic trigeminal nucleus (MTN) neurons transmit somatosensory information from proprioceptors in the oral-facial region. Several mechanisms of sensory transduction in these specialized receptors have been proposed, but the neurotransmitters which are responsible for mediating proprioceptive information are still unknown. The current study concentrates on the distribution of one putative neurotransmitter system, serotonin (SER), in the cat MTN. A second objective was to clarify the location and sources of serotoninergic projections on the MTN neurons.

METHODS

To determine whether SER was localized in the MTN, the peroxidase-antiperoxidase (PAP) immunocytochemical technique was applied at light and electron microscopic levels in colchicine-treated animals. The origin of SER-containing fibers in the MTN was studied using a double-labeling method combining retrograde transport with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and SER immunocytochemistry.

RESULTS

There were no SER-containing neurons in the MTN. The cell bodies of immunonegative MTN neurons were closely surrounded by fine SER-positive fibers and terminals. The labeled fibers were in most cases very thin and sometimes varicose. Ultrastructurally, direct synaptic contacts between SER-containing terminals and perikarya of MTN neurons of all sizes could be seen. The majority of SER-labeled structures were synaptic terminals in which the immunoreactive material was located within the small round clear as well as the small granular vesicles (diameter 50-80 nm) and a few large dense-cored vesicles (up to 150 nm). Retrograde tracing demonstrated that most of cells in the nuclei raphe dorsalis, pontis and magnus were WGA-HRP-labeled.

CONCLUSIONS

These results indicated that MTN neurons received serotoninergic projections from the raphe nuclei of the brainstem. In light of these morphological data, it is concluded that the MTN of the cat is under the influence of SER-containing axons and this serotoninergic input may modulate MTN neuronal activity at the first synaptic relay.

The sites of origin of serotoninergic afferent fibers in the trigeminal motor, facial, and hypoglossal nuclei in the rat

The sites of origin of serotoninergic afferents in the trigeminal motor (Vm), facial (VII), and hypoglossal nuclei (XII) were studied in the rat by fluorescent retrograde labeling with Fluoro-Gold, in combination with immunofluorescence histochemistry for serotonin (5-HT). The results indicated: (1) The nucleus raphe magnus, nucleus raphe pallidus, and nucleus raphe obscurus contained 5-HT neurons projecting to the Vm, VII or XII. (2) The nucleus raphe dorsalis sends 5-HT fibres to the Vm and VII, but not to the XII. (3) The gigantocellular reticular nucleus pars alpha contained 5-HT neurons projecting to the VII.