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Neural circuits engaged in mastication and orofacial nociception
Umeå University, Faculty of Medicine, Integrative Medical Biology, Physiology. Umeå University, Faculty of Medicine, Odontology.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A deeper understanding of both movement control and the effects of nociceptor inputs on our motor systems is critical for proper clinical diagnosis of musculo-skeletal dysfunctions and for development of novel rehabilitation schemes. In the jaw system, masticatory movements are produced by a central pattern generator (CPG) located in the brainstem. Considerable efforts have been made in deciphering this neuronal network. The present thesis contributes towards an increasingly detailed understanding of its essential elements, and presents a hypothesis of how deep somatic pain (i.e. muscle pain) may be evoked and interferes with the masticatory CPG circuitry.

In Paper I, the expression of c-Fos-like protein was used as a molecular marker to visualize brainstem neurons that were active during induced fictive mastication in the anesthetized and paralyzed rabbit. Our findings provide a previously lacking detailed record of the neuronal populations that form the masticatory motor pattern. Certain cells were located in brainstem areas previously suggested to be involved in the masticatory CPG. However, it was a new finding that neurons in the dorsal part of the trigeminal main sensory nucleus (NVsnpr-d) may belong to this circuitry. Paper II focused on the discovered neurons in NVsnpr in an in vitro slice preparation from young rats.  Intracellular recordings allowed us to define two cell types based on their response to depolarizing current. Microstimulation applied to the trigeminal motor nucleus, its reticular border, the parvocellular reticular formation and the nucleus reticularis pontis caudalis, elicited postsynaptic potentials in 81% of the neurons tested. Responses obtained were predominately excitatory and sensitive to gluta-matergic antagonists DNQX or/and APV. Some inhibitory and biphasic responses were also evoked. Bicuculline methiodide or strychnine blocked the IPSPs indicating that they were mediated by GABAA or glycinergic receptors. About one third of the stimulations activated both types of neurons antidromically. Neurons in NVsnpr-d seem to gather all the conditions that can theoretically account for a role in masticatory rhythm generation.

In Paper III, the masticatory model system was used to investigate the possible role of muscle spindle primary afferents in development of persistent musculoskeletal pain. Following intramuscular acidic (pH 4.0) saline injections of rat masseter muscles, in vitro whole cell recordings were done from jaw closing muscle spindle somata located in the trigeminal mesencephalic nucleus (NVmes). Compared to control neurons, the somata of afferents exposed to acid had more hyperpolarized membrane potentials, more hyperpolarized thresholds for firing, high frequency membrane oscillations and ectopic bursting of action potentials. These changes in membrane properties lasted for up to 35 days. Within the same time frame experi-mental animals showed hypersensitivity to touch on the skin covering the injected muscle. Similar saline injections also resulted in a significant increase of activity dependent c-Fos expression in NVmes neurons compared to controls. Immuno-fluorescence and lectin binding studies indicated that small-caliber muscle afferents containing known nociceptor markers (CGRP, SP, P2X3, TRPV1 and IB4) and expressing glutamate receptors are found close to the annulo-spiral endings of the NVmes afferents. Combined, our new observations support the hypothesis that excessive release of glutamate, within muscle spindles due to ectopically evoked antidromic action potentials, could lead to development of persistent musculoskeletal pain by activation and/ or sensitization of adjacent muscle afferent nociceptors.

Place, publisher, year, edition, pages
Umeå: Umeå university , 2009. , 58 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1288
Keyword [en]
rhythmical jaw movements; central pattern generator; trigeminal system; rabbit; Rat; c-Fos, nociceptors; muscle pain; muscle spindle afferents; glutamate
National Category
Physiology
Research subject
Physiology
Identifiers
URN: urn:nbn:se:umu:diva-26342ISBN: 978-91-7264-847-0 (print)OAI: oai:DiVA.org:umu-26342DiVA: diva2:241956
Distributor:
Fysiologi, 901 87, Umeå
Public defence
2009-10-28, Biologihuset, Bi201, Umeå universitet, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2009-10-07 Created: 2009-10-06 Last updated: 2010-01-18Bibliographically approved
List of papers
1. Identification of c-Fos immunoreactive brainstem neurons activated during fictive mastication in the rabbit
Open this publication in new window or tab >>Identification of c-Fos immunoreactive brainstem neurons activated during fictive mastication in the rabbit
2005 (English)In: Experimental Brain Research, ISSN 0014-4819, E-ISSN 1432-1106, Vol. 165, no 4, 478-489 p.Article in journal (Refereed) Published
Abstract [en]

In the present study we used the expression of the c-Fos-like protein as a "functional marker" to map populations of brainstem neurons involved in the generation of mastication. Experiments were conducted on urethane-anesthetized and paralyzed rabbits. In five animals (experimental group), rhythmical bouts of fictive masticatory-like motoneuron activity (cumulative duration 60-130 min) were induced by electrical stimulation of the left cortical "masticatory area" and recorded from the right digastric motoneuron pool. A control group of five animals (non-masticatory) were treated in the same way as the experimental animals with regard to surgical procedures, anesthesia, paralysis, and survival time. To detect the c-Fos-like protein, the animals were perfused, and the brainstems were cryosectioned and processed immunocytochemically. In the experimental group, the number of c-Fos-like immunoreactive neurons increased significantly in several brainstem areas. In rostral and lateral areas, increments occurred bilaterally in the borderzones surrounding the trigeminal motor nucleus (Regio h); the rostrodorsomedial half of the trigeminal main sensory nucleus; subnucleus oralis-gamma of the spinal trigeminal tract; nuclei reticularis parvocellularis pars alpha and nucleus reticularis pontis caudalis (RPc) pars alpha. Further caudally-enhanced labeling occurred bilaterally in nucleus reticularis parvocellularis and nucleus reticularis gigantocellularis (Rgc) including its pars-alpha. Our results provide a detailed anatomical record of neuronal populations that are correlated with the generation of the masticatory motor behavior.

Keyword
Anesthesia, Animals, Brain Stem/*metabolism, Data Interpretation; Statistical, Efferent Pathways/cytology/physiology, Electric Stimulation, Immunohistochemistry, Male, Mastication/*physiology, Masticatory Muscles/innervation/physiology, Microelectrodes, Motor Cortex/physiology, Motor Neurons/physiology, Neurons/*metabolism, Paralysis/physiopathology, Proto-Oncogene Proteins c-fos/*metabolism, Rabbits
Identifiers
urn:nbn:se:umu:diva-12592 (URN)10.1007/s00221-005-2319-5 (DOI)15887006 (PubMedID)
Available from: 2008-01-09 Created: 2008-01-09 Last updated: 2017-12-14Bibliographically approved
2. Physiological characterization, localization and synaptic inputs of bursting and nonbursting neurons in the trigeminal principal sensory nucleus of the rat
Open this publication in new window or tab >>Physiological characterization, localization and synaptic inputs of bursting and nonbursting neurons in the trigeminal principal sensory nucleus of the rat
2005 (English)In: European Journal of Neuroscience, ISSN 0953-816X, E-ISSN 1460-9568, Vol. 22, no 12, 3099-3110 p.Article in journal (Refereed) Published
Abstract [en]

A population of neurons in the trigeminal principal sensory nucleus (NVsnpr) fire rhythmically during fictive mastication induced in the in vivo rabbit. To elucidate whether these neurons form part of the central pattern generator (CPG) for mastication, we performed intracellular recordings in brainstem slices taken from young rats. Two cell types were defined, nonbursting (63%) and bursting (37%). In response to membrane depolarization, bursting cells, which dominated in the dorsal part of the NVsnpr, fired an initial burst followed by single spikes or recurring bursts. Non-bursting neurons, scattered throughout the nucleus, fired single action potentials. Microstimulation applied to the trigeminal motor nucleus (NVmt), the reticular border zone surrounding the NVmt, the parvocellular reticular formation or the nucleus reticularis pontis caudalis (NPontc) elicited a postsynaptic potential in 81% of the neurons tested for synaptic inputs. Responses obtained were predominately excitatory and sensitive to glutamatergic antagonists DNQX and/or APV. Some inhibitory and biphasic responses were also evoked. Bicuculline methiodide or strychnine blocked the IPSPs indicating that they were mediated by GABA(A) or glycinergic receptors. About one-third of the stimulations activated both types of neurons antidromically, mostly from the masseteric motoneuron pool of NVmt and dorsal part of NPontc. In conclusion, our new findings show that some neurons in the dorsal NVsnpr display both firing properties and axonal connections which support the hypothesis that they may participate in masticatory pattern generation. Thus, the present data provide an extended basis for further studies on the organization of the masticatory CPG network.

Keyword
Action Potentials/drug effects/*physiology/radiation effects, Animals, Animals; Newborn, Biotin/analogs & derivatives/metabolism, Dose-Response Relationship; Radiation, Drug Interactions, Electric Stimulation/methods, Evoked Potentials/drug effects/physiology/radiation effects, Excitatory Amino Acid Antagonists/pharmacology, Glycine Agents/pharmacology, Neurons/*classification/drug effects/*physiology/radiation effects, Patch-Clamp Techniques/methods, Quinoxalines/pharmacology, Rats, Rats; Sprague-Dawley, Strychnine/pharmacology, Synaptic Transmission/drug effects/*physiology/radiation effects, Trigeminal Nuclei/*cytology
Identifiers
urn:nbn:se:umu:diva-12591 (URN)10.1111/j.1460-9568.2005.04479.x (DOI)16367776 (PubMedID)
Available from: 2008-01-09 Created: 2008-01-09 Last updated: 2017-12-14Bibliographically approved
3. Evidence that muscle spindle mechanoreceptor afferents play a role in chronic muscle pain
Open this publication in new window or tab >>Evidence that muscle spindle mechanoreceptor afferents play a role in chronic muscle pain
Show others...
(English)Manuscript (preprint) (Other academic)
Keyword
pain, muscle spindle, nociceptor, trigeminal, glutamate receptor, VGLUT1, P2X3, CGRP, substance P, TRPV1
Identifiers
urn:nbn:se:umu:diva-26340 (URN)
Available from: 2009-10-06 Created: 2009-10-06 Last updated: 2010-06-23Bibliographically approved

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