Barry Sessle - US grants

The overall objective of the research of program is the clarification of trigeminal nociceptive mechanisms, especially in the brainstem, related to acute and chronic pain conditions that afflict the face and mouth. Our recent NIH-supported studies, which have produced 10 papers and 7 abstracts in 1981 and 1982, have helped provide some of the insights into the brainstem mechanisms involved in dental and facial pain in particular. Major gaps or uncertainties in our knowledge however still exist in the neural mechanisms underlying dental, joint and muscle pain especially and in the processes associated with pain conditions that may be related to muscle dysfunction (e.g. TMJ or myofascial pain dysfunction syndrome) or to sensory loss (e.g. painful sensory neuropathies, neuralgias, causalgias). We will use our expertise and experience gained over the last 18 years with the trigeminal system of the cat and monkey and continue to examine functionally identified single noticeptive and nonnociceptive neurons recorded electrophysiologically in the trigeminal spinal tract nucleus of the cat. In particular, we now propose to determine which types of neurons relay joint and musle nociceptive information and to study the response properties and convergent patterns seen in these neurons with stimulation of muscle, joint and other orofacial afferents (Aim i); to determine if these responses can be modulated by other sensory inputs and by descending influences from brainstem, cortex and thalamic sites implicated in pain and analgesia and gain insights into the underlying neurochemical mechanisms by studying the effects on the modulatory influences of antagonists to the possible neurochemicals involved (Aim ii); and continue to investigate the effects of sensory loss on the functional organization of these neurons by delineating further the changes in their functional properties that we have shown to occur with tooth pulp extirpation and comparing them with likely changes that may be induced by deafferentation of other orofacial structures or interruption of the modulatory interaction that has been demonstrated between different components of the nucleus (Aim iii). We anticipate that our findings will lead to future studies in kittens and chronic recording investigations in adult animals to examine maturational and behavioral changes that may be associated with orofacial sensory alterations.

The long-term objectives of our research program are to clarify the central mechanisms underlying acute and chronic dental and orofacial pain and its control. Our recent NIH-supported research has resulted in major new insights into the neuroplasticity of the trigeminal (V) brainstem complex, and into brainstem mechanisms underlying deep (e.g. muscle; and temporomandibular joint, TMJ) as well as cutaneous (facial) pain. THE EXPRESSION OF NEUROPLASTICITY THAT WE DOCUMENTED IN THE ADULT v BRAINSTEM COMPLEX WAS REVEALED BY ENDODONTIC DEAFFERENTATION OF THE TOOTH PULP. IN THE SPINAL SOMATOSENSORY SYSTEM, PERIPHERAL DEAFFERENTATION CAN ALSO INDUCE ALTERATIONS IN SOMATOSENSORY PATHWAYS THAT HAVE BEEN VIEWED AS A REFLECTION OF CNS NEUROPLASTICITY AND AS PROCESSES CONTRIBUTING TO THE DEVELOPMENT OF CHRONIC PAIN. THE MECHANISMS AS PROCESSES CONTRIBUTING TO THE DEVELOPMENT OF CHRONIC PAIN. THE MECHANISMS UNDERLYING THESE CHANGES IN INHIBITION OF EXISTING AFFERENT INPUTS. OUR STUDIES OF V BRAINSTEM CELLS INDICATED THAT MAXILLARY OR MANDIBULAR PULP DEAFFERENTATION IN ADULT ANIMALS RESULTED IN STATISTICALLY SIGNIFICANT ALTERATIONS IN MECHANORECEPTIVE FIELD AND RESPONSE PROPERTIES THAT WERE ESPECIALLY APPARENT IN ORALS NEURONS; THESE EFFECTS WERE SIGNIFICANTLY PROLONGED WITH MORE EXTENSIVE PULP DEAFFERENTATION. AS FOR THE SPINAL SYSTEM, HOWEVER, the mechanisms underlying our documented physiological changes after deafferentation of the pulp are unclear. THEREFORE, we will test hypotheses that mandibular pulp deafferentation is associated with (A) SPROUTING OF MAXILLARY AFFERENTS INTO REGIONS OF V SUBNUCLEUS ORALIS NORMALLY DEVOTED TO THE REPRESENTATION OF THE MANDIBULAR DIVISION; (B) a decrease in afferent inhibition; and (C) a decrease in primary afferent depolarization (PAD) WHICH IS CONSIDERED A REFLECTION OF PRESYNAPTIC INHIBITION. Comparison of these features between normal and deafferented adult animals will be made in V brainstem neurons or primary afferents IN QUANTITATIVE ANALYSES UTILIZING, FOR HYPOTHESES B & C, OUR WELL-DOCUMENTED EXPERTISE IN EXTRACELLULAR SINGLE UNIT RECORDING. TO ADDRESS HYPOTHESIS A, NEW APPROACHES FOR OUR LAB INVOLVING HRP LABELLING OF V NERVE BRANCHES AND INTRACELLULAR HRP LABELLING OF PHYSIOLOGICALLY IDENTIFIED SINGLE UNITS WILL BE USED. THESE STUDIES WILL CLARIFY SOME OF THE MECHANISMS THAT MAY BE INVOLVED IN NEUROPLASTIC RESPONSES OF THE V BRAINSTEM COMPLEX TO INJURY OF THE TOOTH PULP AND THAT MAY UNDERLIE THE DEVELOPMENT OF CHRONIC OROFACIAL PAIN.

Our long-term objectives is to elucidate the central mechanisms and neuroblastic underlying acute and chronic pain. While alterations in C- fiber function have been implicated in several chronic pain conditions, the role of C-fiber afferent in the normal development of neuronal properties in central somatosensory pathways, let alone in the development of disordered pain behavior, is still unclear. C-fibers do provide nociceptive afferent excitatory inputs to V brainstem and spinal and spinal neurons, but recent studies have drawn attention to the possible neuroeffector and neurotrophic influences that C fibers may also exert; these neuroblastic influence including shaping neuronal receptive field and response properties in both nociceptive and non-nociceptive pathways. Do C-fibers have a similar role in the normal development of these properties i the V somatosensory system? We will address this question, by testing: Hypothesis I, that the neonatal depletion of C- fiber afferent produces alterations in the receptive field and response properties of nociceptive neurons in subnucleus caudalis of the adult V brainstem complex; Hypothesis II, that the neonatal depletion of C-fiber afferent produces alterations in the receptive field and response properties of low-threshold mechanoreceptive (LTM) neurons in the subnucleus caudalis of the adult V brainstem complex; and Hypothesis III, that the neonatal depletion of C-fiber afferent produces alterations in the receptive field and response properties of low-threshold mechanoreceptive (LTM) neurons in the main sensory nucleus of the adult V brainstem complex. Since C-fiber primary afferent do not project to the V main sensory nucleus changes associated with neonatal capsaicin administration is an alteration to the modulatory influence that caudalis exerts on main sensory neurons; we will therefore also test: Hypothesis IV, that the neonatal depletion of C-fiber afferent produces alteration in the modulatory influences of subnucleus caudalis on the receptive field and response properties of low-threshold mechanoreceptive (LTM) neurons in the main sensory nucleus of the adult V brainstem complex. For each series of experiments, neonatal rats will be injected with capsaicin to deplete their C-fiber afferent and then at 2-3 months of age we will characterize, and compare with control animals, the properties of neurons in V subnucleus caudalis, which acts as the as the primary brainstem relay of nociceptive information, or in the main sensory nucleus which acts as the principal LTM brainstem relay in the vibrissa pathway to cortex. In both groups of animals, main sensory neuronal properties will also be tested during manipulation of the ascending caudalis modulatory influences by local anesthetic or glutamate injections of caudalis. The significance of these studies lies in their integral link to neuroblastic processes operating in the V brainstem complex and the determinants of receptive field and response properties of V somatosensory neurons, and to the view that altered C-fiber function may be involved in the development of several chronic pain conditions.

DESCRIPTION (adapted from applicant's abstract): The long-term objective of our NIH-supported research is to elucidate the central mechanisms and neuroplastic processes underlying acute and chronic dental and orofacial pain conditions and their control. Our recent data have revealed tooth pulp-induced neuroplastic changes in nociceptive brainstem neurons of the rat subnucleus oralis and caudalis that involve N-methyl-D-aspartate (NMDA) mechanisms. These changes appear to reflect a process analogous to the "central sensitization" recently described in spinal nociceptive pathways that has been implicated in the development of the hyperalgesia and spread and referral of pain that may occur after injury and inflammation of peripheral tissues. The relative importance of oralis and caudalis to the central expression and modulation of these nociceptive phenomena is however still unclear, and there is very limited information available of thalamic nociceptive mechanisms and neuroplasticity associated with the central mediation of pulp pain. Given the well-documented role of caudalis in orofacial pain mechanisms and its direct projections to both thalamus and oralis, and the limited information on brainstem and thalamic neuroplasticity, it is proposed to use single neuron recordings in anesthetized rats, to address Hypothesis I: The pulp-induced neuroplastic changes in subnucleus oralis nociceptive neurons can be manifested in ventrobasal thalamic neurons and are dependent on subnucleus caudalis; and Hypothesis II: Pulp-evoked neuronal discharges but not pulp-induced neuroplastic changes in ventrobasal thalamic neurons are primarily dependent on subnucleus oralis. The properties of ventrobasal thalamic neurons will be documented before and after molar pulp stimulation in rats with or without disruption of caudalis or oralis to determine if neuroplastic changes are manifested in thalamic nociceptive neurons and non-nociceptive neurons and if these changes an other neuronal properties are dependent on caudalis or oralis. Hypothesis III: Pulp-induced neuroplastic changes in subnucleus oralis nociceptive neurons but not pulp-evoked oralis neuronal discharges are dependent on subnucleus. The properties of oralis neurons will similarly be assessed to determine if the pulp-induced oralis neuroplastic changes and other neuronal properties are dependent on caudalis. This project will provide further new insights into the central processing of pulp afferent information and its relationship to orofacial pain and inflammation.

DESCRIPTION (adapted from applicant's abstract): The long-term objective of our NIH-supported research is to elucidate the central mechanisms and neuroplastic processes underlying acute and chronic dental and orofacial pain conditions and their control. Our recent data have revealed tooth pulp-induced neuroplastic changes in nociceptive brainstem neurons of the rat subnucleus oralis and caudalis that involve N-methyl-D-aspartate (NMDA) mechanisms. These changes appear to reflect a process analogous to the "central sensitization" recently described in spinal nociceptive pathways that has been implicated in the development of the hyperalgesia and spread and referral of pain that may occur after injury and inflammation of peripheral tissues. The relative importance of oralis and caudalis to the central expression and modulation of these nociceptive phenomena is however still unclear, and there is very limited information available of thalamic nociceptive mechanisms and neuroplasticity associated with the central mediation of pulp pain. Given the well-documented role of caudalis in orofacial pain mechanisms and its direct projections to both thalamus and oralis, and the limited information on brainstem and thalamic neuroplasticity, it is proposed to use single neuron recordings in anesthetized rats, to address Hypothesis I: The pulp-induced neuroplastic changes in subnucleus oralis nociceptive neurons can be manifested in ventrobasal thalamic neurons and are dependent on subnucleus caudalis; and Hypothesis II: Pulp-evoked neuronal discharges but not pulp-induced neuroplastic changes in ventrobasal thalamic neurons are primarily dependent on subnucleus oralis. The properties of ventrobasal thalamic neurons will be documented before and after molar pulp stimulation in rats with or without disruption of caudalis or oralis to determine if neuroplastic changes are manifested in thalamic nociceptive neurons and non-nociceptive neurons and if these changes an other neuronal properties are dependent on caudalis or oralis. Hypothesis III: Pulp-induced neuroplastic changes in subnucleus oralis nociceptive neurons but not pulp-evoked oralis neuronal discharges are dependent on subnucleus. The properties of oralis neurons will similarly be assessed to determine if the pulp-induced oralis neuroplastic changes and other neuronal properties are dependent on caudalis. This project will provide further new insights into the central processing of pulp afferent information and its relationship to orofacial pain and inflammation.

[unreadable] DESCRIPTION (provided by applicant): This NIH-supported research program has the long-term objective of elucidating the central mechanisms and neuroplastic processes underlying acute and chronic dental and orofacial pain conditions and their control. Recent data indicate that stimulation of the tooth pulp with an inflammatory irritant induces a 'central sensitization' of nociceptive neurons in the rat brainstem and thalamus and that the brainstem subnucleus caudalis ('medullary dorsal horn') is strategically involved. NMDA receptors (NMDAR) are also involved in this process that has been implicated in the allodynia, hyperalgesia and spread and referral of pain that may occur after injury and inflammation. Purinergic receptor (P2XR) mechanisms have been recently identified as another modulatory process in spinal nociceptive transmission that may function through a powerful presynaptic regulation of glutamate release in the spinal dorsal horn. There has been no study of purinergic mechanisms in central nociceptive processing in the orofacial region, other than some recent preliminary data from our laboratory that indirectly suggests these mechanisms may operate in caudalis. We therefore propose in vivo and in vitro experiments to determine (i) if the pulp-induced central sensitization in caudalis nociceptive neurons involves endogenous purinergic mechanisms; if so, (ii) whether these mechanisms are presynaptic; and (iii) whether they are NMDA receptor dependent. Our experimental design will allow us to test in vivo the involvement of endogenous purinergic mechanisms in pulp-induced central sensitization in functionally identified single neurons in Vc and their potential for regulation of glutamate release and NMDAR activation. The in vivo experiments will be supplemented by in vitro experiments that will provide important additional insights into the P2XR subtype involved and whether the purinergic receptor mechanisms are operating presynaptically. These studies will provide new information on a novel chemical mediator of nociceptive transmission, and new insights will be gained of the central processing of sensory information from the tooth pulp. These insights could be important in the development of improved therapeutic approaches for the prevention of pain associated with pulpal inflammation and for the relief of pain once it has been initiated.

DESCRIPTION (provided by applicant): The etiology and pathogenesis of temporomandibular joint disorders, including rheumatoid arthritis (RA) and temporomandibular disorders, are poorly understood. We have found that artificial elevation of peripheral glutamate levels in masseter muscle or TMJ tissues evokes nociceptive afferent discharges and jaw reflexes involving brainstem subnucleus caudalis in rats and induces pain in humans; sex-related differences occur in the responses to glutamate. This has led to the general hypothesis that activation of peripheral NMDA receptors may contribute to masseter muscle or TMJ pain and the female predominance in TMJ disorders. To address this, we will test (A) if rat nociceptive masseter or TMJ afferent fibers can be excited by glutamate application to masseter or TMJ that is mediated through activation of peripheral NMDA receptors and blocked by peripherally applied NMDA receptor antagonist, and if there are sex-related differences in these effects that are dependent on sex steroid hormones; (B) if glutamate levels in rat masseter or TMJ are elevated by excitation of masseter or TMJ afferents, and if there are sex-related differences in these levels that are dependent on sex steroid hormones; (C) if the excitability of rat caudalis nociceptive neurons is enhanced by glutamate-induced activation of peripheral NMDA receptors in masseter or TMJ and blocked by peripherally applied NMDA receptor antagonist, and if there are sex-related differences in this excitability increase that are dependent on sex steroid hormones; (D) if injection of glutamate into the human masseter or TMJ evokes pain in humans through activation of peripheral NMDA receptors, and if there are sex-related differences in this effect and in the efficacy of peripheral NMDA receptor antagonist to block glutamate-induced pain in these healthy subjects as well as ongoing clinical pain in RA or myofascial TMD patients that are dependent on sex steroid hormones; and (E)) if there is a difference in human masseter or TMJ glutamate levels between healthy subjects and the patients, and if there are sex-related differences in peripheral glutamate levels that are dependent on sex steroid hormones. Insights gained by our multidisciplinary investigations will lead to a better understanding of musculoskeletal pain mechanisms and could provide a basis for the development of novel approaches for the management of pain in TMJ disorders.

DESCRIPTION (provided by applicant): This NIH-supported research program has the long-term objective of elucidating the mechanisms and neuroplastic processes underlying acute and chronic dental and orofacial pain conditions and their control. Our recent data indicate that stimulation of the tooth pulp with an inflammatory irritant induces a NMDA receptor(NMDAR)-dependent 'central sensitization'of nociceptive neurons in the rat brainstem and thalamus and that the brainstem subnucleus caudalis ('medullary dorsal horn') is strategically involved in this process that has been implicated in the allodynia, hyperalgesia and spread and referral of pain that may occur after injury and inflammation. Purinergic receptor (P2XR) mechanisms have been recently identified as another modulatory process in spinal nociceptive transmission that may function through a powerful presynaptic regulation of glutamate release in the spinal dorsal horn. Recently, we have provided the first findings of a role for purinergic mechanisms in central nociceptive processing in the orofacial region. We propose to build upon these findings and use immunocytochemical and in vivo and in vitro electrophysiological techniques to test the hypotheses A) Afferent inputs to Vc from tooth pulp and dura include P2XR-expressing afferents that are sufficient to induce Vc central sensitization and associated sensorimotor behavior by central P2XR mechanisms;and B) Central P2XR produce a presynaptic facilitation of primary afferent transmission in Vc that is dependent on specific NMDAR subunits. Our experimental design will allow us to determine the afferents expressing P2XR, if these afferent inputs to brainstem are sufficient to induce central sensitization by central P2XR mechanisms, and if central P2XR presynaptically facilitate primary afferent transmission and act via NMDAR subunits. These studies will provide new information on a novel chemical mediator of nociceptive transmission, and new insights will be gained of the processing of sensory information from the tooth pulp and dura. These insights could be important in the development of improved therapeutic approaches for the prevention of pain associated with pulpal inflammation and for the relief of pain once it has been initiated.