Phrenic afferent input excites C1-C2 spinal neurons in rats

Phrenic afferent activation modulates cardiorespiratory output in the adult rat

Phrenic afferents project to brainstem areas responsible for cardiorespiratory control and the mid-cervical spinal cord containing the phrenic motor nucleus. Our purpose was to quantify the impact of small- and large-diameter phrenic afferent activation on phrenic motor output. Anesthetized and ventilated rats received unilateral phrenic nerve stimulation while contralateral phrenic motor output and blood pressure were recorded. Twelve currents of 40-Hz inspiratory-triggered stimulation were delivered (20 s on, 5 min off) to establish current response curves. Stimulation pulse width was varied to preferentially activate large-diameter phrenic afferents (narrow pulse width) and recruit small-diameter fibers (wide pulse width). Contralateral phrenic amplitude was elevated immediately poststimulation at currents above 35 µA for wide and 70 µA for narrow pulse stimulation when compared with animals not receiving stimulation (time controls). Wide pulse width stimulation also increased phrenic burst frequency at currents ≥35 µA, caused a transient decrease in mean arterial blood pressure at currents ≥50 µA, and resulted in a small change in heart rate at 300 µA. Unilateral dorsal rhizotomy attenuated stimulation-induced cardiorespiratory responses indicating that phrenic afferent activation is required. Additional analyses compared phrenic motor amplitude with output before stimulation and showed that episodic activation of phrenic afferents with narrow pulse stimulation can induce short-term plasticity. We conclude that the activation of phrenic afferents 1) enhances contralateral phrenic motor amplitude when large-diameter afferents are activated, and 2) when small-diameter fibers are recruited, the amplitude response is associated with changes in burst frequency and cardiovascular parameters.NEW & NOTEWORTHY Acute, inspiratory-triggered stimulation of phrenic afferents increases contralateral phrenic motor amplitude in adult rats. When small-diameter afferents are recruited, the amplitude response is accompanied by an increase in phrenic burst frequency, a transient decrease in mean arterial blood pressure, and a slight increase in heart rate. Repeated episodes of large-diameter phrenic afferent activation may also be capable of inducing short-term plasticity.

Targeted activation of spinal respiratory neural circuits

Spinal interneurons which discharge in phase with the respiratory cycle have been repeatedly described over the last 50 years. These spinal respiratory interneurons are part of a complex propriospinal network that is synaptically coupled with respiratory motoneurons. This article summarizes current knowledge regarding spinal respiratory interneurons and emphasizes chemical, electrical and physiological methods for activating spinal respiratory neural circuits. Collectively, the work reviewed here shows that activating spinal interneurons can have a powerful impact on spinal respiratory motor output, and can even drive rhythmic bursting in respiratory motoneuron pools under certain conditions. We propose that the primary functions of spinal respiratory neurons include 1) shaping the respiratory pattern into the final efferent motor output from the spinal respiratory nerves; 2) coordinating respiratory muscle activation across the spinal neuraxis; 3) coordinating postural, locomotor and respiratory movements, and 4) enabling plasticity of respiratory motor output in health and disease.

Anatomy and physiology of phrenic afferent neurons

Large-diameter myelinated phrenic afferents discharge in phase with diaphragm contraction, and smaller diameter fibers discharge across the respiratory cycle. In this article, we review the phrenic afferent literature and highlight areas in need of further study. We conclude that 1) activation of both myelinated and nonmyelinated phrenic sensory afferents can influence respiratory motor output on a breath-by-breath basis; 2) the relative impact of phrenic afferents substantially increases with diaphragm work and fatigue; 3) activation of phrenic afferents has a powerful impact on sympathetic motor outflow, and 4) phrenic afferents contribute to diaphragm somatosensation and the conscious perception of breathing. Much remains to be learned regarding the spinal and supraspinal distribution and synaptic contacts of myelinated and nonmyelinated phrenic afferents. Similarly, very little is known regarding the potential role of phrenic afferent neurons in triggering or modulating expression of respiratory neuroplasticity.

Histological identification of phrenic afferent projections to the spinal cord

Limited data are available regarding the spinal projections of afferent fibers in the phrenic nerve. We describe a method that robustly labels phrenic afferent spinal projections in adult rats. The proximal end of the cut phrenic nerve was secured in a microtube filled with a transganglionic tracer (cholera toxin β-subunit, CT-β, or Cascade Blue) and tissues harvested 96-h later. Robust CT-β labeling occurred in C3-C5 dorsal root ganglia cell bodies and phrenic afferent projections were identified in the mid-cervical dorsal horn (laminae I-III), intermediate grey matter (laminae IV, VII) and near the central canal (laminae X). Afferent fiber labeling was reduced or absent when CT-β was delivered to the intrapleural space or directly to the hemidiaphragm. Soaking the phrenic nerve with Cascade Blue also produced robust labeling of mid-cervical dorsal root ganglia cells bodies, and primary afferent fibers were observed in spinal grey matter and dorsal white matter. Our results show that the 'nerve soak' method effectively labels both phrenic motoneurons and phrenic afferent projections, and show that primary afferents project throughout the ipsilateral mid-cervical gray matter.

Prostaglandin E2 potentiates the excitability of small diameter trigeminal root ganglion neurons projecting onto the superficial layer of the cervical dorsal horn in rats

The aim of the present study was to investigate how prostaglandin E2 (PGE2) affects the excitability of trigeminal root ganglion (TRG) neurons, projecting onto the superficial layer of the cervical dorsal horn, using fluorescence retrograde tracing and perforated patch-clamp techniques. TRG neurons were retrogradely labeled with fluorogold (FG). The cell diameter of FG-labeled neurons was small (< 30 microm). Under the voltage-clamp mode, application of PGE2 (0.01-10 microM) concentration-dependently increased the magnitude of the peak tetrodotoxin-resistant sodium current (TTX-R I(Na)) and this current was maximal at a concentration of 1 microM. One micromolar PGE2 application caused a hyperpolarizing shift of 8.3 mV in the activation curve for TTX-R I(Na). In the current-clamp mode, the PGE2 (1 microM) application significantly increased the number of action potentials during the depolarizing step pulses as well as the level of overshoot but had no significant effect on the resting membrane potential. These results suggest that the excitability of small diameter TRG neurons seen after 1 microM PGE2 application is involved in an increase in the

Convergence of cutaneous, musculoskeletal, dural and visceral afferents onto nociceptive neurons in the first cervical dorsal horn

The convergence of cutaneous, musculoskeletal, dural and visceral afferents onto nociceptive neurons in the first cervical dorsal horn was investigated in urethane/chloralose-anesthetized rats. Electrical stimulation was applied to facial, neck, shoulder and forepaw skin, cornea (COR), dura, second cervical (C2) nerve, hypoglossal nerve, temporomandibular joint, masseter (MAS) muscle and superior laryngeal nerve. In addition, acetic acid was injected intraperitoneally and microinjection of glutamate was applied to the tongue, MAS muscle, splenius cervicis muscle, dura and intrapericardial area. A total of 52 nociceptive neurons classified as wide dynamic range (n = 28) or nociceptive-specific (n = 24) was studied. All nociceptive neurons received afferent input from the skin and at least one COR, musculoskeletal, dural or visceral afferent source in the trigeminal (V) or cervical area but input from afferent sources caudal to the C2 innervation territory was sparse. The proportion of neurons responding to COR, dural, C2 nerve, hypoglossal nerve, temporomandibular joint, MAS muscle and superior laryngeal nerve stimulations was 87, 54, 85, 52, 73, 64 and 31%, respectively. Electrical stimulation of all tested sites showed a double logarithmic stimulus-response relation, and cluster analysis of the excitability to COR, musculoskeletal, dural and visceral stimulations revealed two groups of neurons, one mainly containing wide dynamic range neurons and one mainly containing nociceptive-specific neurons. These findings indicate that afferent convergence in first cervical dorsal horn nociceptive neurons may be limited to the craniofacial area and that they may play an important role in the integration of craniofacial and upper cervical nociceptive inputs.

N-Methyl-D-Aspartate (NMDA) and non-NMDA receptor antagonists suppress the superior sagittal sinus-evoked activity of C1 spinal neurons responding to tooth pulp electrical stimulation in rats

The aim of the present study was to determine whether there is a convergence of inputs from tooth pulp (TP) and the superior sagittal sinus (SSS) on rat C1 spinal neurons, and to examine the effects of iontophoretically applied N-methyl-D: -aspartate (NMDA) and non-NMDA receptor antagonists on the SSS-evoked activity of C1 neurons. Extracellular single unit-recordings were made from 20 C1 units responding to TP electrical stimulation with a constant temporal relationship to a digastric electromyogram signal, using a multibarrel electrode in pentobarbital-anesthetized rats. Ninety percent of C1 neurons (18/20) responding to TP stimulation also responded to the SSS stimulation. These neurons were considered to be SSS-afferent inputs from Adelta-fibers (5.8 +/- 0.6 m/s; n = 18), based on the calculation of nerve conduction velocity. After the iontophoretic application (30, 50, and 70 nA) of an NMDA receptor blocker (5R-10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cycloheptene-5,10-imine hydrogen maleate (MK801) or a non-NMDA receptor blocker (6-cyano-7-nitroquinoxaline-2,3-dione) (CNQX), the mean number of spikes responding to the SSS stimulation significantly decreased (30, 50, and 70 nA; P < 0.05). These results suggest that there is a convergence of inputs from SSS and TP afferents on C1 neurons; it is possible that both NMDA and non-NMDA receptors located on C1 neurons may be targets for the treatment of the trigeminal referred pain associated with migraine.

Role of capsaicin-sensitive primary afferent inputs from the masseter muscle in the C1 spinal neurons responding to tooth-pulp stimulation in rats

The aim of the present study was to demonstrate the convergence of inputs from masseter muscle (MM) and tooth pulp (TP) onto C1 spinal neurons and to determine whether the afferent fibers express the functional vanilloid receptor (VR1). Extracellular single-unit recordings were made from 61 C1 units responding to TP electrical stimulation with a constant temporal relationship to a digastric electromyogram signal in pentobarbital anesthetized rats. Eighty-four percent of C1 neurons responding to TP stimulation also responded to the ipsilateral MM stimulation. Of these neurons, 61% were considered to be afferent inputs from Adelta-fibers and the remaining units (39%) were C-fibers, based on calculation of the nerve conduction velocity. Intramuscular injection of capsaicin (0.05 and 0.1%) produced a reduction in a MM-induced C1 neuronal activity in a dose-dependent manner and this effect was antagonized by pretreatment with an antagonist of VR1, capsazepine. Some of these units were also excited by noxious heat stimulation (> 43 degrees C). The trigeminal root ganglion (TRG) neurons that innervated the MM were retrogradely labeled with Fluorogold (FG) and the small-diameter FG-labeled TRG neurons expressed the immunoreactivity for VR1. After intramuscular mustard oil injection (noxious chemical stimulation), the C1 neuronal activity induced by both touch and pinch stimuli was enhanced and their receptive field sizes were significantly expanded. These changes were reversed within 15-20 min. These results suggest that there may be the convergence of noxious afferents inputs from the MM and TP afferents on the same C1 neurons in rats, and that the afferent fibers expressing the functional VR1 may contribute to the hyperalgesia and/or referred pain associated with temporomandibular joint disorder.

Craniofacial inputs to upper cervical dorsal horn: implications for somatosensory information processing

The aim of this study was to characterize the properties of somatosensory neurons in the first 2 cervical spinal dorsal horns (C1 and C2 DHs) and compare them with those previously described for the rostral subnucleus caudalis (rVc). A total of 74 nociceptive neurons classified as wide-dynamic-range (WDR) or nociceptive-specific (NS), as well as 72 low-threshold mechanoreceptive (LTM) neurons, was studied in urethane/chloralose-anesthetized rats. The majority of LTM neurons were located in laminae III/IV and had a small mechanoreceptive field (RF) that included the posterior face and cervical tissues. In contrast, the nociceptive neurons were located in laminae I/II or V/VI, and the RF of each C1 and C2 DH nociceptive neuron included a part of the face and in 47% of them the RF included a region supplied by upper cervical afferents. There was a gradual caudal shift in the neuronal RF from nasal/intraoral tissues towards the neck as recording sites progressed from rVc to C1 and C2 DHs. In contrast to LTM neurons, many C1 and C2 DH nociceptive neurons received mechanosensitive convergent afferent inputs from cervical and craniofacial deep tissues (e.g., tongue muscles or temporomandibular joint), and over 50% could be activated by hypoglossal (XII) nerve electrical stimulation. We propose that C1 and C2 DHs represent part of the caudal extension of the Vc, and that Vc and C1 and C2 DHs may act together as one functional unit to process nociceptive information from craniofacial and cervical tissues, including that from deep craniofacial tissues.

Convergence of nociceptive information from temporomandibular joint and tooth pulp afferents on C1 spinal neurons in the rat

The aim of the present study was to test the hypothesis that there is a convergence of afferent inputs from the temporomandibular joint (TMJ) on C1 spinal neurons responding to electrical stimulation of the tooth pulp (TP). In 14 pentobarbital anesthetized rats, the extracellular single unit activity of 31 C1 spinal neurons and the amplitude in a digastric muscle electromyogram (n = 31) increased proportionally during 1.0-3.5 times the threshold for the jaw-opening reflex (JOR). Of 31 C1 spinal neurons responsive to TP afferents, 28 (approximately 90%) were also excited by electrical stimulation of the ipsilateral TMJ capsule. All neurons tested were divided into three categories of nociceptive specific, wide dynamic range and non-responsive as to their responsiveness to mechanical stimuli (pin prick and touch) of the somatic receptive field (skin of the face, neck, jaw and upper forearm) and TMJ capsule. Nineteen (68%) of 28 C1 spinal neurons received nociceptive information from C fibers of the TMJ capsule. These results suggest that there is a convergence of noxious information from the TMJ and TP afferents on the same C1 spinal neurons, which importantly contribute to pain perception from the TMJ region.

Opioidergic modulation of excitability of rat trigeminal root ganglion neuron projections to the superficial layer of cervical dorsal horn

The aim of the present study was to investigate the effect of a micro-opioid receptor agonist DAMGO (Tyr-d-Ala-Gly-NMe-Phe-Gly-ol) on the excitability of trigeminal root ganglion (TRG) neurons, projecting onto the superficial layer of the cervical dorsal horn, by using the perforated-patch technique and to determine whether TRG neurons show the expression of mRNA or functional protein for micro-opioid receptors by using reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. TRG neurons projecting onto the superficial layer of the cervical dorsal horn were retrogradely labeled with Fluorogold (FG). The cell diameter of FG-labeled TRG neurons was small (<30 microm). Under voltage-clamp (V(h)=-60 mV), voltage-dependent K(+) currents were recorded in the TRG neurons and isolated by blocking Na(+) and Ca(2+) currents with appropriate ion replacement. Separation of the K(+) current components was achieved by the response to variation in the conditioning voltage. Two distinct K(+) current components, a transient (I(A)) and sustained (I(K)), were identified. DAMGO significantly increased I(A) by 57% (20 microM) and in a dose-dependent manner (1-50 microM). Similarly, I(K) was also enhanced by DAMGO administration (42%, 20 microM). The augmentation of both I(A) and I(K) was antagonized by a micro-opioid receptor antagonist, CTOP (d-Phe-Cys-Thr-d-Trp-Orn-Thr-Pen-Thr-NH(2)). Hyperpolarization of the membrane potential was elicited by DAMGO (20 microM) and the response was associated with a decrease in the input resistance. DAMGO induced hyperpolarization was blocked by CTOP. DAMGO-sensitive I(A) and I(K) currents were antagonized by K(+) channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). In the presence of both 4-AP and TEA, no significant changes in membrane potential induced by DAMGO application were observed. In the presence of BaCl(2), DAMGO evoked hyperpolarization with decreased resistance was observed. The firing rate of action potentials and the first spike duration induced by depolarizing step pulses were decreased in the presence of DAMGO. RT-PCR analysis demonstrated the expression of mRNA for micro-opioid receptors in the trigeminal ganglia. The micro-opioid receptor immunoreactivity was expressed in the small diameter FG-labeled TRG neurons. These results suggest that the activation of micro-opioid receptors inhibits the excitability of rat small diameter TRG neurons projecting on the superficial layer of the cervical dorsal horn and this inhibition is mediated by potentiation of voltage-dependent K(+) currents. We therefore concluded that modulation of nociceptive transmission in the trigeminal system, resulting in the functional activation of micro-opioid receptors, occurs at the level of small TRG cell bodies and/or their primary afferent terminals, which contribute to opioid analgesia in the trigeminal pain.

Propriospinal neurons in the C1-C2 spinal segments project to the L5-S1 segments of the rat spinal cord

Physiological studies indicate that neurons in the upper cervical spinal cord have descending projections to the lumbosacral spinal cord and mediate inhibition of dorsal horn neurons activated from afferent input. In the present study, retrograde tracing techniques were used to examine the distribution of propriospinal neurons in C1-C2 spinal segments that project to lumbosacral spinal segments. Fluorogold or horseradish peroxidase were injected unilaterally or bilaterally into the L5-S1 spinal segments. After 2-4 days, rats were perfused with fixative and C1-C2 spinal segments were processed for retrograde labeling. Numerous neurons were found in the C1-C2 segments. In unilaterally and bilaterally injected rats, retrogradely labeled neurons were located on both the ipsilateral and contralateral sides. Retrogradely labeled neurons were located in the following locations: lateral cervical and spinal nuclei, nucleus proprius, ventral horn and the central gray region (area X). These studies demonstrate a descending projection from C1-C2 segments to the lower lumbar and sacral spinal cord. We hypothesize that many of these C1-C2 propriospinal neurons are important in modulating responses of spinal neurons at lower segmental levels to various peripheral stimuli.

Phrenic nerve inputs to upper cervical (C1-C3) spinothalamic tract neurons in monkeys

Afferent input from the phrenic nerve enters mid-cervical spinal segments, and previous results in monkeys show that mid-cervical spinothalamic (STT) neurons are activated by groups II and III phrenic afferent input arising from the diaphragm. In rats, dorsal horn neurons in C1-C2 segments receive phrenic input stimulated above the heart, but are not activated by phrenic input arising from diaphragm or abdomen. It is not known if this differential organization occurs in primate STT neurons. We hypothesized that high cervical STT neurons in monkeys also would be preferentially activated by phrenic inputs from thoracic structures, such as the pericardium. We examined extracellular discharge rate of C1-C3 STT neurons for responses to electrical stimulation of phrenic nerve fibers. Responses to stimulation of ipsilateral phrenic fibers above the heart were compared to effects of input stimulated below the heart and also to effects of contralateral phrenic input. We concluded that upper cervical STT neurons are most strongly excited by ipsilateral input from small diameter phrenic fibers arising from thoracic structures, but that group III and IV input from diaphragmatic fibers as well as input from contralateral phrenic fibers have a lesser effect on C1-C3 STT neurons and thus might be involved in nociceptive processing.

Cardiopulmonary sympathetic afferent input does not require dorsal column pathways to excite C1-C3 spinal cells in rats

Effects of electrically stimulating the left stellate ganglion to activate cardiopulmonary sympathetic afferent (CPSA) fibers were determined on C1-C3 dorsal horn neurons in anaesthetized rats. Fifty-two of 53 dorsal horn neurons affected by CPSA stimulation were excited and one neuron was inhibited. In 6 experiments, dorsal columns and ventrolateral funiculi were sequentially lesioned to determine neuronal pathways involved in CPSA activation of C1-C3 neurons. In 6 additional experiments, spinal transection at rostral C1 was used to determine the contribution of supraspinal relays. We concluded that CPSA input to C1-C3 segments travelled bilaterally in ventrolateral pathways, and that supraspinal relays were not required for CPSA excitation of C1-C3 neurons. These results suggest that neurons in C1-C3 segments might play an important role in processing visceral spinal afferent information.

Chest cancer refers pain to face and jaw: a case review

The dentist's sphere of treatment is generally limited by state law to the teeth, alveolar process, gums, cheeks, jaws or oral cavity and associated structures. However today, an appreciation of the total person is necessary. Pain referred from the chest to the face and jaw is presented to the new student when first assuming responsibility for treatment of the clinic patient. With contemporary dental practice expanded to orofacial pain and the temporomandibular joint area, general medical knowledge is essential. Without hesitancy, when the situation is not distinct, referral for medical consultation may be the best part of patient care.