Projection of nodose ganglion cells to the upper cervical spinal cord in the rat

Do the psychological effects of vagus nerve stimulation partially mediate vagal pain modulation?

There is preclinical and clinical evidence that vagus nerve stimulation modulates both pain and mood state. Mechanistic studies show brainstem circuitry involved in pain modulation by vagus nerve stimulation, but little is known about possible indirect descending effects of altered mood state on pain perception. This possibility is important, since previous studies have shown that mood state affects pain, particularly the affective dimension (pain unpleasantness). To date, human studies investigating the effects of vagus nerve stimulation on pain perception have not reliably measured psychological factors to determine their role in altered pain perception elicited by vagus nerve stimulation. Thus, it remains unclear how much of a role psychological factors play in vagal pain modulation. Here, we present a rationale for including psychological measures in future vagus nerve stimulation studies on pain.

Localization of receptors for calcitonin-gene-related peptide to intraganglionic laminar endings of the mouse esophagus: peripheral interaction between vagal and spinal afferents?

The calcitonin-gene-related peptide (CGRP) receptor is a heterodimer of calcitonin-receptor-like receptor (CLR) and receptor-activity-modifying protein 1 (RAMP1). Despite the importance of CGRP in regulating gastrointestinal functions, nothing is known about the distribution and function of CLR/RAMP1 in the esophagus, where up to 90 % of spinal afferent neurons contain CGRP. We detected CLR/RAMP1 in the mouse esophagus using immunofluorescence and confocal laser scanning microscopy and examined their relationship with neuronal elements of the myenteric plexus. Immunoreactivity for CLR and RAMP1 colocalized with VGLUT2-positive intraganglionic laminar endings (IGLEs), which were contacted by CGRP-positive varicose axons presumably of spinal afferent origin, typically at sites of CRL/RAMP1 immunoreactivity. This provides an anatomical basis for interaction between spinal afferent fibers and IGLEs. Immunoreactive CLR and RAMP1 also colocalized in myenteric neurons. Thus, CGRP-containing spinal afferents may interact with both vagal IGLEs and myenteric neurons in the mouse esophagus, possibly modulating motility reflexes and inflammatory hypersensitivity.

Neural Mechanisms That Underlie Angina-Induced Referred Pain in the Trigeminal Nerve Territory: A c-Fos Study in Rats

The present study was designed to determine whether the trigeminal sensory nuclear complex (TSNC) is involved in angina-induced referred pain in the trigeminal nerve territory and to identify the peripheral nerve conducting nociceptive signals that are input into the TSNC. Following application of the pain producing substance (PPS) infusion, the number of Fos-labeled cells increased significantly in the subnucleus caudalis (Sp5C) compared with other nuclei in the TSNC. The Fos-labeled cells in the Sp5C disappeared when the left and right cervical vagus nerves were sectioned. Lesion of the C1-C2 spinal segments did not reduce the number of Fos-labeled cells. These results suggest that the nociceptive signals that conduct vagal afferent fibers from the cardiac region are input into the Sp5C and then projected to the thalamus.

Neurobiology of estrogen status in deep craniofacial pain

Pain in the temporomandibular joint (TMJ) region often occurs with no overt signs of injury or inflammation. Although the etiology of TMJ-related pain may involve multiple factors, one likely risk factor is female gender or estrogen status. Evidence is reviewed from human and animal studies, supporting the proposition that estrogen status acts peripherally or centrally to influence TMJ nociceptive processing. A new model termed the "TMJ pain matrix" is proposed as critical for the initial integration of TMJ-related sensory signals in the lower brainstem that is both modified by estrogen status, and closely linked to endogenous pain and autonomic control pathways.

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.

Responses and afferent pathways of C1-C2 spinal neurons to cervical and thoracic esophageal stimulation in rats

Because vagal and sympathetic inputs activate upper cervical spinal neurons, we hypothesized that stimulation of the esophagus would activate C(1)-C(2) neurons. This study examined responses of C(1)-C(2) spinal neurons to cervical and thoracic esophageal distension (CED, TED) and afferent pathways for CED and TED inputs to C(1)-C(2) spinal neurons. Extracellular potentials of single C(1)-C(2) spinal neurons were recorded in pentobarbital-anesthetized male rats. Graded CED or TED was produced by water inflation (0.1-0.5 ml) of a latex balloon. CED changed activity of 48/219 (22%) neurons; 34 were excited (E), 12 were inhibited (I), and 2 were E-I. CED elicited responses for 18/18 neurons tested after ipsilateral cervical vagotomy, for 12/14 neurons tested after bilateral vagotomy and for 9/11 neurons tested after bilateral vagotomy and C(6)-C(7) spinal cord transection. TED changed activity of 31/190 (16%) neurons (28E, 3 I). Ipsilateral cervical vagotomy abolished TED-evoked responses of 5/12 neurons. Bilateral vagotomy eliminated responses of 2/4 neurons tested, and C(6)-C(7) spinal transection plus bilateral vagotomy eliminated responses of 2/2 neurons. Thus inputs from CED to C(1)-C(2) neurons most likely entered upper cervical dorsal roots, whereas inputs from TED were dependent on vagal pathways and/or sympathetic afferent pathways that entered the thoracic dorsal roots. These results supported a concept that C(1)-C(2) spinal neurons play a role in integrating visceral information from cervical and thoracic esophagus.

Cardiopulmonary sympathetic and vagal afferents excite C1-C2 propriospinal cells in rats

The purpose of this study in anesthetized rats was to determine the effects of stimulating cardiopulmonary sympathetic afferents (CPSA) and vagal afferents on C1-C2 descending propriospinal neurons. We hypothesized that inhibition of spinal sensory neurons produced by CPSA or vagus activation might relay in C1-C2 spinal segments. Extracellular action potentials were recorded from 73 C1-C2 neurons whose axons were antidromically activated in lumbar segments. CPSA input excited 22 cells, inhibited two cells and excited/inhibited one cell, whereas vagal input excited eight cells and inhibited two cells. Results are consistent with the hypothesis that CPSA input can be processed in C1-C2 segments to produce neural modulation in distant spinal segments.

Response properties of TMJ units in superficial laminae at the spinomedullary junction of female rats vary over the estrous cycle

Neurons responsive to stimulation of the temporomandibular joint (TMJ) region were recorded from superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C(2)) junction region of cycling female rats under barbiturate anesthesia. To determine if receptive field (RF) properties or sensitivity to algesic chemicals of TMJ units vary over the estrous cycle, animals were selected from proestrous (high estrogen) or early diestrous (low estrogen) stages. More than 90% of TMJ units from each group received convergent nociceptive input [wide dynamic range (WDR) or nociceptive specific (NS)-like] from facial skin. The cutaneous high-threshold RF areas of WDR units from proestrous rats were 30% larger than diestrous units, while RF areas of NS units were similar. Bradykinin (BK, 0.1-10 microM) injection into the TMJ region excited a high percentage of units (>80% of total) from both groups in a dose-related manner. However, BK-evoked response magnitude (R(mag), +140%) and duration (+64%) were greater for proestrous than diestrous units. Both WDR and NS-like TMJ units of proestrous females displayed enhanced BK-evoked R(mag) values and response duration. Glutamate or mustard oil excitation of TMJ units was not affected by stage of the estrous cycle. Several TMJ units from proestrous and diestrous females were activated antidromically from the contralateral posterior thalamus, indicating that projection and nonprojection units were included in the sample population. These results were consistent with the hypothesis that factors related to stage of the estrous cycle modify the processing of deep craniofacial inputs by superficial dorsal horn neurons at the spinomedullary junction, a key region for the initial integration of sensory signals from the TMJ.

Sex differences in amino acid release from rostral trigeminal subnucleus caudalis after acute injury to the TMJ region

The neurological basis for painful temporomandibular disorders (TMD) and the higher prevalence of TMD pain in women than men is not known. To better define the circuitry and neurochemical mechanisms in the lower brainstem associated with noxious sensory inputs from the temporomandibular joint (TMJ) region a microdialysis method was used to measure the release of amino acid transmitters from the ventral trigeminal subnucleus interpolaris/caudalis transition region (Vi/Vc-vl). The irritant chemical, mustard oil, was injected into the TMJ region (TMJ-MO) under barbiturate anesthesia in males and normal cycling female rats. Males displayed significant increases in glutamate, serine, and glycine within 15 min after TMJ-MO and increases in citrulline occurred after a delay of 15-30 min. TMJ-MO did not enhance amino acid release in diestrus or proestrus females. GABA release was not affected by TMJ-MO in males or females. Pretreatment with morphine (3 mg/kg, i.v.) prevented the increase in amino acid release seen after TMJ-MO in males. Amino acid release at the Vi/Vc-vl transition region evoked by TMJ-MO also was prevented by prior microinjection of the GABA(A) receptor agonist, muscimol, into the most caudal portion of Vc suggesting this region acted as a critical relay for nociceptive inputs from the TMJ region. These results suggest that glutamatergic mechanisms acting at the Vi/Vc-vl transition region contribute to processing of nociceptive signals that arise from the TMJ region. These results also are consistent with the hypothesis that central neural mechanisms that integrate nociceptive inputs from deep craniofacial tissues are different in males and females.

Vagotomy prevents morphine-induced reduction in Fos-like immunoreactivity in trigeminal spinal nucleus produced after TMJ injury in a sex-dependent manner

Acute injury to the temporomandibular joint (TMJ) region activates neurons in multiple, but spatially discrete, areas of the trigeminal spinal nucleus as seen by an increase in Fos-like immunoreactive neurons (Fos-LI). Pretreatment with morphine greatly reduces Fos-LI produced in the dorsal paratrigeminal area (dPa5), ventrolateral pole of the subnucleus interpolaris/caudalis (Vi/Vc-vl) transition region, and laminae I-II at the subnucleus caudalis/upper cervical cord junction (Vc/C2) suggesting a role for these areas in processing pain signals from the TMJ region. To determine if vagal afferents contribute to neural activation after TMJ injury or reduction of activity after morphine, Fos-LI was quantified in the lower brainstem and upper cervical spinal cord of intact and vagotomized male and female rats under barbiturate anesthesia. Bilateral cervical vagotomy (VgX) did not affect Fos-LI produced by TMJ injury in males or females in the absence of morphine. By contrast, morphine-induced reduction in Fos-LI produced at the Vi/Vc-vl transition region was prevented by prior VgX in males and diestrus females, but not in proestrus females. Morphine inhibition of Fos-LI produced in laminae I-II at the Vc/C2 junction region was diminished in vagotomized males compared to intact animals, but not affected in females. In an autonomic control area, the caudal ventrolateral medulla (CVLM), VgX reversed the morphine-induced reduction in Fos-LI in males and females similarly compared to their respective intact controls. These results were consistent with the hypothesis that the Vi/Vc-vl transition region plays a unique role in deep craniofacial pain processing and may integrate autonomic and opioid-related modulatory signals in a manner dependent on sex hormone status.

Involvement of the central nervous system in the protective effect of melanocortins in myocardial ischaemia/reperfusion injury

Melanocortin peptides exert, in rats, a protective effect in myocardial ischaemia followed by reperfusion, or permanent occlusion of a coronary artery. Moreover, melanocortins have an anti-shock effect. Since the mechanism of the life-saving effect of these peptides in haemorrhagic shock includes an essential brain loop, we aimed to determine whether the central nervous system (CNS) is also involved in the protective effect against the outcome of short-term myocardial ischaemia followed by reperfusion. Ischaemia was produced in anaesthetized rats by ligature of the left anterior descending coronary artery for 5 min. Reperfusion-induced ventricular tachycardia (VT), ventricular fibrillation (VF) and lethality, and the time-course of arterial blood pressure over 5 min following reperfusion were evaluated. Groups of 8-14 rats were used. Intravenous (i.v.) injection of ACTH-(1-24) (0.16-0.48 mg/kg) during the ischaemic period dose dependently reduced the incidence of VT, VF and of lethality. In saline-treated rats, coronary reperfusion caused VT in 100% animals, VF in 86%, and death in 86%. The highest dose of ACTH-(1-24) (0.48 mg/kg) completely prevented the occurrence of VT, VF and death in all rats (P<0.005). Moreover, the melanocortin peptide prevented the fall in mean arterial pressure (MAP) occurring during reperfusion. Treatment with ACTH-(1-24) by the intracerebroventricular (i.c.v.) route also reduced the incidence of VT, VF and lethality, and prevented the fall in MAP in a dose dependent manner. Complete (100%) protection occurred with an i.c.v. dose (0.048 mg/kg) 10 times less than that needed by the i.v. route. The present data show that in the protective effect of melanocortin peptides against the injury after myocardial ischaemia/reperfusion, the i.c.v. route of administration is more effective than the i.v. route. They suggest that a CNS mechanism, not yet identified, may be involved.

Intensity coding by TMJ-responsive neurons in superficial laminae of caudal medullary dorsal horn of the rat

Temporomandibular disorders (TMD) represent a family of recurrent conditions that often cause pain in the temporomandibular joint (TMJ) region and muscles of mastication. To determine if TMJ-responsive neurons encoded the intensity of pro-inflammatory chemical signals, dose-effect relationships were assessed after direct injection bradykinin into the joint space and compared with responses after injection of glutamate or saline. Neurons were recorded from superficial laminae of the trigeminal subnucleus caudalis/upper cervical cord junction region (Vc/C(2)) and identified by palpation of the TMJ region in barbiturate-anesthetized male rats. The majority (62 of 84) of units received convergent input from facial skin, while 26% were driven only by deep input from the TMJ region. Conduction-velocity based on the latency to firing after electrical stimulation of the TMJ region indicated 64% of units were driven by A-delta fiber input only. Bradykinin (0.1-10 microM) excited 69% of neurons tested, and 70% (19 of 27) of these units were activated by the lowest dose (0.1 microM). Glutamate (50-200 mM) excited 27% of units; however, when tested after bradykinin, 58% of units were activated by glutamate. Some TMJ units (17%) were excited by saline injection alone and not enhanced further by bradykinin or glutamate. Most (88%) TMJ units were activated by injection of the small fiber excitant, mustard oil (20% solution), into the TMJ region. Units responsive to bradykinin or glutamate were not restricted to particular classes [e.g., wide dynamic range (WDR), nociceptive specific (NS), deep only]. A small percentage of TMJ units (approximately 15%) were activated antidromically from the contralateral posterior thalamus. In parallel studies using c-fos immunocytochemistry, bradykinin (1 microM) injection into the TMJ region produced a greater number of Fos-positive neurons at the Vc/C(2) region than glutamate (200 mM) or saline. These results revealed two broad classes of TMJ units that encoded the intensity of pro-inflammatory chemical stimuli applied to the TMJ region, units that received convergent nociceptive input from facial skin (i.e., WDR and NS units) and units that responded only to deep input from the TMJ region. On the basis of encoding properties and efferent projection status, it is concluded that activation of TMJ units within the superficial laminae at the Vc/C(2) region contribute to the diffuse and spreading nature of TMD pain sensation.

Functional and chemical anatomy of the afferent vagal system

The results of neural tracing studies suggest that vagal afferent fibers in cervical and thoracic branches innervate the esophagus, lower airways, heart, aorta, and possibly the thymus, and via abdominal branches the entire gastrointestinal tract, liver, portal vein, billiary system, pancreas, but not the spleen. In addition, vagal afferents innervate numerous thoracic and abdominal paraganglia associated with the vagus nerves. Specific terminal structures such as flower basket terminals, intraganglionic laminar endings and intramuscular arrays have been identified in the various organs and organ compartments, suggesting functional specializations. Electrophysiological recording studies have identified mechano- and chemo-receptors, as well as temperature- and osmo-sensors. In the rat and several other species, mostly polymodal units, while in the cat more specialized units have been reported. Few details of the peripheral transduction cascades and the transmitters for signal propagation in the CNS are known. Glutamate and its various receptors are likely to play an important role at the level of primary afferent signaling to the solitary nucleus. The vagal afferent system is thus in an excellent position to detect immune-related events in the periphery and generate appropriate autonomic, endocrine, and behavioral responses via central reflex pathways. There is also good evidence for a role of vagal afferents in nociception, as manifested by affective-emotional responses such as increased blood pressure and tachycardia, typically associated with the perception of pain, and mediated via central reflex pathways involving the amygdala and other parts of the limbic system. The massive central projections are likely to be responsible for the antiepileptic properties of afferent vagal stimulation in humans. Furthermore, these functions are in line with a general defensive character ascribed to the vagal afferent, paraventricular system in lower vertebrates.

Thoracic visceral inputs use upper cervical segments to inhibit lumbar spinal neurons in rats

We tested the hypothesis that cardiopulmonary sympathetic afferent (CPSA) input entering upper thoracic spinal segments relays in the cervical spinal cord to inhibit activity of lumbar spinothalamic tract (SST) cells and dorsal horn (DH) cells. Two sequential spinal transections in the same animal were made, one at rostral C1 and one at C4-C6 segments, to determine neuronal pathways involved in the inhibition. We concluded that inhibitory effects induced by CPSA and somatic stimulation might be mediated by propriospinal mechanisms located in upper cervical segments. Vagal inhibition required supraspinal pathways.

Phrenic afferent input excites C1-C2 spinal neurons in rats

Effects of electrical stimulation of the ipsilateral phrenic nerve above the heart were determined on cells in the C1-C2 spinal cord segments of 27 rats anesthetized with pentobarbital. Forty-five cells that responded to this stimulus were included in this study. These cells then were examined at the same stimulus parameters for effects of stimulating the ipsilateral phrenic nerve below the heart, the contralateral phrenic nerve above the heart, and/or the left (ipsilateral) cervical vagus nerve. Ipsilateral phrenic nerve stimulation below the heart had no effect on 20 of 20 cells tested. Seven of 16 cells tested for effects of contralateral phrenic nerve stimulation above the heart were excited and activity of 9 cells was unaffected. Activity changes and activation latencies were not significantly different in the 7 cells excited by both ipsilateral and contralateral phrenic nerve stimulation. Thirty-seven of 45 cells excited by ipsilateral phrenic nerve stimulation also were excited by stimulation of the left cervical (ipsilateral) vagus nerve. The mean increase in cell activity was significantly greater following vagal nerve stimulation compared to phrenic nerve stimulation, and mean activation latency was significantly longer for vagal stimulation. Excitatory receptive somatic fields were classified for 35 cells. Somatic field locations for most cells (80%) included the ipsilateral neck and jaw. Activity of 26 cells was increased by both noxious pinch and brushing the hair, activity of 6 cells was increased only by noxious pinch, and activity of 3 cells was increased only by brushing the hair. Results of this study indicate that there are afferent fibers in the phrenic nerve above the heart, but not below the heart, that excite cells in the C1-C2 segments of the spinal cord. Most cells also were excited by noxious stimuli applied to their somatic receptive fields. Thus, the phrenic nerve may provide a pathway for referral of pain to the neck and jaw from thoracic structures.

Gastric distension modulates hypothalamic neurons via a sympathetic afferent path through the mesencephalic periaqueductal gray

The effects of gastric distension on extracellularly recorded single neuronal activity in the lateral hypothalamuslateral preoptic area-medial forebrain bundle (LPA-LH-MFB), other areas of the hypothalamus, mesencephalic periaqueductal gray (PAG), and other areas associated with the mesencephalic reticular formation were determined in the anesthetized rat. Gastric distension was produced by filling a gastric balloon with water using a calibrated infusion pump. Experimental conditions were based on previous studies that simulated gastric distension during fluid consumption in the rat. The effects of stomach distension using water at body temperature and room temperature were compared. Neurons in both the hypothalamus and mesencephalon were modulated by gastric distension. Hypothalamic neurons exhibited responses associated with gastric distension and exhibited interactions between distension and temperature stimulation of the stomach. Neurons in the mesencephalic periaqueductal gray (PAG) and associated reticular formation also were modulated by these gastric stimuli. When the PAG was electrically stimulated, similar responses to gastric distension and PAG stimulation were observed for hypothalamic neurons. The effects of gastric distension on hypothalamic neurons were reduced or eliminated when the PAG stimulating electrode site was destroyed by electrocoagulation. In addition, the microiontophoretic application of horseradish peroxidase at hypothalamic neuronal recording sites where gastric distension effects were observed resulted in the retrograde labeling of neurons in the PAG. These gastric stimulation-induced effects on hypothalamic and mesencephalic neuronal activity were attenuated but were not permanently eliminated by bilateral cervical vagotomy. However, these effects were significantly reduced or eliminated by bilateral transection of the cervical sympathetic chain or spinal transection at the first cervical level. Because the filling of balloons placed into the abdominal cavity close to the stomach had no similar effects on neural activity, these results can be attributed primarily to the activation of gastric mechano-and temperature-sensitive receptors. These results indicate that the effects of gastric temperature/distension stimulation under these conditions are mediated to a large degree by sympathetic afferents. The PAG is clearly involved as one of the mesencephalic relays for gastric afferent input to the hypothalamus.

Responses of neurons in caudal solitary nucleus of female rats to stimulation of vagina, cervix, uterine horn and colon

Neurons in the caudal part of the solitary nucleus (NTS) are known for their processing of information derived from many viscera, including cardiovascular, respiratory and alimentary tract organs. This study characterized responses of NTS neurons in female rats in estrus to mechanical stimulation of four pelvic visceral organs (i.e. the vaginal canal, cervix, uterine horn and colon) as well to gentle mechanical skin stimulation. Of the 90 neurons tested, 31% responded with excitation or inhibition to one (22%) or more (9%) visceral stimuli. Responses included 13% to vaginal distension, 12% to cervix stimulation, 10% to uterine distension and 4% to colon distension. None responded to gentle cutaneous stimuli. These results expand the domain of visceral functions of NTS neurons to include pelvic female reproductive organs. The failure of NTS neurons to respond to gentle cutaneous stimuli contrasts with convergent responses of neurons in the gracile nucleus to skin and pelvic visceral stimuli [13] indicating the two nuclei are involved in different aspects of visceral function.

Effects of vagal afferent nerve stimulation on noxious heat-evoked Fos-like immunoreactivity in the rat lumbar spinal cord

Electrophysiological and behavioral studies have described modulation of nociception by vagal afferent fibers. The objectives of this study were to 1) use Fos-like immunoreactivity as a marker for neuronal activity to examine populations of neurons in the spinal cord that are activated by a noxious heat stimulus, 2) determine whether heat-evoked Fos-like immunoreactivity can be modulated by vagal afferent stimulation, and 3) determine whether vagally-mediated effect on heat-evoked Fos-like immunoreactivity can be blocked by intrathecally administered serotoninergic receptor and alpha-adrenergic receptor antagonists. Neurons demonstrating Fos-like immunoreactivity were located in the ipsilateral superficial and deep dorsal horn laminae extending from the caudal L3 through the rostral L6 region of the spinal cord. Stimulation of the right cervical vagus nerve attenuated significantly (42%) heat-evoked Fos-like immunoreactivity in the superficial laminae. The reduction in Fos-like immunoreactivity by vagal stimulation was abolished by intrathecal administration of methysergide, a nonselective serotoninergic receptor antagonist, but not by phentolamine, a nonselective alpha-adrenoceptor antagonist. These results suggest that vagal afferent modulation of spinal nociceptive transmission is mediated, at least in part, by serotonin receptors.

Short latency excitation of upper cervical respiratory neurons by vagal stimulation in the rat

Extracellular recordings were made from 29 respiration-phased neurons in the upper cervical spinal cord (C1-C3) in nine anesthetized rats while ipsilateral and contralateral vagi were stimulated via platinum hook electrodes. Neuronal responses to vagal stimulation were recorded using peristimulus histograms. An accumulation of spikes with an average latency of 4.0 +/- 0.8 (S.D.) ms occurred in 11 cells after ipsilateral stimulation. These results indicate that there are fibers in the vagus which oligosynaptically excite respiratory neurons in the upper cervical spinal cord.

Vagal afferent fibers excite upper cervical neurons and inhibit activity of lumbar spinal cord neurons in the rat

Effects of electrical stimulation of the cervical vagus nerve were determined in cervical or lumbar spinal neurons in 27 rats anesthetized with pentobarbital. Ipsilateral cervical vagus stimulation (ICVS) increased activity of 44 neurons in the C1 segment. At the same stimulation parameters, contralateral cervical vagus stimulation (CCVS) either increased, decreased or did not affect activity of C1 neurons that were excited by ICVS. For C1 cells excited by both ICVS and CCVS, the mean latency for activation was significantly longer for CCVS than for ICVS, and ICVS produced a greater degree of excitation than CCVS. In segments C2-C6, 16 of 18 neurons were excited by ICVS and 2 were inhibited. However, CCVS did not excite the C2-C6 neurons but either inhibited or did not affect activity. In 6 cervical cells, a CCVS conditioning stimulus reduced the level of excitation by ICVS (test stimulus). Transection of the C2 or C3 dorsal roots did not significantly affect the excitatory vagal input to C1 cells. Excitatory somatic receptive fields were classified for 60 cervical spinal cells that responded to vagal stimulation. Most (87%) cells were excited by noxious pinch; 29 were wide dynamic range (WDR) cells and 21 were high threshold cells. In contrast to upper cervical neurons, spinothalamic tract (STT) and spinal cells in lumbar segments were not excited by ICVS or CCVS at the stimulation parameters used in this study, but were primarily inhibited by vagal stimulation. Results of this study showed that a group of cells in upper cervical segments were excited by vagal afferents. This excitatory vagal input reaches the C1 segment primarily via an ipsilateral, supraspinal route.