Distribution of somatic and visceral primary afferent fibres within the thoracic spinal cord of the cat

Muscular mechanical hyperalgesia revealed by behavioural pain test and c-Fos expression in the spinal dorsal horn after eccentric contraction in rats

Delayed onset muscle soreness (DOMS) is quite common, but the mechanism for this phenomenon is still not understood; even the existence of muscle tenderness (mechanical hyperalgesia) has not been demonstrated in experimental models. We developed an animal model of DOMS by inducing eccentric contraction (lengthening contraction, ECC) to the extensor digitorum longus muscle (EDL), and investigated the existence of mechanical hyperalgesia in the EDL by means of behavioural pain tests (Randall-Selitto test and von Frey hair test, applied to/through the skin on the EDL muscle) and c-Fos expression in the spinal dorsal horn. We found that the mechanical withdrawal threshold measured with the Randall-Selitto apparatus decreased significantly between 1 and 3 days after ECC, while that measured by von Frey hairs did not. The group that underwent stretching of the muscle only (SHAM group) showed no change in mechanical pain threshold in either test. These results demonstrated that the pain threshold of deep tissues (possibly of the muscle) decreased after ECC. c-Fos immunoreactivity in the dorsal horn (examined 2 days after ECC/SHAM exercise) was not changed by either ECC or compression (1568 mN) to the EDL muscle by itself, but it was significantly increased by applying compression to the EDL muscle 2 days after ECC. This increase was observed in the superficial dorsal horn of the L4 segment of the ipsilateral side, and was clearly suppressed by morphine treatment (10 mg kg(-1), i.p.). These results demonstrated the existence of mechanical hyperalgesia in the muscle subjected to ECC. This model could be used for future study of the neural mechanism of muscle soreness.

Discharge patterns of somatosensitive neurons in the nucleus tractus solitarius of the cat

Encoding of sensory information by nucleus of the solitary tract (NTS) neurons is incompletely understood. Using extracellular single-unit recording in alpha-chloralose-urethane anesthetized cats, we have examined the discharge characteristics of NTS neurons to activation of somatic Adelta and C fiber afferents by skeletal muscle contraction evoked by electrical stimulation of lower lumbar/upper sacral ventral roots. Generally, somatic afferent stimulation evoked two distinct firing patterns. The first population (36/43 cells) increased their firing rate to brief somatic stimuli. A subset (21/27 cells) exhibited a rapid decay of their firing rate during sustained somatic stimulation. Peak instantaneous firing frequency (F(p)) increased proportionally with the intensity of somatic stimulation (105+/-4 vs. 119+/-4 vs. 139+/-4 Hz, 10, 20 and 40 Hz, respectively, P<0.0001), whereas steady-state firing frequency (F(ss)) was not altered (25+/-2 vs. 27+/-2 vs. 27+/-2 Hz, 10, 20 and 40 Hz, respectively, P=0.72). Two indices were derived to quantify the decay properties. The decay rate constant (obtained from exponential curve fitting) was not altered by stimulation frequency (461+/-10 vs. 442+/-14 vs. 429+/-26 ms, 10, 20 and 40 Hz, respectively, P=0.415), nor was the decay index (derived to express the percent reduction in firing rate with respect to the initial peak firing rate; 76+/-2 vs. 77+/-2 vs. 81+/-2%, 10, 20 and 40 Hz, respectively, P=0.187). In contrast, the second population (seven of 43 cells) decreased their firing rate to stimulation. Of the NTS neurons tested for barosensitivity (29/36), none responded to pressure stimulation. These results have identified a population of somatosensitive NTS neurons that exhibit rapid firing rate decay properties during sustained stimulation. However, this population could faithfully encode phasic excitation during rhythmic somatosensory input. These results are discussed in relation to the role of somatosensory input on baroreflex function.

Acute nociceptive somatic stimulus sensitizes neurones in the spinal cord to colonic distension in the rat

The common co-existence of fibromyalgia and chronic abdominal pain could be due to sensitization of spinal neurones (SNs), as a result of viscero-somatic convergence. The objective of this study is to explore the influence of acute nociceptive somatic stimulation in the form of acid injections, into the ipsilateral somatic receptive field of neurones responsive to colorectal distension (CRD), and the potential role of ionotropic glutamate receptors on sensitization. Action potentials of CRD-sensitive SNs were recorded extracellularly from the lumbar (L(2)-L(5)) spinal cord. Stimulus-response functions (SRFs) to graded CRD (10-80 mmHg, 30 s) were constructed before and 30 min after ipsilateral injection of low pH (4.0, 100 microl) saline into the somatic receptive fields. In some experiments, cervical (C(1)-C(2)) spinalization was performed to eliminate supraspinal influence. The selective NMDA receptor antagonist CGS 19755 and AMPA receptor antagonist NBQX were injected (25 micromol kg(-1), i.v.) to examine their influence on sensitization. Three types of neurones were characterized as short-latency abrupt (SLA, n = 24), short latency sustained (SLS, n = 12), and long-latency (LL, n = 6) to CRD. Ipsilateral injection of low pH (4.0) in the somatic receptive field, but not the contralateral gastrocnemius (GN) or front leg muscles, sensitized responses of these neurones to CRD. Spinalization had no influence on the development of low pH-induced sensitization. Both CGS 19755 and NBQX significantly attenuated the sensitized response to CRD in intact and spinalized animals. Acute nociceptive somatic stimulus sensitizes CRD-sensitive SNs receiving viscero-somatic convergence. The sensitization occurs at the spinal level and is independent of supraspinal influence. Ionotropic glutamate receptors in the spinal cord are involved in sensitization.

Neurotrophins in spinal cord nociceptive pathways

Neurotrophins are a well-known family of growth factors for the central and peripheral nervous systems. In the course of the last years, several lines of evidence converged to indicate that some members of the family, particularly NGF and BDNF, also participate in structural and functional plasticity of nociceptive pathways within the dorsal root ganglia and spinal cord. A subpopulation of small-sized dorsal root ganglion neurons is sensitive to NGF and responds to peripheral NGF stimulation with upregulation of BDNF synthesis and increased anterograde transport to the dorsal horn. In the latter, release of BDNF appears to modulate or even mediate nociceptive sensory inputs and pain hypersensitivity. We summarize here the status of the art on the role of neurotrophins in nociceptive pathways, with special emphasis on short-term synaptic and intracellular events that are mediated by this novel class of neuromessengers in the dorsal horn. Under this perspective we review the findings obtained through an array of techniques in naïve and transgenic animals that provide insight into the modulatory mechanisms of BDNF at central synapses. We also report on the results obtained after immunocytochemistry, in situ hybridization, and monitoring intracellular calcium levels by confocal microscopy, that led to hypothesize that also NGF might have a direct central effect in pain modulation. Although it is unclear whether or not NGF may be released at dorsal horn endings of certain nociceptors in vivo, we believe that these findings offer a clue for further studies aiming to elucidate the putative central effects of NGF and other neurotrophins in nociceptive pathways.

Afferent pathways and responses of T3–T4 spinal neurons to cervical and thoracic esophageal distensions in rats

The purposes of this study were to (1) compare responses of T(3)-T(4) spinal neurons to thoracic and cervical esophageal distension (TED, CED) and (2) determine afferent pathways for esophageal input to these neurons. Extracellular potentials of single superficial and deeper T(3)-T(4) neurons were recorded in pentobarbital anesthetized male rats. Graded TED or CED was produced by water inflation (0.1-0.5 ml) of a latex balloon. TED changed activity of 121/432 (28%) neurons (114 were excited); CED activated 69/269 (26%) neurons (56 were excited). Of 151 neurons that were tested for responses to both TED and CED, 40 (26%) neurons responded to both TED and CED. Mean duration of excitatory responses in convergent neurons to TED was significantly longer than the duration of responses to CED (31.4+/-2.8 vs. 25.4+/-1.0 s, n=34, P<0.05). A total of 105 out of 121 (87%) and 66 out of 69 (96%) neurons responsive to TED and CED had somatic fields. Spinal transection at rostral C(1) and at C(7)-C(8) indicated that excitatory responses to TED resulted from activation of afferent input that entered thoracic spinal segments; whereas, excitatory responses to CED resulted from afferent inputs entering cervical or thoracic spinal segments. These data showed that the upper thoracic spinal cord received sensory information from the esophagus through cervical and/or thoracic spinal visceral afferent pathways.

Central projection of unmyelinated (C) primary afferent fibers from gastrocnemius muscle in the guinea pig

We have demonstrated the central projections of muscle C or group IV afferent fibers in the guinea pig by tracing arborizations in the spinal cord. C afferent fibers from the gastrocnemius muscle (GCM) were electrophysiologically identified by conduction velocity (less than 1 m/second). A single neuron in the lumbar 5 dorsal root ganglion (L5 DRG) was intracellularly labeled with Phaseolus vulgaris leucoagglutinin (PHA-L). After iontophoretic injection of PHA-L, we processed the lumbar cord and L5 DRG for PHA-L immunohistochemistry. Six muscle C afferent fibers from 40 animals were labeled, and whole trajectories were recovered. Labeled fibers were reconstructed by tracing of the arbor in serial parasagittal sections. The GCM C afferents projected rostrocaudally for two or three segments and ran at the surface of the dorsal funiculus, giving off collaterals into laminae I and II and sometimes into parts of lamina III. We determined, based on the branching pattern and form of the terminal plexus, that the branching of muscle C afferent fibers showed an intermediate pattern that fell morphologically between the terminal patterns of somatic and visceral afferents. The numbers and sizes of fiber swellings and terminal swellings were measured on all collateral branches. We found that the area of distribution of the terminal swellings of muscle C afferent fibers is larger than that of somatic terminals but that the density of terminal swellings in the terminal area was lower than that of the somatic terminals.

Central projections of thoracic splanchnic and somatic nerves and the location of sympathetic preganglionic neurons in Xenopus laevis

The central and peripheral organization of thoracic visceral and somatic nervous elements was studied by applying dextran amines to the proximal cut ends of the thoracic splanchnic and somatic nerves in Xenopus laevis. Many labeled dorsal root ganglion cells of visceral afferents, and all somatic afferents, were located in a single ganglion of one spinal segment, and the two types of cells were distributed topographically within the ganglion. The labeled sympathetic preganglionic neurons were located predominantly in the same area of the thoracic spinal gray as in other frogs and in mammals. The labeled visceral afferents projected to Lissauer's tract and the dorsal funiculus. The visceral fibers of the tract ascended to the level of the subcerebellar area, supplying collateral branches to the lateral one-third of the dorsal horn and to the area of brainstem nuclei, including lateral cervical and descending trigeminal nucleus, and descended to the filum terminale. The visceral fibers of the dorsal funiculus were distributed to the dorsal column nucleus and the solitary tract. A similar longitudinal projection was also seen in the somatic afferents. The dual central pathway of thoracic primary afferents in the anuran spinal cord is a property held in common with mammals, but the widespread rostrocaudal projection through Lissauer's tract may be a characteristic of the anuran central nervous system. In frogs, the direct transmission of primary afferent information to an extremely wide area of the central nervous system may be important for prompt assessment of environmental factors and control of body functions.

A comparative reappraisal of projections from the superficial laminae of the dorsal horn in the rat: The forebrain

Projections to the forebrain from lamina I of spinal and trigeminal dorsal horn were labeled anterogradely with Phaseolus vulgaris-leucoagglutinin (PHA-L) and/or tetramethylrhodamine-dextran (RHO-D) injected microiontophoretically. Injections restricted to superficial laminae (I/II) of dorsal horn were used primarily. For comparison, injections were also made in deep cervical laminae. Spinal and trigeminal lamina I neurons project extensively to restricted portions of the ventral posterolateral and posteromedial (VPL/VPM), and the posterior group (Po) thalamic nuclei. Lamina I also projects to the triangular posterior (PoT) and the ventral posterior parvicellular (VPPC) thalamic nuclei but only very slightly to the extrathalamic forebrain. Furthermore, the lateral spinal (LS) nucleus, and to a lesser extent lamina I, project to the mediodorsal thalamic nucleus. In contrast to lamina I, deep spinal laminae project primarily to the central lateral thalamic nucleus (CL) and only weakly to the remaining thalamus, except for a medium projection to the PoT. Furthermore, the deep laminae project substantially to the globus pallidus and the substantia innominata and more weakly to the amygdala and the hypothalamus. Double-labeling experiments reveal that spinal and trigeminal lamina I project densely to distinct and restricted portions of VPL/VPM, Po, and VPPC thalamic nuclei, whereas projections to the PoT appeared to be convergent. In conclusion, these experiments indicate very different patterns of projection for lamina I versus deep laminae (III-X). Lamina I projects strongly onto relay thalamic nuclei and thus would have a primary role in sensory discriminative aspects of pain. The deep laminae project densely to the CL and more diffusely to other forebrain targets, suggesting roles in motor and alertness components of pain.

Psychophysical analysis of visceral and cutaneous pain in human subjects

Clinical evidence suggests that cutaneous and visceral pain differ in sensory, affective, and motivational realms, yet there has been little comparative characterization of these types of pain. This study uses psychophysical measures to compare directly visceral and cutaneous pain and sensitivity. Healthy subjects (10 males, seven females, age 19-29) evaluated perceptions evoked by balloon distention of the distal esophagus and contact heat on the upper chest. Subjects gave continuous ratings of pain intensity using an on-line visual analog scale (VAS), reported maximum pain intensity and unpleasantness on printed VASs, chose phrases from the McGill Pain Questionnaire and Spielberger State-Trait Anxiety Inventory, and drew the area of perceived sensation. For esophageal distention, the threshold for pain intensity was higher than that observed for unpleasantness, whereas for contact heat, pain and unpleasantness thresholds did not differ for either phasic (10s) or tonic (36s) stimulus application. The relative unpleasantness, calculated as the difference between the unpleasantness and the intensity ratings, was higher during esophageal distention than during either phasic or tonic cutaneous heat; this difference in relative unpleasantness was seen at all intensities of esophageal stimulation. Subjects chose significantly more affective words and reported more anxiety during visceral pain than during phasic cutaneous heat pain. A similar tendency was observed when visceral pain was compared to tonic cutaneous heat pain. Subjects also chose a wider range of words to describe visceral than cutaneous pain. On-line VAS ratings revealed greater pain sensation after stimulus termination during visceral than during phasic cutaneous pain; likewise, a similar tendency was observed between visceral and tonic cutaneous pain. Finally, visceral pain led to a more spatially diffuse sensation and was referred to the entire chest and sometimes to the back. Our results show that visceral pain is more unpleasant, diffuse, and variable than cutaneous pain of similar intensity, independent of the duration of the presented stimuli. The data suggest the likelihood of both similarities and differences in the neural substrates underlying visceral and cutaneous pain.

Up-regulation of substance P and NMDA receptor mRNA in dorsal horn and preganglionic sympathetic neurons during adjuvant-induced noxious stimulation in rats

Substance P (SP) and glutamate-containing terminals are found in the dorsal horn and preganglionic sympathetic neurons (PSNs) in the intermedio-lateral nucleus of the spinal cord. SP receptor (SPR) and N-methyl-D-aspartate type glutamate receptor (NMDAR) were also recognized in portions of the dorsal horn and PSNs. Primary sensory nerve fibers containing SP and glutamate terminated around PSNs, or partly on PSNs directly as well as on dorsal horn neurons (DHNs). The present study was performed to investigate the changes in SPR and NMDAR mRNA expressions during nociception in rats. Upon the injection of complete Freund's adjuvant (CFA) into the front paw, edema and hyperalgesia occurred immediately, with the difference in latency score between injected and non-injected paws continuing to day 10. The up-regulation of SPR and NMDAR mRNAs in DHNs and PSNs was recognized using in situ hybridization and northern blot techniques. CFA injection increased SPR mRNA expression in PSNs at days 1 and 4, and NMDAR mRNA expression at days 1, 4 and 7. At day 14, the mRNA expression of both receptors decreased to the control level. These changes in the amount of receptor mRNAs in DHNs and PSNs may cause hyperalgesia and sympathetically mediated pain.

Visceral pain in humans: lessons from animals

Acute and persistent neuropathic and inflammatory injuries of healthy animals have contributed importantly to our current understanding of nociception and pain. Studies have differentiated somatic from visceral nociceptive input, and elucidated the pathways of transduction, transmission, perception, and modulation of the input. Other animal studies have identified important genetic and environmental influences on responses to nociception. Studies of naturally occurring visceral pain syndromes in animals also have added to our understanding of comparable syndromes in humans. Because of the aversive nature of pain, use of healthy animals to study pain in the service of other animals and humans is a decision to be taken carefully, and carries with it the responsibility of treating the animals as humanely as possible.

Morphological change of substance P receptor immunoreactive dendrites of preganglionic sympathetic neurons

Substance P (SP)-containing terminals and SP receptors (SPRs) are found on the dendrites of preganglionic sympathetic neurons (PSNs) in the intermedio-lateral nucleus (IML) of the spinal cord. The SP-containing fibers were thought to be of supraspinal origin. However, the primary sensory nerve fibers terminated around PSNs, and some of them directly on PSNs. We observed approximately 150 SPR-immunoreactive (ir) varicosities on the dendrites of PSNs in slices of the first thoracic segment (T1) in control rats. The number of varicosities decreased to 41% 14 days after hemisection of the spinal cord at the fourth cervical segment (C4), and to 55% 14 days after sectioning the spinal dorsal roots at the C8 and T1 levels. The number of varicosities decreased by 33% in 8-week-old rats which had been administered capsaicin subcutaneously within 24 hours after birth to eradicate unmyelinated sensory fibers. However, varicosities increased by 15% 15 minutes after injection of capsaicin into the plantar surface of the front paw to stimulate somatosensory nerve fibers in adult rats. The results demonstrate that SPR-ir varicosities on the dendrites of PSNs were modulated not only by the supraspinal nervous system but also directly by the primary sensory nerve terminals.

Fos expression in the rat brain and spinal cord evoked by noxious stimulation to low back muscle and skin

STUDY DESIGN

Acute noxious stimulation delivered to lumbar muscles and skin of rats was used to study Fos expression patterns in the brain and spinal cord.

OBJECTIVES

The present study was conducted to determine the differences in Fos expression in the brain and spinal cord as evoked by stimuli delivered to lumbar muscles and skin in rats.

SUMMARY OF BACKGROUND DATA

Patients with low back pain sometimes show psychological symptoms, such as quiescence, loss of interest, decreased activities, appetite loss, and restlessness. The pathway of deep somatic pain to the brain has been reported to be different from that of cutaneous pain. However, Fos expression has not been studied in the central nervous systems after stimulation of low back muscles.

METHODS

Rats were injected with 100 L of 5% formalin into the multifidus muscle (deep pain group; n = 10) and into the back skin of the L5 dermatome (cutaneous pain group; n = 10). Two hours after injection, the distribution of Fos-immunoreactive neurons was studied in the brain and spinal cord.

RESULTS

Fos-immunoreactive neurons were observed in laminae I-V in the spinal cord in the cutaneous pain group, but they were not seen in lamina II in the deep pain group. In the brain, Fos-immunoreactive neurons were significantly more numerous in the deep pain group than in the cutaneous pain group in the piriform cortex, the accumbens nucleus core, the basolateral nucleus of amygdala, the paraventricular hypothalamic nucleus, the ventral tegmental area, and the ventrolateral periaqueductal gray.

CONCLUSION

The finding that Fos-immunoreactive neurons were absent from lamina II of the spinal cord in the deep pain group is similar to that of the projection pattern of the visceral pain pathway. Fos expression in the ventrolateral periaqueductal gray in the deep pain group may represent a reaction of quiescence and a loss of interest, activities, or appetite. Furthermore, the detection of large numbers of Fos-immunoreactive neurons in the core of accumbens nucleus, basolateral nucleus of amygdala, paraventricular hypothalamic nucleus, and ventral tegmental area in the deep pain group may suggest a dominant reaction of dopaminergic neurons to stress, and a different information processing pathway than from that of cutaneous pain.

Spinal sources of noxious visceral and noxious deep somatic afferent drive onto the ventrolateral periaqueductal gray of the rat

Studies utilizing the expression of Fos protein as a marker of neuronal activation have revealed that pain of deep somatic or visceral origin selectively activates the ventrolateral periaqueductal gray (vlPAG). Previous anatomical tracing studies revealed that spinal afferents to the vlPAG arose from the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus at all spinal segmental levels, with approximately 50% of vlPAG-projecting spinal neurons found within the upper cervical spinal cord. This study utilized detection of Fos protein to determine the specific populations of vlPAG-projecting spinal neurons activated by noxious deep somatic or noxious visceral stimulation. Pain of cardiac or peritoneal (i.e., visceral) origin activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the thoracic cord, whereas pain of hindlimb (i.e., deep somatic) origin activated neurons in the same laminar regions but in the lumbosacral cord. Each of these deep noxious manipulations also activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the upper cervical spinal cord. In a second set of experiments, the combination of retrograde tracing and Fos immunohistochemistry revealed that vlPAG-projecting spinal neurons activated by deep somatic pain were located in both the upper cervical and lumbosacral cord, whereas those activated by visceral pain were restricted to the thoracic spinal cord. Thus pain arising from visceral versus deep somatic body regions influences neural activity within the vlPAG via distinct spinal pathways. The findings also highlight the potential significance of the upper cervical cord in integrating pain arising from deep structures throughout the body.

Physiological properties of the lamina I spinoparabrachial neurons in the rat

Single-unit extracellular recordings of spino-parabrachial (spino-PB) neurons (n = 53) antidromically driven from the contralateral parabrachial (PB) area were performed in the lumbar cord in anesthetized rats. All the spino-PB neurons were located in the lamina I of the dorsal horn. Their axons exhibited conduction velocities between 2.8 and 27.8 m/s, in the thin myelinated fibers range. They had an extremely low spontaneous activity (median = 0. 064 Hz) and a small excitatory receptive field (</=2 toes or pads). They were all activated by both peripheral A (mainly Adelta) and C fibers after intense transcutaneous electrical stimulation. Their discharge always increased in response to noxious natural stimuli of increasing intensities. The great majority (75%) of spino-PB neurons were nociceptive specific, i.e., they were excited only by noxious stimuli. The remaining (25%) still were excited primarily by noxious stimuli but also responded moderately to innocuous stimuli. Almost all spino-PB neurons (92%, 49/53) were activated by both mechanical and heat noxious stimuli. Among them, 35% were in addition moderately activated by noxious cold (thresholds between +20 and -10 degrees C). Only (8%, 4/53) responded exclusively to noxious heat. Spino-PB neurons clearly encoded the intensity of mechanical (n = 39) and thermal (n = 38) stimuli in the noxious range, and most of the individual stimulus-response functions were monotonic and positive up to 40/60 N. cm(-2) and 50 degrees C, respectively. For the mechanical modality, the mean threshold was 11.5 +/- 1.25 N. cm(-2) (mean +/- SE), the response increased almost linearly with the logarithm of the pressure between 10 and 60 N. cm(-2), the mean p(50) (pressure evoking 50% of the maximum response) and the maximum responsiveness were: 30 +/- 2.4 N. cm(-2) and 40.5 +/- 5 Hz, respectively. For the thermal modality, the mean threshold was 43.6 +/- 0.5 degrees C, the mean curve had a general sigmoid aspect, the steepest portion being in the 46-48 degrees C interval, the mean t(50) and the maximum responsiveness were: 47.4 +/- 0.3 degrees C and 40 +/- 4.4 Hz, respectively. Most of the spino-PB neurons tested (13/16) had their noxiously evoked responses clearly inhibited by heterotopic noxious stimuli. The mean response to noxious stimuli during heterotopic stimuli was 31.7 +/- 6.1% of the control response. We conclude that the nociceptive properties of the lamina I spino-PB neurons are reflected largely by those of PB neurons that were suggested to be involved in autonomic and emotional/aversive aspects of pain.

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.

Primary afferent fibers of the pelvic nerve terminate in the gracile nucleus of the rat

After injection of horseradish peroxidase (HRP) into the pelvic nerve of the rat, a small number of HRP-labeled axon terminals were found in the gracile nucleus. Double labeling experiments were also performed: Fluoro-Gold (FG) was injected into the pelvic nerve, while cholera toxin B subunit (CTb) was injected into the gracile nucleus or dorsal faciculus at the fifth and sixth cervical cord segments ipsilateral to the FG injection. About 5% of FG-labeled neurons were labeled with CTb in the L6-, S1- and S2-dorsal root ganglia ipsilateral to the tracer injection. The results suggest that some primary afferent information from pelvic visceral organs may be directly conveyed to gracile nucleus by the primary afferent neurons.

Synaptic inhibition of cat phrenic motoneurons by internal intercostal nerve stimulation

Intracellular recordings from 65 phrenic motoneurons (PMNs) in the C5 segment and recordings of C5 phrenic nerve activity were made in 27 pentobarbitone-anesthetized, paralyzed, and artificially ventilated adult cats. Inhibition of phrenic nerve activity and PMN membrane potential hyperpolarization (48/55 PMNs tested) was seen after stimulation of the internal intercostal nerve (IIN) at a mean latency to onset of 10.3 +/- 2.7 ms. Reversal of IIN-evoked hyperpolarization (n = 14) by injection of negative current or diffusion of chloride ions occurred in six cases, and the hyperpolarization was reduced in seven others. Stimulation of the IIN thus activates chloride-dependent inhibitory synaptic inputs to most PMNs. The inhibitory phrenic nerve response to IIN stimulation was reduced by ipsilateral transection of the lateral white matter at the C3 level and was converted to an excitatory response by complete ipsilateral cord hemisection at the same level. After complete ipsilateral hemisection of the spinal cord at C3 level, stimulation of the IIN evoked both excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) in PMNs (n = 10). It was concluded that IIN stimulation can evoke both excitatory and inhibitory responses in PMNs using purely spinal circuitry, but that excitatory responses are normally suppressed by a descending pathway in intact animals. Fifteen PMNs were tested for possible presynaptic convergence of inputs in these reflex pathways, using test and conditioning stimuli. Significant enhancement (>20%) of IPSPs were seen in seven of eight IIN-evoked responses using pericruciate sensorimotor cortex (SMC) conditioning stimuli, but only one of five IIN-evoked responses were enhanced by superior laryngeal nerve (SLN) conditioning stimuli. The IIN-evoked IPSP was enhanced in one of two motoneurons by stimulation of the contralateral phrenic nerve. It was concluded that presynaptic interneurons were shared by the IIN and SMC pathways, but uncommonly by other pathways. These results indicate that PMNs receive inhibitory synaptic inputs from ascending thoracocervical pathways and from spinal interneurons. These inhibitory reflex pathways activated by afferent inputs from the chest wall may play a significant role in the control of PMN discharge, in parallel with disfacilitation following reduced activity in bulbospinal neurons projecting to PMNs.

Increased c-fos expression in spinal lumbosacral projection neurons and preganglionic neurons after irritation of the lower urinary tract in the rat

Chemical irritation of the lower urinary tract (LUT) induces c-fos expression in neurons in the lumbosacral (L(6) and S(1)) spinal cord. This study used axonal tracing with fluorescent dyes to identify the types of spinal neurons expressing Fos immunoreactivity (IR) after LUT irritation in the rat. Fos-IR was detected in lateral and medial superficial dorsal horn, the sacral parasympathetic nucleus (SPN) and lamina X around the central canal. Fos-IR was detected in spinal neurons projecting to supraspinal sites (brainstem and hypothalamus), in preganglionic neurons (PGN) and in unlabeled segmental interneurons. A substantial percentage (20%) of dye labeled PGN exhibited Fos-IR after LUT irritation; and a larger percentage (36%) exhibited Fos-IR after electrical stimulation of the pelvic nerve which contains afferent pathways from all of the pelvic organs. The majority (average 55%) of Fos-positive neurons projecting to supraspinal sites were also located in the region of the SPN. A selective distribution of different types of neurons was detected in this region: PGN were located ventral to the spinal projection neurons which in turn were located ventral to the majority of unidentified Fos-positive neurons. The distribution of Fos-positive PGN and projection neurons was similar in spinal intact and spinal transected animals indicating that c-fos expression was mediated by monosynaptic afferent input or input from segmental interneurons and was not due to activation of supraspinal micturition reflex pathways.

Visceral pain

Visceral pain is the most common form of pain produced by disease and one of the most frequent reasons why patients seek medical attention. Yet much of what we know about the mechanisms of pain derives from experimental studies of somatic not visceral nociception. The conventional view is that visceral pain is simply a variant of somatic pain, a view based on the belief that a single neurological mechanism is responsible for all pain. However, the more we learn about the mechanisms of somatic and visceral pain, the more we realise that although these two processes have much in common, they also have important differences. Although visceral pain is an important part of the normal sensory repertoire of all human beings and a prominent symptom of many clinical conditions, not much clinical research has been done in this field and there are few clinical scientists with expertise in the management of visceral pain. Instead, visceral pain is usually treated by a range of specialists who take quite different approaches to the management of this type of pain. Thus, the management of visceral pain is frequently unsatisfactory. In this review, we consider visceral pain as a separate form of pain and examine its distinct sensory properties from a clinical perspective. We describe recent research findings that may change the way we think about visceral pain and, more importantly, may help develop new procedures for its management.

Substance P receptor-expressing neurons in the medullary and spinal dorsal horns projecting to the nucleus of the solitary tract in the rat

By using substance P receptor (SPR) immunofluorescence histochemistry combined with fluorescent retrograde labeling, we examined the distribution of the trigeminal and spinal neurons with SPR-like immunoreactivity (-LI) projecting to the nucleus of the solitary tract in the rat. After injection of Fluoro-Gold (FG) into the nucleus of the solitary tract, FG-labeled neurons showing SPR-LI were mainly seen in lamina I of the medullary and spinal dorsal horns, lamina V and the lateral spinal nucleus of the spinal cord. The present results suggest that the trigeminal and spinal neurons with SPR-LI, especially those in lamina I may be involved in the transmission of somatic and/or visceral nociceptive information from the medullary and spinal dorsal horns to the nucleus of the solitary tract.

Induction of c-fos gene expression by spinal cord transection in the rat

Sympathetic nerve activity is maintained after high spinal injury through circuits that remain in question. We evaluated patterns of c-fos gene induction as a monitor of spinal neurons responding to high spinal cord transection in the rat. Rats were anesthetized with isofluorane. Lower cervical or upper thoracic spinal segments were exposed, immersed in warm mineral oil and transected. Spinal cords were exposed but not transected in anesthetized controls. After 2.5 h, spinalized and control rats were perfused for immunocytochemistry. Cervical and thoracolumbar spinal segments and dorsal root ganglia were sectioned coronally. Tissues were incubated in primary, polyclonal antisera raised in rabbit or sheep against a peptide sequence unique to the N-terminal domain of Fos, and processed immunocytochemically. Neurons were induced to express Fos-like immunoreactivity (FLI), bilaterally, in the spinal gray, but not in primary sensory ganglia. Spinal cord transection induced neurons to express FLI in thoracic laminae I, IIo (outer substantia gelatinosa), Vre (lateral reticulated division), VII (lamina intermedia) and X, and the intermediolateral cell column. Lamina VIII was also labeled in spinal-injured but not in control animals. Immunolabeled nuclei were prominent in lumbar segments and were concentrated in the medial third of laminae I and IIo, and in laminae VII and X. Few cells were labeled in upper cervical or sacral segments. FLI was sparse in the spinal gray of controls and expressed mainly within the dorsal root entry zone of upper thoracic segments. Patterns of c-fos gene expression were site-specific and correlated with laminae that respond predominantly to noxious stimulation and that contain sympathetic interneurons. Laminae that are responsive to non-noxious stimuli and activated by walking, IIi, nucleus proprius, medial V and layer VI were not induced to express FLI. We conclude that neurons in specific spinal laminae that process high threshold afferents and that harbor neurons with sympathetic nerve-related activity are activated selectively by spinal cord transections. We hypothesize that peripheral afferents processed by spinal-sympathetic circuit neurons may regulate sympathetic discharge in the absence of supraspinal drive.

Induction of c-fos gene expression by spinal cord transection in Sus scrofa

Functional responses of primary sensory afferents and spinal cord were monitored in swine subjected to a high cervical (C1) spinal transection. Two and a half hours after transection, dorsal root ganglia and cervical and thoracolumbar spinal segments were processed immunocytochemically for the c-fos gene product, Fos and related antigens. In spinal-transected animals, Fos-like immunoreactivity (FLI) was induced in spinal laminae I, V, VII and X and the intermediolateral cell column but not in sensory ganglia as compared to controls: spinal-intact age-matched littermates. Spinal laminae expressing FLI harbor sympathetic and somatic interneurons and may aid in maintaining sympathetic outflow.

Differential expression of c-fos messenger RNA in the rat spinal cord after mucosal and serosal irritation of the stomach

Expression of the immediate early gene c-fos is considered to be a marker for neuronal activation in the spinal cord in response to afferent input. Since the stomach is continually exposed to injurious chemicals, the present study examined whether application of acid (0.15 M HCl) and formalin (5%) to the gastric mucosa or serosal surface of the stomach stimulates c-fos transcription in the caudal thoracic spinal cord of anaesthetized rats. The spinal cord was removed 15, 45 or 120 min after exposure of the stomach to the noxious chemicals and processed for quantitative in situ hybridization autoradiography of c-fos messenger RNA. Exposure of the gastric mucosa to acid or formalin failed to increase the expression of c-fos messenger RNA in the thoracic spinal cord. Application of acid to the serosal surface of the stomach was also unable to stimulate c-fos transcription, whereas serosal application of formalin led to substantial expression of c-fos messenger RNA in the superficial but also deeper laminae of the spinal dorsal horn when examined 45 min, but not 15 or 120 min, post-stimulation. The highest expression of c-fos messenger RNA was seen when formalin was injected subcutaneously into one hindpaw and c-fos transcription was examined in the lumbar spinal cord. These data indicate that acute exposure of the gastric mucosa to chemical injury does not provide the afferent input which is necessary to cause appreciable c-fos transcription in second order neurons within the spinal cord. Stimulation of the gastric mucosa by acid and formalin was followed, however, by gastric hyperaemia in which spinal afferents releasing vasodilator peptides have been implicated. It is concluded, therefore, that acute stimulation of nociceptive afferents in the stomach causes local homoeostatic reactions but does not necessarily provide afferent input sufficient to recruit spinal nociceptive circuits.

Spinal mu and delta, but not kappa, opioid-receptor agonists attenuate responses to noxious colorectal distension in the rat

The antinociceptive efficacy of different opioid-receptor agonists following their intrathecal (i.t.) administration was examined in awake, unanesthetized rats in a model of visceral pain. Cumulative i.t. doses of the mu-preferring opioid-receptor agonist morphine produced dose-dependent attenuation of the change (increase) in mean arterial pressure (delta MAP) and elevation of the visceromotor threshold to colorectal distension (CRD). Similar dose-dependent antinociceptive effects were produced after i.t. administration of the mu opioid-receptor-selective agonist DAMPGO. Morphine and DAMPGO were equipotent against the delta MAP to phasic CRD (80 mm Hg, 20 sec), but DAMPGO was more than 6 times more potent than morphine in elevating the visceromotor threshold to an incrementing CRD. Intrathecal administration of the delta opioid-receptor-selective agonist DPDPE produced, like morphine and DAMPGO, a dose-dependent attenuation of the delta MAP to CRD; DPDPE was one-tenth as potent as morphine or DAMPGO. DPDPE also dose-dependently elevated the visceromotor threshold to CRD, but its efficacy was only half that of morphine or DAMPGO. The kappa opioid-receptor-selective agonist U 50488H was without antinociceptive efficacy after i.t. administration, but did attenuate responses to CRD after systemic administration. The antinociceptive effects produced by morphine and DAMPGO were antagonized by i.t. pretreatment with naloxone and the effects produced by DPDPE were antagonized by i.t. pretreatment with the delta opioid-receptor-selective antagonist naltrindole. These data indicate that local mu and delta, but not kappa, opioid receptors can modulate visceral nociceptive transmission in the spinal cord.

Multichannel recording of cerebral potentials evoked by esophageal balloon distension in humans

In order to obtain insight in the generator localization of esophageal evoked potentials, cerebral responses were recorded from 32 scalp electrodes in five healthy volunteers (two male; three female; age 20-30 years), using series of 50 balloon inflations with 15 ml of air. Sequential topographical mapping of waveforms was performed in each subject. Biphasic waveforms were recorded. At Fpz, a positive deflection at 300 and a negative deflection at 465 msec (P300 and N465) were recorded and at Pz, N300 and P465. At Cz the first peak (N270) was slightly earlier than 300 msec. Waveforms were left to right symmetrical. At distal electrodes, biphasic waveforms were recorded (P300 and N465). In four subjects, a gradual phase shift occurred in between the waveforms at midline electrode Cz and the left and right mastoids. Brain mapping showed phase reversals between central negativity and surrounding positivity at about 300 msec, and between central positivity and surrounding negativity at 400-500 msec. Our data suggest the presence of more than one generator in the anterior and dorsal part of the insula and/or dorsal periinsular cortex.

Spinal cord lamina V and lamina VII interneuronal projections to sympathetic preganglionic neurons

This light and electron microscopic study sought to localize spinal cord interneurons that contribute to the normal and abnormal physiological regulation of spinal sympathetic preganglionic function. Sympathetic preganglionic neurons in caudal C8 through T4 of rat spinal cord were retrogradely labeled with wheat germ agglutinin (WGA) and/or cholera beta subunit (CT beta) following injections into the superior cervical ganglion (SCG). With two exceptions, the observed locations of retrogradely WGA- and CT beta-labeled sympathetic preganglionic neurons were as expected from previous studies. The exceptions were restricted populations of cells in caudal C8 and rostral T1 spinal segments. These neurons were classified as ventrolateral (vlSPN) and ventromedial (vmSPN) sympathetic preganglionic neurons; their somata and dendrites encircled dorsolateral lamina IX motoneurons. Only WGA was transported transneuronally following the retrograde labeling of sympathetic preganglionic neurons. Transneuronally WGA-labeled spinal interneurons were located principally in the reticulated division of lamina V and dorsolateral lamina VII. A strict segmental organization was observed. All transneuronally labeled interneurons were ipsilateral to, and coextensive with, retrogradely WGA-labeled sympathetic preganglionic neurons. Electron microscopic observations suggested that retrograde transsynaptic passage of WGA occurred within the sympathetic preganglionic neuropil and showed further that similar classes of organelles were WGA immunoreactive in retrogradely labeled sympathetic preganglionic neurons and in transneuronally labeled lamina V and lamina VII neurons: 1) cisternae and vesicles at the trans face of the Golgi apparatus, 2) large endosomes/dense bodies, and 3) multivesicular bodies. The data are consistent with two hypotheses: 1) Somatic and visceral primary afferent inputs to thoracic spinal cord modify segmental sympathetic preganglionic function through activation of a disynaptic pathway involving lamina V and/or lamina VII interneurons, and 2) long-loop propriospinal pathways access sympathetic preganglionic neurons through symmetrical, segmental interneuronal circuitry.

Anatomical properties of brainstem trigeminal neurons that respond to electrical stimulation of dural blood vessels

Single unit recording studies in anesthetized cats have identified a population of neurons in the brainstem trigeminal complex that can be activated by stimulation of major dural blood vessels. Such dura-responsive neurons exhibit response properties that are appropriate for a role in the mediation of vascular head pain in that they typically exhibit nociceptive facial receptive fields whose periorbital distribution is similar to the region of referred pain evoked by dural stimulation in humans. In the present study, intracellular labelling with horseradish peroxidase was used to examine the anatomical characteristics of brainstem trigeminal neurons that respond to dural stimulation. A total of 17 neurons was labelled that responded to electrical stimulation of dural sites overlying the superior sagittal sinus or middle meningeal artery. Fourteen of these neurons also responded to electrical stimulation of the cornea. The neurons in this sample were located in the rostral two-thirds of the trigeminal nucleus caudalis and the caudalmost part of the nucleus interpolaris. Within caudalis, the neurons were located in the deeper part of the nucleus, primarily lamina V, and were concentrated ventrolaterally. The dendritic arborizations of the dura-responsive neurons typically exhibited a dorsolateral-to-ventromedial orientation and did not extend into the superficial laminae of caudalis. Dura-responsive neurons had axonal collaterals and boutons in the nucleus caudalis, nucleus interpolaris, the infratrigeminal region ventral to nucleus interpolaris, the nucleus of the solitary tract, and the medullary reticular formation. The axonal boutons within the trigeminal complex exhibited a ventrolateral distribution which largely overlapped the distribution of the somata. The results are consistent with previous evidence that dura-responsive brainstem trigeminal neurons may have a role in the mediation of dural vascular head pain and also indicate that such neurons may contribute to nociceptive processing within the dorsal horn.

Effects of viscerosensory stimulation on hypothalamically elicited predatory behavior in cats

Hypnogenic (HS) or arousing (AS) stimulations of the small intestine (INT), splanchnic (SPL), and vagal (VAG) nerves were used to modify the predatory behavior (PB) elicited by stimulating the lateral hypothalamus (LHS). HS induced EEG synchronization and sleep. AS aroused the cat from slow-wave sleep. LHS induced the cat to attack an anesthetized rat and bite its neck after an exploratory activity. The following parameters of PB were determined: biting latency (BL), the interval between the beginning of LHS and the touching the rat by the cat's muzzle; exploratory time (ET), which begins with an environmental search and culminates in orienting toward the rat; attack time (AT), in which the cat stalks and bites the rat. HS, delivered for 5, 10, 15 min to INT, SPL, and VAG prior to LHS, increased BL and ET and did not affect AT. AS, delivered for 10 s to INT or VAG prior to LHS, decreased BL by reducing ET. SPL AS shortened BL by decreasing both ET and AT. The viscerosensory effects on PB were decreased by increasing the intensity of LHS; a ferocious attack with BL less than 10 s was not influenced by either HS or AS. These results indicate that the viscerosensory influence can modify PB by inhibiting or facilitating the priming events of the attack.

Distribution of immunoreactivity for enkephalin, substance P and vasoactive intestinal peptide in fibres surrounding splanchnic sympathetic preganglionic neurons in rats

The distribution of substance P, enkephalin and vasoactive intestinal peptide in fibres and cells was examined in the autonomic nuclei of the lower thoracic and lumbar segments of the rat spinal cord. Attention was focussed on the location of the peptides in sympathetic preganglionic neurons contributing to the greater and lesser splanchnic nerves and in fibres surrounding these neurons. To identify splanchnic preganglionic neurons, Fluoro-Gold was applied to the left splanchnic nerve in anaesthetized rats and some of these animals received intrathecal administration of colchicine at thoracic segments 6, 9 and 12, 24-48 h before perfusion with fixative. Immunoreactivity for substance P, enkephalin and vasoactive intestinal peptide in fibres and cells of the sixth thoracic to second lumbar spinal cord was detected with fluorescent immunocytochemical techniques. Most retrogradely labelled cells (90%) were located in the intermediolateral nucleus and the rest were situated in the nucleus intercalatus and the central autonomic nucleus of the gray matter. Terminals of fibres containing immunoreactivity to all three peptides were found in all autonomic regions. Fibres immunoreactive for substance P and enkephalin were seen projecting in the white matter to the region of the intermediolateral nucleus and extending from this nucleus to the central autonomic nucleus. Terminals containing each of the three peptides were also found surrounding the retrogradely labelled cells in the intermediolateral nucleus. Approximately two cells immunoreactive for vasoactive intestinal peptide were found per section and 80% were located in the autonomic regions. Fewer cells immunoreactive for substance P and enkephalin were observed (approximately one per section) and 70% were outside laminae VII and X. Although cells immunoreactive for substance P, enkephalin and vasoactive intestinal peptide were located in all autonomic regions of the spinal cord, cells doubly labelled with retrograde dye and with the antisera to either of the peptides could not be identified. The data suggest that (i) substance P, enkephalin and vasoactive intestinal peptide are contained in fibres of neurons regulating preganglionic sympathetic control of the abdominal viscera and its vasculature; and (ii) these peptides may not be major transmitters within splanchnic preganglionic neurons.

Spinal projections of pelvic visceral afferents of the rat: a calcitonin gene-related peptide (CGRP) immunohistochemical study

Little information is available concerning the pelvic visceral afferent system, in view of its terminal location in the spinal cord and its associated transmitter substances in the rat. By utilizing an immunostaining method to examine the transneuronal neurotransmitter depletion resulting from peripheral sensory nerve injury, the spinal projections of primary afferent fibers containing calcitonin gene-related peptide (CGRP) and originating from pelvic viscera were studied in the lumbosacral spinal cord of the rat. After unilateral or bilateral pelvic nerve section, CGRP immunoreactivity in the lumbosacral spinal cord was decreased greatly in the sacral parasympathetic nucleus (SPN), the dorsolateral fasiculus, the medial border of the dorsal horn, the dorsal gray commissure (DGC), and the intermediate gray connecting the SPN and DGC. Fine structural analysis showed that the CGRP-immunoreactive terminals made synaptic contact with dendrites and, rarely, with somata. Although there was some incidence of a synaptic contact between a CGRP-IR terminal and a vesicle-containing profile, definite evidence of axo-axonal synapse has not been confirmed. These data indicate that CGRP-containing pelvic visceral primary afferent fibers project to autonomic areas of the lumbosacral spinal cord by way of the pelvic nerve and make synaptic contact with dendrites and somata.

Spinal and hindbrain structures involved in visceroception and visceronociception as revealed by the expression of Fos, Jun and Krox-24 proteins

We have used the evoked expression of the immediate early gene-encoded proteins (Krox-24, c-Fos, Fos B, Jun D, Jun B, c-Jun) to monitor visceral processing in both the spinal cord and hindbrain structures of rats undergoing either mechanical colorectal or chemical intraperitoneal stimulation. Experiments were conducted under controlled volatile anaesthesia to suppress affective reactions that visceral stimulations may induce. The results refer to the effects of anaesthesia alone, and of both innocuous and noxious stimulations. Non-nociceptive and nociceptive stimulation but not anaesthesia were effective in evoking c-Fos, c-Jun, Jun B and Krox-24 expressions in the spinal cord. Intraperitoneal injections labelled cells mostly at the thoracolumbar junction levels, while colorectal distension labelled cells mostly at the lumbrosacral junction levels. Labelling was widely distributed throughout the gray matter including superficial layers, deep dorsal horn, lamina X and sacral parasympathetic columns. Krox-24- and, to a lesser degree, c-Jun-labelled cells were quite numerous in the superficial layers of the dorsal horn; Jun B, and especially c-Fos, were very effective in demonstrating inputs to all parts of the spinal cord. Both anaesthesia and noxious visceral stimulation were effective in evoking c-Fos, Krox-24 and Jun B expressions in discrete hindbrain subregions. The structures which are primarily labelled under anaesthesia are the rostral ventrolateral medulla, the external medial and lateral nuclei of the parabrachial area, the medial and dorsal subnuclei of the nucleus of the solitary tract, the area postrema, the central gray including pars alpha and nucleus O, the nucleus beta of the inferior olive, the locus coeruleus, and the inferior colliculi and adjacent parts of central gray. The structures which are primarily labelled following noxious visceral stimulation are the caudal intermediate reticular nucleus as part of the caudalmost ventrolateral medulla and the superior lateral nucleus of the rostrolateral parabrachial area. Labelling in the caudal intermediate reticular nucleus was maximal for colorectal distension. Labelling in the superior lateral nucleus was specific to peritoneal inflammation. The Edinger-Westphal nucleus is a structure in which noxious-evoked labelling was superposed onto the anaesthesia-evoked labelling. Nociception-evoked overexpression in this nucleus was maximal for intraperitoneal inflammation. The present work demonstrates that the central effects induced by either anaesthesia or visceroception including pain can be effectively monitored through the induction of an array of immediate early genes.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative analysis of central terminal projections of visceral and somatic unmyelinated (C) primary afferent fibers in the guinea pig

In guinea pigs, intracellular labeling of the dorsal root ganglion (DRG) cells with Phaseolus vulgaris-leucoagglutinin (PHA-L) was used to demonstrate the central projections of somatic and visceral afferent C-fibers. The terminations of the afferent fibers were analyzed qualitatively and quantitatively with the aid of camera lucida drawings. Terminal branches of C-fibers of both somatic and visceral origin were, in general, distributed in accord with the organization of the neuropil in lamina of the spinal cord. Terminal boutons arranged from longitudinally coursing fibers were distributed in lamina I, while boutons in lamina II were scattered in an apparent random fashion. The synaptic enlargements were counted in gray matter of the spinal dorsal horn and measured on each terminal branch of a fiber. All synaptic boutons (over one thousand) of somatic fibers were found in the superficial dorsal horn (laminae I and II). More than 60% of the synaptic enlargements of the visceral afferents also were localized superficially (lamina I and adjacent dorsal funiculus) while 10-20% of the visceral enlargements appeared in deeper layers of the spinal cord. Boutons of somatic C-fibers were larger than those of visceral origin. Quantitative data of the unmyelinated afferent fibers are discussed in the context of the sensory functions of myelinated afferent fibers.

Propriospinal input to thoracolumbar sympathetic nuclei from cervical and lumbar lamina I neurons in the cat and the monkey

The possibility that specific thermoreceptive and nociceptive influences on sympathetic outflow are conveyed directly to spinal sympathetic regions by lamina I neurons was investigated anatomically with the immunofluorescent PHA-L technique in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-II in the cervical (C6-8) or lumbar (L6-7) enlargement. Bilateral (symmetric) terminal arborizations were observed (with an ipsilateral predominance) in the intermediolateral, intermediomedial, and intervening regions of the thoracolumbar intermediate zone. In serial horizontal sections, patches of terminal labeling appeared at regular longitudinal intervals in the intermediolateral region. Longitudinally coursing fibers that had multiple varicosities and gave off small terminal branches were observed in the intermediolateral and the intermediomedial regions. Mediolateral strips of labeling that extended from labeling in the intermediolateral region to labeling in the intermediomedial region occurred at fairly regular longitudinal intervals. Because the longitudinal distribution of these terminations corresponds very well with the characteristic (ladder-like) longitudinal pattern of organization of the neuropil of the thoracolumbar sympathetic nuclei, i.e., the principal part of the intermediolateral cell column, the central autonomic n., and the intervening n. intercalatus, it is inferred that these lamina I terminations occur within these nuclei. After cervical injections, the labeling was most dense in the upper thoracic T2-4 spinal cord segments in both the cat and the monkey; labeling was also present in the T10-12 segments. After lumbar injections, labeling in the cat was located in the L4 segment; labeling in the monkey was present in the T4-6 and T10-12 segments. The labeling obtained was much more dense in the monkey than in the cat. These observations reveal a spinal lamina I projection that could provide a direct pathway for the somatosympathetic reflex effects of thermal and noxious stimuli. Considered together with reports that lamina I and the sympathetic nuclei both receive descending input from certain key autonomic regions, this result emphasizes the importance of lamina I for homeostasis, in addition to its probable roles in behavioral arousal, affect, and sensation. These observations thus support the proposed concept that lamina I processes and distributes in a functionally specific manner the sensory input relevant to the physiological status of the tissues and organs of the entire organism.

Projection patterns of primary sensory neurons studied by transganglionic methods: somatotopy and target-related organization

The anatomical organization of the centrally projecting branches of different peripheral sensory nerves was not possible to investigate efficiently until the development of the axonal tracing methods. Horseradish peroxidase applied peripherally could be visualized in central projection areas provided a sensitive histochemical method was used; this created the basis for transganglionic tracing from the periphery. This has permitted the investigation of large-scale projections from peripheral sensory nerves. The use of conjugates of horseradish peroxidase and lectins with affinities for different populations of primary sensory neurons, as well as the use of different postoperative survival times, has offered the possibility for selective visualization of projections from subsets of primary sensory neurons. For detailed studies of single afferent fiber projections, a combined physiological-anatomical approach using single-unit recording followed by intraaxonal application of horseradish peroxidase, has become the method of choice. This chapter will focus on results which have been achieved by transganglionic tracing methods, in regard to the organization of the central projections of peripheral sensory nerves.

The representation of prolonged and intense, noxious somatic and visceral stimuli in the ventrolateral orbital cortex of the cat

The responses of single neurones in the ventrolateral orbital (VLO) cortex to noxious pinch, heating of the skin, twisting of the joints and distension of the gall bladder were studied in cats anaesthetized with halothane. Of 60 neurones studied, 44 responded to prolonged (greater than 10 sec) stimuli that were well within the noxious range. Neurones were relatively unresponsive to innocuous stimuli or to the transient application of noxious stimuli. Many single neurones responded to a variety of modalities of noxious stimuli (e.g., skin heating and gall bladder distension). Many neurones studied showed a fluctuating level (5-15 Hz) of ongoing spontaneous activity. Neurones responded with either an increased frequency of spikes (excitation) or an inhibition of spontaneous discharge, irrespective of the source of noxious stimulation. Noxious stimuli delivered simultaneously to two different tissues (e.g., skin and visceral) sometimes produced excitation of the neurone under study, to levels above that produced by the application a noxious stimulus to only one of the tissues. Receptive fields were often large involving both contralateral and ipsilateral areas of the body, as well as both fore and hind limbs. No evidence of somatotopic organization was obtained. The responses of some neurones outlasted the application of the stimuli by many minutes. It is concluded that single neurones in the ventrolateral orbital cortex respond to the prolonged application of intensely noxious stimuli to a variety of body tissues, in a manner that is in keeping with the involvement of this cortical area in both the physiological, autonomic and experiential components of the affective-motivational aspect of pain. Furthermore, from the consequences of lesion studies in man and animals, it is proposed that the activation of cells in the orbital cortex by a variety of noxious stimuli reflects its more general role in the development and maintenance of behaviour in response to negative reinforcement of both social and physical origins.

Spinal distribution of ascending lamina I axons anterogradely labeled with Phaseolus vulgaris leucoagglutinin (PHA-L) in the cat

The location of the ascending axons of spinal lamina I cells was studied in cats that received injections of Phaseolus vulgaris leucoagglutinin (PHA-L) in the superficial dorsal horn of the cervical or lumbosacral enlargement. Lamina I axons that could be ascribed to the spinothalamic tract (STT) were of particular interest. The cases were divided into three sets: in seven optimal cases the injections were restricted to lamina I; in ten nominal cases the injections involved laminae I-II or laminae I-III and occasionally lamina IV; and in eight mixed cases laminae I-V were injected. Since ipsilateral propriospinal and bilateral supraspinal axons originate from laminae I and V, but only ipsilateral propriospinal axons from laminae II-IV, this categorization facilitated a comparative analysis. Ascending axons labeled immunohistochemically with avidin/Texas Red were observed in oblique transverse sections from the C1, C3/4, T6, T12, and L3/4 levels. Incidental axonal labeling occurred in the ipsilateral dorsal columns because of passing primary afferent fiber uptake and, in nominal and mixed cases with involvement of laminae III-IV, in the superficial dorsolateral funiculus at the location of the spinocervical tract. Ipsilateral ascending lamina I axons in optimal cases were located in Lissauer's tract and in the white matter adjacent to the dorsal horn. Since these appeared to terminate in lamina I, and few remained at C1, they were ascribed to propriospinal projections. Contralateral ascending lamina I axons in optimal and nominal cases were distributed throughout the dorsal and ventral portions of the lateral funiculus (LF), but, despite considerable variability between animals in their location and dispersion, they were consistently concentrated in the middle of the LF (i.e., at the level of the central canal). This concentration was observed in a slightly more ventral location at C1, and a similar but weaker concentration of lamina I axons was located slightly more dorsally in C1 on the ipsilateral side. These supraspinal lamina I projections were ascribed to the spinomesencephalic tract (SMT) and to the STT. In mixed cases, additional ascending axons ascribed to lamina V cells were labeled in the ventrolateral and ventral funiculi. Many labeled axons were found in this region following a large injection of biocytin into lumbosacral laminae V-VIII in a supplementary case. These results thus together support previous descriptions of a dorsoventral distribution of STT axons according to laminar origin, but they contradict recent reports that lamina I axons ascend in the dorsolateral funiculus.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensory innervation of the canine esophagus, stomach, and duodenum

The sensory innervation of the postpharyngeal foregut was investigated by injecting the enzyme horseradish peroxidase (HRP) into the walls of the esophagus, stomach, or duodenum. The transported HRP was identified histochemically, labeled neurons in the spinal and vagal ganglia were counted, and the results were plotted using an SAS statistical program. The spinal sensory fields of each viscus were defined using three determinations: craniocaudal extent, principal innervation field, and peak innervation field. The data revealed that innervation fields are craniocaudally extensive, the sensory field of each viscus overlaps significantly with its neighbor, yet each viscus can be characterized by a field of peak innervation density. Craniocaudal innervation of the esophagus spans as many as 22-23 paired spinal ganglia (C1-L2). There are two peak innervation fields for the cervical (C2-C6 and T2-T4) and for the thoracic (T2-T4 and T8-T12) sectors of the esophagus. The sensory innervation of the stomach extends craniocaudally over as many as 25 paired spinal ganglia (C2-L5). The peak innervation field of the stomach spans a large area comprising the cranial, middle, and the immediately adjoining caudal thoracic ganglia (T2-T10). The duodenum is innervated craniocaudally by as many as 15 paired thoracolumbar ganglia (T2-L3). Peak innervation originates in the middle and caudal thoracic ganglia and cranial lumbar (T6-L1) ganglia. There is a recognizable viscerotopic organization in the sensory innervation of the postpharyngeal foregut; successively more caudal sectors of this region of the alimentary canal are supplied with sensory fibers from successively more caudal spinal dorsal root ganglia. Vagal afferent innervation of the esophagus, stomach, and duodenum is bilateral and originates predominantly, but not exclusively, from vast numbers of neurons in the nodose (distal) ganglia. The esophagus is innervated bilaterally and more abundantly by jugular (proximal) ganglia neurons than is either the stomach or duodenum. The physiological significance of the findings are discussed in relation to the phenomena of visceral pain and referred pain.

[125I]vasoactive intestinal polypeptide binding sites: quantitative autoradiographic distribution in the rat spinal cord

The quantitative autoradiographic distribution of [125I]vasoactive intestinal polypeptide (VIP) receptor binding sites was investigated in the rat spinal cord. [125I]VIP binding sites are discretely distributed, with a rostro-caudal gradient, along the longitudinal length of the cord; highest densities of sites being observed in its lumbar and sacral segments. In transverse sections, highest levels of [125I]VIP sites are present in laminae I and II, around the central canal, and in the parasympathetic lateral horn of the sacral segment. Moderate densities are seen along the medial border of the dorsal horn and the sympathetic lateral horn of the thoracic cord. Low amounts of labeling are observed in most structures of the ventral horn while white matter areas are apparently devoid of specific [125I]VIP binding. Thus, the distribution of spinal [125I]VIP receptor sites correlates well with that of VIP-like immunoreactive materials and support possible roles for this peptide in sensory neurotransmission and in the control of autonomic functions.

Differential c-fos-like protein expression in mechanically versus chemically induced visceral nociception

The expression of c-fos-like protein has been suggested to be a marker for neuronal activity in nociceptive processing. The immunohistochemical detection of this protein was used to determine if different visceral noxious stimuli induce distinct patterns in the rat spinal cord. We have developed a mechanical visceral pain model which is based on the acute distention of the duodenum yielding a quantifiable behavioral endpoint, writhing-like activity. One hour following either intraperitoneal injection of acetic acid or the distention of the duodenum via a chronically implanted balloon catheter, the animals were processed for the immunocytochemical detection of c-fos-like protein in the spinal cord. Characteristic patterns of c-fos-like immunoreactivity were observed following each type of stimulus that differed in spinal laminar and segmental distribution, number of neurons expressing fos-like immunoreactivity and staining intensity. The chemical noxious stimulus induced c-fos bilaterally in laminae I and X predominantly in the thoraco-lumbar region of the spinal cord. In contrast, the mechanical noxious stimulus induced a greater number and more intense neuronal c-fos-like protein expression in laminae I-VI, IX and X. These data provide further evidence that there is a differential nociceptive modulation in mechanical noxious visceral stimulation.

Ultrastructural localization of substance P, met-enkephalin, and somatostatin immunoreactivity in lamina X of the primate spinal cord

The ultrastructural localization of substance P (SP), met-enkephalin (MENK), and somatostatin (SS) in the lamina X area surrounding the central canal of the macaque monkey was examined by the indirect peroxidase-antiperoxidase method. The most common synaptic terminals in lamina X were simple terminals (S) with small rounded or pleomorphic clear vesicles; one to two dense-core vesicles were occasionally also present. These were found on soma, dendrites, and dendritic spines, in all regions of lamina X. A second class of terminal with round or oval clear vesicles was glomerular (G) in shape, with scalloped edges, and contained many mitochondria. These large terminals had several synaptic contacts onto dendrites, spines, and small terminals and were found mainly in the lateral region. The third class (L) contained small clear vesicles and several vesicles with large, dense cores (100-125 nm), and also contacted dendrites, mainly lateral to the canal. The fourth class of terminal (D) contained small clear vesicles and several vesicles with small, dense cores (75-100 nm); these contacted dendrites and somata in all areas. Very few terminals with flat vesicles were identified. There was an unequal distribution of immunoreactivity among the several terminal classes identified in lamina X. Most SP terminals were S terminals, but SP L terminals were also common; few were D terminals. MENK terminals were usually either S terminals or D terminals; L terminals were rarely MENK positive. SS terminals were commonly D terminals or S terminals; L terminals were also rarely SS positive. Only SP terminals were identified as G terminals. Synaptic targets of SP, MENK, and SS terminals were most commonly dendrites. In addition to unlabelled neurons, peptidergic neurons and their processes were also synaptic targets of terminals containing the same peptide. The distributions of these peptides in primate lamina X differ from that of the same peptides in primate superficial dorsal horn. These differences are important, in consideration of some of the parallels that may be drawn between the lamina X area and the superficial dorsal horn; both areas have high concentrations of the same peptides, receive nociceptive primary afferents, and contain spinothalamic and other projection neurons. Nevertheless, comparison of the distribution of immunoreactivity among terminal classes indicates that neurochemical organization at the ultrastructural level is quite distinct in each of the two areas. This may also reflect other roles of the lamina X area, including its involvement in visceral functions, although it would be expected that this element might be less prominent at the cervical levels we investigated.

Splanchnic input to thoracic spinal neurons and its supraspinal modulation in the rat

The urinary responses of 62 T8-T11 spinal neurons were recorded extracellularly following electrical stimulation of the greater splanchnic nerve (GSN) in chloralose-anesthetized rats. Recorded neurons were found in both the dorsal and ventral horns. Fifty-seven neurons increased their firing rate in response to GSN stimulation; 8 of these exhibited biphasic responses consisting of excitations followed by inhibitions. Excitatory responses to GSN stimulation consisted of either one or two bursts with latencies consistent with activation by either A delta or C fibers. GSN stimulation inhibited 5 neurons. The effects of reversible spinalization on spontaneous activity and on both synchronous and non-synchronous (afterdischarge) GSN-evoked responses were investigated using a cooling probe on the spinal cord between C1 and C2. Of 19 neurons tested in this way, 9 exhibited opposite directional changes in their spontaneous activities and their GSN-evoked responses upon spinalization. Differential effects of cold-block on first and second bursts, or on A delta- and C-fiber mediated responses, were not usually observed. However, differential effects of cold-block on synchronous and non-synchronous portions of the overall GSN-evoked response were often observed in that their magnitudes often changed independently of one another. Supraspinal pathways contributed to GSN-evoked responses of several neurons because their responses were diminished during cooling while spontaneous activity was increased or unchanged. These decreases in the magnitude of the GSN-evoked response were not always accounted for by decreases in the synchronous portions of the responses. However, most neurons did exhibit decreases in the number of non-synchronous responses, or afterdischarges, during spinal cooling, exhibiting in some cases biphasic responses. This study provides evidence for strong supraspinal regulation of splanchnic afferent input to the spinal cord of the rat. Further, this regulation exhibits some specificity toward different portions of splanchnic-evoked responses in spinal neurons.