Interneurons presynaptic to rat tail-flick motoneurons as mapped by transneuronal transport of pseudorabies virus: few have long ascending collaterals

Regional Differences Within the Anterior Cingulate Cortex in the Generation Versus Suppression of Pain Affect in Rats

The anterior cingulate cortex (ACC) modulates emotional responses to pain. Whereas, the caudal ACC (cACC) promotes expression of pain affect, the rostral ACC (rACC) contributes to its suppression. Both subdivisions receive glutamatergic innervation, and the present study evaluated the contribution of N-methyl-d-aspartic acid (NMDA) receptors within these subdivisions to rats' expression of pain affect. Vocalizations that follow a brief noxious tail shock (vocalization afterdischarges, VAD) are a validated rodent model of pain affect. The threshold current for eliciting VAD was increased in a dose-dependent manner by injecting NMDA into the rACC, but performance (latency, amplitude, and duration) at threshold was not altered. Alternately, the threshold current for eliciting VAD was not altered following injection of NMDA into the cACC, but its amplitude and duration at threshold were increased in a dose-dependent manner. These effects were limited to Cg1 of the rACC and cACC, and blocked by pretreatment of the ACC with the NMDA receptor antagonist d-2-amino-5-phosphonovalerate. These findings demonstrate that NMDA receptor agonism within the cACC and rACC either increases or decreases emotional responses to noxious stimulation, respectively. PERSPECTIVE: NMDA receptor activation of the rostral and caudal ACC respectively inhibited or enhanced rats' emotional response to pain. These findings mirror those obtained from human neuroimaging studies; thereby, supporting the use of this model system in evaluating the contribution of ACC to pain affect.

The Influence of Placebo Analgesia Manipulations on Pain Report, the Nociceptive Flexion Reflex, and Autonomic Responses to Pain

Expectations for pain relief and experience/conditioning are psychological factors that contribute to placebo analgesia, yet few studies have studied the physiological mechanisms underlying their effects. This study randomized 133 participants to 4 groups: an expectation only (E-only) group, a conditioning only (C-only) group, an expectation plus conditioning (E+C) group, and a natural history (NH) control group. Painful electric stimulations were delivered before and after an inert cream was applied to the site of stimulation. Pain-related outcomes (pain ratings, nociceptive flexion reflex [NFR], skin conductance response, and heart rate acceleration) were recorded after each stimulation. NFR (a measure of spinal nociception) assessed if placebo analgesia inhibited spinal processing of pain. E+C was the only manipulation that significantly inhibited pain and skin conductance response. Surprisingly, NFR was facilitated in the E+C and E-only groups. No effects were noted for C-only. Mediation analysis suggested 2 descending processes were engaged during E+C that influenced spinal nociception: 1) descending facilitation and 2) descending inhibition that was also responsible for pain reduction. These results suggest that E+C manipulations produce the strongest analgesia and have a complex influence on spinal nociception involving both inhibitory and facilitatory processes. PERSPECTIVE: This study assessed whether placebo analgesia manipulations that include expectations, conditioning, or both modulate the NFR (measure of spinal nociception). Only the manipulation that involved expectations and conditioning inhibited pain, but both expectation manipulations facilitated NFR. This suggests a complex modulation of spinal neurons by placebo manipulations.

Anatomical evidence of pruriceptive trigeminothalamic and trigeminoparabrachial projection neurons in mice

Itch is relayed to higher centers by projection neurons in the spinal and medullary dorsal horn. We employed a double-label method to map the ascending projections of pruriceptive and nociceptive trigeminal and spinal neurons. The retrograde tracer fluorogold (FG) was stereotaxically injected into the right thalamus or lateral parabrachial area (LPb) in mice. Seven days later, mice received intradermal (id) microinjection of histamine, chloroquine, capsaicin, or vehicle into the left cheek. Histamine, chloroquine, and capsaicin intradermally elicited similar distributions of Fos-positive neurons in the medial aspect of the superficial medullary and spinal dorsal horn from the trigeminal subnucleus caudalis to C2. Among neurons retrogradely labeled from the thalamus, 43%, 8%, and 22% were Fos-positive following id histamine, chloroquine, or capsaicin. Among the Fos-positive neurons following pruritic or capsaicin stimuli, ∼1-2% were retrogradely labeled with FG. Trigeminoparabrachial projection neurons exhibited a higher incidence of double labeling in the superficial dorsal horn. Among the neurons retrogradely labeled from LPb, 36%, 29%, and 33% were Fos positive following id injection of histamine, chloroquine, and capsaicin, respectively. Among Fos-positive neurons elicited by id histamine, chloroquine, and capsaicin, respectively, 3.7%, 4.3%, and 4.1% were retrogradely labeled from LPb. The present results indicate that, overall, relatively small subpopulations of pruriceptive and/or nociceptive neurons innervating the cheek project to thalamus or LPb. These results imply that the vast majority of pruritogen- and algogen-responsive spinal neurons are likely to function as interneurons relaying information to projection neurons and/or participating in segmental nocifensive circuits.

How are we going to make progress treating bladder pain syndrome? ICI-RS 2013

AIMS

To look at the current state of knowledge in bladder pain syndrome and ascertain how we can make advances in the near term.

METHODS

A compendium of the ideas presented at the International Consultation on Incontinence Research Society 2013 meeting of clinicians and basic scientists.

RESULTS

The meeting included the following topics: potential connection between defined and undefined IC/BPS; association between psychiatric disorders and IC/BPS; rationale for multimodal therapy approach in IC/BPS; and issues of a placebo control in human studies.

CONCLUSIONS

Translational research studies are still in need of improved animal models to study IC/BPS mechanisms and development of novel methods to objectively measure bladder pain in rodents. The need to try and develop better clinical therapies will best be met by proper phenotyping of this heterogeneous population and avoiding premature publication of clinical trials that are anecdotal and do not include randomized placebo control populations. Patients with Hunner's lesions should be identified prior to or in the course of clinical trials so that results in this subgroup can be evaluated.

Models of inflammation of the lower urinary tract

Inflammation of the lower urinary tract occurs frequently in people. The causes remain obscure, with the exception of urinary tract infection. Animal models have proven useful for investigating and assessing mechanisms underlying symptoms associated with lower urinary tract inflammation and options for suppressing these symptoms. This review will discuss various animal models of lower urinary tract inflammation, including feline spontaneous (interstitial) cystitis, neurogenic cystitis, autoimmune cystitis, cystitis induced by intravesical instillation of chemicals or bacterial products (particularly lipopolysaccharide or LPS), and prostatic inflammation initiated by transurethral instillation of bacteria. Animal models will continue to be of significant value in identifying mechanisms resulting in bladder inflammation, but the relevance of some of these models to the causes underlying clinical disease is unclear. This is primarily because of the lack of understanding of causes of these disorders in people. Comparative and translational studies are required if the full potential of findings obtained with animal models to improve prevention and treatment of lower urinary tract inflammation in people is to be realized.

Hyperactivity, startle reactivity and cell-proliferation deficits are resistant to chronic lithium treatment in adult Nr2e1(frc/frc) mice

The NR2E1 region on Chromosome 6q21-22 has been repeatedly linked to bipolar disorder (BP) and NR2E1 has been associated with BP, and more specifically bipolar I disorder (BPI). In addition, patient sequencing has shown an enrichment of rare candidate-regulatory variants. Interestingly, mice carrying either spontaneous (Nr2e1(frc) ) or targeted (Tlx(-) ) deletions of Nr2e1 (here collectively known as Nr2e1-null) show similar neurological and behavioral anomalies, including hypoplasia of the cerebrum, reduced neural stem cell proliferation, extreme aggression and deficits in fear conditioning; these are the traits that have been observed in some patients with BP. Thus, NR2E1 is a positional and functional candidate for a role in BP. However, no Nr2e1-null mice have been fully evaluated for behaviors used to model BP in rodents or pharmacological responses to drugs effective in treating BP symptoms. In this study we examine Nr2e1(frc/frc) mice, homozygous for the spontaneous deletion, for abnormalities in activity, learning and information processing, and cell proliferation; these are the phenotypes that are either affected in patients with BP or commonly assessed in rodent models of BP. The effect of lithium, a drug used to treat BP, was also evaluated for its ability to attenuate Nr2e1(frc/frc) behavioral and neural stem cell-proliferation phenotypes. We show for the first time that Nr2e1-null mice exhibit extreme hyperactivity in the open field as early as postnatal day 18 and in the home cage, deficits in open-field habituation and passive avoidance, and surprisingly, an absence of acoustic startle. We observed a reduction in neural stem/progenitor cell proliferation in Nr2e1(frc/frc) mice, similar to that seen in other Nr2e1-null strains. These behavioral and cell-proliferation phenotypes were resistant to chronic-adult-lithium treatment. Thus, Nr2e1(frc/frc) mice exhibit behavioral traits used to model BP in rodents, but our results do not support Nr2e1(frc/frc) mice as pharmacological models for BP.

Acid-sensing ion channel 3 expression in mouse knee joint afferents and effects of carrageenan-induced arthritis

Arthritis is associated with decreases in local pH. Of the acid-sensing ion channels (ASIC), ASIC3 is most sensitive to such a pH change, abundantly expressed in dorsal root ganglion (DRG), and critical for the development of secondary hyperalgesia. The purpose of this study was to investigate the upregulation of ASIC3, using an acute arthritic pain model in mice. We examined ASIC3 expression in DRG neurons innervating the knee joint with and without carrageenan-induced arthritis by means of retrograde labeling and immunohistochemistry. We also examined the difference of DRG phenotype between ASIC3+/+ and ASIC3-/- mice. ASIC3 immunoreactivity was present in 31% of knee joint afferents and dominantly in small cells. After joint inflammation, ASIC3-immunoreactive neurons significantly increased in number by 50%. Calcitonin gene-related peptide (CGRP) increased similarly in both ASIC3+/+ and ASIC3-/- mice. Soma size distribution of ASIC3-immunoreactive neurons without CGRP expression was shifted to smaller-diameter neurons. Our results suggest that ASIC3 plays an important role in acute arthritic pain. Specifically, we propose that ASIC3 upregulation along with CGRP and phenotypic change in ASIC3-immunoreactive neurons without CGRP are responsible for the development of secondary hyperalgesia after carrageenan-induced arthritis.

PERSPECTIVE

This article shows that ASIC3 is upregulated along with CGRP in knee joint afferents and that there is a phenotypic change in ASIC3-immunoreactive nonpeptidergic neurons in an animal model of acute arthritis. Understanding the basic neurobiology after acute arthritis could lead to future new pharmacological management of arthritis.

Differential effects of the cannabinoid receptor agonist, WIN 55,212-2, on lamina I and lamina V spinal trigeminal nucleus caudalis neurons

Direct application of cannabinoids to the medullary dorsal horn (MDH) inhibits lamina V nociceptive neurons. The present study compared the effect of the cannabinoid receptor agonist, WIN 55,212-2 (WIN-2) on the activity of lamina I and lamina V MDH neurons using extracellular single unit recording in anesthetized rats. Activity evoked by a contact thermode was measured before and after local application of WIN-2 (0.5-2.0 microg/microl) to the brainstem. Fast and slow heat ramps were used to differentiate between activity evoked primarily by A-delta and C primary afferent fibers, respectively. In lamina V neurons, WIN-2 produced a concentration dependent decrease in activity evoked by both fast and slow heat, reaching significance at 1.0 microg/microl. In lamina I neurons, WIN-2 administration inhibited slow heat-evoked activity beginning at 1.0 microg/microl, but had no significant effect on fast heat-evoked activity, even at the highest concentration (2.0 microg/microl). In separate experiments, the effect of intrathecal administration of WIN-2 to the MDH on head withdrawal latencies elicited by fast and slow heat ramps applied to the whisker pad was assessed in lightly anesthetized rats. Head withdrawal latencies elicited by slow but not by fast heat stimulation were increased by WIN-2. Taken together, these results emphasize the importance of lamina I neurons in the control of a nociceptive heat-evoked reflex.

Multi-tasking in the spinal cord--do 'sympathetic' interneurones work harder than we give them credit for?

The role of interneurones in the control of sympathetic activity has been somewhat of a mystery since, for many years, it was difficult to target these cells for study. Recently scientists have started to unravel the action potential properties of these neurones, where they receive their inputs from and where they project to. This review looks at the information known to date about sympathetic interneurones. The locations of these neurones and their local axonal ramifications suggest that they play a more widespread function than previously thought. Therefore the data to support such a theory are also examined.

Long ascending propriospinal projections from lumbosacral to upper cervical spinal cord in the rat

The retrograde tracer cholera toxin beta-subunit (CTB) was used to trace long ascending propriospinal projections from neurons in the lumbosacral spinal cord to the upper cervical (C3) gray matter in adult male Sprague-Dawley rats. Following large 0.5 microl CTB injections restricted mainly to the upper cervical ventral horn (n=5), there were many lumbosacral CTB-positive neurons (14-17/section) in the intermediate gray and ventral horn (dorsal lamina VIII, medial VII extending into X) contralaterally, with fewer at corresponding ipsilateral locations. Labeled cells (4-8/section) were also observed in contralateral laminae IV-VI and the lateral spinal nucleus, with fewer ipsilaterally. Few labeled cells (<2/section) were observed in superficial laminae I-II. Smaller (0.15 microl) microinjections of CTB restricted to the upper cervical ventral gray matter labeled cells in contralateral laminae VII-VIII (approximately 6-9/section) with fewer ipsilaterally. There were relatively fewer (<2/section) in the intermediate dorsal horn and very few (<1/section) in lamina I. Larger (0.5 microl) CTB injections encompassing the C3 dorsal and ventral gray matter on one side labeled significantly more CTB-positive neurons (>6/section) in contralateral lamina I compared to ventral horn injections. These results suggest direct projections from ventromedially located neurons of lumbar and sacral segments to the contralateral ventral gray matter of upper cervical segments, as well as from neurons in the intermediate but not superficial dorsal horn. They further suggest that some lumbosacral superficial dorsal horn neurons project to the upper cervical dorsal horn. These propriospinal projections may be involved in coordinating head and neck movements during locomotion or stimulus-evoked motor responses.

Afferent pathways to the region of the vestibular nuclei that participates in cardiovascular and respiratory control

Prior experiments have shown that a region of the medial and inferior vestibular nuclei contributes to cardiovascular and respiratory regulation. In addition to labyrinthine inputs, the majority of neurons in this region of the vestibular nuclei receive signals from the skin, muscle, and viscera, although the pathways conveying these nonlabyrinthine inputs to the vestibular nucleus neurons are unknown. To gain further insight into the afferent pathways to this functionally distinct subdivision of the vestibular complex, we combined monosynaptic mapping with viral transneuronal tracing in the ferret. First order afferent projections were defined by retrograde transport of the beta-subunit of cholera toxin (CTbeta), and the extended polysynaptic circuitry was defined in the same animals by injection of a recombinant of pseudorabies virus Bartha (PRV) into the contralateral vestibular nuclei. Neurons containing CTbeta or infected by retrograde transneuronal transport and replication of PRV were distributed throughout the spinal cord, but were 10 times more prevalent in the cervical cord than the lumbar cord. The labeled spinal neurons were most commonly observed in Rexed's laminae IV-VI and the dorsal portions of laminae VII-VIII. Both the CTbeta and PRV injections also resulted in labeling of neurons in all four vestibular nuclei, the prepositus hypoglossi, the reticular formation, the inferior olivary nucleus, the medullary raphe nuclei, the spinal and principal trigeminal nuclei, the facial nucleus, and the lateral reticular nucleus. Following survival times >/=3 days, PRV-infected neurons were additionally present in nucleus solitarius and the gracile and cuneate nuclei. These data show that an anatomical substrate is present for somatosensory and visceral inputs to influence the activity of cells in the autonomic region of the vestibular nuclei and suggest that these signals are primarily transmitted through brainstem relay neurons.

Modulation of sympathetic and somatomotor function by the ventromedial medulla

The ventromedial medulla is implicated in a variety of functions including nociceptive and cardiovascular modulation and the control of thermoregulation. To determine whether single microinjections into the ventromedial medulla elicit changes in one or multiple functional systems, the GABA(A) receptor antagonist bicuculline was microinjected (70 nl, 5-50 ng) into the ventromedial medulla of lightly anesthetized rats, and cardiovascular, respiratory, and nociceptive measures were recorded. Bicuculline microinjection into either the midline raphe or the laterally adjacent reticular nucleus simultaneously increased interscapular brown adipose tissue temperature, heart rate, blood pressure, expired [CO(2)], and respiration rate and elicited shivering. Bicuculline microinjection also decreased the noxious stimulus-evoked changes in heart rate and blood pressure, decreased the frequency of heat-evoked sighs, and suppressed the cortical desynchronization evoked by noxious stimulation. Although bicuculline suppressed the motor withdrawal evoked by noxious tail heat, it enhanced the motor withdrawal evoked by noxious paw heat, evidence for specifically patterned nociceptive modulation. Saline microinjections into midline or lateral sites had no effect on any measured variable. All bicuculline microinjections, midline or lateral, evoked the same set of physiological effects, consistent with the lack of a topographical organization within the ventromedial medulla. Furthermore, as predicted by the isodendritic morphology of cells in the ventromedial medulla, midline bicuculline microinjection increased the number of c-fos immunoreactive cells in both midline raphe and lateral reticular nuclei. In summary, 70-nl microinjections into ventromedial medulla activate cells in multiple nuclei and elicit increases in sympathetic and somatomotor tone and a novel pattern of nociceptive modulation.

Affective analgesia following the administration of morphine into the amygdala of rats

The amygdala processes stimuli that threaten the individual and organizes the execution of affective behaviors that permit the individual to cope with the threat. The prototypical threat to an individual is exposure to a noxious stimulus. The present study evaluated the contribution of the amygdala in modulating the affective response of rats to noxious stimulation. Vocalization afterdischarges (VADs) are a validated model of the affective response of rats to noxious tailshock. The antinociceptive action of morphine microinjected into the amygdala on VAD thresholds was compared to its effect on the thresholds of other tailshock-elicited responses (vocalizations during shock, VDS and spinal motor reflexes, SMRs). Whereas VADs are organized within the forebrain, VDSs and SMRs are organized at medullary and spinal levels of the neuraxis, respectively. The bilateral administration of morphine into the basolateral complex of the amygdala (BLC) produced dose-dependent increases in VAD and VDS thresholds, although increases in VAD thresholds were significantly greater than increases in VDS thresholds. Administration of morphine into BLC was ineffective in elevating SMR thresholds. Morphine-induced increases in vocalization thresholds were reversed in a dose-dependent manner by microinjection of the opiate receptor antagonist methylnaloxonium into BLC. Microinjection of morphine in the vicinity to the BLC did not alter vocalization thresholds. The present results provide further evidence for the preferential involvement of the amygdala in modulation of the affective component of the pain experience.

Transneuronal tracing from sympathectomized lumbar epaxial muscle in female rats

Pseudorabies virus (PRV) has been used as a transneuronal tracer to study central neural networks, including the central control of the lordosis-producing, lumbar epaxial muscles. Within muscles, however, the sympathetic innervation of blood vessels poses a confounding source of tracer labeling in the CNS. The present study destroyed sympathetic nerves before injection of PRV, thereby allowing for a more selective uptake by somatic motoneurons. Specifically, a focal sympathectomy was created by the injection of dopamine-beta-hydroxylase immunotoxin (DHIT). When PRV was injected into control rats, both somatic motoneurons within the ventral horn of the spinal cord and sympathetic preganglionic neurons within the intermediolateral column (IML) of the spinal cord became labeled. Additionally, labeled neurons were observed in many brain regions, including those previously implicated in the control of the lordosis reflex (e.g., the medullary reticular formation; MRF) and those previously implicated in the control of vasomotor tone (e.g., the rostral ventrolateral medulla; RVLM). When injected into DHIT-pretreated animals, PRV labeling in ventral horn neurons persisted in many animals; however, labeling in IML was eliminated in almost every case. In these animals, PRV labeling was absent in brain areas traditionally associated with vasomotor tone, such as RVLM, whereas labeling persisted in brain areas previously implicated in the control of the lordosis response, such as MRF. The results support the connectivity of spinal and medullary structures with the somatic control of the lordosis-producing muscles and provide a more detailed description of these portions of the putative lordosis-relevant neurocircuitry.

Contributions of the medullary raphe and ventromedial reticular region to pain modulation and other homeostatic functions

The raphe magnus is part of an interrelated region of medullary raphe and ventromedial reticular nuclei that project to all areas of the spinal gray. Activation of raphe and reticular neurons evokes modulatory effects in sensory, autonomic, and motor spinal processes. Two physiological types of nonserotonergic cells are observed in the medullary raphe and are thought to modulate spinal pain processing in opposing directions. Recent evidence suggests that these cells may modulate stimulus-evoked arousal or alerting rather than pain-evoked withdrawals. Nonserotonergic cells are also likely to modulate spinal autonomic and motor circuits involved in thermoregulation and sexual function. Medullary serotonergic cells have state-dependent discharge and are likely to contribute to the modulation of pain processing, thermoregulation, and sexual function in the spinal cord. The medullary raphe and ventromedial reticular region may set sensory, autonomic, and motor spinal circuits into configurations that are appropriate to the current behavioral state.

Effects of intravenous and intrathecal sufentanil on a C-fibre reflex elicited by a wide range of stimulus intensities in the rat

A C-fibre reflex elicited by electrical stimulation within the territory of the sural nerve was recorded from the ipsilateral biceps femoris muscle in anaesthetised, intact rats, and in anaesthetised rats whose brains had been transected at the level of the obex. The temporal evolution of the response was studied by recording recruitment curves built with stimulus intensities from 0 to 10 times threshold. Both i.v. and i.t. sufentanil resulted in dose-dependent depressions of the reflex. Increasing the stimulus intensity from 1.5 to 10 times threshold resulted in an increase in the ED(50) from 0.58 (0.40-0.86) to 2.40 (1.87-3.31) microgram/kg for i.v. sufentanil and from 0.64 (0.46-0.79) to 1.63 (1.29-3.31) microgram/kg for i.t. sufentanil. With increasing stimulus intensity, the dose-response curves showed a progressive shift to the right, but this shift was only slight with the highest intensity stimuli. The ratios for the ED(50)s for i.v. to i.t. sufentanil were near 1. Following i.v. administration, sufentanil also facilitated the C-fibre reflex and produced tonic inter-stimulus discharges. They disappeared after the i.v. injection of naloxone. In the obex-transected rats, the depressive effect of sufentanil increased, while the facilitations and tonic inter-stimulus discharges disappeared. These findings suggest that the analgesic effects of i.v. ant i.t. sufentanil are similar, probably because sufentanil is highly soluble in lipids. Sufentanil-induced facilitations relate to supraspinal actions on motor controls and/or on the descending control of nociceptive transmission.

Anatomical evidence for two spinal 'afferent-interneuron-efferent' reflex pathways involved in micturition in the rat: a 'pelvic nerve' reflex pathway and a 'sacrolumbar intersegmental' reflex pathway

We labeled interneurons in the L1-L2 and L6-S1 spinal cord segments of the rat that are involved in bladder innervation using transneuronal retrograde transport of pseudorabies virus (PRV) in normal animals and in animals with selected nerve transections. Preganglionic neurons were identified using antisera against choline acetyltransferase (ChAT). In some experiments we labelled parasympathetic preganglionic neurons (PPNs) in the L6-S1 spinal cord by retrograde transport of Fluorogold from the major pelvic ganglion. We identified bladder afferent terminals using the transganglionic transport of the anterograde tracer cholera toxin subunit b. We present anatomical evidence for two spinal pathways involved in innervation of the bladder. First, in the intact rat, afferent information from the bladder connects, via interneurons in L6-S1, to the PPNs that provide the efferent innervation of the bladder. The afferent terminals were located mainly in close apposition to interneurons located dorsal to the retrogradely labeled PPNs. Second, using L6-S1 ganglionectomies or L6-S1 ventral root rhizotomies we limited viral transport to the sympathetic pathways innervating the bladder. This procedure also labelled interneurons (but not PPNs) with PRV in the L6-S1 spinal cord in a location very similar to those described in the intact rat. These interneurons also receive bladder afferent terminals but we propose that they project to sympathetic preganglionic neurons, most of which are in the L1-L2 spinal segments. Based on this anatomical evidence, we propose the existence of two spinal reflex pathways involved in micturition: a pathway limited to a reflex arc in the pelvic nerve (presumably excitatory to the detrusor muscle); and a pathway involving the pelvic nerve and sympathetic nerve fibers, some of which may travel in the hypogastric (presumably inhibitory to the detrusor muscle).

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Spatial and temporal patterns of transneuronal labeling in CNS neurons after injection of pseudorabies virus into the sciatic nerve of adult rats

The distribution of labeled neurons in the brain and spinal cord was studied after injecting the Bartha strain of pseudorabies virus (PRV) into the sciatic nerve to provide a baseline for studying neural circuitry after spinal cord injury (SCI) and regeneration. Following a single injection of viral particles into the left sciatic nerve, PRV labeling was found in the spinal cord at 2 days post-injection (p.i.). Increasing complexity in viral labeling from the spinal cord to supraspinal regions became apparent with increasing survival time. In brain regions, several neuronal groups that regulate sympathetic outflow, such as the rostroventrolateral medulla, the lateral paragigantocellular nuclei, and the A5 cells, were densely labeled. However, relatively sparse labeling was noticed in the lateral vestibular nuclei, the red nucleus and the motor cortex whose spinal projections regulate somatic motor function, although those areas were abundantly labeled with Fast blue (FB) in a double-labeling experiment in which FB was co-injected into the lumbar cord. The pattern of viral labeling became more complex beyond 5 days p.i. when increased numbers of cell groups were labeled with PRV but not FB. In addition, some infected neurons started to lyse, as evidenced by a decrease in viral labeling at 7 days p.i. Thus, the 5th day post-viral injection would appear to be an appropriate survival time to obtain maximal labeling with acceptable specificity. We suggest that transneuronal labeling using PRV should be appropriate for studying multi-neural circuitry after SCI and regeneration.

Secondary hyperalgesia to punctate mechanical stimuli. Central sensitization to A-fibre nociceptor input

Tissue injury induces enhanced pain sensation to light touch and punctate stimuli in adjacent, uninjured skin (secondary hyperalgesia). Whereas hyperalgesia to light touch (allodynia) is mediated by A-fibre low-threshold mechanoreceptors, hyperalgesia to punctate stimuli may be mediated by A- or C-fibre nociceptors. To disclose the relative contributions of A- and C-fibres to the hyperalgesia to punctate stimuli, the superficial radial nerve was blocked by pressure at the wrist in nine healthy subjects. Secondary hyperalgesia was induced by intradermal injection of 40 microg capsaicin, and pain sensitivity in adjacent skin was tested with 200 micron diameter probes (35-407 mN). The progress of conduction blockade was monitored by touch, cold, warm and first pain detection and by compound sensory nerve action potential. When A-fibre conduction was blocked completely but C-fibre conduction was fully intact, pricking pain to punctate stimuli was reduced by 75%, but burning pain to capsaicin injection remained unchanged. In normal skin without A-fibre blockade, pain ratings to the punctate probes increased significantly by a factor of two after adjacent capsaicin injection. In contrast, pain ratings to the punctate probes were not increased after capsaicin injection when A-fibre conduction was selectively blocked. However, hyperalgesia to punctate stimuli was detectable immediately after block release, when A-fibre conduction returned to normal. In conclusion, the pricking pain to punctate stimuli is predominantly mediated by A-fibre nociceptors. In secondary hyperalgesia, this pathway is heterosynaptically facilitated by conditioning C-fibre input. Thus, secondary hyperalgesia to punctate stimuli is induced by nociceptive C-fibre discharge but mediated by nociceptive A-fibres.

Central mechanisms of pain modulation

Bulbospinal serotonergic neurons and two physiological classes of bulbospinal nonserotonergic cells interact to modulate pain transmission. Recent studies have begun to elaborate targets of descending pain modulation other than the well-studied flexion withdrawal pathways. Site-specific, naloxone-sensitive placebo analgesia, which is hard to reconcile with current models of descending pain modulation, presents an exciting challenge to the field.

Central neuronal circuit innervating the lordosis-producing muscles defined by transneuronal transport of pseudorabies virus

The lordosis reflex is a hormone-dependent behavior displayed by female rats during mating. This study used the transneuronal tracer pseudorabies virus (PRV) to investigate the CNS network that controls the lumbar epaxial muscles that produce this posture. After PRV was injected into lumbar epaxial muscles, the time course analysis of CNS viral infection showed progressively more PRV-labeled neurons in higher brain structures after longer survival times. In particular, the medullary reticular formation, periaqueductal gray (PAG), and ventromedial nucleus of the hypothalamus (VMN) were sequentially labeled with PRV, which supports the proposed hierarchical network of lordosis control. Closer inspection of the PRV-immunoreactive neurons in the PAG revealed a marked preponderance of spheroid neurons, rather than fusiform or triangular morphologies. Furthermore, PRV-immunoreactive neurons were concentrated in the ventrolateral column, rather than the dorsal, dorsolateral, or lateral columns of the PAG. Localization of the PRV-labeled neurons in the VMN indicated that the majority were located in the ventrolateral subdivision, although some were also in other subdivisions of the VMN. As expected, labeled cells also were found in areas traditionally associated with sympathetic outflow to blood vessels and motor pathways, including the intermediolateral nucleus of the spinal cord, the paraventricular hypothalamic nucleus, the red nucleus, and the motor cortex. These results suggest that the various brain regions along the neuraxis previously implicated in the lordosis reflex are indeed serially connected.

Physiological properties of raphe magnus neurons during sleep and waking

Neurons in the medullary raphe magnus (RM) that are important in the descending modulation of nociceptive transmission are classified by their response to noxious tail heat as ON, OFF, or NEUTRAL cells. Experiments in anesthetized animals demonstrate that RM ON cells facilitate and OFF cells inhibit nociceptive transmission. Yet little is known of the physiology of these cells in the unanesthetized animal. The first aim of the present experiments was to determine whether cells with ON- and OFF-like responses to noxious heat exist in the unanesthetized rat. Second, to determine if RM cells have state-dependent discharge, the activity of RM neurons was recorded during waking and sleeping states. Noxious heat applied during waking and slow wave sleep excited one group of cells (ON-U) in unanesthetized rats. Other cells were inhibited by noxious heat (OFF-U) applied during waking and slow wave sleep states in unanesthetized rats. NEUTRAL-U cells did not respond to noxious thermal stimulation applied during either slow wave sleep or waking. ON-U and OFF-U cells were more likely to respond to noxious heat during slow wave sleep than during waking and were least likely to respond when the animal was eating or drinking. Although RM cells rarely respond to innocuous stimulation applied during anesthesia, ON-U and OFF-U cells were excited and inhibited, respectively, by innocuous somatosensory stimulation in the unanesthetized rat. The spontaneous activity of >90% of the RM neurons recorded in the unanesthetized rat was influenced by behavioral state. OFF-U cells discharged sporadically during waking but were continuously active during slow wave sleep. By contrast, ON-U and NEUTRAL-U cells discharged in bursts during waking and either ceased to discharge entirely or discharged at a low rate during slow wave sleep. We suggest that OFF cell discharge functions to suppress pain-evoked reactions during sleep, whereas ON cell discharge facilitates pain-evoked responses during waking.

Paradoxical inhibition of nociceptive neurons in the dorsal horn of the rat spinal cord during a nociceptive hindlimb reflex

Nociceptive-specific and multireceptive neurons in the lumbar dorsal horn are excited by noxious stimuli applied to the hindpaw and inhibited by noxious stimuli applied to distant body regions. Given that at least a subset of these neurons are part of the circuit for nociceptive reflexes, inhibition of nociceptive-specific and multireceptive neurons should inhibit nociceptive reflexes. Unfortunately, previous attempts to test this hypothesis have been inconclusive because of methodological differences between electrophysiological and behavioral experiments. The present study overcame this problem by recording neural and reflex activity simultaneously. Rats were anesthetized with halothane and surgically prepared for single-unit recording from the lumbar dorsal horn. Hindpaw heat caused a burst of activity that reliably preceded hindpaw withdrawal in 10 nociceptive-specific and 17 multireceptive neurons. A distant noxious stimulus (tail in 50 degrees C water or ear pinch) inhibited the evoked activity of both nociceptive-specific and multireceptive neurons and simultaneously changed the topography of the hindpaw reflex from flexion to extension without altering reflex latency. The present data are consistent with previous reports of inhibition of nociceptive-specific and multireceptive neurons during application of a distant noxious stimulus. However, inhibition of nociceptive-specific and multireceptive neurons concomitant with a shift in the hindlimb reflex from flexion to extension suggests that these neurons are part of the circuit for flexor reflexes specifically. Presumably, lateral inhibition from the flexor to extensor circuit allows for the release of hindlimb extension when neurons in the flexion circuit are inhibited by a distant noxious stimulus. Such a system reduces the chance of injury by allowing for withdrawal reflexes to a single noxious stimulus and escape reactions, such as running and jumping, to multiple noxious stimuli.

Activation of CNS circuits producing a neurogenic cystitis: evidence for centrally induced peripheral inflammation

We present a model of neurogenic cystitis induced by viral infection of specific neuronal circuits of the rat CNS. Retrograde infection by pseudorabies virus (PRV) of neuronal populations neighboring those that innervate the bladder consistently led to a localized immune response in the CNS and bladder inflammation. Infection of bladder circuits themselves or of circuits distant from these rarely produced cystitis. Absence of virus in bladder and urine ruled out an infectious cystitis. Total denervation of the bladder, selective C-fiber deafferentation, or bladder sympathectomy prevented cystitis without affecting the CNS disease, indicating a neurogenic component to the inflammation. The integrity of central bladder-related circuits is necessary for the appearance of bladder inflammation, because only CNS lesions affecting bladder circuits, i.e., bilateral dorsolateral or ventrolateral funiculectomy, as well as bilateral lesions of Barrington's nucleus/locus coeruleus area, prevented bladder inflammation. The close proximity in the CNS of noninfected visceral circuits to infected somatic neurons would thus permit a bystander effect, leading to activation of the sensory and autonomic circuits innervating the bladder and resulting in a neurogenic inflammation localized to the bladder. The present study indicates that CNS dysfunction can bring about a peripheral inflammation.

Efferent connections of the lamina terminalis, the preoptic area and the insular cortex to submandibular and sublingual gland of the rat traced with pseudorabies virus

Neurones situated in the lamina terminalis (organum vasculosum of the lamina terminalis, median preoptic nucleus and subfornical organ) as well as within medial and lateral parts of the preoptic area and in the insular cortex become transneuronally labelled following pseudorabies virus injections into the submandibular or the sublingual gland. These neurones are efferently connected to a chain of central neurones directed to secretory or vascular tissue of the submandibular or the sublingual gland. By varying the postinoculation time a stepwise infection of different forebrain nuclei was registered, with the hypothalamic paraventricular nucleus and the lateral hypothalamic area being the first forebrain structures labelled. Such early infected neurones within these hypothalamic nuclei are in all likelihood third order neurones regulating salivary secretion and might have functioned as relays transmitting virus to other forebrain structures. The above mentioned forebrain areas together with several other hypothalamic nuclei as well as the bed nucleus of the stria terminalis, the central nucleus of the amygdala and the substantia innominata, seem to be the widespread anatomical basis for the central regulation of salivary gland function.

Analgesia and abuse potential: an accidental association or a common substrate?

The fact that centrally acting analgesics have abuse potential commensurate with their analgesic activity raises the question of whether these effects are related. The abuse potential of drugs depends on their ability to produce reinforcing effects, which are mediated by a neural system that includes the ventral tegmental dopamine cells and their connections with the ventral striatum. Morphine and amphetamine are both powerful analgesics and have high abuse potential. Their analgesic and reinforcing effects are mediated by similar receptors, similar sites of action, and overlapping neural substrates. These coincidences suggest that reinforcers may produce analgesia by transforming the aversive affective state evoked by pain into a more positive affective state. The implications of this hypothesis and its relation to other known mechanisms of analgesia are discussed. The hypothesis predicts that drugs with reinforcing effects should produce analgesia. A survey of drugs acting through 21 classes of receptors reveals that in 13 classes there is evidence for both analgesic and reinforcing effects that are approximately equipotent. The GABA(A) agonists were found to be the only drugs with confirmed abuse potential that lack analgesic activity. The interpretation of this and several other anomalous cases is discussed.

Secondary hyperalgesia and perceptual wind-up following intradermal injection of capsaicin in humans

Wind-up and secondary hyperalgesia both are related to central sensitization, but whereas the former is explained by homosynaptic facilitation, the latter is due to heterosynaptic facilitation. To investigate possible interactions between both types of facilitation, we tested for alterations of perceptual wind-up in the secondary hyperalgesic skin zone adjacent to a capsaicin injection with light touch (by a cotton wisp) and punctate stimuli (calibrated von Frey hairs and pin pricks). Temporal summation of pain sensation (perceptual wind-up) was only observed with a clearly noxious stimulus (pin prick) presented at a repetition frequency of 0.6 s(-1), but not 0.2 s(-1). Pain ratings to trains of pin pricks reached a plateau after 3-4 repetitions, which was 1.65 times the initial rating ('wind-up ratio'). Injection of capsaicin induced a tenderness to mechanical stimuli in adjacent uninjured skin (secondary hyperalgesia), including hyperalgesia to light touch (allodynia) and hyperalgesia to punctate stimuli. Hyperalgesia to punctate stimuli was characterized by a leftward shift of the stimulus response function, corresponding to a decrease in pain threshold and an increase of painfulness of suprathreshold stimuli by a factor of 3-4. After capsaicin, the difference between the ratings of the first and last stimuli of trains of pin pricks was increased, but the ratio was unchanged. This behavior is equivalent to an increase in effective stimulus intensity, and could be mimicked by increasing the pin prick force from 20 mN to 40 and 80 mN in normal skin. Thus, the leftward shift of the stimulus response function fully accounts for all alterations of pain sensitivity to punctate stimuli in the zone of secondary hyperalgesia. We conclude that when the gain of spinal transmission was changed in secondary hyperalgesia, the gain of wind-up remained unchanged. These findings indicate that secondary hyperalgesia (heterotopic facilitation) and wind-up of pain sensation (homotopic facilitation) are independent phenomena.

Direct comparison of heat-evoked activity of nociceptive neurons in the dorsal horn with the hindpaw withdrawal reflex in the rat

Although the sensory coding of nociceptive neurons in the dorsal horn has been studied extensively, surprisingly little is known about how these neurons contribute to nociceptive reflexes. The objective of the present study was to examine the characteristics of dorsal horn neurons capable of initiating hindpaw withdrawal. To this end, neural and reflex activity were measured simultaneously in response to noxious radiant heat applied to the hindpaw in lightly anesthetized rats. Subsets of both multireceptive (MR; 52/95) and nociceptive-specific (NS; 19/46) neurons showed a consistent burst of activity that preceded the reflex. However, when compared with NS neurons, MR neurons as a group were: more likely to be active before the reflex (55 vs. 41%); more active before the reflex (31 vs. 23 Hz); and active earlier (2.8 vs. 2.3 s before the reflex). Subsets of MR neurons were active before the reflex regardless of receptive field size or location in the dorsal horn. In contrast, NS neurons with small receptive fields or those located outside of superficial laminae were rarely active before the reflex and thus unlikely to be part of the reflex circuit. These results suggest that current classification schemes, in particular MR and NS categories, cannot be used as the sole criterion to predict involvement in nociceptive reflexes. However, simultaneous measurement of neural and reflex activity provides an opportunity to determine the characteristics of nociceptive neurons involved in withdrawal reflexes.

Transneuronal labeling of a nociceptive pathway, the spino-(trigemino-)parabrachio-amygdaloid, in the rat

Transneuronal tracing of a nociceptive pathway, the spino-(trigemino)-parabrachio-amygdaloid pathway, was performed using an alpha-herpes virus, the Bartha strain of pseudorabies virus (PRV). Microinjection of PRV into the central nucleus of the amygdala (Ce) resulted in progressive retrograde and transneuronal infection of a multisynaptic circuit involving neurons in the brainstem and spinal cord as detected immunocytochemically. At short survival (26 hr), retrogradely labeled neurons were concentrated in the external lateral nucleus of the parabrachial complex (elPB) but were absent from both the trigeminal nucleus caudalis (TNC) and the spinal cord. At longer survivals (52 hr), labeled cells were present in lamina I of both the TNC and spinal dorsal horn. Retrograde labeling from the Ce with Fluoro-gold demonstrated that elPB neurons have long dendrites extending laterally into the terminal field of spinal and trigeminal afferents, where transneuronal passage of PRV to these afferents could occur. Even longer survivals (76 hr) resulted in a columnar pattern of cell labeling in the TNC and spinal dorsal horn that extended from lamina I into lamina II. At this longest survival, primary sensory neurons became infected. Bilateral excitotoxic lesions of the elPB blocked almost all viral passage from the Ce to superficial laminae of the TNC and spinal dorsal horn. These results demonstrate that nociceptive input to the amygdala is relayed from neurons in lamina I through the elPB. We propose that this modular arrangement of lamina I and II neurons may provide the basis for spinal processing of peripheral input to the amygdala.