Tonic descending inhibition affects intensity coding of nociceptive responses of spinal dorsal horn neurones in the cat

Short-term plasticity in the spinal nociceptive system

ABSTRACT

Somatosensory information is delivered to neuronal networks of the dorsal horn (DH) of the spinal cord by the axons of primary afferent neurons that encode the intensity of peripheral sensory stimuli under the form of a code based on the frequency of action potential firing. The efficient processing of these messages within the DH involves frequency-tuned synapses, a phenomenon linked to their ability to display activity-dependent forms of short-term plasticity (STP). By affecting differently excitatory and inhibitory synaptic transmissions, these STP properties allow a powerful gain control in DH neuronal networks that may be critical for the integration of nociceptive messages before they are forwarded to the brain, where they may be ultimately interpreted as pain. Moreover, these STPs can be finely modulated by endogenous signaling molecules, such as neurosteroids, adenosine, or GABA. The STP properties of DH inhibitory synapses might also, at least in part, participate in the pain-relieving effect of nonpharmacological analgesic procedures, such as transcutaneous electrical nerve stimulation, electroacupuncture, or spinal cord stimulation. The properties of target-specific STP at inhibitory DH synapses and their possible contribution to electrical stimulation-induced reduction of hyperalgesic and allodynic states in chronic pain will be reviewed and discussed.

Development of a traumatic cervical dislocation spinal cord injury model with residual compression in the rat

BACKGROUND

Preclinical spinal cord injury models do not represent the wide range of biomechanical factors seen in human injuries, such as spinal level, injury mechanism, velocity of spinal cord impact, and residual compression. These factors may be responsible for differences observed between experimental and clinical study results, especially related to the controversial issue of timing of surgical decompression.

NEW METHOD

Somatosensory Evoked Potentials were used to: a) characterize residual compression depths in a dislocation model, and b) evaluate the physiological effect of whether or not the spinal cord was decompressed following the initial injury, prior to the application of residual compression. Modifications to vertebral clamps and the development of a novel surgical frame allowed us to conduct surgical and injury procedures in a controlled manner without the risk of additional damage to the spinal cord. Behavioural outcomes were evaluated following varying dislocation displacements, in addition to the survivability of 4 h of residual compression following a traumatic injury.

RESULTS

Residual compression immediately following the initial dislocation demonstrated significantly different electrophysiological response compared to when the residual compression was delayed.

COMPARISON WITH EXISTING METHOD

There are currently no other residual compression models that utilize a dislocation injury mechanism. Many residual compression studies have demonstrated the effectiveness of early decompression, however the compression of the spinal cord is often not representative of clinical traumatic injuries. Preclinical studies typically model residual compression using a sustained force through quasi-static application, when human injuries often occur at high velocities, followed by a sustained displacement occlusion of the spinal canal.

CONCLUSIONS

This study has validated several novel procedural approaches and injury parameters, and provided critical details to implement in the development of a traumatic cervical dislocation SCI model with residual compression.

Effects of pruritogens and algogens on rostral ventromedial medullary ON and OFF cells

Rostroventromedial medulla (RVM) ON and OFF cells are thought to facilitate and inhibit spinal nociceptive transmission, respectively. However, it is unknown how ON and OFF cells respond to pruritic stimuli or how they contribute to descending modulation of spinal itch signaling. In pentobarbital sodium-anesthetized mice, single-unit recordings were made in RVM from ON and OFF cells identified by their respective increase or decrease in firing that occurred just before nocifensive hindlimb withdrawal elicited by paw pinch. Of RVM ON cells, 75% (21/28) were excited by intradermal histamine, 50% (10/20) by intradermal chloroquine, and 75% (27/36) by intradermal capsaicin. Most chemically responsive units also responded to a scratch stimulus applied to the injected hindpaw. Few ON cells responded to intradermal injection of vehicle (saline: 5/32; Tween 2/17) but still responded to scratching. For OFF cells, intradermal histamine and scratching inhibited 32% (6/19) with no effect of histamine in the remainder. Intradermal chloroquine inhibited 44% (4/9) and intradermal capsaicin inhibited 61% (11/18) of OFF cells. Few OFF cells were affected by vehicles (Tween: 1 inhibited, 7 unaffected; saline: 3 excited, 1 inhibited, 8 unaffected). Both ON and OFF cells that responded to one chemical usually also responded to others, whereas units unresponsive to the first-tested chemical tended not to respond to others. These results indicate that ascending pruriceptive signals activate RVM ON cells and inhibit RVM OFF cells. These effects are considered to facilitate and disinhibit spinal pain transmission, respectively. It is currently not clear if spinal itch transmission is similarly modulated. NEW & NOTEWORTHY The rostroventromedial medulla (RVM) contains ON and OFF cells that are, respectively, excited and inhibited by noxious stimuli and have descending projections that facilitate and inhibit spinal nociceptive transmission. Most RVM ON cells were excited, and OFF cells inhibited, by intradermal injection of the pruritogens histamine and chloroquine, as well as the algogen capsaicin. These results indicate that itchy stimuli activate RVM neurons that presumably give rise to descending modulation of spinal itch transmission.

Opposing effects of cervical spinal cold block on spinal itch and pain transmission

Inactivation of descending pathways enhanced responses of spinal dorsal horn neurons to noxious stimuli, but little is known regarding tonic descending modulation of spinal itch transmission. To study effects of cervical spinal cold block on responses of dorsal horn neurons to itch-evoking and pain-evoking stimuli, single-unit recordings were made from superficial dorsal horn wide dynamic range and nociceptive-specific-type neurons in pentobarbital-anesthetized mice. Intradermal histamine excited 17 units. Cold block starting 1 minute after intradermal injection of histamine caused a marked decrease in firing. The histamine-evoked response during and following cold block was significantly lower compared with control histamine-evoked responses in the absence of cold block. A similar but weaker depressant effect of cold block was observed for dorsal horn unit responses to chloroquine. Twenty-six units responded to mustard oil allyl isothiocyanate (AITC), with a further significant increase in firing during the 1-minute period of cold block beginning 1 minute after AITC application. Activity during cold block was significantly greater compared with the same time period of control responses to AITC in the absence of cold block. Ten units' responses to noxious heat were significantly enhanced during cold block, while 6 units' responses were reduced and 18 unaffected. Cold block had no effect on mechanically evoked responses. These results indicate that spinal chemonociceptive transmission is under tonic descending inhibitory modulation, while spinal pruriceptive transmission is under an opposing, tonic descending facilitatory modulation.

Individual differences in temperature perception: Evidence of common processing of sensation intensity of warmth and cold

The longstanding question of whether temperature is sensed via separate sensory systems for warmth and cold was investigated by measuring individual differences in perception of nonpainful heating and cooling. Sixty-two subjects gave separate ratings of the intensity of thermal sensations (warmth, cold) and nociceptive sensations (burning/stinging/pricking) produced by cooling (29 degrees C) or heating (37 degrees C) local regions of the forearm. Stimuli were delivered via a 4 x 4 array of 8 mm x 8 mm Peltier thermoelectric modules that enabled test temperatures to be presented sequentially to individual modules or simultaneously to the full array. Stimulation of the full array showed that perception of warmth and cold were highly correlated (Pearson r = 0.83, p < 0.05). Ratings of nonpainful nociceptive sensations produced by the two temperatures were also correlated, but to a lesser degree (r = 0.44), and the associations between nociceptive and thermal sensations (r = 0.35 and 0.22 for 37 and 29 degrees C, respectively) were not significant after correction for multiple statistical tests. Intensity ratings for individual modules indicated that the number of responsive sites out of 16 was a poor predictor of temperature sensations but a significant predictor of nociceptive sensations. The very high correlation between ratings of thermal sensations conflicts with the classical view that warmth and cold are mediated by separate thermal modalities and implies that warm-sensitive and cold-sensitive spinothalamic pathways converge and undergo joint modulation in the central nervous system. Integration of thermal stimulation from the skin and body core within the thermoregulatory system is suggested as the possible source of this convergence.

Nociceptive sensations evoked from 'spots' in the skin by mild cooling and heating

It was recently found that nociceptive sensations (stinging, pricking, or burning) can be evoked by cooling or heating the skin to innocuous temperatures (e.g., 29 and 37 degrees C). Here, we show that this low-threshold thermal nociception (LTN) can be traced to sensitive 'spots' in the skin equivalent to classically defined warm spots and cold spots. Because earlier work had shown that LTN is inhibited by simply touching a thermode to the skin, a spatial search procedure was devised that minimized tactile stimulation by sliding small thermodes (16 and 1mm(2)) set to 28 or 36 degrees C slowly across the lubricated skin of the forearm. The procedure uncovered three types of temperature-sensitive sites (thermal, bimodal, and nociceptive) that contained one or more thermal, nociceptive, or (rarely) bimodal spots. Repeated testing indicated that bimodal and nociceptive sites were less stable over time than thermal sites, and that mechanical contact differentially inhibited nociceptive sensations. Intensity ratings collected over a range of temperatures showed that LTN increased monotonically on heat-sensitive sites but not on cold-sensitive sites. These results provide psychophysical evidence that stimulation from primary afferent fibers with thresholds in the range of warm fibers and cold fibers is relayed to the pain pathway. However, the labile nature of LTN implies that these low-threshold nociceptive inputs are subject to inhibitory controls. The implications of these findings for the roles of putative temperature receptors and nociceptors in innocuous thermoreception and thermal pain are discussed.

The role of mu-opioid receptors in inflammatory hyperalgesia and alpha 2-adrenoceptor-mediated antihyperalgesia

The purpose of the present study was to investigate the role of mu-opioid receptor in inflammatory hyperalgesia in intact and in spinalized animals and the interaction between mu-opioid and alpha2-adrenergic receptor in acute pain and inflammatory hyperalgesia. Behavioral responses to mechanical and heat stimuli were studied in mu-opioid receptor knockout mice and wildtype control mice. Thermal nociception was evaluated by measuring paw withdrawal latencies to radiant heat applied to the hindpaws. Mechanical nociception was measured by von Frey monofilament applications to the hindpaws. Intraplantar carrageenan-induced (1 mg/40 microl) mechanical and heat hyperalgesia were compared in micro-opioid knockout and wildtype mice. The effect of systemically administered alpha2-adrenergic receptor agonist dexmedetomidine (1-10 microg/kg) was evaluated on mechanical and thermal withdrawal responses under normal and inflammatory state in knockout and wildtype mice. The role of micro-opioid receptor in descending modulation of nociception was studied by assessing mechanical and heat withdrawal responses before and after mid-thoracic spinalization. Withdrawal responses to radiant heat and von Frey monofilaments were similar in mu-opioid knockout and wildtype mice before and after the carrageenan induced hindpaw inflammation. Also, antinociceptive effects of dexmedetomidine in thermal and mechanical nociceptive tests were similar before carrageenan induced hindpaw inflammation. However, the potency of dexmedetomidine was significantly reduced in carrageenan-induced mechanical hyperalgesia in mu-opioid knockout mice compared to the wildtype control mice. Thermal and mechanical withdrawal responses were similar between mu-opioid knockout and wildtype mice before and after mid-thoracic spinalization. Our observations indicate that the mu-opioid receptors do not play an important role in alpha2-adrenergic receptor agonist-mediated acute antinociception. In addition, micro-opioid receptors are not tonically involved in the modulation of inflammation-induced mechanical and thermal hyperalgesia, and the supraspinal control of spinal reflexes. However, in the presence of inflammation, mu-opioid receptors play an important role in the antihyperalgesic actions of an alpha2-adrenergic receptor agonist.

Cholinergic, noradrenergic, and serotonergic inhibition of fast synaptic transmission in spinal lumbar dorsal horn of rat

It is known that spinal nociceptive sensory transmission receives descending inhibitory and facilitatory modulation from supraspinal structures. Glutamate is the major fast excitatory transmitter between primary afferent fibers and spinal dorsal horn neurons. In whole-cell patch clamp recordings from dorsal horn neurons in spinal slices, we investigated synaptic mechanisms for inhibitory modulation at the lumbar level of the spinal cord. Application of the cholinergic receptor agonist carbachol produced a dose-dependent inhibition of glutamate-mediated excitatory postsynaptic currents (EPSCs) (IC(50) 13 microM). Postsynaptic injection of two different types of G-protein inhibitors, guanosine 5'-O-2-thiophosphate or guanosine 5'-O-3-thiotriphosphate, blocked the inhibition produced by carbachol. Clonidine, a selective alpha-adrenergic receptor agonist, also produced a dose-dependent inhibition of EPSCs (IC(50) 7 microM) that was reduced by postsynaptic inhibition of G-proteins. The inhibitory effect of serotonin was likewise mediated by postsynaptic G-proteins. Our results suggest that activation of postsynaptic neurotransmitter receptors plays a critical role in inhibition of glutamate mediated sensory responses by acetylcholine, norepinephrine, and serotonin. Our results support the hypothesis that descending sensory modulation may be mediated by multiple neurotransmitter receptors in the spinal cord.

Induction of long-term potentiation of single wide dynamic range neurones in the dorsal horn is inhibited by descending pathways

Previous studies have shown that long-term potentiation (LTP) in the dorsal horn may be induced by noxious stimuli. In this study it is investigated whether induction of LTP in the dorsal horn may be affected by the descending pathways. Extracellular recordings of wide dynamic range (WDR) neurones in the lumbar dorsal horn in intact urethane-anaesthetized Sprague--Dawley rats were performed, and the electrically evoked neuronal responses in these neurones were defined as A-fibre and C-fibre responses according to latencies. Using a short-term cold block of the thoracic spinal cord, which produced a completely reversible increase of the A-fibre and C-fibre responses, the influence of the descending inhibitory system on the induction of LTP by electrical high-frequency conditioning applied to the sciatic nerve was examined. As previously shown the A-fibre responses were almost unchanged following the conditioning. In contrast, the C-fibre responses following the same conditioning were strongly increased. Thus, a clear LTP of the nociceptive transmission in the dorsal horn was observed following electrical high-frequency conditioning. Interestingly, we found that the LTP was more powerful when the effects of the descending pathways were temporarily eliminated during conditioning. It is concluded that induction of LTP by electrical high-frequency conditioning stimulation, which may be part of the wider term central sensitization, is inhibited by descending pathways.

Stage-dependent changes in the modulation of spinal nociceptive neuronal activity during the course of inflammation

Spinal and supraspinal controls can tonically or phasically modulate the output of spinal nociceptive neurons. Alterations of these modulatory systems have been described during the acute stage of inflammation. In the present study in the rat, tonic descending controls were assessed during acute (24--48 h) and chronic (3--4 weeks) stages of monoarthritis of the ankle. The electrophysiological properties of spinal convergent neurons with ankle input were compared before and after spinalization. In a parallel series of experiments, spinal convergent neurons were recorded from the normal side in order to assess the propriospinal and supraspinal inhibitory controls triggered by nociceptive stimulation of the inflamed ankle. Tonic descending inhibition of convergent neurons with input from the inflamed ankle was enhanced during the acute stage and then decreased during the chronic stage of monoarthritis. Contralateral-induced inhibitions exhibited a similar temporal evolution. Time-dependent changes in the spinal transmission of nociceptive signals were shown by removing descending modulation in animals with monoarthritis; sensitization of spinal neurons with input from the inflamed ankle was demonstrated during the acute stage of monoarthritis, whereas a crossed transmission between inflamed and normal sides was observed during the chronic stage of the disease. These results show that dynamic and stage-dependent modifications of descending controls tend to dampen the central changes associated with inflammation.

Influence of spinalization on spinal withdrawal reflex responses varies depending on the submodality of the test stimulus and the experimental pathophysiological condition in the rat

The influence of midthoracic spinalization on thermally and mechanically induced spinal withdrawal reflex responses was studied in the rat. There were three experimental groups of rats: healthy controls, rats with a spinal nerve ligation-induced unilateral neuropathy, and rats with a carrageenan-induced inflammation of one hindpaw. Tail flick response was induced by radiant heat. Hindlimb withdrawal was induced by radiant heat, ice water, and innocuous or noxious mechanical stimulation of the paw. Prior to spinalization, spinal nerve ligated and carrageenan-treated animals had a marked unilateral allodynia and hyperalgesia. Spinalization tended to induce a facilitation of noxious heat-evoked reflexes. This spinalization-induced facilitation was stronger on tail than hindlimb withdrawal. Spinalization-induced skin temperature change did not explain the facilitation of noxious heat-evoked reflexes. In contrast, spinal withdrawal responses induced by noxious cold or mechanical stimulation were significantly suppressed following spinalization. The spinalization-induced facilitatory effects as well as inhibitory ones on spinal reflexes were enhanced in inflamed/neuropathic animals. The results indicate that the tonic descending control of spinal nocifensive responses varies depending on the submodality of the test stimulus, the segmental level of the reflex (tail vs. hindlimb), and on the pathophysiological condition.

Classification of dorsal horn neurons based on somatic receptive fields in cats with intact and transected spinal cords: neural plasticity

Classification of dorsal horn neurons based on cell activity responses to somatic receptive fields stimulation, was compared between anesthetized cats with transected or intact cords. Results showed a significant (P < or = 0.001) difference. In animals with transected cords, dorsal horn neurons responded with less specificity to noxious and innocuous stimulation. The results are consistent with the proposition that loss of supraspinal influences plays a significant role in determining response characteristics of dorsal horn neurons.

The organization and function of endogenous antinociceptive systems

Much progress has been made the understanding of endogenous pain-controlling systems. Recently, new concepts and ideas which are derived from neurobiology, chaos research and from research on learning and memory have been introduced into pain research and shed further light on the organization and function of endogenous antinociception. These most recent developments will be reviewed here. Three principles of endogenous antinociception have been identified, as follows. (1) Supraspinal descending inhibition: the patterns of neuronal activity in diencephalon, brainstem and spinal cord during antinociceptive stimulation in midbrain periaqueductal gray (PAG) or medullary nucleus raphe magnus have now been mapped on the cellular level, using the c-Fos technique. Results demonstrate that characteristic activity patterns result within and outside the PAG when stimulating at its various subdivisions. The descending systems may not only depress mean discharge rates of nociceptive spinal dorsal horn neurons, but also may modify harmonic oscillations and nonlinear dynamics (dimensionality) of discharges. (2) Propriospinal, heterosegmental inhibition: antinociceptive, heterosegmental interneurons exist which may be activated by noxious stimulation or by supraspinal descending pathways. (3) Segmental spinal inhibition: a robust long-term depression of primary afferent neurotransmission in A delta fibers has been identified in superficial spinal dorsal horn which may underlie long-lasting antinociception by afferent stimulation, e.g. by physical therapy or acupuncture.

Differential effects of spinalization on discharge patterns and discharge rates of simultaneously recorded nociceptive and non-nociceptive spinal dorsal horn neurons

Recordings were made simultaneously from 2-5 neurons at the same site in the lumbar spinal dorsal horn of pentobarbital-anesthetized rats. Neurons were classified as low-threshold (LT) or multireceptive (MR) according to their responses to non-noxious mechanical or noxious radiant heat stimuli of the skin. At the same recording sites neurons could be encountered which belong to different classes and/or which had mechanoreceptive fields which did not overlap. Cold blocks of the upper or lower thoracic cord or transsections of the upper cervical cord were made to evaluate the effects of spinalization on both the rate and pattern of background activity and/or noxious heat-evoked responses of different dorsal horn neurons under identical experimental conditions. At 24 of 27 recording sites, spinalization had qualitatively or quantitatively different effects on the rate of background activity of simultaneously recorded neurons. Interspike interval (ISI) means of background activity were significantly reduced in 29 of 65 (44.6%) neurons, prolonged in 23 of 65 (35.4%) neurons, or unchanged in 13 of 65 (20%) neurons. MR neurons displayed a significantly higher incidence of decreased background activity 17 of 45 (37.8%) and a lower incidence of increased background activity (18 of 45, 40%) during spinalization than the LT neurons from which 1 of 12 (8.3%) decreased and 8 of 12 (66.6%) increased background activity. Almost all (95.4%) neurons changed their discharge patterns after spinalization. At 9 of 27 recording sites, the discharge patterns of simultaneously recorded neurons were affected differently by spinalization as revealed by the coefficient of dispersion of the interspike intervals (ISI), indicating changes in the tendency to discharge action potential in clusters (bursts). At the same recording sites the level of noxious heat-evoked responses of simultaneously recorded MR neurons was also differentially affected by spinalization. Nociceptive responses were significantly enhanced in 19 of 37 (51.4%) neurons (137.8 +/- 142.6% of control, mean +/- SD), reduced in 13 of 37 neurons (35.1%) (by 58.9 +/- 20.9%) and/or unchanged in 5 of 37 (13.5%) neurons. It is concluded that no general 'tone' of descending antinociception exists and that tonic descending excitatory and inhibitory systems may be active simultaneously modulating both the level and pattern of neuronal discharges.

Descending influences on the cutaneous receptive fields of postsynaptic dorsal column neurones in the cat

1. The influence of activity in descending systems on the cutaneous receptive field properties of postsynaptic dorsal column (PSDC) neurones has been investigated in chloralose-anaesthetized cats. The main aim of the study was to determine whether the receptive field boundaries of PSDC neurones are under the control of systems descending from the brain. 2. Single-unit recordings were made from the ascending axons of PSDC units in the dorsal columns. Receptive fields were analysed using light tactile and noxious mechanical and thermal stimuli, both before and during a reversible block of spinal conduction produced by cooling the cord rostral of the recording site. 3. The light tactile excitatory fields of PSDC neurones were largely unaffected by the cold-block procedure. 4. In contrast, both the sensitivity of PSDC neurones to noxious stimuli and the area of skin from which they could be effectively excited by such stimuli were found to be profoundly modified by interruption of descending activity. Two-thirds of the units excited by noxious pinch responded more vigorously in the cold-blocked state and one-half from an expanded area of skin. Responses to noxious radiant heat were similarly modified. 5. Inhibition evoked in PSDC neurones, whether by light tactile or noxious stimuli, involved predominantly segmental mechanisms since it remained effective in the cold-blocked state. 6. It is concluded that neurones of the PSDC system are amongst those dorsal horn neurones with receptive field geometries which may be modified by activity in descending systems.

Brain-stem areas tonically inhibiting dorsal horn neurones: studies with microinjection of the GABA analogue piperidine-4-sulphonic acid

In barbiturate anaesthetized cats, tonic inhibition of the excitation of lumbar dorsal horn neurones by impulses in unmyelinated primary afferents was measured by reversibly cooling the spinal cord at the thoraco-lumbar junction. Tonic inhibition was reduced by microinjection of the GABA analogue, piperidine-4-sulphonic acid (2.5 nM in 0.5 microliter) mainly at AP -7, L 2-5 and V -8 to -10. This area in the ventrolateral medulla is just ventral to the facial nucleus and has been shown to be important in cardiovascular control, particularly in relation to fear-defence reactions. It is proposed that tonic inhibition of the nociceptive responses of spinal neurones is part of such a reaction in response to the trauma of surgery. Since previous experiments had shown that the ventrolateral medulla was important in spinal inhibition produced by PAG stimulation, these experiments support the proposal that analgesia does not occur in isolation but is part of a complex behavioural response of an animal in a potentially injurious environment.