WDR response profiles of spinal dorsal horn neurons may be unmasked by barbiturate anesthesia

Temporal and spatial dynamics of spinal sensorimotor processing in an intersegmental cutaneous nociceptive reflex

The cutaneus trunci muscle (CTM) reflex produces a skin "shrug" in response to pinch on a rat's back through a three-part neural circuit: 1) A-fiber and C-fiber afferents in segmental dorsal cutaneous nerves (DCNs) from lumbar to cervical levels, 2) ascending propriospinal interneurons, and 3) the CTM motoneuron pool located at the cervicothoracic junction. We recorded neurograms from a CTM nerve branch in response to electrical stimulation. The pulse trains were delivered at multiple DCNs (T₆-L₁), on both sides of the midline, at two stimulus strengths (0.5 or 5 mA, to activate Aδ fibers or Aδ and C fibers, respectively) and four stimulation frequencies (1, 2, 5, or 10 Hz) for 20 s. We quantified both the temporal dynamics (i.e., latency, sensitization, habituation, and frequency dependence) and the spatial dynamics (spinal level) of the reflex. The evoked responses were time-windowed into Early, Mid, Late, and Ongoing phases, of which the Mid phase, between the Early (Aδ fiber mediated) and Late (C fiber mediated) phases, has not been previously identified. All phases of the response varied with stimulus strength, frequency, history, and DCN level/side stimulated. In addition, we observed nociceptive characteristics like C fiber-mediated sensitization (wind-up) and habituation. Finally, the range of latencies in the ipsilateral responses were not very large rostrocaudally, suggesting a myelinated neural path within the ipsilateral spinal cord for at least the A fiber-mediated Early-phase response. Overall, these results demonstrate that the CTM reflex shares the temporal dynamics in other nociceptive reflexes and exhibits spatial (segmental and lateral) dynamics not seen in those reflexes.NEW & NOTEWORTHY We have physiologically studied an intersegmental reflex exploring detailed temporal, stimulus strength-based, stimulation history-dependent, lateral and segmental quantification of the reflex responses to cutaneous nociceptive stimulations. We found several physiological features in this reflex pathway, e.g., wind-up, latency changes, and somatotopic differences. These physiological observations allow us to understand how the anatomy of this reflex may be organized. We have also identified a new phase of this reflex, termed the "mid" response.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Activation of lumbar spinal wide-dynamic range neurons by a sanshool derivative

The enigmatic sensation of tingle involves the activation of primary sensory neurons by hydroxy-alpha-sanshool, a tingly agent in Szechuan peppers, by inhibiting two-pore potassium channels. Central mechanisms mediating tingle sensation are unknown. We investigated whether a stable derivative of sanshool-isobutylalkenyl amide (IBA)-excites wide-dynamic range (WDR) spinal neurons that participate in transmission of chemesthetic information from the skin. In anesthetized rats, the majority of WDR and low-threshold units responded to intradermal injection of IBA in a dose-related manner over a >5-min time course and exhibited tachyphylaxis at higher concentrations (1 and 10%). Almost all WDR and low-threshold units additionally responded to the pungent agents mustard oil (allyl isothiocyanate) and/or capsaicin, prompting reclassification of the low-threshold cells as WDR. The results are discussed in terms of the functional role of WDR neurons in mediating tingle sensation.

The influence of sex and ovarian hormones on temporomandibular joint nociception in rats

The aim of this study was to investigate the influence of sex and ovarian hormones on formalin- and glutamate-induced temporomandibular joint (TMJ) nociception in rats. The influence of sex and ovarian hormones on the nociceptive behavior induced by formalin or glutamate was virtually the same. The nociceptive behavior of males was similar to that of females in the proestrus phase of the estrous cycle but was significantly lower than that in the diestrus phase. Since the serum level of estradiol but not of progesterone was significantly higher in the proestrus than in the diestrus phase, these data suggest that females with lower endogenous serum level of estradiol have an exacerbation of TMJ nociception. The nociceptive behavior of ovariectomized rats was similar to that of diestrus females and significantly greater than that of proestrus females. Although the administration of estradiol or progesterone in ovariectomized females significantly reduced TMJ nociception, the combination of both hormones did not increase the antinociceptive effect induced by each of them. These findings suggest that estradiol and progesterone decrease TMJ nociception in an independent way.

PERSPECTIVE

We report that ovarian hormones have an antinociceptive effect on the TMJ formalin and glutamate nociceptive behavior models. Therefore, the greater prevalence and severity of TMJ pain in women of reproductive age may be a consequence of hormonal fluctuation during the reproductive cycle, in that during low endogenous estradiol serum level TMJ pain sensitivity is increased, enhancing the risk of females experiencing TMJ pain.

Exposure to intermittent nociceptive stimulation under pentobarbital anesthesia disrupts spinal cord function in rats

RATIONALE

Spinal cord plasticity can be assessed in spinal rats using an instrumental learning paradigm in which subjects learn an instrumental response, hindlimb flexion, to minimize shock exposure. Prior exposure to uncontrollable intermittent stimulation blocks learning in spinal rats but has no effect if given before spinal transection, suggesting that supraspinal systems modulate nociceptive input to the spinal cord, rendering it less susceptible to the detrimental consequences of uncontrollable stimulation.

OBJECTIVE

The present study examines whether disrupting brain function with pentobarbital blocks descending inhibitory systems that normally modulate nociceptive input, making the spinal cord more sensitive to the adverse effect of uncontrollable intermittent stimulation.

MATERIALS AND METHODS

Male Sprague-Dawley rats received uncontrollable intermittent stimulation during pentobarbital anesthesia after (experiment 1) or before (experiment 2) spinal cord transection. They were then tested for instrumental learning at a later time point. Experiment 3 examined whether these manipulations affected nociceptive (thermal) thresholds.

RESULTS

Experiment 1 showed that pentobarbital had no effect on the induction of the learning deficit after spinal cord transection. Experiment 2 showed that intact rats anesthetized during uncontrollable intermittent stimulation failed to learn when later transected and tested for instrumental learning. Experiment 3 found that uncontrollable intermittent stimulation induced an antinociception in intact subjects that was blocked by pentobarbital.

CONCLUSIONS

The results suggest a surgical dose of pentobarbital (50 mg/kg) suppresses supraspinal (experiment 2) but not spinal (experiment 1) systems that modulate nociceptive input to the spinal cord by blocking the antinociception that is induced by this input (experiment 3).

Tuning of membrane properties regulates subliminal synapses in dorsal horn neurons of intact rats

Functional plasticity in receptive field properties underlies the mechanism whereby spinal dorsal horn neurons encode changes in pain sensitivity following peripheral injury. Activation of "silent" or subliminal excitatory synapses was hypothesized to account for this injury-induced neural plasticity. To better characterize the mechanisms governing subliminal inputs, we adapted whole-cell patch clamp to the study of dorsal horn neurons in intact, anesthetized rats. In this report we show that the membrane properties of spinal cells correlate to functional class defined by action potential responses to cutaneous stimuli. In addition, we report the discovery of a novel "silent" population of neurons with solely subliminal excitatory inputs at rest that can be activated by membrane depolarization. Finally, an induced change in baseline membrane potential to a level nearer that of a different functional class results in a corresponding change in the responses to cutaneous stimuli of a given cell to that of the new functional class. In summary our findings suggest that biophysical membrane properties are key factors determining the functional profile of spinal neurons. The rapid change of such properties may regulate the function of silent synapses in spinal neurons and underlie rapid development of neural plasticity.

Endogenous mechanisms of sensory modulation

We provide evidence supporting the idea that the relationship between tissue damage, or the threat of tissue damage, and the response to such stimuli is variant and dependent on neuronal networks by which attentional, emotional and cognitive components of pain experience activate endogenous descending modulatory systems. Most previous studies have focused on responses to transient noxious stimuli with little information on the influence of descending modulation on behavioral responses to persistent pain and hyperalgesia after tissue or nerve injury. Utilizing correlative behavioral and neuronal studies we have demonstrated that (1) behavioral context modulates neuronal activity in nociceptive and non-nociceptive somatosensory pathways, supporting the hypothesis that responses in these pathways are not immutable; (2) descending modulation influences behavior and neuronal activity at spinal cord levels after inflammation and persistent pain; and (3) there are descending facilitatory as well as inhibitory influences on behavior and spinal cord neuronal activity that may impact on persistent pain particularly of deep muscle and visceral origin. Cortical as well as subcortical pathways are available by which dorsal horn activity can be modulated by attentional, motivational and cognitive factors. It appears that the same neuronal mechanisms in the forebrain and brain stem are available for behavioral modulation in a learned task involving the threat of tissue damage (transient noxious stimuli) as are available in the development and amplification of persistent pain produced by inflammation. These parallel brain mechanisms emphasize the saliency of pain experience as an important learned behavior for the survival of the organism, similar to sequential goal-directed behaviors in an operant task.

Cutaneous responsiveness of lumbar spinal dorsal horn neurons is reduced by general anesthesia, an effect dependent in part on GABAA mechanisms

Extracellular activity was recorded from single spinal dorsal horn neurons in both chronic cat and acute rat models. This was done to define the effects of anesthesia on the processing of sensory information elicited by nonnoxious tactile stimulation of peripheral receptive fields (RFs). In the chronic cat model, baseline data were obtained in physiologically intact, awake, drug-free animals before anesthetic administration (halothane 1.0-2.0%). This made it possible to compare and contrast activity of each cell in the drug-free and anesthetized state. Halothane effects were confirmed in the acute rat model (anesthetized, spinally transected, and in some cases decerebrate). In addition, the gamma-aminobutyic acid-A (GABAA)-receptor antagonist picrotoxin (2 mg/kg) was administered intravenously to verify that the observed halothane effect on spinal dorsal horn neurons was mediated by an interaction with GABAA-receptor systems. Halothane effects on three separate measures of response to nonnoxious tactile stimuli were observed in the chronic cat model. Halothane produced a significant, dose-dependent reduction in the low-threshold RF area of the neurons studied. Halothane also caused a significant reduction in neuronal response to RF brushing (dynamic stimulus) and to maintained contact with the RF (static stimulus). A dose dependency was not observed with these latter two effects. Neurons with a predominant rapidly adapting response seemed to be less susceptible to halothane suppression than slowly adapting cells. In the acute rat model an increase in halothane caused a reduction in neuronal response similar to that seen in the cat. The intravenous administration of 2 mg/kg of picrotoxin by itself caused no significant change in RF size or response to brushing. However, the same amount of picrotoxin did cause a 50% reversal of the halothane-induced reduction in RF size without causing a significant change in the halothane effect on response to RF brushing. In contrast to work recently reported in a chronic sheep model, halothane causes a significant reduction in spinal dorsal horn neuronal response to tactile stimulation of peripheral RFs. This effect is caused by, in part, but not exclusively, to GABAA-neurotransmitter systems. However, the relative influence of GABAA systems may vary with the nature of the stimulus.

Effects of noradrenergic agonists on superficial dorsal horn neurons in the cat spinal cord

The objectives of the present study were to determine the effects of iontophoretically-applied noradrenergic agonists on the unit activity of physiologically characterized superficial dorsal horn neurons in the barbiturate-anesthetized cat spinal cord, and to determine if a relationship exists between the effects produced by these agents and neuron modality. The effects of norepinephrine (NE), clonidine (CLON, a selective alpha2-agonist) and phenylephrine (PE, a selective alpha1-agonist) on spontaneous and D,L-homocysteic acid (DLH)-evoked unit activity were examined for 68 superficial dorsal horn neurons. Iontophoretically applied NE inhibited (40% of the cells examined) and excited (39% of the cells examined) unit activity. Mixed effects (i.e., inhibition and excitation) on unit activity also were observed (10% of the cells examined), and NE had no effect on the unit activity of some cells (11% of the cells examined). Excitation was the predominant effect produced by CLON (62% of the cells examined); however, inhibition (19% of the cells examined), mixed effects (5% of the cells examined), and no effects on unit activity (14% of the cells examined) also were observed. Iontophoretically applied PE inhibited (46% of the cells examined) and excited (36% of the cells examined) unit activity. Mixed effects on unit activity also were observed (4% of the cells examined), and PE had no effect on the unit activity of some cells (14% of the cells examined). Whether NE, CLON or PE exerted excitatory or inhibitory effects on unit activity did not depend on neuron modality.

Control of size and excitability of mechanosensory receptive fields in dorsal column nuclei by homolateral dorsal horn neurons

Both accidental and experimental lesions of the spinal cord suggest that neuronal processes occurring in the spinal cord modify the relay of information through the dorsal column-lemniscal pathway. How such interactions might occur has not been adequately explained. To address this issue, the receptive fields of mechanosensory neurons of the dorsal column nuclei were studied before and after manipulation of the spinal dorsal horn. After either a cervical or lumbar laminectomy and exposure of the dorsal column nuclei in anesthetized cats, the representation of the hindlimb or of the forelimb was defined by multiunit recordings in both the dorsal column nuclei and in the ipsilateral spinal cord. Next, a single cell was isolated in the dorsal column nuclei, and its receptive field carefully defined. Each cell could be activated by light mechanical stimuli from a well-defined cutaneous receptive field. Generally the adequate stimulus was movement of a few hairs or rapid skin indentation. Subsequently a pipette containing either lidocaine or cobalt chloride was lowered into the ipsilateral dorsal horn at the site in the somatosensory representation in the spinal cord corresponding to the receptive field of the neuron isolated in the dorsal column nuclei. Injection of several hundred nanoliters of either lidocaine or cobalt chloride into the dorsal horn produced an enlargement of the receptive field of the neuron being studied in the dorsal column nuclei. The experiment was repeated 16 times, and receptive field enlargements of 147-563% were observed in 15 cases. These data suggest that the dorsal horn exerts a tonic inhibitory control on the mechanosensory signals relayed through the dorsal column-lemniscal pathway. Because published data from other laboratories have shown that receptive field size is controlled by signals arising from the skin, we infer that the control of neuronal excitability, receptive field size and location for lemniscal neurons is determined by tonic afferent activity that is relayed through a synapse in the dorsal horn. This influence of dorsal horn neurons on the relay of mechanosensory information through the lemniscal pathways must modify our traditional views concerning the relative independence of these two systems.

A neuronal correlate of secondary hyperalgesia in the rat spinal dorsal horn is submodality selective and facilitated by supraspinal influence

Tissue injury produces hyperalgesia not only in the injured area (primary hyperalgesia) but also outside of it (secondary hyperalgesia). In the present investigation, the submodality selectivity and the contribution of supraspinal influence to a neural correlate of the secondary hyperalgesia induced by neurogenic inflammation was studied in the presumed pain relay neurons of the rat spinal dorsal horn. Mechanically and thermally evoked responses to wide-dynamic range (WDR) neurons of the spinal dorsal horn were recorded under sodium pentobarbital anesthesia in rats. Neurogenic inflammation was induced by application of mustard oil outside of the receptive fields of WDR neurons. To study the contribution of supraspinal influence to mustard oil-induced changes in neuronal responses, the spinal cord was transected at a midthoracic level or lidocaine was microinjected into the rostroventromedial medulla (RVM). Furthermore, the antidromically evoked compound volley in the sural nerve was determined to reveal excitability changes in the central terminals of primary afferent A-fibers induced by mustard oil. The results indicate that mustard oil adjacent to the receptive fields of spinal WDR neurons significantly enhanced their responses to mechanical but not to noxious heat stimuli, without a significant influence on their spontaneous activity. Both high- and low-threshold mechanoreceptive input to WDR neurons was equally facilitated, whereas mechanoreceptive input to spinal dorsal horn neurons mediating innocuous messages (low-threshold mechanoreceptive neurons) was not changed. Mustard oil in a remote site (forepaw) did not produce any hyperexcitability to responses evoked by hindpaw stimulation. Spinal transection or lidocaine block of the RVM significantly attenuated the mustard oil-induced mechanical hyperexcitability in spinal dorsal horn neurons. Mustard oil had no significant effect on a compound volley in the sural nerve induced by intraspinal stimulation of sural nerve terminals at a submaximal intensity. The selective mechanical hyperexcitability in spinal WDR neurons, without a change in their spontaneous activity, can be explained by a heterosynaptic facilitatory action on presynaptic terminals mediating mechanical signals to these nociceptive spinal neurons. These findings indicate that brain stem-spinal pathways, involving the RVM, do not only suppress nociception but under some pathophysiological conditions concurrent facilitatory influence may predominate and lead to enhancement of mechanical hyperexcitability. The descending facilitatory feed-back loop to nociceptive spinal neurons may help to protect the wounded tissue and thus promote healing.

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.

Correlation of effects of general anesthetics on somatosensory neurons in the primate thalamus and cortical EEG power

The effects of two types of general anesthetic on the neurophysiological properties of the primate somatosensory thalamus were correlated with effects on frontal cortex electroencephalographic (EEG) power and spectral properties. Graded doses of the intravenous agent methohexital sodium (METH) were studied in 12 cells in three monkeys on a halothane baseline anesthetic. Low doses of METH (0.2-1.0 mg/kg) produced a reduction of EEG power but had no effects on spontaneous or evoked thalamic activity. EEG power showed maximal attenuation after 2.0 mg/kg METH, whereas decreases in thalamic activity were first noted over a similar moderate dose range (2.0-5.0 mg/kg). The physiological parameter most sensitive to METH was the spontaneous activity, which showed initial changes in rate and moderate doses followed by marked inhibition at higher doses. Finally, the high dose of METH (10.0 mg/kg) produced marked reduction in all neurophysiological parameters with recovery over the following 30-45 min. The effects of the volatile anesthetic halothane were studied on 15 cells in four monkeys anesthetized with pentobarbital sodium. The low dose of halothane (0.25%) produced a facilitation of responses to cutaneous stimuli as well as decrease in the rate and burst patterns in the spontaneous activity. The power in the EEG was not affected at this concentration. The responses of the cells to the mechanical stimuli at moderate doses (0.5-1.0%) of halothane returned to the baseline magnitude, whereas spontaneous activity remained unaffected compared with initial effects. EEG power was reduced by 1% halothane. Finally, all neurophysiological parameters showed profound reduction at the highest halothane concentrations (2.0-3.0%) with recovery over the next 30-45 min. In conclusion, the two classes of anesthetics most commonly used for acute neurophysiological studies in the primate show well-defined thresholds at which changes in the response properties of thalamic neurons are produced. This threshold for the barbiturates and halothane can be predicted by monitoring of cortical EEG.

Thiopentone induced enhancement of somatic motor responses to noxious stimulation: influence of GABAA receptor modulation

PURPOSE

This study was conducted to determine whether hyperalgesic effects of subanaesthetic concentrations of thiopentone could be attributed to GABAA receptor effects.

METHODS

All studies were performed on 50 rats in a prospective, randomized, blinded fashion using saline-injected animals as controls. Using a modified Randall-Selitto technique, the motor behavior stimulated by noxious stimulation was quantified by determining the lowest tail pressure required to provoke a withdrawal response (somatic motor response threshold, SMRT). In the first protocol (21 rats), we studied the effects of 0.5, 1.5 and 5 mg.kg-1 i.v. of the GABAA agonist, muscimol, on SMRT. In the second protocol (20 rats), the effects of administration of saline, muscimol 0.5 mg.kg-1, or the competitive GABAA antagonist, bicuculline 0.25 mg.kg-1, upon the SMRT-reducing effects of a standardized thiopentone infusion were observed.

RESULTS

No dose of muscimol produced hyperalgesia. The highest dose of muscimol used (5 mg.kg-1) produced pronounced analgesic effects, raising the SMRT above 750 g. No change in SMRT was detected with the smaller doses of muscimol. Given in combination with muscimol (0.5 mg.kg-1), thiopentone produced analgesia, as shown by an increase in SMRT (P = 0.009). In the bicuculline treated animals, SMRT decreased linearly with increasing plasma thiopentone concentrations (P < 0.001). The slope of the relationship in the bicuculine group was not significantly different from that observed in the saline-treated group, indicating that bicuculline did not block the hyperalgesic effects of thiopentone.

CONCLUSION

The results of these studies suggest that hyperalgesia associated with thiopentone is not mediated primarily by GABAA receptors.

Descending inhibition in neonatal rat spinal cord: actions of pentobarbital and morphine

Descending inhibition plays an important role in modulating spinal nociceptive neurotransmission. Barbiturates have been suggested to be poor analgesics or anti-analgesic because they block descending inhibition from supraspinal centers to the spinal cord. Opiate analgesics, on the other hand, are postulated to increase descending inhibition. We tested this hypothesis in an isolated brain stem-spinal cord preparation from neonatal rats, using as the test response a nociceptive-related slow ventral root potential (sVRP) recorded in the lumbar region. Brain stem and spinal cord were separately perfused. Transecting the spinal cord, applying the local anesthetic lidocaine to the brain stem, or cooling the brain stem increased the area of the sVRP, thus demonstrating that tonic descending inhibition is present in this preparation. Pentobarbital (Pb) (1-10 microM) applied to the spinal cord depressed the sVRP in a dose-dependent fashion. Spinal cord transection did not significantly change Pb potency. Pb (5-10 microM) applied to the brain stem alone did not significantly increase sVRP amplitude. Morphine (15-35 nM) applied to the spinal cord also depressed the sVRP but had no effect when applied to the brain stem. The results show that there are functional synaptic connections mediating tonic descending inhibition in the neonatal rat. They do not support interaction with tonic descending inhibition as an explanation for morphine analgesia or as a reason for lack of analgesic properties in the barbiturates.

The dominant class of somatosensory neurone recorded in the spinal dorsal horn of awake sheep has wide dynamic range properties

In order to investigate the properties of dorsal horn neurones in the absence of the distorting influences of anaesthesia, preparative surgery, prior training or excessive restraint, recordings have been made in sheep chronically prepared for single-cell recording. Within the limitations of sampling error of dorsal horn neurones with cutaneous receptive fields, the cell type most frequently encountered had wide dynamic range (WDR; convergent; multireceptive) properties; these accounted for 59% of the 46 neurones that were examined in detail. High-threshold mechanoreceptive (nocispecific) and low-threshold mechanoreceptive neurones formed 11% and 30% of the sample, respectively. These and other data indicate that under normal physiological conditions in the awake state, many spinal neurones do indeed have WDR properties, implying that these cells have an important function in nociceptive processing.

Role of WDR neurons in a hind limb noxious heat evoked flexion withdrawal reflex

Behavioral experiments and neurophysiological experiments, the two major types of preclinical studies which have paved the way for the development of spinal analgesia were compared under identical conditions utilizing the same animals. The results demonstrate that the activation of the wide dynamic range (WDR) neurons preceded the behavioral withdrawal reflexes, and that the activation of the WDR neurons occurred at lower stimulus temperature than that for the withdrawal reflex. The results suggest that the neuronal activation began before the behavioral reflex but also that the firing frequency of the WDR neurons at the time of the withdrawal reflex could not distinguish between non-noxious and noxious stimuli. Further study is needed to elucidate the neuronal mechanisms of the activation of the behavioral reflex.

Inflammation and hyperalgesia in rats neonatally treated with capsaicin: effects on two classes of nociceptive neurons in the superficial dorsal horn

To address the mechanisms of hyperalgesia and dorsal horn plasticity following peripheral tissue inflammation, the effects of adjuvant-induced inflammation of the rat hindpaw on behavioral nociception and nociceptive neuronal activity in the superficial dorsal horn were examined in neonatally capsaicin-treated rats 6-8 weeks of age. Capsaicin treatment resulted in an 82% loss of unmyelinated fibers in L5 dorsal roots, a dramatic reduction of substance P-like immunoreactivity in the spinal cord, and a significant decrease in the percentage of dorsal horn nociceptive neurons that responded to C-fiber stimulation and noxious heating of the skin. The thermal nociceptive threshold was significantly increased in capsaicin-treated rats, but behavioral hyperalgesia to thermal stimuli still developed in response to inflammation. Following inflammation, there was a significant decrease in mechanical threshold and an increase in response duration to mechanical stimuli in both vehicle- and capsaicin-treated rats, suggesting that a state of mechanical hyperalgesia was also induced. The capsaicin treatment appears to have differential effects on nociceptive specific (NS) and wide-dynamic-range (WDR) neurons in inflamed rats. Expansion of the receptive fields of nociceptive neurons, a measure of the effect of inflammation-induced CNS plasticity, was less extensive for NS than for WDR neurons in capsaicin-treated rats. Compared to vehicle-treated rats, a smaller population of NS neurons, but a similar percentage of WDR neurons, had background activity in inflamed capsaicin-treated rats. C-fiber strength electrical stimulation of the sciatic nerve produced expansion of the receptive fields in a greater portion of NS neurons (53%, P < 0.05) in capsaicin- than in vehicle-treated rats (32%). There was no difference in stimulation-induced expansion of the receptive fields for WDR neurons between vehicle- or capsaicin-treated rats. An N-methyl-D-aspartate receptor antagonist, MK-801, attenuated the behavioral hyperalgesia and reduced the receptive field size of dorsal horn neurons in inflamed capsaicin- and vehicle-treated rats. The data suggest that while capsaicin-sensitive primary afferents may be involved in neuronal plasticity induced by peripheral tissue inflammation, changes in the capsaicin-insensitive WDR and NS populations are sufficient to produce thermal and mechanical hyperalgesia after the loss of capsaicin-sensitive primary afferents.

Diazepam attenuates hyperexcitability and mechanical hypersensitivity of dorsal horn convergent neurones during reperfusion of the rat's tail following ischaemia

We have investigated the involvement of the GABAA-benzodiazepine receptor complex in nociceptive activity of convergent neurones in the spinal cord during ischaemia and reperfusion of their receptive fields on the rat's tail. In enflurane anaesthetized rats, extracellular recordings were made from convergent neurones located throughout the dorsal horn before, during and after 30 min of ischaemia. Following intrathecal saline pretreatment, there was a significant increase in spontaneous firing rate during ischaemia (219 +/- 21%, P < 0.02, n = 10) which persisted during reperfusion. After 10 min of reperfusion, the neurones exhibited a greater response than before ischaemia to both innocuous brush (54 +/- 11%, P < 0.05, n = 10) and noxious pinch (72 +/- 14%, P < 0.02, n = 10) and the enhanced sensitivity persisted over the 60-min reperfusion period. During reperfusion, receptive field size increased in most neurones tested. Intrathecal diazepam (100 and 500 micrograms) abolished the hyperexcitability and the hypersensitivity to both innocuous and noxious mechanical stimulation during reperfusion. The highest dose of diazepam (500 micrograms) also attenuated the increase in spontaneous firing rate during ischaemia. Diazepam, at the doses tested, had no effect on receptive field enlargements during reperfusion. The effect of 100 micrograms of diazepam was partially reversed by flumazenil (1 mg/kg i.p.) but not by naloxone (1 mg/kg i.p.). In the absence of ischaemia, diazepam had no effect on spontaneous firing rate nor on the responses to innocuous or noxious mechanical stimulation. Our results support an antinociceptive role for benzodiazepines in the dorsal horn elements responsible for reperfusion hyperalgesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Pentobarbital prevents the development of C-fiber-induced hyperalgesia in the rat

Noxious stimuli applied to the skin can produce long-lasting, C-fiber-dependent, secondary hyperalgesia that is mediated by central mechanisms. NMDA receptor antagonists and low doses of morphine can preferentially block the development of hyperalgesia without significantly altering unpotentiated responses to nociceptive stimuli. The aim of our study was to determine if low doses of pentobarbital can also preferentially alter either hyperalgesic or unpotentiated responses to nociceptive heat stimuli in spinalized and intact rats. Our results demonstrate the following. (1) Mustard oil applied above the ankle joint or electrical stimulation of the sciatic nerve at C-fiber intensity in spinalized, unanesthetized rats decreased the latency to withdrawal of the foot from water maintained at 47-49 degrees C. This secondary hyperalgesia to thermal stimulation persisted for at least 1 h and was most likely mediated by central mechanisms. (2) Pentobarbital in both spinalized and spinal cord-intact rats prevented the development of the late component (42-120 min) but only partially decreased the early (2-6 min) component of hyperalgesia. In contrast, pentobarbital had relatively minimal effects on unpotentiated withdrawal responses. Thus, pentobarbital is similar to morphine in its ability to prevent hyperalgesia, but may differ from the anesthetic isoflurane, which does not interfere with the development of hyperalgesia.

Effects of anesthetic agents on somatosensory responses of raphespinal neurons in the cat

The goal of this study was to examine the effects of alpha-chloralose and pentobarbital on the somatosensory responses of medullary raphespinal neurons. With alpha-chloralose, 98% of neurons responded to innocuous stimuli, particularly tapping, but only 1/64 responded selectively to nociceptive stimuli. In contrast, 73% of neurons responded selectively to noxious stimuli with pentobarbital, and none responded selectively to innocuous stimuli. In addition, raphespinal neurons studied with pentobarbital had higher spontaneous discharge rates, and a higher incidence of spontaneity, than neurons studied with alpha-chloralose. Thus, different anesthetics produce raphespinal neurons with different somatosensory response characteristics.

Characterization of spinal somatosensory neurons having receptive fields in lumbar tissues of cats

In pentobarbital anesthetized cats, extracellular unitary recordings were made from neurons in the extreme lateral dorsal horn of spinal segments L4-5. All 118 units reported had receptive fields in deep somatic tissues and/or skin of the lumbar region, hip and/or proximal leg. Neurons were functionally characterized according to their responses to non-noxious and noxious mechanical stimuli and to injections of algogens. Most neurons (92%) were either wide-dynamic range (WDR) or nociceptive specific (NS), and most of these had very large nociceptive receptive fields in the back/hip/leg that included both skin and deep somatic tissues innervated through both the dorsal (back/hip) and ventral (leg/ventral spine) rami. Most (72%) were 'hyperconvergent' in that they were responsive to stimulation of many different somatic tissues including skin, muscles, facet joint capsules, ligaments, and periosteum. Some units were tested and found also to be activated by noxious stimulation of spinal dura and ventral annulus fibrosis and ventral longitudinal ligament. Twelve of 22 neurons tested were found to have ascending axons extending beyond Th10. The nocireceptive neurons (NS and WDR) in the population tested are suitable for processing information about tissue damage in deep somatic tissues in the back, hip and proximal leg. The apparent relative paucity of such neurons and their very large hyperconvergent receptive fields suggest that sensations served by these neurons, such as low back and referred leg pain, would be neither well localized nor attributable to pathology in a specific tissue. These deductions, based on physiological characteristics in cats, are consistent with clinical reports from humans who experience pain as a consequence of spinal or paraspinal injuries.

A model system to determine the effects of specific neurotransmitters on segmental reflexes in the spinal cord of the rat

A simple, repeatable, and quantifiable method to evaluate the effects of neurotransmitters on spinal cord reflexes is described. This model involves selective stimulation of A beta and A delta afferent fibers in the hind limb of the rat. The resulting evoked hind limb withdrawal reflex consists of short and long-latency EMG components along with their associated force production. This model was used to evaluate the effects of noradrenaline on the evoked reflex activity. The results are consistent with those obtained using much more invasive techniques and demonstrate the value of these techniques in evaluating the effects on spinal cord circuitry of other neurotransmitters and pharmacologic agents.

Drastic changes of ventromedial medulla neuronal properties induced by barbiturate anesthesia. I. Comparison of the single-unit types in the same awake and pentobarbital-treated rats

By means of single-unit recordings, as we have already performed in other studies, we have found that in the awake, drug-free, freely moving rat, there is only one neuronal class potentially involved in nociception and its control at the ventromedial medulla level (VMM, a structure involved in the spinal descending control systems of nociception): the 'multireceptive multimodal' units. These neurons are always activated by very light mechanical (air puff, light touch) and mechanical (pinch, pin-prick) or thermal noxious stimuli, in addition to an auditory stimulus. During identical VMM penetrations, performed in the same animals tested first awake and then anesthetized a few days later with 30 mg/kg of i.p. pentobarbital, we once again found the 'multireceptive multimodal' units, but this time with physiological properties that were strongly modified: in particular, we noted a disappearance of the nociceptive responses consecutive to a strong noxious heat pulse application (36-51 degrees C), associated sometimes with a reduction of the responses due to innocuous stimulation. This is in agreement with the classical effects of barbiturates. In light of previous observations reported in the literature devoted to the VMM physiology in the anesthetized rat, the most important observation in our study was that, with pentobarbital anesthesia, we recorded 'new' neuronal classes as compared to the awake condition. In these classes, which appeared to be qualitatively similar to those already reported under anesthesia, we found the units exclusively driven by innocuous stimulation (excited for the majority), the units specifically driven by noxious stimulation (half excited, half inhibited) and a 'multireceptive multimodal' group inhibited or excited-inhibited by non-noxious and noxious stimuli (half of the multireceptive group). All these data demonstrate that barbiturate anesthesia strongly modifies the VMM physiology in relation to nociception. Furthermore, since our results, that were obtained in anesthetized rats, were qualitatively identical to those described in the literature under similar experimental conditions, they raise the question of the appropriateness of using a barbiturate anesthetic in order to study the cellular mechanisms related to nociception at this level. In addition, these findings indicate that the obtention of only one neuronal class in the awake, drug-free, freely moving rat (the excited 'multireceptive' neurons) is not due to an experimental bias, which strongly emphasizes the reliability of using awake animals. However, it remains to be determined by which mechanisms pentobarbital 'distorts' the VMM physiology as compared to the normal, standard physiological conditions of the awake animal.

Drastic changes of ventromedial medulla neuronal properties induced by barbiturate anesthesia. II. Modifications of the single-unit activity produced by Brevital, a short-acting barbiturate in the awake, freely moving rat

In the preceding study, we have found that pentobarbital, a powerful barbiturate substance, strongly modified the ventromedial medulla (VMM) physiology in relation to nociception: indeed, in the same rats, during time-separated similar VMM penetrations, we have recorded, under pentobarbital, 'new' neuronal groups as compared to the awake state, such as the units exclusively driven (excited or inhibited) by cutaneous innocuous or noxious stimulations and the multimodal multireceptive neurons inhibited by non-noxious and noxious stimuli. Still under pentobarbital, we have also recorded the same units found as the rats were awake, i.e., the multimodal multireceptive neurons exclusively excited by various innocuous and noxious stimuli. However, the spontaneous and nociceptive activities of these units were strongly modified as compared to awake animals. Using Brevital (a short-acting barbiturate substance) administration, we have, in the present study, tried to understand the mechanisms underlying these drastic modifications. In particular, one of the questions was whether or not the 'new' neuronal classes recorded under anesthesia resulted from a modification of the physiological properties of the unique VMM neuronal group potentially involved in nociception in awake animals: the multimodal multireceptive units. By following the VMM neuronal activities either before and after or after Brevital administration until recovery from anesthesia, we have determined that the units exclusively driven by innocuous stimulation might result from a modification of the multimodal multireceptive neurons. Alternatively, the multireceptive units inhibited by peripheral stimulations are possibly totally different neurons, silent when the animals are awake.

Preparative surgery enhances the direct spinal actions of three injectable anaesthetics in the anaesthetized rat

In this study we have investigated the influence of preparative surgery on the potency with which a range of injectable anaesthetics depressed nociceptive withdrawal reflexes in anaesthetized, spinalized rats. Drug effects were compared on 2 different preparations, each requiring differing degrees of preparatory surgery. Recordings were made in each case of unitary motoneurone responses to controlled noxious stimuli. The dose-dependent effects of the general anaesthetics alpha-chloralose (20-80 mg/kg i.v.) and alphaxalone/alphadolone (0.5-2 mg/kg) and of the dissociative anaesthetic ketamine (0.5-16 mg/kg) were studied. When the degree of surgical intervention was increased, the reflex response to a uniform mechanical pinch stimulus was facilitated. This enhanced response was more susceptible to the reflex depressant actions of all the compounds studied.

Evidence for GABA-mediated control of putative nociceptive modulating neurons in the rostral ventromedial medulla: iontophoresis of bicuculline eliminates the off-cell pause

This study was undertaken to test the hypothesis that gamma-aminobutyric acid (GABA) is an endogeneous neurotransmitter regulating the activity of a class of putative nociceptive modulatory neurons (termed "off-cells") in the rostral ventromedial medulla (RVM) of the barbiturate-anesthetized rat. Off-cells, which are believed to correspond to the RVM output neuron that inhibits nociceptive processing at the level of the spinal cord, exhibit an abrupt pause in firing that begins immediately prior to the occurrence of the tail flick response (TF), a nocifensive reflex evoked by application of noxious heat to the tail. Single-unit recording and iontophoretic techniques were used to examine the ability of the GABAA receptor antagonist bicuculline methiodide (BIC) to antagonize selectively the characteristic off-cell pause. Iontophoretic application of BIC (5-30 nA) blocked the TF-related pause in each of the off-cells tested. This effect of BIC was generally slow in onset, and outlasted the period of application by several minutes. BIC iontophoresis also eliminated the cyclic alternation between active and silent periods that is often displayed by off-cells in lightly anesthetized rats. BIC application did not have a consistent effect on the firing of two other classes of RVM neurons ("on-cells" and "neutral cells"). Iontophoretically applied BIC antagonized the inhibitory effect of iontophoretically applied GABA, but not that produced by glycine. The glycine receptor antagonist strychnine did not mimic the action of BIC on off-cell activity. These data demonstrate antagonism of a synaptically evoked response using iontophoretic application of BIC, and provide strong evidence that the inhibitory neurotransmitter GABA mediates the TF-related off-cell pause. Taken together with behavioral experiments demonstrating that a GABA-mediated inhibitory process within RVM is crucial in permitting execution of the TF response, the present observations point to the significant functional relevance of GABA transmission within RVM in modulation of nociception.

Functional activity mapping of the rat brainstem during formalin-induced noxious stimulation

Functional activity changes in 35 selected structures of the rat brainstem elicited by subcutaneous formalin injection in a forepaw were investigated by the [14C]2-deoxyglucose method in unanesthetized, freely moving animals. Experiments were initiated 2 min ("early" group) or 60 min ("late" group) after the injection. Treatment induced a significant increase of [14C]2-deoxyglucose uptake relative to controls in 17 structures of the "early" group, including portions of the bulbar, pontine and mesencephalic reticular formation, nucleus raphe magnus, median and dorsal raphe nuclei, the ventrolateral and dorsal subdivisions of the periaqueductal gray matter, deep layers of the superior colliculus and the anterior pretectal nucleus. Most changes were bilateral, with the exception of the increases observed in the nucleus reticularis paragigantocellularis and the lateral parabrachial area, which were contralateral, and the one in the mesencephalic reticular formation, which was ipsilateral to the injected paw. In pentobarbital-anesthetized rats a significant difference in metabolic activity values between formalin- and saline-injected animals was only detected at the medullary level. In the "late" unanesthetized formalin group functional activity levels were higher than controls in four structures, including the lateral reticular and paragigantocellular nuclei, contralaterally, and nucleus cuneiformis and ventrolateral periaqueductal gray matter, bilaterally. No between-groups difference was observed in visual or auditory structures. These results provide evidence for activation of several brainstem regions, which are conceivably involved in different sensory, motivational or motor circuits, during the initial phase of formalin-evoked noxious stimulation in unanesthetized animals. Functional changes blunted over time as did pain-related behavior integrated at the supraspinal level, but they persisted in some brainstem regions for which involvement in endogenous antinociceptive systems have been suggested. The mechanisms underlying these time-related changes need to be clarified.

Functional activity mapping of the rat spinal cord during formalin-induced noxious stimulation

The functional activity pattern in the cervical enlargement of the spinal cord (as expressed by changes in local glucose utilization) was investigated by the semi-quantitative [14C]2-deoxyglucose technique 2 min ("early" group) or 60 min ("late" group) after injection of a small amount of dilute formalin (0.06-0.08 ml, 5%) in a forepaw of unanesthetized, freely-moving rats. Control animals were either injected with an equivalent volume of saline or simply handled. In both formalin groups a tonic flexion of the injected limb was present during the experiments, while supraspinal-integrated behavior (such as licking the affected paw) was sharply reduced in the late group. A bilateral increase of metabolic activity indexes, more pronounced on the ipsilateral side, was found in the "early" formalin-injected animals. The highest increase over control values was found in the medial part of the superficial (laminae I-II) region of the ipsilateral dorsal horn. However, the [14C]2-deoxyglucose uptake was found to be elevated over the whole extent of the dorsal horns, as well as in the gray matter surrounding the central canal, anterior horns and ipsilateral dorsolateral funiculus. In a parallel group of experiments performed in pentobarbital-anesthetized rats metabolic increases in the early period after formalin injection were less pronounced; they were only found in the ipsilateral side of the cord. In the "late" formalin group the overall metabolic changes were less conspicuous. They were mainly observed in the side ipsilateral to the injection, the highest increase being found in the deep portion (laminae V-VI) of the dorsal horn. Therefore, the spatial distribution of functional activation elicited during prolonged noxious stimulation in the spinal cord gray matter of unanesthetized rats varies according to time and changes in animal behavior.

Neural pathways in chronic pain

The evidence for changes in function of the central nervous system in cases of chronic pain is persuasive. We are not dealing with a passively wired system but one which changes structure and function and even connectivity in response to incoming sensory information. Whether these changes are capable of reversal with time and treatment remains to be shown. An optimist would suggest that physiological changes without abnormalities are indeed capable of reversal given time and appropriate neural input that matches normal non-painful afferent stimulation. That this is feasible is suggested by strategies of management with successful outcomes in patients with chronic pain, especially when pain is due to intermittent or limited ongoing stimulation of nociceptors. Clinical experience suggests, however, that deafferentation pain syndromes where pain is a consequence of damage directly to the nervous system cannot be viewed in such an optimistic light. A great deal more knowledge is required of how both the peripheral and central nervous system react to damage before we will be in a position to manage this source of chronic pain successfully.

A single-unit recording system, contact thermal probe and electromechanical stimulator for studying cellular mechanisms related to nociception at brain stem level of awake, freely moving rats

The purpose of this paper is to describe a simple, light-weight (3 g) device bearing a fine platinum-irridium Teflon-coated wire (50 microns) used to record single-unit activity extracellularly at brain stem level in the totally conscious freely moving rat. The up and down movements of the electrode through a guide cannula are insured by a small nut and a spring; the distance between the electrode and the end of the guide cannula is measured with a nut index. The system is directly connected to an amplifier (no FET or preamplifier) and allows for long term recordings necessary for a complete neuronal characterization and pharmacological experiments. The device is easy to make, entirely recoverable, and can be implanted from an animal to another. Further improvements are possible such as tungsten microelectrodes and telemetric or microinjection systems. In order to study some neuronal brain stem mechanisms involved in nociception, we have also designed a contact thermal probe and an electromechanical stimulator. The thermode is stuck to the shaved skin on the back of the rat, allowing heat pulses up to 51 degrees C to be applied. The mechanical stimulator is used manually and delivers reproducible innocuous stimuli to the skin. The fact that both types of stimulations are driven electrically enables the elaboration of cumulated peristimulus histograms which will reflect the neuronal activities in response to the application of noxious and non noxious stimuli.

Plasticity of some spinal dorsal horn neurons as revealed by pentobarbital-induced disinhibition

Extracellular activity was recorded from single spinal dorsal horn neurons in physiologically intact, awake, drug-free cats before and after the intravenous administration of 20 mg/kg pentobarbital (Pb). Pb produced a series of changes in response properties that reflect a significant moment-to-moment plasticity of some spinal dorsal horn neurons. Pb administration unmasked the ability of some low-threshold (LT) neurons to respond to noxious mechanical or thermal stimuli resulting in their being reclassified as wide dynamic range (WDR) neurons. Pb also appeared to unmask an afterdischarge in some neurons following noxious mechanical stimulation. In addition, some neurons appeared to be better able to signal changes in the intensity of mechanical stimulation after Pb. Neuronal receptive fields for low threshold stimulation were reduced in many instances but enlargement was also observed. The responses of some neurons to peripheral stimulation were unchanged by Pb. We hypothesize that the relatively low doses of Pb used in the study reduced tonic inhibition of some spinal dorsal neurons although the observed effects could have been produced by excitation.

Tonic 5-HT modulation of spinal dorsal horn neuron activity evoked by both noxious and non-noxious stimuli: a source of neuronal plasticity

The influence of tonic serotonergic modulation on the responses of spinal dorsal horn neurons to natural peripheral stimulation was examined in physiologically intact, awake, drug-free cats. Systemically administered methysergide (maximum cumulative dose 2 mg/kg) caused significant changes in responses of some dorsal horn neurons to both mildly noxious and non-noxious stimulation. Individual changes provide evidence, in this model, for tonic 5-HT modulation of many aspects of sensory transmission at the level of the spinal cord. Taken together, the changes demonstrate the significant degree of plasticity that exists for some spinal dorsal horn neurons. It is clear that the plasticity of some spinal dorsal horn neurons allows for a much broader response profile than would be apparent under the restricted circumstances of a normal neurophysiologic study. Removal of tonic inhibition on responses to noxious stimuli may be an aspect of neuronal plasticity that functions to provide an immediate change in the way that the nervous system responds to a noxious stimulus.

Different classes of cat spinal neurons display differential sensitivity to sodium pentobarbital

The effect of graded doses of systemically injected sodium pentobarbital on several classes of spinal neurons was studied using spinal cats. Classes of spinal neurons included unidentified dorsal horn cells, ascending tract dorsal horn cells, and motoneurons. Single unit activity of spinal neurons was evoked by electrically stimulating a peripheral nerve with an intensity strong enough to excite both A and C fibers. The A- and C-fiber evoked activity was compared before and after intravenous injections of small incremental doses of sodium pentobarbital. The activity of different classes of spinal neurons showed different sensitivities to graded doses of systemically injected pentobarbital. The reflex activity of motoneurons elicited by stimulation of peripheral nerve was much more sensitive to pentobarbital than that of dorsal horn cells. In general, activity evoked by peripheral unmyelinated fibers was more susceptible to pentobarbital than was that evoked by myelinated fibers. However, intravenous injections of pentobarbital produced nondifferential suppression of dorsal horn cell activity evoked by noxious and innocuous mechanical stimuli applied to the peripheral receptive fields.

Behavioral and autonomic correlates of the tactile evoked allodynia produced by spinal glycine inhibition: effects of modulatory receptor systems and excitatory amino acid antagonists

Intrathecal administration of glycine (strychnine) or GABA (bicuculline) but not opioid (naloxone), adrenergic (phentolamine) or serotonin (methysergide) receptor antagonists resulted in a dose-dependent organized agitation response to light tactile stimulation. This effect was maximally evoked by oscillating but not continuous stimulation applied to a dermatome corresponding to the levels of spinal cord acted upon by the intrathecal antagonist. Similar results were observed in chloralose-urethane anesthetized rats in which tactile stimulation evoked hypertensive responses following local tactile stimuli. The effects were only mildly depressed by even high doses of spinal morphine or DADL and not at all by ST-91 or baclofen. In contrast, intrathecal injections of glutamate receptor antagonists resulted in a dose-dependent depression of the strychnine evoked hyperesthesia with the ordering of activity being MK-801, AP-5, kynurenic acid, SKF10047 and ketamine. At doses below those which produced motor dysfunction, however, these agents had no effects on the hot-plate response latency. These data emphasize that low threshold afferent input is likely subject to an ongoing modulation, the loss of which results in a miscoding of the afferent stimulus yielding a pain relevant message. The lack of effect of agents having a powerful effect on somatic pain stimuli and the converse effects of glutamate receptor antagonists on the strychnine hyperesthesia at doses which do not affect the somatic pain response indicate discriminable processing systems, the characteristics of which resemble the clinical phenomenon observed in patients suffering from sensory dysesthesia following central and peripheral horn injury.

Long-term changes in discharge behaviour of cat dorsal horn neurones following noxious stimulation of deep tissues

Certain pathological types of afferent input are supposed to lead to long-term changes in the responsiveness of dorsal horn neurones. This mechanism might be of importance for the development of neurological disturbances such as chronic pain. The present study was undertaken in order to find out whether dorsal horn neurones--particularly those processing input from deep tissues--exhibit long-lasting changes in response behaviour after a short-lasting noxious stimulation of deep tissue. In anaesthetized cats, the impulse activity of single dorsal horn cells was recorded extracellularly with glass microelectrodes. In a small number of cells that had multiple receptive fields (RFs), the algesic agent bradykinin was injected into a muscle RF and the properties of all RFs retested at regular time intervals. Following noxious chemical stimulation of one RF, the injected and the other RFs of the same neurone often showed changes which consisted of an increase in size, a lowering of mechanical threshold and appearance of new RFs. In an attempt to assess the influence of a single noxious stimulus on the entire population of dorsal horn cells, the properties of a greater sample of neurones were compared before and after injection of bradykinin into the deep tissues of the hind limb. Every cell encountered was classified as being driven by (1) cutaneous receptors only, (2) deep receptors only, (3) both input sources, or (4) electrical stimulation only (cell without receptive field). Following injection of bradykinin, the proportion of cells with both deep and cutaneous input and of those having background activity rose, and the percentage of cells without a receptive field decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Putative nociceptive modulatory neurons in the rostral ventromedial medulla of the rat display highly correlated firing patterns

Recent work in this laboratory has identified two classes of putative nociceptive modulating neurons in the rostral ventromedial medulla (RVM) of the rat: "off-cells," which pause beginning just prior to the tail flick response (TF) evoked by noxious heat, and "on-cells," which accelerate shortly before the occurrence of the TF. In the unstimulated, lightly anesthetized rat, the spontaneous firing pattern of individual on- and off-cells consists of alternating periods of silence and activity lasting from several seconds to a few minutes. In the present study, simultaneous recordings were made from pairs of TF-related neurons, and the relationships among the firing patterns of cells within a class and between cells of different classes were determined. All cells of a given class showed fluctuations in spontaneous discharge that were in phase. On the other hand, there was a striking reciprocity of firing between the two cell classes, such that a decrease in activity of cells of one class was accompanied by an increase in activity of cells of the other class. These observations point to the existence of integrating mechanisms that coordinate the activity of all members of each class of TF-related neurons. Thus, the pattern of activity of any single on- or off-cell provides a useful index of the excitability of all cells of that class. Moreover, because of the highly reciprocal nature of the firing of the two classes, it is possible to infer the current state of both cell populations from the pattern of activity of any single TF-related neuron.

Medial hypothalamic stimulation suppresses nociceptive spinal dorsal horn neurons but not the tail-flick reflex in the rat

This study investigated the potential analgesic effects of medial hypothalamic stimulation (HS) on a measure of nocifensive behavior (tail-flick test (TF] in awake rats, and potential inhibitory effects of identical HS on spinal dorsal horn neuronal responses to noxious skin heating in the same animals anesthetized with sodium pentobarbital. Sixty-five male Sprague-Dawley rats implanted with a bipolar stimulation electrode in histologically verified medial hypothalamic sites were tested behaviorally for TF suppression during HS (100 ms trains at 100 Hz, 3/s, 100-1100 microA) in 2-4 consecutive weekly test sessions. Thirty-three of these rats were then used in electrophysiological experiments to record responses of 36 dorsal horn units to noxious skin heating (48-54 degrees C, 10 s/2 min) of the hindfoot pad in the absence of and during HS. Behaviorally, 31/65 rats had no TF suppression at the highest HS intensity tested (1100 microA), 24/65 rats exhibited aversive behavior or motor activity which disallowed reliable TF testing, and only 10/65 rats showed TF suppression in at least one test session. In electrophysiological experiments, the heat-evoked responses of 25/36 dorsal horn units were inhibited to at least 50% of control during HS. The responses of 11 units remained at 65-100% of the control responses during HS of up to 1100 microA. In rats demonstrating TF suppression, 4/7 units were inhibited. In rats with no TF suppression, 10/15 units were inhibited, and in rats showing aversive behavior, 11/14 units were inhibited by HS. These data indicate that although HS suppresses spinal nociceptive neurons, it does not cause reliable TF suppression in unanesthetized rats and bring into question the often-held assumption that stimulation-evoked descending inhibition of spinal nociceptive neurons implies behavioral analgesia.

Lack of spontaneous activity of cutaneous spinal dorsal horn neurons in awake, drug-free, spinally transected cats

Extracellular single-unit activity was recorded from neurons with cutaneous input in the dorsal horn of the spinal cord (L4-L6) of awake, drug-free cats before (for several weeks) and after (day 1 through day 7) cord transection (T12). The spontaneous activity of the neurons was minimal or nonexistent in both recording conditions. The lack of spontaneous activity following spinal cord transection contrasts sharply with activity recorded in acute spinal-cord transected preparations in which high rates of spontaneous activity have been reported. This discrepancy may reflect an important difference between the chronic, awake, drug-free and acute preparations.