Depression of A and C fibre-evoked segmental reflexes by morphine and clonidine in the in vitro spinal cord of the neonatal rat

The left-right side-specific endocrine signaling in the effects of brain lesions: questioning of the neurological dogma

Each cerebral hemisphere is functionally connected to the contralateral side of the body through the decussating neural tracts. The crossed neural pathways set a basis for contralateral effects of brain injury such hemiparesis and hemiplegia as it has been already noted by Hippocrates. Recent studies demonstrated that, in addition to neural mechanisms, the contralateral effects of brain lesions are mediated through the humoral pathway by neurohormones that produce either the left or right side-specific effects. The side-specific humoral signaling defines whether the left or right limbs are affected after a unilateral brain injury. The hormonal signals are released by the pituitary gland and may operate through their receptors that are lateralized in the spinal cord and involved in the side-specific control of symmetric neurocircuits innervating the left and right limbs. Identification of features and a proportion of neurological deficits transmitted by neurohormonal signals vs. those mediated by neural pathways is essential for better understanding of mechanisms of brain trauma and stroke and development of new therapies. In a biological context, the left–right side-specific neuroendocrine signaling may be fundamental for the control of the left- and right-sided processes in bilaterally symmetric animals.

Left-Right Side-Specific Neuropeptide Mechanism Mediates Contralateral Responses to a Unilateral Brain Injury

Neuropeptides are implicated in control of lateralized processes in the brain. A unilateral brain injury (UBI) causes the contralesional sensorimotor deficits. To examine whether opioid neuropeptides mediate UBI induced asymmetric processes we compared effects of opioid antagonists on the contralesional and ipsilesional hindlimb responses to the left-sided and right-sided injury in rats. UBI induced hindlimb postural asymmetry (HL-PA) with the contralesional hindlimb flexion, and activated contralesional withdrawal reflex of extensor digitorum longus (EDL) evoked by electrical stimulation and recorded with EMG technique. No effects on the interossei (Int) and peroneaus longus (PL) were evident. The general opioid antagonist naloxone blocked postural effects, did not change EDL asymmetry while uncovered cryptic asymmetry in the PL and Int reflexes induced by UBI. Thus, the spinal opioid system may either mediate or counteract the injury effects. Strikingly, effects of selective opioid antagonists were the injury side-specific. The μ-antagonist β-funaltrexamine (FNA) and κ-antagonist nor-binaltorphimine (BNI) reduced postural asymmetry after the right but not left UBI. In contrast, the δ-antagonist naltrindole (NTI) inhibited HL-PA after the left but not right-side brain injury. The opioid gene expression and opioid peptides were lateralized in the lumbar spinal cord, and coordination between expression of the opioid and neuroplasticity-related genes was impaired by UBI that together may underlie the side-specific effects of the antagonists. We suggest that mirror-symmetric neural circuits that mediate effects of left and right brain injury on the contralesional hindlimbs are differentially controlled by the lateralized opioid system.

Ipsilesional versus contralesional postural deficits induced by unilateral brain trauma: a side reversal by opioid mechanism

Unilateral traumatic brain injury and stroke result in asymmetric postural and motor deficits including contralateral hemiplegia and hemiparesis. In animals, a localized unilateral brain injury recapitulates the human upper motor neuron syndrome in the formation of hindlimb postural asymmetry with contralesional limb flexion and the asymmetry of hindlimb nociceptive withdrawal reflexes. The current view is that these effects are developed due to aberrant activity of motor pathways that descend from the brain into the spinal cord. These pathways and their target spinal circuits may be regulated by local neurohormonal systems that may also mediate effects of brain injury. Here, we evaluate if a unilateral traumatic brain injury induces hindlimb postural asymmetry, a model of postural deficits, and if this asymmetry is spinally encoded and mediated by the endogenous opioid system in rats. A unilateral right-sided controlled cortical impact, a model of clinical focal traumatic brain injury was centred over the sensorimotor cortex and was observed to induce hindlimb postural asymmetry with contralateral limb flexion. The asymmetry persisted after complete spinal cord transection, implicating local neurocircuitry in the development of the deficits. Administration of the general opioid antagonist naloxone and μ-antagonist β-funaltrexamine blocked the formation of postural asymmetry. Surprisingly, κ-antagonists nor-binaltorphimine and LY2444296 did not affect the asymmetry magnitude but reversed the flexion side; instead of contralesional (left) hindlimb flexion the ipsilesional (right) limb was flexed. The postural effects of the right-side cortical injury were mimicked in animals with intact brain via intrathecal administration of the opioid κ-agonist (2)-(trans)-3,4-Dichloro-N-methyl-N-[2-(1-pyrrolidiny)-cyclohexyl]benzeneacetamide that induced hindlimb postural asymmetry with left limb flexion. The δ-antagonist naltrindole produced no effect on the contralesional (left) flexion but inhibited the formation of the ipsilesional (right) limb flexion in brain-injured rats that were treated with κ-antagonist. The effects of the antagonists were evident before and after spinal cord transection. We concluded that the focal traumatic brain injury-induced postural asymmetry was encoded at the spinal level, and was blocked or its side was reversed by administration of opioid antagonists. The findings suggest that the balance in activity of the mirror symmetric spinal neural circuits regulating contraction of the left and right hindlimb muscles is controlled by different subtypes of opioid receptors; and that this equilibrium is impaired after unilateral brain trauma through side-specific opioid mechanism.

Differential suppression of the ipsi- and contralateral nociceptive reflexes in the neonatal rat spinal cord by agonists of µ-, δ- and κ-opioid receptors

Nociceptive discharges caused by the unilateral tissue damage are processed in the spinal cord by both ipsi- and contralateral neuronal circuits. The mechanisms of the neurotransmitter control of this bilateral excitation spread is poorly understood. Spinally administered opiates are known to suppress nociceptive transmission and nociceptive withdrawal reflexes. Here we investigated whether three major types of opioid receptors are involved in the bilateral control of the spinal nociceptive sensorimotor processing. Effects of the µ-, δ- and κ-opioid receptor agonists on the ipsi- and contralateral nociceptive reflexes were studied by recording slow ventral root potentials in an isolated spinal cord preparation of the new-born rat. Absolute levels of expression of the opioid genes were analyzed by the droplet digital PCR. Ipsi- and contralateral slow ventral root potentials were most strongly suppressed by the µ-opioid receptor agonist DAMGO, by 63% and 85%, followed by the κ-opioid receptor agonist U-50488H, by 44% and 73%, and δ-opioid receptor agonist leucine-enkephalin, by 27% and 49%, respectively. All these agonists suppressed stronger contra- than ipsilateral responses. Naloxone prevented effects of the agonists indicating that they act through opioid receptors, which, as we show, are expressed in the neonatal spinal cord at the levels similar to those in adults. Thus, opioid receptor agonists suppress the segmental nociceptive reflexes. Stronger contralateral effects suggest that the endogenous opioid system regulates sensorimotor processing in the spinal commissural pathways. These effects of opioids may be relevant for treatment of symmetric clinical pain symptoms caused by unilateral tissue injury.

Spinal Reflexes and Windup In Vitro: Effects of Analgesics and Anesthetics

The spinal cord is the first relay center for nociceptive information. Following peripheral injury, the spinal cord sensitizes. A sign of spinal sensitization is the hyper-reflexia which develops shortly after injury and can be detected in the isolated spinal cord as a "memory of pain." In this context, it is easy to understand that many analgesic compounds target spinally located sites of action to attain analgesia. In vitro isolated spinal cord preparations have been used for a number of years, and experience on the effects of compounds of diverse pharmacological families on spinal function has accumulated. Recently, we have proposed that the detailed study of spinal segmental reflexes in vitro may produce data relevant to the evaluation of the analgesic potential of novel compounds. In this review, we describe the main features of segmental reflexes obtained in vitro and discuss the effects of compounds of diverse chemical nature and pharmacological properties on such reflexes. Our aim was to compare the different profiles of action of the compounds on segmental reflexes in order to extract clues that may be helpful for pharmacological characterization of novel analgesics.

Effects of centrally acting analgesics on spinal segmental reflexes and wind-up

BACKGROUND

The spinal cord is a prime site of action for analgesia. Here we characterize the effects of established analgesics on segmental spinal reflexes. The aim of the study was to look for the pattern of action or signature of analgesic effects on these reflexes.

METHODS

We used a spinal cord in vitro preparation of neonate mice to record ventral root responses to dorsal root stimulation. Pregabalin, clonidine, morphine and duloxetine and an experimental sigma-1 receptor antagonist (S1RA) were applied to the preparation in a cumulative concentration protocol. Drug effects on the wind-up produced by repetitive stimulation of C-fibres and on responses to single A- and C-fibre intensity stimuli were analysed.

RESULTS

All compounds produced a concentration-dependent inhibition of total spikes elicited by repetitive stimulation. Concentrations producing ∼50% reduction in this parameter were (in μM) clonidine (0.01), morphine (0.1), pregabalin (1), duloxetine (10) and S1RA (30). At these concentrations clonidine, pregabalin and S1RA had significant effects on the wind-up index and little depressant effects on responses to single stimuli. Morphine and duloxetine did not depress wind-up index and showed large effects on responses to single stimuli. None of the compounds had strong effects on the amplitude of the non-nociceptive monosynaptic reflex.

CONCLUSIONS

morphine and duloxetine had general depressant effects on spinal reflexes, whereas the effects of clonidine, pregabalin and S1RA appeared to be restricted to signals originated by strong repetitive activation of C-fibres. Results are discussed in the context of reported behavioural effects of the compounds studied.

Contribution of α2A-adrenoceptor subtype to effect of dexmedetomidine and xylazine on spinal synaptic transmission of mice

Alpha-2A adrenergic receptor (AR) subtype plays an important role in the analgesic effect of α2-AR agonists. Here, we examined the effects of α2-AR agonists, dexmedetomidine and xylazine, on spinal synaptic transmission in newborn C57BL/6J and α2A-AR mutant mice. Spinal reflex potentials, the monosynaptic reflex potential (MSR) and the slow ventral root potential (sVRP), were measured in isolated spinal cords. The compound action potential was measured in isolated lumbar nerve. Dexmedetomidine and xylazine suppressed both the MSR and sVRP in a concentration-dependent manner. In α2A-AR mutant mice, sVRP suppression by dexmedetomidine was greatly weakened, while that by xylazine (30-100μM) showed only slight attenuation. A high concentration (300μM) of xylazine completely suppressed the sVRP, even in α2A-AR mutant mice spinal cords, and also suppressed the compound action potential. MSR suppression by these α2-AR agonists had no difference between wild-type and α2A-AR mutant mice. These results suggest that sVRP suppression by dexmedetomidine and xylazine is mainly mediated by α2A-AR. In addition, a high concentration of xylazine inhibits conduction of the action potential, which is not mediated by α2A-AR. α2-AR is not responsible for the dexmedetomidine- and xylazine-mediated inhibition of the MSR.

Endogenously released 5-HT inhibits A and C fiber-evoked synaptic transmission in the rat spinal cord by the facilitation of GABA/glycine and 5-HT release via 5-HT(2A) and 5-HT(3) receptors

Serotonin (5-HT) released from descending fibers plays important roles in spinal functions such as locomotion and nociception. 5-HT2A and 5-HT3 receptors are suggested to contribute to spinal antinociception, although their activation also contributes to neuronal excitation. In the neonatal spinal cord, DL-p-chloroamphetamine (pCA), a 5-HT releaser, inhibited both A fiber-evoked monosynaptic reflex potential (MSR) and C fiber-evoked slow ventral root potential (sVRP). The pCA-mediated inhibition was reversed by ketanserin (a 5-HT2A receptor antagonist) and tropisetron (a 5-HT3 receptor antagonist). Bath-applied 5-HT also inhibited MSR and sVRP; in this case, the actions of 5-HT were antagonized by ketanserin, but not by tropisetron. The pCA-evoked inhibition of sVRP was reduced by bicuculline (a GABAA receptor antagonist) and strychnine (a glycine receptor antagonist). Furthermore, ketanserin inhibited the pCA-evoked release of gamma-aminobutyric acid (GABA) and glycine, while tropisetron inhibited the pCA-evoked release of 5-HT. These results suggest that 5-HT released by pCA activates 5-HT2A receptors, which in turn stimulates the release of GABA/glycine and thereby blocks the spinal nociceptive pathway. 5-HT3 receptors may be involved in the facilitation of 5-HT release via a positive feedback process.

Inhibitory effects of dopamine on spinal synaptic transmission via dopamine D1-like receptors in neonatal rats

BACKGROUND AND PURPOSE

Dopamine released from the endings of descending dopaminergic nerve fibres in the spinal cord may be involved in modulating functions such as locomotion and nociception. Here, we examined the effects of dopamine on spinal synaptic transmissions in rats.

EXPERIMENTAL APPROACH

Spinal reflex potentials, monosynaptic reflex potential (MSR) and slow ventral root potential (sVRP), were measured in the isolated spinal cord of the neonatal rat. Dopamine release was measured by HPLC.

KEY RESULTS

Dopamine at lower concentrations (<1 µM) depressed sVRP, which is a C fibre-evoked polysynaptic response and believed to reflect nociceptive transmission. At higher concentrations (>1 µM), in addition to a potent sVRP depression, dopamine depolarized baseline potential and slightly depressed MSR. Depression of sVRP by dopamine was partially reversed by dopamine D(1) -like but not by D(2) -like receptor antagonists. SKF83959 and SKF81297, D(1) -like receptor agonists, and methamphetamine, an endogenous dopamine releaser, also caused the inhibition of sVRP. Methamphetamine also depressed MSR, which was inhibited by ketanserin, a 5-HT(2A/2C) receptor antagonist. Methamphetamine induced the release of dopamine and 5-HT from spinal cords, indicating that the release of endogenous dopamine and 5-HT depresses sVRP and MSR respectively.

CONCLUSION AND IMPLICATIONS

These results suggested that dopamine at lower concentrations preferentially inhibited sVRP, which is mediated via dopamine D(1) -like and other unidentified receptors. The dopamine-evoked depression is involved in modulating the spinal functions by the descending dopaminergic pathways.

Differential contributions of adenosine to hypoxia-evoked depressions of three neuronal pathways in isolated spinal cord of neonatal rats

BACKGROUND AND PURPOSE

Hypoxic effects on neuronal functions vary significantly with experimental conditions, but the mechanism for this is unclear. Adenosine has been reported to play a key role in depression of neuronal activities in the CNS during acute hypoxia. Hence, we examined the effect of acute hypoxia on different spinal reflex potentials and the contribution of adenosine to them.

EXPERIMENTAL APPROACH

Spinal reflex potentials, monosynaptic reflex potential (MSR), slow ventral root potential (sVRP) and dorsal root potential (DRP), were measured in the isolated spinal cord of the neonatal rat. Adenosine release was measured by using enzymatic biosensors.

KEY RESULTS

In the spinal cord preparation isolated from postnatal day 5-8 rats at 27°C, acute hypoxia induced adenosine release and depressed three reflex potentials. However, in postnatal day 0-3 rats at 27°C, the hypoxic-induced adenosine release and depression of MSR were negligible, while the depression of sVRP and DRP were perceptible responses. In postnatal day 0-3 rats at 33°C, hypoxia evoked adenosine release and depression of MSR. An adenosine A(1) receptor selective antagonist and a high [Ca(2+)](o), which suppressed adenosine release, abolished the hypoxic-induced depression of MSR but not those of sVRP and DRP.

CONCLUSIONS AND IMPLICATIONS

Hypoxic-induced depression of MSR depends on adenosine release, which is highly susceptible to age, temperature and [Ca(2+)](o). However, a large part of the depressions of DRP and sVRP are mediated via adenosine-independent mechanisms. This differential contribution of adenosine to depression is suggested to be an important factor for the variable effects of hypoxia on neuronal functions.

Neuropeptide modulation of a lumbar spinal reflex: potential implications for female sexual function

INTRODUCTION

Neuropeptides are known to modulate female receptivity. However, even though receptivity is a spinal reflex, the role of neuropeptides within the spinal cord remains to be elucidated.

AIM

The aims were to (i) investigate neuropeptides in the lumbosacral region; and (ii) determine how neuropeptides modulate glutamate release from stretch Ia fibers, touch sensation Abeta fibers and Adelta/C pain fibers.

MAIN OUTCOME MEASURES

Neuropeptide modulation of the lumbosacral dorsal-root ventral-root reflex in vitro.

METHODS

Spinal cords were removed from Sprague-Dawley rats in compliance with UK Home Office guidelines. Hemisected cords were superfused with aCSF and the dorsal root (L4-S1) was stimulated to evoke glutamate release. A biphasic reflex response was evoked from the opposite ventral root consisting of a monosynaptic (Ia fibers) and polysynaptic (Abeta, Adelta/C fibers) component.

RESULTS

The micro opioid receptor (MOR) agonist DAMGO inhibited the monosynaptic (EC(50) 0.02 +/- 0.02 nM) and polysynaptic area (EC(50) 125 +/- 167 nM) but not polysynaptic amplitude. Oxytocin and corticotrophin releasing factor (CRF) inhibited the monosynaptic amplitude (EC(50), 1.4 +/- 1.0 nM and EC(50) 4.3 +/- 3.5 nM, respectively), polysynaptic amplitude (EC(50) 18.2 +/- 28.0 nM and EC(50), 9.5 +/- 13.3 nM, respectively), and area (EC(50) 11.6 +/- 13.0 nM and EC(50), 2.8 +/- 3.3 nM, respectively); effects that were abolished by oxytocin and CRF(1) antagonists, L-368899 and 8w. Melanocortin agonists solely inhibited the monosynaptic component, which were blocked by the MC(3/4) receptor antagonist SHU9119.

CONCLUSION

These data suggest endogenous neuropeptides are released within the lumbosacral spinal cord. Melanocortin agonists, oxytocin, CRF, and DAMGO via MC(4), oxytocin, CRF(1), and MOR inhibit glutamate release but with differing effects on afferent fiber subtypes. Melanocortins, oxytocin, CRF, and DAMGO have the ability to modulate orgasm whereas oxytocin, CRF and DAMGO can increase pain threshold. Oxytocin and CRF may dampen touch sensation.

Neonatal morphine enhances nociception and decreases analgesia in young rats

The recognition of the impact of neonatal pain experience on subsequent sensory processing has led to the increased advocacy for the use of opioids for pain relief in infants. However, following long-term opioid exposure in intensive care units more than 48% of infants exhibited behaviors indicative of opioid abstinence syndrome, a developmentally equivalent set of behaviors to opioid withdrawal as seen in adults. Little is known about the long-term influence of repeated neonatal morphine exposure on nociception and analgesia. To investigate this, we examined mechanical and thermal nociception on postnatal days 11, 13, 15, 19, 24, 29, 39 and 48 following subcutaneous administration of morphine (3 mg/kg) once daily on postnatal days 1-9. The cumulative morphine dose-response was assessed on postnatal days 20 and 49, and stress-induced analgesia was assessed on postnatal days 29 and 49. Both basal mechanical and thermal nociception in neonatal, morphine-exposed rats were significantly lower than those in saline-exposed, handled-control rats and naive rats until P29. A rightward-shift of cumulative dose-response curves for morphine analgesia upon chronic neonatal morphine was observed both on P20 and P49. The swim stress-induced analgesia was significantly decreased in neonatal morphine-exposed rats on P29, but not on P49. These data indicate that morphine exposure equivalent to the third trimester of gestation produced prolonged pain hypersensitivity, decreased morphine antinociception, and decreased stress-induced analgesia. The present study illustrates the need to examine the long-term influence of prenatal morphine exposure on pain and analgesia in the human pediatric population.

Synergistic depression of NMDA receptor-mediated transmission by ketamine, ketoprofen and L-NAME combinations in neonatal rat spinal cords in vitro

BACKGROUND AND PURPOSE

Spinal N-methyl-D-aspartate (NMDA) receptor/cyclooxygenase (COX) and nitric oxide synthase (NOS) pathways play a major role in nociceptive processing, and influencing them simultaneously may induce synergistic analgesia. This study determined the spinal antinociceptive interactions between ketamine (NMDA receptor channel blocker), ketoprofen (COX inhibitor) and L-NAME (NOS inhibitor) combinations.

EXPERIMENTAL APPROACH

Using an in vitro neonatal rat spinal cord preparation, two A-fibre-mediated reflexes, the monosynaptic reflex (MSR) and the low-intensity excitatory postsynaptic potential (epsp), and one C-fibre-mediated reflex, the high-intensity epsp, were evoked electrically. The effect of drugs and drug combinations on these reflexes was assessed and the type of interaction determined by isobolographic analysis.

KEY RESULTS

Infusion of ketamine alone decreased all three reflexes. That of ketoprofen decreased both the low and the high-intensity epsp only. Infusion of L-NAME alone produced no significant effects. Co-infusion of fixed ratios of IC(40) fractions of both (ketamine+ketoprofen) and (ketamine+L-NAME) were synergistic for depressing the low and the high-intensity epsps. The interaction was sub-additive for both combinations on the MSR. The only significant effect for the (ketoprofen+L-NAME) combination was synergism on the high-intensity epsp.

CONCLUSIONS AND IMPLICATIONS

All three combinations synergistically depressed nociceptive spinal transmission, and both ketamine and ketoprofen and ketamine and L-NAME combinations did so with potentially decreased motor side effects. If such combination profiles also occur in vivo, the present findings raise the possibility of ultimate therapeutic exploitation of increased analgesia with fewer side effects.

Developmental changes in threshold, conduction velocity, and depressive action of lignocaine on dorsal root potentials from neonatal rats are associated with maturation of myelination

Conduction velocity of primary afferent fibres and blocking actions of local anaesthetics seem to be developmentally regulated. The current work investigated physiological (threshold, conduction velocity, and myelination) and pharmacological (lignocaine (0.0625 to 2 mmol/L) and capsaicin (2 micromol/L)) ontogenic changes on in vitro sciatic nerve-dorsal root preparations from 0- to 12-day-old rats. As rats aged, stimulus intensities necessary to evoke A-fibre thresholds significantly decreased and A-fibre conduction velocities significantly increased. For C-fibres, thresholds significantly increased and conduction velocities significantly varied with age. The blocking potency of lignocaine varied with age: A-fibres from 4-day-old rats and younger were significantly more resistant than those from older rats, and C-fibres were blocked more uniformly amongst ages. Capsaicin significantly depressed C-fibres irrespective of age, and A-fibres were significantly reduced during the first postnatal week only. Myelination significantly increased as rats aged. A-fibre physiological parameters were significantly correlated with both other A-fibre physiological and pharmacological parameters, but C-fibre parameters were not. Peripheral A-fibre transduction mechanisms seem to require time to acquire their full stimulus-response sensitivity, which coincides with development of myelination. In contrast, peripheral C-fibres seem to have mature transduction mechanisms from the first days of postnatal life.

Involvement of adenosine in depression of synaptic transmission during hypercapnia in isolated spinal cord of neonatal rats

Adenosine is one of the most important neuromodulators in the CNS, both under physiological and pathological conditions. In the isolated spinal cord of the neonatal rat in vitro, acute hypercapnic acidosis (20% CO2, pH 6.7) reversibly depressed electrically evoked spinal reflex potentials. This depression was partially reversed by 8-cyclopentlyl-1,3-dimethylxanthine (CPT), a selective A1 adenosine receptor antagonist. Isohydric hypercapnia (20% CO2, pH 7.3), but not isocapnic acidosis (5% CO2, pH 6.7), depressed the reflex potentials, which were also reversed by CPT. An ecto-5'-nucleotidase inhibitor did not affect the hypercapnic acidosis-evoked depression. An inhibitor of adenosine kinase, but not deaminase, mimicked the inhibitory effect of hypercapnic acidosis on the spinal reflex potentials. Accumulation of extracellular adenosine and inhibition of adenosine kinase activity were caused by hypercapnic acidosis and isohydric hypercapnia, but not isohydric acidosis. These results indicate that the activation of adenosine A1 receptors is involved in the hypercapnia-evoked depression of reflex potentials in the isolated spinal cord of the neonatal rat. The inhibition of adenosine kinase activity is suggested to cause the accumulation of extracellular adenosine during hypercapnia.

Depression of NMDA-receptor-mediated segmental transmission by ketamine and ketoprofen, but not L-NAME, on the in vitro neonatal rat spinal cord preparation

Activation of spinal N-methyl-D-aspartate (NMDA) receptors and then the nitric oxide and the arachidonic acid pathways is important in pain transmission. This study assessed the effects of the NMDA receptor channel blocker ketamine, the nitric oxide synthase inhibitor L-NAME, and the cyclooxygenase inhibitor ketoprofen in nociceptive transmission using an in vitro neonatal rat spinal cord preparation. Supramaximal electrical stimulation of the dorsal root evoked the A-fibre- and C-fibre-mediated high intensity excitatory postsynaptic potential (EPSP) in the ipsilateral ventral root. Low intensity stimulation evoked the A-fibre-mediated monosynaptic compound action potential (MSR) superimposed on the low intensity EPSP. Both the low intensity EPSP and the high intensity EPSP contain NMDA-receptor-mediated components. Only ketamine and ketoprofen depressed the synaptic responses. Ketamine depressed all three spinal reflexes with IC(50) values (with 95% CI) of 10.80 microM (5.97 to 19.54 microM) for the MSR, 8.29 microM (4.53 to 14.17 microM) for the low intensity EPSP, and 5.35 microM (3.05 to 9.40 microM) for the high intensity EPSP. Ketoprofen depressed the low intensity EPSP and the high intensity EPSP only; IC(50) values (with 95% CI) were 354.5 microM (217.5 to 576.8 microM) and 302.7 microM (174.0 to 526.7 microM), respectively. Reflexes recovered after drug washout. These data demonstrated that ketamine and ketoprofen, but not L-NAME, depressed NMDA-mediated nociceptive transmission in spinal cord preparations from neonatal rats.

Acute and chronic morphine alters formalin pain in neonatal rats

The present study tested the hypothesis that morphine exposure during the human developmental equivalent of the third trimester would alter inflammatory pain. This study examined whether acute or continuous opioid exposure in the neonatal rat alters formalin-induced nociception after 4 days of abstinence. Rats were exposed to a single acute administration of morphine on postnatal day 7 or 72 h of opioid infusion from postnatal days 5-7 via osmotic pump. When challenged with intraplantar formalin on postnatal day 11, rats exposed to acute or chronic morphine had increased phase II pain-associated behaviors. These findings suggest that neonatal morphine exposure may have unintended consequences on inflammatory pain.

Effects of opioid receptor and alpha2-adrenoceptor agonists on slow ventral root potentials and on capsaicin and formalin tests in neonatal rats

The inhibitory effects of morphine and alpha2-adrenoceptor agonists on slow ventral root potentials (slow VRP) following ipsilateral dorsal root stimulation in neonatal rat spinal cord were compared with the analgesic effects of these drugs on formalin and capsaicin tests in neonatal rats. Morphine, (D-Phe2, D-Pen5)-enkephalin (DPDPE), dexmedetomidine, clonidine and xylazine showed concentration-related inhibition of slow VRP. The order of potency was dexmedetomidine>morphine=DPDPE>clonidine>xylazine. The inhibitory effects of opioid agonists and alpha2-adrenoceptor agonists were abolished by naloxone, an opioid antagonist, and atipamezole, an alpha2-adrenoceptor antagonist, respectively. There was no cross antagonism. Morphine, dexmedetomidine and xylazine dose-dependently inhibited body movement induced by formalin or capsaicin. The order of potency was dexmedetomidine>morphine>xylazine. Although morphine and dexmedetomidine inhibited formalin- and capsaicin-induced body movement in the same dose range, xylazine inhibited formalin-induced body movement at lower concentrations than capsaicin-induced one. The inhibitory potency for slow VRP by these drugs seems to be correlated with that for capsaicin-induced body movement but not that for formalin-induced one. Dexmedetomidine and morphine in combination inhibited slow VRP and body movement induced by capsaicin in an additive manner. It is suggested that the antinociceptive effects of dexmedetomidine and morphine but not xylazine on the capsaicin test are mainly due to spinal effects and that there is no synergistic interaction between dexmedetomidine and morphine in the neonatal rat.

The functional expression of mu opioid receptors on sensory neurons is developmentally regulated; morphine analgesia is less selective in the neonate

Opioid requirements in neonatal patients are reported to be lower than older infants and this may be a reflection of the developmental regulation of opioid receptors. In this study we have investigated the postnatal regulation of Mu opioid receptor (MOR) function in both rat lumbar dorsal root ganglion (DRG) cultures and behavioural mechanical and thermal reflex tests in rat pups. Immunostaining with MOR and selective neurofilament (NF200) antibodies was combined with calcium imaging of MOR function in cultured neonatal and adult rat dorsal root ganglion cells. Calcium imaging showed that a significantly greater number of neonatal DRG neurons expressed functional MOR compared to adult (56.5+/-3.4 versus 39.9+/-1.5%, n=8, mean+/-SEM, P<0.001). This expression is confined to the large, neurofilament positive sensory neurons, while expression in small, nociceptive, neurofilament negative neurons remains unchanged. Sensory threshold testing in rat pups showed that the analgesic potency of systemic morphine to mechanical stimulation is significantly greater in the neonate and declines with postnatal age. Morphine analgesic potency in thermal nociceptive tests did not change with postnatal age. These experiments show that the MOR expressed on large DRG neurons in neonates are functional and are subject to postnatal developmental regulation. This changing functional receptor profile is consistent with greater morphine potency in mechanical, but not thermal, sensory tests in young animals. These results have important clinical implications for the use of morphine in neonates and provide a possible explanation for the differences in morphine requirements observed in the youngest patients.

Spinal vs. supraspinal antinociceptive activity of the adenosine A1 receptor agonist cyclopentyl-adenosine in rats with inflammation

The adenosine A(1) receptor is involved in spinal cord antinociception. As its role at supraspinal sites is not well known, we studied the systemic effects of its agonist N-cyclopentyl-adenosine (CPA) in single motor units from adult-spinalized, intact and sham-spinalized rats. CPA was not effective after spinalization, but it was very effective in intact animals (ID50: 92+/-1.3 microg/kg, noxious pinch) and over 10-fold more potent in sham-spinalized animals (ID50 of 8.3+/-1 microg/kg). Wind-up was also inhibited by CPA. We also studied the effect of CPA in the immature spinal cord preparation, where CPA dose-dependently inhibited responses to low (IC50s: 9+/-0.7 and 7.7+/-1.3 nM) and high intensity stimulation (IC50s: 4.9+/-0.5 and 12.1+/-2 nM). We conclude that the integrity of the spinal cord is crucial for the antinociceptive activity of systemic CPA in adult rats but not in immature rats, not yet influenced by a completely developed supraspinal control.

Mechanical allodynia and thermal hyperalgesia upon acute opioid withdrawal in the neonatal rat

Upon withdrawal from opioids many patients experience a heightened sensitivity to stimuli and an exaggerated pain response. We present evidence that neonatal rats exhibit allodynia and hyperalgesia on acute opiate withdrawal. Postnatal 7 and 21 day rats were used to approximately model a full term human infant and a human child, respectively. The opiate antagonist naloxone was used to precipitate withdrawal at 30 or 120 min after a single acute administration of morphine. Alternatively, rats were allowed to undergo spontaneous withdrawal. Behavioral manifestations of withdrawal syndrome were not observed when naloxone was administered at 30 min post-morphine, but were present when withdrawal was precipitated at 120 min. Spontaneous and precipitated withdrawal from a single acute administration of morphine produced mechanical allodynia and thermal hyperalgesia in postnatal day 7 rats and mechanical allodynia in postnatal day 21 rats. A higher dose of morphine was required to produce mechanical allodynia in postnatal day 21 versus 7 rats but this increase was independent of the analgesic efficacy of morphine at these two ages. The present work illustrates the need to examine the phenomenon of hypersensitivity upon opioid withdrawal in the human pediatric population.

Inhibition of N-type voltage-activated calcium channels in rat dorsal root ganglion neurons by P2Y receptors is a possible mechanism of ADP-induced analgesia

Patch-clamp recordings from small-diameter rat dorsal root ganglion (DRG) neurons maintained in culture demonstrated preferential inhibition by ATP of high-voltage-activated, but not low-voltage-activated, Ca2+ currents (I(Ca)). The rank order of agonist potency was UTP > ADP > ATP. ATP depressed the omega-conotoxin GVIA-sensitive N-type current only. Pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate tetraammonium, two P2Y1 receptor antagonists, almost abolished the ATP-induced inhibition. Both patch-clamp recordings and immunocytochemistry coupled with confocal laser microscopy indicated a colocalization of functional P2X3 and P2Y1 receptors on the same DRG neurons. Because the effect of ATP was inhibited by intracellular guanosine 5'-O-(2-thiodiphosphate) or by applying a strongly depolarizing prepulse, P2Y1 receptors appear to block I(Ca) by a pathway involving the betagamma subunit of a G(q/11) protein. Less efficient buffering of the intracellular Ca2+ concentration ([Ca2+]i) by reducing the intrapipette EGTA failed to interfere with the ATP effect. Fura-2 microfluorimetry suggested that ATP raised [Ca2+]i by a Galpha-mediated release from intracellular pools and simultaneously depressed the high external potassium concentration-induced increase of [Ca2+]i by inhibiting I(Ca) via Gbetagamma. Adenosine 5'-O-(2-thiodiphosphate) inhibited dorsal root-evoked polysynaptic population EPSPs in the hemisected rat spinal cord and prolonged the nociceptive threshold on intrathecal application in the tail-flick assay. These effects were not antagonized by PPADS. Hence, P2Y receptor activation by ADP, which is generated by enzymatic degradation of ATP, may decrease the release of glutamate from DRG terminals in the spinal cord and thereby partly counterbalance the algogenic effect of ATP.

Modulation of spinal nociception by GluR5 kainate receptor ligands in acute and hyperalgesic states and the role of gabaergic mechanisms

GluR5 receptors modulate spinal nociception, however, their role in nociceptive hypersensitivity remains unclear. Using behavioural and electrophysiological approaches, we have investigated several GluR5 ligands in acute and hyperalgesic states. Furthermore, as the GABAergic system plays a role in GluR5 mediated effects in the brain, we also analysed the interaction between GluR5 agonists and GABA(A) antagonists in the spinal cord. In young rats in vivo, the GluR5 selective agonist ATPA was antinociceptive and antihyperalgesic in a model of inflammatory hyperalgesia (ED(50) approximately 4.6 and approximately 5.2 mg/kg, respectively), whereas the GluR5/GluR6 agonist SYM2081 was only antihyperalgesic. ATPA, but not SYM2081, was also able to inhibit nociceptive motoneurone responses in anaesthetised adult rats after intrathecal administration. In hemisected spinal cords in vitro, SYM2081 was inactive, whereas ATPA and another GluR5 agonist, (S)-5-iodowillardiine, inhibited nociceptive reflexes (EC(50) 1.1+/-0.4 micro M and 0.36+/-0.05 micro M, respectively). Both GluR5 agonists also inhibited motoneurone responses to repetitive dorsal root stimulation and their cumulative depolarisation, a correlate of wind-up. The GABA(A) antagonists bicuculline (10 micro M) and SR95531 (1 micro M) enhanced polysynaptic responses to single stimuli but abolished the cumulative depolarisation. Both bicuculline and SR95531 significantly attenuated the inhibition of nociceptive responses by 1 micro M ATPA (by approximately 50%). We conclude that selective GluR5 kainate receptor activation inhibits spinal nociception and its sensitisation caused by ongoing peripheral nociceptive drive. GABA(A) receptors are involved in tonic inhibition of segmental responses, but contribute to their sensitisation by repetitive primary afferent stimulation. Furthermore, there is a cross-talk between the two systems, presumably due to GluR5-mediated activation of GABAergic inhibitory interneurones in the spinal cord.

High concentrations of adrenergic antagonists prolong sciatic nerve blockade by tetrodotoxin

BACKGROUND

Millimolar-range concentrations of some adrenergic antagonists have been shown to have local anesthetic-like properties, and to stimulate GTPase activity in vitro. In this report, we investigate whether these agents can potentiate the effect of tetrodotoxin (TTX) and bupivacaine, a conventional local anesthetic, and whether GTPase activation plays a role.

METHODS

Rats received sciatic nerve blockade with tetrodotoxin or bupivacaine co-injected with adrenergic antagonists and/or agonists, or pertussis toxin. Thermal nociceptive blockade was quantified with modified hotplate testing.

RESULTS

Nerve block from TTX alone lasted 153 (99-223) min (median and 25th and 75th percentiles). Co-injection with 20 mM phentolamine, propranolol, and yohimbine prolonged TTX block to 856 (765-862), 486 (444-510), and 465 (413-495) min respectively (P<0.005 in all cases, compared to TTX alone). Micromolar concentrations of adrenergic antagonists (which inhibited the prolongation of TTX block by epinephrine) did not prolong TTX block. Injection of adrenergic antagonists alone did not produce specific nerve block. They did not prolong TTX block when injected at a remote subcutaneous site. Prolongation of TTX block by phentolamine was not inhibited by co-injection with pertussis toxin. Adrenergic antagonists did not prolong bupivacaine block.

CONCLUSIONS

High concentrations of adrenergic antagonists markedly prolonged TTX block, but not bupivacaine block. This locally mediated action does not appear to be adrenergic-receptor-specific, or mediated by GTPase activation.

The neurobiology of pain: developmental aspects

Invasive procedures that would be painful in children and adults are frequently performed on infants admitted to the neonatal intensive care unit. This article discusses sensory responses to these procedures in the immature nervous system and highlights the fact that, in addition to causing distress and delayed recovery, pain in infancy is also a developmental issue. First, the immaturity of sensory processing within the newborn spinal cord leads to lower thresholds for excitation and sensitization, therefore potentially maximizing the central effects of these tissue-damaging inputs. Second, the plasticity of both peripheral and central sensory connections in the neonatal period means that early damage in infancy can lead to prolonged structural and functional alterations in pain pathways that can last into adult life.

Mu- and delta-opioid receptors are downregulated in the largest diameter primary sensory neurons during postnatal development in rats

The aim of this study was to investigate the postnatal development of mu-(MOR) and delta-opioid receptor (DOR) immunoreactivity in rat dorsal root ganglia. Lumbar dorsal root ganglia (DRG) from postnatal day (P) 0, 3, 7 and 21 rat pups were immunostained for MOR and DOR. Proportions of MOR +ve and DOR +ve cells were calculated from profile counts. Diameters of MOR +ve and DOR +ve cells were measured and compared to -ve cells. The coexpression of MOR and neurofilament (NF200) in DRG over this postnatal period was also investigated. A greater proportion of cells were immunoreactive for MOR and DOR in neonatal rat DRG at P0, P3 and P7 compared to P21. At P3, 39.5+/-1.7% of cells were MOR +ve and 30.3+/-1.5% were DOR +ve, whereas at P21, the values were 30.1+/-1.7% and 21.8+/-1.6% (mean+/-SEM), respectively. During the first postnatal week both opioid receptors were expressed in cells across the whole diameter range but by 3 weeks of age, expression was restricted to small and medium diameter cells. Furthermore, a significantly higher proportion of NF200 +ve cells expressed MOR in new-born compared to P21 rats. The results show that MOR and DOR expression is downregulated in the largest diameter, NF200 +ve primary sensory neurons postnatally. Since these neurons are mainly non-nociceptive, this may explain previous reports of opioid agonists affecting reflex responses to both innocuous and noxious stimuli in rat pups. The results highlight an important difference between opioid function in the immature and adult nervous system.

Differential actions of spinal analgesics on mono-versus polysynaptic Adelta-fibre-evoked field potentials in superficial spinal dorsal horn in vitro

Processing of nociceptive information can be modulated at various levels in spinal cord that may range from changes of neurotransmitter release from primary afferent Adelta- or C-fibres to excitability changes of spinal interneurones or motoneurones. The site and mechanism of action of spinal analgesics has been assessed with a number of in vivo and in vitro methods with sometimes conflicting results. Here, we have used transverse spinal cord slices with attached dorsal roots to simultaneously record mono- and polysynaptic Adelta-fibre-evoked field potentials in superficial spinal dorsal horn. Two classical spinal analgesics, morphine and clonidine, and the metabotropic glutamate receptor agonist (IS,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) differentially affected mono- and polysynaptic Adelta-fibre-evoked transmission in spinal dorsal horn. Polysynaptic responses were dose-dependently inhibited while the monosynaptic response remained unaffected. These results suggest that spinal analgesics may preferentially affect polysynaptic but not monosynaptic Adelta-fibre-evoked responses in superficial spinal dorsal horn.

Synaptic control of motoneuronal excitability

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

Actions of kainate and AMPA selective glutamate receptor ligands on nociceptive processing in the spinal cord

Kainate receptors expressing the GluR5 subunit of glutamate receptor are present at high levels on small diameter primary afferent neurones that are considered to mediate nociceptive inputs. This suggests that GluR5 selective ligands could be novel analgesic agents. The role of kainate receptors on C fibre primary afferents has therefore been probed using three compounds that are selective for homomeric GluR5 receptors. The agonist, ATPA, and the antagonists, LY294486 and LY382884, have been tested in four models of nociception: responses evoked by noxious stimulation of the periphery have been recorded electrophysiologically (1) from hemisected spinal cords from neonatal rats in vitro, (2) from single motor units in adult rats in vivo, (3) from dorsal horn neurones in adult rats in vivo, and (4) in hotplate tests with conscious mice. In some protocols comparisons were made with the AMPA selective antagonist GYKI 53655. The agonist ATPA reduced nociceptive reflexes in vitro, but failed to have effects in vivo. In all tests, the GluR5 antagonists reduced nociceptive responses but only at doses that also affected responses to exogenous AMPA. The AMPA antagonist reduced nociceptive responses at doses causing relatively greater reductions of responses to exogenous AMPA. The results indicate that GluR5 selective ligands do reduce spinal nociceptive responses, but they are not strongly analgesic under these conditions of acute nociception.

Postnatal development of multiple opioid receptors in the spinal cord and development of spinal morphine analgesia

The postnatal ontogeny of mu, delta and kappa opioid receptor binding sites in the spinal cord of rat pups at various postnatal days was determined using in vitro autoradiographical methods. The functional effect of spinal morphine was also assessed using in vivo electrophysiological methods in rats at P14, P21 and adults (P56). Both mu and kappa opioid receptor binding-sites are present from P0 and spread relatively diffusely throughout the spinal cord. Overall binding peaks at P7 and subsequently decreases to adult levels with the mu opioid receptor binding sites regressing to become denser in the superficial dorsal horn. delta Opioid receptor binding was first seen at P7, and no distinction between superficial and deeper laminae was seen. In the adult, the relative proportions of the opiate receptors in the superficial dorsal horn are 63%, 22% and 15%, for mu, delta and kappa receptor binding sites, respectively. C-fibre evoked dorsal horn neuronal responses recorded from anaesthetized rat pups were highly sensitive to spinal morphine at P21, (EC50 0.005 microgram), compared to the adult (EC50 0.9 microgram). However, the EC50 (0.2 microgram) at P14 was greater than at P21 despite the fact that mu receptor binding was greater at P14. Opioid receptor binding is developmentally regulated and undergoes substantial postnatal reorganization. However, the number of mu receptor binding sites appears not to be the only determinant of functional sensitivity to spinal morphine. Other factors, such as coupling of the receptors are likely to be important.

Depression of NMDA receptor-mediated synaptic transmission by four alpha2 adrenoceptor agonists on the in vitro rat spinal cord preparation

1. Alpha2-adrenoceptor agonists have a spinal site of analgesic action. In the current study the synaptic depressant actions of xylazine, detomidine, romifidine and dexmedetomidine have been compared on segmental reflexes containing NMDA receptor-mediated components in the neonatal rat hemisected spinal cord preparation in vitro. 2. Reflexes were evoked in the ventral root following either supramaximal electrical stimulation of the corresponding ipsilateral lumbar dorsal root to evoke the high intensity excitatory postsynaptic potential (e.p.s.p.) involving all primary afferent fibres, or low intensity stimulation to evoke the solely A fibre-mediated low intensity e.p.s.p. The high intensity e.p.s.p. contains a greater NMDA receptor-mediated component. 3. Xylazine, romifidine, detomidine and dexmedetomidine all depressed both the high intensity e.p.s.p. and the low intensity e.p.s.p. giving respective EC50 values of 0.91+/-0.2 microM (n=12), 23.4+/-3 nM (n=12), 37.7+/-7 nM (n=8) and 0.84+/-0.1 nM (n=4) for depression of the high intensity e.p.s.p. and 0.76+/-0.1 microM (n=12), 22.0+/-3 nM (n=12), 24.9+/-6 nM (n=4) and 2.7+/-0.6 nM (n=4) for depression of the low intensity e.p.s.p., respectively. Unlike the other three drugs, the two values for dexmedetomidine, showing a greater selectivity for the high intensity e.p.s.p., are significantly different. 4. Each of these depressant actions was reversed by the selective alpha2-adrenoceptor antagonist atipamezole (1 microM). 5. In contrast to previous reports of the actions of alpha2-adrenoceptor agonists on the in vitro spinal cord preparation, at concentrations ten fold higher than the above EC50 values xylazine, romifidine, detomidine and dexmedetomidine depressed the initial population spike of motoneurons (MSR). This depression was not reversed by atipamezole. 6. Comparison of the rank order of the present EC50 values for depression of the high intensity e.p.s.p. with potency ratios from in vivo analgesic tests in previous studies show a close correlation between the present in vitro tests and analgesic potency. There is no correlation between the present data and previously obtained affinities of the agonists at non-adrenergic imidazoline binding sites. 7. The current findings therefore suggest that xylazine, romifidine, detomidine and dexmedetomidine are exerting their central analgesic actions at the spinal level principally through alpha2-adrenoceptors. All four agonists showed the same profile of selective depression of the NMDA receptor-mediated component of reflexes similar to that reported previously for clonidine. However dexmedetomidine, unlike the other ligands, selectively depressed the high intensity e.p.s.p.

Selective depression of dorsal root-evoked high threshold synaptic excitation by the selective kappa opioid receptor agonist enadoline in the neonatal rat hemisected spinal cord in vitro

The present study aimed to compare the actions of the selective kappa opioid receptor agonist enadoline (CI-977) with morphine in order to see if there is a heterogeneity of opioid receptors between spinal reflex pathways. High (C- and A-fibre evoked activity) and low (A-fibres only) intensity electrical stimulation of dorsal roots in the neonatal rat hemisected spinal cord preparation in vitro was used to distinguish between synaptic activity measured in the corresponding ventral root. Enadoline selectively depressed the high intensity-evoked EPSP with an EC50 of 7.6 nM (n = 7), contrasting with our previous finding in this preparation that morphine is an equipotent depressant of A- and C-fibre-mediated synaptic responses. The depressant effects of enadoline and morphine were reversed by naloxone giving apparent Kd values of 14 +/- 3 nM (n = 4) for enadoline-induced and 4.2 +/- 1 nM (n = 4) for morphine-induced depression. These data suggest that activation of kappa opioid receptors has a selective depressant action on C-fibre-mediated synaptic activity. Such a functional difference mediated at a subclass of opioid receptors has not been previously observed in an in vitro spinal preparation.