Spinal mediators of nociceptive neurotransmission and hyperalgesia

Endomorphin-1 and endomorphin-2 modulate responses of trigeminal neurons evoked by N-methyl-D-aspartic acid and somatosensory stimuli

The present study investigated the modulation of N-methyl-D-aspartate (NMDA)-evoked and peripheral cutaneous stimulus-evoked responses of trigeminal neurons by endomorphins, endogenous ligands for the mu-opioid receptor. Effects of endomorphins, administered microiontophoretically, were tested on the responses of nociceptive neurons recorded in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in anesthetized rats. Endomorphin-1 and endomorphin-2 predominantly reduced the NMDA-evoked responses, producing an inhibitory effect of 54.1 +/- 2.96% (mean +/- SE; n = 34, P < 0.001) in 92% (34/37) of neurons and 63.6 +/- 3.61% (n = 32, P < 0.001) in 91% (32/35) of neurons, respectively. The inhibitory effect of endomorphins was modality specific; noxious stimulus-evoked responses were reduced more than nonnoxious stimulus-evoked responses. Naloxone applied at iontophoretic current that blocked the inhibitory effect of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin, reduced the peak inhibitory effect of endomorphins on the NMDA- and natural stimulus-evoked responses. We suggest that endomorphins by acting at micro-opioid receptor selectively modulate noxious stimulus-evoked responses in the medullary dorsal horn.

Alleviation of mechanical and thermal allodynia by CGRP(8-37) in a rodent model of chronic central pain

CGRP(8-37) is a truncated version of calcitonin gene-related peptide (CGRP) that binds to the CGRP receptor with similar affinity but does not activate the receptor and is a highly selective CGRP receptor antagonist. CGRP and activation of its receptor appear to play a role in peripheral inflammatory and neuropathic models of pain although there is considerable controversy. The aim of this study was to examine possible anti-nociceptive effects of CGRP(8-37) on a model of chronic central neuropathic pain known to develop weeks after spinal hemisection. Adult male Sprague-Dawley rats were given a spinal hemisection (N=34) or a sham surgery (N=10) at the T13 spinal segment. An externally accessible PE-10 intrathecal catheter that terminated at T13 was used for drug delivery. Animals were allowed to recover for 4 weeks at which time the hemisected animals displayed mechanical and thermal allodynia bilaterally, in both forelimbs and hindlimbs. CGRP(8-37) was delivered just prior to a testing session in 1, 5, 10, or 50 nM doses in artificial cerebral spinal fluid in 10 microl volumes. CGRP(8-37) was effective in alleviating mechanical and thermal allodynia in a dose-dependent manner (P<0.05). The 50 nM dose was most efficacious for both forelimb and hindlimb responses (P<0.05). The period of efficacy was 10 min to onset for a duration of 20 min. Post-drug washout responses were not statistically significant compared to pre-drug responses. The sham control groups demonstrated no statistically significant difference at any dose of CGRP(8-37) when compared to pre-surgical baseline values. In conclusion, CGRP(8-37) is effective in abolishing mechanical and thermal allodynia produced by spinal hemisection. Consequently, the CGRP receptor may play a role in chronic central neuropathic pain and offers a novel therapeutic approach to managing chronic central pain.

Intrathecal administration of an NMDA or a non-NMDA receptor antagonist reduces mechanical but not thermal allodynia in a rodent model of chronic central pain after spinal cord injury

Spinal cord injuries (SCI) result in a devastating loss of function and chronic central pain syndromes frequently develop in the majority of these patients. The present study uses a rodent spinal hemisection model of SCI in which mechanical and thermal allodynia develops by 24 days after injury. Post-operative paw withdrawal responses to low threshold and high threshold mechanical stimuli compared to pre-operative responses (4.78, 9.96, and 49.9 mN) were increased and were statistically significant (p<0.05) for both forelimbs and hindlimbs indicating the development of mechanical allodynia. By contrast, post-operatively, the temperature at which paw withdrawal accompanied by paw lick occurred was significantly decreased (p<0.05), indicating the development of thermal allodynia. The intrathecal application of either D-AP5, a competitive NMDA receptor antagonist, or NBQX-disodium salt, a competitive non-NMDA AMPA/kainate receptor antagonist, alleviated the mechanical allodynia and lowered the threshold of response for the high threshold mechanical stimuli in a dose-dependent manner, and these decreases were statistically significant (p<0.05). By contrast, neither the D-AP5 nor the NBQX produced a statistically significant change in the thermal allodynia behavior in either forelimbs or hindlimbs in the hemisected group. No significant changes in locomotion scores, and thus no sedation, were demonstrated by the hemisected group for the doses tested. These data support the potential efficacy of competitive excitatory amino acid receptor antagonists in the treatment of chronic central pain, particularly where input from low threshold mechanical afferents trigger the onset of the painful sensation. Furthermore, these data suggest a role for both NMDA and non-NMDA receptors in the development of plastic changes in the spinal cord that provide the underlying mechanisms for central neuropathic pain.

alpha2-adrenoceptors modulate NMDA-evoked responses of neurons in superficial and deeper dorsal horn of the medulla

Extracellular single unit recordings were made from neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in 21 male rats anesthetized with urethan. NMDA produced an antagonist-reversible excitation of 46 nociceptive as well as nonnociceptive neurons. Microiontophoretic application of a preferential alpha2-adrenoceptor (alpha2AR) agonist, (2-[2, 6-dichloroaniline]-2-imidazoline) hydrochloride (clonidine), reduced the NMDA-evoked responses of 86% (6/7) of nociceptive-specific (NS) neurons, 82% (9/11) of wide dynamic range (WDR) neurons, and 67% (4/6) of low-threshold (LT) neurons in the superficial dorsal horn. In the deeper dorsal horn, clonidine inhibited the NMDA-evoked responses of 94% (16/17) of NS and WDR neurons and 60% (3/5) of LT neurons. Clonidine facilitated the NMDA-evoked responses in 14% (1/17) of NS, 9% (1/11) of WDR, and 33% (2/6) of LT neurons in the superficial dorsal horn. Idazoxan, an alpha2AR antagonist, reversed the inhibitory effect of clonidine in 90% (9/10) of neurons, whereas prazosin, an alpha1-adrenoceptor antagonist with affinity for alpha2BAR, and alpha2CAR, were ineffective. We suggest that activation of alpha2ARs produces a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive neurons in the medullary dorsal horn.

Chronic central pain after spinal cord injury

Spinal cord injury (SCI) frequently results in dysesthesias that have remained refractory to clinical treatments despite a variety of interventions. The failure of therapeutic strategies to treat dysesthesias after SCI is due to the lack of attention given to mechanisms that elicit chronic pain following SCI. An overview of the literature with respect to the development of chronic pain in the SCI patient population will be given. In addition, a mammalian model of chronic central pain following spinal cord trauma will be presented. The model is characterized by the development of mechanical and thermal allodynia, as demonstrated by measuring the thresholds of accepted nociceptive tests, the paw withdrawal responses accompanied by changes in behavior consistent with the experience of noxious stimuli. In addition, vocalization responses that are accompanied by postural and behavioral changes consistent with the receipt of a noxious stimulus and involving supraspinal pathways are measured. Locomotor function was also tested and scored using the Basso, Beattie, and Bresnahan (BBB) open field test scale. Our data indicate that somatosensory thresholds for both mechanical and thermal stimuli that elicit paw withdrawal (flexor reflex) or vocalizations, accompanied by complex changes in behavior, are significantly different following SCI. These changes represent the development of mechanical and thermal allodynia. To determine the underlying mechanism for the altered sensory responses, we used electrophysiological techniques to determine if nociceptive dorsal horn neurons demonstrated increased excitability to peripheral stimulation as evidenced by increased responses to natural somatosensory stimuli. The data presented support the development of central sensitization of dorsal horn neurons after spinal cord hemisection. This provides a mechanism for the development of mechanical and thermal allodynia after SCI. Hypotheses that account for the development of the central pain state after SCI, as well as therapeutic interventions to ameliorate the pain state, are discussed.

Neuronal damage following intraspinal injection of a nitric oxide synthase inhibitor in the rat

Intraspinal microinjection of the nonspecific nitric oxide synthase (NOS) inhibitor N-nitro-L-arginine methyl ester (L-NAME) was used to determine if inhibition of NOS results in morphological changes in the rat spinal cord. Following spinal injections of 100-750 mM L-NAME (pH 7.0), 1.0-500 mM L-NAME (pH 2.5-5.4), or L-NAME + L-arginine, quantitative analysis of morphological changes revealed a positive dose-response relationship between L-NAME and neuronal loss. This effect was blocked by L-arginine and was inversely related to spinal levels of NOS enzyme activity. Results of this study have shown the importance of basal NOS activity in maintaining the structural integrity of spinal neurons. It is proposed that the effects of L-NAME on nitric oxide (NO) production leads to decreased blood flow, secondary to vasoconstriction, and a hypoxic-ischemic reaction in spinal tissue. The results suggest that a potential contributing factor to neuronal damage in pathological conditions such as spinal cord injury may be the decreased production of nitric oxide.

A nitric oxide synthesis inhibitor (L-NAME) reduces licking behavior and Fos-labeling in the spinal cord of rats during formalin-induced inflammation

Formalin injected subcutaneously into the hindpaw of the rat produces an animal model of inflammation that exhibits a phasic component and a tonic component of pain. We evaluated the effects of a nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME), on a formalin-induced behavior, hindpaw licking, and on Fos-labeling of nuclei in the fifth lumbar spinal segment. Our results demonstrated that pretreatment with intrathecal doses of 0.3 and 1.0 mg of L-NAME significantly reduced licking behavior associated with injection of formalin into the left hindpaw of the rat. In addition, these same doses of L-NAME reduced formalin-induced Fos-labeling in the ipsilateral dorsal gray matter (as compared to the contralateral gray matter). Qualitative assessment suggested that the reduction in labeling occurred primarily in the superficial dorsal horn. The stereoisomer, D-NAME, administered at the same doses had little to no effect on either formalin-induced licking or Fos-labeling. Finally, our results revealed that total licking time was related to Fos-labeling. Rats that spent less time licking the hindpaw exhibited a smaller increase in Fos-labeling. Our results suggest that the production of nitric oxide is associated with licking behavior resulting from formalin injection into the hindpaw of rats. Our results also suggest that the production of nitric oxide and Fos are associated. Indeed, these substances may be involved in spinal pathways associated with nociception.

Opioids modulate N-methyl-D-aspartic acid (NMDA)-evoked responses of neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis)

Extracellular single unit recordings were made from 74 neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis). N-methyl-D-aspartic acid (NMDA) excited nociceptive as well as non-nociceptive neurons. NMDA receptor antagonist, DL-2-Amino-5-Phosphonovaleric acid (AP-5), blocked the NMDA-evoked excitation. Microiontophoretic application of a selective mu-opioid receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO), reduced the NMDA-evoked responses of 100% of nociceptive specific (NS), 93% of wide dynamic range (WDR) and 86% of low threshold (LT) neurons in the superficial and deeper dorsal horn of the medulla. In contrast, application of a selective delta 1-opioid receptor agonist, [D-Pen2,5]enkephalin (DPDPE), reduced the NMDA-evoked responses of 90% of NS neurons, 72% of WDR neurons and 67% of LT neurons in the superficial and deeper dorsal horn of the medulla. DPDPE also produced excitatory or biphasic effects. The inhibitory actions of DAMGO and DPDPE were reversed by naloxone and/or 7-benzylidenenaltrexone (BNTX), mu- and delta 1-receptor antagonists. It is concluded that mu- and delta-opioid receptor agonists produce a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive and non-nociceptive neurons in the medullary dorsal horn.

Role of substance P in the modulation of C-fiber-evoked responses of spinal dorsal horn neurons

Substance P (SP) as well as excitatory amino acids (EAAs) appear to be released in response to stimulation of primary afferent C-fibers. Activity at N-methyl-D-aspartate (NMDA) receptors is essential for wind-up (the progressive potentiation of C-fiber-evoked responses of single neurons in response to an electrical stimulation), however, the role of SP in wind-up is unclear. To address this, the effects of iontophoretically applied CP-99,994 (a NK-1 receptor antagonist), SP and SP(1-7) (an N-terminal breakdown product of SP), were compared on responses of spinal dorsal horn wide dynamic range (WDR) neurons of the rat. Post-stimulus time histograms (PSTH) were summed over 12 responses to low frequency (0.5 Hz) electrical stimulation of the cutaneous receptive field. Changes in responses of dorsal horn neurons were evaluated by monitoring C-fiber input, wind-up, and the total number of spikes evoked by C-fiber activity in response to the 12 stimuli. The NK-1 receptor antagonist CP-99,994 significantly inhibited the total number of C-spikes and caused a significant reduction in wind-up without changing the C-fiber input, indicating the involvement of NK-1 receptors in wind-up. Application of SP led to an overall increase in the total number of C-fiber evoked responses of dorsal horn neurons and C-fiber input, however, wind-up, as defined, was significantly decreased following SP. In contrast, substance P(1-7) evoked a long-lasting increase in the total number of C-fiber-related spikes which was initially sustained by a transient increase in the input followed by a longer lasting increase in wind-up, an effect opposite that of CP-99,994. As NMDA activity has been previously shown to be inhibited and then potentiated by SP N-terminal activity over a similar time interval, the present data are consistent with the mediation of wind-up by NMDA and its modulation by SP N-terminal activity. Release of SP in response to noxious stimulation may, therefore, increase primary afferent C-fiber activity (input) whereas an accumulation of SP N-terminal metabolites appears to potentiate wind-up, perhaps via positive modulation of EAA activity.

Mechanisms of hyperalgesia and morphine tolerance: a current view of their possible interactions

Over the last several years, compelling evidence has accumulated indicating that central hyperactive states resulting from neuronal plastic changes within the spinal cord play a critical role in hyperalgesia associated with nerve injury and inflammation. Such neuronal plastic changes may involve activation of central nervous system excitatory amino acid (EAA) receptors, subsequent intracellular cascades including protein kinase C translocation and activation as well as nitric oxide production, leading to the functional modulation of receptor-ion channel complexes. Similar EAA receptor-mediated cellular and intracellular mechanisms have now been implicated in the development of tolerance to the analgesic effects of morphine, and a site of action involved in both hyperalgesia and morphine tolerance is likely to be in the superficial laminae of the spinal cord dorsal horn. These observations suggest that hyperalgesia and morphine tolerance, two seemingly unrelated phenomena, may be interrelated by common neural substrates that interact at the level of EAA receptor activation and related intracellular events. This view is supported by recent observations showing that thermal hyperalgesia develops when animals are made tolerant to morphine antinociception and that both hyperalgesia and reduction of the antinociceptive effects of morphine occur as a consequence of peripheral nerve injury. The demonstration of interrelationships between neural mechanisms underlying hyperalgesia and morphine tolerance may lead to a better understanding of the neurobiology of these two phenomena in particular and pain in general. This knowledge may also provide a scientific basis for improved pain management with opiate analgesics.

Infusion of substance P or neurokinin A by microdialysis alters responses of primate spinothalamic tract neurons to cutaneous stimuli and to iontophoretically released excitatory amino acids

The responses of 25 spinothalamic tract (STT) neurons to mechanical and thermal stimulation of the skin, as well as to a battery of iontophoretically applied excitatory amino acids (EAAs), were tested before and then during microdialysis of substance P (SP) or neurokinin A (NKA) into the dorsal horn of anesthetized monkeys. Neither peptide had significant effects on the background activity or the responses to mechanical or thermal stimulation of the skin. However, each peptide produced significant increases in the responses to iontophoretic application of one or more EAAs. In addition, following combined application of the EAAs and either SP or NKA, the responses of the cells to mechanical stimulation of the skin increased. Combined application of SP and NKA failed to produce an increase in responses to either the EAAs or to cutaneous stimuli that was greater than that produced by either peptide alone. It is concluded that SP and NKA produce an increase in the responses of STT cells to iontophoretic applications of EAAs and the combined effects of these compounds produce sustained increases in responses to mechanical stimulation of the skin. These changes mimic those observed when STT cells are sensitized by peripheral noxious stimuli, suggesting that the mechanism of induction and expression of sensitization involves the facilitation of dorsal horn neuron responses to EAAs by tachykinins.

Effects of nitric oxide availability on responses of spinal wide dynamic range neurons to excitatory amino acids

The role of nitric oxide (NO) in responses of spinal dorsal horn neurons to excitatory amino acids and to cutaneous mechanical stimuli was examined. Extracellular recordings were made from wide dynamic range neurons excited with iontophoretically applied excitatory amino acid agonists, N-methyl-D-aspartate (NMDA) and (R,S)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) or kainic acid. Nitric oxide availability was decreased by iontrophoretic application of NO synthase inhibitors, N omega-nitro-L-arginine methyl ester (L-NAME) or L-N5-(1-iminoethyl)ornithine (L-NIO), or elevated by the NO donating compound, S-nitroso-N-penicillamine (SNAP). When cells were excited with successive application of NMDA and non-NMDA excitatory amino acid receptor agonists, application of NO synthase inhibitors led to a decrease in responses to NMDA in 60% of neurons. In more than a third of the cells tested, inhibition of NO synthase caused reciprocal changes in responses to glutamate receptor agonists: NMDA-evoked responses were significantly decreased whereas responses to the non-NMDA receptor agonists (AMPA or kainic acid) were increased. Application of the NO donating compound, S-nitroso-N-penicillamine, revealed an opposite tendency, increasing responses to NMDA in more than half of the neurons tested. In approximately 40% of the cells, reciprocal changes in responses to excitatory amino acid receptor agonists of NMDA versus non-NMDA types were observed after application of S-nitroso-N-penicillamine, such that the increase in NMDA responses was accompanied by decreases in the responses to kainic acid.(ABSTRACT TRUNCATED AT 250 WORDS)