Andrea Ebersberger

Mechanisms of Pain in Arthritis

Abstract: Inflammation in the joint causes peripheral sensitization (increase of sensitivity of nociceptive primary afferent neurons) and central sensitization (hyperexcitability of nociceptive neurons in the central nervous system). The processes of sensitization are thought to be the basis of arthritic pain that appears as spontaneous pain (joints at rest) and hyperalgesia (augmented pain response on noxious stimulation and pain on normally nonpainful stimulation). Sensitization also facilitates efferent neuronal processes through which the nervous system influences the inflammatory process. Peripheral sensitization is produced by the action of inflammatory mediators such as bradykinin, prostaglandins, neuropeptides, and cytokines which activate corresponding receptors in proportions of nerve fibers. In addition, the expression of receptors, for example, bradykinin and neurokinin 1 receptors, is upregulated during inflammation. The development of hyperexcitability of spinal cord neurons is produced by various transmitter/receptor systems that constitute and modulate synaptic activation of the neurons. The key transmitter is glutamate that activates N‐methyl‐d‐aspartate (NMDA) and non‐NMDA receptors on spinal cord neurons. Blockade of these receptors prevents and reduces central sensitization. Excitatory neuropeptides (substance P and calcitonin gene‐related peptide) further central sensitization. Central sensitization also is facilitated by mediators that have complex actions (e.g., prostaglandin E2). Spinal PGE2 binds to receptors at presynaptic endings of primary afferent neurons (thus influencing synaptic release) and to receptors on postsynaptic spinal cord neurons. The administration of PGE2 to the spinal cord surface produces changes of responsiveness of spinal neurons similar to peripheral inflammation, and spinal indomethacin to the spinal cord attenuates development of hyperexcitability significantly.

Release of substance P, calcitonin gene-related peptide and prostaglandin E2 from rat dura mater encephali following electrical and chemical stimulation in vitro.

Neurogenic inflammation of the dura, expressed in plasma extravasation and vasodilatation, putatively contributes to different types of headache. A novel in vitro preparation of the fluid-filled skull cavities was developed to measure mediator release from dura mater encephali upon antidromic electrical stimulation of the trigeminal ganglion and after application of a mixture of inflammatory mediators (serotonin, histamine and bradykinin, 10(-5) M each, pH 6.1) to the arachnoid side of rat dura. The release of calcitonin gene-related peptide, substance P and prostaglandin E2 from dura mater was measured in 5-min samples of superfusates using enzyme immunoassays. Orthodromic chemical and antidromic electrical stimulation of dural afferents caused significant release of calcitonin gene-related peptide (2.8- and 4.5-fold of baseline). The neuropeptide was found to be increased during the 5-min stimulation period and returned to baseline (20.9 +/- 12 pg/ml) in the sampling period after stimulation. In contrast, release of substance P remained at baseline levels (19.3 +/- 11 pg/ml) throughout the experiment. Prostaglandin E2 release was elevated during chemical and significantly also after antidromic electrical stimulation (6- and 4.2-fold of baseline, which was 305 +/- 250 pg/ml). Prostaglandin E2 release outlasted the stimulation period for at least another 5 min. The data support the hypothesis of neurogenic inflammation being involved in headaches and provide new evidence for prostaglandin E2 possibly facilitating meningeal nociceptor excitation and, hence, pain.

Is there a correlation between spreading depression, neurogenic inflammation, and nociception that might cause migraine headache?

The time course of propagation of scotoma and blood flow changes during migraine aura parallels the phenomenon of cortical spreading depression (CSD). It was proposed that CSD generates a sterile neurogenic inflammation in the meninges, which may then lead to the activation or sensitization of nociceptors, thus generating headache. We performed rat experiments in which the effect of CSD on plasma extravasation in the dura mater and on neuronal activity in deep laminae of the trigeminal nucleus was assessed in vivo. CSD did not alter dural plasma extravasation measured by means of bovine serum albumin–coupled flourescein (n = 17 rats) compared to the CSD‐free contralateral side. In an in vitro model, the application of KCl to the dura at concentrations extracellularly found during CSD did not alter the release of calcitonin gene–related peptide and prostaglandin E2 from the dura. In 33 rats, neither single CSDs nor a series of CSDs altered ongoing neuronal activity or mechanical and/or thermal sensitivity of the deeply located neurons to stimulation of their receptive fields in the dura mater. These results are at variance with data that showed increased c‐Fos labeling in superficial laminae of the trigeminal nucleus following CSD. They do not suggest that CSD initiates migraine headache via neurogenic inflammation. Ann Neurol 2001;49:7–13

Update on peripheral mechanisms of pain: beyond prostaglandins and cytokines

The peripheral nociceptor is an important target of pain therapy because many pathological conditions such as inflammation excite and sensitize peripheral nociceptors. Numerous ion channels and receptors for inflammatory mediators were identified in nociceptors that are involved in neuronal excitation and sensitization, and new targets, beyond prostaglandins and cytokines, emerged for pain therapy. This review addresses mechanisms of nociception and focuses on molecules that are currently favored as new targets in drug development or that are already targeted by new compounds at the stage of clinical trials - namely the transient receptor potential V1 receptor, nerve growth factor, and voltage-gated sodium channels - or both.

Spinal antinociceptive effects of cyclooxygenase inhibition during inflammation: Involvement of prostaglandins and endocannabinoids

&NA; Both cyclooxygenase‐1 and ‐2 are expressed in the spinal cord, and the spinal COX product prostaglandin E2 (PGE2) contributes to the generation of central sensitization upon peripheral inflammation. Vice versa spinal COX inhibition is considered an important mechanism of antihyperalgesic pain treatment. Recently, however, COX‐2 was shown to be also involved in the metabolism of endocannabinoids. Because endocannabinoids can have analgesic actions it is conceivable that inhibition of spinal COX produces analgesia not only by inhibition of PG synthesis but also by inhibition of endocannabinoid breakdown. In the present study, we recorded from spinal cord neurons with input from the inflamed knee joint and we measured the spinal release of PGE2 and the endocannabinoid 2‐arachidonoyl glycerol (2‐AG) in vivo, using the same stimulation procedures. COX inhibitors were applied spinally. Selective COX‐1, selective COX‐2 and non‐selective COX inhibitors attenuated the generation of spinal hyperexcitability when applied before and during development of inflammation but, when inflammation and spinal hyperexcitability were established, only selective COX‐2 inhibitors reversed spinal hyperexcitability. During established inflammation all COX inhibitors reduced release of spinal PGE2 almost equally but only the COX‐2 inhibitor prevented breakdown of 2‐AG. The reversal of spinal hyperexcitability by COX‐2 inhibitors was prevented or partially reversed by AM‐251, an antagonist at the cannabinoid‐1 receptor. We conclude that inhibition of spinal COX‐2 not only reduces PG production but also endocannabinoid breakdown and provide evidence that reversal of inflammation‐evoked spinal hyperexcitability by COX‐2 inhibitors is more related to endocannabinoidergic mechanisms than to inhibition of spinal PG synthesis.

Changes in the Effect of Spinal Prostaglandin E2 during Inflammation: Prostaglandin E (EP1-EP4) Receptors in Spinal Nociceptive Processing of Input from the Normal or Inflamed Knee Joint

Inflammatory pain is caused by sensitization of peripheral and central nociceptive neurons. Prostaglandins substantially contribute to neuronal sensitization at both sites. Prostaglandin E2 (PGE2) applied to the spinal cord causes neuronal hyperexcitability similar to peripheral inflammation. Because PGE2 can act through EP1-EP4 receptors, we addressed the role of these receptors in the spinal cord on the development of spinal hyperexcitability. Recordings were made from nociceptive dorsal horn neurons with main input from the knee joint, and responses of the neurons to noxious and innocuous stimulation of the knee, ankle, and paw were studied after spinal application of recently developed specific EP1-EP4 receptor agonists. Under normal conditions, spinal application of agonists at EP1, EP2, and EP4 receptors induced spinal hyperexcitability similar to PGE2. Interestingly, the effect of spinal EP receptor activation changed during joint inflammation. When the knee joint had been inflamed 7-11 hr before the recordings, only activation of the EP1 receptor caused additional facilitation, whereas spinal application of EP2 and EP4 receptor agonists had no effect. Additionally, an EP3α receptor agonist reduced responses to mechanical stimulation. The latter also attenuated spinal hyperexcitability induced by spinal PGE2. In isolated DRG neurons, the EP3α agonist reduced the facilitatory effect of PGE2 on TTX-resistant sodium currents. Thus pronociceptive effects of spinal PGE2 can be limited, particularly under inflammatory conditions, through activation of an inhibitory splice variant of the EP3 receptor. The latter might be an interesting target for controlling spinal hyperexcitability in inflammatory pain states.

Blockade of voltage-gated calcium channels in rat inhibits repetitive cortical spreading depression.

Blockers of L-, N-, and P/Q-type voltage-gated calcium channels (VGCCs) were topically applied to the cortical surface of anaesthetized adult rats to study their role in cortical spreading depression (CSD), a correlate of the migraine aura. By pricking the brain, single CSD could still be elicited after blockade of the three different types of VGCCs as in the untreated brain. Topical KCl application to the untreated cortex resulted in repetitive CSD. However, after application of blockers at either L-, or N-, or P/Q-type VGCCs to the cortical surface, application of KCl elicited only one or very few CSD, and their repetition rate was dramatically reduced. The results suggest that cortical excitability resulting in repetitive CSD is markedly influenced by N- and P/Q-type VGCCs and less by L-type VGCCs.

Noradrenergic Agonists and Antagonists Influence Migration of Cortical Spreading Depression in Rat—a Possible Mechanism of Migraine Prophylaxis and Prevention of Postischemic Neuronal Damage

Cortical spreading depression (CSD) is thought to be a neuronal mechanism that expands the penumbra zone after focal brain ischemia and that causes migraine aura. Both adrenergic agonists and antagonists significantly influence the size of the penumbra zone and decline the frequency of migraine. To study whether these compounds act by influencing CSD, we applied different drugs topically to an area of the exposed cortex of anesthetized adult rats and observed the migration of CSD-related DC potential deflections across the treated area. The adrenergic agonist norepinephrine (1 mmol/L) and the α2-agonist clonidine (0.56 mmol/L) blocked reversibly the migration of CSD. The β-blocker propranolol (250 μmol/L to 1 mmol/L) dose-dependently diminished migration velocity or even blocked migration of CSD. The CSD blockade by the α2-antagonist yohimbine (1.75 mmol/L) was because of its action on inhibitory 5-HT1A receptors. None of the substances in the concentrations used had influence on regional cerebral blood flow or on systemic arterial blood pressure. The data suggest that the interference of these compounds with CSD may contribute to their beneficial therapeutic effect. The effect of β-receptor antagonists in human migraine needs further exploration, since these drugs also work in migraine without aura.

Effects of N-, P/Q- and L-type Calcium Channel Blockers on Nociceptive Neurones of the Trigeminal Nucleus with Input from the Dura

In anaesthetized rats, extracellular recordings were made from neurones of the spinal trigeminal nucleus, involved in the processing of nociceptive input from the dura. Blockers of voltage-gated calcium channels (VGCCs) were administered topically to the exposed brainstem. Blockade of N-type (CaV2.2) channels reduced spontaneous activity and responses of the neurones to cold and chemical stimuli applied to the dura, suggesting that N-type channels regulate excitatory synaptic activation. Blockade of L-type (CaV1) channels enhanced spontaneous discharges of the neurones. Blockade of P/Q-type (CaV2.1) channels slightly decreased responses to chemical and cold stimuli but markedly increased spontaneous activity, an effect which was absent during concomitant application of GABA to the brainstem. The data suggest that P/Q-type VGCCs regulate a tonic synaptic inhibitory control of the brainstem neurones. The risk of migraine by genetic modifications of P/Q-type channels may thus be sought in disturbed inhibition in the network that processes nociceptive dura input.

Spinal interleukin-6 is an amplifier of arthritic pain in the rat

OBJECTIVE Significant joint pain is usually widespread beyond the affected joint, which results from the sensitization of nociceptive neurons in the central nervous system (central sensitization). This study was undertaken to explore whether the proinflammatory cytokine interleukin-6 (IL-6) in the joint induces central sensitization, whether joint inflammation causes the release of IL-6 from the spinal cord, and whether spinal IL-6 contributes to central sensitization. METHODS In anesthetized rats, electrophysiologic recordings of spinal cord neurons with sensory input from the knee joint were made. Neuronal responses to mechanical stimulation of the rat knee and leg were monitored. IL-6 and soluble IL-6 receptor (sIL-6R) were applied to the knee joint or the spinal cord. Spinal release of IL-6 was measured by enzyme-linked immunosorbent assay. Soluble gp130, which neutralizes IL-6/sIL-6R, was spinally applied during the development of joint inflammation or during established inflammation. RESULTS A single injection of IL-6/sIL-6R into the rat knee joint as well as application of IL-6/sIL-6R to the rat spinal cord significantly increased the responses of spinal neurons to mechanical stimulation of the knee and ankle joint, i.e., induced central sensitization. Application of soluble gp130 to the rat spinal cord attenuated this effect of IL-6. The development of knee inflammation in the rat caused spinal release of IL-6. Spinal application of soluble gp130 attenuated the development of inflammation-evoked central sensitization but did not reverse it. CONCLUSION Our findings indicate that the generation of joint pain in the rat involves not only IL-6 in the joint but also IL-6 released from the spinal cord. Spinal IL-6 contributes to central sensitization and thus promotes the widespread hyperalgesia observed in the course of joint disease.