Pharmacologic treatment of neuropathic pain

Neuropathic pain, or pain after nervous system injury, can be very refractory to pharmacologic interventions. Through a better understanding of the pathophysiology of neuropathic pain, it has been suggested that nonopioid agents, such as antidepressants and anticonvulsants, may be more efficacious in the treatment of neuropathic pain than common analgesics, such as opioids or nonsteroidal anti-inflammatory drugs. However, this has not been consistently demonstrated in clinical studies. Conversely, many confounding factors of neuropathic pain make it difficult to interpret clinical studies. Therefore, we must develop a better understanding of the preclinical models of neuropathic pain to better understand the application of new and old drugs to the human neuropathic pain state. This article provides an overview of the commonly used preclinical neuropathic pain models, followed by a summary of the efficacy of currently available agents in preclinical pain models and human correlates.

Demyelinating disorder of the central nervous system occurring in black South Africans

OBJECTIVES

To investigate the nature and cause in eight black South African patients of a recurrent (multiphasic), remitting, and relapsing demyelinating disease of the CNS.

METHODS

The clinical and laboratory investigations and radiological manifestations of these patients were documented.

RESULTS

Each patient had two or more acute attacks of demyelinating disease affecting the CNS. The clinical presentations of the patients were predominantly those of multiphasic neuromyelitis optica (NMO). Brain MRI in these patients showed features consistent with those described for acute disseminated encephalomyelitis (ADEM), as well as lesions that are described in multiple sclerosis. There was involvement of the corpus callosum in addition to typical ADEM lesions. Laboratory investigations excluded all other known causes of multiphasic CNS demyelination. Oligoclonal antibodies were not detected in these patients at any time. The patients were all from a population with a low risk for MS (black South Africans).

CONCLUSION

The patients described here represent a new phenotypic expression of a recurrent (multiphasic), steroid sensitive, inflammatory demyelinating disorder of the CNS occurring in black South Africans. The disorder is either a distinct inflammatory demyelinating disorder of the CNS of as yet unknown aetiology, or a varied form of MS (ADEM/NMO) occurring in a population with a low risk (where the genetic trait and environmental risk factors for MS do not exist) for MS.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Selected contribution: chemoreflex responses to CO2 before and after an 8-h exposure to hypoxia in humans

The ventilatory sensitivity to CO2, in hyperoxia, is increased after an 8-h exposure to hypoxia. The purpose of the present study was to determine whether this increase arises through an increase in peripheral or central chemosensitivity. Ten healthy volunteers each underwent 8-h exposures to 1) isocapnic hypoxia, with end-tidal PO2 (PET(O2)) = 55 Torr and end-tidal PCO2 (PET(CO2)) = eucapnia; 2) poikilocapnic hypoxia, with PET(O2) = 55 Torr and PET(CO2) = uncontrolled; and 3) air-breathing control. The ventilatory response to CO2 was measured before and after each exposure with the use of a multifrequency binary sequence with two levels of PET(CO2): 1.5 and 10 Torr above the normal resting value. PET(O2) was held at 250 Torr. The peripheral (Gp) and the central (Gc) sensitivities were calculated by fitting the ventilatory data to a two-compartment model. There were increases in combined Gp + Gc (26%, P < 0.05), Gp (33%, P < 0.01), and Gc (23%, P = not significant) after exposure to hypoxia. There were no significant differences between isocapnic and poikilocapnic hypoxia. We conclude that sustained hypoxia induces a significant increase in chemosensitivity to CO2 within the peripheral chemoreflex.

Vaccinations and multiple sclerosis

Two problems must be considered in regard to the relationship between vaccinations and MS: Do vaccinations favour the first attack of MS? Do they increase the short- or long-term risk in patients with known disease? Answers to these questions are difficult due to the paucity of reported cases, our ignorance of the precise frequency of neurological adverse events in vaccines based on prospective studies, and finally by the lack of a well established pathophysiology. In most instances, the role of the vaccine is based on a temporal link between the injection and the onset of neurological disease, and more rarely to a positive reintroduction. Acute disseminated encephalomyelitis (ADEM), a monophasic and multifocal illness of the white and grey matter, has been observed following various viral or bacterial infections as well as vaccine injections for diseases such as pertussis, tetanus and yellow fever. The similarities between ADEM and experimental allergic encephalitis (EAE) are suggestive of an immunological process. In addition to the dramatic presentation of ADEM, more limited white matter involvement, such as optic neuritis or myelitis, has been reported following vaccine injections, and has occasionally been counted as the first attack of MS. In France, 25 million inhabitants, almost half of the population, were vaccinated against hepatitis B (HB) between 1991 and 1999. Several hundred cases of an acute central demyelinating event following HB vaccination were reported to the pharmacovigilance unit, leading to a modification of vaccination policy in the schools and the initiation of several studies designed to examine the possible relationship between the vaccine and the central demyelinating events. The results of these studies failed to establish the causality of the HB vaccine. Nevertheless, molecular mimicry between HB antigen(s) and one or more myelin proteins, or a non-specific activation of autoreactive lymphocytes, could constitute possible pathogenetic mechanisms for these adverse neurological events.

Non-conventional MR techniques to monitor the evolution of multiple sclerosis

In multiple sclerosis (MS), conventional magnetic resonance imaging (MRI) has proved to be sensitive for detecting lesions and their changes over time. However, conventional MRI cannot characterize and quantify the tissue damage within and outside such lesions. Other quantitative MR techniques, including MR spectroscopy (MRS), magnetization transfer imaging (MTI) and diffusion-weighted imaging (DWI) have the potential to overcome this limitation and, as a consequence, to provide complementary information to conventional MRI. MTI- and DWI-derived measurements quantify the structural changes occurring within and outside lesions seen on conventional MRI scans, and MRS adds information on the biochemical nature of such changes. The application of these MR techniques to the study of MS is dramatically increasing our understanding of how MS causes irreversible deficits, and it is likely to provide useful insights into the pathophysiology of other white matter diseases in the future. The present review summarizes the major contributions made by these three MR techniques in the understanding of MS evolution.

Pathophysiologic mechanisms of neuropathic pain

New animal models of peripheral nerve injury have facilitated our understanding of neuropathic pain mechanisms. Nerve injury increases expression and redistribution of newly discovered sodium channels from sensory neuron somata to the injury site; accumulation at both loci contributes to spontaneous ectopic discharge. Large myelinated neurons begin to express nociceptive substances, and their central terminals sprout into nociceptive regions of the dorsal horn. Descending facilitation from the brain stem to the dorsal horn also increases in the setting of nerve injury. These and other mechanisms drive various pathologic states of central sensitization associated with distinct clinical symptoms, such as touch-evoked pain.

Complex regional pain syndromes

Complex regional pain syndromes (CRPS) (formerly reflex sympathetic dystrophy and causalgia) are neuropathic pain conditions that are initiated by an extremity trauma or peripheral nerve lesion. Clinical definition and scientific understanding of CRPS are still evolving; however, both the clinical picture and therapeutic options are significantly influenced by a dysfunction of the sympathetic nervous system. Recent investigations suggest functional central abnormalities and a peripheral inflammatory component in the pathophysiology of CRPS. Interdisciplinary treatment includes physical, pharmacologic, and invasive interventional therapy, as well as stimulation techniques.

Depression in the elderly: a beta-adrenergic receptor dysfunction?

Depression is a significant problem in the elderly population and is often accompanied by cognitive changes and agitation. Hypothalamic-pituitary axis dysfunction may occur in some depressed elderly patients and represent a non-specific physiological hyperarousal state. Moreover, recent evidence confirms that antidepressants down-regulate beta-adrenergic receptors in the central nervous system. These findings provide the background for proposing that depression may result from an age-specific, stress-induced dysfunction of the beta-adrenergic receptor system in a subgroup of the geriatric population. A bio-psycho-social model for understanding and developing innovative treatment strategies for depression in the elderly is presented.

Central and peripheral mechanisms of migraine

Central (cortex and upper brainstem) and peripheral (trigeminovascular) mechanisms are involved in the pathophysiology of migraine. We propose that unknown migraine triggers initiate depolarization of cortical neurons followed concomitantly by a transient increase in blood flow. This transient hyperemia spreads at a rate of 3-6 mm/min followed by hypoperfusion which is more long-lasting. Through unknown mechanisms (probably chemical), we hypothesize that this wave activates or sensitizes trigeminovascular axons which is a consequence of neuropeptide release and neurogenic inflammation. Orthodromic conduction along trigeminovascular fibers transmits information centrally via the trigeminal caudalis and other brainstem nuclei to higher cortical structures for registration and modulation of nociceptive information.

Central and peripheral mechanisms in migraine: a neurophysiological approach

The author briefly reviews the main evidence in favour of both central and peripheral factors supposed to take part in the complex pathomechanisms leading to the activation of the trigemino-vascular system in the migraine attack.

Miller Fisher syndrome: toward a more comprehensive understanding

PURPOSE

To review recent knowledge on the clinical features, pathology and pathophysiology, diagnosis and treatment of Miller Fisher syndrome (MFS).

DATA SOURCES

Clinical and laboratory studies on MFS in the past 10 years were included.

RESULTS

A viral infection preceded neurological symptoms in 71.8% of MFS patients. Typical MFS consists of the triad of ataxia, areflexia and ophthalmoplegia. Other cranial nerves are also involved, which may overlap with limb weakness in typical Guillain-Barre syndrome (GBS). Lower cranial nerve variants of GBS, atypical MFS and ataxic neuropathies may overlap, and are thought of as variant forms of MFS. Recurrence and CNS involvement is found more frequently in MFS than in GBS. Antibody to GQ1b, a tetrasyaloganglioside (GQ1b antibody) which is found in close relation to ophthalmoplegia in MFS, is also associated with Campylobacter jejuni (C. jejuni) serotype Penner 2. This suggests that C. jejuni may induce MFS via the GQ1b structure. The GQ1b antibody may lead to the failure of acetylcholine release from motor nerve terminals, which has been confirmed by clinical neurophysiological results.

CONCLUSIONS

Many studies have shown similarities in the pathogenesis of MFS and GBS. However, there are still some differences between them, especially in the areas of sensory and CNS involvement. The GQ1b antibody is thought of as one of the key factors in the pathogenesis of MFS, especially with ophthalmoplegia, and it may prove a useful clinical marker in the diagnosis of MFS.

Fibromyalgia, chronic fatigue syndrome, and myofascial pain syndrome

The prevalence of chronic widespread pain in the general population in Israel was comparable with reports from the USA, UK, and Canada. Comorbidity with fibromyalgia (FM) resulted in somatic hyperalgesia in patients with irritable bowel syndrome. One sixth of the subjects with chronic widespread pain in the general population were also found to have a mental disorder. Mechanisms involved in referred pain, temporal summation, muscle hyperalgesia, and muscle pain at rest were attenuated by the N-methyl-D-aspartate (NMDA) antagonist, ketamine, in FM patients. Delayed corticotropin release, after interleukin-6 administration, in FM was shown to be consistent with a defect in hypothalamic corticotropin-releasing hormone neural function. The basal autonomic state of FM patients was characterized by increased sympathetic and decreased parasympathetic systems tones. The severity of functional impairment as assessed by the Medical Outcome Survey Short Form (SF-36) discriminated between patients with widespread pain alone and FM patients. Chronic fatigue syndrome (CFS) occurred in about 0.42% of a random community-based sample of 28,673 adults in Chicago, Illinois. A significant clinical overlap between CFS and FM was reported. Cytokine dysregulation was not found to be a singular or dominant factor in the pathogenesis of CFS. A favorable outcome of CFS in children was reported; two thirds recovered and resumed normal activities. No major therapeutic trials in FM and CFS were reported over the past year.

Central neuroplasticity and pathological pain

The traditional specificity theory of pain perception holds that pain involves a direct transmission system from somatic receptors to the brain. The amount of pain perceived, moreover, is assumed to be directly proportional to the extent of injury. Recent research, however, indicates far more complex mechanisms. Clinical and experimental evidence shows that noxious stimuli may sensitize central neural structures involved in pain perception. Salient clinical examples of these effects include amputees with pains in a phantom limb that are similar or identical to those felt in the limb before it was amputated, and patients after surgery who have benefited from preemptive analgesia which blocks the surgery-induced afferent barrage and/or its central consequences. Experimental evidence of these changes is illustrated by the development of sensitization, wind-up, or expansion of receptive fields of CNS neurons, as well as by the enhancement of flexion reflexes and the persistence of pain or hyperalgesia after inputs from injured tissues are blocked. It is clear from the material presented that the perception of pain does not simply involve a moment-to-moment analysis of afferent noxious input, but rather involves a dynamic process that is influenced by the effects of past experiences. Sensory stimuli act on neural systems that have been modified by past inputs, and the behavioral output is significantly influenced by the "memory" of these prior events. An increased understanding of the central changes induced by peripheral injury or noxious stimulation should lead to new and improved clinical treatment for the relief and prevention of pathological pain.

Neural substrates for the perception of acutely induced dyspnea

Little is currently known about the brain regions involved in central processing of dyspnea. We performed a functional imaging study with positron emission tomography (PET) to assess brain activation associated with an important component of dyspnea, respiratory discomfort during loaded breathing. We induced respiratory discomfort in eight healthy volunteers by adding external resistive loads during inspiration and expiration. Brain activation was characterized by a significant increase in regional cerebral blood flow (rCBF) (Z score of peak activation > 3.09). As compared with the unloaded control condition, high loaded breathing was associated with neural activation in three distinct brain regions, the right anterior insula, the cerebellar vermis, and the medial pons (respective Z scores = 4.75, 4.44, 4.41). For these brain regions, we further identified a positive correlation between rCBF and the perceived intensity of respiratory discomfort (respective Z scores = 4.45, 4.75, 4.74) as well as between rCBF and the mean amplitude of mouth pressure swings (DeltaPm), the index of the main generating mechanism of the sensation (respective Z scores = 4.67, 4.36, 4.31), suggesting a common activation by these two parameters. Furthermore, we identified an area in the right posterior cingulate cortex where neural activation was specifically associated with perceived intensity of respiratory discomfort that is not related to DeltaPm (Z score = 4.25). Our results suggest that respiratory discomfort related to loaded breathing may be subserved by two distinct neural networks, the first being involved in the concomitant processing of the genesis and perception of respiratory discomfort and the second in the modulation of perceived intensity of the sensation by various factors other than its main generating mechanism, which may include emotional processing.

Central pontine myelinolysis--a rare manifestation of CNS Sjogren's syndrome

Central nervous involvement in Sjogren's syndrome (CNS-SS) is not uncommon and has a variety of manifestations. We describe a 47-year-old woman with Sjogren's syndrome who presented with distal renal tubular acidosis with severe hypokalemia and hypokalemic myopathy. She developed progressive obtundation after years of stable disease. ANA, anti-Ro antibodies were positive. Brain MRI showed a cleft in the mid pons which was hypointense on T1 and hyperintense on T2 which was considered to be classical of central pontine myelinolysis. Serial MRI showed initial enlargement of the lesion which persisted despite successful immunosuppressive therapy with pulse methylprednisolone, pulse cyclophosphamide, plasmapheresis and IVIG.

Neurotransplantation-induced plasticity in the recipient CNS: focusing on the recipient response

In the last two decades, neurotransplantation has gained the status of a potentially valuable treatment option in various central nervous system (CNS) disorders. This technique has provided considerable functional improvement in animal models of neurodegenerative diseases, stroke and trauma. In order to make the best therapeutic use of this treatment option, mechanisms of neurotransplantation-induced recovery need to be better understood. Specific interactions of transplants with the recipient brain, which are prominent in embryonic neural cell grafts, include formation of synapses between grafted neurons and recipient neuronal population with controlled release of neurotransmitters. Production and release of specific trophic substances for the recipient neurons by the grafted cells also play a considerable role in some transplants. In the above cases, the functional recovery seems to correlate well with the number of surviving cells in the transplant. There is, however, another component to the graft-induced recovery, best revealed in those graft recipients who display functional improvement although only few or no grafted cells can be found at the post mortem analysis. While psychological factors (placebo effect) have been proposed to play a central role in human graft recipients with functional recovery in the absence of surviving grafts, animal models of neurodegenerative disease have consistently shown the same phenomenon. Our recent results point to the local inflammatory and immune response to transplantation as a key element which induces a trophic response in the CNS parenchyma and stimulates plastic changes of the recipient neural connections. Findings by other investigators, who studied the connections between the inflammatory and neurotrophic responses in vitro and in vivo, and glial reaction to CNS trauma and trophic factor synthesis in vivo, support such conclusions. Accumulated evidence point to the need for further studies that would elucidate the role of the immune response in connection with CNS transplantation outcome.

Chemical preconditioning in mice is not mediated by upregulation of nitric oxide synthase isoforms

Ischemic preconditioning requires increased nitric oxide (NO) production. However, NO may also trigger delayed neuronal death cascades. The goal therefore was to investigate nitric oxide synthase (NOS) isoforms (neuronal NOS: nNOS; endothelial NOS: eNOS; inducible NOS: iNOS) with reverse transcriptase-polymerase chain reaction in hippocampal slices from control mice and slices prepared upon preconditioning in vivo (single intraperitoneal injection of 20 mg/kg body weight 3-nitropropionate (3NP)). One hour after preconditioning nNOS (108+/-34%, mean+/-SD), eNOS (93+/-34%), and iNOS (282+/-261%) remained at control levels. Similarly, nNos, eNOS, and iNOS stayed at control level 12, 24, and 72 h after preconditioning with 3NP. Incubation of slices, however, drastically increased iNOS (1676+/-818, P<0.01). We conclude that chemical preconditioning other than ischemic preconditioning may not increase potentially harmful nitric oxide synthase isoforms.

The mGlu(2/3) agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate, is anti- and proconvulsant in DBA/2 mice

The anticonvulsant activity of the selective group II metabotropic glutamate receptor (mGlu) agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC) has been evaluated in chemoconvulsant and sound-induced models of epileptic seizures in DBA/2 mice. 2R,4R-APDC (> or =10 nmol, intracerebroventricularly (i.c.v.), -15 min) transiently reduced sound-induced seizure activity including clonic seizures to 40% of vehicle at 20 nmol (i.c.v.) and 30% of vehicle at 100 mg/kg (intraperitoneally (i.p.), -15 min). 2R,4R-APDC inhibited clonic seizures induced by the group III mGlu antagonist (R,S)-alpha-methylserine-O-phosphate (2.5 micromol, i.c.v.) when co-injected at 20-40 nmol and inhibited limbic seizure activity induced by the mGlu(1/5) agonist (R,S)-3,5-dihydroxyphenylglycine (1.5 micromol, i.c.v.) when co-injected at 10-40 nmol. A reversal of the anticonvulsant activity of 2R,4R-APDC was observed at (>20 nmol) in each of the chemoconvulsant and sound-induced models of epileptic seizures. 2R,4R-APDC (0.1-1 micromol, i.c.v.) induced stimulus-independent, rapid and dose-dependent clonic seizures. Selective mGlu(2/3) agonists represent a novel class of potential anti-epileptic drugs, however due to the proconvulsant activity observed here, 2R,4R-APDC is obviously limited in this regard.