André Muller

Nogo expression in muscle correlates with amyotrophic lateral sclerosis severity

Nogo, a protein inhibiting axonal regeneration, exhibits a characteristic isoform‐specific pattern of expression in skeletal muscle of transgenic mice and patients with amyotrophic lateral sclerosis. Here, the increased levels of Nogo‐A or Nogo‐B in muscle biopsies of 15 amyotrophic lateral sclerosis patients significantly correlated with the severity of clinical disability and with the degree of muscle fiber atrophy. Nogo‐A immunoreactivity was observed selectively in atrophic slow‐twitch type I fibers. These results suggest that Nogo expression in muscle is a marker of amyotrophic lateral sclerosis severity. Ann Neurol 2005;57:553–556

β2-adrenoceptors are critical for antidepressant treatment of neuropathic pain†

Tricyclic antidepressants (TCAs) are one of the first‐line pharmacological treatments against neuropathic pain. TCAs increase the extracellular concentrations of noradrenaline and serotonin by blocking the reuptake transporters of these amines. However, the precise downstream mechanism leading to the therapeutic action remains identified. In this work, we evaluated the role of adrenergic receptors (ARs) in the action of TCAs.

Delta-opioid receptors are critical for tricyclic antidepressant treatment of neuropathic allodynia.

BACKGROUND The therapeutic effect of antidepressant drugs against depression usually necessitates a chronic treatment. A large body of clinical evidence indicates that antidepressant drugs can also be highly effective against chronic neuropathic pain. However, the mechanism by which these drugs alleviate pain is still unclear. METHODS We used a murine model of neuropathic pain induced by sciatic nerve constriction to study the antiallodynic properties of a chronic treatment with the tricyclic antidepressants nortriptyline and amitriptyline. Using knockout and pharmacological approaches in mice, we determined the influence of delta-opioid receptors in the therapeutic action of chronic antidepressant treatment. RESULTS In our model, a chronic treatment by tricyclic antidepressant drugs totally suppresses the mechanical allodynia in neuropathic C57Bl/6J mice. This therapeutic effect can be acutely reversed by an injection of the delta-opioid receptor antagonist naltrindole. Moreover, the antiallodynic property of antidepressant treatment is absent in mice deficient for the delta-opioid receptor gene. CONCLUSIONS The antiallodynic effect of chronic antidepressant treatment is mediated by a recruitment of the endogenous opioid system acting through delta-opioid receptors.

Chronic, but not acute, tricyclic antidepressant treatment alleviates neuropathic allodynia after sciatic nerve cuffing in mice

Antidepressant drugs act mainly by blocking the noradrenaline and/or serotonin uptake sites and require a chronic treatment. Tricyclic antidepressants are among the first line treatments clinically recommended against neuropathic pain. As observed against depression, a chronic treatment is required for a therapeutic effect. However, both in depression‐related and pain‐related research in rodents, it is difficult to design models that reproduce the clinical conditions and are sensitive to chronic but not to acute treatment by antidepressant drugs. In this study, we used a murine neuropathic pain model induced by the unilateral insertion of a polyethylene cuff around the main branch of the sciatic nerve. This model induced a long‐lasting ipsilateral mechanical allodynia. We evidenced that chronic, but not acute, treatment with the tricyclic antidepressants nortriptyline or amitriptyline suppressed the cuff‐induced mechanical allodynia. On the contrary, fluoxetine, a selective serotonin reuptake inhibitor, remained ineffective. To understand which mechanism is recruited downstream in order to alleviate the allodynia, we tested the opioid receptor antagonist naloxone, the delta‐opioid receptor antagonist naltrindole and the kappa‐opioid receptor antagonist nor‐BNI. We show that the therapeutic effect of notriptyline implicates the endogenous opioid system, in particular the delta‐ and the kappa‐opioid receptors. For comparison, we tested the anticonvulsant gabapentin and showed that it alleviates neuropathic allodynia after 3 days of treatment. Naloxone had no effect on gabapentin therapeutic benefit, showing that antidepressants and anticonvulsants alleviate neuropathic allodynia through independent mechanisms. Our work provides a clinically relevant model to understand the mechanism by which chronic antidepressant treatment can alleviate neuropathic pain.

Antidepressants and gabapentinoids in neuropathic pain: Mechanistic insights.

Neuropathic pain arises as a consequence of a lesion or disease affecting the somatosensory system. It is generally chronic and challenging to treat. The recommended pharmacotherapy for neuropathic pain includes the use of some antidepressants, such as tricyclic antidepressants (TCAs) (amitriptyline…) or serotonin and noradrenaline re-uptake inhibitors (duloxetine…), and/or anticonvulsants such as the gabapentinoids gabapentin or pregabalin. Antidepressant drugs are not acute analgesics but require a chronic treatment to relieve neuropathic pain, which suggests the recruitment of secondary downstream mechanisms as well as long-term molecular and neuronal plasticity. Noradrenaline is a major actor for the action of antidepressant drugs in a neuropathic pain context. Mechanistic hypotheses have implied the recruitment of noradrenergic descending pathways as well as the peripheral recruitment of noradrenaline from sympathetic fibers sprouting into dorsal root ganglia; and importance of both α2 and β2 adrenoceptors have been reported. These monoamine re-uptake inhibitors may also indirectly act as anti-proinflammatory cytokine drugs; and their therapeutic action requires the opioid system, particularly the mu (MOP) and/or delta (DOP) opioid receptors. Gabapentinoids, which target the voltage-dependent calcium channels α2δ-1 subunit, inhibit calcium currents, thus decreasing the excitatory transmitter release and spinal sensitization. Gabapentinoids also activate the descending noradrenergic pain inhibitory system coupled to spinal α2 adrenoceptors. Gabapentinoid treatment may also indirectly impact on neuroimmune actors, like proinflammatory cytokines. These drugs are effective against neuropathic pain both with acute administration at high dose and with repeated administration. This review focuses on mechanistic knowledge concerning chronic antidepressant treatment and gabapentinoid treatment in a neuropathic pain context.

Loss of prion protein in a transgenic model of amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is a motor neuron degenerative disorder caused in a proportion of cases by missense mutations in the gene encoding Cu/Zn superoxide dismutase (Cu/Zn-SOD) which result in unknown, lethal enzymatic activity. Based on a differential screening approach, we show here that the gene encoding the cellular prion protein (PrP(C)) was specifically repressed in a transgenic model of ALS overexpressing the mutant G86R Cu/Zn-SOD. Analysis by Northern blot, semiquantitative RT-PCR, and Western blot revealed that PrP(C) down-regulation, which appeared early in the asymptomatic phase of the pathology, occurred preferentially in those tissues primarily affected by the disease (spinal cord, sciatic nerve, and gastrocnemius muscle). This down-regulation was not accompanied by refolding of the aberrant PrP(Sc) isoform, the agent which causes transmissible spongiform encephalopathies. Furthermore, modification of PrP(C) expression was specifically linked to the presence of the G86R mutant since no changes were observed in transgenic mice overexpressing wild-type Cu/Zn-SOD. PrP(C) has been shown to play a role in the protection against oxidative stress, and we therefore propose that its down-regulation may contribute at least in part to ALS pathogenesis.

Melanocortin receptors and δ-opioid receptor mediate opposite signalling actions of POMC-derived peptides in CATH.a cells

The locus cœruleus is innervated by proopiomelanocortin (POMC)‐derived peptide immunoreactive fibres. The biological effects of α melanocyte‐stimulating hormone (αMSH) and β‐endorphin on second messengers (cAMP, inositol phosphates) and gene transcription were studied in the locus cœruleus‐derived cell line CATH.a.

Pituitary Adenylate Cyclase‐Activating Polypeptide Triggers Dual Transduction Signaling in CATH.a Cells and Transcriptionally Activates Tyrosine Hydroxylase and c‐fos Expression

Abstract: We used a catecholaminergic neuron‐like cell line (CATH.a cells) as a model system to investigate the likelihood that pituitary adenylate cyclase‐activating polypeptide (PACAP) may participate in the regulation of specific gene expression in catecholaminergic neurons. Analysis by reverse transcriptase‐PCR amplification revealed the presence in these cells of type I PACAP receptors, with a short isoform, together with a heavier so‐called Hop splice variant. PACAP38 and PACAP27 enhanced, in a dose‐dependent manner, both cyclic AMP formation and phosphoinositide breakdown, with EC50 values of, respectively, 0.6 × 10−10 and 2 × 10−9M. These peptides, in addition, also elevated [Ca2+]i by mobilizing intracellular calcium pools. Vasoactive intestinal peptide (VIP) was ∼1,000‐fold less potent in stimulating cyclic AMP (with EC50 = 2 × 10−7M) and failed to change the turnover of phosphoinositides and to alter [Ca2+]i. Both forms of PACAP, as well as forskolin, stimulated transcriptional induction of tyrosine hydroxylase (TH) and c‐fos promoters fused to a chloramphenicol acetyltransferase (CAT) reporter gene in transiently transfected cells (p < 0.01 vs. controls). Induction of CAT activity linked to both TH and c‐fos promoters was obliterated upon coexpression of a dominant inhibitory mutant (Mt‐RAB) of cyclic AMP‐dependent protein kinase. We conclude that CATH.a cells do express functional PACAP type I receptors, the activation of which impinges on TH and c‐fos transcription according to a process that is primarily dependent on the cyclic AMP‐PKA pathway.

Continuous activation of pituitary adenylate cyclase-activating polypeptide receptors elicits antipodal effects on cyclic AMP and inositol phospholipid signaling pathways in CATH.a cells: role of protein synthesis and protein kinases.

Abstract: Continuous exposure of cells to agonists develops a process that determines the extent to which the cells eventually respond to further stimuli. Here we used CATH.a cells (a catecholaminergic neuron‐like cell line), which express pituitary adenylate cyclase‐activating polypeptide (PACAP) receptors linked to both adenylyl cyclase and phospholipase C‐β pathways, to investigate the influence of prolonged hormonal treatment on dual signaling and gene transcription. Prolonged incubation of cells with PACAP failed to down‐regulate the density and affinity of membrane binding sites and caused opposite changes in messenger systems: PACAP‐stimulated cyclic AMP accumulation was attenuated in a time‐ and dose‐dependent fashion (t1/2 = 6.7 h and IC50 = 0.1 nM), whereas phosphoinositide turnover was overstimulated. Both effects were insensitive to pertussis toxin, whereas the drop in cyclic AMP concentration was also unchanged in the presence of 3‐isobutyl‐1‐methylxanthine, indicating that neither Gi‐like proteins nor cyclic nucleotide phosphodiesterases play a critical role in these processes. Blockade of protein synthesis with cycloheximide, as well as inhibition by H89 of cyclic AMP‐dependent protein kinase (but not by bisindolylmaleimide of protein kinase C) antagonized the influences exerted by PACAP on adenylyl cyclase activity and inositol phosphate formation. Transcription of the chimeric GAL4‐CREB construct, transiently transfected into CATH.a cells, was stimulated by PACAP, and this effect was potentiated as a result of chronic PACAP treatment. The results of the present investigation provide new insight into the possible differential regulation and cross‐talks of transduction signals of receptors linked to multiplex signaling. They demonstrate that prolonged exposure of CATH.a cells to PACAP results in the desensitization of the cyclic AMP pathway and superinduction of the inositol phosphate signal, through protein neosynthesis and cyclic AMP‐dependent protein kinase activation. At the same time, they show that desensitization of cyclic AMP signaling not only fails to hamper, but actually amplifies PACAP‐stimulated CREB‐regulated transcription.

Early Activation of Antioxidant Mechanisms in Muscle of Mutant Cu/Zn‐Superoxide Dismutase‐Linked Amyotrophic Lateral Sclerosis Mice

Abstract: A subset of familial ALS cases is associated with missense mutations in the gene encoding Cu/Zn‐superoxide dismutase (SOD1), a free radical scavenging enzyme that protects cells against oxidative stress. Overexpression of these ALS‐linked mutations confers an unidentified gain of function to the enzyme that triggers a series of neurological disorders characteristic of human ALS. To understand how skeletal muscle may counteract the progression of the disease, we explored the expression of different molecular effectors involved in antioxidant pathways. Our results are strongly indicative of the early and long‐lasting activation of a series of molecular effectors thought to act coordinately in preventing the increased oxidative stress characteristic of ALS.