Spinal cord pathology is ameliorated by P2X7 antagonism in a SOD1-mutant mouse model of amyotrophic lateral sclerosis

In recent years there has been an increasing awareness of the role of P2X7, a receptor for extracellular ATP, in modulating physiopathological mechanisms in the central nervous system. In particular, P2X7 has been shown to be implicated in neuropsychiatry, chronic pain, neurodegeneration and neuroinflammation. Remarkably, P2X7 has also been shown to be a ‘gene modifier’ in amyotrophic lateral sclerosis (ALS): the receptor is upregulated in spinal cord microglia in human and rat at advanced stages of the disease; in vitro, activation of P2X7 exacerbates pro-inflammatory responses in microglia that have an ALS phenotype, as well as toxicity towards neuronal cells. Despite this detrimental in vitro role of P2X7, in SOD1-G93A mice lacking P2X7, the clinical onset of ALS was significantly accelerated and disease progression worsened, thus indicating that the receptor might have some beneficial effects, at least at certain stages of disease. In order to clarify this dual action of P2X7 in ALS pathogenesis, in the present work we used the antagonist Brilliant Blue G (BBG), a blood-brain barrier permeable and safe drug that has already been proven to reduce neuroinflammation in traumatic brain injury, cerebral ischemia-reperfusion, neuropathic pain and experimental autoimmune encephalitis. We tested BBG in the SOD1-G93A ALS mouse model at asymptomatic, pre-symptomatic and late pre-symptomatic phases of disease. BBG at late pre-onset significantly enhanced motor neuron survival and reduced microgliosis in lumbar spinal cord, modulating inflammatory markers such as NF-κB, NADPH oxidase 2, interleukin-1β, interleukin-10 and brain-derived neurotrophic factor. This was accompanied by delayed onset and improved general conditions and motor performance, in both male and female mice, although survival appeared unaffected. Our results prove the twofold role of P2X7 in the course of ALS and establish that P2X7 modulation might represent a promising therapeutic strategy by interfering with the neuroinflammatory component of the disease.

Adenosine A(2A) receptors are required for normal BDNF levels and BDNF-induced potentiation of synaptic transmission in the mouse hippocampus

Brain-derived neurotrophic factor (BDNF), a member of neurotrophin family, enhances synaptic transmission and regulates neuronal proliferation and survival. Both BDNF and its tyrosine kinase receptors (TrkB) are highly expressed in the hippocampus, where an interaction with adenosine A(2A) receptors (A(2A)Rs) has been recently reported. In the present paper, we evaluated the role of A(2A)Rs in mediating functional effects of BDNF in hippocampus using A(2A)R knock-out (KO) mice. In hippocampal slices from WT mice, application of BDNF (10 ng/mL) increased the slope of excitatory post-synaptic field potentials (fEPSPs), an index of synaptic facilitation. This increase of fEPSP slope was abolished by the selective A(2A) antagonist ZM 241385. Similarly, genetic deletion of the A(2A)Rs abolished BDNF-induced increase of the fEPSP slope in slices from A(2A)R KO mice The reduced functional ability of BDNF in A(2A)R KO mice was correlated with the reduction in hippocampal BDNF levels. In agreement, the pharmacological blockade of A(2)Rs by systemic ZM 241385 significantly reduced BDNF levels in the hippocampus of normal mice. These results indicate that the tonic activation of A(2A)Rs is required for BDNF-induced potentiation of synaptic transmission and for sustaining a normal BDNF tone in the hippocampus.

Behavioral and electrophysiological effects of the adenosine A2A receptor antagonist SCH 58261 in R6/2 Huntington's disease mice

The effect of chronic treatment with the selective adenosine A2A receptor antagonist SCH 58261 on the behavioral and electrophysiological alterations typical of R6/2 mice (a transgenic mouse model of Huntington's disease, HD), has been studied. Starting from 5 weeks of age, R6/2 and wild type (WT) mice were treated daily with SCH 58261 (0.01 mg/kg i.p.) for 7 days. In the following weeks, the ability of mice to perform in the rotarod, plus maze and open field tests were evaluated. In addition, with electrophysiological experiments in corticostriatal slices we tested whether the well-known increased NMDA vulnerability of R6/2 mice was prevented by SCH 58261 treatment. We found that chronic treatment with SCH 58262: i) fully prevented the alterations in emotional/anxious responses displayed by R6/2 mice; ii) did not prevent the impairment in motor coordination; iii) abolished the increase in NMDA-induced toxicity observed in the striatum of HD mice. On balance, targeting A2A receptors seems to have some beneficial effects in HD even though, given the complexity of A2A receptor pharmacology and HD pathogenesis, further studies are necessary to clarify whether A2A receptor antagonists have therapeutic potential in HD.

Permissive role of adenosine A2A receptors on metabotropic glutamate receptor 5 (mGluR5)-mediated effects in the striatum

The metabotropic glutamate receptors 5 (mGlu5Rs) and the adenosine A2A receptors (A2ARs) have been reported to functionally interact in the striatum. The aim of the present work was to verify the hypothesis that the state of activation of A2A Rs could influence mGlu5R-mediated effects in the striatum. In electrophysiological experiments (extracellular recording in rat corticostriatal slices), the ability of the selective mGlu5R agonist CHPG to potentiate the reduction of the field potential amplitude induced by NMDA was prevented not only by the selective mGlu5R antagonist MPEP, but also by the selective A2AR antagonist ZM 241385. Analogously, the application of CHPG potentiated NMDA-induced toxicity (measured by LDH release) in cultured striatal neurons, an effect that was abolished by both MPEP and ZM 241385. Finally, the A2AR agonist CGS 21680 potentiated CHGP effects, an action that was reproduced and abolished, respectively, by forskolin (an activator of the cAMP/protein kinase A, PKA, pathway) and KT 5720 (a PKA inhibitor). The results indicate that A2ARs exert a permissive role on mGlu5R-induced effects in the striatum. Such an interaction may represent an additional target for the development of therapeutic strategies towards striatal disorders.

Modulation of glutamate release and excitotoxicity by adenosine A2A receptors

Because an increased glutamate outflow is thought to play a crucial role in triggering excitotoxic neuronal death, drugs able to regulate glutamate release could be effective for the management of neurodegenerative diseases. In this article, the authors discuss the hypothesis that adenosine A2A receptor antagonists (A2A antagonists) may belong to the aforementioned category. In rats bilaterally lesioned with the excitotoxin quinolinic acid (QA) in the striatum, the A2A antagonist SCH 58261 significantly reduced the motor, EEG, and neuropathologic changes induced by the lesion. Such effects of SCH 58261 occurred only at low doses and were paralleled by an inhibition of QA-stimulated glutamate release. The role played by A2A antagonists in the regulation of glutamate outflow was also confirmed by preliminary results obtained in the model of paired-pulse stimulation in corticostriatal slices. Conversely, based on data obtained in cultured striatal neurons, A2A antagonists appear unable to directly inhibit NMDA effects. In conclusion, A2A antagonists show clear neuroprotective effects in models of brain injury, although their actual therapeutic potential needs to be confirmed in a wider range of doses and in models of neurodegenerative diseases in which presynaptic and postsynaptic effects play different relative roles.

Inhibitory effect of somatostatin on neutral amino acid transport in isolated brain microvessels

In the presence of somatostatin-14 or some of its receptorial agonists, the uptake of large neutral amino acids by isolated brain microvessels was found to be inhibited up to 50%, no other transport system being affected. Although the luminal and abluminal sides of brain endothelial cells are both capable of taking up large neutral amino acids, only uptake from the abluminal side appears to be inhibited by somatostatin. The involvement of a type-2 somatostatin receptor was suggested by assays with a series of receptor-specific somatostatin agonists, and was confirmed by the release of inhibition caused by a specific type-2 receptor antagonist. A type-2-specific mRNA was indeed shown to be present in both bovine brain microvessels ex vivo and primary cultures of endothelial cells from rat brain microvessels.

Age-related decline in the functional response of striatal group I mGlu receptors

In order to verify whether striatal group I metabotropic glutamate (mGlu) receptors undergo functional alteration in ageing, the effects induced by the selective agonist 3,5-dihydroxyphenylglycine (DHPG) in the striatum of young (3 months) and aged (24-25 months old) rats were compared. The ability of DHPG to stimulate phosphoinositide (PI) hydrolysis (striatal slices), to influence striatal dopamine release (in vivo microdialysis) and to potentiate the effects of NMDA on extracellular field potential amplitude (extracellular recordings on striatal slices) was reduced in the striatum of old vs young rats. These results show an age-dependent reduction in the functional response of striatal group I mGlu receptors, which may be one of the factors underlying the reduced ability aged striatum to integrate information.

Postnatal development of delta-opioid receptor subtypes in mice

1. The density and affinity of binding sites for the delta-selective opioid ligands [3H]-[D-Ala2, Asp4]deltorphin (DELT-I), [3H]-[D-Ala2Glu4]-deltorphin (DELT-II), [3H]-[D-Pen2,D-Pen5]enkephalin (DPDPE), and [3H]-naltrindole (NTI) were determined in whole brain from 10, 15, 25 and 60 day-old C57BL mice. 2. At all ages, the analyses of the homologous displacement curves, gave best fits to single rather than to multiple site models. The binding capacity (Bmax) labelled by [3H]-NTI was about one half that labelled by [3H]-DELT-I, [3H]-DELT-II and [3H]-DPDPE. In 25 and 60 day-old mouse brain the DPDPE Bmax was 25% less than the deltorphin-II Bmax. 3. In saturation experiments, specific binding of [3H]-DELT-I on adult mouse brain homogenates was best fitted by a two-site model (34%, high affinity site, Kd = 1.08 nM and 66% low affinity sites, Kd = 39.9 nM). 4. DPDPE produced a biphasic inhibition of specific [3H]-DELTI-I binding, from 15 days of age onwards. The relative percentage of high and low affinity sites was 72% and 28% in 15 day-, 65% and 35% in 25 day- and 30% and 70% in 60 day-old mice. 5. In adult mouse brain labelled with [3H]-DELT-I, DELT-II recognized 71% of high-affinity and 29% of low-affinity sites DELT-I and DPDPE produced monophasic inhibition of specific [3H]-DELT-II binding to brain homogenates of adult mice. 6. These data suggest that a sub-population of delta-sites (probably the delta 2-subtype), recognized by DELT-I, with high affinity for DELT-II and low affinity for DPDPE develops from 25 days onward. 7. In electrically stimulated mouse vas deferens (MVD) the rank order of potency of the three delta-agonists was: DELT-I > DELT-II > DPDPE in 10 day-old mice: and DELT-I- DELT-II > DPDPE, from 25 days onward. During this time, the potency of DELT-II increased about 15 fold whereas the potency of DELT-I and DPDPE increased only 5 times. The higher efficacy of DELT-II could depend on receptor maturation towards the delta 2-subtype.

Interaction between the mu-agonist dermorphin and the delta-agonist [D-Ala2, Glu4]deltorphin in supraspinal antinociception and delta-opioid receptor binding

1. In rats, the interaction between the mu-opioid agonist dermorphin and the delta-opioid agonist [D-Ala2, Glu4]deltorphin was studied in binding experiments to delta-opioid receptors and in the antinociceptive test to radiant heat. 2. When injected i.c.v., doses of [D-Ala2, Glu4]deltorphin higher than 20 nmol produced antinociception in the rat tail-flick test to radiant heat. Lower doses were inactive. None of the doses tested elicited the maximum achievable response. This partial antinociception was accomplished with an in vivo occupancy of more than 97% of brain delta-opioid receptors and of 17% of mu-opioid receptors. Naloxone (0.1 mg kg-1, s.c.), and naloxonazine (10 mg kg-1, i.v., 24 h before), but not the selective delta-opioid antagonist naltrindole, antagonized the antinociception. 3. In vitro competitive inhibition studies in rat brain membranes showed that [D-Ala2, Glu4]deltorphin displaced [3H]-naltrindole from two delta-binding sites of high and low affinity. The addition of 100 microM Gpp[NH]p produced a three fold increase in the [D-Ala2, Glu4]deltorphin Ki value for both binding sites. The addition of 10 nM dermorphin increased the Ki value of the delta-agonist for the high affinity site five times. When Gpp[NH]p was added to the incubation medium together with 10 nM dermorphin, the high affinity Ki of the delta-agonist increased 15 times. 4. Co-administration into the rat brain ventricles of subanalgesic doses of dermorphin and [D-Ala2, Glu4]deltorphin resulted in synergistic antinociceptive responses. 5. Pretreatment with naloxone or with the non-equilibrium mu-antagonists naloxonazine and beta-funaltrexamine completely abolished the antinociceptive response of the mu-delta agonist combinations. 6. Pretreatment with the delta-opioid antagonists naltrindole and DALCE reduced the antinociceptive response of the dermorphin-[D-Ala2, Glu4]deltorphin combinations to a value near that observed after the mu-agonist alone. At the dosage used, naltrindole occupied more than 98% of brain delta-opioid receptors without affecting mu-opioid-receptors. 7. These data suggest that in the rat tail-flick test to radiant heat, mu- and delta-opioid agonists co-operate positively in evoking an antinociceptive response. Although interactions between different opioid pathways cannot be excluded, in vitro binding results indicate that this co-operative antinociception is probably mediated by co-activation of the delta-opioid receptors at the cellular level by the mu- and delta-agonist.

Pharmacological studies of 'sapo' from the frog Phyllomedusa bicolor skin: a drug used by the Peruvian Matses Indians in shamanic hunting practices

The dried skin secretion from Phyllomedusa bicolor, 'sapo', is used by the Matses Indians of the Northern Peru, in shamanic rites mainly designed to improve luck in hunting. When rubbed into burned, exposed areas of the skin, the drug causes the prompt appearance of violent peripheral gastrointestinal and cardiovascular effects soon followed by remarkable central effects (increase in physical strength, heightening of senses, resistance to hunger and thirst, exalted capacity to face stress situations). All the peripheral and most of the central effects of 'sapo' can be ascribed to the exceptionally high content of the drug (up to 7% of its weight) in potently active peptides, easily absorbed through the burned, inflamed areas of the skin. The concentration in 'sapo' of the single peptides (phyllocaerulein, phyllomedusin, phyllokinin, demorphins and deltorphins) has been determined by bioassay, and peptide contents were correlated with the different symptoms of the 'sapo' intoxication.

Dermorphin-related peptides from the skin of Phyllomedusa bicolor and their amidated analogs activate two mu opioid receptor subtypes that modulate antinociception and catalepsy in the rat

Three naturally occurring dermorphin-like peptides from the skin of the frog Phyllomedusa bicolor, the related carboxyl-terminal amides, and some substituted analogs were synthesized, their binding profiles to opioid receptors were determined, and their biological activities were studied in isolated organ preparations and intact animals. The opioid binding profile revealed a very high selectivity of these peptides for mu sites and suggested the existence of two receptor subtypes, of high and low affinity. The peptides tested acted as potent mu opioid agonists on isolated organ preparations. They were several times more active in inhibiting electrically evoked contractions in guinea pig ileum than in mouse vas deferens. When injected into the lateral brain ventricle or peritoneum of rats, the high-affinity-site-preferring ligand, [Lys7-NH2]dermorphin, behaved as a potent analgesic agent. By contrast, the low-affinity-site-preferring ligand, [Trp4,Asn7-NH2]dermorphin, produced a weak antinociception but an intense catalepsy.