Roberto Maggio

Nicotine prevents experimental Parkinsonism in rodents and induces striatal increase of neurotrophic factors

Abstract: The repeated finding of an apparent protective effect of cigarette smoking on the risk of Parkinsons disease is one of the few consistent results in the epidemiology of this disorder. Among the numerous substances that originate from tobacco smoke, nicotine is by far the most widely studied. Nicotine is a natural alkaloid that has considerable stimulatory effects on the CNS. Its effects on the CNS are mediated by the activation of neuronal heteromeric acetylcholine‐gated ion channel receptors (nAChRs, also termed nicotinic acetylcholine receptors). In the present study, we describe the neuroprotective effects of (−)‐nicotine in two animal models of parkinsonism: diethyldithiocarbamate‐induced enhancement of 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine toxicity in mice and methamphetamine‐induced neurotoxicity in rats and mice. The neuroprotective effect of (−)‐nicotine was very similar to that of the noncompetitive NMDA receptor antagonist (+)‐MK‐801. In parallel experiments, we found that (−)‐nicotine induces the basic fibroblast growth factor‐2 (FGF‐2) and the brain‐derived neurotrophic factor in rat striatum. The effect of (−)‐nicotine on the induction of FGF‐2 was prevented by the nAChR antagonist mecamylamine. We also found that (+)‐MK‐801 was able to induce FGF‐2 in the striatum. As trophic factors have been reported to be neuroprotective for dopaminergic cells, our data suggest that the increase in neurotrophic factors is a possible mechanism by which (−)‐nicotine protects from experimental parkinsonisms.

Functional Role of the Third Cytoplasmic Loop in Muscarinic Receptor Dimerization

By means of the expression of two chimeric receptors, α2/m3 and m3/α2, in which the carboxyl-terminal receptor portions, containing transmembrane (TM) domains VI and VII, were exchanged between the α2C adrenergic and the m3 muscarinic receptor, Maggio et al. (Maggio, R., Vogel, Z., and Wess, J. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 3103-31073) demonstrated that G protein-linked receptors are able to interact functionally with each other at the molecular level to form (hetero)dimers. In the present study we tested the hypothesis that interaction between receptors might depend on the presence of a long third intracellular (i3) loop and that shortening this loop could impair the capability of receptors to form dimers. To address this question, we initially created short chimeric α2 adrenergic/m3 muscarinic receptors in which 196 amino acids were deleted from the i3 loop (α2/m3-short and m3/α2-short). Although co-transfection of α2/m3 and m3/α2 resulted in the appearance of specific binding, the co-expression of the two short constructs (α2/m3-short and m3/α2-short), either together or in combination, respectively, with m3/α2 and α2/m3 did not result in any detectable binding activity. In another set of experiments, a mutant m3 receptor, m3/m2(16aa), containing 16 amino acids of the m2 receptor sequence at the amino terminus of the third cytoplasmic loop, which was capable of binding muscarinic ligands but was virtually unable to stimulate phosphatidylinositol hydrolysis, was also mutated in the i3 loop, resulting in the m3/m2(16aa)-short receptor. Although co-transfection of m3/m2(16aa) with a truncated form of the m3 receptor (m3-trunc, containing an in frame stop codon after amino acid codon 272 of the rat m3 sequence) resulted in a considerable carbachol-stimulated phosphatidylinositol breakdown, the co-transfection of m3/m2(16aa)-short with the truncated form of the m3 receptor did not result in any recovery of the functional activity. Thus, these data suggest that intermolecular interaction between muscarinic receptors, involving the exchange of amino-terminal (containing TM domains I-V) and carboxyl-terminal (containing TM domains VI and VII) receptor fragments depends on the presence of a long i3 loop. One may speculate that when alternative forms of receptors with a different length of the i3 loop exist, they could have a different propensity to dimerize.

Striatal increase of neurotrophic factors as a mechanism of nicotine protection in experimental parkinsonism.

SummaryThe repeated finding of an apparent protective effect of cigarette smoking on the risk of Parkinsons disease is one of the few consistent results in the epidemiology of this disorder. Among the innumerous substances that originate from tobacco smoke, nicotine is by far the most widely studied, and the most likely candidate for a protective effect against neuronal degeneration in Parkinsons disease. Nicotine is a natural alkaloid that has considerable stimulatory effects on the central nervous system (CNS). Its effects on the CNS are mediated by the activation of neuronal heteromeric acetylcholine-gated ion channel receptors (nAChR, also termed nicotinic acetylcholine receptors). In the present study, we describe the neuroprotective effects of (−)nicotine in two animal models of parkinsonism: the diethyldithiocarbamate (DDC)-induced enhancement of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in mice, and the methamphetamine-induced neurotoxicity in rats and mice. In parallel experiments, we found that (−)nicotine induces the basic fibroblast growth factor (FGF-2) and the brain-derived neurotrophic factor (BDNF) in rat striatum. As FGF-2 and BDNF have been reported to be neuroprotective for dopaminergic cells, our data indicate that the increase in neurotrophic factors is a possible mechanism by which (−)nicotine protects from experimental parkinsonisms. Moreover, they suggest that nAChR agonists could be of potential benefit in the progression of Parkinsons disease.

The impact of G‐protein‐coupled receptor hetero‐oligomerization on function and pharmacology

Although highly controversial just a few years ago, the idea that G‐protein‐coupled receptors (GPCRs) may undergo homo‐oligomerization or hetero‐oligomerization has recently gained considerable attention. The recognition that GPCRs may exhibit either dimeric or oligomeric structures is based on a number of different biochemical and biophysical approaches. Although much effort has been spent to demonstrate the mechanism(s) by which GPCRs interact with each other, the physiological relevance of this phenomenon remains elusive. An additional source of uncertainty stems from the realization that homo‐oligomerization and hetero‐oligomerization of GPCRs may affect receptor binding and activity in different ways, depending on the type of interacting receptors. In this brief review, the functional and pharmacological effects of the hetero‐oligomerization of GPCR on binding and cell signaling are critically analyzed.

Inhibition of nitric oxide synthase dramatically potentiates seizures induced by kainic acid and pilocarpine in rats

We investigated whether the severity of convulsions evoked by kainic acid and pilocarpine is modified in nitric oxide synthase inhibitor-treated rats. We found that chronic treatment (4 days) with NW-nitro-L-arginine greatly potentiates seizures induced by both convulsants suggesting a potential role for nitric oxide in mechanisms regulating seizure induction and propagation.

Potent activation of dopamine D3/D2 heterodimers by the antiparkinsonian agents, S32504, pramipexole and ropinirole

Recombinant, human dopamine D3 and D2 receptors form functional heterodimers upon co‐expression in COS‐7 cells. Herein, actions of the antiparkinsonian agents, S32504, ropinirole and pramipexole, at D3/D2L heterodimers were compared to their effects at the respective monomers and at split, chimeric D3trunk/D2tail and D2trunk/D3tail receptors: the trunk incorporated transmembrane domains (TDs) I–V and the tail TDs VI and VII. In binding assays with the antagonist [3H]nemonapride, all agonists were potent ligands of D3 receptors showing, respectively, 100‐, 18‐ and 56‐fold lower affinity at D2L receptors, mimicking the selective D3 receptor antagonist, S33084 (100‐fold). At D3trunk/D2tail receptors, except for ropinirole, all drugs showed lower affinities than at D3 sites, whereas for D2trunk/D3tail receptors, affinities of all drugs were higher than at D2L sites. The proportion of high affinity binding sites recognized by S32504, pramipexole and ropinirole in membranes derived from cells co‐expressing D3 and D2L sites was higher than in an equivalent mixture of membranes from cells expressing D3 or D2L sites, consistent with the promotion of heterodimer formation. In contrast, the percentage of high and low affinity sites (biphasic isotherms) recognized by S33084 was identical. Functional actions were determined by co‐transfection of a chimeric adenylyl cyclase (AC)‐V/VI insensitive to D3 receptors. Accordingly, D3 receptor‐transfected cells were irresponsive whereas, in D2L receptor‐transfected cells, agonists suppressed forskolin‐stimulated cAMP production with modest potencies. In cells co‐transfected with D3 and D2L receptors, S32504, ropinirole and pramipexole potently suppressed AC‐V/VI with EC50s 33‐, 19‐ and 11‐fold lower than at D2L receptors, respectively. S32504 also suppressed AC‐V/VI activity at split D3trunk/D2tail and D2trunk/D3tail chimeras transfected into COS‐7 cells. In conclusion, antiparkinson agents behave as potent agonists at D3/D2‘heterodimers’, though any role in their actions in vivo remains to be demonstrated.

S33138 [N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1]-benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenylacetamide], A Preferential Dopamine D3 versus D2 Receptor Antagonist and Potential Antipsychotic Agent: I. Receptor-Binding Profile and Functional Actions at G-Protein-Coupled Receptors

The novel, potential antipsychotic, S33138 (N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1]benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenylacetamide), displayed ∼25-fold higher affinity at human (h) dopamine D3 versus hD2L (long isoform) and hD2S (short isoform) receptors (pKi values, 8.7, 7.1, and 7.3, respectively). Conversely, haloperidol, clozapine, olanzapine, and risperidone displayed similar affinities for hD3, hD2L, and hD2S sites. In guanosine-5′-O-(3-[35S]thio)-triphosphate ([35S]-GTPγS) filtration assays, S33138 showed potent, pure, and competitive antagonist properties at hD3 receptors, displaying pKB and pA2 values of 8.9 and 8.7, respectively. Higher concentrations were required to block hD2L and hD2S receptors. Preferential antagonist properties of S33138 at hD3 versus hD2L receptors were underpinned in antibody capture/scintillation proximity assays (SPAs) of Gαi3 recruitment and in measures of extracellular-regulated kinase phosphorylation. In addition, in cells cotransfected with hD3 and hD2L receptors that assemble into heterodimers, S33138 blocked (pKB, 8.5) the inhibitory influence of quinpirole upon forskolin-stimulated cAMP formation. S33138 had low affinity for hD4 receptors (<5.0) but revealed weak antagonist activity at hD1 receptors (Gαs/SPA, pKB, 6.3) and hD5 sites (adenylyl cyclase, 6.5). Modest antagonist properties were also seen at human serotonin (5-HT)2A receptors (Gαq/SPA, pKB, 6.8, and inositol formation, 6.9) and at 5-HT7 receptors (adenylyl cyclase, pKB, 7.1). In addition, S33138 antagonized hα2C adrenoceptors ([35S]GTPγS, 7.2; Gαi3/SPA, 6.9; Gαo/SPA, 7.3, and extracellular-regulated-kinase, 7.1) but not hα2A or hα2B adrenoceptors (<5.0). Finally, in contrast to haloperidol, clozapine, olanzapine, and risperidone, S33138 displayed negligible affinities for multiple subtypes of α1-adrenoceptor, muscarinic, and histamine receptor. In conclusion, S33138 possesses a distinctive receptor-binding profile and behaves, in contrast to clinically available antipsychotics, as a preferential antagonist at hD3 versus hD2 receptors.

Genetic Deletion of Trace Amine 1 Receptors Reveals Their Role in Auto-Inhibiting the Actions of Ecstasy (MDMA)

“Ecstasy” [3,4-methylenedioxymetamphetamine (MDMA)] is of considerable interest in light of its prosocial properties and risks associated with widespread recreational use. Recently, it was found to bind trace amine-1 receptors (TA1Rs), which modulate dopaminergic transmission. Accordingly, using mice genetically deprived of TA1R (TA1-KO), we explored their significance to the actions of MDMA, which robustly activated human adenylyl cyclase-coupled TA1R transfected into HeLa cells. In wild-type (WT) mice, MDMA elicited a time-, dose-, and ambient temperature-dependent hypothermia and hyperthermia, whereas TA1-KO mice displayed hyperthermia only. MDMA-induced increases in dialysate levels of dopamine (DA) in dorsal striatum were amplified in TA1-KO mice, despite identical levels of MDMA itself. A similar facilitation of the influence of MDMA upon dopaminergic transmission was acquired in frontal cortex and nucleus accumbens, and induction of locomotion by MDMA was haloperidol-reversibly potentiated in TA1-KO versus WT mice. Conversely, genetic deletion of TA1R did not affect increases in DA levels evoked by para-chloroamphetamine (PCA), which was inactive at hTA1 sites. The TA1R agonist o-phenyl-3-iodotyramine (o-PIT) blunted the DA-releasing actions of PCA both in vivo (dialysis) and in vitro (synaptosomes) in WT but not TA1-KO animals. MDMA-elicited increases in dialysis levels of serotonin (5-HT) were likewise greater in TA1-KO versus WT mice, and 5-HT-releasing actions of PCA were blunted in vivo and in vitro by o-PIT in WT mice only. In conclusion, TA1Rs exert an inhibitory influence on both dopaminergic and serotonergic transmission, and MDMA auto-inhibits its neurochemical and functional actions by recruitment of TA1R. These observations have important implications for the effects of MDMA in humans.

Stimulatory role of dopamine on fibroblast growth factor-2 expression in rat striatum

We have previously shown that systemic injection of (–)nicotine produces a selective up‐regulation of fibroblast growth factor (FGF)‐2 mRNA levels in rat striatum. Because (–)nicotine can increase striatal release of dopamine and glutamate, in the present study we have investigated the contribution of these neurotransmitters in the modulation of FGF‐2 expression. We found that coinjection of dopaminergic D1 (SCH23390) or D2 (haloperidol) receptor antagonists prevents nicotine‐induced elevation of FGF‐2 expression. However, injection of the NMDA receptor antagonist MK‐801 produced a significant increment of FGF‐2 mRNA and protein levels in rat striatum similar to the effect produced by (–)nicotine alone. Interestingly this effect of MK‐801 could also be prevented by D1 or D2 receptor antagonists, suggesting that an elevation of dopamine levels may be required for the regulation of the trophic molecule. Accordingly we found that the non‐selective dopaminergic agonist apomorphine can similarly increase striatal FGF‐2 mRNA levels. Despite the observation that both D1 and D2 receptors appear to contribute to the modulation of FGF‐2 expression, only a direct activation of D2 receptors, through quinpirole administration, was able to mimic the effect of apomorphine. On the basis of FGF‐2 neurotrophic activity, these results suggest that direct or indirect activation of dopaminergic system can be neuroprotective and might reduce cell vulnerability in degenerative disorders.

Dopamine D2-D3 receptor heteromers: pharmacological properties and therapeutic significance.

Heteromerization of dopamine receptors has been shown for both the D(1)/D(5) and D(2)/D(3)/D(4) receptor families, which couple positively and negatively, respectively, to adenylyl cyclase. The present article reviews data on dopamine heteromers formed by D(3), focusing in particular on associations with their D(2) counterparts. Certain antiparkinsonian agents, like the preferential and high efficacy D(3)>D(2) agonists, pramipexole, and ropinirole, show amplified potency at D(2)-D(3) heteromers versus constituent monomers. Accordingly, in cells cotransfected with D(2) and D(3) receptors, pramipexole, and ropinirole suppress forskolin (FK)-stimulated cAMP production with higher potencies as compared to cells transfected with D(2) or D(3) receptors only. Furthermore, in cells cotransfected with D(2) and an excess of D(3) receptors, the partial agonists aripiprazole, S33592, bifeprunox, N-desmethylclozapine, and preclamol lose agonist properties and abolish the actions of quinpirole. Then, partial agonists are transformed into antagonists upon cotransfection of D(2) with an excess of D(3) receptors. A hypothetical relationship of the above observations to the pathophysiology and possibly treatment of neuropsychiatric diseases is discussed.