Nitric oxide is involved in acetylcholinesterase inhibitor-induced myopathy in rats

ABSTRACT Excess activation of muscle nicotinic acetylcholine receptors due to genetic mutations, as seen in slow channel congenital myasthenic syndrome, or acetylcholinesterase (AChE) inhibition results in muscle cell degeneration. Our recent work showed that nitric oxide synthase (NOS) inhibitors prevent nicotine-induced muscle cell death in culture. In the present study, we examined the effects of NOS inhibition on nicotinic receptor-mediated myopathy in vivo. Rats injected with the AChE inhibitor paraoxon demonstrate a 90-fold increase in the number of dying muscle cells compared with control as evidenced histologically by centralized nuclei and the presence of degenerating profiles. Coadministration of the nonspecific NOS inhibitor nitro-L-arginine methyl ester or the neuronal NOS-specific inhibitor 7-nitroindazole dramatically reduced the presence of such degenerating profiles to ϳ20% of that seen with paraoxon alone. These results show that inhibition of NOS, as well as neuronal NOS, significantly reduces AChE inhibitor-induced muscle cell degeneration, suggesting that increased nitric oxide production mediates such myopathy

Cortical nNOS neurons co-express the NK1 receptor and are depolarized by Substance P in multiple mammalian species

We have previously demonstrated that Type I neuronal nitric oxide synthase (nNOS)-expressing neurons are sleep-active in the cortex of mice, rats, and hamsters. These neurons are known to be GABAergic, to express Neuropeptide Y (NPY) and, in rats, to co-express the Substance P (SP) receptor NK1, suggesting a possible role for SP in sleep/wake regulation. To evaluate the degree of co-expression of nNOS and NK1 in the cortex among mammals, we used double immunofluorescence for nNOS and NK1 and determined the anatomical distribution in mouse, rat, and squirrel monkey cortex. Type I nNOS neurons co-expressed NK1 in all three species although the anatomical distribution within the cortex was species-specific. We then performed in vitro patch clamp recordings in cortical neurons in mouse and rat slices using the SP conjugate tetramethylrhodamine-SP (TMR-SP) to identify NK1-expressing cells and evaluated the effects of SP on these neurons. Bath application of SP (0.03–1 μM) resulted in a sustained increase in firing rate of these neurons; depolarization persisted in the presence of tetrodotoxin. These results suggest a conserved role for SP in the regulation of cortical sleep-active neurons in mammals

Decrease in ␣3*/␣6* Nicotinic Receptors but Not Nicotine- Evoked Dopamine Release in Monkey Brain after Nigrostriatal Damage

ABSTRACT Nicotinic acetylcholine receptors (nAChRs) are decreased in the striata of patients with Parkinson's disease (PD) or in experimental models after nigrostriatal damage. Because presynaptic nAChRs on striatal dopamine terminals mediate dopamine release, receptor loss may contribute to behavioral deficits in PD. The present experiments were done to determine whether nAChR function is affected by nigrostriatal damage in nonhuman primates, because this model shares many features with PD. Initial characterization of nicotine-evoked [ 3 H]dopamine release from monkey striatal synaptosomes revealed that release was calcium-dependent and inhibited by selective nAChR antagonists. It is noteworthy that a greater proportion (ϳ70%) of release was inhibited by the ␣3*/␣6* antagonist ␣-conotoxinMII (␣-CtxMII) compared with rodents. Monkeys were lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and [ 3 H]dopamine release, dopamine transporter, and nAChRs were measured. As anticipated, lesioning decreased the transporter and ␣3*/␣6* nAChRs in caudate and putamen. In contrast, ␣3*/␣6* nAChR-evoked [ 3 H]dopamine release was reduced in caudate but not putamen, demonstrating a dissociation between nAChR sites and function. A different pattern was observed in the mesolimbic dopamine system. Dopamine transporter levels in nucleus accumbens were not reduced after MPTP, as expected; however, there was a 50% decline in ␣3*/␣6* nAChR sites with no decrease in ␣3*/␣6* receptor-evoked dopamine release. No declines in ␣-CtxMIIresistant nAChR (␣4*) binding or nicotine-evoked release were observed in any region. These results show a selective preservation of ␣3*/␣6* nAChR-mediated function in the nigrostriatal and mesolimbic dopamine systems after nigrostriatal damage. Maintenance of function in putamen, a region with a selective loss of dopaminergic terminals, may be important in PD. Parkinson's disease (PD) is characterized by motor and cognitive deficits and is manifested by reductions in dopaminergic neurons in substantia nigra and dopamine levels in striatum Receptor binding and immunoprecipitation studies have revealed multiple nAChR subtypes on striatal dopaminergic terminals, including both ␣4* and ␣6* in rodents ABBREVIATIONS: PD, Parkinson's disease; ␣-CtxMII, ␣-conotoxinMII; nAChR, nicotinic acetylcholine receptor; MPTP, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; BSA, bovine serum albumin; RTI-121, 2␤-carboxylic acid isopropyl ester-3␤-(4-iodophenyl)tropane); ANOVA, analysis of variance; *, nicotinic receptors containing the indicated ␣ and ␤ subunit and additional undefined subunits

The regulation of adrenal tyrosine hydroxylase.

Administration of apomorphine, a dopamine (DA) agonist, led to a short-term decrease in adrenal catecholamines (CA) and a long-term increase in adrenal tyrosine hydroxylase (TH) activity. The lowered CA were not responsible for the increase in enzyme activity. Instead, the increase was the result of changes in sympathetic activity to the adrenal medulla. DA agonists and a DA precursor increased adrenal TH activity in rats; this increase was prevented by blockade off receptors. Destruction or blockade of serotonin (5-HT) neurons also increased adrenal TH; this effect was partially reversed by a 5-HT precursor. Central dopaminergic and serotoninergic mechanisms are thus involved in the regulation of adrenal TH and have net excitatory and inhibitory roles, respectively. Destruction or blockade of the dopaminergic system partially eliminated the influence of the 5-HT system; however, destruction of 5-HT neurons did not prevent DA agonist-induced increases. This favors the view that interactions between the two systems are sequential, with the serotoninergic system preceding the dopaminergic.L'injection d'apomorphine, un agoniste de la dopamine (DA), a causé, à court terme, une diminution des concentrations des catécholamines (CA) et, à long terme, une élévation de l'activité, de la tyrosine hydroxylase (TH). Cette élévation n'est pas due à la diminution des CA mais semble plutôt être la résultat d'altérations dans l'activité du système orthosympathique. Les agonistes de la DA et DOPA ont causé une élévation de l'activité de la TH chez le rat, élévation qui est antagonisée par le blocage des récepteurs de la DA. La destruction des cellules sérotoninergiques ou le blocage des récepteurs de la sérotonine ont aussi provoqué une élévation de l'activité de la TH; cet effet est partiellement éliminé par le 5-hydroxytryptophane, précurseur immédiat de la sérotonine. Les systèmes dopaminergique et sérotoninergique sont donc impliqués dans la régulation de l'activité de la TH dans les glandes surrénales et jouent, respectivement, un rôle d'excitation et d'inhibition. La destruction ou le blocage du système dopaminergique a partiellement annulé l'influence du système sérotoninergique; cependant, la destruction des neurones sérotoninergiques n'a pas emphêché l'augmentation induite par les agonistes de la DA. Cela semble indiquer que les deux systèmes agissent successivement, le système sérotoninergique précedant le système dopaminergique

Nicotine Reduces L-DOPA-Induced Dyskinesias by Acting at ␤2* Nicotinic Receptors

ABSTRACT L-DOPA-induced dyskinesias or abnormal involuntary movements (AIMs) are a debilitating adverse complication associated with prolonged L-DOPA administration for Parkinson's disease. Few treatments are currently available for dyskinesias. Our recent data showed that nicotine reduced L-DOPA-induced AIMs in parkinsonian animal models. An important question is the nicotinic acetylcholine receptor (nAChR) subtypes through which nicotine exerts this beneficial effect, because such knowledge would allow for the development of drugs that target the relevant receptor population(s). To address this, we used ␤2 nAChR subunit knockout [␤2(Ϫ/Ϫ)] mice because ␤2-containing nAChRs are key regulators of nigrostriatal dopaminergic function. All of the mice were lesioned by intracranial injection of 6-hydroxydopamine into the right medial forebrain bundle. Lesioning resulted in a similar degree of nigrostriatal damage and parkinsonism in ␤2(Ϫ/Ϫ) and wild-type mice. All of the mice then were injected with L-DOPA (3 mg/kg) plus benserazide (15 mg/kg) once daily for 4 weeks until AIMs were fully developed. L-DOPA-induced AIMs were approximately 40% less in the ␤2(Ϫ/Ϫ) mice compared with the wild-type mice. It is interesting to note that nicotine (300 g/ml in drinking water) reduced L-DOPA-induced AIMs by 40% in wild-type mice but had no effect in ␤2(Ϫ/Ϫ) mice with partial nigrostriatal damage. The nicotine-mediated decline in AIMs was much less pronounced in wild-type mice with near-complete degeneration, suggesting that presynaptic nAChRs on dopaminergic terminals have a major influence. These data demonstrate an essential role for ␤2* nAChRs in the antidyskinetic effect of nicotine and suggest that drugs targeting these subtypes may be useful for the management of L-DOPA-induced dyskinesias in Parkinson's disease

α6β2* and α4β2* Nicotinic Receptors Both Regulate Dopamine Signaling with Increased Nigrostriatal Damage: Relevance to Parkinson's Disease

Nicotinic receptors (nAChRs) are important modulators of dopaminergic transmission in striatum, a region critical to Parkinson's disease. The nAChRs mainly involved are the α6β2* and α4β2* subtypes. Lesion studies show that the α6β2* receptor is decreased to a much greater extent with nigrostriatal damage than the α4β2* subtype raising the question whether this latter nAChR population is more important with increased nigrostriatal damage. To address this, we investigated the effect of varying nigrostriatal damage on α6β2* and α4β2* receptor-modulated dopamine signaling using cyclic voltammetry. This approach offers the advantage that changes in dopamine release can be observed under different neuronal firing conditions. Total single-pulse-evoked dopamine release decreased in direct proportion to declines in the dopamine transporter and dopamine uptake. We next used α-conotoxinMII and mecamylamine to understand the role of the α4β2* and α6β2* subtypes in release. Single-pulse–stimulated α6β2* and α4β2* receptor dopamine release decreased to a similar extent with increasing nigrostriatal damage, indicating that both subtypes contribute to the control of dopaminergic transmission with lesioning. Total burst-stimulated dopamine release also decreased proportionately with nigrostriatal damage. However, the role of the α4β2* and α6β2* nAChRs varied with different degrees of lesioning, suggesting that the two subtypes play a unique function with burst firing, with a somewhat more prominent and possibly more selective role for the α6β2* subtype. These data have important therapeutic implications because they suggest that drugs directed to both α4β2* and α6β2* nAChRs may be useful in the treatment of neurological disorders such as Parkinson's disease