Correlation between dopaminergic neurons, acetylcholinesterase and nicotinic acetylcholine receptors containing the α3- or α5-subunit in the rat substantia nigra

Neural circuits and nicotinic acetylcholine receptors mediate the cholinergic regulation of midbrain dopaminergic neurons and nicotine dependence

Midbrain dopaminergic (DA) neurons are governed by an endogenous cholinergic system, originated in the mesopontine nuclei. Nicotine hijacks nicotinic acetylcholine receptors (nAChRs) and interferes with physiological function of the cholinergic system. In this review, we describe the anatomical organization of the cholinergic system and the key nAChR subtypes mediating cholinergic regulation of DA transmission and nicotine reward and dependence, in an effort to identify potential targets for smoking intervention. Cholinergic modulation of midbrain DA systems relies on topographic organization of mesopontine cholinergic projections, and activation of nAChRs in midbrain DA neurons. Previous studies have revealed that α4, α6, and β2 subunit-containing nAChRs expressed in midbrain DA neurons and their terminals in the striatum regulate firings of midbrain DA neurons and activity-dependent dopamine release in the striatum. These nAChRs undergo modification upon chronic nicotine exposure. Clinical investigation has demonstrated that partial agonists of these receptors elevate the success rate of smoking cessation relative to placebo. However, further investigations are required to refine the drug targets to mitigate unpleasant side-effects.

Neuroprotective effect of crocin on substantia nigra in MPTP-induced Parkinson's disease model of mice

Parkinson's disease is caused by damage to substantia nigra dopaminergic neurons. Factors such as oxidative stress, inflammatory factors, and acetylcholinesterase activity may induce this disease. On the other hand, crocin-one of the active ingredients of saffron-has anti-oxidant and anti-inflammatory properties. This study was performed to evaluate the protective effect of crocin to decrease dopaminergic neuron damage and Parkinson's disease complications induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). A set of 24 male BALB/c Mice were divided randomly into four groups: (1) MPTP group receiving 30 mg/kg MPTP for 5 days; (2) MPTP + crocin group receiving 30 mg/kg MPTP for 5 days and 30 mg/kg crocin for 15 days; (3) NS group receiving normal saline for 5 days; and (4) NSIG group receiving normal saline intraperitoneally for 5 days and also normal saline by gavage for 15 days. After the treatment period, pole and hanging motor tests were performed in all groups. Then, the brains of all the animals were removed and fixed in formalin, prepared according to routine histologic methods and cut into sections of 5 µm thickness. Prepared sections were stained by immunohistochemistry techniques and toluidine blue to detect tyrosine-hydroxylase (TH)-positive neurons and dark neurons, respectively. Finally, the mean number of these cells were calculated by stereological methods and compared with the statistical tests in different groups. The results showed a significant increase in the time taken for the animal to fall from the pole in the MPTP group in comparison with other groups (P < 0.001). The time taken for them to stay on the wire in the hanging test decreased significantly in the MPTP group compared to the other groups (P < 0.001).,while in the MPTP + crocin group, the time to falling decreased (P < 0.05) and the time staying on the wire increased (P < 0.001) compared to the MPTP group. The number of TH-positive neurons in the MPTP group also decreased significantly in comparison with saline and MPTP + crocin groups (P < 0.001). The number of dark neuron sin the MPTP group increased significantly as compared with saline and the MPTP + Crocin groups (P < 0.001). Our results showed that crocin improves MPTP-induced Parkinson's disease complications and decreases cell death in the substantia nigra.

Acetylcholinesterase deficiency decreases apoptosis in dopaminergic neurons in the neurotoxin model of Parkinson's disease

The apoptosis pathway has been proposed to be involved in causing neuronal cell death in the pathogenesis of Parkinson's disease. However, the details of this pathway are poorly understood. Previous research has shown increased acetylcholinesterase expression during apoptosis in various cell types, which suggests that acetylcholinesterase has a potential role in neuronal cell death. In this study, we found that acetylcholinesterase protein expression increased and caspase-3 was activated in PC12 cells treated with 1-methyl-4-phenylpyridinium. Furthermore, the genetic or pharmacological inhibition of acetylcholinesterase was shown to protect PC12 cells from MPP+ induced apoptotic cell death. To study the function of acetylcholinesterase as a mechanism of neuronal cell death in vivo, we subsequently established a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine Parkinson's disease mouse model utilizing acetylcholinesterase-deficient mice. Studies in these mice revealed reduced dopaminergic neuron loss and lower expression levels of apoptotic proteins in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated heterozygous mice compared to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated wild-type mice. We conclude that it is highly probable that acetylcholinesterase is involved in the pathogenesis of the neurotoxin model of Parkinson's disease via apoptosis. Specifically, a deficiency or inhibition of acetylcholinesterase can decrease apoptosis and protect dopaminergic neurons in the neurotoxin model of Parkinson's disease.

Nicotine modulates GABAergic transmission to dopaminergic neurons in substantia nigra pars compacta

AIM

Dopaminergic neurons in the substantia nigra pars compacta (SNc) play important roles in motor control and drug addiction. As the major afferent, GABAergic innervation controls the activity of SNc dopaminergic neurons. Although it is clear that nicotine modulates SNc dopaminergic neurons by activating subtypes of somatodendritic nicotinic acetylcholine receptors (nAChRs), the detailed mechanisms of this activation remain to be addressed.

METHODS

In the current study, we recorded GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) from dissociated SNc dopaminergic neurons that were obtained using an enzyme-free procedure. These neurons preserved some functional terminals after isolation, including those that release GABA.

RESULTS

We found that both extra- and intra-cellular calcium modulates sIPSCs in these neurons. Furthermore, both nicotine and endogenous acetylcholine enhance the frequency of sIPSCs. Moreover, endogenous acetylcholine tonically facilitates sIPSC frequency, primarily by activating the alpha4beta2* nAChRs on the GABAergic terminals.

CONCLUSION

Nicotine facilitates GABA release onto SNc dopaminergic neurons mainly via the activation of presynaptic alpha4beta2* nAChRs.

Responses of limbic and extrapyramidal substance P systems to nicotine treatment

RATIONALE

Neuropeptides are linked to the psychopathology of stimulants of abuse, principally through dopamine mechanisms. Substance P (SP) is one of these neuropeptides and is associated with both limbic and extrapyramidal dopaminergic pathways and likely contributes to the pharmacology of these stimulants. The effects of nicotine on these dopamine systems have also been extensively studied; however, its effects on the associated SP pathways have received little attention.

OBJECTIVES

In the present study, we elucidated the effects of nicotine treatment on limbic and extrapyramidal SP systems by measuring changes in associated SP tissue concentrations.

MATERIALS AND METHODS

Male Sprague-Dawley rats received (+/-)nicotine 4.0 mg/kg/day (0.8 mg/kg, intraperitoneally; five injections at 2-h intervals) in the presence or absence of selective dopamine D1 and D2 receptor antagonists or a nonselective nicotinic acetylcholine receptor antagonist.

RESULTS

The nicotine treatment significantly but temporarily decreased substance P-like immunoreactivity (SPLI) content in the ventral tegmental area (VTA) and substantia nigra 12-18 h after drug exposure. The nicotine-mediated changes in SPLI were selectively blocked by pretreatment with mecamylamine as well as a dopamine D1, D2, or both receptor antagonists. Other brain areas that also selectively demonstrated nicotine-related declines in SPLI content included prefrontal cortex, the nucleus accumbens shell, and the very posterior caudate.

CONCLUSIONS

These findings indicate that some limbic and basal ganglia SP systems are significantly affected by exposure to nicotine through processes mediated by nicotinic and dopaminergic receptors, suggesting a role for SP pathways in nicotine's limbic and extrapyramidal effects.

Nicotinic and dopamine D2 receptors mediate nicotine-induced changes in ventral tegmental area neurotensin system

Neuropeptides have been implicated in the psychopathology of stimulants of abuse. Neurotensin is a neuropeptide associated with the regulation of the nigrostriatal and mesolimbic dopamine pathways. In addition, the ventral tegmental area, a midbrain region implicated in the rewarding effects of most, if not all, addictive drugs, appears to be a particularly critical target for nicotine action. Because neurotensin has been linked with both mesolimbic and mesocortical dopamine function, we examined the impact of nicotine treatment on central nervous neurotensin systems by measuring changes in neurotensin tissue content because it has been shown that such changes reflect alterations in release and activity of this peptide system. Male Sprague-Dawley rats received multiple administrations of (+/-) nicotine 4.0 mg/kg/day (0.8 mg/kg, i.p.; 5 x 2-h intervals) in the presence or absence of selective dopamine receptor antagonists (dopamine D(1); SCH 23390 or dopamine D(2); eticlopride) or two doses of the non-selective nicotinic acetylcholine receptor antagonist (mecamylamine; 3.0 and 6.0 mg/kg, s.c.). The nicotine treatment significantly decreased neurotensin-like immunoreactivity content in the ventral tegmental area, as well as related regions such as prefrontal cortex, substantia nigra, and anterior striatal region 12-18 h after drug treatment, but not the nucleus accumbens. The nicotine-mediated decrease in the neurotensin-like immunoreactivity of the ventral tegmental area was selectively blocked by a specific dopamine D(2), but not a dopamine D(1), receptor antagonist, while mecamylamine attenuated at the low (3.0 mg/kg) and completely blocked at high (6.0 mg/kg) dose this nicotine effect. These findings with previous studies, suggest that nicotine-mediated dopamine release activates D(2) receptors which in turn increases neurotensin release, turnover and acutely reduces tissue levels in the ventral tegmental area and other limbic and basal ganglia structures.

Postnatal development of noradrenergic terminals in the rat trigeminal motor nucleus: A light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation in the trigeminal motor nucleus (Vmot) of postnatal and adult rats was examined by light and electron microscopic immunocytochemistry using antibodies against dopamine-beta-hydroxylase or tyrosine hydroxylase. NA fibers were identified in the Vmot as early as the day of birth (postnatal day 0; P0). A continuous increase in the density of labeled fibers was observed during development up to P20, with a slight decrease at P30 and in the adult. Electron microscopic analysis of serial ultrathin sections revealed that, at P5, nearly half (46%) of the examined NA terminals made synaptic contact with other neuronal elements with membrane specializations. The percentage of examined NA varicosities engaged in synaptic contacts increased at P15 (74%), then decreased in the adult (64%). At all developmental ages, the majority of contacts made by these boutons were symmetrical, the postsynaptic elements being mainly dendrites and occasionally somata. Interestingly, some of the NA terminals made axo-axon contacts with other unidentified boutons. These results show that, although the density of NA fibers increases during postnatal development, functional NA boutons are present in the Vmot at early postnatal ages. Some of these fibers might exert their effects via nonsynaptic release of noradrenaline, the so-called volume transmission, but, in the main, they form conventional synaptic contacts with dendrites, somata, and other axonal terminals in the Vmot. These results are consistent with previous electrophysiological studies that propose an important role for the NA system in modulating mastication.

Developmental regulation of nicotinic acetylcholine receptors within midbrain dopamine neurons

We have combined anatomical and functional methodologies to provide a comprehensive analysis of the properties of nicotinic acetylcholine receptors (nAChRs) on developing dopamine (DA) neurons of Sprague-Dawley rats. Double-labeling in situ hybridization was used to examine the expression of nAChR subunit mRNAs within developing midbrain DA neurons. As brain maturation progressed there was a change in the pattern of subunit mRNA expression within DA neurons, such that alpha3 and alpha4 subunits declined and alpha6 mRNA increased. Although there were strong similarities in subunit mRNA expression in substantia nigra (SNc) and ventral tegmental area (VTA), there was higher expression of alpha4 mRNA in SNc than VTA at gestational day (G) 15, and of alpha5, alpha6 and beta3 mRNAs during postnatal development. Using a superfusion neurotransmitter release paradigm to functionally characterize nicotine-stimulated release of [(3)H]DA from striatal slices, the properties of the nAChRs on DA terminals were also found to change with age. Functional nAChRs were detected on striatal terminals at G18. There was a decrease in maximal release in the first postnatal week, followed by an increase in nicotine efficacy and potency during the second and third postnatal weeks. In the transition from adolescence (postnatal days (P) 30 and 40) to adulthood, there was a complex pattern of functional maturation of nAChRs in ventral, but not dorsal, striatum. In males, but not females, there were significant changes in both nicotine potency and efficacy during this developmental period. These findings suggest that nAChRs may play critical functional roles throughout DA neuronal maturation.

Mefloquine Enhances Nigral γ-Aminobutyric Acid Release via Inhibition of Cholinesterase

Mefloquine, a widely used antimalarial drug, has many neuropsychiatric effects. Although the mechanisms underlying these side effects remain unclear, recent studies show that mefloquine enhances spontaneous transmitter release and inhibits cholinesterases. In this study, we examined the effect of mefloquine on GABA receptor-mediated, spontaneous inhibitory postsynaptic currents (sIPSCs) of dopaminergic neurons, mechanically dissociated from the substantia nigra pars compacta of rats aged 6 to 17 postnatal days. Mefloquine (0.1-10 microM) robustly and reversibly increased the frequency of sIPSCs with an EC50 of 1.3 microM. Mefloquine also enhanced the frequency of miniature inhibitory postsynaptic currents in the presence of tetrodotoxin but without changing their mean amplitude. This suggests that mefloquine acts presynaptically to increase GABA release. Mefloquine-induced enhancement of sIPSCs was significantly attenuated in medium containing low Ca2+ (0.5 mM) or following pretreatment with 1,2-bis (2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester (30 microM), a membrane-permeable Ca2+ chelator. In contrast, 100 microM Cd2+ did not alter the action of mefloquine. This suggests that mefloquine-induced facilitation of GABA release depends on extracellular and intraterminal Ca2+ but not on voltage-gated Ca2+ channels. Mefloquine-induced enhancement of sIPSCs was significantly attenuated in the presence of the anticholinesterase agent physostigmine or blockers of non-alpha7 nicotinic acetylcholine receptors. Taken together, these data suggest that mefloquine enhances GABA release through its inhibition of cholinesterase. This allows accumulation of endogenously released acetylcholine, which activates neuronal nicotinic receptors on GABAergic nerve terminals. The resultant increase of Ca2+ entry into these terminals enhances vesicular release of GABA. This action may contribute to the neurobehavioral effects of mefloquine.