Codistribution of nicotinic acetylcholine receptor subunit alpha3 and beta4 mRNAs during rat brain development

Nicotinic receptors and Parkinson's disease

Accumulating evidence indicates that nicotinic receptors play a role in basal ganglia function. Furthermore, nicotine administration may be neuroprotective in animal models of nigrostriatal degeneration, while cigarette smoking is inversely correlated with Parkinson's disease. Because nicotinic receptors are decreased in Parkinson's disease, these observations may suggest that nicotinic agonists are beneficial in this disorder. We used two model systems to investigate this possibility. One involved non-human primates, which represent a good model because their neuroanatomical organization resembles that of man and nigrostriatal degeneration leads to biochemical and behavioral deficits similar to Parkinson's disease. To identify the subunits that comprise basal ganglia nicotinic receptors, we investigated alpha4, alpha6, alpha7, beta2, beta3 and beta4 transcript distribution in monkey substantia nigra. All mRNAs were expressed with a selective alteration in some transcripts after 1-methyl-4-phenyl-1,2,3,6-tetrahydropteridine (MPTP) induced nigrostriatal degeneration. As an approach to evaluate neuroprotective effects of nicotine against nigral neuron damage, we used mesencephalic neurons in culture, treated with a selective dopaminergic neurotoxin. The results show that nicotine pretreatment protected against dopaminergic nigral neural degeneration. These data suggest that nicotinic receptor ligands may be useful in Parkinson's disease therapy.

Exposure to nicotine during a defined period in neonatal life induces permanent changes in brain nicotinic receptors and in behaviour of adult mice

Neonatal exposure to low doses of nicotine has been shown to prevent the development of low-affinity nicotine-binding sites, and to elicit a different behaviour response to nicotine in the mice as adults. This study has identified a defined period during the development of neonatal mouse brain for the induction of these permanent changes. Neonatal mice, aged either 3, 10, or 19 days were exposed to nicotine, 66 micrograms nicotine-base/kg b.wt., s.c. twice daily, on 5 consecutive days. In the cerebral cortex, high- and low-affinity (HA and LA) nicotine-binding sites were assayed (3H-nicotine/nicotine) in neonatal male mice aged 8, 15, and 24 days and in adult mice aged 4 months. Spontaneous behaviour and nicotine-induced behaviour were observed in 4-month-old male mice. The spontaneous behaviour test did not indicate any difference between saline- and nicotine-treated mice, whereas the nicotine-induced behaviour test revealed a hypoactive response to nicotine, though only in mice given nicotine on days 10-14. The response of controls and the other age categories to nicotine was an increased activity. At no time during the neonatal period could LA nicotine-binding sites be found following nicotine treatment, but the persistence of this effect was evident only in adult mice exposed on days 10-14.

Diminution of nicotinic receptor alpha 3 subunit mRNA expression in aged rat brain

Losses in nicotinic acetylcholine receptors (nAChRs) have been linked to a decline in cognitive function in patients with neurodegenerative diseases, but the impact of normal aging on the different neuronal nicotinic receptor subunits has yet to be fully characterized. The expression pattern of nine nAChR subunits mRNA (alpha2-7 and beta2-4) was investigated in this study in young and aged rat brains, 5 weeks and 30 months old, respectively. Microtissue samples were dissected from brain slices and nAChR subunit mRNA expression was analyzed by reverse transcription polymerase chain reaction (RT-PCR) from eight different brain areas. In several regions, a loss of PCR signal was found for the alpha3, and to a lesser extent, for alpha2 subunit mRNA in aged rat brain. A relative quantification of alpha3 and alpha4 mRNA expression was then carried out in four of these brain regions. A significant diminution of alpha3 expression level was observed in all regions tested while, in comparison, much less modification in alpha4 mRNA was detected. This decrease in alpha3 subunit mRNA may represent a selective degradation of neurons expressing the alpha3 subunit or a diminution of alpha3-containing nAChR subtypes in those neurons during aging.

Presence of functional neuronal nicotinic acetylcholine receptors in brainstem motoneurons of the rat

In mammals, nicotinic acetylcholine receptors (nAChRs) play a crucial role in motor control. Muscle-type nAChRs mediate synaptic excitation of skeletal muscle by motoneurons, and nAChRs are present on Renshaw cells, where they produce recurrent inhibition of spinal motoneurons. We asked whether nAChRs are also present in motoneurons. Whole-cell recordings were performed on various motor nuclei in brainstem slices of young rats. Neurons were visualized using infrared (IR) videomicroscopy. Acetylcholine (ACh) or the nicotinic agonist, epibatidine, were delivered by pressure microinjection. Facial (VII), hypoglossal (XII) and vagal (X) motoneurons responded to ACh by generating a fast inward current. In VII motoneurons, the ACh effect was mimicked by epibatidine, and nicotine induced a slow inward current and desensitized the ACh-evoked current. In VII and XII motoneurons, the ACh-evoked current was blocked by the nicotinic antagonist dihydro-beta-erythroidine (DHbetaE), but was unaffected by methyllycaconitine (MLA), an alpha7-specific antagonist. By contrast, the ACh-induced current in X motoneurons was sensitive to MLA. Current-voltage relationships indicated that the currents mediated by either alpha7-containing (X) or non-alpha7-containing (VII, XII) nAChRs displayed inward rectification. In accordance with the electrophysiological data, autoradiography revealed that VII, X and XII nuclei of young rats contained binding sites for [3H]epibatidine; binding sites for [125I]alpha-bungarotoxin, a selective ligand of alpha7-containing nAChRs, were present in X nucleus but were almost undetectable in VII and XII nuclei. Thus, brainstem motoneurons of young rats possess functional nAChRs. They could promote fast synaptic coupling between motoneurons, and thus play a role in somatic and visceral motor functions.

Alpha3beta4 subunit-containing nicotinic receptors dominate function in rat medial habenula neurons

Regional-specific differences in nicotinic acetylcholine receptors (nAChRs) were examined using the whole-cell patch clamp technique in rat medial habenula (MHb) slices. The majority of cells in the ventral two thirds of the MHb responded robustly to local pressure application of nAChR agonists. Mean agonist potency profiles in the middle and ventral thirds of the MHb were similar: cytisine was the most potent agonist and DMPP the weakest, consistent with a significant contribution of the beta4 subunit to functional nAChRs in all areas of the MHb. In acutely isolated MHb neurons, the alpha3beta4-selective toxin alpha-CTx-AuIB (1 microM) reversibly blocked approximately 75% of the nicotine-induced currents, as expected for cells solely expressing alpha3beta4 nAChRs. However, the alpha3beta2-selective toxin, alpha-CTx-MII (100 nM), blocked a variable fraction (0-90%) of the MHb nicotinic response implying that beta2 subunits may contribute to some functional receptors. We suggest that the effects of alpha-CTx-MII may arise from interaction with alpha3beta2beta4 subunit-containing nAChRs. This idea is supported by the findings (1) that alpha-CTx-MII antagonizes receptors comprised of alpha3, beta2 and beta4 subunits in Xenopus oocytes, and (2) that a mutant alpha-CTx-MII toxin[H12A], which blocks alpha3beta2beta4 receptors but not alpha3beta2 or alpha3beta4 nAChRs, also reduces nicotinic currents in some MHb neurons. Overall these data imply that most functional nAChRs on MHb cells contain at least alpha3 and beta4 subunits, and that a variable subpopulation additionally contains the beta2 subunit.

Highly sensitive radioactive in situ hybridization using full length hydrolyzed riboprobes to detect alpha 2 adrenoceptor subtype mRNAs in adult and developing rat brain

The mRNA expression of highly homologous alpha 2 adrenoceptor subtypes was determined using a highly sensitive in situ hybridization protocol that allowed the detection of low abundance mRNA. Full-length 35S-labeled riboprobes specific for alpha 2A, alpha 2B and alpha 2C adrenoceptors were used for maximal sensitivity. The probes were hydrolyzed to an average length of 600 bp which, in combination with proteinase K digestion, resulted in optimal probe penetration in developmental and adult tissue. The expression intensity could be quantified and the ontogeny of receptor mRNA expression determined. At the same time receptor binding sites or functional proteins could be detected simultaneously in adjacent sections, because fresh frozen and post-fixed tissue was used.

Differential effects of heterogeneous nuclear ribonucleoprotein K on Sp1- and Sp3-mediated transcriptional activation of a neuronal nicotinic acetylcholine receptor promoter

The neuronal nicotinic acetylcholine receptor gene family consists of 11 members, alpha2-alpha9 and beta2-beta4. Three of the genes, those encoding the alpha3, alpha5, and beta4 subunits, are clustered tightly within the genome. These three subunits constitute the predominant acetylcholine receptor subtype expressed in the peripheral nervous system. The genomic proximity of the three genes suggests a regulatory mechanism ensuring their coordinate expression. However, it is likely that gene-specific regulatory mechanisms are also functioning because the expression patterns of the three genes, although similar, are not identical. Previously we identified regulatory elements within the beta4 promoter region and demonstrated that these elements interact specifically with nuclear proteins. One of these elements, E1, interacts with the regulatory factor Puralpha as well as three other unidentified DNA-binding proteins with molecular masses of 31, 65, and 114 kDa. Another element, E2, interacts with Sp1 and Sp3. Because E1 and E2 are immediately adjacent to one another, we postulated that the proteins that bind to the elements interact to regulate beta4 gene expression. Here we report the identification of the 65-kDa E1-binding protein as heterogeneous nuclear ribonucleoprotein K and demonstrate that it affects the transactivation of beta4 promoter activity by Sp1 and Sp3 differentially.

Non-acetylcholine displaceable 3[H]epibatidine binding in the rat brain

Using autoradiography two types of 3[H]epibatidine binding in rat brain was found, the by far most widespread was nicotine displaceable, and represented nicotinic cholinergic receptors, probably the two subtypes alpha4beta2 and alpha3beta4. Using acetylcholine and nicotine in concentrations able to displace 3[H]epibatidine from the alpha4beta2 and alpha3beta4 subunits, in a few brain structures a non-acetylcholine and non-nicotine displaceable 3[H]epibatidine binding was found. The binding was almost exclusively localised to the medial and lateral habenula, interpeduncular nucleus and pineal gland; at 0.1 nM 3[H]epibatidine it represented about 40% of the total 3[H]epibatidine binding. A literature search directed at neurotransmitters in habenula did not demonstrate any neurotransmitter with this anatomical distribution in the brain, suggesting that the non-acetylcholine displaceable 3[H]epibatidine binding represents a binding site, not previously described.

Allosteric nicotinic receptors, human pathologies

Nicotinic acetylcholine receptors are ligand-gated ion channels present in muscle and brain. These allosteric oligomers may exist in several conformational states which include a resting state, an open-channel state, and a desensitized refractory state. Recent work has shown that point mutations in the nicotinic receptor may, altogether, abolish desensitization, increase apparent affinity for agonists and convert the effect of a competitive antagonist into an agonist response. These pleiotropic effects are interpreted in terms of the allosteric model. This paper reviews recent evidence that such mutations occur spontaneously in humans and may cause diseases such as congenital myasthenia or familial frontal lobe epilepsy. In addition, nicotinic receptors are involved in tobacco smoking. Accumulating evidence, including experiments with knock-out animals, indicates that addiction to nicotine is linked to the activation of beta 2-subunit containing nicotinic receptors in the dopaminergic mesolimbic neurons which are part of the reward systems in the brain. Current research also indicates that nicotinic agonists might serve as therapeutic agents for Alzheimer's disease and Tourette's syndrome, as well as for schizophrenia. This paper extends and updates a recently published review.