Modulation of nicotinic acetylcholine receptor activity in submucous neurons by intracellular messengers

Alzheimer's disease: Targeting the Cholinergic System

Acetylcholine (ACh) has a crucial role in the peripheral and central nervous systems. The enzyme choline acetyltransferase (ChAT) is responsible for synthesizing ACh from acetyl-CoA and choline in the cytoplasm and the vesicular acetylcholine transporter (VAChT) uptakes the neurotransmitter into synaptic vesicles. Following depolarization, ACh undergoes exocytosis reaching the synaptic cleft, where it can bind its receptors, including muscarinic and nicotinic receptors. ACh present at the synaptic cleft is promptly hydrolyzed by the enzyme acetylcholinesterase (AChE), forming acetate and choline, which is recycled into the presynaptic nerve terminal by the high-affinity choline transporter (CHT1). Cholinergic neurons located in the basal forebrain, including the neurons that form the nucleus basalis of Meynert, are severely lost in Alzheimer’s disease (AD). AD is the most ordinary cause of dementia affecting 25 million people worldwide. The hallmarks of the disease are the accumulation of neurofibrillary tangles and amyloid plaques. However, there is no real correlation between levels of cortical plaques and AD-related cognitive impairment. Nevertheless, synaptic loss is the principal correlate of disease progression and loss of cholinergic neurons contributes to memory and attention deficits. Thus, drugs that act on the cholinergic system represent a promising option to treat AD patients.

Rapid upregulation of alpha7 nicotinic acetylcholine receptors by tyrosine dephosphorylation

Alpha7 nicotinic acetylcholine receptors (nAChRs) modulate network activity in the CNS. Thus, functional regulation of alpha7 nAChRs could influence the flow of information through various brain nuclei. It is hypothesized here that these receptors are amenable to modulation by tyrosine phosphorylation. In both Xenopus oocytes and rat hippocampal interneurons, brief exposure to a broad-spectrum protein tyrosine kinase inhibitor, genistein, specifically and reversibly potentiated alpha7 nAChR-mediated responses, whereas a protein tyrosine phosphatase inhibitor, pervanadate, caused depression. Potentiation was associated with an increased expression of surface alpha7 subunits and was not accompanied by detectable changes in receptor open probability, implying that the increased function results from an increased number of alpha7 nAChRs. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated exocytosis was shown to be a plausible mechanism for the rapid delivery of additional alpha7 nAChRs to the plasma membrane. Direct phosphorylation/dephosphorylation of alpha7 subunits was unlikely because mutation of all three cytoplasmic tyrosine residues did not prevent the genistein-mediated facilitation. Overall, these data are consistent with the hypothesis that the number of functional cell surface alpha7 nAChRs is controlled indirectly via processes involving tyrosine phosphorylation.

Distribution of neuronal nicotinic acetylcholine receptors containing different alpha-subunits in the submucosal plexus of the guinea-pig

The subunit composition and localisation of nicotinic acetylcholine receptors (nAChRs) in the submucosal plexus of the guinea-pig ileum were studied using both affinity-purified monoclonal and polyclonal antibodies against alpha3, alpha4, alpha5 and alpha7 nAChR subunits and specific alpha7-containing nAChRs blocker methyllycaconitine (MLA). By means of immunohistochemistry performed in non-dissociated preparations, it was found that only 4% of submucosal ganglia expressed nAChRs. Specific staining, associated with cell membranes, was found with alpha3-, alpha5- and alpha7-, but not alpha4-specific antibodies. Double staining using alpha5- and alpha7-specific antibodies demonstrated that about one-half of the nAChR-positive ganglia contained neurons immunoreactive to both antibodies, while the others possessed either alpha5- or alpha7-immunoreactivity. Nanomolar concentrations of MLA prevented alpha7-specific antibody binding and did not influence the alpha5-specific antibody binding even when applied in micromolar concentrations. In electrophysiological experiments performed using a patch-clamp 'whole-cell' recording method, the neurons were identified by their sensitivity to MLA. In conclusion, submucosal neurons of the guinea-pig ileum express nAChRs containing alpha3-, alpha5- and alpha7-subunits. The co localisation of alpha5- and alpha7-subunits found in immunohistochemical experiments as well as kinetic characteristics of MLA-blocked receptors found by electrophysiological experiments allow us to suggest the presence of heteromeric alpha7-containing nAChRs in the submucosal plexus of the guinea-pig ileum.

Differential modulation of nicotinic acetylcholine receptor subtypes and synaptic transmission in chick sympathetic ganglia by PGE(2)

The diversity of neuronal nicotinic acetylcholine receptors (nAChRs) is likely an important factor in the modulation of synaptic transmission by acetylcholine and nicotine. We have tested whether postsynaptic nAChRs are modulated in a subtype-specific manner by prostaglandin E(2) (PGE(2)), a regulator of neuronal excitability in both the central and peripheral nervous systems, and examined the effects of PGE(2) on nicotinic transmission. Somatodendritic nAChRs in chick lumbar sympathetic ganglia include four nAChR subtypes distinguished on the basis of conductance and kinetic profile. Nanomolar PGE(2) applied to the extrapatch membrane differentially regulates opening probability (Po), frequency and the opening duration of each nAChR channel subtype in cell-attached patches. PGE(2) decreases the Po of the predominant nAChR subtype (36 pS) and significantly increases Po and open duration of the 23 pS subtype. The 23 pS subtype is gated by the alpha 7-selective agonist choline, and choline-gated currents are inhibited by alpha-bungarotoxin. To examine whether PGE(2) modulates nAChRs at synaptic sites, we studied the effects of PGE(2) on amplitude and decay of synaptic currents in visceral motoneuron-sympathetic neuron co-cultures. PGE(2) significantly decreases the amplitude of miniature excitatory postsynaptic currents (mEPSCs), consistent with the predominant inhibition by PGE(2) of all but the 23 pS subtype. The time constant of mEPSCs at PGE(2)-treated synapses is prolonged, which is also consistent with an increased contribution of the longer open duration of the 23 pS nAChR subtype with PGE(2) treatment. To examine the presynaptic effect of PGE(2), nanomolar nicotine was used. Nicotine induces facilitation of synaptic transmission by increasing mEPSC frequency, an action thought to involve presynaptic, alpha 7-containing nAChRs. In the presence of PGE(2), nicotine-induced synaptic facilitation persists. Thus the net effect of PGE(2) is to alter the profile of nAChRs contributing to synaptic transmission from larger conductance, briefer opening channels to smaller conductance, longer opening events. This subtype-specific modulation of nAChRs by PGE(2) may provide a mechanism for selective activation and suppression of synaptic pathways mediated by different nAChR subtype(s) at both pre- and postsynaptic sites.

Action of polygodial on agonist-induced contractions of the rat portal vein in vitro

This study investigated the vasorelaxant action of the sesquiterpene polygodial, isolated from the bark of Drymis winteri, on rat portal vein in vitro, contracted by various agonists. Polygodial (21-342 microM) preincubated 20 min before, produced graded antagonism of the contractile responses caused by bradykinin, endothelin-1, noradrenaline, the stable analogue of thromboxane A2 U46619, substance P, neurokinin B, and senktide (an NK3-selective agonist). Polygodial, at the same concentration, also produced graded inhibition of the contractile response induced by potassium chloride and by phorbol ester. At the median inhibitory concentration (IC50) level, polygodial was approximately 114- to 177-fold more active in inhibiting mediated contractions to senktide and phorbol ester. When assessed in the tonic contraction induced by endothelin-1 (0.5 nM) or by phorbol (3 microM), polygodial (0.1-100 microM) produced concentration-dependent relaxation, with maximal inhibition (E(max)) of 62 +/- 2% and 100%, respectively. Finally, polygodial (0.1-100 microM) inhibited the rhythmic spontaneous contractions of the rat portal vein (E(max) of 75 +/- 2%). Taken together, these results suggest that the vasorelaxant actions caused by polygodial in rat portal vein are, at least in part, associated with inhibition of calcium influx through voltage-sensitive channels and interaction with protein kinase C-dependent mechanisms. In addition, these data confirm and extend our previous suggestion that polygodial preferentially antagonizes tachykinin-mediated contraction, especially the NK3-mediated responses.