Effects of Pertussis Toxin Suggest a Role for G-Proteins in the Inhibition of Acetylcholine Release from Rat Myenteric Plexus by Opioid and Presynaptic Muscarinic Receptors

Functional expression of choline transporter-like protein 1 (CTL1) in human neuroblastoma cells and its link to acetylcholine synthesis

We examined the molecular and functional characterization of choline uptake into human neuroblastoma cell lines (SH-SY5Y: non-cholinergic and LA-N-2: cholinergic neuroblastoma), and the association between choline transport and acetylcholine (ACh) synthesis in these cells. Choline uptake was saturable and mediated by a single transport system. Removal of Na(+) from the uptake buffer strongly enhanced choline uptake. Choline uptake was inhibited by the choline analogue hemicholinium-3 (HC-3) and various organic cations, and was significantly decreased by acidification of the extracellular medium. The increase in choline uptake under Na(+)-free conditions was inhibited by a Na(+)/H(+) exchanger (NHE) inhibitor. Real-time PCR revealed that choline transporter-like protein 1 (CTL1), NHE1 and NHE5 mRNA are mainly expressed. Western blot and immunocytochemical analysis indicated that CTL1 protein was expressed in plasma membrane. ChAT mRNA was expressed at a much higher level in LA-N-2 cells than in SH-SY5Y cells. The conversion of choline to ACh was confirmed in both cells, and was enhanced in Na(+)-free conditions. These findings suggest that CTL1 is functionally expressed in both SH-SY5Y and LA-N-2 cells and is responsible for choline uptake that relies on a directed H(+) gradient as a driving force, and this transport functions in co-operation with NHE1 and NHE5. Furthermore, choline uptake through CTL1 is associated with ACh synthesis in cholinergic neuroblastoma cells.

Calcium channels involved in the inhibition of acetylcholine release by presynaptic muscarinic receptors in rat striatum

1. The mechanism of the inhibitory action of presynaptic muscarinic receptors on the release of acetylcholine from striatal cholinergic neurons is not known. We investigated how the electrically stimulated release of [3H]-acetylcholine from superfused rat striatal slices and its inhibition by carbachol are affected by specific inhibitors of voltage-operated calcium channels of the L-type (nifedipine), N-type (omega-conotoxin GVIA) and P/Q-type (omega-agatoxin IVA). 2. The evoked release of [3H]-acetylcholine was not diminished by nifedipine but was lowered by omega-conotoxin GVIA and by omega-agatoxin IVA, indicating that both the N- and the P/Q-type (but not the L-type) channels are involved in the release. The N-type channels were responsible for approximately two thirds of the release. The release was >97% blocked when both omega-toxins acted together. 3. The inhibition of [3H]-acetylcholine release by carbachol was not substantially affected by the blockade of the L- or P/Q-type channels. It was diminished but not eliminated by the blockade of the N-type channels. 4. In experiments on slices in which cholinesterases had been inhibited by paraoxon, inhibition of [3H]-acetylcholine release by endogenous acetylcholine accumulating in the tissue could be demonstrated by the enhancement of the release after the addition of atropine. The inhibition was higher in slices with functional N-type than with functional P/Q-type channels. 5. We conclude that both the N- and the P/Q-type calcium channels contribute to the stimulation-evoked release of acetylcholine in rat striatum, that the quantitative contribution of the N-type channels is higher, and that the inhibitory muscarinic receptors are more closely coupled with the N-type than with the P/Q-type calcium channels.

Pertussis toxin increases isoproterenol induced relaxation in field-stimulated ileum

We investigated the effects of pertussis toxin on contraction in field-stimulated guinea pig ileum in the absence and presence of isoproterenol. Field-stimulation elicited pertussis toxin-insensitive contractions. Cumulative addition of isoproterenol produced a maximal 52% reduction in the contractile response. Following pertussis toxin-treatment, the maximal inhibitory effect of isoproterenol increased to 83%. Pertussis toxin had no effect on the ability isoproterenol to inhibit contractions elicited by histamine agonists. Our results suggest that the increased effectiveness of isoproterenol in pertussis toxin-treated ileum is due to an uncoupling of the muscarinic M2 receptor contractile mechanism.

The effects of brucine and alcuronium on the inhibition of [3H]acetylcholine release from rat striatum by muscarinic receptor agonists

1. Radioligand binding experiments indicate that the affinity of muscarinic receptors for their agonists may be enhanced by allosteric modulators. We have now investigated if brucine can enhance the inhibitory effects of muscarinic receptor agonists on the electrically evoked release of [3H]acetylcholine ([3H]ACh) from superfused slices of rat striatum. 2. The evoked release of [3H]ACh was inhibited by all agonists tested (i.e., furmethide, oxotremorine-M, bethanechol and oxotremorine). 3. Brucine enhanced the inhibitory effects of furmethide, oxotremorine-M and bethanechol on the evoked [3H]ACh release without altering the inhibitory effect of oxotremorine. 4. Alcuronium was applied for comparison and found to diminish the inhibitory effect of furmethide on the evoked [3H]ACh release. 5. The results demonstrate that it is possible both to enhance and diminish the functional effects of muscarinic receptor agonists by allosteric modulators. 6. The direction of the observed effects of brucine and alcuronium on [3H]ACh release fully agrees with the effects of these modulators on the affinities of human M4 receptors for furmethide, oxotremorine-M, bethanechol and oxotremorine, as described by Jakubik et al. (1997). This supports the view that the presynaptic muscarinic receptors responsible for the autoinhibition of ACh release in rat striatum belong to the M4 muscarinic receptor subtype.

Direct interaction of gbetagamma with a C-terminal gbetagamma-binding domain of the Ca2+ channel alpha1 subunit is responsible for channel inhibition by G protein-coupled receptors

Several classes of voltage-gated Ca2+ channels (VGCCs) are inhibited by G proteins activated by receptors for neurotransmitters and neuromodulatory peptides. Evidence has accumulated to indicate that for non-L-type Ca2+ channels the executing arm of the activated G protein is its betagamma dimer (Gbetagamma). We report below the existence of two Gbetagamma-binding sites on the A-, B-, and E-type alpha1 subunits that form non-L-type Ca2+ channels. One, reported previously, is in loop 1 connecting transmembrane domains I and II. The second is located approximately in the middle of the ca. 600-aa-long C-terminal tails. Both Gbetagamma-binding regions also bind the Ca2+ channel beta subunit (CCbeta), which, when overexpressed, interferes with inhibition by activated G proteins. Replacement in alpha1E of loop 1 with that of the G protein-insensitive and Gbetagamma-binding-negative loop 1 of alpha1C did not abolish inhibition by G proteins, but the exchange of the alpha1E C terminus with that of alpha1C did. This and properties of alpha1E C-terminal truncations indicated that the Gbetagamma-binding site mediating the inhibition of Ca2+ channel activity is the one in the C terminus. Binding of Gbetagamma to this site was inhibited by an alpha1-binding domain of CCbeta, thus providing an explanation for the functional antagonism existing between CCbeta and G protein inhibition. The data do not support proposals that Gbetagamma inhibits alpha1 function by interacting with the site located in the loop I-II linker. These results define the molecular mechanism by which presynaptic G protein-coupled receptors inhibit neurotransmission.

3,4-Diaminopyridine masks the inhibition of noradrenaline release from chick sympathetic neurons via presynaptic alpha 2-adrenoceptors: insights into the role of N- and L-type calcium channels

We have investigated the participation of the N-type (omega-conotoxin GVIA-sensitive) and L-type (nifedipine-sensitive) calcium channels in the alpha 2-adrenoceptor mediated autoinhibition of the release of [3H]noradrenaline from chick sympathetic neurons in culture. Blockade of 3,4-diaminopyridine-sensitive potassium channels resulted in tetrodotoxin-sensitive and calcium-dependent increase of the release of [3H]noradrenaline evoked by electrical stimulation. Nifedipine attenuated the evoked release under control conditions by 20%, but in the presence of 3,4-diaminopyridine by 51%, while omega-conotoxin decreased the release under control conditions by 87% and in the presence of 3,4-diaminopyridine by only 43%. The L-type calcium channel activator Bay k 8644 increased the evoked release of the transmitter both in the absence and in the presence of 3,4-diaminopyridine. Under control conditions, the alpha 2-adrenoceptor agonist UK 14304 decreased the evoked release by 57% and the alpha 2-adrenoceptor antagonist rauwolscine increased it by 14%. Nifedipine did not prevent this modulation. In the presence of 3,4-diaminopyridine, UK 14304 lost its effect on the release of noradrenaline, but its inhibitory action was restored when nifedipine, but not omega-conotoxin, was added. Changes in the increase of intracellular calcium concentration ([Ca2+]i) evoked by electrical stimulation, measured in the cell processes by microfluorimetry, paralleled the changes in the release of [3H]noradrenaline. Under control conditions, nifedipine attenuated the rise of intracellular calcium by only 16%, while omega-conotoxin did so by 66%. 3,4-Diaminopyridine enhanced the evoked rise of [Ca2+]i; in its presence the rise of intracellular calcium was about equally reduced by nifedipine and omega-conotoxin (by 46 and 36%, respectively). These effects were additive. UK 14304 diminished the peak concentration of [Ca2+]i elicited by the standard electrical stimulation by 31% and rauwolscine antagonised this effect. UK 14304 did not measurably inhibit the stimulation-evoked rise of intraterminal [Ca2+]i in the presence of 3,4-diaminopyridine but it produced an inhibition by 26% if nifedipine had been applied together with 3,4-diaminopyridine. Our observations show that, under control conditions, the stimulated release of [3H]noradrenaline is mainly associated with the opening of N-type channels, while in the presence of 3,4-diaminopyridine the contribution of L-type channels becomes more important. The alpha 2-adrenoceptor stimulation by UK 14304 inhibits the release of [3H]noradrenaline but, in the presence of 3,4-diaminopyridine, the inhibition of release can only be observed if the massive influx through L-type calcium channels is prevented. These data suggest that presynaptic alpha 2-adrenoceptors of chick sympathetic neurons preferentially influence the N-type calcium channels.

Muscarine receptors regulating electrically evoked release of acetylcholine in hippocampus are linked to pertussis toxin-sensitive G proteins but not to adenylate cyclase

[3H]Acetylcholine release elicited with 360 pulses/3 Hz from slices of rabbit hippocampus is facilitated in the presence of the muscarine (M) receptor antagonist atropine (indicating the existence of autoinhibition) and diminished by the M receptor agonists carbachol and oxotremorine. N-Ethylmaleimide (30 microM) and pertussis toxin (8 micrograms/ml) counteracted antagonist-induced facilitation and agonist-induced inhibition of release, suggesting that a pertussis toxin-sensitive GTP-binding protein is involved in the chain of events mediating activation of M receptors to inhibition of release. Neither 8-bromo-cyclic AMP (300 microM), a membrane analogue of cyclic AMP, nor rolipram (10 microM), a phosphodiesterase inhibitor, affected electrically evoked release of [3H]acetylcholine. They also did not influence the oxotremorine-induced inhibition of transmitter release. In conclusion, no evidence was found for the assumption that activation of M autoreceptors is linked to inhibition of adenylate cyclase.

Presynaptic muscarinic receptors and the release of acetylcholine from cerebrocortical prisms: roles of Ca2+ and K+ concentrations

The mechanism by which presynaptic muscarinic autoreceptors inhibit the release of acetylcholine (ACh) from cerebrocortical cholinergic fibres has not been clarified. To test the view that muscarinic autoreceptors act by decreasing Ca2+ influx, we performed experiments in which rat cerebrocortical prisms were preloaded with (14C)choline, washed, depolarized with 14-65 mM K+ in the absence of Ca2+ and then exposed (still under depolarization) to various concentrations of Ca2+ to evoke the release of (14C)ACh. The muscarinic agonist, oxotremorine, used at a 100 microM concentration, inhibited the release of (14C)ACh by 59-86% in experiments with 14 and 26.5 mM K+ but had no significant effect at 65.5 mM K+. No systematic changes in the inhibitory effects of oxotremorine could be found at any of the K+ concentrations used when the concentration of Ca2+ was varied in the range of 0.25-4.0 mM. At 2 mM Ca2+ and K+ concentrations above 14 mM, the inhibitory effect of oxotremorine was inversely related to the concentration of K+. The inhibitory effect of oxotremorine on (14C)ACh release was not blocked by 100 microM 4-amino-pyridine. The fact that the inhibitory effect of oxotremorine could not be overcome by an increase in the concentration of Ca2+ suggests that, under the conditions used, a restriction of the influx of Ca2+ did not play a major role in the muscarinic inhibition of ACh release; rather, oxotremorine appeared to act by decreasing membrane depolarization.2+ of the Ca(2+)-voltage hypothesis of neurotransmitter release, supposing