Block by 5-hydroxytryptamine and apomorphine of recombinant human neuronal nicotinic receptors

Dopamine triggers skeletal muscle tone by activating D1-like receptors on somatic motoneurons

The dopamine system plays an integral role in motor physiology. Dopamine controls movement by modulation of higher-order motor centers (e.g., basal ganglia) but may also regulate movement by directly controlling motoneuron function. Even though dopamine cells synapse onto motoneurons, which themselves express dopamine receptors, it is unknown whether dopamine modulates skeletal muscle activity. Therefore, we aimed to determine whether changes in dopaminergic neurotransmission at a somatic motor pool affect motor outflow to skeletal muscles. We used microinjection, neuropharmacology, electrophysiology, and histology to determine whether manipulation of D(1)- and D(2)-like receptors on trigeminal motoneurons affects masseter and/or tensor palatini muscle tone in anesthetized rats. We found that apomorphine (a dopamine analog) activated trigeminal motoneurons and triggered a potent increase in both masseter and tensor palatini tone. This excitatory effect is mediated by D(1)-like receptors because specific D(1)-like receptor activation strengthened muscle tone and blockade of these receptors prevented dopamine-driven activation of motoneurons. Blockade of D(1)-like receptors alone had no detectable effect on basal masseter/tensor palatini tone, indicating the absence of a functional dopamine drive onto trigeminal motoneurons, at least during isoflurane anesthesia. Finally, we showed that D(2)-like receptors do not affect either trigeminal motoneuron function or masseter/tensor palatini muscle tone. Our results provide the first demonstration that dopamine can directly control movement by manipulating somatic motoneuron behavior and skeletal muscle tone.

5-Hydroxytryptamine directly inhibits neuronal nicotinic acetylcholine receptors in rat trigeminal ganglion neurons

In the present study, whole-cell patch clamp recording technique was used to investigate the action of 5-hydroxytryptamine (5-HT) on the function of native neuronal nicotinic acetylcholine receptors expressed in the rat trigeminal ganglion neurons. Inward currents (I(nic)) caused by externally-applied nicotine were observed in majority of the examined neurons, which were mediated by alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors. We found that 5-HT could reversibly inhibit I(nic) in a concentration-dependent manner, and the inhibition did not involve 5-HT receptors. Other serotonergic agents, such as 2-methyl-5-HT, alpha-methyl-5-HT, sumatriptan and ICS-205,930, also had similar inhibitory effects on I(nic). 5-HT inhibited nicotinic acetylcholine receptors in a non-competitive manner, as 5-HT decreased the maximal current response to nicotine but had no effect on the threshold and EC(50). The inhibition of I(nic) by 5-HT was voltage-dependent and became stronger at hyperpolarized potentials. These results indicated that 5-HT directly inhibited nicotinic acetylcholine receptors in the trigeminal ganglion neurons. As a local modulator of the nicotinic acetylcholine receptor, 5-HT might play a role in the modulation of sensory information.

Antiemetics of the 5-hydroxytryptamine 3A antagonist class inhibit muscle nicotinic acetylcholine receptors

Antagonists of the serotonergic 5-hydroxytryptamine 3A receptor (5-HT(3A)R) and muscle nicotinic acetylcholine receptors (nAChR) are widely used in anesthesia practice. Both 5-HT(3A)R and nAChR are ligand-gated ion channels with known pharmacological overlap between some of their agonists and antagonists. We studied the actions of clinically used 5-HT(3A)R antagonist antiemetics and nondepolarizing muscle blockers on ionic currents elicited by the activation of mammalian 5-HT(3A)R and muscle nAChR, expressed in Xenopus laevis oocytes. Currents were recorded using a whole-cell two-electrode voltage clamp technique. Dolasetron, ondansetron, and granisetron reversibly inhibited 5-HT(3A)R function at nanomolar concentrations with 50% inhibitory concentrations (IC(50)) of 11.8, 6.4, and 0.2 nM; the rank order of inhibition correlated well with their clinical antiemetic potencies. The principal metabolite of dolasetron, hydrodolasetron, was 40 times more potent than the parent compound on 5-HT(3A)R (IC(50) = 0.29 nM). The potency of the nondepolarizing muscle blocker d-tubocurarine in blocking 5-HT(3A)R was similar to that of the antiemetics and significantly more than vecuronium and rapacuronium (IC(50) = 11.4 nM, 18.9 microM, 60.5 microM). Conversely, ondansetron, dolasetron, and granisetron also reversibly inhibited nAChR currents in a dose-dependent manner with IC(50)s of 14.2, 7.8, and 4.4 microM for the adult nAChR and 16.0, 18.6, and 13.9 microM for the embryonic nAChR. Again, hydrodolasetron showed significantly (10 times) more inhibitory potency on the adult nAChR than the parent compound dolasetron. These results indicate that drugs that target specific ligand-gated ion channels may also affect other ion channel types.

Block by phytoestrogens of recombinant human neuronal nicotinic receptors

The effects of phytoestrogens on neuronal nicotinic acetylcholine receptor/channels were examined by expressing recombinant channels in Xenopus oocytes. When functional channels were expressed with human alpha4 and beta2 subunits, daidzein (10 and 100 microM) partially inhibited the ionic current activated by acetylcholine. The current inhibition was also observed when functional channels were expressed with human alpha3 and beta4 subunits or rat homologues. Genistin (100 microM) also inhibited the acetylcholine-activated current. Tamoxifen (100 microM), an antiestrogen did not antagonize the inhibition by daidzein. The results suggest that phytoestrogens, like estrogens and xenoestrogens, block human neuronal acetylcholine receptors through non-genomic mechanisms.

Octopaminergic modulation of synaptic transmission between an identified sensory afferent and flight motoneuron in the locust

The role of the biogenic amine octopamine in modulating cholinergic synaptic transmission between the locust forewing stretch receptor neuron (fSR) and the first basalar motoneuron (BA1) was investigated. The amines 5-hydroxytryptamine (5-HT, serotonin) and dopamine were also studied. Bath application of octopamine, 5-HT, and dopamine at concentrations of 10(-4) M reversibly decreased the amplitude of monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in BA1 by electrically stimulating the fSR axon. These effects occurred without any detectable change in either input resistance or membrane potential of BA1. The amines also reversibly decreased the amplitude of responses to acetylcholine (ACh) pressure-applied to the soma of BA1. The muscarinic antagonist scopolamine (10(-6) M) had no significant effect on the octopamine-induced decrease in ACh responses. These observations suggest that these amines potentially could physiologically depress cholinergic transmission between fSR and BA1, at least in part, by altering nicotinic rather than muscarinic cholinergic receptor function. Although the octopaminergic agonists naphazoline and tolazoline both mimicked the actions of octopamine, the receptor responsible for octopamine-mediated modulation could not be characterized since amine receptor antagonists tested on the preparation had complex actions. Confocal immunocytochemistry revealed intense octopamine immunoreactivity in the anterior lateral association center, thus confirming the presence of octopamine in neuropil regions containing fSR/BA1 synapses and therefore supporting a role for this amine in the modulation of synaptic transmission between the fSR and BA1. 5-HT-immunoreactivity, conversely, was concentrated within the ventral association centers; very little staining was observed in the dorsal neuropil regions in which fSR/BA1 synapses are located.

Modulation by 5-hydroxytryptamine of nicotinic acetylcholine responses recorded from an identified cockroach (Periplaneta americana) motoneuron

Recordings from the soma of the cockroach (Periplaneta americana) fast coxal depressor motoneuron (Df) were made while acetylcholine (ACh) was regularly pressure-applied locally from a micropipette. The modulatory effects upon these nicotinic ACh responses of bath-applied 5-hydroxytryptamine (5-HT, serotonin), dopamine and octopamine were investigated under either current-clamp or voltage-clamp conditions. The biogenic amines reversibly suppressed, but never totally abolished, ACh responses, 5-HT being the most potent, with a threshold near 10(-6) m (EC50 = 5 x 10(-5) m). Occlusion experiments indicate that the amines share a common mechanism at the level of either receptors or second messenger pathways. The amines also modulated responses to nicotine or carbachol (each of which resists hydrolysis by acetylcholinesterases), indicating that the amines did not act by accelerating ACh degradation. Pharmacological antagonists were used in an attempt to characterize the receptor responsible for amine-mediated modulation. Although a number of antagonists mimicked the action of amines rather than producing blockade, the antagonistic actions of LSD and RS23597 pointed strongly to a receptor-mediated mechanism, but did not allow receptor identification. The magnitude of the modulatory effect of 5-HT was significantly reduced by intracellular guanosine-5'-O-(2-thiodiphosphate) (GDP-beta-S), indicating involvement of a G-protein. Intracellular injection of the calcium chelator BAPTA did not block the modulatory effect of 5-HT, showing that the amines do not operate through the calcium-dependent pathway by which muscarinic receptors act on nicotinic currents. The adenylate cyclase inhibitor dideoxyadenosine (DDA), on the other hand, did attenuate the action of 5-HT, suggesting involvement of cyclic AMP.

Modulation by estrogens and xenoestrogens of recombinant human neuronal nicotinic receptors

The effects of estrogens and xenoestrogens on human neuronal nicotinic acetylcholine receptor/channels were examined by expressing recombinant channels in Xenopus oocytes. When functional channels were expressed with alpha3 and beta4 subunits, estrogens (17beta-estradiol, 17alpha-estradiol, 17alpha-ethynylestradiol and diethylstilbestrol) and xenoestrogens (bisphenol A, p-nonylphenol and p-octylphenol) inhibited an ionic current activated by acetylcholine at concentrations up to 100 microM. When the subunit combination was changed to alpha4beta2, diethystilbestrol and the xenoestrogens inhibited the acetylcholine-activated current, but 17beta-estradiol or 17alpha-estradiol did not. For 17alpha-ethynylestradiol, the current through the alpha4beta2 receptor/channel was inhibited at 1 microM, but it was markedly enhanced at 10 and 100 microM. Tamoxifen (10 microM), an antiestrogen, itself inhibited the acetylcholine-activated current but did not antagonize the current modulations induced by the estrogens and the xenoestrogens. These and additional results suggest that human neuronal nicotinic acetylcholine receptors are the targets of non-genomic actions of estrogens and xenoestrogens.

Nicotine attenuates DOI-induced head-twitch response in mice: implications for Tourette syndrome

Tourette syndrome (TS), a chronic neuropsychiatric disorder, is characterized by motor and vocal tics. Preliminary clinical studies indicate possible therapeutic benefits of nicotine in the treatment of Tourette's syndrome (TS). It has been proposed that twitches of the head in mice or twitches of head and shoulders in rats following administration of the selective 5HT(2A/C) agonist DOI (1-)2,5-dimethoxy-4-iodophenyl-2-aminopropane, can serve as an animal model of tics in TS. In this study, the effects of acute and chronic administration of nicotine on DOI-induced head twitch response (HTR) in male albino ICR mice were evaluated. Both acute and chronic nicotine (daily injections for 10 days) reduced the DOI-induced HTR. Moreover, chronic administration of DOI (1 mg/kg/day for 10 days) resulted in 65% increase in [125I]alpha-bungarotoxin binding in cerebellum and 41% increase in striatal [3H]cytisine binding. However, the acute inhibitory effects of nicotine were not blocked by pretreatment with the nicotinic antagonist, mecamylamine. Indeed, at higher doses, mecamylamine also reduced the DOI-induced HTR. The data suggest that both nicotine and mecamylamine may be of therapeutic potential in the treatment of some symptoms of TS.

Localization of agonist and competitive antagonist binding sites on nicotinic acetylcholine receptors

Identification of all residues involved in the recognition and binding of cholinergic ligands (e.g. agonists, competitive antagonists, and noncompetitive agonists) is a primary objective to understand which structural components are related to the physiological function of the nicotinic acetylcholine receptor (AChR). The picture for the localization of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are located mainly on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are identical, the observed high and low affinity for different ligands on the receptor is conditioned by the interaction of the alpha subunit with other non-alpha subunits. This molecular interaction takes place at the interface formed by the different subunits. For example, the high-affinity acetylcholine (ACh) binding site of the muscle-type AChR is located on the alphadelta subunit interface, whereas the low-affinity ACh binding site is located on the alphagamma subunit interface. Regarding homomeric AChRs (e.g. alpha7, alpha8, and alpha9), up to five binding sites may be located on the alphaalpha subunit interfaces. From the point of view of subunit arrangement, the gamma subunit is in between both alpha subunits and the delta subunit follows the alpha aligned in a clockwise manner from the gamma. Although some competitive antagonists such as lophotoxin and alpha-bungarotoxin bind to the same high- and low-affinity sites as ACh, other cholinergic drugs may bind with opposite specificity. For instance, the location of the high- and the low-affinity binding site for curare-related drugs as well as for agonists such as the alkaloid nicotine and the potent analgesic epibatidine (only when the AChR is in the desensitized state) is determined by the alphagamma and the alphadelta subunit interface, respectively. The case of alpha-conotoxins (alpha-CoTxs) is unique since each alpha-CoTx from different species is recognized by a specific AChR type. In addition, the specificity of alpha-CoTxs for each subunit interface is species-dependent. In general terms we may state that both alpha subunits carry the principal component for the agonist/competitive antagonist binding sites, whereas the non-alpha subunits bear the complementary component. Concerning homomeric AChRs, both the principal and the complementary component exist on the alpha subunit. The principal component on the muscle-type AChR involves three loops-forming binding domains (loops A-C). Loop A (from mouse sequence) is mainly formed by residue Y(93), loop B is molded by amino acids W(149), Y(152), and probably G(153), while loop C is shaped by residues Y(190), C(192), C(193), and Y(198). The complementary component corresponding to each non-alpha subunit probably contributes with at least four loops. More specifically, the loops at the gamma subunit are: loop D which is formed by residue K(34), loop E that is designed by W(55) and E(57), loop F which is built by a stretch of amino acids comprising L(109), S(111), C(115), I(116), and Y(117), and finally loop G that is shaped by F(172) and by the negatively-charged amino acids D(174) and E(183). The complementary component on the delta subunit, which corresponds to the high-affinity ACh binding site, is formed by homologous loops. Regarding alpha-neurotoxins, several snake and alpha-CoTxs bear specific residues that are energetically coupled with their corresponding pairs on the AChR binding site. The principal component for snake alpha-neurotoxins is located on the residue sequence alpha1W(184)-D(200), which includes loop C. In addition, amino acid sequence 55-74 from the alpha1 subunit (which includes loop E), and residues gammaL(119) (close to loop F) and gammaE(176) (close to loop G) at the low-affinity binding site, or deltaL(121) (close to the homologous region of loop G) at the high-affinity binding site, are i

5-hydroxytryptamine interaction with the nicotinic acetylcholine receptor

The present study examines the interaction of the neurotransmitter 5-hydroxytryptamine (5-HT) with muscle-type nicotinic acetylcholine receptors. 5-HT inhibits the initial rate of [125I]alpha-bungarotoxin binding to Torpedo acetylcholine receptor membranes (IC(50)=8.5+/-0.32 mM) and [3H]5-HT can be photoincorporated into acetylcholine receptor subunits, with labeling of the alpha-subunit inhibitable by both agonists and competitive antagonists. Within the agonist-binding domain, [3H]5-HT photoincorporates into alphaTyr(190), alphaCys(192) and alphaCys(193). Functional studies using the human clonal cell line TE671/RD, show that 5-HT is a weak inhibitor (IC(50)=1.55+/-0.25 mM) of acetylcholine receptor activity. In this regard, agonist-response profiles in the absence and presence of 5-HT indicate a noncompetitive mode of inhibition. In addition, 5-HT displaces high affinity [3H]thienylcyclohexylpiperidine binding to the desensitized Torpedo acetylcholine receptor channel (IC(50)=1.61+/-0.07 mM). Collectively, these results indicate that 5-HT interacts weakly with the agonist recognition site and inhibits receptor function noncompetitively by binding to the acetylcholine receptor channel.