An action of 5-hydroxytryptamine on the frog motor end-plate

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.

Antagonism of nicotinic acetylcholine receptors by inhibitors of monoamine uptake

A study was made of the effects of several monoamine-uptake inhibitors on membrane currents elicited by acetylcholine (ACh-currents) generated by rat neuronal alpha2beta4 and mouse muscle nicotinic acetylcholine receptors (AChRs) expressed in Xenopus laevis oocytes. For the two types of receptors the monoamine-uptake inhibitors reduced the ACh-currents albeit to different degrees. The order of inhibitory potency was norfluoxetine > clomipramine > indatraline > fluoxetine > imipramine > zimelidine > 6-nitro-quipazine > trazodone for neuronal alpha2beta4 AChRs, and norfluoxetine > fluoxetine > imipramine > clomipramine > indatraline > zimelidine > trazodone > 6-nitro-quipazine for muscle AChRs. Thus, the most potent inhibitor was norfluoxetine, whilst the weakest ones were trazodone, 6-nitro-quipazine and zimelidine. Effects of the tricyclic antidepressant imipramine were studied in more detail. Imipramine inhibited reversibly and non-competitively the ACh-current with a similar inhibiting potency for both neuronal alpha2beta4 and muscle AChRs. The half-inhibitory concentrations of imipramine were 3.65 +/- 0.30 microM for neuronal alpha2beta4 and 5.57 +/- 0.19 microM for muscle receptors. The corresponding Hill coefficients were 0.73 and 1.2 respectively. The inhibition of imipramine was slightly voltage-dependent, with electric distances of approximately 0.10 and approximately 0.12 for neuronal alpha2beta4 and muscle AChRs respectively. Moreover, imipramine accelerated the rate of decay of ACh- currents of both muscle and neuronal AChRs. The ACh-current inhibition was stronger when oocytes, expressing neuronal alpha2beta4 or muscle receptors, were preincubated with imipramine alone than when it was applied after the ACh-current had been generated, suggesting that imipramine acts also on non-activated or closed AChRs. We conclude that monoamine-uptake inhibitors reduce ACh-currents and that imipramine regulates reversibly and non- competitively neuronal alpha2beta4 and muscle AChRs through similar mechanisms, perhaps by interacting externally on a non-conducting state of the AChR and by blocking the open receptor-channel complex close to the vestibule of the channel. These studies may be important for understanding the regulation of AChRs as well as for understanding antidepressant- and side-effects of monoamine-uptake inhibitors.

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

Threonine-for-leucine mutation within domain M2 of the neuronal alpha(7) nicotinic receptor converts 5-hydroxytryptamine from antagonist to agonist

A study was made of the effects of 5-hydroxytryptamine (5HT) on homomeric neuronal nicotinic receptors (nAcChoR) expressed in Xenopus oocytes after injection of cDNA encoding the wild-type chicken alpha(7) subunit. Acetylcholine (AcCho) elicited large currents (IAcCho) that were reduced by 5HT in a reversible and dose-dependent manner, with a half-inhibitory concentration (IC50) of 56 microM and a Hill coefficient (nH) of 1.2. The inhibition of IAcCho by 5HT was noncompetitive and voltage independent, a behavior incompatible with a channel blockade mechanism. 5HT alone did not elicit membrane currents in oocytes injected with the wild-type alpha(7) subunit cDNA. In contrast, 5HT elicited membrane currents (I5HT) in oocytes injected with cDNA encoding an alpha(7) mutant subunit with a threonine-for-leucine-247 substitution (L247T alpha(7)). I5HT was inhibited by the potent nicotinic receptor blockers alpha-bungarotoxin (100 nM) and methyllycaconitine (1 microM). Furthermore, the characteristics of I5HT, including its voltage dependence, were similar to those of IAcCho. The 5HT dose-I5HT response gave an apparent dissociation constant EC50 of 23.5 microM and a Hill coefficient nH of 1.7, which were not modified by the presence of AcCho. Similarly, the apparent affinity of L247T alpha(7) for AcCho as well as its cooperativity were not influenced by 5HT, indicating a lack of mutual interactions between 5HT and AcCho. These results show that 5HT is a potent noncompetitive antagonist of neuronal alpha(7) nAcChoR, but it becomes a noncompetitive agonist following mutation of the highly conserved leucine residue 247 located in the channel domain M2.

Serotonergic modulation of muscle acetylcholine receptors of different subunit composition

Modulation of muscle acetylcholine (AcCho) receptors (AcChoRs) by serotonin [5-hydroxytryptamine (5HT)] and other serotonergic compounds was studied in Xenopus laevis oocytes. Various combinations of alpha, beta, gamma, and delta subunit RNAs were injected into oocytes, and membrane currents elicited by AcCho were recorded under voltage clamp. Judging by the amplitudes of AcCho currents generated, the levels of functional receptor expression were: alpha beta gamma delta > alpha beta delta > alpha beta gamma > alpha gamma delta. The alpha beta gamma delta and alpha beta delta AcChoR Subtypes were strongly blocked by 5HT, whereas the alpha beta gamma receptor was blocked only slightly. The order of blocking potency of AcChoRs by 5HT was: alpha beta delta > alpha beta gamma delta > alpha beta gamma. 5HT receptor antagonists, such as methysergide and spiperone, were even more potent blockers of AcChoRs than 5HT but did not show much subunit selectivity. Blockage of alpha beta gamma delta and alpha beta delta receptors by 5HT was voltage-dependent, and the voltage dependence was abolished when the delta subunit was omitted. These findings may need to be taken into consideration when trying to elucidate the mode of action of many clinically important serotonergic compounds.

Effects of serotonergic agents on neuronal nicotinic acetylcholine receptors

In Xenopus oocytes expressing neuronal nicotinic acetylcholine receptors (nAcChoRs), made up of alpha 2 and beta 4 subunits, acetylcholine (AcCho) elicited ionic membrane currents (AcCho currents) that were modulated by serotonergic agents. Both agonists and antagonists specific for various serotonin (5-hydroxytryptamine, 5HT) receptor subtypes interacted directly with alpha 2 beta 4 nAcChoRs: 5HT, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin, methysergide, spiperone, and ketanserin reversibly reduced the amplitude of AcCho currents and accelerated their decay. The AcCho-current time course decayed with two exponential functions. In the presence of 5HT, the fast time constant of current decay (tau f) was not greatly modified, but the slow time constant (tau s) was reduced. With AcCho and 5HT both at 100 microM, tau s was reduced from 140 s to 85 s. The order of potency for inhibition of AcCho current amplitudes was (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin > methysergide > spiperone > ketanserin > 5HT. The inhibition was voltage-dependent but the magnitude of the voltage dependence for the different blockers did not correspond to their blocking potency: e.g., the block with spiperone was stronger than with 5HT, but it was less voltage-dependent. Our results suggest that serotonergic agents block neuronal nAcChoRs in a noncompetitive manner, similar to the block of muscle nAcChoR by curare and other substances. These results show that neuronal nAcChoR channels that have been activated by their specific neurotransmitter may be modulated by nonspecific neurotransmitters and their antagonists. These effects may help to better understand brain functions as well as the mode of action of the many serotonergic agents that are used in medical practice.

Serotonin modulates nicotinic responses of adrenal chromaffin cells

5-Hydroxytryptamine (5-HT) specifically and reversibly inhibits nicotine-induced currents and catecholamine release in bovine adrenal chromaffin cells in culture. Pharmacological analysis indicates that the inhibition is not mediated by known 5-HT receptor subtypes. The inhibition is noncompetitive over a range of nicotine concentrations between 1 and 100 microM. Preincubation with either 5-HT or substance P significantly protects the response from nicotine-induced desensitization. It is concluded that 5-HT inhibits nicotinic acetylcholine receptors on bovine adrenal chromaffin cells, probably by binding to a noncompetitive site on the receptor itself. Because both blood and the chromaffin cells contain 5-HT, the inhibition provides an opportunity for negative control of catecholamine secretion from the adrenals.

Blockage of nicotinic acetylcholine receptors by 5-hydroxytryptamine

The action of 5-hydroxytryptamine (5HT) on nicotinic acetylcholine receptor (nAChR) channels was investigated in mouse myotubes, human cloned TE671/RD cells, and Xenopus laevis oocytes. The decay of the ACh-activated whole-cell currents was reversibly accelerated in the presence of 5HT (10(-5) to 10(-3) M), in a dose-dependent manner. 5HT also reduced the size and accelerated the decay of currents elicited by ACh in Xenopus oocytes injected with mRNA extracted from C2 myotubes or Torpedo electroplaques, or oocytes injected with cloned mouse muscle AChR subunit mRNAs. The effect of 5HT was promptly reversed after washout, or by depolarizing the oocyte beyond -10 mV. In patch-clamp recordings from myotubes, bath-application of 5HT did not exert an indirect influence on the ACh-activated channels within the patch membrane. In contrast, when the patch membrane was exposed to 5HT (10(-6) M), ACh unit responses appeared as bursts of short pulses. It is concluded that the regulation of ACh responses by 5HT results from a fast noncompetitive blocking action of nAChR-channels. These results show that ligand-gated channels, activated by their specific neurotransmitter, may be regulated by a different neurotransmitter through a direct action on the receptor molecule.

Modulation of the sensitivity of nicotinic receptors in autonomic ganglia

This article reviews some of the evidence suggesting that a variety of endogenous substances either facilitates or inhibits the sensitivity of nicotinic acetylcholine (ACh) receptors at the subsynaptic membrane of cholinergic synapses. It is noteworthy that 5-hydroxytryptamine and histamine act as competitive antagonists, like curare, presumably changing the affinity of ACh for the specific binding site on the nicotinic receptor. Catecholamine, neuropeptides, prostaglandin and glucocorticoids act as non-competitive antagonists on an allosteric site on the receptor-ionic channel complex. ATP and LH-RH (in a subpopulation of sympathetic neurons) caused a facilitation of the sensitivity of nicotinic receptors. The mode of actions of endogenous substances which modulate the nicotinic receptor-sensitivity is similar to those of pharmacological agents. Therefore, these neurotransmitters and neurohormones have been termed endogenous 'antagonists' or 'sensitizers'.

5-Hydroxytryptamine decreases the sensitivity of nicotinic acetylcholine receptor in bull-frog sympathetic ganglion cells

The post-synaptic effects of 5-hydroxytryptamine (5-HT) were examined in neurones of bull-frog sympathetic ganglia with intracellular micro-electrode and voltage-clamp recording techniques. Atropine (1 microM) was used to block the muscarinic cholinoceptors. 5-HT reduced the amplitude of the fast excitatory post-synaptic potential (fast e.p.s.p.). 5-HT also reduced the mean amplitude of the miniature excitatory post-synaptic potentials (m.e.p.s.p.s) without affecting their frequency. Voltage-clamp studies showed that 5-HT decreased in a dose-dependent manner the amplitude of the acetylcholine (ACh) current produced by ionophoretic application of ACh to sympathetic neurones. The relationship between the log of the ACh dose, applied ionophoretically, and the peak ACh current (the dose-response curve) was examined in voltage-clamped neurones. 5-HT caused a parallel shift to the right of the dose-response curve for ACh. Analysis using a double reciprocal plot (Lineweaver-Burk plot) revealed that 5-HT increased the apparent dissociation constant (Km) of ACh for the receptor without changing the maximum ACh current (Vmax), suggesting a competitive antagonism. The relationship between the 5-HT dose and the magnitude of inhibition of the ACh current was obtained using two different amplitudes for the ACh response. The dose-response curve of 5-HT-induced inhibition using a relatively high amplitude ACh current, S1, was parallel with that for a relatively low amplitude ACh current, S2. The Dixon plot of these two curves yielded an apparent inhibition constant (Ki) of 42 microM. Both fast excitatory post-synaptic currents (fast e.p.s.c.s) and miniature excitatory post-synaptic currents (m.e.p.s.c.s) had single-exponential decay time courses. The time constants of fast e.p.s.c. decay (tau e) and m.e.p.s.c. decay (tau m) were not altered by 5-HT, suggesting that 5-HT does not change the kinetics of opening and closing of the ionic channel associated with the nicotinic receptor. 5-HT did not alter the reversal potential of the fast e.p.s.c. These results suggest that 5-HT decreases the sensitivity of the nicotinic receptor of sympathetic neurones, by interfering with ACh binding at the active site on the receptor-ionic-channel complex. 5-HT may physiologically inhibit cholinergic transmission as it is an endogenous substance which antagonizes the nicotinic receptor in post-ganglionic neurones of bull-frog sympathetic ganglia.

Histamine as an endogenous antagonist of nicotinic ACh-receptor

The mechanism of an inhibitory effect of histamine on the sensitivity of frog skeletal muscle endplate was analyzed by studying the dose-response relation between the quantity of ACh applied iontophoretically and the ACh-induced postsynaptic current (ACh current), and also the interaction between histamine and erabutoxin-b (ETX-b). The results obtained show that histamine, like curare, decreased the sensitivity of ACh-receptor in a competitive manner.

Effects of serotonin (5-hydroxytryptamine) on amphibian neuromuscular junction

A study of the effects of serotonin transmission was carried out on the frog neuromuscular junction by means of microelectrode methods. Serotonin was employed in concentrations of 5-100 microM. Serotonin did not affect membrane characteristics or the resting potential whether at non-neuronal (muscular fiber) or endplate segments of the junction. While serotonin did not affect the frequency of the miniature endplate potentials (MEPPs), it significantly decreased evoked release of acetylcholine. Serotonin significantly decreased, in a dose-dependent fashion, the amplitude of acetylcholine potentials, endplate currents (EPCs), endplate potentials (EPPs) and MEPPs. Also, serotonin shortened significantly the EPC time course and half-decay time, and caused loss of membrane voltage sensitivity of the half-decay time. While it did not affect the null potential, serotonin changed the voltage-EPC relationship from linear to non-linear, and markedly attenuated the dependence of EPC amplitude on membrane potential. These results demonstrate that serotonin induces depressant effects at both pre- and post-synaptic sites of amphibian neuromuscular junction and that its post-synaptic action is directed at the receptor-channel macromolecule rather than at either the channel or the receptor alone.

Modulation of nicotinic transmission by biogenic amines in bullfrog sympathetic ganglia

Studies of transmission in isolated paravertebral sympathetic ganglia of the bullfrog and at the sciatic-sartarius muscle synapse in the frog yielded evidence that biogenic amines such as catecholamines or 5-hydroxytryptamine can modulate transmission in sympathetic ganglia and at the neuromyal junction. These two transmitter substances are able to modulate transmission by affecting the amount of ACh release from presynaptic terminals and also by affecting the sensitivity of nicotinic Ach receptors of the subsynaptic membrane. Information is presented as to how these compounds exert their modulatory effects on these synapses.

5-hydroxytryptamine controls ACh-receptor sensitivity of bullfrog sympathetic ganglion cells

Experimental evidences showing that 5-hydroxytryptamine (5-HT) is directly interacting with nicotinic acetylcholine (ACh) receptors and thereby depresses the sensitivity of these receptors to ACh, are presented by making use of bullfrog sympathetic ganglion cells and frog skeletal muscle endplates. It was suggested that 5-HT might decrease the affinity of ACh to nicotinic receptor sites, since the mode of 5-HT action was comparable to that of D-tubocurarine action.

The actions of 5-hydroxytryptamine on isolated skeletal muscle preparations

The effects of 5-hydroxytryptamine (5HT) on the mouse phrenic nerve--hemidiaphragm and frog sartorius muscle preparations were studied using twitch tension and intracellular recording methods. Between 1--7.5 mM 5HT transiently augmented the twitch response of the diaphragm to indirect, but not direct, stimulation. Above 5 mM 5HT the augmentation was followed by a concentration-dependent twitch blockade. The blockade was more apparent in anticholinesterase-treated preparations, and was seen to a lesser extent in directly stimulated muscles. Intracellular recording from diaphragm muscles revealed that 5HT depressed the amplitudes of both miniature endplate potentials (m.e.p.p.s.) and endplate potentials (e.p.p.s.), this effect being more pronounced in the presence of neostigmine. 5HT also greatly prolonged the duration of the e.p.p.s. In the sartorius muscles, 1--5 mM 5HT slowed and depressed action potentials. It is suggested that 5HT facilitates transmission by an anticholinesterase action and inhibits by blockade of receptor ion channels.

The effect of 5-hydroxytryptamine on the chick biventer cervicis muscle

The actions of 5-hydroxytryptamine have been studied in the chick biventer cervicis nerve--muscle preparation. At concentrations between 2.5 X 10(-3) and 3.7 X 10(-3) M, 5HT produced a transient increase in responses to indirect stimulation, but not to direct stimulation. At higher concentrations, 5HT produced a reversible depression of twitches, which was much more marked in indirectly than in directly stimulated preparations. In a concentration range of 2.5-3.7 X 10(-3) M, 5HT increased the response to exogenous acetylcholine but reduced the response to carbachol. After pretreatment with neostigmine, 5HT inhibited responses to both acetylcholine and carbachol. It is suggested that the facilitatory effects of 5HT are due to its anticholinesterase activity. The blocking actions of 5HT are thought to be curare-like. In addition, irreversible toxic effects on the muscle preparations were found after repeated exposures to high concentrations of 5HT.

On the conformations of hallucinogenic molecules and their correlation

There are only a few possible conformations of D-lysergic acid diethylamide and hallucinogenic derivatives of tryptamine and phenylethylamine. Of these possible conformations there is a high structural correlation among the probable conformations of active hallucinogenic molecules and between these conformations and the known conformations of several central nervous system transmitter molecules.