Possible allosteric interaction of 4-aminopyridine with rat brain muscarinic acetylcholine receptors

Alpha1 -adrenergic stimulation selectively enhances endothelium-mediated vasodilation in rat cremaster arteries

We have systematically investigated how vascular smooth muscle α₁‐adrenoceptor activation impacts endothelium‐mediated vasodilation in isolated, myogenically active, rat cremaster muscle 1A arteries. Cannulated cremaster arteries were pressurized intraluminally to 70 mmHg to induce myogenic tone, and exposed to vasoactive agents via bath superfusion at 34°C. Smooth muscle membrane potential was measured via sharp microelectrode recordings in pressurized, myogenic arteries. The α₁‐adrenergic agonist phenylephrine (25–100 nmol/L) produced further constriction of myogenic arteries, but did not alter the vasorelaxant responses to acetylcholine (0.3 μmol/L), SKA‐31 (an activator of endothelial Ca²⁺‐dependent K⁺ channels) (3 μmol/L) or sodium nitroprusside (10 μmol/L). Exposure to 0.25–1 μmol/L phenylephrine or 1 μmol/L norepinephrine generated more robust constrictions, and also enhanced the vasodilations evoked by acetylcholine and SKA‐31, but not by sodium nitroprusside. In contrast, the thromboxane receptor agonist U46619 (250 nmol/L) dampened responses to all three vasodilators. Phenylephrine exposure depolarized myogenic arteries, and mimicking this effect with 4‐aminopyridine (1 mmol/L) was sufficient to augment the SKA‐31‐evoked vasodilation. Inhibition of L‐type Ca²⁺ channels by 1 μmol/L nifedipine decreased myogenic tone, phenylephrine‐induced constriction and prevented α₁‐adrenergic enhancement of endothelium‐evoked vasodilation; these latter deficits were overcome by exposure to 3 and 10 μmol/L phenylephrine. Mechanistically, augmentation of ACh‐evoked dilation by phenylephrine was dampened by eNOS inhibition and abolished by blockade of endothelial KCa channels. Collectively, these data suggest that increasing α₁‐adrenoceptor activation beyond a threshold level augments endothelium‐evoked vasodilation, likely by triggering transcellular signaling between smooth muscle and the endothelium. Physiologically, this negative feedback process may serve as a “brake” to limit the extent of vasoconstriction in the skeletal microcirculation evoked by the elevated sympathetic tone.

Neuronal Voltage Gated Potassium Channels May Modulate Nitric Oxide Synthesis in Corpus Cavernosum

Potassium channels (K⁺Ch) in corpus cavernosum play an important role in the regulation of erection. Nitric oxide (NO) acts through opening of K⁺Ch leading to hyperpolarization and relaxation.

Aim : This study aims to update knowledge about the role of voltage-gated K⁺Ch (K_V) channels in erectile machinery and investigate their role in the control of NO action &/or synthesis in the corpus cavernosum.

Methods : Tension studies using isolated rabbit corpus cavernosum (CC) strips and rat anococcygeus muscle were conducted. Results are expressed as mean ± SEM.

Results : Electric field stimulation (EFS, 2–16 Hz) evoked frequency-dependent relaxations of the PE (phenylephrine)-precontracted CC strips. At 2 Hz, EFS-induced relaxation amounted to 73.17 ± 2.55% in presence 4-AP (10⁻³ M) compared to 41.98 ± 1.45% as control. None of the other selective K⁺Ch blockers tested inhibited EFS-induced relaxation. 4-AP (10⁻³M) significantly attenuated ACh-induced relaxation of rabbit CC where dose-response curve was clearly shifted upward, and attenuated SNP- induced relaxation, for example, to 49.28 ± 4.52% compared to 65.53 ± 3.01% as control at 10⁻⁶ M SNP. The potentiatory effect of 4-AP on EFS was abolished or reversed in presence of N^G-nitro-L-arginine (L-NNA, non-selective nitric oxide synthase inhibitor, 10⁻⁵M, and 2 × 10⁻⁴M). Same results were observed in rat anococcygeus muscle which is a part of the erectile machinery in rats.

Conclusion : This study provides evidence for the presence of prejunctional voltage-gated K⁺Ch in CC, the blockade of which may increase the neuronal synthesis of NO.

Allosteric Modulation of Muscarinic Acetylcholine Receptors

An allosteric modulator is a ligand that binds to an allosteric site on the receptor and changes receptor conformation to produce increase (positive cooperativity) or decrease (negative cooperativity) in the binding or action of an orthosteric agonist (e.g., acetylcholine). Since the identification of gallamine as the first allosteric modulator of muscarinic receptors in 1976, this unique mode of receptor modulation has been intensively studied by many groups. This review summarizes over 30 years of research on the molecular mechanisms of allosteric interactions of drugs with the receptor and for new allosteric modulators of muscarinic receptors with potential therapeutic use. Identification of positive modulators of acetylcholine binding and function that enhance neurotransmission and the discovery of highly selective allosteric modulators are mile-stones on the way to novel therapeutic agents for the treatment of schizophrenia, Alzheimer’s disease and other disorders involving impaired cognitive function.

Amiridin and tacrine modulation of the activity and plasticity of the cholinoreceptors of neurons of the common snail: phenomenology and mechanisms

The influence of amiridin and tacrine on the membrane potential, activity, and plasticity of cholinoreceptors was investigated using the methods of recording of intracellular potentials and transmembrane currents in identified RPa3 and LPa3 neurons of the common snail. Amiridin and tacrine (1-100 mumole/liter) do not exert appreciable influences on the membrane potential of the cells. Both compounds modulate the activity of cholinoreceptors, judging from their influence on the inward current induced by local application of the acetylcholine: the duration of the ACh current increases and its amplitude varies biphasically (a short-latency increase is succeeded by a decrease). Amiridin and tacrine intensify the extinction of the ACh current induced by repeated applications of ACh to the soma. The acetylcholinesterase inhibitor, physostigmine, exerts a similar modulatory influence on the ACh current and on its extinction, and also prevents the manifestation of the effects of amiridin and tacrine. Amiridin and physostigmine directly influence the cholinoreceptors and the ionic membrane channels controlled by them by similarly altering the voltage-current characteristics (VAC) of the ACh current, and by shifting the reversal potential of the ACh current in the negative direction, bringing it closer to the equilibrium potential for chloride ions; this may be interpreted as a result of the nonidentical action of these compounds on the various ionic currents controlled by cholinoreceptors. The hypothesis is advanced that the modulatory influence of amiridin, tacrine, and physostigmine on the activity and plasticity of the cholinoreceptors is governed by their direct membrane-cytoplasmic action.

Effect of carbachol in the absence and presence of phenylephrine on Rb+ efflux and tension in rabbit left atria

The muscarinic agonist carbachol produced a concentration-dependent increase in 86Rb+ efflux and decrease in tension in isolated, electrically stimulated rabbit left atria. However, the lowest concentration of carbachol tested produced only a very small increase in 86Rb+ efflux, while it caused a relatively greater decrease in tension. 4-Aminopyridine and pertussis toxin attenuated the carbachol-stimulated 86Rb+ efflux and negative inotropic effect. However, 4-aminopyridine had a greater inhibitory effect on carbachol-stimulated 86Rb+ efflux than on carbachol-induced decreases in tension. Pre-treatment of rabbits with pertussis toxin completely abolished the increase in 86Rb+ efflux and decrease in tension produced by carbachol in the presence of the alpha-adrenoceptor agonist phenylephrine. 4-Aminopyridine attenuated the negative inotropic response to carbachol in the presence of phenylephrine, but had less effect on the carbachol-induced increase in 86Rb+ efflux under these conditions. These results suggest that carbachol-induced increases in K+ efflux may contribute at least in part to the negative inotropic responses to carbachol in the presence and absence of phenylephrine. However, this may not be sufficient to explain the direct negative inotropic response of left atria to carbachol.

Comparison of the effects of four dendrotoxin peptides, 4-aminopyridine and tetraethylammonium on the electrically evoked [3H]noradrenaline release from rat hippocampus

We have examined the effects of four dendrotoxin (DaTX) peptides, alpha-, beta-, gamma- and delta-DaTX, separated from the venom of the green mamba (Dendroaspis angusticeps), on field stimulation-evoked [3H]noradrenaline (NA) release from rat hippocampus and compared their effects with those of two other inhibitors of K+ channels, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 4-AP (10-300 microM) and TEA (0.1-5 mM) facilitated the evoked [3H]NA release in a concentration-dependent manner. The evoked [3H]NA release was reduced to about half by alpha 2-adrenoceptor stimulation (UK 14,304; 100 nM) and this reduction was antagonized by 4-AP (10-100 microM), whereas TEA even at 5 mM was a poor inhibitor of alpha 2-effects. alpha-DaTX (10-200 nM) mimicked 4-AP in increasing the electrically evoked [3H]NA release and diminishing the inhibitory effects of UK 14,304 in a concentration-dependent manner. delta-DaTX did not itself alter the electrically evoked [3H]NA release, but at 200 nM, it reduced the effects of alpha 2-receptor stimulation. beta- and gamma-DaTX (up to 200 nM) had no significant effects. 4-AP, 3,4-diaminopyridine (3,4-DAP), TEA and the four dendrotoxins displaced the binding of [3H]p-aminoclonidine ([3H]PAC) from alpha 2-receptors. The IC50 values were 6.6 x 10(-4), 1.42 x 10(-3), 5.6 x 10(-2) for 4-AP, 3,4-DAP and TEA, respectively, and 3.19 x 10(-6) M for alpha-DaTX. Thus, their potency as inhibitors of alpha 2-receptors is apparently too low to account alone for the antagonism by K+ channel inhibitors of alpha 2-effects on NA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Naltrexone potentiates 4-aminopyridine seizures in the rat

The effects of a pharmacological blockade of the mu opiate receptors on the manifestation of tonic-clonic seizures were investigated in freely moving animals. 4-aminopyridine, a specific blocker of the neuronal K+ channels was used to produce generalized convulsions. After pretreatment of adult rats with 1 mg/kg naltrexone HCl, 3, 5, 7, 9, 14 mg/kg 4-aminopyridine was injected intraperitoneally, and the latencies of the symptoms generated by 4-aminopyridine were measured. Naltrexone HCl decreased these latencies and enhanced the seizures significantly. The experiments provided further evidence for the existence of a tonic anticonvulsant opioid system in the brain.

Mixed competitive and allosteric antagonism by gallamine of muscarinic receptor-mediated second messenger responses in N1E-115 neuroblastoma cells

The antagonistic effects of gallamine on muscarinic receptor-linked responses were investigated in N1E-115 neuroblastoma cells. M1 muscarinic receptor-mediated phosphoinositide hydrolysis induced by carbamylcholine was antagonized by gallamine, with a Ki value of 33 microM. By comparison, gallamine was four- to fivefold less potent in blocking noncardiac M2 muscarinic receptor-mediated inhibition of cyclic AMP formation, with a Ki value of 144 microM. The resulting Arunlakshana-Schild plots of the antagonism of both responses by gallamine were linear and exhibited slopes not differing from 1, a result indicative of a competitive mechanism. To elucidate further the nature of gallamine's inhibitory actions, experiments were performed where the effects of gallamine in combination with the known competitive muscarinic antagonist, N-methylscopolamine (NMS), were studied. In the presence of both antagonists, a supraadditive shift in the carbamylcholine dose-response curve was demonstrated for the two responses, a result suggestive of an allosteric mode of interaction between gallamine and NMS binding sites. Confirmation that gallamine allosterically modifies the muscarinic receptor was provided by radioligand binding studies. Gallamine competition curves with either [N-methyl-3H]scopolamine methyl chloride ([3H]NMS) or [N-methyl-3H]quinuclidinyl benzilate methyl chloride ([3H]NMeQNB) were unusually shallow. Furthermore, gallamine decelerated the rate of dissociation of receptor-bound [3H]NMS greater than [3H]NMeQNB in a dose-dependent manner. The present study demonstrates that whereas gallamine antagonizes carbamylcholine-mediated responses in N1E-115 cells in a competitive manner, an allosteric component of its action is revealed in the presence of muscarinic antagonists such as NMS.

Enhancement by potassium of carbachol-stimulated inositol phospholipid breakdown in rat cerebral cortical miniprisms: comparison with other depolarising agents

Increasing the [K+] in the assay medium from 5.7 to 17.8 mM produces a large enhancement of the inositol phospholipid breakdown response to the muscarinic agonist carbachol in rat cerebral cortical miniprisms, with minor effects on basal inositol phospholipid breakdown. This effect is also found with Rb+. The enhancement by a raised [K+] is not accompanied by a change in the composition of the labelled polyphosphoinositides. The carbachol-stimulated inositol phospholipid breakdown at 17.8 and 42.7 mM K+ was antagonised by veratrine (5-80 microM), 4-aminopyridine (5 mM), and tetraethylammonium (20 mM). These compounds, however, also inhibited the binding of [3H]quinuclidinyl benzilate to cortical membranes. BRL 34915 (0.2-20 microM) was without significant effect on carbachol-stimulated inositol phospholipid breakdown at either 5.7 or 17.8 mM K+.Mg2+ (10 mM) considerably reduced the carbachol-stimulated inositol phospholipid breakdown at 17.8, but not 42.7, mM K+. Inositol phospholipid breakdown was also stimulated, albeit to a small extent, by L-glutamate (100-3,000 microM) and quisqualate (1-100 microM), with the stimulation being additive to that produced by carbachol at both 5.7 and 17.8 mM K+. N-Methyl-D-aspartate (10-1,000 microM in Mg2+-free medium) had no significant effect on basal inositol phospholipid breakdown and had little or no effect on carbachol-stimulated inositol phospholipid breakdown at either 5.7 or 17.8 mM K+. It is concluded that it may not be correct to ascribe wholly the enhancement by K+ of carbachol-stimulated inositol phospholipid breakdown to the tissue-depolarising actions of this ion and that other actions of K+ may be involved.

Effects of calcium antagonists on muscarinic receptor subtypes in the rat brain

The effects of several calcium antagonists, including nicardipine, nifedipine, verapamil, and diltiazem, on muscarinic acetylcholine antagonist binding were studied in the P2 fraction of the rat cerebral cortex using either [3H]quinuclidinyl benzilate ([3H]QNB) or [3H]pirenzipine as the radioactive ligand. (1) The potency of [3H]QNB binding inhibition was in the order nicardipine greater than verapamil greater than diltiazem greater than nifedipine. The IC50 values of nicardipine, verapamil, and diltiazem were 2.56 X 10(-6) M, 1.28 X 10(-5) M, and 6.00 X 10(-5) M, respectively. (2) The inhibition of [3H]QNB binding by nicardipine was significantly decreased in the presence of Ca ions. (3) In saturation experiments of [3H]QNB binding in the presence of nicardipine, the Kd value appeared to be significantly affected, but the Bmax value was unchanged. This indicated that nicardipine probably inhibited [3H]QNB binding allosterically. On the other hand, (4) nicardipine inhibited [3H]pirenzipine binding completely with an IC50 value of 7.87 X 10(-7) M. It was concluded that nicardipine had an inhibitory effect on M1-receptor binding.

Competitive interaction of pirenzepine with rat brain muscarinic acetylcholine receptors

In the present work, we studied the details of the interaction of the nonclassical muscarinic receptor antagonist pirenzepine with [3H]quinuclidinyl benzilate binding sites in rat brain homogenates. Pirenzepine showed biphasic competition curves with a Hill coefficient lower than unity, and these curves were better described according to a two-site receptor model. The affinities and the relative preponderance of these sites were constant at different ligand concentrations, in accordance with a competitive type of interaction. Similarly, pirenzepine did not influence the rate of dissociation of the [3H]quinuclidinyl benzilate-receptor complex, even at relatively high concentrations. However, although low concentrations of pirenzepine decreased the affinity of [3H]quinuclidinyl benzilate for the receptor without affecting the density of the binding sites, higher concentrations of the antagonist decreased the receptor number in a reversible fashion. Schild plots of these data indicated an apparent deviation from simple competition in this experimental design, an observation which can be attributed to the selectivity of pirenzepine for different receptor subtypes. Furthermore, pirenzepine, at concentrations high enough to saturate both its high- and low-affinity sites protected [3H]quinuclidinyl benzilate binding sites in the brain against irreversible alkylation by propylbenzilylcholine mustard. Therefore, our data support a competitive nature of interaction of pirenzepine with rat brain muscarinic receptors.

Interaction of 4-aminopyridine with [3H]phencyclidine receptors in rat brain homogenates

The effect of 4-aminopyridine and its analogs on the specific binding of [3H]phencyclidine was investigated in rat brain homogenates. 4-Aminopyridine (4-AP) and 3,4-diaminopyridine displaced [3H]phencyclidine binding, while 3-aminopyridine was without effect. The concentrations of 4-AP required for inhibition of binding increased with increasing the ligand concentration, and the resultant Dixon plots indicated a competitive type of interaction. However, 4-AP also accelerated the dissociation rate of the ligand-receptor complex, suggesting that the effect of 4-AP on phencyclidine receptors in the brain might not be purely competitive.