Comparative in vivo effects of parathion on striatal acetylcholine accumulation in adult and aged rats

Aged rats are more sensitive to the acute toxicity of the prototype organophosphate insecticide, parathion. We compared the acute effects of parathion on diaphragm and brain regional cholinesterase activity, muscarinic receptor binding and striatal acetylcholine levels in 3- and 18-month-old male Sprague-Dawley rats. Adult and aged rats were surgically implanted with a microdialysis cannula into the right striatum 5-7 days prior to parathion treatment. Rats were given either vehicle (peanut oil, 2 ml/kg) or one of a range of dosages of parathion (adult: 1.8, 3.4, 6.0, 9.0, 18 and 27 mg/kg, s.c.; aged: 1.8, 3.4, 6 and 9 mg/kg, s.c.) and body weight, functional signs of toxicity, and nocturnal motor activity were recorded for seven days. Three and seven days after parathion treatment, microdialysis samples were collected and rats were subsequently sacrificed for biochemical measurements. Higher dosages of parathion led to significant time-dependent reductions in body weight in both age groups. Rats in both age groups treated with lower dosages showed few overt signs of cholinergic toxicity while equitoxic high dosages (adult, 27 mg/kg; aged, 9 mg/kg) elicited marked signs of cholinergic toxicity (involuntary movements and SLUD [i.e., acronym for Salivation, Lacrimation, Urination and Defecation] signs) with peak effects being noted 3-4 days after treatment. Nocturnal activity (ambulation and rearing) was reduced in both age groups following parathion dosing, with more prominent effects in adults and rearing being more consistently affected. Dose- and time-dependent inhibition of cholinesterase activity was noted in both diaphragm and striatum. Total muscarinic receptor ([(3)H]quinuclidinyl benzilate, QNB) binding was significantly lower in aged rats, and both total binding and muscarinic agonist ([(3)H]oxotremorine methiodide] binding was significantly reduced in both age-groups treated with the highest dosages of parathion (adult, 27 mg/kg; aged, 9 mg/kg). In contrast to relatively similar levels of cholinesterase inhibition, striatal extracellular acetylcholine levels were significantly lower (2.2- to 2.9-fold) in aged rats at both 3 and 7 day time-points compared to adult rats treated with equitoxic dosages (i.e., 9 and 27 mg/kg, respectively). No age-related differences in in vitro striatal acetylcholine synthesis or in vivo acetylcholine accumulation following direct infusion of the cholinesterase inhibitor neostigmine (1 microM) were noted. While aged rats are more sensitive than adults to the acute toxicity of parathion, lesser acetylcholine accumulation was noted in the striatum of aged rats exhibiting similar levels of cholinesterase inhibition. These findings suggest that lesser acetylcholine accumulation may be required to elicit cholinergic signs in the aged rat, possibly based on aging-associated changes in muscarinic receptor density.

Neuronal nitric oxide synthase activity in rat urinary bladder detrusor: participation in M3 and M4 muscarinic receptor function

1. The aim of this paper was to determine the different signalling cascades involved in contraction of the rat urinary bladder detrusor muscle mediated via muscarinic acetylcholine receptors (muscarinic AChR). Contractile responses, phosphoinositides (IPs) accumulation, nitric oxide synthase (NOS) activity and cyclic GMP (cGMP) production were measured to determine the reactions associated with the effect of cholinergic agonist carbachol. The specific muscarinic AChR subtype antagonists and different inhibitors of the enzymatic pathways involved in muscarinic receptor-dependent activation of NOS and cGMP were tested. 2. Carbachol stimulation of M(3) and M(4) muscarinic AChR increased contractility, IPs accumulation, NOS activity and cGMP production. All of these effects were selectively blunted by 4-DAMP and tropicamide, M(3) and M(4) antagonists respectively. 3. The inhibitors of phospholipase C (PLC), calcium/calmodulin (CaM), neuronal NOS (nNOS) and soluble guanylate cyclase, but not of protein kinase C and endothelial NOS (eNOS), inhibited the carbachol action on detrusor contractility. These inhibitors also attenuated the muscarinic receptor-dependent increase in cGMP and activation of NOS. 4. In addition, sodium nitroprusside and 8-bromo-cGMP, induced negative relaxant effect. 5. The results obtained suggest that carbachol activation of M(3) and M(4) muscarinic AChRs, exerts a contractile effect on rat detrusor that is accompanied by an increased production of cGMP and nNOS activity. The mechanism appears to occur secondarily to stimulation of IPs turnover via PLC activation. This in turn, triggers cascade reactions involving CaM, leading to activation of nNOS and soluble guanylate cyclase. They, in turn, exert a modulator inhibitory cGMP-mediated mechanism limiting the effect of muscarinic AChR stimulation of the bladder.

Muscarinic receptors autoantibodies purified from mammary adenocarcinoma-bearing mice sera stimulate tumor progression

The ability of tumor cells to stimulate adaptive immunity, particularly by inducing anti-tumor antibodies (Abs), has been extensively reviewed. LM3 is a tumorigenic cell line derived from a murine mammary metastatic adenocarcinoma that spontaneously overexpressed mAchR. Here we investigate the ability of Abs purified from the sera of LM3 tumor-bearing mice, directed against muscarinic acetylcholine receptors (mAchR) to modulate tumor cells' proliferation and angiogenesis. We observed that IgG from early tumor bearers (ETB), 14-day LM3 tumor, and from late tumor bearers (LTB), 28-day LM3 tumor, displaced tritiated quinuclidinyl benzilate binding to LM3 tumor cells, confirming Abs interaction with cholinoceptors, while IgG from normal mice did not modify the antagonist binding to mAchR at any concentration tested. In addition, Abs from ETB and LTB immunoblotted a protein of 70 kDa on murine tumor cells and on heart homogenates that was also recognized by a specific anti-M(2) receptor monoclonal antibody. We also observed that IgG purified from ETB-stimulated LM3 cells' proliferation in a more effective manner than the muscarinic agonist carbachol (CARB) did. IgG from LTB-potentiated LM3 cells induced angiogenesis by increasing the number of blood vessels and VEGF-A production in peritumoral skin "via" mAchR, in an agonist similar manner. All effects were blocked by preincubating cells with the non-selective antagonist atropine. In conclusion, autoAbs purified from LM3 tumor-bearing mice sera exert different pro-tumor actions depending on the stage of tumor development: in ETB, they stimulate tumor cells' proliferation, while in LTB they potentiate tumor neovascularization.

Preparation and biological effects of pure stereoisomeric novel soft anticholinergics

A series of pure stereoisomeric soft glycopyrrolate analogues 3, 4 and 5 was synthesized using chiral intermediates and by careful separation of the stereoisomers formed during the last quaternization step of the synthesis. The stereochemistry of the products was elucidated using various 1D and 2D NMR techniques. Anticholinergic activity of the new compounds was determined by receptor binding studies and performing tests on isolated organs and by in vivo tests. Receptor binding revealed that in the higher alkyl ester series the (2R, 1'R, 3'R) and the (2R, 1'S, 3'S) isomers were the compounds showing the highest receptor affinity furthermore it demonstrated the confines of the length of the alkyl chain. In vitro isolated organ experiments correlated well with the receptor binding results, and in vivo investigations indicated the soft character of the compounds.

In vivo co-ordinated interactions between inhibitory systems to control glutamate-mediated hippocampal excitability

We present an overview of the long-term adaptation of hippocampal neurotransmission to cholinergic and GABAergic deafferentation caused by excitotoxic lesion of the medial septum. Two months after septal microinjection of 2.7 nmol alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), a 220% increase of GABA(A) receptor labelling in the hippocampal CA3 and the hilus was shown, and also changes in hippocampal neurotransmission characterised by in vivo microdialysis and HPLC. Basal amino acid and purine extracellular levels were studied in control and lesioned rats. In vivo effects of 100 mm KCl perfusion and adenosine A(1) receptor blockade with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on their release were also investigated. In lesioned animals GABA, glutamate and glutamine basal levels were decreased and taurine, adenosine and uric acid levels increased. A similar response to KCl infusion occurred in both groups except for GABA and glutamate, which release decreased in lesioned rats. Only in lesioned rats, DPCPX increased GABA basal level and KCl-induced glutamate release, and decreased glutamate turnover. Our results evidence that an excitotoxic septal lesion leads to increased hippocampal GABA(A) receptors and decreased glutamate neurotransmission. In this situation, a co-ordinated response of hippocampal retaliatory systems takes place to control neuron excitability.

Effects of mercury on neurochemical receptors in wild river otters (Lontra canadensis)

Fish-eating wildlife, such as river otters (Lontra canadensis), accumulate mercury (Hg) at concentrations known to impair animal behavior, but few studies have explored the underlying biochemical changes that precede clinical neurotoxicity. The objective of this study was to determine if Hg exposure can be related to concentrations of neurochemical receptors in river otters. River otter carcasses (n = 66) were collected in Ontario and Nova Scotia (Canada) by local trappers in 2002-2004. Concentrations of Hg (total and organic) were measured in the cerebral cortex and cerebellum. Saturation binding curves for the cholinergic muscarinic acetylcholine (mACh) receptor and dopamine-2 (D2) receptor were completed for each animal to calculate receptor density (Bmax) and ligand affinity (Kd). Negative correlations were found between concentrations of Hg and mACh receptor Bmax (r(total) Hg = -0.458, r(inorganic) Hg = -0.454, r(organic) Hg = -0.443) in the cerebral cortex. A negative correlation was also found between concentrations of total Hg and D2 receptor Bmax (r = -0.292) in the cerebral cortex. These results suggest that neurochemical receptors may prove useful as novel biomarkers of Hg exposure and neurotoxic effects in wildlife. Given the importance of cholinergic and dopaminergic systems in animal physiology, the ecological implications of these changes need to be investigated.

Pharmacological profiles of an anticholinergic agent, phencynonate hydrochloride, and its optical isomers

AIM

To comparatively study the pharmacological profiles of 3-methyl-3-azabicyclo(3,3,1)nonanyl-9-alpha-yl-alpha-cyclopentyl-alpha-phenyl-alpha-glycolate (phencynonate hydrochloride, CPG), an anticholinergic agent, and its enantiomers [R(-)-and S(+)-CPG].

METHODS

The affinity and relative efficacy were tested using radioligand-binding assay with muscarinic acetylcholine receptors from rat cerebral cortex. The pharmacological activities were assessed in three individual experiments: (1) potentiating the effect of subthreshold hypnotic dose of sodium pentobarbital; (2) inhibiting oxotremorine-induced salivation; and (3) inhibiting the contractile response to carbachol.

RESULTS

The order of potency of phencynonate hydrochloride and its optical isomers to inhibit the binding of [3H]quinuclidinyl benzilate ([3H]QNB) was R(-)-CPG (K(i)=46.49+/-1.27 nmol/L)>CPG(K(i)=271.37+/-72.30nmol/L)>S(+)-CPG(K(i)=1263.12+/-131.64 nmol/L). The results showed that R(-)-CPG had the highest affinity to central muscarinic receptors among the three compounds, but did not show any central depressant effects at dose from 10.00 to 29.15 mg/kg. CPG increased the effects of subthreshold hypnotic dose of sodium pentobarbital induced-sleeping [the ED50+/-95% LC value was 21.06+/-3.04 mg/kg]. CPG and R(-)-CPG displayed nearly equipotent effect in depressing oxotremorine-induced salivation [the ED50 +/-95% LC for R(-) and CPG were 1.10+/-0.28 and 1.07+/-0.15 mg/kg, respectively], and the contractile response to carbachol (pA(2) values for R (-) and CPG were 6.84 and 6.80, respectively). S(+)-CPG presented the lowest anticholinergic profiles, but could potentate effects of its enantiomers in some manner.

CONCLUSIONS

These data suggested that R(-)-CPG acted as an eutomer in racemate and a competitive antagonist to acetylcholine muscarinic receptors, but S(+)-CPG was less active in comparison to R(-)-CPG and its racemate. The central depressant effects of R(-)-CPG and S(+)-CPG were lower in comparison to its racemate.

An interspecies comparison of mercury inhibition on muscarinic acetylcholine receptor binding in the cerebral cortex and cerebellum

Mercury (Hg) is a ubiquitous pollutant that can disrupt neurochemical signaling pathways in mammals. It is well documented that inorganic Hg (HgCl(2)) and methyl Hg (MeHg) can inhibit the binding of radioligands to the muscarinic acetylcholine (mACh) receptor in rat brains. However, little is known concerning this relationship in specific anatomical regions of the brain or in other species, including humans. The purpose of this study was to explore the inhibitory effects of HgCl(2) and MeHg on [(3)H]-quinuclidinyl benzilate ([(3)H]-QNB) binding to the mACh receptor in the cerebellum and cerebral cortex regions from human, rat, mouse, mink, and river otter brain tissues. Saturation binding curves were obtained from each sample to calculate receptor density (B(max)) and ligand affinity (K(d)). Subsequently, samples were exposed to HgCl(2) or MeHg to derive IC50 values and inhibition constants (K(i)). Results demonstrate that HgCl(2) is a more potent inhibitor of mACh receptor binding than MeHg, and the receptors in the cerebellum are more sensitive to Hg-mediated mACh receptor inhibition than those in the cerebral cortex. Species sensitivities, irrespective of Hg type and brain region, can be ranked from most to least sensitive: river otter > rat > mink > mouse > humans. In summary, our data demonstrate that Hg can inhibit the binding [(3)H]-QNB to the mACh receptor in a range of mammalian species. This comparative study provides data on interspecies differences and a framework for interpreting results from human, murine, and wildlife studies.

Differential modulation of muscarinic receptors in the rat brain by repeated exposure to methyl parathion

The neurochemical and behavioral effects of repeated subdermal administration of methyl parathion (MP) at low doses were investigated. Adult male rats were treated repeatedly with either vehicle or MP subcutaneously (3 mg/kg/day) and observed for the signs of toxicity during the treatment period. The toxic sign, tremor, reached maximum right after 9th injection in MP-treated rats, and declined thereafter. Animals were sacrificed and brains were taken 1 week or 3 weeks after the daily treatment for measurement of acetylcholinesterase (AChE) activity and binding of radioligands, [3H]QNB (nonselective), [3H]pirenzepine (M1-selective), and [3H]AF-DX384 (M2-selective) to muscarinic receptors. With this treatment regimen, the AChE activity in the blood dropped quickly and maintained at 30% of the control level after 6 injections. After 3 weeks of treatment, MP caused 80-90% AChE inhibition and substantial reductions in [3H]QNB binding (9-33%), [3H]pirenzepine binding (9-22%) and [3H]AF-DX384 binding (6-38%) in different brain regions, including striatum, hippocampus, frontal cortex, thalamus and midbrain. After 1 week of treatment, the inhibition of AChE in brain regions was from 54 to 74%, whereas receptor densities were only marginally affected in a few regions. The timing of the changes in receptor population correlates well with the changes in behaviors during the repeated MP exposure. Our findings suggest that down-regulation of muscarinic receptors plays a role in the development of tolerance to MP. And, the regulations of muscarinic receptors were different among receptor subtypes and brain regions.

Characterization of radioligand binding to a transmembrane receptor reconstituted into Lipobeads

Lipobeads are hydrogel beads surrounded by a lipid bilayer membrane and have been developed to act as a cell analogue. The FLAG-tagged M(2) muscarinic receptor was incorporated onto the surface of the Lipobead by incubating pre-Lipobeads with proteoliposomes containing the receptor. Receptors reconstituted onto the surface of the Lipobeads were functional in that they bound the antagonists quinuclidinylbenzilate and scopolamine with characteristic muscarinic affinities. This demonstrates the feasibility of using Lipobeads to study the binding properties of the M(2) muscarinic receptor and offers a promising approach to the study of transmembrane protein biology in general.

Mechanisms of altered vagal control in heart failure: influence of muscarinic receptors and acetylcholinesterase activity

Parasympathetic control of the heart is attenuated in heart failure (HF). We investigated possible mechanisms and sites of altered vagal control in dogs with HF induced by rapid pacing. Muscarinic blockade reduced the R-R interval by 308 ms in controls but only by 32 ms in HF, indicating low levels of resting vagal tone. Vagomimetic doses of atropine sulfate prolonged the R-R interval by 109 ms in controls and increased standard deviation of the R-R interval by 66 ms but only by 46 and 16 ms, respectively, in HF. Bradycardia elicited by electrical stimulation of the vagus nerve was also attenuated in the HF group. Conversely, muscarinic receptor activation by bethanechol, and indirectly by neostigmine, elicited exaggerated R-R interval responses in HF. To investigate possible mechanisms, we measured muscarinic receptor density (Bmax) and acetylcholinesterase activity in different areas of the heart. In sinoatrial nodes, Bmax was increased (230 +/- 75% of control) and acetylcholinesterase decreased (80 +/- 6% of control) in HF. We conclude that muscarinic receptors are upregulated and acetylcholinesterase is reduced in the sinus node in HF. Therefore, reduced vagal control in HF is most likely due to changes of presynaptic function (ganglionic), because postsynaptic mechanisms augment vagal control in HF.

[Screening method for compounds acting at super low concentrations]

At present, there are no simple and reliable methods for selection of compounds with physiological activity in super-low concentrations (SLC). The screening system with high predictability is proposed. At the beginning, experiment is carried out on infusorians--spirostoma. Their motion activity, behavior, living ability and shape are registered at concentrations of substances in a range of 10(-3)-10(-13) mol/l. If the effect is detected on spirostoma, then experiments are performed on laboratory animals or their isolated tissues. Specific deviations on injection substance in SLC are detected, depending on compounds class. Nine representatives of different classes of biologically active compounds are investigated in the work: carbofos, deltametrin, 3-quinuclidinyl benzylate, atropine, phenosan, fenazepam, dopamine receptor antagonist SCH-23390, ciprine, and hexenal. It is accepted that if compound action in SLC is observed on spirostoma, then effects in super-low doses are also detected in a whole animal.

A mechanism for rapacuronium-induced bronchospasm: M2 muscarinic receptor antagonism

BACKGROUND

A safe and effective ultra-short-acting nondepolarizing neuromuscular blocking agent is required to block nicotinic receptors to facilitate intubation. Rapacuronium, which sought to fulfill these criteria, was withdrawn from clinical use due to a high incidence of bronchospasm resulting in death. Understanding the mechanism by which rapacuronium induces fatal bronchospasm is imperative so that newly synthesized neuromuscular blocking agents that share this mechanism will not be introduced clinically. Selective inhibition of M2 muscarinic receptors by muscle relaxants during periods of parasympathetic nerve stimulation (e.g., intubation) can result in the massive release of acetylcholine to act on unopposed M3 muscarinic receptors in airway smooth muscle, thereby facilitating bronchoconstriction.

METHODS

Competitive radioligand binding determined the binding affinities of rapacuronium, vecuronium, cisatracurium, methoctramine (selective M2 antagonist), and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; selective M3 antagonist) for M2 and M3 muscarinic receptors.

RESULTS

Rapacuronium competitively displaced 3H-QNB from the M2 muscarinic receptors but not from the M3 muscarinic receptors within clinically relevant concentrations. Fifty percent inhibitory concentrations (mean +/- SE) for rapacuronium were as follows: M2 muscarinic receptor, 5.10 +/- 1.5 microm (n = 6); M3 muscarinic receptor, 77.9 +/- 11 microm (n = 8). Cisatracurium and vecuronium competitively displaced 3H-QNB from both M2 and M3 muscarinic receptors but had affinities at greater than clinically achieved concentrations for these relaxants.

CONCLUSIONS

Rapacuronium in clinically significant doses has a higher affinity for M2 muscarinic receptors as compared with M3 muscarinic receptors. A potential mechanism by which rapacuronium may potentiate bronchoconstriction is by blockade of M2 muscarinic receptors on prejunctional parasympathetic nerves, leading to increased release of acetylcholine and thereby resulting in M3 muscarinic receptor-mediated airway smooth muscle constriction.

Therapeutic use of muscarinic acetylcholine receptor peptide to prevent mice chagasic cardiac dysfunction

Therapeutic use of a peptide corresponding to the aminoacid sequence of the second extracellular loop of human M2 muscarinic acetylcholine receptor (M2 mAChR peptide) was studied. Expression and biological activity of M2 mAChR in association with circulating M2 mAChR-related antibodies in cardiac tissue from chagasic mice were evaluated. Mice infected or not with trypomastigotes Tulahuen strain either treated or not treated with M2 mAChR peptide were sacrificed at 8-9 weeks post-infection. Morphological, binding and contractility studies were performed on all animal groups. Hearts from infected mice showed a mAChR-related dysfunction, with a decrease in heart contractility, impaired response to exogenous mAChR agonist (carbachol) and a significant reduction of mAChR binding sites. Treating infected mice with M2 mAChR peptide reversed those effects. Moreover, autoantibodies from infected mice recognized the M2 mAChR peptide. In addition, serum from infected mice and the corresponding affinity purified IgG was capable of interacting with cardiac mAChR, reducing the number of binding sites and inhibiting the contractile response to exogenous agonist. In conclusion, (1) the development of alterations in mAChR related to cardiac dysfunction, may be associated with the presence of circulating antibodies against these receptors and (2) the chronic treatment with M2 mAChR peptide prevented infected mice heart dysfunction. The mechanism could be explained by the ability of the M2 mAChR peptide to inhibit the chronic interaction of autoantibodies specific to mAChR. The implication of M2 mAChR peptide treatment in the host's immune response is discussed.

Time-dependent changes in rat brain cholinergic receptor expression after experimental brain injury

Alterations in neurotransmitter receptor expression in the central nervous system may contribute to physiological and behavioral deficits that follow traumatic brain injury (TBI). Previous studies from our laboratory have demonstrated significant and widespread deficits in alpha7* nicotinic cholinergic receptor (alpha7* nAChr) expression 2 days following cortical contusion brain injury. The purpose of this study was to evaluate changes in alpha7* nAChr expression over a wider range of post-TBI recovery intervals. Animals were anesthetized and subjected to a moderate cortical contusion brain injury (2 mm cortical compression). Animals were euthanatized at various post-TBI time intervals, ranging from 1 h to 21 days, and quantitative autoradiography was used to evaluate cholinergic receptor subtype expression in the cerebral cortex and hippocampus. As previously reported, the alpha7* nAChr was the most sensitive target of TBI-induced plasticity. Significant decreases in alpha-[(125)I]-bungarotoxin (BTX) binding occurred as early as 1 h post-TBI, and persisted in some brain regions for up to 21 days. A kinetic analysis of changes in BTX binding, performed 2 days following brain injury, indicated that the binding deficits are not due to significant changes in receptor affinity. TBI-induced changes in alpha3*/alpha4* nACh receptors, muscarinic cholinergic receptors, and NMDA-type glutamate receptor expression were lower in magnitude, restricted to fewer brain regions and more transient in nature. Persistent deficits in alpha7* nAChr expression following TBI may contribute to impaired functional outcome following brain injury.

The inhibitory effects of tramadol on muscarinic receptor-induced responses in Xenopus oocytes expressing cloned M(3) receptors

Tramadol is a widely used analgesic, but its mechanism of action is not completely understood. Muscarinic receptors are involved in neuronal function in the brain and autonomic nervous system, and much attention has been paid to these receptors as targets of analgesic drugs in the central nervous system. In this study, we investigated the effects of tramadol on type-3 muscarinic (M(3)) receptors using the Xenopus oocyte expression system. Tramadol (10 nM-100 micro M) inhibited acetylcholine-induced currents in oocytes expressing M(3) receptor. Although GF109203X, a protein kinase C inhibitor, increased the basal current, it had little effect on the inhibition of acetylcholine-induced currents by tramadol. Moreover, tramadol inhibited the specific binding sites of [(3)H]quinuclidinyl benzilate. These findings suggest that tramadol at clinically relevant concentrations inhibits M(3) function via quinuclidinyl benzilate-binding sites. This may explain the modulation of neuronal function and the anticholinergic effects of tramadol.

IMPLICATIONS

Muscarinic receptors are involved in neuronal function and are targets of analgesic drugs. We here report that tramadol inhibits type-3 muscarinic receptors function via quinuclidinyl benzilate-binding sites at clinically relevant concentrations. These findings may explain the modulation of neuronal function and the anticholinergic effects of tramadol.

Effects of N-ethylmaleimide on conformational equilibria in purified cardiac muscarinic receptors

Muscarinic receptors purified from porcine atria and devoid of G protein underwent a 9-27-fold decrease in their apparent affinity for the antagonists quinuclidinyl benzilate, N-methylscopolamine, and scopolamine when treated with the thiol-selective reagent N-ethylmaleimide. Their apparent affinity for the agonists carbachol and oxotremorine-M was unchanged. Conversely, the rate of alkylation by N-ethylmaleimide, as monitored by the binding of [(3)H]quinuclidinyl benzilate, was decreased by antagonists while agonists were without effect. The receptor also underwent a time-dependent inactivation that was hastened by N-ethylmaleimide but slowed by quinuclidinyl benzilate and N-methylscopolamine. The destabilizing effect of N-ethylmaleimide was counteracted fully or nearly so at saturating concentrations of each antagonist and the agonist carbachol. Similar effects occurred with human M(2) receptors differentially tagged with the c-Myc and FLAG epitopes, coexpressed in Sf9 cells, and extracted in digitonin/cholate. The degree of coimmunoprecipitation was unchanged by N-ethylmaleimide, which therefore was without discernible effect on oligomeric size. The data are quantitatively consistent with a model in which the purified receptor from porcine atria interconverts spontaneously between two states (i.e. R R*). Antagonists favor the R state; agonists and N-ethylmaleimide favor the comparatively unstable R* state, which predominates after purification. Occupancy by a ligand stabilizes both states, and antagonists impede alkylation by favoring R over R*. Similarities with constitutively active receptors suggest that R and R* are akin to the inactive and active states, respectively. Purified M(2) receptors therefore appear to exist predominantly in their active state.

Altered expression of autonomic neurotransmitter receptors and proliferative responses in lymphocytes from a chronic mild stress model of depression: effects of fluoxetine

We studied beta-adrenergic and muscarinic cholinergic receptor (MR) expression and proliferative response in lymphocytes from animals under chronic mild stress (CMS) model of depression (CMS animals). Animals were subjected to CMS (periods of food or water deprivation, changes in lighting conditions, tilted cage, etc.) for 12 weeks. CMS lymphocytes showed an altered mitogen-induced proliferation. CMS-B and -T lymphocytes showed an increment on beta-adrenoceptor number and on intracellular responses to a beta-agonist. CMS-T cells showed higher MR expression and lower cGMP responses than normal lymphocytes. MR were not detectable in normal B cells while CMS-B cells showed both MR expression and cGMP response. Beta and muscarinic stimulation influenced lymphocyte proliferative responses, in accordance with cAMP and cGMP responses. After 12 weeks of the CMS procedure, animals were treated with fluoxetine while the CMS procedure continued. Fluoxetine treatment reverted the alterations induced by CMS. These findings suggest a possible mechanism for the immune alterations found in depressive disorders and for the effect of fluoxetine treatment on immune response.

Supersensitivity of the cholinergic muscarinic system in the rat's brain is induced by high concentrations of Cu+2

Transition metals have been described as regulators of receptor's function. here, we studied the effects of chronic administration of Cu2+ or the Cu2+ chelator penicillamine (PA) on the functional and binding properties of the muscarinic receptors (MR) on selected areas of rat's brain. Groups of 10 Sprague-Dawley rats were treated daily, for 45 days with either 1) 1 mg/Kg CuSO4 (Cu2+), 2) 100 mg/Kg PA, or 3) saline solution. Double T-maze and motility cages were used for behavioral testing and the binding assays were performed using [3H]-QNB or [3H]-N-MSCP as MR's ligands. Cu2+ brain levels were measured in the cerebral cortex by atomic absorption spectrophotometer. Results showed that PA treated rats displayed a significant decrease of locomotor's activity (LA) and rearing behavior (RB), but a significant increases in memory efficiency (ME). Cu2+ treated rats displayed diminished RB with no significant changes in LA. Cu2+ treated rats displayed higher MR's density (Bmax) in cortex (C), striatum (S), and hippocampus (H). An increase in Bmax was also observed in PA treated rats, but only in C and S. Finally, Cu2+ tissue concentration was significantly higher in C of both Cu2+ and with PA treated animals. In conclusion, 45 days of Cu2+ or PA treatment induced brain hypercuprosis, which was associated with MR binding supersensitivity; however, change in ME was only observed in PA treated rats suggesting that might be still another factor in these experiments besides Cu2+ (i.e., Zn2+ or PA itself) involved in memory modulation.

Analysis of ligand-receptor interactions from the molecular level to the whole-body level

A system for the quantitative analysis of ligand-receptor interactions is presented, based on models of different levels of complexity. For two pools of receptors, binding of a radioactive ligand is described by b = [(Bml x A(nl))/(K(nl)dl + A(nl))] + [(Bm2 x A(n2))/(Kn2(d2) + A(n2))], (1) where b is the number of bound receptors at a ligand concentration [A], Bml and Bm2 are the receptor concentrations. Kdl and Kd2 are dissociation constants for the ligand-receptor complex, and n1 and n2 are Hill coefficients. The magnitude of the physiological response for a system consisting of two discrete pools of receptors with different affinities is given by p = [(Pm x A(nl))/(EC50(nl) + A(nl))] + [(Pm2 x A(n2)/(EC50(n2)2 + A(n2))], (2) where p is the magnitude of the response to an agonist (or antagonist) at concentration [A], Pml and Pm2 are the maximal magnitudes of the responses for the individual pools of receptors, EC50(1) and EC50(2) are the agonist concentrations giving responses of magnitudes Pm1/2 and Pm2/2, and n1 and n2 are Hill coefficients. The parameters of these equations show: the number of pools of receptors with different affinities for the ligand (Kd or EC50), the number of active receptors (Bmax) or the magnitudes of the maximal response (Pmax), and the numbers of ligand molecules binding with the receptor (n, the Hill coefficient). E is the efficiency (E = Bmax/2Kd, or E = Pmax/2EC50) and gives the overall characteristics of the activity of the effector system. This method of analysis can be applied to any biological reactions whose results can be presented quantitatively.

Altered cardiovascular responses in mice lacking the M(1) muscarinic acetylcholine receptor

Although the M(2) muscarinic acetylcholine receptor (mAChR) is the predominant functional mAChR subtype in the heart, some responses of the cardiovascular system to acetylcholine (ACh) may be mediated by other mAChR subtypes. The potential effect of M(1) mAChR on heart function was investigated using M(1) knockout (M(1)-KO) mice. In vivo cardiodynamic analysis showed that basal values of heart rate (HR), developed left ventricular pressure (DLVP), left ventricular dP/dt(max) (LV dP/dt(max)), and mean blood pressure (MBP) were similar between wild-type (WT) and M(1)-KO mice. Injection of the putative M(1)-selective agonist 4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium (McN-A-343) produced an increase in LV dP/dt(max), DLVP, HR, and MBP in WT mice but did not affect hemodynamic function in the M(1)-KO mice. The stimulatory effect of McN-A-343 in WT mice was blocked by pretreatment with propranolol, indicating that stimulation of the M(1) mAChRs on sympathetic postganglionic neurons evoked release of catecholamines. Intravenous injection of ACh in both WT and M(1)-KO mice caused atrioventricular conduction block, without a significant change in the frequency of atrial depolarization, or atrial fibrillation. Immunoprecipitation and reverse transcriptase-polymerase chain reaction failed to detect the expression of M(1) mAChR in cardiac tissue from WT mice. The carbachol-induced increase of phospholipase C activity in cardiac tissues was not different between WT and M(1)-KO mice. These results demonstrate that 1) activation of M(1) mAChR subtype on sympathetic postganglionic cells results in catecholamine-mediated cardiac stimulation, 2) M(1) mAChR is not expressed in mouse heart, and 3) administration of ACh to mice induces arrhythmia.

Activation of nitric oxide synthase through muscarinic receptors in rat parotid gland

Muscarinic receptors play an important role in secretory and vasodilator responses in rat salivary glands. Nitric oxide synthase (NOS) activity was found coupled to muscarinic receptor activation as well as to nitric oxide-mediated amylase secretion elicited by carbachol. Parotid glands presented a predominant M(3) and a minor muscarinic M(1) acetylcholine receptor population, though carbachol stimulated NOS activity only through muscarinic M(3) receptors as revealed in the presence of 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and pirenzepine. Amylase secretion induced by carbachol appeared to be partly mediated by nitric oxide and nitric oxide-induced signaling since N-nitro-L-arginine methyl ester (L-NAME) inhibited the effect as well as did methylene blue. A negative regulation of NOS by protein kinase C activation in the presence of a high concentration of carbachol was seen in parotid glands and this inhibition was paralleled by amylase secretion.

Doxorubicin affects the cardiac muscarinic system in the rat

During the study on the mechanism of doxorubicin-induced cardiotoxicity, we observed that a long incubation (4 hr) with doxorubicin reduced the maximal negative inotropic effects of a muscarinic receptor agonist, carbachol. The mechanism responsible for this doxorubicin-induced reduction of the efficacy of carbachol was examined in isolated guinea pig hearts. In isolated left atrial muscle preparations, 1 hr incubation with 100 microM doxorubicin caused a parallel right-ward shift of the concentration-response curves for carbachol, but a longer (4 hr) incubation with this agent (30, 100 or 200 microM), caused a significant reduction of the magnitude of the negative inotropic effect of carbachol in addition to the concentration-dependent parallel right-ward shift. The 4-hr incubation with these concentrations of doxorubicin also reduced the maximal negative inotropic effect of an adenosine A1 receptor agonist, R-phenylisopropyl adenosine (R-PIA), without affecting the potency of this agonist. Doxorubicin (1 to 100 microM) reduced [3H]quinuclidinyl benzilate (QNB) binding in a concentration dependent manner, but failed to alter [3HIR-PIA binding. The decrease in the magnitude of the maximal negative inotropic effect by doxorubicin was caused by changes in the muscarinic system at steps common to the transduction of muscarinic and adenosine A1 receptor mechanisms.

Inhibition by tramadol of muscarinic receptor-induced responses in cultured adrenal medullary cells and in Xenopus laevis oocytes expressing cloned M1 receptors

Tramadol is a widely used, centrally acting analgesic, but its mechanisms of action are not completely understood. Muscarinic receptors are known to be involved in neuronal function in the brain and autonomic nervous system, and much attention has been paid to these receptors as targets of analgesic drugs in the central nervous system. This study investigated the effects of tramadol on muscarinic receptors by using two different systems, i.e., a Xenopus laevis oocyte expression system and cultured bovine adrenal medullary cells. Tramadol (10 nM-100 microM) inhibited acetylcholine-induced currents in oocytes expressing the M1 receptor. Although GF109203X, a protein kinase C inhibitor, increased the basal current, it had little effect on the inhibition of acetylcholine-induced currents by tramadol. On the other hand, tramadol did not inhibit the current induced by AlF4-, a direct activator of GTP-binding protein. In cultured bovine adrenal medullary cells, tramadol (100 nM-100 microM) suppressed muscarine-induced cyclic GMP accumulation. Moreover, tramadol inhibited the specific binding of [3H]quinuclidinyl benzilate (QNB). Scatchard analysis showed that tramadol increases the apparent dissociation constant (Kd) value without changing the maximal binding (Bmax), indicating competitive inhibition. These findings suggest that tramadol at clinically relevant concentrations inhibits muscarinic receptor function via QNB-binding sites. This may explain the neuronal function and anticholinergic effect of tramadol.

Sequential changes of cholinergic and dopaminergic receptors in brains after 6-hydroxydopamine lesions of the medial forebrain bundle in rats

We studied sequential changes in muscarinic cholinergic receptors, high-affinity choline uptake sites and dopamine D2 receptors in the brain after 6-hydroxydopamine lesions of the medial forebrain bundle in rats. The animals were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks postlesion. [3H]Quinuclidinylbenzilate (QNB), [3H]hemicholinum-3 (HC-3) and [3H]raclopride were used to label muscarinic cholinergic receptors, high-affinity choline uptake sites and dopamine D2 receptors, respectively. The degeneration of nigrostriatal pathway produced a transient decrease in [3H]QNB binding in the parietal cortex of both ipsilateral and contralateral sides at 2 and 8 weeks postlesion. [3H]QNB binding also showed a mild but insignificant decrease in the ipsilateral striatum throughout the postlesion periods. No significant change was observed in the substantia nigra (SN) of both ipsilateral and contralateral sides throughout the postlesion periods. In contrast, [3H]HC-3 binding showed no significant change in the parietal cortex of both ipsilateral and contralateral sides during the postlesion. However, [3H]HC-3 binding was upregulated in the ipsilateral dorsolateral striatum throughout the postlesion periods. The ventromedial striatum also showed a significant increase in [3H]HC-3 binding at 1 week and 2 weeks postlesion. On the other hand, no significant change in [3H]raclopride binding was found in the parietal cortex of both ipsilateral and contralateral sides during the postlesion. [3H]Raclopride binding showed a conspicuous increase in the ipsilateral striatum (35-52% of the sham-operated values in the lateral part and 39-54% in the medial part) throughout the postlesion periods. In the contralateral side, a mild increase in [3H]raclopride binding was also found in the striatum (10-15% of the sham-operated values in the lateral part and 22% in the medial part) after lesioning. However, a significant decline in [3H]raclopride binding was observed in the ipsilateral SN and ventral tegmental area during the postlesion. The present study indicates that 6-hydroxydopamine injection of medial forebrain bundle in rats can cause functional changes in high-affinity choline uptake site in the striatum, as compared with muscarinic cholinergic receptors. Furthermore, our studies demonstrate an upregulation in dopamine D2 receptors in the striatum and a decrease in the receptors in the SN and ventral tegmental area after the 6-hydroxydopamine injection. Thus, these findings provide further support for neurodegeneration of the nigrostriatal pathway that occurs in Parkinson's disease.

EGF inhibits muscarinic receptor-mediated calcium signaling in a human salivary cell line

The effects of epidermal growth factor (EGF) on intracellular calcium ([Ca(2+)](i)) responses to the muscarinic agonist carbachol were studied in a human salivary cell line (HSY). Carbachol (10(-4) M)-stimulated [Ca(2+)](i) mobilization was inhibited by 40% after 48-h treatment with 5 x 10(-10) M EGF. EGF also reduced carbachol-induced [Ca(2+)](i) in Ca(2+)-free medium and Ca(2+) influx following repletion of extracellular Ca(2+). Under Ca(2+)-free conditions, thapsigargin, an inhibitor of Ca(2+) uptake to internal stores, induced similar [Ca(2+)](i) signals in control and EGF-treated cells, indicating that internal Ca(2+) stores were unaffected by EGF; however, in cells exposed to thapsigargin, Ca(2+) influx following Ca(2+) repletion was reduced by EGF. Muscarinic receptor density, assessed by binding of the muscarinic receptor antagonist L-[benzilic-4,4'-(3)HCN]quinuclidinyl benzilate ([(3)H]QNB), was decreased by 20% after EGF treatment. Inhibition of the carbachol response by EGF was not altered by phorbol ester-induced downregulation of protein kinase C (PKC) but was enhanced upon PKC activation by a diacylglycerol analog. Phosphorylation of mitogen-activated protein kinase (MAP kinase) and inhibition of the carbachol response by EGF were both blocked by the MAP kinase pathway inhibitor PD-98059. The results suggest that EGF decreases carbachol-induced Ca(2+) release from internal stores and also exerts a direct inhibitory action on Ca(2+) influx. A decline in muscarinic receptor density may contribute to EGF inhibition of carbachol responsiveness. The inhibitory effect of EGF is mediated by the MAP kinase pathway and is potentiated by a distinct modulatory cascade involving activation of PKC. EGF may play a physiological role in regulating muscarinic receptor-stimulated salivary secretion.

Characterisation of muscarinic receptor subtypes in avian smooth muscle

The identity of the muscarinic receptor subtype in the chick ileum was investigated in functional and binding studies. Preliminary studies [Choo, L.-K., Mitchelson, F., Napier, P. 1988. J. Auton. Pharmacol. 8, 259-266] suggested apparent avian and mammalian family differences in the muscarinic receptor profile of ileal smooth muscle. In the current study, further characterisation was undertaken using a greater range of antagonists exhibiting high affinity for specific muscarinic receptor subtypes. Dissociation constants from functional and binding experiments were compared with published values for antagonists at each of the five muscarinic receptor subtypes. Linear regression and correlation analyses revealed the receptor initiating the contractile response was most likely of the muscarinic M(3) receptor subtype as the slope of the linear regression was 1.01+/-0.14 and the corresponding correlation coefficient (r) was 0.95. The mammalian muscarinic M(5) receptor subtype also showed a high correlation with the data giving a slope of 0.89+/-0.27 and r value of 0.76. These findings were in direct contrast to those from binding experiments in which the single binding site detected was of the muscarinic M(2) receptor subtype. The slope of the linear regression was 1.14+/-0.24 with an r value of 0.87. Thus, these results suggest that there exists a high proportion of the muscarinic M(2) receptor subtype within the tissue that does not contribute to the functional response.

Zinc and ascorbic acid coordinately promote lipid peroxidation in brain membranes

Zn2+ is present at high concentrations in mammalian brain, and is released in chelatable form after excitation of certain glutamatergic neurons. Recent observations suggest that it may play an important role in excitotoxic-induced neural injury. Ascorbic acid has been widely studied as a stimulator or an inhibitor of lipid-peroxide formation, depending on concentration, and lipid peroxidation has been postulated to be involved in both acute and chronic neurogenerative diseases. We find that ascorbic acid and Zn2+, at concentrations that are achieved in the brain after prolonged synaptic depolarization, coordinately promote lipid-peroxide formation and cause dysfunction of membrane-bound proteins. This effect is unique to Zn2+, and other divalent cations do not share a similar synergism with ascorbate. We propose that the Zn2+-ascorbate interaction may be an overlooked mechanism of lipid-peroxide formation in brain injury.

Preclinical testing of N-[(11)c]-methyl-piperidin-4-yl 2-cyclohexyl-2-hydroxy-2-phenylacetate, a novel radioligand for detection of cerebral muscarinic receptors using PET

The muscarinic antagonist N-[(11)C]methyl-piperidin-4-yl 2-cyclohexyl-2-hydroxy-2-phenylacetate (VC-004) 1 was tested for visualization of muscarinic receptors in the brain. The active (R)-isomer (pKb = 10.92) was labeled by reacting [(11)C]-CH(3)I with the secondary amine precursor (40-60% decay-corrected radiochemical yield, specific activity 13.0-34.3 TBq/mmol, 45 min after end of bombardment). Biodistribution studies were performed in male Wistar rats. Brain uptake of (R)-[(11)C]-VC-004 was high, standard uptake values (SUVs) ranging from 1.6 in cerebellum to 3.3 in frontal cortex. In all brain regions, the nonsubtype selective muscarinic antagonist scopolamine (2.5 mg/kg) blocked (R)-[(11)C]-VC-004 binding to the same extent (84.6 +/- 3.3%) as nonlabeled (R)-VC-004 (2.0 mg/kg, 83.2 +/- 4.6%). In contrast, the fraction of [(11)C]VC-004 binding which was blocked by atropine (2.5 mg/kg) was significantly smaller (54 +/- 17%). The reduction of (R)-[(11)C]-VC-004 binding by low-dose atropine (0.5 mg/kg) was not significantly different from that caused by (R)-(-)-QNB (20 microg/kg). The decrease in specific binding of (R)-[(11)C]VC-004 after (R)-(-)-QNB block correlated well with literature values for the percentages of M(2) receptors in the brain regions studied. (R)-[(11)C]VC-004 was rapidly cleared from plasma (92% with a half-life of 0.27 min) and the fraction of total plasma radioactivity representing parent compound decreased from 99% to 42% at 10 min postinjection. Although (R)-[(11)C]VC-004 can visualize muscarinic receptors in the brain, it does not show selectivity for the M(2)-subtype. A low dose (0.5 mg/kg) of atropine seems to preferentially block M(2)-receptors in vivo, as has been reported for (R)-(-)-QNB.

Pharmacological characterisation of the enantiomers of BM-5, a muscarinic partial agonist with opposed enantioselectivity between affinity and efficacy

The interaction of (R) and (S) enantiomers of the chiral oxotremorine analogue BM-5 with muscarinic acetylcholine receptors was studied in vitro using radioligand binding and isolated tissue preparations. The in vivo effects of (R)-BM-5 were also studied in anaesthetised cat. No receptor or tissue selectivity was found for either enantiomer in radioligand binding studies in cells expressing human muscarinic receptors (M1-M5) or in guinea pig tissues. The affinity of (R)-BM-5 was about 40 times, or 15-60 times higher than that of (S)-BM-5 in recombinant cells or in guinea pig tissues, respectively. Both enantiomers induced contraction of the guinea pig isolated urinary bladder and ileum. (R)-BM-5 was more potent than (S)-BM-5 in bladder (EC50 590 and 3500 nM, respectively) and in ileum (EC50 39 and 2600 nM, respectively). The maximal agonist effect was lower for (R)-BM-5 than for (S)-BM-5 in bladder (2.7% and 6.6%, respectively) and in ileum (32% and 48%, respectively). Contractions were completely inhibited by atropine (1 microM). In vivo, (R)-BM-5 induced bladder contraction and salivation after intravenous administration in the anaesthetised cat (ED50 4.1 and 6.2 microg kg(-1), respectively). In conclusion, (R)- and (S)-BM-5 act as partial muscarinic agonists in the isolated bladder and ileum. (R)-BM-5 was the more potent enantiomer but had a lower maximal agonist effect giving an opposed enantioselectivity for affinity and efficacy. (R)-BM-5 showed agonist activity in vivo, confirming in vitro findings. From affinity and efficacy data it can be concluded that the effects of racemic BM-5 are mediated by the (R)-enantiomer.

An approach to analysis of radiolabeled ligand interactions with specific receptors

The aim of the study was to reveal general characteristics of the ligand-receptor interaction in the binding and displacement of radiolabeled ligands. The binding and displacement of DL-[3H]propranolol hydrochloride ([3H]propranolol) and L-[propyl-2,3, -3H]dihydroalprenolol ([3H]dihydroalprenolol), beta-adrenoceptor antagonists, were studied with isolated rat red blood cells, their membranes and ghosts. The binding of [3H]dihydroalprenolol and L-quinuclidinyl-[phenyl-4-3H]-benzylate ([3H]quinuclidinyl benzylate), a muscarinic acetylcholine receptor antagonist, was studied with cerebral cortex membranes. The ligand-receptor interaction corresponded to that for a model of two pools of receptors in the same effector system, with the binding of two ligand molecules to the receptors and was described by the following equation: b=[(B(m1)A(2))/(K(d1)(2)+A(2))]+[(B(m2)A(2))/(K(d2)(2)+A(2))]. The parameters of [3H]propranolol binding to beta-adrenoceptors were as follows: K(d1)=0.74 nM, K(d2)=14.40 nM, B(m1)=24 unit/cell, and B(m2)=263 unit/cell for native red blood cells from rats; K(d1)=0.70 nM, K(d2)=19.59 nM, B(m1)=9 fmol/mg protein, and B(m2)=39 fmol/mg protein for blood ghosts. The parameters of [3H]quinuclidinyl benzylate binding to muscarinic acetylcholine receptors of cerebral cortex membrane were as follows: K(d1)=0.43 nM, K(d2)=2.83 nM, B(m1)=712 fmol/mg, B(m2)=677 fmol/mg. The analysis of the equilibrium binding and displacement of [3H]propranolol and [3H]dihydroalprenolol at beta-adrenoceptors of membranes, ghosts and native red cells of rats, [3H]dihydroalprenolol at beta-adrenoceptors and [3H]quinuclidinyl benzylate at muscarinic acetylcholine receptors of synaptosomal membranes from rat cerebral cortex demonstrated that the receptors bound two ligand molecules each and consisted of two discrete pools of high- and low-affinity receptors. Similar results were obtained for the displacement of [3H]propranolol, [3H]dihydroalprenolol and [3H]quinuclidinyl benzylate by propranolol, dihydroalprenolol and quinuclidinyl benzylate.

Alanine-scanning mutagenesis of transmembrane domain 6 of the M(1) muscarinic acetylcholine receptor suggests that Tyr381 plays key roles in receptor function

Transmembrane domain 6 of the muscarinic acetylcholine (ACh) receptors is important in ligand binding and in the conformational transitions of the receptor but the roles of individual residues are poorly understood. We have carried out a systematic alanine-scanning mutagenesis study on residues Tyr381 to Val387 within the binding domain of the M(1) muscarinic ACh receptor. The seven mutations were then analyzed to define the effects on receptor expression, agonist and antagonist binding, and signaling efficacy. Tyr381Ala produced a 40-fold reduction in ACh affinity and a 50-fold reduction in ACh-signaling efficacy. Leu386Ala had similar but smaller effects. Asn382Ala caused the largest inhibition of antagonist binding. The roles of the hydroxyl group and benzene ring of Tyr381 were probed further by comparative analysis of the Tyr381Phe and Tyr381Ala mutants using three series of ligands: ACh analogs, azanorbornane- and quinuclidine-based ligands, and atropine analogs. These data suggested that the hydroxyl group of Tyr381 is primarily involved in forming hydrogen bond interactions with the oxygen atoms present in the side chain of ACh. We propose that this interaction is established in the ground state and preserved in the activated state of the receptor. In contrast, the Tyr381 benzene ring may form a cation-pi interaction with the positively charged head group of ACh that contributes to the activated state of the receptor but not the ground state. However, the hydroxyl group and benzene ring of Tyr381 both participate in interactions with azanorbornane- and quinuclidine-based ligands and atropine analogs in the ground state as well as the activated state of the receptor.

Autoradiographic demonstration of quinuclidinyl benzilate binding sites in the vestibular organs of the gerbil

Gerbil vestibular tissues were isolated by microdissection and incubated in vitro with 3H-quinuclidinyl benzilate (3H-QNB). Control tissues were incubated in medium containing unlabeled atropine to differentiate non-specific from specific binding. Autoradiographic grain densities were determined by morphometric techniques and evaluated by two-tailed t-test. The label densities of sensory epithelia from experimental preparations of ampulla, utricle and saccule were found to be significantly higher than those in the adjacent endolymphatic compartment and also higher than those of adjacent stromal tissue comprising connective tissue, nerve fibers and capillaries. In contrast, no tissue region in atropine controls showed label density significantly above that of the endolymphatic compartment. Label density of ampullar sensory epithelium incubated with 3H-QNB alone was significantly higher than that of sensory epithelium from utricle or saccule. Grain density was greater in the peripheral regions of the ampullar crista compared to the vertex. Appreciable label was also present in nerve bundles beneath the sensory epithelium of the ampulla. The current study demonstrates the existence of putative muscarinic neurotransmitter/neuromodulator receptor sites in mammalian vestibular sense organs at locations corresponding to efferent innervation, with particularly significant concentrations in the ampulla.

Inhibitory cholinergic effects on the autonomously contractile bulbus cordis branchialis of the cephalopod Sepia officinalis L

In vitro experiments were performed on a standardized preparation of the autonomously contractile bulbus cordis branchialis of the branchial heart of Sepia officinalis to investigate its cholinergic neuroregulation. Apart from acetylcholine, nicotine and carbachol (nicotinic agonists), the muscarinic agonists muscarine, arecoline, pilocarpine, and oxotremorine also exerted concentration-dependent negative inotropic effects on the preparations. As both the muscarinic antagonist quinuclidinylbenzilate and the nicotinic antagonist alpha-bungarotoxin blocked the ACh action there might be a special, possibly mixed muscarinic/nicotinic ACh-receptor system in the myocytes of the bulbus cordis branchialis, which is different from the cholinergic receptor in the central part of the branchial heart.

Effects of mitoxantrone on action potential and membrane currents in isolated cardiac myocytes

1. The effects of mitoxantrone (MTO), an anticancer drug, on the membrane electrical properties of cardiac myocytes were investigated using the whole-cell clamp technique. 2. In isolated guinea-pig ventricular myocytes, 30 microM MTO induced a time-dependent prolongation of action potential duration (APD) which was occasionally accompanied by early afterdepolarizations. APD prolongation was preserved in the presence of 10 microM tetrodotoxin and showed reverse rate-dependence. 3. Both the inward rectifier K+ current (I(KI)) and the delayed rectifier K+ current (I(K)) of guinea-pig ventricular myocytes were significantly depressed by 30 microM MTO. The rapidly activating component of I(k) (I(Kr)) seemed to be preferentially blocked by MTO. The transient outward current was not affected by MTO in rat ventricular myocytes. 4. Thirty microM MTO had no direct effect on the L-type Ca2+ current (I(Ca(L))), but reversed the inhibitory effect of 1 microM carbamylcholine but not the A1-adenosine receptor agonist (-)-N6-phenylisopropyladenosine (1 microM) on I(Ca(L)) enhanced by 50 nM isoprenaline in guinea-pig ventricular myocytes. In guinea-pig atrial mycotyes, 30 microM MTO inhibited by 93% the muscarinic receptor gated K+ current (I(K,ACh)) evoked by 1 microM carbamylcholine, whereas I(K,ACh) elicited by 100 microM GTPgammaS, a nonhydrolysable GTP analogue, was only decreased by 12%. 5. The specific binding of [3H]QNB, a muscarinic receptor ligand, to human atrial membranes was concentration-dependently displaced by MTO (1-1000 microM). 6. In conclusion, MTO blocks cardiac muscarinic receptors and prolongs APD by inhibition of I(KI) and I(Kr). The occasionally observed early afterdepolarizations may signify a potential cardiac hazard of the drug.

[Effects of propiverine hydrochloride (propiverine) on the muscarinic receptor binding affinity in guinea pig tissues and on salivation in conscious dogs]

Propiverine is a drug for the treatment of incontinence and pollakiuria. Such micturitional disorders are principally caused by a hyperactive bladder. The effects of propiverine, its active metabolite, 1-methyl-4-piperidyl benzilate N-oxide (DPr-P-4 (N-->O)), oxybutynin and terodiline on muscarinic receptors in guinea pig urinary bladder, salivary glands, cerebral cortex, ileal longitudinal muscle and heart were compared. Both propiverine and DPr-P-4 (N-->O) competitively inhibited specific binding of 3H-quinuclidinyl benzilate (3H-QNB) to membrane fractions of these tissues. Oxybutynin, terodiline, pirenzepine and atropine also competitively inhibited the binding of 3H-QNB. The order of these drugs in terms of their affinity for muscarinic receptors was as follows: atropine > oxybutynin > pirenzepine, DPr-P-4 (N-->O), terodiline > propiverine. Propiverine and DPr-P-4 (N-->O) had no selectivity for muscarinic receptors in these tissues, the same as atropine. In contrast, pirenzepine, a M1-selective drug, had 10.1 times greater affinity for muscarinic receptors in the cerebral cortex than in urinary bladder, and the affinity of oxybutynin for muscarinic receptors in salivary glands and in cerebral cortex was 10.9 times and 13.9 times higher, respectively, than in urinary bladder. The affinity of terodiline for muscarinic receptors in the cerebral cortex was 4.4 times higher than in urinary bladder. In this study, the effect of propiverine and oxybutynin on pilocarpine (1 mg/kg, s.c.)-induced salivation in conscious dogs was also compared. Propiverine (5 mg/kg, i.v.) had no effect on pilocarpine-induced salivation, whereas oxybutynin (0.1 mg-0.5 mg/kg, i.v.) inhibited it significantly and dose-dependently. The ID50 values (95% confidence limits) for propiverine and oxybutynin during the 20 min after intravenous administration were 6.88 mg/kg (4.71-15.67) and 0.154 mg/kg (0.115-0.205), respectively. These findings suggest that although propiverine, its active metabolite DPr-P-4 (N-->O), oxybutynin and terodiline competitively inhibit the binding of 3H-QNB to muscarinic receptors, the affinity of these drugs for the muscarinic receptors of these tissues is very different and that propiverine has less effect on salivation than oxybutynin.

Rapid eye movement sleep induction by vasoactive intestinal peptide infused into the oral pontine tegmentum of the rat may involve muscarinic receptors

In rats, rapid eye movement sleep can be induced by microinjection of either the cholinergic agonist carbachol or the neuropeptide vasoactive intestinal peptide into the oral pontine reticular nucleus. Possible involvement of cholinergic mechanisms in the effect of vasoactive intestinal peptide was investigated using muscarinic receptor ligands. Sleep-waking cycles were analysed after infusion into the oral pontine reticular nucleus of vasoactive intestinal peptide (10 ng in 0.1 microl), carbachol (20 ng), atropine (200 ng) and pirenzepine (50, 100 ng), performed separately or in combination at 15-min intervals. The increase in rapid eye movement sleep due to the combined infusion of vasoactive intestinal peptide and carbachol (+58.7+/-4.6% for 8 h, P<0.05) was not significantly different from that induced by each compound separately. The enhancement of rapid eye movement sleep by vasoactive intestinal peptide was totally prevented by infusion of atropine, but not pirenzepine, a relatively selective M1 antagonist. On their own, none of the latter two compounds affected the sleep-waking cycle. Quantitative autoradiographic studies using [3H]quinuclidinyl benzylate (1 nM) and pirenzepine (0.5 microM) indicated that muscarinic receptors correspond to pirenzepine-insensitive binding sites in the oral pontine reticular nucleus. In vitro, vasoactive intestinal peptide (1-100 nM) significantly increased (+30-40%) the specific binding of [3H]quinuclidinyl benzylate to the oral pontine reticular nucleus in rat brain sections. This effect appeared to be due to an increased density, with no change in affinity, of pirenzepine-insensitive binding sites in this area. These data suggest that pirenzepine-insensitive muscarinic binding sites are involved in the induction of rapid eye movement sleep by vasoactive intestinal peptide at the pontine level in the rat.

The presence of a muscarinic receptor on canine erythrocyte membranes

Muscarinic receptors were present on erythrocyte ghosts of the domestic dog (Canis familiaris) at a concentration of 0.61 nmol/mg protein, and had a KD of 62.5 nmol. Properties including a mean nonspecific binding of 80%, the high risk that adherence of additional molecules of radioligand would occur, and the necessity for extensive dilution of erythrocyte ghosts to obtain a practical quantity of muscarinic receptors, lead us to conclude that the canine erythrocyte ghost is not an efficient or practical model to study muscarinic receptors and their interactions with cholinergic agonists or antagonists.

Nerve growth factor preserves a critical motor period in rat striatum

We previously found the occurrence of a critical motor period during rat postnatal development where circling training starting the 7-day schedule at 30 days-but not before or after-induces a lifetime drop in the binding to cholinergic muscarinic receptors (mAChRs) in striatum. Here, we studied whether nerve growth factor (NGF) participates in this restricted period of muscarinic sensitivity. For this purpose, we administered mouse salival gland 2.5S NGF (1.4 or 0.4 microg/day, infused by means of ALZA minipumps) by intrastriatal unilateral route between days 25 and 39, and then trained rats starting at 40 days. Under these conditions, NGF induced a long-term reduction in the striatal [3H] quinuclidilbenzylate (QNB) binding sites despite the fact that motor training was carried out beyond the natural critical period. Thus, at day 70, measurement of specific QNB binding in infused striata of trained rats showed decreases of 42% (p < .0004) and 33% (p < .02) after administration of the higher and lower NGF doses, respectively, with respect to trained rats treated with cytochrome C, for control. Noncannulated striata of the NGF-treated rats also showed a decrease in QNB binding sites (44%; p < .0001) only at the higher infusion rate. This effect was not found in the respective control groups. Our observations show that NGF modulates the critical period in which activity-dependent mAChR setting takes place during rat striatal maturation.

Effects of an agonist, allosteric modulator, and antagonist on guanosine-gamma-[35S]thiotriphosphate binding to liposomes with varying muscarinic receptor/Go protein stoichiometry

We investigated whether alcuronium, an allosteric modulator of muscarinic acetylcholine receptors, can induce receptor-mediated activation of Go proteins in liposomal membranes incorporating purified M2 receptors and Go proteins and whether its action is affected by the receptor/Go protein (R/Go) ratio. The binding of guanosine-gamma-[35S]thiotriphosphate ([35S]GTPgammaS) served as the indicator of G protein activation. It was stimulated by empty receptors at high receptor densities, and the dose-response curve was shifted to the left by the agonist carbachol and to the right by the antagonist atropine. At an R/Go ratio of 300:100, the rate of [35S]GTPgammaS binding was the same in the presence or absence of 0. 1 mM carbachol. Alcuronium increased the binding of [35S]GTPgammaS at R/Go ratios of <3:100 and diminished it at R/Go ratios of >10:100, similar to previous observations on intact cells expressing muscarinic receptors at different densities. The apparent biphasicity of alcuronium action indicates that the allosteric modulator has at least two effects on muscarinic receptor/G protein interaction but its mechanistic basis is unclear. The "active state" of muscarinic receptors induced by alcuronium probably is different from that induced by carbachol. Changes in the densities of receptors and Go proteins had little effect on the kinetics of [35S]GTPgammaS binding and on receptor affinity for carbachol, provided the R/Go ratio was kept constant. This suggests that the receptors and G proteins are located in microdomains in which their concentrations remain constant, despite variations in the amounts of lipidic membranes in the system.

M3-type muscarinic receptors predominantly mediate neurogenic quick contraction of bovine ciliary muscle

1. The present experiments were designed to investigate which subtypes of muscarinic receptors are involved in the neurogenic quick contraction of bovine ciliary muscle in connection to quick eye focal accommodation. 2. Transmural electrical stimulation (TES) produced a transient contraction, which was abolished in the presence of 3 x 10(-7) M tetrodotoxin and 10(-6) M atropine, but greatly augmented by 3 x 10(-7) M physostigmine. 3. The exogenously applied acetylcholine (ACh: 10(-9) to 3 x 10(-6) M) produced a concentration-dependent contraction, which was competitively antagonized by 10(-6) M atropine and augmented by 3 x 10(-7) M physostigmine, but unaffected by 3 x 10(-7) M tetrodotoxin. 4. The magnitude and time to peak of the maximal contraction produced by TES were significantly greater (1267.5 +/- 86.0 mg, P < 0.005) and shorter (9.0 +/- 0.2 sec, P < 0.005) than corresponding values (97.0 +/- 9.9 mg and 20.3 +/- 2.1 sec, respectively) of the phasic contraction caused by exogenously applied 10(-5) M ACh, at which concentration the agonist caused the maximal contraction. The velocity (140.6 +/- 7.8 mg/sec) of the transient contraction caused by TES was approximately 28-fold greater than that of the phasic contraction caused by ACh (5.1 +/- 0.9 mg/sec). 5. The contractions produced by TES were greatly attenuated by 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) as an M3 antagonist and slightly by pirenzepine as an M1 antagonist (20.2 +/- 7.9% inhibition at the highest concentration), but not by methoctramine (MET) as an M2 antagonist. The IC50 value (-log M) for 4-DAMP was determined to be 7.17 +/- 0.14. 6. Scatchard plot analysis of [3H]-quinuclidinylbenzilate (QNB) binding revealed that the binding sites constituted a single population with a Kd of 31.2 +/- 0.8 pM and a Bmax of 895.5 +/- 93.2 fmol/mg protein. The activity in inhibiting [3H]-QNB binding was most potent with 4-DAMP (-log Ki = 7.98 +/- 0.02), but less potent with pirenzepine (-log Ki = 6.43 +/- 0.04) and MET (-log Ki = 7.32 +/- 0.16). 4-DAMP was approximately 35- and 5-fold more potent than pirenzepine and MET in terms of -log Ki values, respectively, suggesting the predominant localization of M3 receptor subtypes in the bovine ciliary muscle membrane. 7. These results suggest that TES produces a neurogenic quick contraction of the bovine ciliary muscle, which would be mediated mainly by ACh released from the intramural nerve terminals and subsequent excitation of M3 receptor subtypes localized on the ciliary muscle cells, and that neurogenic quick contraction of the ciliary muscle is possibly involved in part in eye focal accommodation.

Internalization and down-regulation of human muscarinic acetylcholine receptor m2 subtypes. Role of third intracellular m2 loop and G protein-coupled receptor kinase 2

Internalization and down-regulation of human muscarinic acetylcholine m2 receptors (hm2 receptors) and a hm2 receptor mutant lacking a central part of the third intracellular loop (I3-del m2 receptor) were examined in Chinese hamster ovary (CHO-K1) cells stably expressing these receptors and G protein-coupled receptor kinase 2 (GRK2). Agonist-induced internalization of up to 80-90% of hm2 receptors was demonstrated by measuring loss of [3H]N-methylscopolamine binding sites from the cell surface, and transfer of [3H]quinuclidinyl benzilate binding sites from the plasma membrane into the light-vesicle fractions separated by sucrose density gradient centrifugation. Additionally, translocation of hm2 receptors with endocytic vesicles were visualized by immunofluorescence confocal microscopy. Agonist-induced down-regulation of up to 60-70% of hm2 receptors was demonstrated by determining the loss of [3H]quinuclidinyl benzilate binding sites in the cells. The half-time (t1/2) of internalization and down-regulation in the presence of 10(-4) M carbamylcholine was estimated to be 9.5 min and 2.3 h, respectively. The rates of both internalization and down-regulation of hm2 receptors in the presence of 10(-6) M or lower concentrations of carbamylcholine were markedly increased by coexpression of GRK2. Agonist-induced internalization of I3-del m2 receptors was barely detectable upon incubation of cells for 1 h, but agonist-induced down-regulation of up to 40-50% of I3-del m2 receptors occurred upon incubation with 10(-4) M carbamylcholine for 16 h. However, the rate of down-regulation was lower compared with wild type receptors (t1/2 = 9.9 versus 2.3 h). These results indicate that rapid internalization of hm2 receptors is facilitated by their phosphorylation with GRK2 and does not occur in the absence of the third intracellular loop, but down-regulation of hm2 receptors may occur through both GRK2-facilitating pathway and third intracellular loop-independent pathways.

Characteristic subcellular distribution, in brain, heart and lung, of biperiden, trihexyphenidyl, and (-)-quinuclidinyl benzylate in rats

The subcellular distribution of biperiden (BP), trihexyphenidyl (TP) and (-)-quinuclidinyl benzylate (QNB) in brain, heart and lung following high dose (3.2 mg/kg) i.v. administration was investigated in rats. The subcellular distribution of BP or TP used clinically conformed with that of QNB, a typical potent central muscarinic antagonist. The concentration-time courses of the brain subcellular fractions for these drugs were of two types which decreased slowly and in parallel to the plasma concentration. The subcellular distribution in the brain and heart was dependent on the protein amount of each fraction. The percent post-nuclear fraction (P2) of the total concentration in the lung was characteristically about 3-5 times larger than that in the heart. It was elucidated that the distribution in the lung differs from that in the brain and heart, with high affinity which is not dependent on the protein amount in the P2 fraction containing lysosomes. On the other hand, at a low dose (650 ng/kg) of 3H-QNB, each fraction as a percentage of the total concentration in the brain increased in synaptic membrane and synaptic vesicles and decreased in nuclei and cytosol as compared with the high dose. These results show that although the tissue concentration-time courses of anticholinergic drugs appear to decrease simply in parallel to plasma concentration, the subcellular distribution exhibits a variety of patterns among various tissues.

Diazepam protects against rat hippocampal neuronal cell death induced by antisense oligodeoxynucleotide to GABA(A) receptor gamma2 subunit

Antisense oligodeoxynucleotides (ODNs) are used for the selective inhibition of gene expression. Antisense ODNs are promising tools for the investigation of physiological implications of proteins in the central nervous system of rodents in vivo. We have previously demonstrated that a phosphorothioate antisense ODN to the GABA(A) receptor gamma2 subunit, but not sense or mismatch control ODNs, induces a decrease in ex vivo benzodiazepine receptor radioligand binding in rat hippocampus when infused into the hippocampus in vivo [Karle et al., Neurosci. Lett., 202 (1995) 97-100]. This effect is parallelled by a decrease in the number of GABA(A) receptors and an extensive loss of hippocampal neurones. There is increasing awareness of risks of toxic 'non-antisense' effects induced by ODNs, and in particular phosphorothioate ODNs. The present experiments were designed to investigate the specificity of effects induced by the gamma2 subunit antisense ODN. The temporal development of changes in [3H]flunitrazepam and [3H]quinuclidinyl benzilate binding as well as in tissue protein levels supports the notion that the antisense ODN primarily acts by blocking the expression of the targeted receptor subunit protein. Furthermore, it is shown that a threshold for the elicitation of neurodegenerative changes exists. Finally, it is demonstrated that diazepam treatment of rats protects against the development of neuronal cell death induced by the antisense ODN. Collectively, the results support the hypothesis that the neurodegeneration induced by the antisense ODN is a consequence of diminished GABAergic inhibitory tonus following a selective down-regulation of gamma2 subunit-containing GABA(A) receptor complexes.

Intracellular signals coupled to different rat ileal muscarinic receptor subtypes

Taking into account that the activation of different subtypes of ileal muscarinic acetylcholine receptors (mAChR) regulate gut functions such as tone, motility, and electrolyte secretion, we characterized the expression of mAChR in ileal-purified membranes. We also studied intracellular signals triggered by mAChR activation. Binding parameters obtained from saturation assays with the nonselective tritiated muscarinic antagonist, quinuclidynil benzilate ([3H]-QNB), were maximal number of binding sites (Bmax): 30 +/- 2 fmol/mg prot and dissociation constant (Kd): 0.2 +/- 0.03 nM. The competitive inhibition of [3H]-QNB specific binding by various nonlabelled muscarinic antagonists was measured and the rank order of potency was: atropine (ATROP) > 4-DAMP > AF-DX 116 > pirenzepine (PZ). The activation of mAChR by carbachol (CARB) increased ileal motility in a concentration-dependent manner (EC50 2 x 10[-7] M). The antagonists' order of potency to displace dose-response curve of CARB was: ATROP > 4-DAMP > AF-DX116 > PZ. Optimal concentration of CARB on ileal strips increased phosphoinositide turnover and cGMP levels by activating ml receptor subtype and decreased isoproterenol (ISO) stimulated levels of cAMP due to M2 receptor activation. We can conclude that the activation of different mAchR subtypes triggers different intracellular signals that could regulate intestinal tone and motility.

[The effect of a new cholinolytic-[3H] tricyclopinate on human brain muscarinic receptors]

The binding characteristics of the novel cholinergic antagonist [3H] tricyclopinate with muscarinic receptors from human cerebral cortex were investigated in comparison with [3H] QNB by performing radioligand binding assays. As revealed by saturation experiments, the binding parameters of [3H] tricyclopinate (Kd = 0.044 nmol.L-1, Bmax = 514 fmol.mg-1) were almost identical with those of [3H]QNB (Kd = 0.040 nmol.L-1, Bmax = 508 fmol.mg-1). Both ligands fit a one site model of receptor-ligand interaction. Tricyclopinate showed a potency comparable to QNB on muscarinic receptors in inhibition experiments. However, some differences also existed between tricyclopinate and QNB. Kinetic experiments showed that both the association and dissociation of tricyclopinate (K1 = 1.40 (nmol.L-1)-1.min-1, K2 = 0.39 min-1) with muscarinic receptors were quicker than QNB (K1 = 0.65 (nmol.L-1)-1.min-1, K2 = 0.005 min-1). In addition, tricyclopinate behaved differently from QNB in the response of the dissociation profile to the allosteric modulation of gallamine. These results demonstrated that tricyclopinate has comparable affinity to muscarinic receptors with QNB but might interact with them in a different way. The introduction of [3H] tricyclopinate might complement the use of [3H] QNB in the study of central muscarinic receptors.

Muscarinic acetylcholine receptors on rat thymocytes: their possible involvement in DNA fragmentation

Several studies have shown that the nervous (and hormonal) system controls immune functions. In the present study, we examined the presence of muscarinic acetylcholine receptors and the effect of carbachol on DNA fragmentation in adult rat thymocytes. Rat thymocytes possessed high affinity binding sites for the muscarinic antagonist [3H]3-quinuclidinyl benzilate (QNB). The average number of binding sites per cells was 3000, and the equilibrium dissociation constant of [3H]QNB on intact cell was approximately 80 nM. The binding was inhibited by an M1- and M3-selective antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodine (4-DAMP). Hydrocortisone (100 mg/kg, s.c.) treatment of rats for 2 days prior to sacrifice increased the average number of [3H]QNB binding sites on thymocytes by 82 +/- 33%. The gel electrophoresis of DNA extracted from carbachol-treated thymocytes revealed a ladder pattern typical of intranucleosomal fragmentation. The addition of oxotremorine-M also induced DNA fragmentation and the effects of muscarinic agonists were inhibited by the addition of atropine or 4-DAMP. The results suggest the existence of muscarinic receptors and the possible involvement in apoptosis in thymocytes.

Correction of the stereochemical assignment of the benzilic acid center in (R)-(-)-3-quinuclidinyl (S)-(+)-4-iodobenzilate [(R,S)-4-IQNB]

Radioiodinated (R)-quinuclidinyl-4-iodobenzilate (4IQNB) is a high affinity muscarinic antagonist which has been utilized for in vitro and in vivo assays, and for SPECT imaging in humans. 4IQNB exists in four different diastereomeric forms, since there are two asymmetric centers at the quinuclidinyl and benzilic acid centers. Based upon our in vivo studies, we have determined that the absolute stereochemistry previously assigned to the benzilic center was incorrect for the diastereomer that had been previously referred to as '(R)-quinuclidinyl-(R)-4-iodobenzilate' [(R,R)-4IQNB]. The correct designation for this diastereomer is '(R)-quinuclidinyl-(S)-4-iodobenzilate' [(R,S)-4IQNB].

Reversal of a selective decrease in hippocampal acetylcholine release, but not of the persistence of kindling, after discontinuation of long-term pentylenetetrazol administration in rats

The time course of the effect of pentylenetetrazol (PTZ)-induced kindling on acetylcholine release in the hippocampus of freely moving rats was investigated with the transversal microdialysis technique. The basal extracellular concentration of acetylcholine in the hippocampus was reduced significantly (-29%, P < 0.05) after 3 weeks, and the effect was maximal (-52%, P < 0.01) after 4 weeks and remained essentially unchanged during the remaining 4 weeks of PTZ treatment (30 mg/kg, i.p., 3 times/week), relative to vehicle-treated rats. The basal release of acetylcholine in the prefrontal cortex and in the striatum of kindled rats was unchanged compared with that of vehicle-treated rats. The specific binding of [3H]quinuclidinyl benzilate, a non-selective ligand of muscarinic receptors, was significantly increased (+29%, P < 0.01) in hippocampal membrane, but not in membranes prepared from the prefrontal cortex or striatum, of PTZ-kindled rats. Thirty days after discontinuation of PTZ treatment, both hippocampal acetylcholine output and the density of muscarinic receptors had returned to values characteristic of vehicle-treated rats, whereas seizure susceptibility did not differ significantly from that apparent 4 days after PTZ administration. These results suggest that the selective and transient decrease in acetylcholine output and the parallel increase in the density of postsynaptic muscarinic receptors in the hippocampus may play a role in facilitating the development of kindling rather than in the maintenance of the kindled state.