Pharmacological differences between two muscarinic responses of the rat superior cervical ganglion in vitro

Cell-Specific Cholinergic Modulation of Excitability of Layer 5B Principal Neurons in Mouse Auditory Cortex

The neuromodulator acetylcholine (ACh) is crucial for several cognitive functions, such as perception, attention, and learning and memory. Whereas, in most cases, the cellular circuits or the specific neurons via which ACh exerts its cognitive effects remain unknown, it is known that auditory cortex (AC) neurons projecting from layer 5B (L5B) to the inferior colliculus, corticocollicular neurons, are required for cholinergic-mediated relearning of sound localization after occlusion of one ear. Therefore, elucidation of the effects of ACh on the excitability of corticocollicular neurons will bridge the cell-specific and cognitive properties of ACh. Because AC L5B contains another class of neurons that project to the contralateral cortex, corticocallosal neurons, to identify the cell-specific mechanisms that enable corticocollicular neurons to participate in sound localization relearning, we investigated the effects of ACh release on both L5B corticocallosal and corticocollicular neurons. Using in vitro electrophysiology and optogenetics in mouse brain slices, we found that ACh generated nicotinic ACh receptor (nAChR)-mediated depolarizing potentials and muscarinic ACh receptor (mAChR)-mediated hyperpolarizing potentials in AC L5B corticocallosal neurons. In corticocollicular neurons, ACh release also generated nAChR-mediated depolarizing potentials. However, in contrast to the mAChR-mediated hyperpolarizing potentials in corticocallosal neurons, ACh generated prolonged mAChR-mediated depolarizing potentials in corticocollicular neurons. These prolonged depolarizing potentials generated persistent firing in corticocollicular neurons, whereas corticocallosal neurons lacking mAChR-mediated depolarizing potentials did not show persistent firing. We propose that ACh-mediated persistent firing in corticocollicular neurons may represent a critical mechanism required for learning-induced plasticity in AC.

SIGNIFICANCE STATEMENT

Acetylcholine (ACh) is crucial for cognitive functions. Whereas in most cases the cellular circuits or the specific neurons via which ACh exerts its cognitive effects remain unknown, it is known that auditory cortex (AC) corticocollicular neurons projecting from layer 5B to the inferior colliculus are required for cholinergic-mediated relearning of sound localization after occlusion of one ear. Therefore, elucidation of the effects of ACh on the excitability of corticocollicular neurons will bridge the cell-specific and cognitive properties of ACh. Our results suggest that cell-specific ACh-mediated persistent firing in corticocollicular neurons may represent a critical mechanism required for learning-induced plasticity in AC. Moreover, our results provide synaptic mechanisms via which ACh may mediate its effects on AC receptive fields.

Effects of the muscarinic agonist, 5-methylfurmethiodide, on contraction and electrophysiology of Ascaris suum muscle

Contraction and electrophysiological effects of 5-methylfurmethiodide (MFI), a selective muscarinic agonist in mammals, were tested on Ascaris suum muscle strips. In a contraction assay, MFI produced weak contraction and was less potent than levamisole and acetylcholine. Atropine (3microM) a non-selective muscarinic antagonist in mammalian preparations, did not affect contractions produced by MFI. Mecamylamine (3microM) a nicotinic antagonist in A. suum preparations, blocked the MFI contractions indicating that MFI had weak nicotinic agonist actions. In two-micropipette current-clamp experiments MFI, at concentrations greater than 10microM, produced concentration-dependent depolarizations and small increases in membrane conductance. The depolarizing effects were not abolished by perfusing the preparation in a calcium-free Ascaris Ringer solution to block synaptic transmission, suggesting that MFI effects were mediated by receptors on the muscle and were calcium-independent. A high concentration of mecamylamine, 30microM, only reduced the depolarizing responses by 42%, indicating that MFI also had effects on non-nicotinic receptors. Three non-nicotinic effects in the presence of 30microM mecamylamine were identified using voltage-clamp techniques: (i) MFI produced opening of mecamylamine-resistant non-selective-cation channel currents; (ii) MFI inhibited opening of voltage-activated potassium currents; and (iii) MFI increased the threshold of voltage-activated calcium currents. We suggest that a drug that is more selective for voltage-activated potassium currents, without effects on other channels like MFI, may be exploited pharmacologically as a novel anthelmintic or as an agent to potentiate the action of levamisole. In a larval migration assay we demonstrated that 4-aminopyridine (4-AP: a potassium channel blocker) potentiated the effects of levamisole but MFI did not.

Investigation of the mechanism for the relaxation of rat duodenum mediated via M1 muscarinic receptors

1 Relaxation responses of the rat isolated duodenum to the putative M1 muscarinic receptor agonist, McN-A-343, were examined to determine whether the response was due to the release of known non-adrenergic, non-cholinergic relaxant neurotransmitters and to establish the involvement of M1 muscarinic receptors. 2 The role of ATP was examined with the P2 receptor antagonist, suramin, which at 30 mum antagonized the relaxant responses to alpha,beta-methylene ATP. The same dose, however, failed to inhibit the relaxation by McN-A-343. 3 The role of nitric oxide (NO) was examined with the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 100 microm), which failed to inhibit the responses to McN-A-343. As NO mediates relaxation of the duodenum via cGMP generation through guanylyl cyclase, whether the relaxation by McN-A-343 was also via cGMP was examined with the guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The relaxation responses to the NO donor, S-nitroso-N-acetyl penicillamine, were inhibited in the presence of ODQ (3 microm), but not those by McN-A-343. 4 Release of gamma-aminobutyric acid (GABA) was examined with the GABAA receptor antagonist, bicuculline (10 microm), which shifted the concentration-response curves for the relaxation of the duodenum by GABA to the right. There was a similar degree of shift in the concentration-response curve for McN-A-343 by bicuculline indicating that release of GABA from enteric neurones of the duodenum could explain the relaxation response to McN-A-343. 5 To test whether the muscarinic receptors mediating the relaxation of the duodenum were of the M1 subtype, the susceptibility to the selective competitive antagonist, pirenzepine and the selective muscarinic toxin from green mamba, MT7, was examined. Pirenzepine (1 microm) shifted the concentration-response for McN-A-343 to the right in a parallel fashion with a dose ratio of 33.3 +/- 20.2. This yielded a pA2 value of 7.5, which concords with those for other responses reputed to be mediated via M1 muscarinic receptors. The toxin MT7 was used as an irreversible antagonist and following incubation with the duodenum was washed from the bath. An incubation time of 30 min with 100 nm of MT7 caused a significant parallel shift in the concentration-response to McN-A-343 confirming the involvement of M1 muscarinic receptors. 6 This study has confirmed that McN-A-343 relaxes the rat duodenum via muscarinic receptors of the M1 subtype and that these receptors are probably located on enteric neurones from which their stimulation releases GABA.

Developmental pattern of M1 and M2 muscarinic gene expression and receptor levels in cat carotid body, petrosal and superior cervical ganglion

Using real-time reverse transcriptase polymerase chain reaction, Northern blot, and Western blot analyses, we evaluated the developmental pattern of mRNA and protein expression level of muscarinic M1 and M2 receptors in the carotid body, petrosal ganglion and superior cervical ganglion of 1-day, 15-day, 2-month-old and adult cats. mRNA expression and protein levels of tyrosine hydroxylase, the rate limiting enzyme for dopamine synthesis, were also assessed. Carotid body M1 receptor mRNA, increased significantly by approximately 100% and 300% in 2-month and adult vs. 1- and 15-day-old cats, but protein level decreased gradually being approximately 50% lower compared with 1-day-old cats. In the petrosal ganglion, muscarinic M1 receptor mRNA level was higher in 15-day-old cats vs. 1-day-old, 2-month-old and adult cats and protein levels were about 30% lower than in 1- and 15-day-old cats. In the superior cervical ganglion, muscarinic M1 receptor mRNA was approximately 50% and 80% higher in 2-month-old and adult cats than 1- and 15-day-old, but no changes in the protein level except in 15-day-old cats which was approximately 40% higher than 1-day-old. There was no change of muscarinic M2 receptor mRNA or protein level in the carotid body or petrosal ganglion. However, in the superior cervical ganglion, the significant increase of mRNA of 30% and 50% in 2-month-olds and adults, respectively was not associated with an increase in receptor protein. Tyrosine hydroxylase mRNA and protein level decreased significantly with age in the carotid body and petrosal ganglion. In the superior cervical ganglion, the age dependent increase in tyrosine hydroxylase mRNA was not associated with any changes in the protein level. These results show that the expression of muscarinic M1 and M2 receptors are age and organ-dependent in cats. Consequently, these changes may modulate chemosensory activity during development since muscarinic M1 receptor is predominantly involved in postsynaptic chemosensory activity, while muscarinic M2 receptor modulates acetylcholine and dopamine release from chemosensitive cells.

Inhibition of field stimulation-induced contractions of rabbit vas deferens by muscarinic receptor agonists: selectivity of McN-A-343 for M1 receptors

Inhibition of the field stimulation-induced twitch responses of the rabbit vas deferens by the muscarinic receptor agonist, McN-A-343, has been attributed to presynaptic muscarinic receptors of the M1 subtype located on noradrenergic nerve terminals. Stimulation of these receptors causes inhibition of transmitter release and inhibition of the contractile response. However, the selectivity of McN-A-343 for M1 receptors has been questioned and this throws doubt on whether the prejunctional receptors of the rabbit vas deferens are of the M1 subtype. In this study we have undertaken a comprehensive re-evaluation of the inhibition of prostatic and epididymal portions of the rabbit isolated field-stimulated vas deferens by several agonists, including McN-A-343, and quantified the antagonism by M1-selective antagonists, pirenzepine and telenzepine. Prostatic and epididymal portions of vasa deferentia from New Zealand White rabbits were immersed in a low Ca2+ Krebs solution at 32+/-0.5 degrees C gassed with 5% CO2 in oxygen. Yohimbine (1.0mM) was present throughout to block prejunctional alpha2-adrenoceptors. Field stimulation was applied by repeated application of single pulses (30 V, 0.05 Hz, 0.5 ms) and isometric contractions recorded. Carbachol and oxotremorine initially potentiated the epididymal contractions but at higher concentrations there was inhibition. In the prostatic portion, oxotremorine only inhibited. McN-A-343 produced inhibitory responses only in both epididymal and prostatic portions. Pirenzepine shifted the concentration-response curves forthe inhibitory responses to oxotremorine to the right. However, the potentiation of the twitches also became more apparent with the lower concentrations of oxotremorine. Schild plots for the antagonism by pirenzepine yielded pA2 values of 7.96+/-0.004 and 7.7+/-0.02 for the epididymal and prostatic portions, respectively. The concentration-response curves for the inhibition of twitches by McN-A-343 were displaced to the right in a parallel manner by pirenzepine in both prostatic and epididymal portions with no potentiation of the twitches. The Schild plot for this antagonism generated pA2 values of 7.68+/-0.01 and 8.07+/-0.01, respectively. Telenzepine caused parallel shifts of the McN-A-343 concentration-response curves to the right in prostatic portions, the pA2 value being 8.70+/-0.13. Telenzepine (10(-7) M) abolished the inhibitory effect of carbachol to reveal only concentration-dependent potentiation of the contractions. The Schild plot for antagonism of this contractile effect yielded a pA2 value (7.07+/-0.09) that was significantly less by almost two orders of magnitude (1.70) than the value for the antagonism by telenzepine of the McN-A-343-induced inhibitory response. The pA2 values of pirenzepine and telenzepine against the inhibitory responses of the rabbit vas deferens are consistent with the involvement of M1 receptors. This leads to the conclusion that McN-A-343 causes inhibition through this receptor type. The doubts concerning the selectivity of McN-A-343 for M1 receptors are therefore unfounded. The fact that McN-A-343 does not display a selective binding profile suggests that its selectivity does not arise from affinity differences but probably resides in its intrinsic efficacy.

A new method for recording surface compound potentials in sympathetic ganglia from mouse, rat, and guinea pig--application to muscarinic and nicotinic depolarizations

We present a new method for electrophysiologic investigations in isolated autonomous ganglia of a variety of laboratory animals. This method enables determination of surface compound potentials in ganglia and changes induced by pharmacologic compounds. Advantages of our methods are as following: (1) the method is relatively simple and does not require sophisticated experimental setups, with minor modifications it is adaptable to investigate ganglia of varying sizes; (2) the signal amplitude is comparable or even higher when compared with signals obtained by other methods: (3) the apparatus allows fast addition and removal of the investigational compounds and thus the determination of acute and subacute desensitizing effects; and (4) fast preparation and minor tissue injuries during preparation of the ganglia allow determination of surface potential changes over a time period of up to 2 days without qualitative changes of the parameters. In this report we demonstrate the validity of this method using superior cervical ganglia from rat, mouse, and guinea pig. Agonists used to trigger potential changes are the cholinergic agonists acetylcholine, muscarine, nicotine, and carbachol. The possibility of receptor desensitization by these compounds is investigated by repeated application over 5 h.

Selective activation of heterologously expressed G protein-gated K+ channels by M2 muscarinic receptors in rat sympathetic neurones

1. G protein-regulated inward rectifier K+ (GIRK) channels were over-expressed in dissociated rat superior cervical sympathetic (SCG) neurones by co-transfecting green fluorescent protein (GFP)-, GIRK1- and GIRK2-expressing plasmids using the biolistic technique. Membrane currents were subsequently recorded with whole-cell patch electrodes. 2. Co-transfected cells had larger Ba2+-sensitive inwardly rectifying currents and 13 mV more negative resting potentials (in 3 mM [K+]o) than non-transfected cells, or cells transfected with GIRK1 or GIRK2 alone. 3. Carbachol (CCh, 1-30 microM) increased the inwardly rectifying current in 70 % of GIRK1+ GIRK2-transfected cells by 261 +/- 53 % (n = 6, CCh 30 microM) at -120 mV, but had no effect in non-transfected cells or in cells transfected with GIRK1 or GIRK2 alone. Pertussis toxin prevented the effect of carbachol but had no effect on basal currents. 4. The effect of CCh was antagonized by 6 nM tripitramine but not by 100 nM pirenzepine, consistent with activation of endogenous M2 muscarinic acetylcholine receptors. 5. In contrast, inhibition of the voltage-activated Ca2+ current by CCh was antagonized by 100 nM pirenzepine but not by 6 nM tripitramine, indicating that it was mediated by M4 muscarinic acetylcholine receptors. 6. We conclude that endogenous M2 and M4 muscarinic receptors selectively couple to GIRK currents and Ca2+ currents respectively, with negligible cross-talk.

Exocytotic release from neuronal cell bodies, dendrites and nerve terminals in sympathetic ganglia of the rat, and its differential regulation

Stimulant-induced exocytosis has been demonstrated in sympathetic ganglia of the rat by in vitro incubation of excised ganglia in the presence of tannic acid, which stabilizes vesicle cores after their exocytotic release. Sites of exocytosis were observed along non-synaptic regions of the surfaces of neuron somata and dendrites, including regions of dendrosomatic and dendrodendritic apposition, as well as along the surfaces of nerve terminals About half the exocytoses associated with nerve terminals were parasynaptic or synaptic, and these appeared mostly to arise from the presynaptic terminal, but occasionally from the postsynaptic element. The results demonstrated that the neurons of sympathetic ganglia release materials intraganglionically in response to stimulation, that release from different parts of the neuron is subject to independent regulation, at least via cholinergic receptors, and that release is partly diffuse, potentially mediating autocrine or paracrine effects, and partly targeted toward other neurons, but that the latter mode is not necessarily, and not evidently, synaptic. Specifically, exocytosis from all locations increased significantly during incubation in modified Krebs' solution containing 56 nm potassium. Observation of the effects of cholinergic agonists (nicotine, carbachol, oxotremorine) and antagonists (atropine, AF-DX 116) showed that nicotinic and muscarinic excitation each, independently, increased the incidence of exocytosis from somata and dendrites. Exocytosis from nerve endings was not altered by nicotine, but was enhanced or, at high initial rates of exocytosis, decreased, by muscarinic stimulation. Evidence was obtained for muscarinic auto-inhibition of exocytosis from nerve terminals, occurring under basal incubation conditions, and for a muscarinic excitatory component of somatic exocytosis, elicitable by endogenous acetylcholine. The M2-selective muscarinic antagonist AF-DX 116 was found to modify the exocytotic response of the dendrites to oxotremorine, widening the range of its variation; this effect is consistent with recent evidence for the presence of M2-like muscarinic binding sites, in addition to M1-like binding, upon these dendrites [Ramcharan E. J. and Matthews M. R. (1996) Neuroscience 71, 797-832]. Over all conditions, disproportionately more sites of somatic and dendritic exocytosis were found to be located in regions of dendrosomatic and dendrodendritic apposition than would be expected from the relative extent of the neuronal surface occupied by these relationships. Such mechanisms of intraganglionic release may be expected to contribute to the regulation and integration of the behaviour of the various functionally distinctive populations of neurons in these ganglia, by autocrine, paracrine, and focal, neuroneuronal, routes of action. Similar phenomena of exocytotic soma-dendritic release might prove to subserve integrative neuroneuronal interactions more widely throughout the nervous system.

Assessment of muscarinic transmission in the superior cervical and ciliary ganglion of the cat

This study was undertaken to determine if muscarinic mechanisms are involved in synaptic transmission in the parasympathetic ciliary ganglion as has been clearly shown for sympathetic ganglia. Cats were anesthetized, and following topical ephedrine, pupillary constrictions were elicited by electrical stimulation of the intracranial oculomotor nucleus. Nictitating membrane contractions were evoked by electrical stimulation of the preganglionic cervical nerve. Frequency-response curves were repeated after infusion with hexamethonium (0.6-1.0 mg/kg min-1) and after subsequent administration of atropine (500 micrograms/kg. i.v.). In other experiments, effects of nicotinic (DMPP) and muscarinic (McN-A-343) agonists on postganglionic ciliary nerve activity were measured. Treatment with hexamethonium reduced nictitating membrane responses at all frequencies of stimulation (by about 75% at 16-32 Hz). The residual nictitating membrane contractions were subsequently blocked by the addition of atropine. In contrast, hexamethonium totally abolished miosis produced by CNS preganglionic oculomotor nerve stimulation. The nicotinic agonist, DMPP, produced nictitating membrane contractions, miosis, and increased ciliary nerve firing. In contrast, McN-A-343 contracted the nictitating membrane but failed to increase postganglionic ciliary nerve activity. These results suggest that, unlike sympathetic ganglia, a significant degree of muscarinic transmission does not occur in the parasympathetic ciliary ganglion.

McN-A-343 increases renal sympathetic nerve activity and blood pressure by a muscarinic and a non-muscarinic mechanism in the rat

1. Intravenous administration of the putative M1 muscarinic agonist McN-A-343 to conscious rats evokes an increase in mean arterial pressure (MAP) which can be blocked by muscarinic receptor antagonists. The present study was undertaken to evaluate the increase in MAP and renal sympathetic nerve activity (RSNA) evoked by intravenous administration of McN-A-343 to urethane-anaesthetized rats. 2. McN-A-343 (0.1-0.3 mg kg-1) evoked a concurrent increase in MAP and RSNA which could be inhibited by the nonselective muscarinic receptor antagonist methylatropine or the selective M1 muscarinic receptor antagonist telenzepine. Administration of higher doses of McN-A-343 (0.3-1.2 mg kg-1) in the presence of muscarinic receptor blockade evoked brief bursts in RSNA accompanied by increases in MAP. 3. The increases in MAP, but not the increases in RSNA, evoked by all doses of McN-A-343 could be attenuated by the selective alpha 1-adrenoceptor antagonist prazosin. Adding the selective alpha 2-adrenoceptor antagonist yohimbine to prazosin did not further inhibit the pressor response to the low doses of McN-A-343. 4. The irreversible alpha-adrenoceptor and NPY receptor antagonist benextramine also attenuated the pressor response evoked by the low doses of McN-A-343 but not the increases in RSNA. However, when combined with muscarinic receptor blockade, benextramine completely inhibited the brief bursts in RSNA, and thus also the increases in MAP, evoked by the high doses of McN-A-343. 5. The pressor response remaining after the administration of high doses of McN-A-343 to rats pretreated with prazosin and methylatropine was inhibited by treatment with alpha,beta-methylene ATP. 6. These results show that McN-A-343 evokes increases in RSNA by muscarinic and non-muscarinic mechanisms. Furthermore, the subsequent increase in MAP is primarily dependent upon activation of vascular alpha 1-adrenoceptors, but may also involve activation of P2 alpha receptors.

Autoradiographic localization of functional muscarinic receptors in the rat superior cervical sympathetic ganglion reveals an extensive distribution over non-synaptic surfaces of neuronal somata, dendrites and nerve endings

Fast synaptic transmission in sympathetic ganglia is mediated by acetylcholine, acting on nicotinic receptors, yet muscarinic receptors are also present and are involved in the production of slow postsynaptic potentials. In order further to elucidate the role of muscarinic receptors in ganglionic transmission their distribution in the rat superior cervical sympathetic ganglion was investigated autoradiographically by use of the tritiated irreversible muscarinic ligand propylbenzilylcholine mustard. It was observed that this agent blocked the carbachol-evoked hydrolysis of inositol phospholipids in the ganglion and that this response to carbachol is itself inhibitable by selective muscarinic antagonists with a potency sequence which indicates involvement primarily of M1 receptors. Light microscope autoradiography showed that labelling inhibitable by atropine and by the M1-selective muscarinic antagonist pirenzepine was essentially confined to the margins of neuronal somata and regions of dendritic arborization, which include synaptic contacts. Quantitative electron microscope autoradiography showed that binding of the radioligand, of which approximately 70% was inhibitable by atropine and 68% by pirenzepine, was associated predominantly with surface membranes of neuronal somata, dendrites, other neurites (including axons and uncharacterized dendrites) and nerve terminal profiles, in the approximate ratios 95:85:52:45. Of the inhibitable binding over neuronal membranes in the ganglion little more than 3% was found to be synaptically located, and this involved para- or peri-synaptic regions of nerve terminal contacts rather than the specialized synaptic zone. About 5% of the inhibitable binding over neuronal membranes involved non-synaptic surfaces of nerve terminals and preterminal axon segments; almost 70% was distributed over non-synaptic surfaces of neuronal somata and dendrites, and about 21% upon other neurites. Binding sites were found not to be more highly concentrated at or adjacent to synapses than over other regions of neuronal surface membranes. About 50%, possibly more, of the binding on non-synaptic surfaces of nerve endings, and about 7% of binding upon dendritic membranes, was of non-M1, possibly M2 type, inhibitable by atropine but not by pirenzepine. Non-synaptic neuro-neuronal appositions, which involve dendrites and somata and often lie adjacent to synapses, showed rather more than twice the binding expected for each membrane individually; and neuronal membrane exposed to basal lamina lining ganglionic tissue spaces showed high levels of binding. Little inhibitable binding was seen over membranes of satellite and Schwann cells, or over cytoplasmic territories or ganglionic interstitial tissue. A model was constructed of the distribution of label, which showed that the observed results for total binding could be approximately matched by assuming the following relative densities of ligand binding sites: interstitial tissue space and supporting cells 1, soma cytoplasm 3, cytoplasm of dendrites, neurites and nerve terminals 4.5, surfaces of mesodermal elements 15, surfaces of neurites and nerve endings including sites of synapse 45, surfaces of dendrites 90, surfaces of neuronal somata 120, non-synaptic neuro-neuronal appositions 180. It is concluded that functional muscarinic receptors in this sympathetic ganglion, predominantly of the M1 type linked with slow depolarizations, but including some non-M1 receptors, are widely distributed over non-synaptic surfaces of the neuronal somata and dendrites and are not concentrated at synapses. Presynaptic autoreceptors are also present, of which half or more are of non-M1, possibly M2, type which might be inhibitory. The presence of M4 receptors is not excluded...

Adenosine selectively depresses muscarinic compared with non-muscarinic receptor mediated depolarisation of the rat superior cervical ganglion

1. A grease gap d.c. recording technique was used to measure electrophysiological responses of the isolated rat superior cervical ganglion. 2. Adenosine at 100 microM depressed depolarisations to the muscarinic agonists carbachol, muscarine and methylfurmethide. In contrast adenosine (100 microM) did not alter depolarisations to 1,1-dimethyl-4-phenylpiperazinium, 2-methyl-5-hydroxytryptamine and potassium and enhanced depolarisations to 5-hydroxytryptamine and gamma-aminobutyric acid. 3. Adenosine-induced depressions of the depolarisations to carbachol, muscarine, and methylfurmethide tended to be increased in the presence of 0.3 microM methoctramine (a muscarinic receptor antagonist with slight selectivity for M2 receptors). The increase was statistically significant (P < 0.01) for carbachol. 4. Medium containing 0.1 mM Ca2+ and 0.3 microM pirenzepine augmented the hyperpolarising phase of the response to carbachol. Adenosine (10-300 microM) hyperpolarised ganglia and did not significantly alter the hyperpolarisation to 0.3 or 1 microM carbachol but selectively reduced the depolarisation response to 3 microM carbachol. 5. Adenosine-induced hyperpolarisations (100 microM) were enhanced when applied during depolarisations to muscarinic agonists (muscarine, pilocarpine, N-methyl-N-(l-methyl-4-pyrrolidine-2-butynyl)acetamide (BM-5)), and other M-current inhibitors, barium and eledoisin-related-peptide. Adenosine induced hyperpolarisations were not affected by D-Ala6-luteinizing-hormone-releasing-hormone or uridine 5'-triphosphate which produced small depolarisations. 6. It is concluded that adenosine acts selectively in opposing mechanisms of depolarisation of the rat SCG that are due to the action of muscarinic agonists (acting via M1-receptors) and by other M-current inhibitors.

Bradykinin receptors in mouse and rat isolated superior cervical ganglia

1. The ability of bradykinin and its analogues to depolarize rat and mouse superior cervical ganglia was studied by use of in vitro grease-gap recording techniques, and the ability of antagonists selective for bradykinin receptor subtypes to block their effects was examined. 2. Bradykinin (3 microM) depolarized ganglia from both species, although the magnitude of the maximal response was less in mouse (15 +/- 5%, n = 7) than rat tissue (33 +/- 6%, n = 7), relative to muscarine (1 microM). 3. Interleukin 1 beta (30 u ml-1 for 18 h at 37 degrees C) increased the depolarization caused by bradykinin (3 microM) in mouse ganglia from 15% to 54% (P < 0.001, n = 12). Responses to the B1 receptor agonist, [des-Arg10]-kallidin (3 microM) were similarly potentiated but this was only detected after inhibition of peptidase activity with 10 microM captopril (4% to 35%, n = 5). 4. In ganglia from both species the rank order of agonist potency was bradykinin = [Lys0]-bradykinin >> [des-Arg10]-kallidin. However, like responses to [des-Arg10]-kallidin in mouse tissue, both the potency of bradykinin and the maximal depolarization achieved (EC50 = 912 nM; 80%, n = 11) was enhanced following inhibition of angiotensin converting enzyme with 10 microM captopril (EC50 = 50 nM; 135%, n = 4). 5. Responses to bradykinin were selectively antagonized by the B2 receptor antagonist, Hoe 140 but not by the B1 antagonist, [Leu8]-bradykinin1-8. From Schild analysis the pA2 value for Hoe 140 in mouse tissue was 9.65, although the slope of the regression line was significantly greater than unity, indicating non-competitive kinetics (slope = 1.88 +/- 0.18, n = 9). The depolarization caused by [Lys0]-bradykinin was also antagonized by Hoe 140 (3 nM).6. Thus the predominant bradykinin receptor in mouse superior cervical ganglia is compatible with a B2 subtype. Furthermore the depolarizations caused by B1 and B2 agonists in this tissue can be increased following exposure to interleukin l beta, and by blocking peptide degradation with captopril.

Evidence that the 5-HT3 receptors of the rat, mouse and guinea-pig superior cervical ganglion may be different

1. Using grease-gap recordings from the isolated superior cervical ganglion of mouse, rat and guinea-pig, we have compared the depolarization evoked by 5-hydroxytryptamine (5-HT) with that evoked by the selective 5-HT3 receptor agonist 2-methyl-5-HT (2-Me-5-HT). 2. The maximum depolarization induced by 2-Me-5-HT was smaller than that induced by 5-HT in all three species, and particularly in the guinea-pig. 3. The 5-HT2 receptor antagonist ketanserin (1 microM) caused a clear rightward shift of the dose-response curve to 5-HT on the guinea-pig ganglion, but not on the mouse or rat ganglion. Spiperone (0.03 microM) had a quantitatively similar action to ketanserin (0.1 microM) on the 5-HT dose-response curve of the guinea-pig ganglion. Ketanserin had no significant effect on the dose-response curve to 2-Me-5-HT on any of these ganglia. 4. Using 2-Me-5-HT as the agonist, we determined the pA2 values for two 5-HT3 receptor antagonists. The potency of ICS 205-930 varied by approximately 100 fold between the species and that of (+)-tubocurarine varied by over 1000 fold. The differences in the pA2 values of these compounds varied independently among the species. 5. We conclude that 5-HT3 receptors are present on the superior cervical ganglion from the rat, mouse and guinea-pig, but these receptors may be pharmacologically distinct from each other. In addition, the depolarization of the guinea-pig superior cervical ganglion by low concentrations of 5-HT is largely mediated by ketanserin-sensitive receptors.

Lack of long-term potentiation, non-cholinergic transmission and muscarinic inhibition in cat superior cervical ganglia innervated by nodose ganglion cells

In anesthetized cats, in which a nodose-superior cervical ganglion (SCG) anastomosis had been performed 6-9 months earlier, the nictitating membrane contraction evoked by electrical stimulation of the cervical vagus nerve ipsilateral to the anastomosis was recorded. The competence of nodose neurons in regulation the multiple synaptic mechanisms of the sympathetic ganglion was tested by comparing this response with the responses to stimulation of (self-reinnervated SCG). The response of the nictitating membrane ipsilateral to the anastomosis was smaller and had a lower EC50 for hexamethonium (C6) than the responses of the nictitating membrane ipsilateral to the intact or sutured CST. A 40 Hz 10s stimulus train to the intact or sutured CST produced potentiation of ganglionic transmission lasting 1 hour or longer, while a similar stimulus train to the anastomosed cervical vagus nerve produced no potentiation. During block of ganglionic nicotinic transmission with C6, CST or vagus nerve stimulation evoked responses which increased in amplitude with increasing stimulus frequency and were blocked by the selective muscarinic receptor antagonist pirenzepine. When the anticholinesterase eserine was added, the responses evoked by preganglionic stimulation decreased in amplitude in the intact SCG, as previously shown [7], and in the self-reinnervated SCG. This effect, which is due to inhibition mediated by muscarinic receptors selectively blocked by AF-DX116, was absent in the anastomosed SCG. During block of ganglionic transmission with C6 and atropine, a 40 Hz stimulus train to the intact or to the sutured CST evoked a slow, small amplitude contraction that was enhanced by naloxone. This response, most likely mediated by peptides [6], was absent in the anastomosed SCG.(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonistic properties of McNeil-A-343 at 5-HT4 and 5-HT3 receptors

1. This study describes the in vitro interaction of the muscarinic ligand McNeil-A-343 with two 5-hydroxytryptamine (5-HT) receptor subtypes, the 5-HT4 and 5-HT3 receptors, using functional as well as radioligand binding studies. 2. In the rat oesophageal muscularis mucosae, precontracted with carbachol, McNeil-A-343 was a competitive antagonist (pA2 6.2) of the 5-HT4 receptor which mediates the relaxation induced by 5-HT. The compound per se relaxed the oesophagus at high concentration only (> or = 10 microM), an effect unchanged by desensitization of the 5-HT4 receptor with 10 microM 5-methoxytryptamine. In the same preparation in the absence of tone, McNeil-A-343 displaced the carbachol concentration-response curve to the right, yielding an apparent affinity (pA2) of 4.9 for muscarinic receptors. 3. In the rat isolated superior cervical ganglion preparation, after blockade of muscarinic and nicotinic receptors, McNeil-A-343 caused a concentration-dependent depolarization that was unaffected by 100 nM ondansetron. The concentration-fast depolarization curve to 5-HT, mediated by the 5-HT3 receptor, was displaced to the right by McNeil-A-343, which showed an apparent affinity (pA2) of 4.8 for the 5-HT3 subtype. 4. In binding studies, McNeil-A-343 recognized a single population of 5-HT4 receptors in pig caudate nucleus, with a pKI of 5.9. The binding affinity of McNeil-A-343 for 5-HT3 receptors in NG 108-15 cells was approximately four times lower (pKI 5.3). Binding affinities (pKI) for muscarinic receptor subtypes in rat tissues were 5.3 (M1, cortex), 5.2 (M2, heart) and 4.9 (M3, submandibular glands), respectively. 5. McNeil-A-343 is an antagonist at 5-HT4 and 5-HT3 receptors; the interaction of the compound with these receptor subtypes (notably the 5-HT4) occurs in a range of concentrations which generally overlaps that relevant to the interaction with muscarinic receptors.

Himbacine discriminates between putative muscarinic M1 receptor-mediated responses

This study describes the antagonistic properties of himbacine, in comparison with those of pirenzepine, at muscarinic receptors mediating the depolarization of rat superior cervical ganglion, the inhibition of electrically-induced twitch contractions of rabbit vas deferens and the contraction of dog saphenous vein, currently classified as putative muscarinic M1 sites. The affinity of himbacine for the vas deferens site (pA2 8.08) was nearly ten times higher than those for the M1 receptors of rat ganglion and dog saphenous vein (pA2 7.14 and 7.16, respectively); affinity estimates for pirenzepine were similar throughout the different preparations. The present data are consistent with the allocation of ganglion and saphenous vein receptors into the M1 subclass; the profile of the vas deferens site, conversely, appears to be different, and possibly more closely related to that reported for the M4/m4 receptor.

Action of purine and pyrimidine nucleotides on the rat superior cervical ganglion

1. Using a grease-gap technique, we have investigated the effects of purine and pyrimidine nucleotides on the d.c. potential of the rat isolated superior cervical ganglion (SCG). 2. Of the purines tested, adenosine, adenosine 5'-triphosphate (ATP), beta,gamma-methylene-adenosine 5'-triphosphate (beta,gamma-MeATP) at up to 300 microM produced concentration-dependent hyperpolarizations, whereas 2-methyl-thio-ATP (2-Me.S.ATP) and alpha,beta-methylene-ATP (alpha,beta-MeATP) depolarized ganglia. Of the pyrimidines tested, uridine 5'-triphosphate (UTP) produced concentration-dependent depolarizations and cytosine 5'-triphosphate (CTP) at 1000 microM produced considerably smaller but significant depolarizations. In contrast uridine 5'-monophosphate (UMP) at 1000 microM hyperpolarized ganglia. The relative order of potency of purines and pyrimidines to depolarize ganglia was: UTP > alpha,beta-MeATP >> CTP > 2-Me.S.ATP and to hyperpolarize ganglia was: adenosine = beta,gamma-MeATP > ATP > UMP. 3. The ability of purines and pyrimidines to alter the depolarizing response caused by muscarine and of purines to alter depolarization induced by gamma-aminobutyric acid (GABA) was determined. The relative order of potency of nucleotides in depressing submaximal depolarization caused by muscarine (100 nM) was: adenosine = ATP > beta,gamma-MeATP whereas 2-Me.S.ATP, alpha,beta-MeATP and UTP did not significantly alter depolarization caused by muscarine. At 100 microM beta,gamma-MeATP and adenosine but not ATP potentiated GABA-induced depolarizations. 4. Hyperpolarizations caused by adenosine, ATP, beta,gamma-MeATP and UMP and depolarizations caused by alpha,beta-MeATP were enhanced in medium containing reduced concentrations of calcium (0.1 mM) and potassium (2 mM). In this medium 8-phenyltheophylline abolished hyperpolarizations caused by adenosine and reversed hyperpolarizations caused by ATP into depolarizations. Suramin (300 microM), a P2-purinoceptor antagonist, significantly reduced the depolarizing response caused by alpha,beta-MeATP and significantly increased hyperpolarizations caused by ATP and Beta,gamma-MeATP. Suramin (300 microM) did not significantly alter depolarizations caused by l,l-dimethyl-4-phenylpiperazinium (10 microM), potassium(3 mM) or muscarine (100 nM) and significantly potentiated depolarizations caused by UTP (100 microM).5.It is concluded that the rat SCG contains PI-purinoceptors that hyperpolarize the ganglion and diminish sensitivity to muscarine, and P2X-purinoceptors that depolarize the SCG. There is also some evidence to suggest the presence of receptors for UTP, i.e., pyrimidinoceptors, which depolarize SCG neurones.

Muscarinic acetylcholine receptor subtypes: localization and structure/function

Based on the sequence of the five cloned muscarinic receptor subtypes (m1-m5), subtype selective antibody and cDNA probes have been prepared. Use of these probes has demonstrated that each of the five subtypes has a markedly distinct distribution within the brain and among peripheral tissues. The distributions of these subtypes and their potential physiological roles are discussed. By use of molecular genetic manipulation of cloned muscarinic receptor cDNAs, the regions of muscarinic receptors that specify G-protein coupling and ligand binding have been defined in several recent studies. Overall, these studies have shown that amino acids within the third cytoplasmic loop of the receptors define their selectivities for different G-proteins and that multiple discontinuous epitopes contribute to their selectivities for different ligands. The residues that contribute to ligand binding and G-protein coupling are described, as well as the implied structures of these functional domains.

The design of novel muscarinic partial agonists that have functional selectivity in pharmacological preparations in vitro and reduced side-effect profile in vivo

Antagonist/agonist binding ratios (NMS/Oxo-M ratio) were used as an index of the efficacy of novel compounds acting at muscarinic receptors. These binding ratios have been used with a range of functional pharmacological assays to investigate the effects of varying the efficacy of muscarinic agonists. This strategy has been used as a means of obtaining functional receptor selectivity by exploiting differences in effective receptor reserves. The oxadiazole and pyrazine muscarinic agonists L-670,548 (NMS/Oxo-M ratio 1100) and L-680,648 (NMS/Oxo-M ratio 690) are amongst some of the most potent and efficacious agonists known. Decreasing the efficacy of compounds from these series, resulted in compounds with functional selectivity. The chloropyrazine L-689,660 (NMS/Oxo-M ratio 28) was an agonist on the rat superior cervical ganglion (M1), a partial agonist on the guinea-pig ileum (M3), but was an antagonist in the guinea-pig atria (M2). Synthesis of compounds with even lower predicted efficacy, such as the cyclopropyloxadiazole L-687,306 (NMS/Oxo-M ratio 15), maintained agonist activity in the ganglion, but showed antagonist activity in the M3 ileal, as well as the M2 atrial preparations. When tested in vivo these compounds did not produce many of the side effects associated with more efficacious agonists, particularly those associated with the cardiovascular system. However, they were active in reversing scopolamine-induced deficits in a variety of behavioural paradigms. This approach shows how functional selectivity for muscarinic receptor subtypes can be achieved in vitro, that in vivo reduces the dose-limiting side effects normally associated with muscarinic agonists.

L-689,660, a novel cholinomimetic with functional selectivity for M1 and M3 muscarinic receptors

1. L-689,660, 1-azabicyclo[2.2.2]octane, 3-(6-chloropyrazinyl)maleate, a novel cholinomimetic, demonstrated high affinity binding (pKD (apparent) 7.42) at rat cerebral cortex muscarinic receptors. L-689,660 had a low ratio (34) of pKD (apparent) values for the displacement of binding of the antagonist ([3H]-N-methylscopolamine ([3H]-NMS) compared with the displacement of the agonist [3H]-oxotremorine-M ([3H]-Oxo-M), in rat cerebral cortex. Low NMS/Oxo-M ratios have been shown previously to be a characteristic of compounds that are low efficacy partial agonists with respect to stimulation of phosphatidyl inositol turnover in the cerebral cortex. 2. L-689,660 showed no muscarinic receptor subtype selectivity in radioligand binding assays but showed functional selectivity in pharmacological assays. At M1 muscarinic receptors in the rat superior cervical ganglion, L-689,660 was a potent (pEC50 7.3 +/- 0.2) full agonist in comparison with (+/-)-muscarine. At M3 receptors in the guinea-pig ileum myenteric plexus-longitudinal muscle or in trachea, L-689,660 was again a potent agonist (pEC50 7.5 +/- 0.2 and 7.7 +/- 0.3 respectively) but had a lower maximum response than carbachol. In contrast L-689,660 was an antagonist at M2 receptors in guinea-pig atria (pA2 7.2 (95% confidence limits 7, 7.4)) and at muscarinic autoreceptors in rat hippocampal slices. 3. The putative M1-selective muscarinic agonist, AF102B (cis-2-methylspiro-(1,3-oxathiolane 5,3')-quinuclidine hydrochloride) was found to have a profile similar to L-689,660 but had up to 100 times less affinity in binding and functional assays.RS-86 (2-ethyl-8-methyl-2,8-diazospiro[4,5]decan 1,3-dionehydrochloride) also had lower affinity than L-689,660, and had no binding selectivity for muscarinic receptor subtypes. RS-86 had a higher NMS/Oxo-M ratio than L-689,660 and was a full agonist at MI,M2 and M3 receptors in the functional pharmacological assays.4. The functional selectivity of L-689,660 in muscarinic pharmacological assays is consistent with the effects of a low efficacy partial agonist in tissues with different effective receptor reserves.

AS-5370 potently antagonizes 5-HT3 receptor-mediated responses on NG108-15 cells and on the rat vagus

The action of a novel 5-HT3 receptor antagonist, AS-5370, has been studied on two electrophysiological models for 5-HT3 receptors: whole-cell patch-clamp recordings from mouse neuroblastoma-rat glioma (NG108-15) cells and grease-gap recordings from rat isolated vagus nerve. The 5-hydroxytryptamine (5-HT)-induced fast inward current of voltage-clamped NG108-15 cells was antagonized by 1 nM AS-5370 in an insurmountable manner. On the rat vagus, AS-5370 reduced the maximum depolarizing response to 5-HT in a concentration-dependent manner. The IC50 for AS-5370 on the rat vagus was 0.3-1.0 nM. The EC50 for 5-HT on the rat vagus was not appreciably affected by AS-5370. On the rat vagus, the (R) enantiomer of AS-5370 was about 30 times more potent than the (S) enantiomer. The antagonist action of AS-5370 on these two cell types was compared with that of (+)-tubocurarine. Unlike tubocurarine, the effect of AS-5370 on NG108-15 cells was not readily reversed during wash. On the rat vagus, tubocurarine antagonized in a competitive manner with an IC50 of 0.3-1.0 microM (pKb = 7.2). It is concluded that AS-5370 is a potent 5-HT3 receptor antagonist on both NG108-15 cells and the rat vagus, but it does not act in a competitive manner.

Discriminative stimulus properties of the muscarinic receptor agonists Lu 26-046 and O-Me-THPO in rats: evidence for involvement of different muscarinic receptor subtypes

The discriminative cues induced by the muscarinic receptor agonists Lu 26-046 ((-)-7-methyl-3(2-propynyloxy)-4,5,6,7-tetrahydroisothiazolo [4,5-c]pyridine ) and O-Me-THPO (3-methoxy-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine) were investigated. The results were compared with those obtained for the binding profiles of these agonists at central muscarinic receptors and with results concerning their functional effects at peripheral muscarinic receptors in vitro. Lu 26-046 had preferential affinity for M1 versus M2 receptors (Ki index [3H]quinuclidinyl benzilate ([3H]QNB/[3H]pirenzepine 4.2) and had partial agonistic activity at M1 and M2 receptors in rat superior cervical ganglion and guinea pig left atrim, respectively. A weak antagonistic effect at M3 receptors in guinea pig ileum was observed. O-Me-THPO had non-selective agonistic effects at peripheral M1, M2 and M3 receptors and had a slight preference for central M2 receptors in binding experiments (M2/M1 index 0.31). Lu 26-046 dose dependently substituted for Lu 26-046 and partially substituted for O-Me-THPO in rats trained to discriminate Lu 26-046 and O-Me-THPO from saline, respectively. The (+)-enantiomer of Lu 26-046, Lu 26-047, had weak partial M1 agonistic activity and M2/M3 antagonistic effects at peripheral receptors. Lu 26-047 also had a high M2/M1 index (9.3) in binding experiments. Lu 26-047 substituted for Lu 26-046, but preferentially inhibited the effect of O-Me-THPO. Pilocarpine had a preferential effect in Lu 26-046-trained rats, while oxotremorine and arecoline had preferential effects in O-Me-THPO-trained rats. Large increases in latency times or a disruption of responding was generally observed. These compounds were full agonists at peripheral M1, M2 and M3 receptors. The muscarinic receptor antagonist scopolamine antagonized the effect of O-Me-THPO and partially inhibited the effect of Lu 26-046. Scopolamine partially substituted for Lu 26-046. The quaternary muscarinic receptor agonist N-methyl atropine had no effect, indicating that the cues are mediated by central muscarinic receptors. It is suggested that the discriminative cues of Lu 26-046 and O-Me-THPO are preferentially mediated by central M1 (partial) and M2 receptor stimulation, respectively. The role of central M3 receptors is not known.

Different effects of muscarinic agonists in rat superior cervical ganglion and hippocampal slices

In this study the effects of muscarinic antagonists and agonists on M1 muscarinic receptors in the isolated rat superior cervical ganglion and the rat hippocampal slice were investigated. Oxotremorine and APE but not pilocarpine, McN-A-343 or 4-Cl-McN-A-343 induced small M2 muscarinic receptor-mediated hyperpolarizations in the rat superior cervical ganglion. Nevertheless, for all the agonists investigated the pA2 values of the muscarinic antagonists pirenzepine, AF-DX 116 and p-fluoro-hexahydro-sila-difenidol indicated the presence of only M1 muscarinic receptors in the rat superior cervical ganglion and hippocampal slice. Full agonistic behaviour with respect to depolarization of the rat superior cervical ganglion was observed for pilocarpine, McN-A-343 and 4-Cl-McN-A-343. Oxotremorine and arecaidine propargyl ester were partial agonists in this preparation, with maximal effects of 35 and 46% of the maximum obtained with pilocarpine, respectively. Pilocarpine, oxotremorine and arecaidin propargyl ester displayed full agonistic behaviour on the increase in firing rate of pyramidal cells in rat hippocampal slices. Whereas 4-Cl-McN-A-343 was a partial agonist (maximal effect of 63% of the maximum obtained with pilocarpine), McN-A-343 displayed no agonistic or antagonistic activity in rat hippocampal slices. It remains to be established whether the heterogeneous behaviour of the agonists in both preparations reflects as yet unknown differences in the M1 receptor protein or results from differences in the coupling of receptor to second messenger.

On the histamine-induced depolarization of the isolated superior cervical ganglion of the rat

1. Using a grease-gap technique, we studied the action of histamine on the d.c. potential recorded between the internal carotid nerve and the main body of the isolated superior cervical ganglion of the rat. 2. A small, slow depolarization was evoked by 10-300 microM histamine. This response was not reduced by lowering the calcium concentration in the superfusing medium (from 2.5 to 0.1 mM), or by superfusing tetrodotoxin, N-methylatropine, or propranolol (all at 1 microM). 3. Mepyramine (10 nM) antagonized this depolarization, but cimetidine (10 microM), metiamide (30 microM), burimamide (10 microM) and impromidine (1 microM) did not. Two other agonists also evoked a mepyramine-sensitive slow depolarization. The rank order of potencies was histamine greater than N alpha-methyl-histamine greater than 2-methyl-histamine. 4. At concentrations greater than 1 mM, histamine also evoked a larger, faster depolarization. This response was undiminished by reducing the calcium concentration of the medium to 0.1 mM or by adding 1 microM tetrodotoxin. The rank order of potency for the agonists was N alpha-methyl-histamine greater than histamine approximately 2-methyl-histamine. The histamine-induced fast response was not antagonized by any of the above-mentioned antagonists. It was slightly reduced by (+)-tubocurarine (100 microM) and N-methylbicuculline (100 microM) but such effects were not consistent with the blockade of nicotinic or GABAA receptor-mediated responses. 5. It was concluded that histamine depolarized the isolated superior cervical ganglion of the rat by activating H1 receptors. Relatively high concentrations of histamine also evoked a fast depolarization of this preparation, but this did not appear to be mediated by H1, H2 or H3 receptors.

Selective labelling of muscarinic M1 receptors in calf superior cervical ganglia by [3H](+/-)-telenzepine

A method was developed to determine the affinities of antimuscarinic drugs at M1 receptors. [3H](+/-)-Telenzepine served as radioligand in crude preparations of calf superior cervical ganglia and showed high affinity for a single receptor population, consisting of M1 receptors (KD = 1.12 nM). Kinetic experiments showed monophasic association (k1 = 0.017 min-1 nM-1) and dissociation (k-1 = 0.017 min-1) kinetics, the half-life of dissociation being 41 min at 37 degrees C. The kinetic KD value amounted to 1.00 nM. M1 affinities for pirenzepine, methoctramine, hexahydro-sila-difenidol and p-fluoro-hexahydro-sila-difenidol determined in competition experiments were similar to those found in functional studies with M1 receptors in rabbit isolated vas deferens. The binding assay was used to determine the affinities of the (R) and (S) enantiomers of tertiary (trihexyphenidyl, hexahydro-difenidol, hexbutinol, p-fluoro-hexbutinol) and quaternary muscarinic antagonists (trihexyphenidyl methiodide, hexbutinol methiodide). Comparison of results obtained with the rabbit vas deferens suggested that the ionic environment may influence the affinities.

Cholinergic inositol phosphate formation in striatal neurons is mediated by distinct mechanisms

In murine striatal neurons devoid of functional synapses (6 days in vitro) the cholinergic agonists carbachol and arecoline evoked dose-dependent inositol phosphate (InsP) responses with mean log EC50s of -4.1 +/- 0.5 and -4.48 +/- 0.1, respectively. Carbachol (1 mM) and arecoline (1 mM) responses were insensitive to tetrodotoxin, a voltage-sensitive Na+ channel blocker, and were blocked by pirenzepine with relatively low affinity (logIC50 = -5.9 +/- 0.3 for the carbachol response and logIC50 = -5.8 +/- 0.3 for the arecoline response). After synaptogenesis (13 days in vitro) the maximal carbachol effect doubled whereas the arecoline response remained unchanged. This additional effect was sensitive to tetrodotoxin and the voltage-dependent Ca2+ channel blocker, omega-conotoxin. The tetrodotoxin-sensitive carbachol response was blocked by lower concentrations of pirenzepine than the tetrodotoxin-insensitive carbachol response. More than 75% of the InsP response evoked by low concentrations of muscarine (1 and 10 microM) was sensitive to tetrodotoxin whereas only 38% of the InsP response stimulated by 1 mM of muscarine could be blocked by tetrodotoxin. These results suggest that there are at least two different mechanisms (depending on the stage of development), activated most probably by two different muscarinic receptors responsible for the carbachol-induced InsP formation in striatal neurons.

A novel series of non-quaternary oxadiazoles acting as full agonists at muscarinic receptors

1 A novel series of non-quaternary oxadiazole-based muscarinic agonists demonstrated high affinity for muscarinic receptors. 2. These agonists possessed high efficacy in the nanomolar range at muscarinic receptors in the superior cervical ganglion, atrium and ileum but did not show selectivity across the tissue preparations. 3. Two amino oxadiazoles, one from a quinuclidine series (L-660,863) and one from a 1-azanorbornane series (L-670,207) possessed a high ratio of potency for displacing the binding of [3H]-N-methyl-scopolamine ([3H]-NMS) to potency for displacing the agonist [3H]-oxotremorine-M cortex. 4. The two azanorbornane derivatives L-670,548 and L-670,207 stimulated the turnover of phosphatidylinositol in the cortex with a potency higher than that obtained with any other known muscarinic agonist (ED50 0.26 and 0.18 microM respectively). 5. The maximum response obtained with L-670,207 was greater than that observed for carbachol but was comparable to that of the natural ligand acetylcholine. 6. These oxadiazole muscarinic agonists are among the most potent and efficacious non-quaternary muscarinic agonists ever described.

In vivo characterisation of novel efficacious muscarinic receptor agonists

Although a number of muscarinic agonists have been used in clinical trials for Alzheimer's Disease, many of these compounds are low in potency and have only limited intrinsic efficacy. The present study describes four non-quaternary oxadiazole based muscarinic agonists from a quinuclidine and a 1-azanorbornane series. These displayed up to 1000 fold higher affinity than arecoline and were efficacious muscarinic agonists at cortical receptors. All four compounds produced peripherally mediated salivation and centrally mediated hypothermia at doses 50-50,000 fold lower than arecoline. The most potent was L-670,548, the methyl oxadiazole in the 1-azanorbornane series, which had an ED50 of 0.0016 mg/kg on the hypothermia model. This derivative was also the most potent compound in ex vivo binding studies (ED50 0.0069 mg/kg) and showed excellent brain penetration (3.8% of the administered dose). These derivatives are the first non quaternary efficacious agonists which show good penetration into the CNS (central nervous system), and will prove useful tools in understanding the role of muscarinic receptors in CNS function.

A pharmacological study of the responses induced by muscarinic agonists on the isolated superior cervical ganglion of the guinea-pig

We have studied the muscarinic agonist induced responses on the guinea-pig superior cervical ganglion in vitro, as recorded from the internal carotid nerve using a grease-gap. The principal response was a depolarization, but a small hyperpolarizing response could be revealed under certain conditions. We determined the pA2 of a number of muscarinic antagonists against the muscarine induced depolarization. Four selective antagonists and atropine appeared to act competitively. The rank order of their pA2s was 4-DAMP (8.5), atropine (8.4), pirenzepine (8.0), methoctramine (7.2) and AF-DX 116 (6.3). In addition to muscarine, we assessed the potency and relative maximum response of nine other muscarinic compounds to depolarize this preparation: carbachol, 5-methylfurmethide, oxotremorine, oxotremorine-M, pilocarpine, RS 86, AF102B and two novel compounds L-670548 and L-679512. L-670548 was the most potent and AF102B was the least potent agonist tested. Only AF102B evoked a maximum depolarization that was significantly smaller than muscarine. A hyperpolarizing response to carbachol (1 microM) could be recorded when the superfusing medium contained 0.3 microM pirenzepine and only 0.1 mM CaCl2 (cf. usual 2.5 mM). This response was relatively small compared to that evoked on the superior cervical ganglion of the rat. It was blocked by the cardioselective antagonists methoctramine (0.1-0.3 microM) and AF-DX 116 (0.3-1.0 microM). Of the 10 agonists tested, only carbachol, oxotremorine and oxotremorine-M reproducibly evoked a hyperpolarizing response. It was concluded that muscarinic agonists can induce a depolarization of the guinea-pig superior cervical ganglion mediated by M1 receptors. The activation of cardiac-like M2 receptors resulted in a hyperpolarizing response that was relatively small.

Interaction of the atypical neuroleptic clozapine with 5-HT3 receptors in the cerebral cortex and superior cervical ganglion of the rat

Clozapine, an atypical neuroleptic drug devoid of extrapyramidal side effects, was a moderately potent, competitive inhibitor of the binding of [3H]quaternised ICS 205-930 to 5-HT3 receptor sites in rat cortical membranes, possessing a pKi value of 7.0. In contrast, several other antipsychotic agents, including fluphenazine, alpha-flupenthixol, haloperidol, spiperone and (-)-sulpiride were essentially inactive. Clozapine also antagonised the 2-methyl 5-HT-induced depolarisation of the rat isolated superior cervical ganglion, a response known to be mediated via 5-HT3 receptors. Clozapine (0.1-1 microM) induced parallel displacements to the right of the dose-response curve to 2-methyl 5-HT in this tissue, possessing a pKb value of 7.3. These data suggest that the atypical antipsychotic profile of clozapine may be related, at least, in part to its ability to interact with central 5-HT3 receptor sites.

Hexahydrodifenidol does not distinguish among M1 receptors in rat cerebral cortex, hippocampus and superior cervical ganglion

We have determined the antagonist affinity of hexahydrodifenidol in a range of receptor assays in the rat:-radioreceptor binding and phosphatidyl-inositol turnover assays in cerebral cortex and hippocampus, and electrophysiological experiments on the superior cervical ganglion and hippocampus. We failed to detect any appreciable differences in the affinity of hexahydrodifenidol among any of these assays.

Interaction of telenzepine with muscarinic receptors in mammalian sympathetic ganglia

The interaction of the antimuscarinic drug telenzepine with muscarinic receptors was studied in rabbit and rat isolated superior cervical sympathetic ganglia. Radioligand binding demonstrated two muscarinic receptor sites in rabbit ganglia, with the characteristics of M1- and M2-receptors. Telenzepine bound to the M1 sites with a KI of 0.94 nmol/l and to the M2 sites with a KI of 17.8 nmol/l; the corresponding values for pirenzepine were 18.6 and 588 nmol/l; for AF-DX 116 the values were 891 and 33 nmol/l respectively. [3H]Telenzepine dissociated from the M1-receptors with a half time of 46 min at 37 degrees C. Electrophysiological experiments demonstrated that telenzepine reduced the amplitude of the extracellularly recorded slow excitatory postsynaptic potential and the slow inhibitory postsynaptic potential (ED50: 38 and 253 nmol/l respectively). In rat ganglia, application of muscarine or the M1-receptor agonist McN-A-343 increased the amplitude of submaximal population action potentials. This facilitation of synaptic transmission was potently blocked by telenzepine and pirenzepine but only weakly by AF-DX 116 (ED50: ca. 30, 150 and 20 mumol/l, respectively). It is concluded that telenzepine blocks the generation of the slow excitatory postsynaptic potential and the excitatory action of muscarine and McN-A-343 via an action on muscarinic M1-receptors.

Pharmacological characterization of the aminopyridazine SR 95639A, a selective M1 muscarinic agonist

In order to design a selective M1 muscarinic agonist, we synthesized SR 95639A (morpholinoethylamino-3-benzocyclohepta-(5,6-c)-pyridazine, dihydrochloride), a semi-rigid analogue of the aminopyridazine antidepressant drug minaprine. SR 95639A displaced [3H]pirenzepine from its binding sites in rat hippocampal membranes with an IC50 value of 0.27 microM. It only weakly displaced [3H]N-methylscopolamine from cerebellar, cardiac and ileal membranes (10-48 microM), and, up to 100 microM, did not interact with the main other receptors of the rat brain. In rat isolated sympathetic ganglia, SR 95639A induced dose-dependent depolarizations which were antagonized by pirenzepine, and dose dependently suppressed the M current. These latter effects were also pirenzepine-sensitive. After i.p. or oral treatment in mice, SR 95639A never induced the classical cholinergic syndrome, up to lethal doses. Finally, SR 95639A (i.p. and p.o.) antagonized contralateral rotations induced by intrastriatal injection of pirenzepine, in mice. These results suggest that SR 95639A is a selective agonist at central muscarinic M1 receptors and may represent a useful tool for further characterization of the nature and function of muscarinic receptor subtypes.

Muscarinic receptor differentiation

1. Several selective antagonists are available to differentiate between muscarinic receptors. 2. Further subdivision of M1 and M2 muscarinic receptors appears possible and is supported by studies with cloned receptors. 3. Reasons for differences between affinity constants determined in functional and binding studies and whether receptor subtypes couple exclusively with a particular cellular mechanism are still to be determined.

Methoctramine and hexahydrodifenidol antagonise two muscarinic responses on the rat superior cervical ganglion with opposite selectivity

Two novel muscarinic antagonists, methoctramine and hexahydrodifenidol, have been assessed for their action against two muscarinic agonist-induced responses on the rat superior cervical ganglion in vitro. DC recordings were made between the desheathed ganglion and its internal carotid nerve using the grease-gap technique. Hexahydrodifenidol and methoctramine antagonised the muscarine-induced M1-mediated depolarisation of this preparation with estimated pA2 values of 7.5 and 6.5, respectively. In 0.3 microM pirenzepine and 0.1 mM CaCl2, 1 microM muscarine evoked a hyperpolarisation mediated by cardiac-like M2 receptors. Hexahydrodifenidol and methoctramine antagonised this response with pIC50 values (-log10IC50) of 5.7 and 7.4, respectively. The selectivity of methoctramine for cardiac-like M2 receptors over M1 receptors is therefore confirmed and extended to these two neuronal responses. The selectivity of hexahydrodifenidol was opposite to, and greater than, that seen with methoctramine.

Selective antagonism of muscarinic potentials on the superior cervical ganglion of the rat

Selective antagonists have been used to classify the muscarinic receptors involved in the slow excitatory postsynaptic potential and slow inhibitory postsynaptic potential of the superior cervical ganglia of the rat, as recorded in 1 microM neostigmine, using a grease-gap method. Cardioselective M2 antagonists, e.g. AF-DX 116, depressed the slow inhibitory postsynaptic potential and enhanced the slow excitatory postsynaptic potential. The M1 selective antagonist pirenzepine depressed both potentials equally. The high potency of pirenzepine against the slow excitatory postsynaptic potential, however, indicates that it is mediated by M1 receptors. The slow excitatory and inhibitory postsynaptic potentials were found to be pharmacologically similar to the muscarinic agonist-induced depolarisation and hyperpolarisation of this preparation, respectively. The actions of two muscarinic agonists on the postsynaptic potentials were also studied. It is concluded that the slow excitatory postsynaptic potential is mediated by M1 receptors and the slow inhibitory postsynaptic potential by cardiac-like M2 receptors.

Biphasic dose-response curve to muscarine on the rat superior cervical ganglion

The dose-response curve for the muscarine-induced depolarisation of the rat isolated superior cervical ganglion, studied over the concentration range of 3 nM-1 mM, was biphasic. An apparent maximum was obtained at around 1-3 microM muscarine, but this was only a plateau between the two parts of the curve. Two cardioselective antagonists, gallamine (10 microM) and AF-DX 116 (1 microM) had a complex action on this dose-response curve. The dose-response curve between 0.01 and 0.3 microM was shifted to the right, the responses around 3 microM muscarine were enhanced, but the dose-response curve over 30 microM muscarine was unaffected. The M1-selective antagonist pirenzepine (0.05 microM) depressed all parts of the dose-response curve, but it still appeared biphasic. Pretreatment of the ganglion with pertussis toxin (1 microgram/ml) enhanced the depolarisation to muscarine 0.01-1000 microM and the dose-response curve became less biphasic. Like gallamine and AF-DX 116, pertussis toxin abolished the muscarinic M2-mediated hyperpolarisation of the ganglion recorded in 0.3 microM pirenzepine. It is concluded that the presence of an underlying M2-mediated hyperpolarisation contributes to the biphasic nature of the dose-response curve to muscarine.

Pertussis toxin sensitivity of drug-induced potentials on the rat superior cervical ganglion

We have investigated the action of pertussis toxin on a range of receptor-mediated responses of the rat superior cervical ganglion in vitro. The ganglia were treated with pertussis toxin for 24 h at 37 degrees C using an in vitro method. Appropriate controls were also carried out. Pertussis toxin (1 microgram/ml) reduced ganglionic hyperpolarisations mediated by adenosine, alpha 2, 5-HT1A, M2 and GABAB receptors. The GABAB-mediated hyperpolarisation of this preparation, evoked by baclofen and GABA in a bicuculline-resistant manner, has not previously been reported. Pertussis toxin did not reduce ganglionic depolarisations evoked by potassium chloride and 5-HT3, GABAA and nicotinic receptors. Depolarisations to muscarine and noradrenaline, probably mediated by M1 and beta-receptors, also appeared to be resistant to pertussis toxin. The similar sensitivity of the various ganglionic hyperpolarisations to pertussis toxin indicates that they may all be mediated by similar G-proteins.

5-Hydroxytryptamine evokes three distinct responses on the rat superior cervical ganglion in vitro

In addition to the 5-HT3-mediated fast depolarisation, 5-hydroxytryptamine (5-HT) evoked two additional responses on the rat superior cervical ganglion: a hyperpolarisation and a slow depolarisation. These responses appeared to be direct actions on 5-HT receptors since they were present in a low calcium medium containing tetrodotoxin and were not abolished by a variety of non-serotonin antagonists. The hyperpolarisation was not antagonised by 5-HT3 or 5-HT2 antagonists. The 5-HT1 ligands 5-carboxamidotryptamine (5-CT) and 8-OH-DPAT also evoked a hyperpolarisation. The hyperpolarisation was antagonised by six 5-HT1A antagonists including WB-4101 and spiroxatrine. It was therefore concluded to be mediated by a 5-HT1A receptor. The slow depolarisation was only evoked by 5-HT. The receptor involved in this response, however, could not be determined. We conclude that in addition to 5-HT3 receptors the rat superior cervical ganglion possesses 5-HT1A receptors and another uncharacterised 5-HT receptor.

M1 and M2 receptors mediate different effects on synaptically evoked potentials of the rat superior cervical ganglion

The application of muscarine, carbachol or methylfurmethide to the rat superior cervical ganglion in vitro resulted in a complex mixture of effects as recorded from the internal carotid nerve using a greased-gap technique. In addition to the previously described depolarizing and hyperpolarizing responses, increases and decreases in the amplitudes of the synaptically evoked compound action potential and its related afterhyperpolarization were observed. The depolarization and the enhancing effects of the agonists were selectively antagonized by pirenzepine (0.3 microM) whereas the hyperpolarization and depressant effects were antagonized by gallamine (10 microM). Methylfurmethide only induced the pirenzepine-sensitive effects. It is concluded that M1 and M2 (cardiac-like) receptors exert different effects on the synaptically evoked potentials of this preparation.

A novel muscarinic receptor antagonist AF-DX 116 differentially blocks slow inhibitory and slow excitatory postsynaptic potentials in the rabbit sympathetic ganglia

Muscarinic, slow postsynaptic potentials (s-epsp and s-ipsp) in the rabbit superior cervical ganglia were shown to be differentially depressed by a novel cardioselective M2-type antagonist AF-DX 116: it antagonized the s-ipsp with IC50 value of 1.5 X 10(-7) M, which is 16-fold more potent in depressing the s-ipsp than the s-epsp. A hyperpolarizing component in the biphasic potential changes induced by a muscarinic agonist, methacholine, was selectively eliminated by this antagonist. AF-DX 116 was thus shown to be an useful tool for discriminating the M2-type muscarinic responses from those of M1-type in the nervous system.