Biochemical and behavioral responses of pilocarpine at muscarinic receptor subtypes in the CNS. Comparison with receptor binding and low-energy conformations

Carbachol-Induced theta-like oscillations in the rodent brain limbic system: Underlying mechanisms and significance

Theta oscillations (4-12 Hz) represent one of the most prominent physiological oscillatory activity in the mammalian EEG. They are observed in several areas of the hippocampus and in parahippocampal structures. Theta oscillations play important roles in modulating synaptic plasticity during memory and learning; moreover, they are dependent on septal cholinergic inputs. Theta oscillations can be reproduced in vitro in several regions of the temporal lobe in the absence of the septum by employing the cholinergic agonist carbachol (CCh). Here, we review the mechanisms underlying CCh-induced theta oscillations. We address: (i) the ability of temporal lobe neuronal networks to oscillate independently at theta frequency during CCh treatment; (ii) the contribution of intrinsic ionic currents; (iii) the participation of principal cells and interneurons; and (iv) their pharmacological profiles. We also discuss the similarities between CCh-induced theta oscillations and physiological type II theta activity, as well as their roles in synaptic plasticity. Finally, we consider experimental evidence pointing to the contribution of spontaneous and CCh-induced theta activity to epileptiform synchronization.

Muscarinic and nicotinic modulation of thalamo-prefrontal cortex synaptic plasticity [corrected] in vivo

The mediodorsal nucleus of the thalamus (MD) is a rich source of afferents to the medial prefrontal cortex (mPFC). Dysfunctions in the thalamo-prefrontal connections can impair networks implicated in working memory, some of which are affected in Alzheimer disease and schizophrenia. Considering the importance of the cholinergic system to cortical functioning, our study aimed to investigate the effects of global cholinergic activation of the brain on MD-mPFC synaptic plasticity by measuring the dynamics of long-term potentiation (LTP) and depression (LTD) in vivo. Therefore, rats received intraventricular injections either of the muscarinic agonist pilocarpine (PILO; 40 nmol/µL), the nicotinic agonist nicotine (NIC; 320 nmol/µL), or vehicle. The injections were administered prior to either thalamic high-frequency (HFS) or low-frequency stimulation (LFS). Test pulses were applied to MD for 30 min during baseline and 240 min after HFS or LFS, while field postsynaptic potentials were recorded in the mPFC. The transient oscillatory effects of PILO and NIC were monitored through recording of thalamic and cortical local field potentials. Our results show that HFS did not affect mPFC responses in vehicle-injected rats, but induced a delayed-onset LTP with distinct effects when applied following PILO or NIC. Conversely, LFS induced a stable LTD in control subjects, but was unable to induce LTD when applied after PILO or NIC. Taken together, our findings show distinct modulatory effects of each cholinergic brain activation on MD-mPFC plasticity following HFS and LFS. The LTP-inducing action and long-lasting suppression of cortical LTD induced by PILO and NIC might implicate differential modulation of thalamo-prefrontal functions under low and high input drive.

Cholinergic responses in GABAergic and non-GABAergic neurons in the intermediate gray layer of mouse superior colliculus

Neurons in the intermediate gray layer (SGI) of the mammalian superior colliculus (SC) receive dense cholinergic innervations from the brainstem parabrachial region. Such cholinergic inputs may influence execution of orienting behaviors. To obtain deeper insights into how the cholinergic inputs modulate the SC local circuits, we analysed the cholinergic responses in identified γ-aminobutyric acid (GABA)ergic and non-GABAergic neurons using SC slices obtained from GAD67-GFP knock-in mice. The responses of SGI neurons to cholinergic agonists were various combinations of fast inward currents mediated mainly via α4β2 and partly by α7 nicotinic receptors (nIN), slow inward currents caused by activation of M1 plus M3 muscarinic receptors (mIN), and slow outward currents caused by activation of M2 muscarinic receptors (mOUT). The most common cholinergic responses in non-GABAergic neurons was nIN + mIN + mOUT (38/68), followed by nIN + mIN (16/68), nIN + mOUT (11/68), nIN only (2/68), and no response (1/68). On the other hand, the major response pattern in GABAergic neurons was either nIN only (26/54) or nIN + mIN (21/54), followed by nIN + mOUT (4/54), mOUT only (2/54), and no response (1/54). Thus, major effects of cholinergic inputs to both SGI GABAergic and non-GABAergic neurons are excitatory, but the response patterns in these two types of SGI neurons are different. Thus, actions of the cholinergic inputs to non-GABAergic and GABAergic SGI neurons are not simple push-pull mechanisms, like excitation vs inhibition, but might cooperate to balance the level of excitation and inhibition for setting the state of the response property of the local circuit.

Cholinergic responses in crossed tecto-reticular neurons of rat superior colliculus

Neurons in the intermediate gray layer (SGI) of mammalian superior colliculus (SC) receive cholinergic innervation from the brain stem parabrachial region, which seems to modulate the signal processing in the SC. To clarify its role particularly in orienting behaviors, we studied cholinergic effects on the major output neuron group of the SGI, crossed tecto-reticular neurons (cTRNs), identified by retrograde labeling from the contralateral brain stem gaze center in SC slices obtained from rats (PND 17-22) by whole cell patch-clamp techniques. Bath application of carbachol induced either 1) nicotinic inward (nIN) + muscarinic inward (mIN) (11/24) or 2) nIN + mIN + muscarinic outward (mOUT) (13/24) current responses. Transient pressure application of 1 mM acetylcholine elicited nIN in all neurons tested (n = 58). In a majority of these neurons (52/58), the nIN was completely suppressed by dihydro-beta-erythroidine, a specific antagonist for alpha4beta2 nicotinic receptor subtype. The remaining 6/58 neurons exhibited not only the slower alpha4beta2 receptor-mediated component but also a faster component that was inhibited by a specific antagonist for alpha7 nicotinic receptor, alpha-bungarotoxin. cTRNs expressing alpha7 nicotinic receptors tended to be smaller in size than those lacking alpha7 receptors. Bath application of muscarine induced two response patterns: mIN only (17/38) and mIN+ mOUT (21/38). The mIN and mOUT were mediated by M3 (plus M1) and M2 muscarinic receptors, respectively. These results suggest that a major response to cholinergic inputs to cTRNs is excitatory. This would indicate the facilitatory role of the brain stem cholinergic system in the execution of orienting behaviors including saccadic eye movements.

Properties of cholinergic responses in neurons in the intermediate grey layer of rat superior colliculus

The intermediate grey layer (SGI) of superior colliculus (SC) receives cholinergic innervation from brainstem parabrachial region. To clarify the action of cholinergic inputs to local circuits in the SGI, we investigated the effect of cholinergic agonists and antagonists on a large number of randomly sampled neurons in Wistar rat SGI (n=246) using whole-cell patch clamp technique in slices of the rat SC. Responses of the recorded cells (n=98) to bath application of carbachol were classified into five patterns: (i) nicotinic inward only (n=14); (ii) nicotinic inward+muscarinic inward (n=26); (iii) nicotinic inward+muscarinic inward+muscarinic outward (n=39); (iv) nicotinic inward+muscarinic outward (n=13) and (v) muscarinic outward only (n=4). Among these, a majority of morphologically identified projection neurons exhibited either response pattern (ii) (9/28) or (iii) (15/28), which suggested that the primary action of cholinergic inputs on the SGI output is excitatory. Nicotinic receptor subtypes involved in the nicotinic current were examined by testing the effects of antagonists on the currents induced by bath application of 1,1-dimethyl-4-phenyl-piperazinium or transient pressure application of acetylcholine (ACh). Muscarinic receptor subtypes involved in the muscarinic inward and outward currents were investigated by examining the effects of antagonists on muscarine-induced currents. The results showed that nicotinic inward currents are mediated mainly by alpha4beta2 and partly by alpha7 nicotinic receptors and that muscarinic inward and outward currents are mediated by M3 (plus M1) and M2 muscarinic receptors, respectively.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.

Short-term effects of pilocarpine on rat hippocampal neurons in culture

PURPOSE

Status epilepticus (SE) has been considered an epileptogenic factor in humans. In the pilocarpine (PILO) model, after a brief period marked by SE, the rats exhibit recurrent spontaneous seizures, mimicking the clinical features of temporal lobe epilepsy. The aim of our study was to identify the molecular actions of PILO that could account for its ability to induce SE.

METHODS

Whole-cell mode of the patch-clamp technique was applied to cultured hippocampal neurons (2-3 weeks old) in the absence and in the presence of PILO (1-10 microM), to study the spontaneous activity, the evoked, and the miniature postsynaptic currents. The postsynaptic currents were isolated pharmacologically.

RESULTS

PILO (1 and 10 microM) caused an initial increase followed by a decrease in the frequency of spontaneous activity. The increase in the frequency of excitatory postsynaptic currents (EPSCs) and inhibitory PSCs (IPSCs) was blocked by atropine (1 microM), indicating that this effect is mediated through muscarinic receptors. PILO also promoted a brief increase of the amplitude of IPSCs indirectly evoked by stimulation of a neuron synaptically connected to the neuron under study. Conversely, PILO promoted a sustained increase on the amplitude of electrically evoked EPSCs. In presence of tetrodotoxin (TTX; 300 nM), PILO (1 microM) increased the frequency of miniature EPSCs and IPSCs without changing their amplitude during the first 3 min of application.

CONCLUSIONS

These results indicate that PILO acting through muscarinic receptor causes an imbalance between excitatory and inhibitory transmission that can result in the generation of SE observed in animals acutely treated with PILO.

The utility of muscarinic agonists in the treatment of Alzheimer's disease

Alzheimer's disease is a progressive neurological disorder characterized by amyloid plaques and neurofibrillary tangles along with memory and cognitive deficits associated with a loss of basal forebrain cholinergic neurons. Efforts to treat Alzheimer's disease have focused on compounds that elevate cholinergic activity such as cholinesterase inhibitors and direct acting muscarinic and nicotinic agonists. Low efficacy and poor selectivity of available compounds have limited the clinical utility of muscarinic agonists. Recent studies suggesting a role for muscarinic agonists in regulating the production of A beta raise the possibility that selective M1 agonists could be useful in treating not only the symptoms, but also the underlying cause(s) of Alzheimer's disease. Thus, renewed efforts have focused on the development of compounds with improved selectivity for M1 receptors and lower toxicity. 5-(3-ethyl-1,2,4-oxadiazol-5-yl)-1,4,5,6-tetrahydropyrimidine (CDD-0102) is a potent M1 agonist with a low side effect profile that enhances memory function in animal models of Alzheimer's disease. The available preclinical data suggest that CDD-0102 may be useful in the treatment of Alzheimer's disease.

Pilocarpine modulates the cellular electrical properties of mammalian hearts by activating a cardiac M3 receptor and a K+ current

1. Pilocarpine, a muscarinic acetylcholine receptor (mAChR) agonist, is widely used for treatment of xerostomia and glaucoma. It can also cause many other cellular responses by activating different subtypes of mAChRs in different tissues. However, the potential role of pilocarpine in modulating cardiac function remained unstudied. 2. We found that pilocarpine produced concentration-dependent (0.1-10 microM) decrease in sinus rhythm and action potential duration, and hyperpolarization of membrane potential in guinea-pig hearts. The effects were nearly completely reversed by 1 microM atropine or 2 nM 4DAMP methiodide (an M3-selective antagonist). 3. Patch-clamp recordings in dispersed myocytes from guinea-pig and canine atria revealed that pilocarpine induces a novel K+ current with delayed rectifying properties. The current was suppressed by low concentrations of M3-selective antagonists 4DAMP methiodide (2-10 nM), 4DAMP mustard (4-20 nM, an ackylating agent) and p-F-HHSiD (20-200 nM). Antagonists towards other subtypes (M1, M2 or M4) all failed to alter the current. 4. The affinity of pilocarpine (KD) at mAChRs derived from displacement binding of [3H]-NMS in the homogenates from dog atria was 2.2 microM (65% of the total binding) and that of 4DAMP methiodide was 2.8 nM (70% of total binding), consistent with the concentration of pilocarpine needed for the current induction and for the modulation of the cardiac electrical activity and the concentration of 4DAMP to block pilocarpine effects. 5. Our data indicate, for the first time, that pilocarpine modulates the cellular electrical properties of the hearts, likely by activating a K+ current mediated by M3 receptors.

Roles of threonine 192 and asparagine 382 in agonist and antagonist interactions with M1 muscarinic receptors

Conserved amino acids, such as Thr in transmembrane domains (TM) V and Asn in TM VI of muscarinic receptors, may be important in agonist binding and/or receptor activation. In order to determine the functional roles of Thr192 and Asn382 in human M1 receptors in ligand binding and receptor activation processes, we created and characterized mutant receptors with Thr192 or Asn382 substituted by Ala. HM1 wild-type (WT) and mutant receptors [HM1(Thr192Ala) and HM1(Asn382Ala)] were stably expressed in A9 L cells. The Kd values for 3H-(R)-QNB and Ki values for other classical muscarinic antagonists were similar at HM1(WT) and HM1(Thr192Ala) mutant receptors, yet higher at HM1(Asn382Ala) mutant receptors. Carbachol exhibited lower potency and efficacy in stimulating PI hydrolysis via HM1(Thr192Ala) mutant receptors, and intermediate agonist activity at the HM1(Asn382Ala) mutant receptors. The Asn382 residue in TM VI but not the Thr192 residue in TM V of the human M1 receptor appears to participate directly in antagonist binding. Both Thr192 and Asn382 residues are involved differentially in agonist binding and/or receptor activation processes, yet the Asn382 residue is less important than Thr192 in agonist activation of M1 receptors. Molecular modelling studies indicate that substitution of Thr192 or Asn382 results in the loss of hydrogen-bond interactions and changes in the agonist binding mode associated with an increase in hydrophobic interactions between ligand and receptor.

The effects of choline on body temperature in conscious rats

Choline (75-300 microg) produced dose-dependent hypothermia when injected intracerebroventricularly (i.c.v.). Pre-treatment with the muscarinic receptor antagonist, atropine (10 microg, i.c.v.), blocked the hypothermic effect of choline (150 microg), but the response was only partially attenuated by pre-treatment with the nicotinic receptor antagonist, mecamylamine (20 microg, i.c.v.). Pirenzepine (25 microg), a muscarinic M1 receptor antagonist, or hexahydro-siladifenidol (HHSD) (100 microg), a muscarinic M3 receptor antagonist, also blocked choline-induced hypothermia when injected centrally. Unlike the other muscarinic receptor antagonists, M2-selective 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyri do[2,3-b][1,4]benzodiazepin-6-one (AF-DX116) (10 microg), did not affect choline-induced hypothermia. We also found that choline-induced hypothermia was very sensitive to the ambient temperature. Similar to its effect at room temperature, choline produced dose-dependent hypothermia at 4 degrees C, but this effect was abolished at 32 degrees C. These data suggest that choline produces hypothermia and this effect is mediated by muscarinic receptors.

Synthesis and biological characterization of 1,4,5,6-tetrahydropyrimidine and 2-amino-3,4,5,6-tetrahydropyridine derivatives as selective m1 agonists

Previous studies identified several novel tetrahydropyrimidine derivatives exhibiting muscarinic agonist activity in rat brain. Such compounds might be useful in treating cognitive and memory deficits associated with low acetylcholine levels, as found in Alzheimer's disease. To determine the molecular features of ligands important for binding and activity at muscarinic receptor subtypes, the series of tetrahydropyrimidines was extended. Several active compounds were examined further for functional selectivity through biochemical studies of muscarinic receptor activity using receptor subtypes expressed in cell lines. Several amidine derivatives displayed high efficacy at m1 receptors and lower activity at m3 receptors coupled to phosphoinositide (PI) metabolism in A9 L cells. Four ligands, including 1b, 1f, 2b, and 7b, exhibited marked functional selectivity for m1 vs m3 receptors. Compound 1f also exhibited low activity at m2 receptors coupled to the inhibition of adenylyl cyclase in A9 L cells. Molecular modeling studies also were initiated to help understand the nature of the interaction of muscarinic agonists with the m1 receptor using a nine amino model of the m1 receptor. Several important interactions were identified, including interactions between the ester moiety and Thr192. Additional interactions were found for oxadiazoles and alkynyl derivatives with Asn382, suggesting that enhanced potency and selectivity may be achieved by maximizing interactions with Asp105, Thr192, and Asn382. Taken together, the data indicate that several amidine derivatives display functional selectivity for m1 muscarinic receptors, warranting further evaluation as therapeutic agents for the treatment of Alzheimer's disease. In addition, several amino acid residues were identified as potential binding sites for m1 agonists. These data may be useful in directing efforts to develop even more selective m1 agonists.

Hippocampal and cerebellar extracellular amino acids during pilocarpine-induced seizures in freely moving rats

Limbic seizures were provoked in freely moving rats by intrahippocampal administration of the muscarinic receptor agonist pilocarpine via a microdialysis probe (10 mM for 40 min at 2 microliters/min). Changes in extracellular hippocampal and cerebellar glutamate, aspartate and gamma-aminobutyric acid (GABA) levels were monitored during and after pilocarpine administration. Effects of systemic or local administration of anticonvulsants on the seizures and concomitant changes in amino-acid concentrations, were investigated. Pilocarpine-induced seizures were completely abolished after intraperitoneal premedication for 7 days with phenobarbital (15 mg/kg per day) and after intrahippocampal administration of 10 mM phenobarbital and 1 mM carbamazepine (180 min at 2 microliters/min). Rats premedicated with carbamazepine (5 mg/kg per day) still developed seizures. The changes in extracellular hippocampal amino-acid levels suggest that glutamate, aspartate and GABA are not involved in seizure onset, but may play a role in seizure maintenance and/or spread in the pilocarpine animal model of epilepsy. The increases in extracellular amino acids in ipsi- and contralateral cerebellum following limbic seizures provoked in the hippocampus, probably play a role in the 'reversed' diaschisis phenomenon.

Synthesis and biochemical activity of novel amidine derivatives as m1 muscarinic receptor agonists

As part of a continuing effort aimed at the development of selective, efficacious, and centrally active m1 muscarinic agonists for the treatment of Alzheimer's disease, a series of amide and hydrazide amidine derivatives (2a-e and 3b-d) was synthesized and examined for muscarinic agonist activity. Preliminary biochemical studies indicated that 2b, 2d, and 3d bound to muscarinic receptors in rat brain and stimulated phosphoinositide (PI) metabolism in rat cerebral cortex. Compounds 2b and 2d were also highly efficacious at m1 muscarinic receptors expressed in cultured A9 L cells. Molecular modeling studies suggest slightly different modes of interaction with m1 receptors for the ester and amide derivatives. Also, hydrogen-bond formation with a Thr residue may be important for m1 muscarinic agonist potency. The data suggest that the amide moiety can replace the ester group found in muscarinic agonists and provide further support for the utility of amidine derivatives in the development of efficacious m1 agonists.

Differential behavioral effects of supracallosal and infracallosal lesions of the septohippocampal pathways: no ameliorative effects of oxotremorine or pilocarpine on radial-maze performance

We examined the effects in young adult female Long-Evans rats of single or combined lesions of the infracallosal and supracallosal septohippocampal pathways on a battery of behavioral tasks over two postoperative periods (14-65 and 75-150 days, respectively). During the first period, rats with lesions of the infracallosal pathways, whether given alone or in combination with lesions of the supracallosal pathways, were more active in the open field and in their home cage, and showed increased reactivity to novel extracage stimuli. Behavioral results during the second postoperative period were similar to those of the first except that rats with lesions of the infracallosal pathways (either alone or in combination with lesions of the supracallosal pathways) were no longer hyperactive in their home cage and rats with the infracallosal lesion alone were no longer hyperactive in the open field. We also observed in rats with lesions of the infracallosal pathways impaired performance in the radial-arm maze task, whether conducted under an uninterrupted protocol (first and second postoperative periods) or with a 1-min intratrial interruption (second postoperative period). Thus, behavioral deficits were observed only in rats with a lesion to the infracallosal component of the septohippocampal pathways, the behavior of rats with the combined lesions being similar to that of rats with single lesions of the infracallosal pathways in most measures. The behavior of rats with lesions of the supracallosal pathways did not differ from that of sham-operated controls in any measure at either postoperative period. Acute, systemic injections of oxotremorine (0.03 or 0.1 mg/kg, ip) or pilocarpine (0.32 or 1.0 mg/kg, ip), two muscarinic agonists, did not affect radial-arm maze performance under either the uninterrupted or interrupted protocol. The use of nonspecific muscarinic agonists does not appear to be sufficient to enhance radial-arm maze performance in rats with infracallosal septohippocampal lesions which, in contrast to supracallosal lesions, were shown to induce a deficit in this task.

Involvement of the central nervous system in the salivary secretion induced by pilocarpine in rats

The effect of rats of an anteroventral third ventricle (AV3V) electrolytic lesion on salivary secretion induced by intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) injection of a cholinergic agonist (pilocarpine) was investigated. Sham- or AV3V-lesioned rats anesthetized with urethane and with a stainless steel cannula implanted into the lateral ventricle (LV) were used. The amount of salivary secretion was studied over a seven-minute period after i.c.v. or i.p. injection of pilocarpine. In sham-operated rats, i.p. injection of pilocarpine (1 mg/kg b.w.) (after 6 h, 2, 7, and 15 days) produced salivary secretion (486 +/- 21, 778 +/- 85, 630 +/- 50, and 560 +/- 55 mg/7 min, respectively). This effect was reduced 6 h, 2, and 7 days after an AV3V lesion (142 +/- 22, 113 +/- 32, and 290 +/- 62 mg/7 min, respectively), but not 15 days after an AV3V lesion (516 +/- 19 mg/7 min). I.c.v. injection of pilocarpine (120 micrograms in 1 microL), in sham-operated rats after 6 h, 2, 7, and 15 days also produced salivary secretion (443 +/- 20, 417 +/- 81, 496 +/- 14, and 427 +/- 47 mg/7 min, respectively). The effects of i.c.v. pilocarpine were also reduced 6 h, 2, and 7 days after an AV3V lesion (143 +/- 19, 273 +/- 14, and 322 +/- 17 mg/7 min, respectively), but not after 15 days (450 +/- 28 mg/7 min). The results demonstrate that the central nervous system, and particularly the AV3V region, is important for the effect of pilocarpine on salivary secretion in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of muscarinic receptors by intrahippocampal injections of gallamine

Multiple intrahippocampal injections of gallamine impair performance of a representational memory task in rats. The binding of [3H]-(-)-quinuclidinyl benzilate (QNB) to rat brain sections was measured to determine if changes in receptor binding were associated with the deleterious effects of gallamine. [3H]-(-)-QNB binding to sections taken from gallamine-injected animals was compared with binding in saline-injected control animals. Autoradiographic analyses indicated an increase in [3H]-(-)-QNB binding sites within all layers of the cerebral cortex and in the superior colliculus in gallamine-treated animals as compared to saline-injected controls. Significant increases were noted in cortical layers IV and V (P less than 0.025) in gallamine-treated animals. No significant changes (P greater than 0.05) in the number of binding sites were observed in the hippocampus, neostriatum or various thalamic nuclei. The ability of unlabeled pirenzepine, gallamine and carbamylcholine to inhibit 0.2 nM [3H]-(-)-QNB binding also was measured to determine changes in the distribution of receptor subtypes. No significant changes were observed in any brain region for the binding of the selective antagonists pirenzepine and gallamine or the agonist carbamyl-choline. Although other possibilities are considered, the data suggest that an increase in the number of muscarinic receptors may contribute to the observed behavioral deficits associated with long-term gallamine treatment.