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

Neuromodulation of Persistent Activity and Working Memory Circuitry in Primate Prefrontal Cortex by Muscarinic Receptors

Neuromodulation by acetylcholine plays a vital role in shaping the physiology and functions of cerebral cortex. Cholinergic neuromodulation influences brain-state transitions, controls the gating of cortical sensory stimulus responses, and has been shown to influence the generation and maintenance of persistent activity in prefrontal cortex. Here we review our current understanding of the role of muscarinic cholinergic receptors in primate prefrontal cortex during its engagement in the performance of working memory tasks. We summarize the localization of muscarinic receptors in prefrontal cortex, review the effects of muscarinic neuromodulation on arousal, working memory and cognitive control tasks, and describe the effects of muscarinic M1 receptor stimulation and blockade on the generation and maintenance of persistent activity of prefrontal neurons encoding working memory representations. Recent studies describing the pharmacological effects of M1 receptors on prefrontal persistent activity demonstrate the heterogeneity of muscarinic actions and delineate unexpected modulatory effects discovered in primate prefrontal cortex when compared with studies in rodents. Understanding the underlying mechanisms by which muscarinic receptors regulate prefrontal cognitive control circuitry will inform the search of muscarinic-based therapeutic targets in the treatment of neuropsychiatric disorders.

Muscarinic M1 Receptor Overstimulation Disrupts Working Memory Activity for Rules in Primate Prefrontal Cortex

Acetylcholine release in the prefrontal cortex (PFC), acting through muscarinic receptors, has an essential role in regulating flexible behavior and working memory (WM). General muscarinic receptor blockade disrupts PFC WM representations, while selective stimulation of muscarinic receptor subtypes is of great interest for the treatment of cognitive dysfunction in Alzheimer's disease. Here, we tested selective stimulation and blockade of muscarinic M1 receptors (M1Rs) in macaque PFC, during performance of a cognitive control task in which rules maintained in WM specified saccadic responses. We hypothesized that M1R blockade and stimulation would disrupt and enhance rule representation in WM, respectively. Unexpectedly, M1R blockade did not consistently affect PFC neuronal rule selectivity. Moreover, M1R stimulation suppressed PFC activity, and at higher doses, degraded rule representations. Our results suggest that, in primates, the deleterious effects of general muscarinic blockade on PFC WM activity are not mediated by M1Rs, while their overstimulation deteriorates PFC rule maintenance.

Activations of muscarinic M1 receptors in the anterior cingulate cortex contribute to the antinociceptive effect via GABAergic transmission

Background Cholinergic systems regulate the synaptic transmission resulting in the contribution of the nociceptive behaviors. Anterior cingulate cortex is a key cortical area to play roles in nociception and chronic pain. However, the effect of the activation of cholinergic system for nociception is still unknown in the cortical area. Here, we tested whether the activation of cholinergic receptors can regulate nociceptive behaviors in adult rat anterior cingulate cortex by integrative methods including behavior, immunohistochemical, and electrophysiological methods. Results We found that muscarinic M₁ receptors were clearly expressed in the anterior cingulate cortex. Using behavioral tests, we identified that microinjection of a selective muscarinic M₁ receptors agonist McN-A-343 into the anterior cingulate cortex dose dependently increased the mechanical threshold. In contrast, the local injection of McN-A-343 into the anterior cingulate cortex showed normal motor function. The microinjection of a selective M₁ receptors antagonist pirenzepine blocked the McN-A-343-induced antinociceptive effect. Pirenzepine alone into the anterior cingulate cortex decreased the mechanical thresholds. The local injection of the GABA_A receptors antagonist bicuculline into the anterior cingulate cortex also inhibited the McN-A-343-induced antinociceptive effect and decreased the mechanical threshold. Finally, we further tested whether the activation of M₁ receptors could regulate GABAergic transmission using whole-cell patch-clamp recordings. The activation of M₁ receptors enhanced the frequency of spontaneous and miniature inhibitory postsynaptic currents as well as the amplitude of spontaneous inhibitory postsynaptic currents in the anterior cingulate cortex. Conclusions These results suggest that the activation of muscarinic M₁ receptors in part increased the mechanical threshold by increasing GABAergic transmitter release and facilitating GABAergic transmission in the anterior cingulate cortex.

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.

A study of the interaction between ropranolol and NSAIDs in protein binding by gel filtration method

Drug protein binding phenomena can lead to some interesting drug-drug interactions when one drug displaces another in the binding site. Studies of protein binding are conducted by several methods including equilibrium dialysis, ultra-filtration and chromatographic methods. Gel filtration is a simple chromatographic method in protein binding studies. Propranolol binds to plasma proteins by 90%-95% in circulation system and other drugs with high protein binding may displace it. In this study protein binding of propranolol has been studied using gel filtration to Bovine Serum Albumin (BSA) alone and in the presence of Acetyl salicylic acid (ASA), Indomethacin and mefenamic acid has been studied using gel filtration method. The results indicated that ASA decreased protein binding of propranolol by 20% to BSA and other drugs did not displace propranolol from the binding site. Therefore, ASA may alter pharmacological effects of propranolol.

The spinal muscarinic receptor subtypes contribute to the morphine-induced antinociceptive effects in thermal stimulation in mice

The present study was undertaken to clarify how spinal muscarinic receptors can be involved in the antinociceptive effects induced by morphine in thermal stimulation. The morphine-induced antinociceptive effects (26.6 micromol/kg, s.c.) was inhibited by an intrathecal (i.t.) injection of the muscarinic antagonist (M) atropine and the M(1)/M(4) antagonist pirenzepine in a dose-dependent manner. In contrast, the M(2) antagonist methoctramine and the M(3) antagonist 4-DAMP did not inhibit the morphine-induced antinociceptive effects. Injection (i.t.) of the putative M(1) agonist McN-A-343 resulted in dose-dependent antinociceptive effects in thermal stimuli. In addition, antinociceptive effects induced by the i.t. injection of morphine were not inhibited by the M(1)/M(4) antagonist pirenzepine, although pirenzepine did inhibit the intracerebroventricular (i.c.v.) injection of morphine-induced antinociceptive effects. These results suggest that the morphine-induced antinociceptive effects in thermal stimuli are regulated by the M(1) or M(4) receptor in the spinal cord.

Intrathecal clonidine inhibits mechanical allodynia via activation of the spinal muscarinic M1 receptor in streptozotocin-induced diabetic mice

We examined the involvement of the spinal muscarinic receptors in the clonidine-induced antiallodynic effects. Mechanical sensitivity was assessed by stimulating the hind paw with von Frey filaments. In streptozotocin-treated (200 mg/kg, i.v.) diabetic mice, hypersensitivity to mechanical stimulation appeared 3 days after streptozotocin administration, and persisted for 11 days. This mechanical hypersensitivity (allodynia) was inhibited by the intrathecal (i.t.) injection of clonidine. The muscarinic receptor antagonist atropine (i.t.) and alpha2-adrenoreceptor antagonist yohimbine (i.t. or subcutaneous injection) abolished the antiallodynic effect of clonidine. The effect was mimicked by the muscarinic M1 receptor antagonist pirenzepine, but not by the muscarinic M2 receptor antagonist methoctoramine or the muscarinic M3 receptor antagonist 4-DAMP (4-diphenyl-acetoxy-N-methylpiperidine methiodide). In addition, the mechanical hypersensitivity in diabetic mice was reduced by the selective muscarinic M1 receptor agonist McN-A-343 (4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium chloride) (i.t.). These results suggest that spinal muscarinic M1 receptors participate in the antiallodynic effect of clonidine in diabetic mice.

The spinal antinociceptive effects of cholinergic drugs in rats: receptor subtype specificity in different nociceptive tests

BACKGROUND

Several studies have shown that muscarinic cholinergic agonists cause antinociception in humans and animals when given by both spinal and non-spinal parenteral routes. It is uncertain which subtype of muscarinic receptor is involved in spinally mediated antinociceptive effects caused by these drugs. The cholinergic receptor agonists McN-A-343 (M1 selective; 3.89 to 389 nmol) and carbachol (non-selective; 0.029 to 29 nmol) were used in a rat acute pain model to investigate the involvement of M1 and non-M1 subtypes in spinally mediated antinociception. The drugs were injected intrathecally and results from experiments in which drug actions were carefully confined to the spinal cord were used to construct agonist dose response curves.

RESULTS

McN-A-343 frequently diffused rostrally to the brain, away from the lumbosacral site of injection. Thus, in spite of its receptor subtype selectivity, McN-A-343 is a poor probe to use in attempting to identify receptor subtypes involved in spinal cord antinociceptive systems. However, in some experiments McN-A-343 caused spinally mediated antinociception assessed by the electrical current threshold test. Antinociception assessed by the tail flick latency test with intrathecal McN-A-343 was observed and found to involve supraspinal mechanisms. Carbachol caused spinally mediated antinociception assessed by both electrical current threshold and tail flick latency.

CONCLUSIONS

The results suggest that M1 receptors are involved in spinally mediated antinociception revealed by electrical current threshold; other cholinergic receptors (non-M1) are involved in thermal antinociception at the spinal cord. This contrasts with previous work on spinally mediated cholinergic antinociception. These differences are believed to be due to difficulties in restricting the action of these drugs to the spinal cord.