Stimulating muscarinic M1 receptors in the anterior cingulate cortex reduces mechanical hypersensitivity via GABAergic transmission in nerve injury rats.

Cholinergic systems modulate synaptic transmission across the neuraxis and play an important role in higher brain function including cognition, arousal and nociception. The anterior cingulate cortex (ACC) is a fundamental brain region for nociception and chronic pain, and receives cholinergic projections mainly from basal forebrain. Recently, we found that the activation of muscarinic M1 receptors in the ACC produced antinociceptive behavior in response to mechanical stimulation. However, it has not been tested whether stimulating muscarinic receptors in the ACC can reduce mechanical hypersensitivity in animal models of chronic pain. Here, we tested whether the activation of muscarinic M1 receptors in the ACC can alleviate mechanical hypersensitivity in a nerve injury model. The activation of muscarinic M1/M4 receptors by McN-A-343 injected into the contralateral side of the ACC, but not into the ventral posterolateral nucleus, was found to dose-dependently reduce mechanical hypersensitivity 7 days following partial sciatic nerve ligation in rats. The reduction of mechanical hypersensitivity by McN-A-343, was blocked by a selective muscarinic M1 antagonist, but not a M4 receptor antagonist. Importantly, the nerve injury model did not change the protein expression of muscarinic M1 receptors in the ACC. Additionally, a type A γ-aminobutyric acid (GABAA) receptor agonist injected into the ACC reduced the mechanical hypersensitivity in this injury model. Finally, a GABAA receptor antagonist blocked the reduction of mechanical hypersensitivity by McN-A-343 in the injury model. Collectively, these results suggest that activations of muscarinic M1 receptors in the ACC reduce nerve injury-induced mechanical hypersensitivity through GABAergic transmission via GABAA receptors.

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 GABAA 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.

Behavioral and neurochemical alterations following thiamine deficiency in rodents: relationship to functions of cholinergic neurons.

Memory deficits are induced during the late stage (20-25 days) of thiamine-deficient (TD) feeding. In this review, the role of cholinergic neurons on the memory deficit induced by TD feeding are summarized. Although memory deficit cannot be suppressed by an injection of thiamine once it appears, such impairment was found to be protected by early treatment with thiamine during TD feeding. Administration of muscarinic M(1) agonist McN-A-343 reversed the memory deficit observed in TD mice, although the muscarinic M(2) antagonist methoctramine did not. The "kampo" (traditional herbal) medicine, "kami-untan-to" (KUT), protected against the memory deficit observed in TD mice. Choline acetyltransferase (ChAT) fluorescence intensity, a marker of presynapse of cholinergic neurons, was decreased in the cortex and hippocampus at an early stage (14th day) of TD, and it was decreased in a wide range of brain areas at a late stage (25th day) of TD. Early KUT treatment inhibited the reduction of ChAT in the hippocampus of TD mice. These findings suggested that the memory deficit may be caused by a reduction in the cholinergic function at an early stage of TD, and that the activation of cholinergic neurons may play an important role in the improvement of TD-induced memory deficit.

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.

Infralimbic muscarinic M1 receptors modulate anxiety-like behaviour and spontaneous working memory in mice.

RATIONALE: Spontaneous working memory and anxiety-like behaviour can be concurrently influenced following kappa 1 opioid agonist or antagonist infusions in the infralimbic (IL) area of the ventromedial prefrontal cortex (vmPFC) in CD-1 mice. OBJECTIVE: The present study sought to evaluate whether acetylcholine (ACh) muscarinic (M) receptor drugs can similarly influence these cognitive-behavioural processes in the IL cortex. METHOD: Anxiety was evaluated in the elevated plusmaze and spontaneous working memory was evaluated in the Y-maze following scopolamine, pirenzepine or McN-A-343 infusion in the IL cortex. RESULTS: In experiment 1, the non-specific muscarinic receptor antagonist, scopolamine, was anxiogenic in trial 1 (5, 10 and 20 nmol), but did not influence behaviour in trial 2 (no-injection) in the elevated plus-maze 24 h later. In week 2, scopolamine disrupted spontaneous working memory in the Y-maze at the highest dose (20 nmol). In experiment 2, pretreatment with the M1 antagonist, pirenzepine, was anxiolytic in trial 1 (5 and 10 nmol), as well as in trial 2 (no-injection) in the elevated plus-maze 24 h later (0.25, 1.25, 2.5, 5 and 10 nmol). In week 2, pirenzepine disrupted spontaneous working memory in the Y-maze (2.5, 5 and 10 nmol). In experiment 3, pretreatment with the M1 agonist, McN-A-343, was anxiogenic in trial 1 (2.5, 5, 10 and 20 nmol), as well as in trial 2 (no-injection) in the elevated plus-maze 24 h later (2.5, 5, 10 and 20 nmol). In week 2, McN-A-343 enhanced spontaneous working memory in the Y-maze (2.5, 5, 10 and 20 nmol). CONCLUSIONS: (1) Enhanced ACh transmission in the vmPFC induces anxiety in challenging environments and enhances spontaneous working memory performance. (2) Blocking or activating postsynaptic M1 receptors in the vmPFC may truncate or exaggerate, respectively, afferent anxiety-relevant information. (3) IL pirenzepine and McN-A-343 exert long-term opposite effects on aversive learning during trial 1 in the elevated plus-maze.