Muscarinic receptors are receptors of the neurotransmitter acetylcholine. Muscarinic receptors are often abbreviated as 'M receptors'. There are 2 general types of acetylcholine receptors, which are the muscarinic receptors, which each consist of a single protein and are metabotropic, and the nicotinic receptors, which each consist of multiple protein subunits and are ionotropic. Muscarinic receptors are so named because they bind to the chemical muscarine.

There are 5 different types of M receptor, which are M1, M2, M3, M4, and M5. M1, M3, and M5 are excitatory whereas M2 and M4 are inhibitory. An excitatory receptor is a receptor which causes neuron firing, and in the brain, a specific excitatory receptor produces a specific corresponding sensation. An inhibitory receptor is a receptor that suppresses the firing of neurons, and unlike excitatory receptors, they are not related to any specific sensations.

M receptors exist both in the brain and in the periphery. 'Periphery' means components of the nervous system outside of the brain. All 5 of the M receptors have known substantial functions in the brain, whereas only the M2 and M3 receptors have known substantial functions in the periphery. In the periphery, M2 and M3 receptors are mostly expressed (i.e. occur) in the parasympathetic (i.e. arousal-decreasing) systems of the heart, the lungs, and the digestive system.

The following are the functions of the different M receptors in the brain, with the excitatory receptors listed first:
M1: facilitates spatial memory and factual understanding, causes fine clear perception*, decreases dopamine production, stimulates GABAergic neurons in the nucleus reticularis thalami (NRT) to create thalamocortical rhythm by preventing excess neuron firing, increases NMDA receptor activity in the frontal lobe of the cerebrum and therefore reduces the symptoms of schizophrenia
M3: involved in dreams and recall of personal memories, causes fine clear perception
M5: involved in the behavioral reinforcing effects of dopamine
M2: an inhibitory autoreceptor (i.e. it inhibits the firing of excitatory receptors of the same neurotransmitter, so as to regulate them), disinhibits the sense of smell, filters out unimportant environmental input in the thalamus
M4: involved in memory formation, converts norepinephrine into epinephrine, decreases multitasking ability due to it's action in the striatum

It is notable to mention that the H1 histamine receptor has a particular relationship with the M1 and M3 receptors, such that H1 receptor firing both increases acetylcholine production and release, and causes the M1 and M3 receptors to fire at a higher rate than they normally would for a given level of acetylcholine. The H1 receptor also causes fine clear perception*. The H1, M1, and M3 receptors together constitute all of the receptors of fine clear perception. That makes those 3 receptors of particular interest. It is notable that they are also all involved in exploration behavior.

*'Fine, clear perception' is an abstract psychological manner of perceiving reality. It is the cause of goodwill (defined here as the desire to perceive and act rightly in fundamental ways (as distinguished from superficial ways) ), empathy, and objectivity. For that reason, agonists (i.e. ligands that cause particular receptors to fire) of the receptors that produce fine clear perception have been proposed as components of a neuroactive chemical mixture to be used in the interrogation of known and suspected criminals and terrorists, which would be far more effective than any currently-used interrogation method, chemical or otherwise.

However, many people in the relevant professions, not to mention the even-higher proportion of the people in the general populace, are not materialist determinists, but rather believe in free will, and thus refuse to even consider the validity of the role of such receptors. There are yet many other professionals and non-professionals who themselves have malicious intent (malice being defined here as the desire to perceive and act wrongly in fundamental ways), and have attempted to create the deception that certain psychological and neurological traits of malice are in fact the traits of goodwill. Most notable among the neurological deceptions is the deception that MDMA causes empathy, when in fact it usually causes malicious aggression, mostly antagonistic aggression (effects vary due to individual differences), and that deception was in fact started by the malicious (not to mention chaoticly law-breaking) MDMA users themselves. There are few honest professionals that have the idealism and the bravery to risk their reputation by standing up against all of such opponents.

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