M2 subtypes, while later studies recognize M3, M4, and most recently M5 subtypes. Genetic differences Meanwhile, geneticists and molecular biologists have characterised five genes that appear to encode muscarinic receptors, named m1-m5 (lowercase m; no subscript number). They code for pharmacologic types M1-M5. The receptors m1 and m2 were determined based upon partial sequencing of M1 and M2 receptor proteins. The others were found by searching for homology, using bioinformatic techniques. Difference in G proteins G proteins contain an alpha-subunit that is critical to the functioning of receptors. These subunits can take a number of forms. There are four broad classes of form of G-protein: Gs, Gi, Gq, and G12/13. Muscarinic receptors vary in the G protein to which they are bound, with some correlation according to receptor type. G proteins are also classified according to their susceptibility to cholera toxin (CTX) and pertussis toxin (PTX, whooping cough). Gs and some subtypes of Gi (Gαt and Gαg) are susceptible to CTX. Only Gi is susceptible to PTX, with the exception of one subtype of Gi (Gαz) which is immune. Also, only when bound with an agonist, those G proteins normally sensitive to PTX also become susceptible to CTX. The various G-protein subunits act differently upon secondary messengers, upregulating Phospholipases, downregulating cAMP, and so on. Because of the strong correlations to muscarinic receptor type, CTX and PTX are useful experimental tools in investigating these receptors. M1 receptor This receptor is found mediating slow EPSP at the ganglion in the postganglionic nerve, is common in exocrine glands and in the CNS. It is predominantly found bound to G proteins of class Gq, which use upregulation of phospholipase C and, therefore, inositol trisphosphate and intracellular calcium as a signaling pathway. A receptor so bound would not be susceptible to CTX or PTX. However, Gi (causing a downstream decrease in cAMP) and Gs (causing an increase in cAMP) have also been shown to be involved in interactions in certain tissues, and so would be susceptible to PTX and CTX, respectively. M2 receptor The M2 muscarinic receptors are located in the heart and lungs. In the heart, they act to slow the heart rate down below the normal baseline sinus rhythm, by slowing the speed of depolarization. In humans, under resting conditions, vagal activity dominates over sympathetic activity. Hence, inhibition of M2 receptors (e.g. by atropine) will cause a raise in heart rate. They also