Receptor Subtype

The ANP leceptoi exists in two forms, ANP and ANPg, both of which have been cloned. These membrane-bound guanylate cyclases have a single transmembrane domain, an intracellular protein kinase-like domain, and a catalytic cyclase domain, activation of which results in the accumulation of cychc guanosine monophosphate (cGMP). A third receptor subtype (ANP ) has been identified that does not have intrinsic guanylate cyclase activity and may play a role in the clearance of ANP. 

Two CCK receptor subtypes, CCK and CCKg are known. A related receptor, the gastrin receptor, has also been described. CCK receptors predominate in the gastrointestinal tract and pancreas and are also localized in discrete brain regions. CCKg receptors predominate in the brain. A 71623... 

Two ET GPCR subtypes, ET and ETg, have been cloned from human tissues. Both leceptois utilize IP /DAG for transduction. ET-1 and ET-2 have similai affinities for the ET subtype, whereas the affinity of ET-3 is much lower. All three peptides have similat affinities for the ETg subtype. Both receptor subtypes ate widely distributed, but ET receptors are more abundant in human heart, whereas ETg receptors constitute 70% of the ET receptors found in kidney. BQ 123 [136553-81 -6] cyclo-[D-Asp-Pro-D-Val-Leu-D-Trp], and ER 139317 (136) are selective ET antagonists. 

Three tachykinin GPCRs, NK, NK, and NK, have been identified and cloned. AH are coupled to phosphatidjhnositol hydrolysis. The NK receptor is selective for substance P (SP) and is relatively abundant in the brain, spinal cord, and peripheral tissues. The NK receptor is selective for NKA and is present in the gastrointestinal tract, urinary bladder, and adrenal gland but is low or absent in the CNS. The NIC receptor is selective for NKB and is present in low amounts in the gastrointestinal tract and urinary bladder, but is abundant in some areas of the CNS, ie, the spinal dorsal bom, soUtary nucleus, and laminae IV and V of the cortex with moderate amounts in the interpeduncular nucleus. Mismatches in the distribution of the tachykinins and tachykinin receptors suggest the possibility of additional tachykinin receptor subtypes. 

The opioid peptides vary in their binding affinities for the multiple opioid receptor types. Leu- and Met-enkephalin have a higher affinity for 5-receptors than for the other opioid receptor types (68), whereas the dynorphin peptides have a higher affinity for K-sites (69). P-Endorphin binds with equal affinity to both p- and 5-receptors, but binds with lower affinity to K-sites (70). The existence of a P-endorphin-selective receptor, the S-receptor, has been postulated whether this site is actually a separate P-endorphin-selective receptor or is a subtype of a classical opioid receptor is a matter of controversy (71,72). The existence of opioid receptor subtypes in general is quite controversial although there is some evidence for subtypes of p- (73), 5-(74), and K-receptors (72,75), confirmation of which may be obtained by future molecular cloning studies. 

Acetyl choline is the natural neurotransmitter for the cholinergic receptor. Two distinct receptor subtypes have been characterized based on their binding affinity for either nicotine (189) and (190) or muscarine (191). 

Receptor subtype Most potent natural PG agonist Usual response on smooth muscle... 

Future Outlook for Antidepressants. Third-generation antidepressants are expected to combine superior efficacy and improved safety, but are unlikely to reduce the onset of therapeutic action in depressed patients (179). Many dmgs in clinical development as antidepressive agents focus on estabhshed properties such as inhibition of serotonin, dopamine, and/or noradrenaline reuptake, agonistic or antagonistic action at various serotonin receptor subtypes, presynaptic tt2-adrenoceptor antagonism, or specific monoamine—oxidase type A inhibition. Examples include buspirone (3) (only... 

Compounds that have agonistic properties at glutamate or aspartate receptors are also CNS stimulants, readily cause convulsions, and presumably could also be employed as analeptics. Three separate excitatory amino acid receptor subtypes have been characterized pharmacologically, based on the relative potency of synthetic agonists. These three receptors are named for their respective prototypical agonists A/-methyl-D-aspartate [6384-92-5]... 

Kaumann, A. J., and Marano, M. (1982). On equilibrium dissociation constants for complexes of drag receptor subtypes Selective and nonselective interactions of partial agonists with two P-adrenoceptor subtypes mediating positive chronotropic effects of (-) isoprenaline in kitten atria. Nannyn Schmiedebeberg s Arch. Pharmacol. 219 216—221. 

Richardson, B. P., Engel, G., Donatsch, P., and Stadler, P. A. (1985). Identification of serotonin M-receptor subtypes and their specific blockade by a new class of drugs. Nature 316 126-131. 

FIGURE 9.17 Venn diagram consisting of the various possible activities (agonism and antagonism) on two receptor subtypes (a- and P-adrenoceptors). Letters label the areas of intersection denoting joint activity. The table shows possible therapeutic application of such joint activity. 

FIGURE 9.19 Dimeric antagonist formed by oligoglycyl-based linkage of two opioid receptor subtype antagonists naltrindole and ICI-199,441. From [59],... 

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