Muscarinic receptor GREGOR W. NIETGEN, JOERG SCHMIDT, LUTZ HESSE, CHRISTIAN W. HONEMANN, functioning and MARCEL E. DURIEUX distribution in the eye: molecular basis and implications for clinical diagnosis and therapy The role of neurotransmitters has generated number of receptors (cholinergic, adrenergic considerable interest over the last decade. and others) have been found in all types of Dale's first description in 1914 of the muscarinic ocular tissue: the functional consequence of and nicotinic components of the cholinergic their activation remains elusive but is currently systeml provided an explanation for the effects being investigated with great zeal. Glaucoma of various cholinergic active drugs on the eye. patients are still being treated with pilocarpine G.w. Nietgen J. Schmidt Parasympathetic cholinergic input to the human almost 40 years5 after its introduction to western L. Hesse iris sphincter muscle comes from neurons medicine in 1875.6 The development of specific Zentrum fOr whose axons make up the ciliary nerve, a muscarinic agonists (acetyl-j3-methylcholine) Augenheilkunde branch of the third cranial nerve. Acetylcholine and antagonists (scopolamine) followed. Philipps-Universitat Marburg is released by these neurons onto their target Cholinesterase was discovered in 19267 and Marburg, Germany 8 cells, the smooth muscle surrounding the pupil. named in 1932. At the same time carbachol was C.W. Honemann Muscarinic acetylcholine receptors on the synthesised,9 a drug resistant to cholinesterases M.E. Durieux surface of the muscle cells transduce the with a suitable specificity for glaucoma Departments of chemical signal into a muscle contraction which treatment.IO,ll Over the years various Pharmacology, Anaesthesiology and constricts the pupil. It has also been shown that compounds have been investigated regarding Neurosurgery muscarinic cholinergic receptors exist in the their potential to lower intraocular pressure University of Virginia Health mammalian iris dilator muscle, once thought to (lOP), the most prominent pathophysiological Sciences Center receive only noradrenergic input from the feature of glaucoma. The majority of these Charlottesvile sympathetic system? This double reciprocal substances do not affect the muscarinic system Virginia, USA innervation of the iris sphincter follows the of the eye and intervene at different receptor Gregor W Nietgen, MD � general pattern of innervation: stimulation of sites. Zentrum fOr the parasympathetic nervous system The ciliary muscle, focus of intense Augenheilkunde (cholinergic muscarinic), which functions investigation, contracts through activation of Philipps-Universitat Marburg Robert-Koch-Strasse 4 through the polyphosphoinositide signalling muscarinic receptors. Due to its insertion into 35033 Marburg pathway, leads to contraction. Relaxation is a the trabecular meshwork it increases aqueous Germany outflow facility, thereby reducing IOpy,13 A result of the activation of the sympathetic Tel: +49 6421 282600 nervous system (beta-adrenergic), which variety of drugs can also reduce lOP, yet by Fax: +49 6421 285678 functions through the cAMP system. very different mechanisms of action. This e-mail: The active secretion of aqueous humour is indicates the pathogenetic complexity of [email protected] carried out by the ciliary epithelium and is glaucoma, with its multiple possible causes; Supported in part by therefore a key target for regulation by however, lOP regulation through the National Institutes of Health grant GMS 52387/02 to endogenous regulators and anti-glaucoma muscarinic signalling system appears to be an M.E.D. G.WN. was drugs. Histological evidence indicates that the important component. The ocular muscarinic supported by a grant from ciliary processes receive innervation by both receptor system is not dedicated solely to the the Deutsche sympathetic and parasympathetic nerves. maintenance of pressure homeostasis though. A Forschungsgemeinschaft Further on, ciliary epithelial cells have been wide distribution of these receptors in the (DFG, Ni 482/1-1). C.WH. is supported by a grant from demonstrated to contain both adrenergic and human eye has been found. Muscarinic the Innovative Medizinische cholinergiC receptors? Interactions between the signalling is involved in Signal transduction Forschung, MOnster, two second messenger systems are important in functions of the retina,14 possibly in reparative Germany (lMF Ho-1-6-11/98-8) regulation of smooth muscle tone and are an functions in the corneal and lenticular Received: 14 September important focal point for pharmacological tissue,15,16 and appears to play a major role in 1998 manipulation.4 Besides these well-established the embryonic and postnatal development of Accepted in revised form: functions of the ocular receptor interplay a vast the eye.17 The main distinction between 22 January 1999 Eye (1999) 13, 285-300 © 1999 Royal College of Ophthalmologists 285 muscarinic and nicotinic receptors, though noted very thousand receptor types have now been shown to belong early, hardly explained their distinct roles in cholinergic to the G protein-coupled receptor (GCR) superfamily, of signalling. The past decade, however, has seen the which the muscarinic receptors form a small but molecular cloning of both nicotinic18 and muscarinic19 distinguished cluster. acetylcholine receptors in Numa's laboratory,20 leading GCRs all show the same molecular signature in their to greatly expanded understanding of these systems. In amino acid sequence: most are around SOD amino acids in addition, new research techniques such as patch length and include seven stretches of approximately 20 clamping21 and single channel recording22 have hydrophobic amino acids each. These domains are provided additional insights into the functioning of the thought to form a-helices traversing the membrane, cholinergic signalling system. leading to the designation of these proteins as seven­ We now know that, although acetylcholine is the transmembrane, or, more fancifully, serpentine or physiological agonist on both nicotinic and muscarinic heptahelical receptors (Fig. I). receptors, they are completely different entities: the first The G proteins stimulated by receptor activation a multi-subunit, ligand-gated ion channel (i.e. an control a number of intercellular systems. Best described ionotropic receptor), the second a single-subunit, G are G proteins stimulating (Gs) and inhibiting (Gi) protein-coupled receptor (i.e. a metabotropic receptor). It adenyl ate cyclase, with corresponding changes in cAMP appears likely that all muscarinic receptor subtypes have levels. Phospholipase C, activated by Gq or Go, generates now been cloned, allowing development of specific inositol trisphosphate (IP , which releases Ca2+ from 3 antibodies,23 detailed mapping of tissue distribution, and intracellular stores) and diacylglycerol (which activates synthesis of subtype-specific agonists and antagonists protein kinase C). In addition, G proteins can activate ion (Table la)?4-26 It has become clear that muscarinic channels, as in the case of Gk (a Gi subtype), which closes signalling plays an important role in multiple locations of a neuronal potassium channel in response to muscarinic the eye, and that ocular cholinergic drugs interfere stimulation. significantly with this system. This article will focus on the molecular basis of these findings. It will show that the complex distribution of muscarinic receptors in the eye is Five muscarinic receptor subtypes have been cloned only a part of many interacting signalling systems, all resulting in the development and maintenance of vision. Once the DNA sequence of one muscarinic receptor was Following a brief summary of the molecular biology of known19 other subtypes were isolated in rapid muscarinic receptors, their distribution and function in succession. Thus far, five muscarinic receptor have been the human eye will be described. A description of the cloned,28 designated ml, m2, m3, m4 and mS. The clinical implications of these signalling pathways and existence of this many subtypes was surprising, as their interactions in pathological processes will be pharmacological studies suggested initially only two (MI outlined. and M2). Glandular M2 receptors were designated M3. (Names with a capital 'M' indicate pharmacologically defined subtypes, whereas those with a small 'm' Molecular biology of muscarinic signalling indicate clones.) Four pharmacological subtypes have The first muscarinic receptor was cloned in 1986.19 In the now been defined (MI, M2, M3, M4)?9,30 This apparent 13 years that have passed, a remarkable amount of excess of subtypes is typical for GCRs, and presumably information has been gathered about the molecular allows finer regulation of receptor expression. The five biology of muscarinic signalling. Not only have subtypes fall into two groups - the 'odd' (ml, m3, mS) (presumably) all subtypes of muscarinic receptors been and the 'even' (m2, m4) - based on sequence homology cloned, but detailed information on their structure­ and second messenger signalling. The odd group signals activity relationship is available, which will prove useful primarily through intracellular Ca2+ ; the even group in the development of new, highly selective agonist and through decreases in cAMP production. In the brain or antagonist drugs. retina, where signalling systems eventually have to transduce their actions through changes in membrane potential, ml and m3 inhibit a G protein-coupled