Cholinergic synapses - physiology and pharmacology

1. Autonomic ganglia

2. Nicotinic and muscarinic ACh receptors

3. The cholinergic system in brain•role in cognitive function and learning•effects of cholinergic drugs•treatment of Alzheimer’s disease

See: www.dns.ed.ac.uk/teaching.html(use Internet Explorer on a PC)

The autonomic nervous system

The sympathetic and parasympathetic divisions of the autonomic nervous systems have contrasting effects on a variety of end-organs. Sympathetic activity elicits ‘flight-or-fight’ responses (increased heart rate, decreased gut motility, reduced gastrointestinal secretions), whereas Parsympathetic activity is associate with a sense of satisfaction and well-being (increased gastrointestinal secretions and motility; slow steady heart rate, etc).

Organisation of autonomic ganglia

Both Sympathetic and parasympathetic divisions employ cholinergic preganglionic neurones to signal between the CNS and autonomic ganglia. The signals relayed to the tissues from postganglionic neurones employ ACh and other neurotransmitters, such nor-adrenaline, peptides and nitric oxide.

Recordings from autonomic ganglia show complex responses.

Intracellular recordings from sympathetic ganglia reveal complex synaptic responses mediated by ACh and other neurotransmitters. The Fast EPSP, Slow IPSP, Slow EPSP are all responses to ACh. The differences are due to the types of receptors that expressed in the ganglion cell membranes. (The Late Slow EPSP - lower right - is mediated by a peptide neurotransmitter, not acting on ACh receptors).

Nicotinic ACh Receptors

Nicotinic ACh receptors are responsible for Fast EPSP’s in the cholinergic sysem, including the EPPs produced at the neuromuscular junction. All these receptors are made up of 5 sub-units surrounding a central pore. Each sub-unit comprises a polypeptide that spans the membrane 4 times, and the N- and C- terminals are outside the cell. The number alpha sub-units varies. Nicotine is an agonist and either hexamethonium or tubocurarine are antagonists for all these types of receptor.

Muscarinic ACh Receptors

Muscarinic ACh receptors are responsible for Slow EPSP’s and Slow IPSP’s in the cholinergic sysem. All these receptors are made up of a single sub-unit and there is no channel directly associated with the receptor. The receptor comprises a polypeptide that spans the membrane 7 times, and the N-terminus is outside and the C- terminus is inside the cell. The inside contains a G-protein binding site, which is activated when ACh binds to the receptor. Muscarine is an agonist and atropine is an antagonists for all types of muscarinic ACh receptor.

Muscarinic ACh Receptors utilise G-proteins to effect a response

Activation of the G-proteins associated with muscarinic ACh receptors has diverse effects. The sub-units of the G-protein dissociate, and one of the sub-units can directly activate potassium ion channels, leading to inhibition of cell firing. Other G-protein sub-units activate enzymes in the cell, including protein kinases, and release of Ca ions. Phosphorylation or Ca-activation of ion channels by these kinases can lead to either depolarisation, hyperpolarisation, or metabolic changes in the cells.

Two alternative signalling pathways follow muscarinic receptor activation

A more detailed view of the alternative mechanisms that are activated after ACh binds to muscarinic receptors. Other neurotransmitters act on specific receptors which have similar ‘metabotropic’ properties.

Muscarinic agonists:

Muscarinic antagonists:

The nature of the cholinergic receptor mediating an excitatory or inhibitory response can be most easily distinguished pharmacologically. Muscarine or oxotremorine have no effect on nicotinic ACh receptors. Likewise atropine blocks the response to ACh or ACh agonists by muscarinic, but not nicotinic receptors. Pirenzepine blocks the M1 sub-type of muscarinic receptor (found in the gut, for example) whereas gallamine (which also blocks nicotinic receptors at the NMJ) has no effect on M1 receptors but blocks M2 muscarinic receptors in the heart.

The brain also contains a network of cholinergic neurones. Those with cell bodies in the basal forebrain project to areas of the brain concerned with cognitive function, memory and learning. Degeneration of neurones in the cholinergic system occurs in presenile dementia (Alzheimer’s disease) and this may contribute to impaired cognitive function and learning, characteristics of the disease

Cholinergic receptors are involved in ‘spatial learning’

Morris Water maze

Evidence that cholinergic neurones play a role in memory and learning has been obtained in behavioural experiments on rodents. These animals learn to navigate in the Morris Water Maze - named after Richard G.M. Morris (Professof Neuroscience at Edinburgh University) who devised the method - where the task is to find a hidden platform (red arrow). The map(top right) shows a typical path adopted after several trials at the task. But they take much longer to learn the location of the platform after being given atropine, the muscarinic antagonist. The hippocampus is strongly implicated as the region of the brain responsible for managing this form of learning.

Donepezil(Aricept)

Cholinergic receptors are involved in transmission in the hippocampus - a learning centre in the brain

Intracellular recordings from neurones in the hippocampus show a component of their synaptic responses that is cholinergic. The synaptic potential is prolonged by the anticholinesterase inhibitor physostigmine, and blocked by the muscarinic antagonist atropine.

Donepezil (proprietary name Aricept) is a mild anticholinesterase acting on specific for forms of cholinesterase expressed in the brain. It has been reported to have mildly beneficial effects on the progression of Alzheimer’s disease.

Cholinergic neuronal “Growth factors”

NeurotrophinsNGFBDNFNT-3NT-4

CytokinesCNTFFGFGDNF IGF

Growth factors are polypeptides which promote cell survival and stabilisation of synapses. Different neurones express receptors for the different types of growth factors. Those listed above have been reported to delay cell degeneration when administered to cholinergic neurones from the brain, either in tissue culture or in vivo. Neurotrophins are a related family whose members have homologous amino acid sequences. NGF was the first “Nerve Growth Factor’ to be discovered.

Backman et al. · Systemic Administration of OX-26-NGF Reverses Age-Related Cognitive Dysfunction J. Neurosci., September 1, 1996, 16(17):5437–5442

Nerve growth factor treatment of old animals improves survival of cholinergic neurones and enhances learning.

Old Old-NGF treated Young

In this report, NGF (coupled to a carrier that enabled it to be transported fron the blood into the brain) was demonstrated to both prevent age-related degeneration in cholinergic neurones in the basal forebrain, and to improve Water Maze learning in aged animals. One application of such growth factor research may be the development of rational treatments for degenerative diseases of the nervous system such as Alzheimers Disease.

Summary

1. Acetylcholine is an important neurotransmitter in both the peripheral nervous system (PNS: skeletal and smooth muscle, autonomic sympathetic and parasympathetic ganglia), and in the central nervous system (CNS).

2. Cholinergic responses are mediated bynicotinic receptors (giving EPSPs) and muscarinic receptors (giving either EPSPs or IPSPs). Nicotinic receptors are ligand-gated ion channels; muscarinic receptors are metabotropic, utilising G-proteins and intracellular signalling to activate channels elsewhere in the cell.

3. Nicotinic receptors are activated by carbachol and generally blocked by tubocurarine and/or hexamethonium. Muscarinic receptors are also activated by carbachol but generally blocked by atropine.

3. The cholinergic system in brain is important in cognitive function and learning. Impaired cognition and learning can be alleviated by stimulating cholinergic transmission using anticholinesterases or growth factors.

4. Attempts to treat Alzheimer’s disease, a form of dementia in which cognitive function and learning are impaired, with anticholinesterases or growth factors have met with limited success.