User:KelseyGratton/Cholinergic neuron

Overview

In general, the cholinergic neuron is any neuron which mainly uses the neurotransmitter acetylcholine (ACh). Many neurological systems use acetylcholine to send its messages, making these systems cholinergic. The parasympathetic nervous system uses acetylcholine almost exclusively, along with neuromuscular junctions, preganglionic neurons of the sympathetic nervous system, the basal forebrain, and some brain stem complexes. Cholinergic neurons provide the primary source of acetylcholine to the cerebral cortex, and are known for their role in promoting cortical activation during both wakefulness and during rapid eye movement (REM) sleep. In recent years, the cholinergic system of neurons has been a main focus of research in aging and neural degradation, specifically as it relates to Alzheimer's Disease. In addition, it is known that the dysfunction and loss of basal forebrain cholinergic neurons and their cortical projections are among the earliest pathological events in Alzheimer's Disease. The following Wikipedia article will provide a detailed discussion on the cholinergic neuron and the latest cutting-edge research involving Alzheimer's and other neurodegenerative diseases.

Anatomy of the Cholinergic Neuron

Most research involving cholinergic neurons involves the basal forebrain cholinergic neurons. However, cholinergic neurons only represent about 5% of the total basal forebrain cell population. Most of these neurons have large somata with extensive dendritic fields. These neurons originate in many different areas across the basal forebrain and project into almost all layers of the cortical region. For example, medial septum-diagonal band of the Broca region (MS-DB) cholinergic neurons project to the hippocampus, while magnocellular preoptic nucleus (MCPO), substantia innominate (SI), and magnocellular basal nucleus (MBN) cholinergic neurons project to the cortex and amygdala. Some of these cholinergic neurons in the MBN may also project to the reticular thalamic nuceus, which provides an indirect route to be able to influence the cortex. Interestingly, basal forebrain cholinergic neurons are homogenous within a particular basal forebrain region but are actually heterogenous across regions.

Normal Aging of the Cholinergic Neuron

Normal aging is described as aging not accompanied by behavioral or cognitive dysfunctions associated with the cholinergic basal forebrain system. In normal aging, there are beadlike swellings within the cholinergic fibers with enlarged or thickening of the axons that often occur in grape-like clusters. This fiber swelling can be induced in a laboratory setting by applying damage to the cell body of the cholinergic neuron, which implies a slow cell and fiber degeneration of affected neurons and their projecting axons.

Neuroprotective Effects on Cholinergic Neurons

It is established knowledge that Nerve Growth Factor (NGF) has a neuroprotective effect on cholinergic neurons. In vitro and in vivo models show that NGF prevents the loss of cholinergic neurons. The small non-toxic molecule urea has no neuroprotective effect on cholinergic neurons by itself. However, when experimental brain slices were treated with NGF + urea the number of cholinergic neurons in the brain slices was significantly enhanced when compared to slices treated with NGF only. It is possible that the enhancing effect of urea may be due to inhibition of the nitric oxide (NO)-system within the cholinergic neuron.

Relationship to Mammalian Circadian System

Cholinergic neurons, along with non-cholinergic neurons, have sleep/wake regulatory functions in the basal forebrain that can be categorized based on their firing patterns in different regions. The cholinergic system allows the circadian system to have the cycle of one day and may also play a role in time memory, and the ability of an individual to form a memory around a certain time of day, known as “time stamping”. The cholinergic system is characterized by high acetylcholine (ACh) release during the active phase of an individual’s circadian system.

Cholinergic Neuron Firing Patterns and the Circadian System

In the medial septum-diagonal band of the Broca region (MS-DB), cholinergic neurons have very low firing rates during both wake and non-REM sleep, and show no rhythmic bursts during hippocampal (theta) EEG activity. However, cholinergic neurons in the magnocellular preoptic nucleus (MCPO) and substantia innominate (SI) have increased firing rates with fast cortical (gamma) EEG activity during wake and REM sleep. This indicates these cholinergic neurons may be activated through α1-receptors by noradrenaline, which were released by locus coeruleus neurons during wake cycles. In a basic summary, cholinergic neurons are always wake or REM-active cycle and are more likely to activate the cortex to induce the gamma and theta activities while behaviorally promoting the states of wakefulness and REM sleep.

Time Memory and Time Stamping

Although not abundant, it has been shown in mice, hamsters, and rats, that cholinergic innervation exists in the suprachiasmatic nucleus (SCN). The SCN is also known as the hypothalamic circadian master clock and is the master of the circadian system. Here, “time memory” defines the memory at a specific time of day for which an individual made an association with a certain event or location, whereas “time stamping” refers to the process by which the specific time-of-day is encoded to support the formation of a time memory. The situation must be important and specific, without unnecessary prolonging, for a time stamp to occur. ACh has an excitatory effect on SCN cells, and it is believed that the cholinergic transmission of more ACh into the SCN will support the formation of a time memory.

For reference, the number of free and available muscarinic acetylcholine receptors (mAChRs) is highest when ACh release is at the lowest levels. When a memorable event occurs, there is a massive release of ACh that will attach to mAChRs. Once too many are involved, the mAChRs will reduce or block further cholinergic input, which protects these cells and the networks from additional cholinergic input that could disrupt the signal. This allows the SCN to perform time stamping and produce a time memory of what has just occurred to the individual. This idea still needs to be further explored experimentally, but allows for the explanation of the cholinergic neuron’s role in memory.

Circadian System and Alzheimer’s Disease

The circadian system is one of the first systems to be damaged in AD. A common complaint amongst AD patients is disrupted sleep, shortened REM sleep, and increased night time awakening. These disruptions will steadily worsen as the AD progresses. It is normal in aging for circadian rhythms to deteriorate as choline acetyltransferase (ChAT) fluctuations change in pattern and ACh levels fluctuate more often. As AD has shown drastically changed cholinergic (ACh) function, it is natural that the circadian system would also follow the changed levels. Circadian rhythmicity in ACh release is critical for optimal memory processing and a loss of this rhythmicity will contribute to cognitive problems in AD.

Neurological Disorders

Degeneration of the cholinergic neurons in the basal forebrain has been linked to progressing memory deficits related to aging, which eventually results in cholinergic hypofunction. The dysfunction and loss of basal forebrain cholinergic neurons has been observed in many dementing disorders with Alzheimer’s Disease being the predominant model. Neurodegenerative diseases also exhibit axonal pathology, transport defects, and abnormal phosphorylation/aggregation of the microtubule binding protein tau. Recent findings suggest that aging-related cognitive deficits are probably due to impairments of cholinergic function rather than cholinergic cell loss. This gives hope to the future in being able to reverse cognitive declines, as the cells are not dead, but deteriorating. Slowing the deterioration is key with neurodegenerative diseases.

Alzheimer's Disease (AD)

Alzheimer’s Disease (AD) is the most common form of dementia, and unfortunately, the sixth leading cause of death in the United States. The proportion of deaths associated with AD continues to grow, jumping by 66% from 2000 to 2008. Overall, it is been extensively documented that AD typically has a decline in the activity of choline acetyltransferase (ChAT) and acetylcholine esterase (AChE), a decline in acetylcholine (ACh) release, and a decline in the levels of nicotinic and muscarinic receptors in the brain. As acetylcholine is the main indication of a cholinergic neuron, cholinergic system research may provide the key to treating and/or reversing this devastating neurodegenerative disease.

Histological Hallmarks

Although degeneration of basal forebrain cholinergic cells has been observed in many other dementing disorders, AD includes two other histological hallmarks that other diseases do not have: β-amyloid plaques and neurofibrillary tangles. The β-amyloid plaques are high-molecular weight fibrils and are major components of the senile AD brain. There appears to be a vast, intrinsic microvascular pathology of the brain in these cases, which suggests a link between β-amyloid production, impairments in cerebrovascular function, and basal forebrain cholinergic deficits in AD. There is also data which suggests that β-amyloid(1-42) mediates its cytotoxic action by affecting key proteins that play a role in apoptosis induction in physiological conditions. The other histological hallmarks, neurofibrillary tangles, are the intracellular inclusions formed by aggregates of hyperphosphorylated tau protein. This is found only in select populations of patients with AD. This tau protein has specific pathology, and has been found both in patients with mild cognitive impairment (MCI, a prodromal stage of AD) and AD. The neurofibrillary tangles seem to increase within the basal forebrain cholinergic complex with old age and at a more accelerated pace in patients with AD.

Probable Cause for Vulnerable Cholinergic Neurons

The “cholinergic hypothesis of AD” is a well-established pathology of the involvement of cholinergic neurons on AD due to their role in memory mechanisms. In 2007, there was research done to determine the reasons why cholinergic neurons were becoming more vulnerable to the β-amyloid plaque formation that was ultimately causing AD. It was determined that a dimetabolism pathway exists for both the maturation and degradation of Nerve Growth Factor (NGF) and causes vulnerable cholinergic neurons. Basal forebrain cholinergic neurons are highly dependent on the constant internal supply of NGF throughout life. If the supply of NGF is interrupted, cholinergic atrophy could begin to occur in these neurons and change their phenotype. This supply could be interrupted if there is a filure in the protease cascade and proNGF cannot be converted to NGF. This is caused by an increased NGF degradation, which in turn was caused by a rise in matrix metalloproteinase-9 (MMP-9) activity. An increase in MMP-9 activity also causes diminished production of NGF in general, a double jeopardy situation (lower production and higher degradation). This double failure of NGF stimulation would lead to the progressive atrophy of basal forebrain cholinergic neurons, which would in turn contribute to AD-related learning and memory declines.

Disease Model

For most studies in AD, mice or rat brain models with β-amyloid plaque buildup is used as the disease model. However, the most cutting edge research needs a viable disease model to be able to study different courses of treatment. Ideally, this disease model would be able to mass produced and not harm animal life. Dr. Su-Chun Zhang and his research team have been able to derive cholinergic neurons from stem cells in a laboratory setting. Taking it even a step further, this research team has been able to take neuroepithelial (skin) cells and convert them to cholinergic neurons to be used as a disease model for any number of neurodegenerative diseases. This quick conversion of cells allows for the rapid testing of any number of potential treatments for a disease.

Potential Treatments

In recent years, there has been a vast amount of research pertaining to potential treatments for AD. One such treatment is the use of memantine, a moderate affinity uncompetitive NMDA receptor antagonist that preferentially blocks excessive NMDA receptor activity without disrupting normal activity. This treatment is based on the theory that degenerative neural disorders (such as AD) have excitotoxic processes due to the inappropriate overstimulation of the N-methyl-D-aspartate (NMDA) receptor. In a rat model, memantine treatment given preventively to certain rats pre-β-amyloid(1-42) lesion significantly reduced the loss of cholinergic fibers versus rats without memantine treatment. Memantine treatment (post-lesion) also reversed the attention and learning deficits in β-amyloid(1-42) affected rats. This data indicates the ability of memantine to rescue neocortical cholinergic fibers (originating from basal forebrain cholinergic neurons) from the neurotoxic effects of β-amyloid(1-42) oligomers. It should also be noted that memantine is able to inhibit the truncation of glycogen synthase kinase-3 (triggered by activated calpain), which is believed to play a key role in the pathogenesis of AD as it pertains to tau phosphorylation (the second histological hallmark).

Another treatment involves the use of exogenous choline acetyltransferase (ChAT) as supplementation in cholinergic neurons. Cholinergic neurons have significantly reduced ChAT and acetylcholine (ACh) activity, which is correlated to the severity of the dementia or cognitive impairments. The problem with this therapy is the ability of excess ChAT to get across the blood-brain barrier to be effective. A fusion protein made up of both protein transduction domain (PTD) and ChAT forms PTD-ChAT is capable of passing through the blood-brain barrier and cell membranes (due to PTD). By studying the time- and dose-effect relationships in mice, it has been determined that PTD-ChAT has the ability to regulate ACh levels in the brain. Through various memory and cognitive tests, mice treated with PTD-ChAT were cured from their memory and cognitive deficits. This is a possible future treatment for AD with exogenous proteins that can be delivered to help various deficits in the brain.

Other Potential Diseases

Cholinergic neurons have also been shown to have an effect on other neurodegenerative diseases such as Parkinson’s disease, Huntington’s disease, Down-syndrome, etc. As with AD, the significant degeneration of basal forebrain cholinergic neurons and the decrease in the neurotransmitter acetylcholine have a drastic effect on behavioral and cognitive function of the individual. More research is needed to solidify specific treatments for each of these neurodegenerative diseases.