The neurotransmitter acetylcholine (ACh) is synthesized in cholinergic neurons. Its action at the synapse is terminated by two enzymes, acetylcholinesterase and butyrylcholinesterase which thereby regulate its action. It is well known that the cholinergic system is involved in many functions of the central nervous system such as the sleep and wake cycle, cognitive function and learning.
In mammalian brain, there are two principal cholinesterases, acetylcholinesterase and butyrylcholinesterase. Acetylcholinesterase activity is found in neuronal somata as well as axons while butyrylcholinesterase activity is present mainly in the cell bodies. Acetylcholinesterase is quite widespread in the brain while distribution of butyrylcholinesterase is more restricted.
In different species, for example in the case of the rat brain, butyrylcholinesterase has been identified in cholinergic neurons as well as in non-cholinergic neurons and in neurons which do not contain acetylcholinesterase. Normally, in human brain acetylcholinesterase positive neurons are more abundant than butyrylcholinesterase-positive neurons. In the normal human brain, the activity of acetylcholinesterase is more predominant than butyrylcholinesterase. For example, in temporal cortex the acetylcholinesterase activity is approximately 90% while the butyrylcholinesterase activity is about 10% of cholinesterase activity.
However, in neurodegenerative diseases such as Alzheimer and Parkinson's diseases, butyrylcholinesterase activity shows progressive increases, while acetylcholinesterase activity declines in certain brain regions. These finding were supported by both Wright et al. (1993) and Lane et al. (2006) who demonstrated that butyrylcholinesterase activity progressively increases as the severity of dementia advances, while acetylcholinesterase activity declines. Findings such as these have led to the conclusion that butyrylcholinesterase has specific physiological functions in the CNS and may be involved in neurological diseases.
Choline acetyltransferase (ChAT, choline O-acetyltransferase) catalyzes the rate-limiting step for ACh synthesis from acetyl-COA and choline in the cytoplasm. Its presence can be used as a definitive marker for cholinergic neurons and, consequently, ChATcontaining neurons can be identified and recognized by using antibodies against ChAT.
Another enzyme as a neuron marker related to cholinergic neurotransmission is one responsible for the translocation of acetylcholine from the cytoplasm into synaptic vesicles, namely vesicular acetylcholine transferase (VAChT). With immunostaining for this enzyme, visualization of cholinergic nerve terminals can be superior to that achieved with ChAT immunostaining. Both ChAT and VAChT are essential for cholinergic neurotransmission.
Several groups of brainstem neurons such as the dorsal motor nucleus of the vagus nerve, hypoglossal, facial, the trigeminal motor nuclei and the pedunculopontine tegmental nuclei contain cholinergic neurons, as indicated by positive staining for ChAT and VAChT. Interestingly, most if not all, of these particular neurons are also positive for acetylcholinesterase and butyrylcholinesterase. Such results emphasize that there are overlapping distributions and interactions among these markers, which may suggest there could be up or down regulation of these neurochemicals if one of them is changed or deleted
Another neuron marker that is present in some cholinergic neurons is nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d). It is a co-factor for nitric oxide synthase (NOS). Thus, nitrergic neurons can be visualized by formation of visible insoluble formazan. By using simple histochemistry techniques, NADPH-d can be used as a robust indicator for nitric oxide synthase.