Acetylcholine early studies

Many early studies of transmitter release depended on measuring its concentration in the effluent of a stimulated, perfused nerve/end-organ preparation. This technique is still widely used to study drug-induced changes in noradrenaline release from sympathetic neurons and the adrenal medulla. However, it is important to realise that the concentration of transmitter will represent only that proportion of transmitter which escapes into the perfusate ( overflow ) (Fig. 4.2). Monoamines, for instance, are rapidly sequestered by uptake into neuronal and non-neuronal tissue whereas other transmitters, such as acetylcholine, are metabolised extensively within the synapse. Because of these local clearance mechanisms, the amount of transmitter which overflows into the perfusate will depend not only on the frequency of nerve stimulation (i.e. release rate) but also on the dimensions of the synaptic cleft and the density of innervation. 

Acetylcholine receptors have been classified into subtypes based on early studies of pharmacologic selectivity 186 The intrinsic complexity and the multiplicity of cholinergic receptors became evident upon elucidation of their primary structures 189... 

Acetylcholine Precursors. Your nerve cells produce acetylcholine from certain dietary precursors (choline and lecithin). Many early studies tried dietary supplements of these precursors. A precedent for this approach was established using the dopamine precursor, L-DOPA, a well-established treatment for Parkinson s disease. Unfortunately, this approach is ineffective in dementia. It appears that the daily doses of these fatty acid precursors needed to have any discernible impact on acetylcholine levels far exceed what an individual can reasonably take in a day. This approach has therefore been abandoned. 

Despite early studies suggesting that the acetylcholine concentration was raised in epileptogenic foci, which would be consistent with the finding that anticholinesterases cause seizures in both animals and man, it now appears that overactivity of the central cholinergic system is unlikely to be the cause of seizures in the human epileptic. Other candidates that have been implicated in the aetiology of epilepsy include adenosine and the enkephalins, but conclusive evidence for their involvement is presently lacking. 

As previously mentioned, early in vivo acute toxicity studies indicated that the action of Type II pyrethroids on the nervous system was different from that of the Type I pyrethroids. Deltamethrin decreased the acetylcholine content of the cerebellum, whereas DDT, a well-established voltage-sensitive sodium channel agonist, and cismethrin, caused no significant reduction [2]. 

The work that paved the way toward enzymatic inhibition was published in the early 1990s by Wudl and coworkers (Schinazietal., 1993 Friedmanetal., 1993 Sijbesma et al., 1993) and since then studies regarding antiviral activity, mainly HIV-protease inhibition, have been carried out to find active compounds. Up to now, the most effective fullerene derivatives are the trans-2, -dimethy 1-bis-fulleropyrrolidin-ium salt (Fig. 1.4) (Marchesan et al., 2005) and the dendrofullerene reported by Hirsch (Schuster et al., 2000) both of them present an ECJ0 of 0.2pM. Also HIV reverse transcriptase can be inhibited by, -dimcthyl-bis-fulleropyrrolidinium salts (Mashino et al., 2005). The same compounds are also active against acetylcholine esterase (AChE), an enzyme that hydrolyzes a very important neurotransmitter. 

Perhaps the most prominent and well-studied class of synthetic poisons are so-called cholinesterase inhibitors. Cholinesterases are important enzymes that act on compounds involved in nerve impulse transmission - the neurotransmitters (see the later section on neurotoxicity for more details). A compound called acetylcholine is one such neurotransmitter, and its concentration at certain junctions in the nervous system, and between the nervous system and the muscles, is controlled by the enzyme acetylcholinesterase the enzyme causes its conversion, by hydrolysis, to inactive products. Any chemical that can interact with acetylcholinesterase and inhibit its enzymatic activity can cause the level of acetylcholine at these critical junctions to increase, and lead to excessive neurological stimulation at these cholinergic junctions. Typical early symptoms of cholinergic poisoning are bradycardia (slowing of heart rate), diarrhea, excessive urination, lacrimation, and salivation (all symptoms of an effect on the parasympathetic nervous system). When overstimulation occurs at the so-called neuromuscular junctions the results are tremors and, at sufficiently high doses, paralysis and death. 

Studies of neuromuscular junctions of the autonomic nervous system as early as 1904 led to the suggestion that adrenaline might be released at the nerve endings. Later it was shown that, while adrenaline does serve as a transmitter at neuromuscular junctions in amphibians, it is primarily a hormone in mammals. Nevertheless, it was through this proposal that the concept of chemical communication in synapses was formulated. By 1921, it was shown that acetylcholine is released at nerve endings of the parasympathetic system, and it later became clear the motor nerve endings of the somatic system also release acetylcholine. 

Inhibition of neurotransmitter release mediated by Ai adenosine receptors is a widespread phenomenon. As mentioned in Section 1, it was first described for cholinergic neurons. But presynaptic Ai receptors also inhibit the release of several other neurotransmitters both in CNS and PNS. Table 2 summarizes early relevant studies. As to postganglionic parasympathetic neurons, there is only one study to our knowledge. In that study, carried out in guinea pig atria, no A] receptor-mediated modulation of acetylcholine release was found (Nakatsuka et al. 1995). 

Although it seems somewhat too early to draw any general conclusions from these studies, they appear to produce a number of most interesting results, correlations and predictions. As could have been anticipated, the PCILO results are more satisfactory than the EHT ones. The situation may best be illustrated by an example, for which we shall take the much-investigated case of acetylcholine, III, the natural intercellular effector in nervous transmission systems. 

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