Vesicular storage and release of a false cholinergic transmitted (acetylpyrrolcholine) in the Torpedo electric organ

Cholinergic false transmitters: physiological and biochemical actions in central and peripheral systems

Three N-ethyl substituted analogs of acetylcholine (ACh) were evaluated for potential use as false neurotransmitters to decrease cholinergic transmission. This evaluation included (1) the elevation of arterial blood pressure upon central administration, (2) depression of blood pressure upon intravenous injection and (3) interactions with central muscarinic and peripheral nicotinic receptors. With respect to the central pressor response, ACh, acetylmonoethylcholine (AMECh) and acetyldiethylcholine (ADECh) were full agonists of decreasing potency; acetyltriethylcholine (ATECh) was a partial agonist of considerably lower potency. The duration of response paralleled potency. With respect to the peripheral depressor response, ACh and AMECh were full agonists of equal potency, and ADECh and ATECh were partial agonists of at least 100-fold lower potency. In terms of their affinity for central muscarinic receptors (brainstem and cerebral cortex), the following series was obtained: ACh greater than AMECh much greater than ADECh = ATECh. All of the agents had a greater affinity for muscarinic receptors in the brainstem compared to cortex. Acetylcholine and AMECh recognized multiple receptor binding conformations; the binding of ADECh and ATECh indicated interaction with a single set of equivalent sites. The affinity for nicotinic ACh receptors from the Torpedo electric organ was ACh = AMECh much greater than ADECh; ATECh had little affinity for these receptors. Acetylcholine, AMECh and ADECh stimulated the binding of [3H]phencyclidine to the ion channel of nicotinic receptor (potency series = ACh greater than AMECh = ADECh); ATECh was inactive. Acetylcholine, AMECh and ADECh also induced receptor conversion to a desensitized conformation; ATECh did not.(ABSTRACT TRUNCATED AT 250 WORDS)

Precursors of cholinergic false transmitters: central effects on blood pressure and direct interactions with cholinergic receptors

The purpose of this study was to determine whether a series of analogs of choline depress central cholinergic function in a manner consistent with the activity of their acetylated derivatives. Intracerebroventricular infusion of monoethylcholine (MECh), diethylcholine (DECh) and triethylcholine (TECh) inhibited the pressor response of unanesthetized rats to the subsequent intravenous injection of physostigmine (a well-characterized muscarinic response). The order of blocking potency was TECh greater than DECh greater than MECh greater than choline, directly opposite to the order of potency for elicitation of a central pressor response by their associated acetylated derivative (i.e. ACh greater than AMECh greater than ADECh = ATECh; Aronstam, Marshall and Buccafusco, 1988). In contrast, there was little selectivity between the analogs of choline in terms of their affinity for muscarinic receptors in the brainstem or cortex; the Ki's for inhibition of the binding of [3H]quinuclidinyl benzilate ranged from 0.33 to 0.95 mM). In terms of their affinity for nicotinic receptors (from the electric organ of Torpedo californica) the following order of potency was obtained: choline greater than MECh = DECh greater than TECh. Choline and MEC stimulated the binding of [3H]phencyclidine to the nicotinic ion channel (EC50's = 79 and 115 microM, respectively). At greater concentrations, all of the analogs inhibited ligand binding to the channel (Ki's from 0.2 to 10 mM), with the following order of potency: TECh greater than DECh greater than MECh greater than choline. These findings suggest that the inhibitory actions of these analogs of choline are related to their synthesis and release as false cholinergic neurotransmitters.

Influx of a choline analog to dog brain measured by positron emission tomography

The influx of the 11C-labeled choline analog pyrrolidinocholine into tissue was measured in the brain of three dogs by positron emission tomography (PET). During the first 90 s after the intravenous bolus injection of the tracer, transfer of tracer from plasma to tissue was unidirectional. The influx constant for pyrrolidinocholine into intracranial tissue, Kin, was 0.017 ml/g/min (0.008 SD), and the initial volume of distribution, V0, was 0.08 ml/g (0.03 SD). The influx constant was at least five times larger than the value expected if simple diffusion were to account for tissue uptake. The method presented in this paper can be used to investigate the availability of plasma choline and its analogs to the living human brain and other tissue in degenerative diseases affecting the cholinergic system, and to provide in vivo information on a choline transport system.

Incorrect interpretations of radioactive labelling experiments with false transmitters

In a number of recent studies attempts have been made to determine the subcellular origin of the released transmitter by comparison of the ratio of the radioactivity (14C/3H) of true and false transmitters between those released by stimulation and the subcellular fractions. It is shown that use of the isotopic or pseudo-molar ratios will lead to incorrect conclusions because varying degrees of isotopic dilution render the ratio meaningless. The only correct basis for comparison is the true molar ratio.

Cholinergic actions of false neurotransmitters: acetylpyrrolidinecholine

Acetylpyrrolidinecholine (AcPyCh), a false transmitter at cholinergic synapses, produces an increase in arterial blood pressure upon intraventricular injection that is comparable in extent and time course to that produced by equivalent doses of acetylcholine. AcPyCh binds to central muscarinic receptors with affinities and properties associated with potent muscarinic agonists. AcPyCh also binds to nicotinic receptors of Torpedo electric organs thereby inducing conformational changes in the receptor-ion channel complex, including certain changes associated with postsynaptic activation and desensitization processes. In all of these cholinergic actions AcPyCh is as potent as acetylcholine.

5'-triphosphate recycles independently of acetylcholine in cholinergic synaptic vesicles

The effect of hemicholinium-3 (HC-3) on vesicular contents in acetylcholine (ACh) and 5-triphosphate (ATP) and the vesicular incorporation of 14C-label derived from [14C]choline ,nd 3H-label derived from [3H]adenosine was investigated after low frequency stimulation (with a subsequent rest period) of the Torpedo electric organ. HC-3 (100 microM) caused an increased depletion of vesicular ACh and blocked the incorporation of 14C-label whereas contents in vesicular ATP and 3H-incorporation were identical with and without HC-3. HC-3 also blocked the recovery of electrical response of the tissue after stimulation but did not cause a change in vesicle numbers. The result suggest that synaptic vesicles continue to recycle ATP in the absence of recycling of ACh and that vesicular uptake and storage of the two components are not coupled to each other.

The lipid and protein content of cholinergic synaptic vesicles from the electric organ of Torpedo marmorata purified to constant composition: implications for vesicle structure

The lipid, protein, acetylcholine and ATP content of cholinergic synaptic vesicles isolated from the richly innervated electric organ of Torpedo marmorata and purified to constant composition has been determined. The number of vesicles present in the preparations has been estimated by quantitative electron microscopy and the mean composition of the vesicle deduced. The acetylcholine content of the purest preparations was considerably greater than that previously attained and reached a mean of 6.10 mmole/g of protein and 2.6 X 10(5) molecules/vesicle; the mean values, for all determinations, were 4.1 +/- S.E.M. 0.6 and 2.6 X 10(5) +/- S.E.M. 0.6 X 10(5) respectively. The lipid and protein content of the vesicle (about 140 and 80 ag/vesicle respectively) is relatively low, indicating a thin, lipid-rich membrane and a highly hydrated core of which not more than 1-2% can be occupied by protein. These findings are consistent with conclusions drawn from recent density determinations made at different osmotic pressures using penetrating and non-penetrating gradients.