High affinity choline uptake and acetylcholine release by guinea pig neocortex synaptosomes: inhibition by adenosine derivatives

Potential role of adenosine antagonist therapy in pathological tremor disorders

The continuing lack of effective long-term therapies for Parkinson's disease and other disorders in which a primary symptom is involuntary tremor is leading to a search for alternative pharmacological strategies. Adenosine is a major modulator of neuronal activity and neurotransmitter release in the central nervous system, with A1 receptors inhibiting transmitter release and A2 receptors generally enhancing release of several transmitter systems relevant to the control of movement. The A2a subtype of receptor is especially concentrated in the neostriatum and is co-localised with D2 receptors for dopamine, the affinity of which are reduced by activation of the A2a population. Antagonists of adenosine, such as theophylline, have been reported to improve the tremor in cases of Parkinson's disease and essential tremor, and the development of better and more selective A2a receptor antagonists may prove of value in these disabling disorders.

Effect of adenosine receptor agonists on spontaneous and K(+)-evoked acetylcholine release from the in vivo rat cerebral cortex

Repeated applications of elevated K+ (100 mM) in artificial cerebrospinal fluid (CSF) were used to evoke an efflux of acetylcholine (ACh) from the in vivo rat cerebral cortex using a cortical cup technique. Elevated K+ reproducibly increased the levels of ACh in cup superfusates by a factor of 3-5-fold above basal levels (27.2 +/- 9.7 nM). The adenosine A1 receptor agonist N6-cyclopentyl adenosine (CPA), at a concentration of 10(-8) M, depressed basal, but not K(+)-evoked ACh efflux. 10(-6) M CPA increased basal, but did not alter K(+)-evoked, ACh efflux. The A2 selective agonist CGS 21680 did not alter either basal, or K(+)-evoked, ACh efflux. The inhibitory effects of 10(-8) M CPA on ACh efflux would be consistent with the presence of adenosine A1 receptors on cholinergic nerve terminals in the cerebral cortex. At a higher concentration (10(-6) M) CPA elevated basal release, possibly by activating low affinity A2 receptors. The failure of CGS 21680 (10(-8) M) to alter basal ACh release suggests an absence of high affinity A2 receptors in these terminals. Whereas elevated K+ in cup superfusates consistently enhanced ACh efflux from the cerebral cortex, this increase was not affected by either CPA or CGS 21680. High K(+)-evoked release of cerebral cortical ACh may be an inappropriate model for the study of adenosine's actions on neurotransmitter release.

Substance B: an endogenous brain factor that reverses presynaptic inhibition of acetylcholine release

Evidence is presented of the existence of a substance in brain (substance B) that reverses presynaptic inhibition of evoked acetylcholine (AcCho) release. Addition of guinea pig brain synaptosomal incubation medium or whole brain extracts to assays in which 1,1-dimethyl-4-phenylpiperazinium-evoked release of [3H]AcCho from guinea pig myenteric plexus synaptosomes reversed purinergic, muscarinic, and alpha 2-adrenergic agonist inhibition. However, the extracts did not increase basal or evoked release of AcCho. Results of chromatography on Bio-Gel P-2 of a concentrated whole brain extract suggest this factor has an Mr of 700. Substance B was resistant to boiling under acid or alkaline conditions as well as incubation with phospholipase C and various proteases; ashing, however, completely destroyed activity. This endogenous factor was also found to antagonize agonist-mediated inhibition of electrically evoked contractions of myenteric plexus-longitudinal muscle strips from the guinea pig ileum. The demonstration of substance B with its ability to reverse presynaptic inhibition of AcCho release reveals one mode of presynaptic modulation.

Effect of adenosine, adenosine derivatives, and caffeine on acetylcholine release from brain synaptosomes: interaction with muscarinic autoregulatory mechanisms

Synaptosomes, prepared from rat cerebral cortex and hippocampus, were preincubated with [methyl-3H]choline. The effect of adenosine, cyclohexyladenosine, N-ethylcarboxamide adenosine, 2'-deoxyadenosine, and oxotremorine on K+-evoked 3H efflux was investigated. High-voltage electrophoretic separation showed that in the presence of physostigmine, the K+-evoked 3H efflux from hippocampal synaptosomes was 90% [3H]acetylcholine and 10% [3H]choline. Adenosine (30 microM) and oxotremorine (100 microM) both decreased [3H]acetylcholine release from hippocampal synaptosomes. The effect was inversely proportional to the KCl concentration and disappeared at a KCl concentration of 50 mM. Cyclohexyladenosine was approximately 3,000 times more active than adenosine, whereas N-ethylcarboxamide adenosine and 2'-deoxyadenosine were inactive. This indicates that A1 adenosine receptors were involved in the inhibitory effect. Caffeine antagonized the adenosine effect, and at a concentration of 100 microM, it stimulated [3H]acetylcholine efflux. The inhibitory effect of oxotremorine was as great in cortical as in hippocampal synaptosomes. In contrast, adenosine was much less active in cortical than in hippocampal synaptosomes. When inhibitory concentrations of adenosine and oxotremorine were added together into the incubation medium, the effect of adenosine on [3H]acetylcholine release was consistently reduced. An interaction between muscarinic and A1 adenosine presynaptic receptors at a common site modulating acetylcholine release can be assumed.

Action of hemicholinium-3 on phospholipid metabolism in Krebs II ascites cells

Incorporation of [Me-14C]choline or/and [2-14C]ethanolamine into phospholipids of Krebs II ascites cells in toto have been tested in the presence of hemicholinium-3. With [Me-14C]choline, labelling of cell pellet, intracellular choline, phosphocholine and total lipid extract is inhibited by hemicholinium-3 in a dose-dependent way between 6.25 X 10(-6) M and 10(-3) M. These effects are caused by a diminution of the choline or/and ethanolamine transport across the cell membrane and by a choline-kinase inhibition. In Krebs cells, choline is taken up by a low affinity Na+ sensitive uptake system (KT = 46 X 10(-6) M) which is competitively inhibited by hemicholinium-3 (KTi = 161 X 10(-6) M). Krebs cells exert a counter-transport (i.e. an exchange of choline across the membrane) against a concentration gradient of 10 mM choline whereas 10 mM hemicholinium-3 has no effect. Choline-kinase is also inhibited (I50 = 57 X 10(-6) M) in Krebs cells in toto and time-course data suggest that choline transport and phosphorylation might be tightly coupled. Specific radioactivities of phosphocholine and choline-glycerophospholipids decrease owing to the effect of the drug on the uptake and phosphorylation system. With 4 X 10(-5) M hemicholinium-3 and [Me-14C]choline as a marker, labelled choline-glycerophospholipids are decreased by 22%. With [2-14C]ethanolamine, labelled ethanolamine-phospholipids are decreased by 26% and choline-glycerophospholipids remain unlabelled. With the two markers, the additional effect produces a 35% decrease. It is concluded that hemicholinium-3 might be able to induce a depression of the intracellular choline and phosphocholine pool which could provoke a serious quantitative deficiency of major phospholipids in Krebs cells.

Regulation of depolarization-dependent release of neurotransmitters by adenosine: cyclic AMP-dependent enhancement of release from PC12 cells

We have used pheochromocytoma cells, clone PC12, as a model system for studying the effects of adenosine on neurosecretion. Exposure of the cells to adenosine or 2-chloroadenosine caused immediate activation of adenylate cyclase, increases in cellular cyclic AMP content, and inhibition of SAM-dependent phospholipid N-methylation and protein carboxymethylation. However, the effects on methylation were only observed with concentrations of adenosine 100 times greater than those that elevated cyclic AMP. Exposure of the cells to adenosine and 2-chloroadenosine did not alter the release of [3H]norepinephrine [(3H]NE) in the absence of depolarization. However, depolarization-dependent release of [3H]NE was markedly elevated by short (1-20 min) pretreatments with adenosine or 2-chloroadenosine. The enhancement of release was observed irrespective of the nature of the depolarizing stimulus (elevated K+, carbamylcholine, or veratridine). Release of [3H]acetylcholine in response to elevated K+ also was increased by adenosine pretreatment. These effects of adenosine and 2-chloroadenosine on neurotransmitter release closely paralleled elevation of cellular cyclic AMP but not inhibition of methylation. Taken together, the results show that adenosine, probably acting through adenosine receptors coupled to stimulation of adenylate cyclase, is able to modulate the neurosecretory process in PC12 cells. Furthermore, the enhancement of release occurred even though the extent of depolarization (measured as 86Rb+ flux through the acetylcholine receptor channel) and the amount of 45Ca2+ which entered upon depolarization were unchanged. Therefore, the enhancement of release produced by elevated cyclic AMP appeared to reflect increased efficiency of the stimulus-secretion coupling process.