Age-related reductions in rat atrial high affinity choline uptake, ACh synthesis, and ACh release. A brief note

Overexpression of the vesicular acetylcholine transporter disrupts cognitive performance and causes age-dependent locomotion decline in Drosophila

Acetylcholinergic (ACh) neurotransmission is essential for key organismal functions such as locomotion and cognition. However, the mechanism through which ACh is regulated in the central nervous system is not fully understood. The vesicular acetylcholine transporter (VAChT) mediates the packaging and transport of ACh for exocytotic release and is a critical component of the ACh release machinery. Yet its precise role in the maintenance of cholinergic tone remains a subject of active investigation. Here we use the overexpression of VAChT as a tool to investigate the role of changes in ACh exocytosis on the regulation of synaptic activity and its downstream consequences. We measured the effect of an increase in VAChT expression on locomotion and cognitive performance as well as on organismal survival across the lifespan. We report the surprising finding that increased VAChT expression results in a significantly shorter lifespan in comparison to control flies. Moreover, constructs overexpressing VAChT demonstrate an age-dependent decrease in locomotion performance. Importantly, we report clear deficits in learning and memory which we measured through a courtship conditioning assay. Together, these data provide evidence for the adverse effects of overexpression of the vesicular acetylcholine transporter in the maintenance of normal behavioral abilities in Drosophila and demonstrates for the first time a role for ACh in the regulation of organismal survival.

Assessment of choline uptake for the synthesis and release of acetylcholine in brain slices by a dynamic autoradiographic technique using [11C]choline

The uptake of choline for the synthesis and release of acetylcholine was investigated in brain slices by dynamic positron autoradiography using [11C]choline. Brain slices (330 microm) were incubated with [11C]choline in oxygenated Krebs-Ringer medium at 34 degrees C and serial two-dimensional time-resolved images of the uptake and release of radioactivity were recorded on Storage Phosphor screens. [11C]choline uptake increased with the period of incubation and was 1.9 times higher in the striatum than cerebral cortex. The uptake in the striatum was significantly diminished by hemicholinium-3 (HC-3), an inhibitor of high-affinity choline uptake. Pretreatment of brain slices with 50 mM K(+) for 20 min enhanced the uptake in striatum. The uptake of [11C]choline in brain slices was saturable using nonlabeled choline. Two uptake systems, a high-affinity and a low-affinity system, were confirmed to exist by kinetic analysis using Lineweaver-Burk plots. The 11C radioactivity that had accumulated in the striatum disappeared on treatment with veratridine, a depolarization agent, in the presence of HC-3. This pattern of disappearance was consistent with that of the appearance of unlabeled and labeled acetylcholine in the medium. These results indicate that this method is useful for obtaining information regarding the uptake of choline for the synthesis and release of acetylcholine in live brain tissues.

Effects of peroxidation and aging on rat neocortical ACh-release and protein kinase C

Cerebral cortical synaptosomes were prepared from 2- or 24-month-old Fischer 344 rats and exposed to a peroxidizing condition (50 microM ferrous ions and 2 microM ascorbate ions) before measuring either the release of newly synthesized [3H]acetylcholine (ACh) or protein kinase C activity (PKC). Several secretagogues with different mechanisms of action and different responses to aging were used to trigger release: K+ depolarization (5 mM-60 mM), calcium ionophore A23187 (1-10 micrograms/ml), and 4-aminopyridine (0.1-10 mM). Aging reduced K+ depolarization-induced release at every K+ concentration studied, reduced A23187-induced release at low but not high concentrations and did not affect 4-aminopyridine-induced release. Membrane peroxidation of synaptosomes from 2-month-old rats altered the response to secretagogues to match that seen in 24-month-old rats. Membrane peroxidation also attenuated the A23187-stimulated translocation of free to bound synaptosomal PKC activity in 2-month-old but not 24-month-old animals. These results suggest that membrane peroxidation may mimic some age-related deficits in secretagogue-induced [3H]ACh release.

Aging: effects on chronotropic actions of muscarinic agonists in isolated rat atria

Negative chronotropic effects of acetylcholine and carbachol were compared in right atria isolated from adult and aged Fischer 344 rats. Preparations from aged rats were more sensitive to the action of acetylcholine; however, there was no difference in responsiveness to carbachol or to acetylcholine after pretreatment with diisopropylfluorophosphate, an irreversible cholinesterase inhibitor. These data suggest that the enhanced sensitivity to the direct chronotropic action of acetylcholine is the result of aging-related reductions in acetylcholinesterase activity.

Long-term neuropathological and neurochemical effects of nucleus basalis lesions in the rat

The long-term effects of excitotoxic lesions in the nucleus basalis magnocellularis of the rat were found to mimic several neuropathological and chemical changes associated with Alzheimer's disease. Neuritic plaque-like structures, neurofibrillary changes, and neuronal atrophy or loss were observed in the frontoparietal cortex, hippocampus, amygdala, and entorhinal cortex 14 months after the lesions were made. Cholinergic markers in neocortex were reduced, while catecholamine and indoleamine metabolism was largely unaffected at this time. Bilateral lesions of the nucleus basalis magnocellularis increased somatostatin and neuropeptide Y in the cortex of the rat by at least 138 and 284 percent, respectively, suggesting a functional interaction between cholinergic and peptidergic neurons that may differ from that in Alzheimer's disease.

Effects of choline augmentation on acetylcholine synthesis and release in rat atrial minces

The uptake and acetylation of [3H]-choline, as well as the calcium-dependent release of a newly synthesized [3H]-ACh, was studied in a new rat atrial mince preparation. The hemicholinium-3-sensitive uptake and acetylation of [3H]-choline increased as [3H]-choline concentrations were elevated to 100 microM in atrial minces. In contrast, hemicholinium-3-sensitive [3H]-choline uptake was saturated with 15 microM [3H]-choline in brain synaptosomes. The increased atrial [3H]-ACh synthesized in the presence of [3H]-choline augmentation was releasable by 50 mM K+-depolarization in a 1 mM cobalt-sensitive manner. These results suggest that atrial parasympathetic activity may be more sensitive to circulating choline concentrations than brain cholinergic neurons are.