Evaluation of radiolabeled acetylcholine synthesis and release in rat striatum

Cholinergic transmission underlies higher brain functions such as cognition and movement. To elucidate the process whereby acetylcholine (ACh) release is maintained and regulated in the central nervous system, uptake of [³ H]choline and subsequent synthesis and release of [³ H]ACh were investigated in rat striatal segments. Incubation with [³ H]choline elicited efficient uptake via high-affinity choline transporter-1, resulting in accumulation of [³ H]choline and [³ H]ACh. However, following inhibition of ACh esterase (AChE), incubation with [³ H]choline led predominantly to the accumulation of [³ H]ACh. Electrical stimulation and KCl depolarization selectively released [³ H]ACh but not [³ H]choline. [³ H]ACh release gradually declined upon repetitive stimulation, whereas the release was reproducible under inhibition of AChE. [³ H]ACh release was abolished after treatment with vesamicol, an inhibitor of vesicular ACh transporter. These results suggest that releasable ACh is continually replenished from the cytosol to releasable pools of cholinergic vesicles to maintain cholinergic transmission. [³ H]ACh release evoked by electrical stimulation was abolished by tetrodotoxin, but that induced by KCl was largely resistant. ACh release was Ca²⁺ dependent and exhibited slightly different sensitivities to N- and P-type Ca²⁺ channel toxins (ω-conotoxin GVIA and ω-agatoxin IVA, respectively) between both stimuli. [³ H]ACh release was negatively regulated by M2 muscarinic and D2 dopaminergic receptors. The present results suggest that inhibition of AChE within cholinergic neurons and of presynaptic negative regulation of ACh release contributes to maintenance and facilitation of cholinergic transmission, providing a potentially useful clue for the development of therapies for cholinergic dysfunction-associated disorders, in addition to inhibition of synaptic cleft AChE.