Neurotransmitter and neuropeptide modulation of high affinity choline uptake in Limulus brain

The role of neurotransmitters in the modulation of the sodium-dependent high affinity choline uptake system (HAChUS) of the horseshoe crab, Limulus polyphemus has been investigated utilizing a tissue slice preparation. Choline uptake was significantly decreased by carbachol but unaffected by atropine and d-tubocurarine. The muscarinic agonist oxotremorine decreased choline uptake by 30.4% while the muscarinic antagonist, pirenzepine, increased uptake by 29.6%. Applied in combination, pirenzepine and oxotremorine abolished their individual effects resulting in control values for choline uptake. The non-cholinergic transmitters octopamine and serotonin significantly enhanced choline uptake. The neuropeptide proctolin elicited a 20% increase in choline transport whereas Phe-Met-Arg-Phe (FMRF) amide was without effect. This study demonstrates that neurotransmitters and neuropeptides modulate the HAChUS, possibly through specific receptor-mediated second messenger systems.