Noradrenaline and 123I-meta-iodobenzylguanidine kinetics in the sympathetic nervous system

Abstract

Noradrenaline is a neurotransmitter released from sympathetic nerve endings and is synthesized from the amino acid tyrosine through a series of enzymatic reactions. When sympathetic nerves are excited, noradrenaline is released to increase the heart rate and cardiac contractility through beta-1 adrenergic receptors. Only a fraction of the released noradrenaline binds to receptors, and most of the remainder (80–95%) undergoes active reuptake into nerve endings (uptake-1 mechanism) to be restored in vesicles. 123I-iodobenzylguanidine (MIBG), an analog of guanethidine (an adrenergic neuron-blocking agent), is a radioactive tracer of noradrenaline and is used to evaluate presynaptic sites of the adrenergic system. MIBG can determine the distribution, activity, and disorders of postsynaptic cardiac sympathetic nerves and is widely used to evaluate various kinds of heart diseases, diabetes, and autonomic disorders, such as neurodegenerative disorders. MIBG is transported into the nerve by noradrenaline monoamine transporter 1, trapped in storage vesicles, and released by a mechanism similar to noradrenaline but is not catabolized like noradrenaline, and this is a necessary characteristic as sympathetic nerve-imaging agent. Understanding MIBG kinetics is important in recognizing the characteristics and the difference between early and delayed image of MIBG scintigraphy.