What is noradrenaline?
Noradrenaline is a catecholamine that functions as a neurotransmitter inside the human brain. Once released by the pre-synaptic neuron, it exerts its effects by binding to and activating receptors on the surface of the cell. The two broad families of noradrenaline receptors are alpha and beta-adrenergic. The former is divided into the subtype’s a1 and a2, the beta receptors into b1, b2, and b3. Alpha-2 receptors, often located pre-synaptically, typically have inhibitory effects on noradrenaline release. The other subtypes all have excitatory effectors. When noradrenaline binds to excitatory, post-synaptic receptors at a threshold level, the receptor activation creates a synaptic potential, a response in the post-synaptic neuron. If noradrenaline instead binds to pre-synaptic alpha-2 receptors at a threshold level, further noradrenaline release from the pre-synaptic neuron will be inhibited.
This mechanism is referred to as feedback inhibition. The effects of noradrenaline can be terminated upon reuptake into the pre-synaptic and post-synaptic neurons. In the pre-synaptic neuron specifically, noradrenaline is repackaged into vesicles. In experimental settings, radioisotopes can be used to label ligands; [3H] tritium is most frequently used and will be the label employed in this study. Brain tissue will be incubated in a solution of [3H]-noradrenaline so that neurons can absorb the labeled neurotransmitter. Noradrenaline release can thus be measured as the radiolabeled neurotransmitter is discharged from pre-synaptic neurons. A scintillator is used to measure ionizing radiation, expressed as a rate of counts per unit time such as counts per minute (CPM). The key advantage of [3H] is that it does not change the molecular structure of the ligand. A hydrogen atom is simply exchanged with its radioactive homolog.