What is the 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. Another practical advantage is the long half-life of [3H] which allows it to be stored for extended periods of time. In this experiment, we will utilize the reuptake mechanism of noradrenaline to fill vesicles with the radio-isotope [3H]-noradrenaline and then examine the effect of receptor agonists and antagonists on the [3H]- noradrenaline release. This investigation contains three parts. First, we will observe the effect of stimulation and inhibition of the pre-synaptic alpha-2 receptor during electrically evoked synaptic transmission using clonidine and yohimbine, respectively. Next, we will examine the effect of stimulation and inhibition on the post-synaptic alpha-1 receptor using phenylephrine and prazosin, respectively. In the last portion, we will use desipramine to observe the effect of reuptake inhibitors and tyramine (which displaces noradrenaline from vesicles) on noradrenaline release during synaptic transmission. We hypothesize that stimulation of the pre-synaptic a2-receptor will inhibit further [3H]-noradrenaline release from the pre-synaptic neuron, while inhibition of this receptor will encourage neurotransmitter release. We predict that stimulation of the post-synaptic a1-receptor will increase [3H] release, whereas inhibition of the receptor will decrease neurotransmitter release.