What is the first step in termination of norepinephrine actions at the receptor?

The termination of norepinephrine actions at the receptor primarily involves several mechanisms, with the first step being the diffusion of norepinephrine away from the receptor site. This process is crucial because it reduces the concentration of norepinephrine in the synaptic cleft, thereby allowing the receptor to return to its inactive state. Once the norepinephrine is no longer bound to the receptor, its signaling effects cease.

Following this initial diffusion, other processes come into play to further clear norepinephrine from the synaptic environment, such as reuptake into the nerve terminal and metabolism by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT). However, these subsequent steps rely on the initial diffusion of the neurotransmitter as a means to decrease its immediate signaling effects at the receptor. In summary, the primary and immediate action that leads to the cessation of norepinephrine's effect is its diffusion away from the receptor, making it the correct answer.