Think of neurotransmitters (NT) as a person, and the synapse/neuron as a button. Imagine there are a group of people whose job is to press the buttons (stimulate the neurons) as many times and as often as possible but they get tired and leave. So turnover rate means how often the person gets tired and leaves the button alone. SSRIs do not allow serotonin to be “reuptaken” or in this example, the person in the room pushing buttons isn’t allowed to leave so he’s constantly pushing the buttons (stimulating the receptor). That’s a basic overview. Obviously overstimulation means your body down regulates the receptor (less buttons to push), and the opposite happens when you take something away from the body - your body upregulates the receptor - puts more buttons hoping to find a neurotransmitter (person) to stimulate it (push the button). Sorry if this sucks, hope it helps.