In the diagram of two neurons shown below at which point would a substance that interferes with the action of a neurotransmitter be most effective

The substance should target the synapse. Through competition with the neurotransmitters in the synaptic cleft, the substance should block the postsynaptic receptors from access to the neurotransmitters. This, therefore, prevents that transmission of the electric signal from one neuron to the other.  This is the common concept in pain relief.   

the menstrual cycle – higher tier. negative feedback mechanisms affect the production of hormones in the menstrual cycle. oestrogen also stimulates the release of lh (luteinising hormone) from the pituitary gland, which in turn controls the production of progesterone.