Remember, the primary job of the neurotransmitter is to fit or "lock" into its own particular receptor, and then to initiate specific physiological responses within the body. However, many drugs, such as THC, are able to bind or attach themselves to a specific receptor, thus mimicking or blocking the normal function of the neurotransmitter destined for that receptor. Some drugs are agonists that activate or "turn on" receptors, and some are antagonists that block receptor function. It is not even that straightforward: many receptors in the brain are linked so that activation of one may block the function of another. These linkages are created by a variety of cellular messengers whose function is to relay information from inside neurons or from one neuron to another.
Thus, proper functioning of the nervous system relies on balancing the results of these receptor activations, regardless if the receptor is activated by a drug mimicking or blocking a neurotransmitter or by the specific neurotransmitter itself. This balance is found between the "excitatory" (stimulant) actions of neurotransmitters and the "inhibitory" (depressant) actions of neurotransmitters. As an example, let us look at a drug that acts like an antagonist that blocks the inhibitory action of a neuro-transmitter. This blockade could upset the balance of normal neuronal function by allowing excitatory activity to become the more dominant neuronal action. In a myriad of ways, these types of neuronal imbalances can be translated to the physical and psychological effects that are seen after using drugs such as marijuana. In this manner, we can see that the actions of agonists and antagonists can be quite complex.
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