Final answer:
A drug that acts as both an agonist and an antagonist can impact neurotransmitter activity by mimicking and blocking effects on receptors in the brain, thus affecting neurotransmitter levels and responses.
Step-by-step explanation:
A drug that produces agonist molecules as well as antagonist molecules at the same time can have complex effects on neurotransmitter systems in the brain. Agonists are substances that can mimic a neurotransmitter by binding to and activating the receptors, enhancing the activity of those neurotransmitters. Conversely, antagonists bind to receptors but block or inhibit the normal activity of neurotransmitters, therefore decreasing their activity.
Treating diseases like Parkinson's disease often involves using dopamine agonists, which mimic the effects of dopamine in the brain. At the same time, psychoactive drugs can also function as antagonists, decreasing activity of particular neurotransmitters by blocking their synthesis or receptor binding.