While researchers have discovered that there are an excessive number of receptor sites for it is not the only neurotransmitter involved in schizophrenia.

I think the correct answer is dopamine. While researchers have discovered that there are an excessive number of receptor sites for dopamine, it is not the only neurotransmitter involved in schizophrenia. The main support for the theory that too much dopamine causes schizophrenia is the fact that antipsychotic medications, which are used to treat schizophrenia, block dopamine receptors. Hope this helps.