Final answer:
Dopamine was the first neurotransmitter to receive significant research attention in the context of schizophrenia, due to its role in psychosis and its targetting by early antipsychotic drugs.
Step-by-step explanation:
The first neurotransmitter to receive substantial research attention regarding its role in the etiology of schizophrenia was dopamine. Early research by Snyder in 1976 focused on the dopamine hypothesis of schizophrenia, highlighting the importance of dopamine receptors. This was before GABA's role was acknowledged and well before the neurotransmitters serotonin and acetylcholine received similar attention.
The dopamine hypothesis of schizophrenia was based on observations that drugs which increase dopamine levels can cause psychosis, and drugs that reduce dopamine activity can diminish symptoms of schizophrenia. This research led to the development of antipsychotic drugs targeting the dopamine system. The hypothesis has evolved over time, integrating the role of other neurotransmitters such as GABA, but dopamine was definitively the first to be substantially researched in connection with schizophrenia.