Question: what is the most abundant neurotransmitter in the nervous system
[{'answer': 'Serotonin', 'provenance': [{'wikipedia_id': '21865', 'title': 'Neurotransmitter', 'start_paragraph_id': 74, 'start_character': 11, 'end_paragraph_id': 74, 'end_character': 20, 'bleu_score': 1.0, 'section': 'Section::::Drug effects.:Precursors.:Catecholamine and trace amine precursors.\n'}]}]