<prompt> In which of the following diseases decrease in levels of single neurotransmitter was first identified?Alzheimers diseaseParkinsons diseaseHuntington diseaseSchizophrenia  <response>: Neurons of the substantia nigra communicate with neurons of the basal ganglia by liberating the neurotransmitter dopamine DA. In Parkinsons disease, neurons of the substantia nigra progressively degeneration; as a result, the amount of DA available for neurotransmission in the corpus striatum is lowered. Ref Harrison20th edition pg 2456 