Regional and age‐related differences in GAD67 expression of parvalbumin‐ and calbindin‐expressing neurons in the rhesus macaque auditory midbrain and brainstem

Neurons expressing the calcium binding proteins (CaBPs) parvalbumin (PV) and calbindin (CB) have shown age‐related density changes throughout the ascending auditory system of both rodents and macaque monkeys. In the cerebral cortex, neurons expressing these CaBPs express markers of γ‐aminobutyric acidergic neurotransmission, such as GAD67, and have well‐understood physiological response properties. Recent evidence suggests that, in the rodent auditory brainstem, CaBP‐containing cells do not express GAD67. It is unknown whether PV‐ and CB‐containing cells in subcortical auditory structures of macaques similarly do not express GAD67, and a better understanding of the neurotransmission of neurons expressing these proteins is necessary for understanding the age‐related changes in their density throughout the macaque auditory system. This was investigated with immunofluorescent double‐labeling techniques to coregister PV‐ and CB‐expressing neurons with GAD67 in the superior olivary complex and the inferior colliculus of young and aged rhesus macaques. The proportions of GAD67‐expressing PV‐ and CB‐positive neurons were computed with unbiased sampling techniques. Our results indicate that between 42% and 62% of PV‐ and CB‐positive neurons in the auditory brainstem and midbrain express GAD67, which is significantly less than in the cerebrum. In general, fewer PV⁺ neurons and more CB⁺ neurons expressed GAD67 as a function of age. These results demonstrate that the inhibitory molecular profile of PV‐ and CB‐expressing neurons can change with age in subcortical auditory structures and that these neurons are distinct from the well‐described inhibitory interneurons that express these proteins in the cerebral cortex. J. Comp. Neurol. 522:4074–4084, 2014. © 2014 Wiley Periodicals, Inc.