Parvalbumin-containing chandelier and basket cell boutons have distinctive modes of maturation in monkey prefrontal cortex.

Parvalbumin (PV)-containing cortical GABA neurons include chandelier cells (PVChCs) and basket cells (PVBCs), which innervate the axon initial segment (AIS) and soma/proximal dendrites of pyramidal cells, respectively. In monkey prefrontal cortex (PFC), the density of PVChC axon cartridges detectable by PV immunoreactivity peaks prior to the onset of puberty before declining markedly to adult levels, whereas the density of PV-immunoreactive (IR) puncta (presumed PVBC boutons) increases during adolescence. These inverse developmental changes in bouton density could explain why an electron microscopy study found no change in the density of symmetric, presumably GABAergic, synapses between infancy and adulthood in monkey PFC. Alternatively, the inverse developmental trajectories of PVChC and PVBC boutons could represent cell type-specific differences in the maturation of PV protein levels. To differentiate between these two alternatives, multilabel confocal microscopy was used to quantify the number of PVChC and PVBC boutons per pyramidal neuron in the PFC of 3-month-old and adult monkeys. The mean number of PVChC boutons per pyramidal neuron AIS was, significantly, 32% lower in adult compared with 3-month-old monkeys, whereas the density of PVBC boutons per pyramidal neuron did not differ between age groups. In contrast, relative levels of PV protein were approximately twofold higher in PVBC boutons in adult animals, whereas PV levels in PVChC boutons did not differ between age groups. These findings suggest cell type-specific mechanisms of maturation of PV-containing GABAergic boutons in monkey PFC.