Postnatal Development of Pre- and Postsynaptic GABA Markers at Chandelier Cell Connections with Pyramidal Neurons in Monkey Prefrontal Cortex DIANNE A. CRUZ, 1 STEPHEN M. EGGAN, 2 AND DAVID A. LEWIS 1,2 * 1 Department of Psychiatry, University of Pittsburgh, Pennsylvania 15213 2 Department of Neuroscience, University of Pittsburgh, Pennsylvania 15213 ABSTRACT The protracted postnatal maturation of the primate prefrontal cortex (PFC) is associated with substantial changes in the number of excitatory synapses on pyramidal neurons, whereas the total number of inhibitory synapses appears to remain constant. In this study, we sought to determine whether the developmental changes in excitatory input to pyramidal cells are paral- leled by changes in functional markers of inhibitory inputs to pyramidal neurons. The chandelier subclass of -aminobutyric acid (GABA) neurons provides potent inhibitory control over pyrami- dal neurons by virtue of their axon terminals, which form distinct vertical structures (termed cartridges) that synapse at the axon initial segment (AIS) of pyramidal neurons. Thus, we examined the relative densities, laminar distributions, and lengths of presynaptic chandelier axon cartridges immunoreactive for the GABA membrane transporter 1 (GAT1) or the calcium- binding protein parvalbumin (PV) and of postsynaptic pyramidal neuron AIS immunoreactive for the GABA A receptor  2 subunit (GABA A  2 ) in PFC area 46 of 38 rhesus monkeys (Macaca mulatta). From birth through 2 years of age, the relative densities and laminar distributions of these three markers exhibited different trajectories, suggesting developmental shifts in the weighting of at least some factors that determine inhibition at the AIS. In contrast, from 2 to 4 years of age, all three markers exhibited similar declines in density and length that paralleled the periadolescent pruning of excitatory synapses to pyramidal neurons. Across development, the predominant laminar location of PV-labeled cartridges and GABA A  2 -immunoreactive AIS shifted from the middle to superficial layers, whereas the laminar distribution of GAT1-positive cartridges did not change. Together, these findings suggest that the maturation of inhibitory inputs to the AIS of PFC pyramidal neurons is a complex process that may differentially affect the firing patterns of subpopulations of pyramidal neurons at specific postnatal time points. J. Comp. Neurol. 465:385– 400, 2003. © 2003 Wiley-Liss, Inc. Indexing terms: parvalbumin; GABA transporters; GABA A receptors The postnatal maturation of circuitry in the primate prefrontal cortex (PFC) is a complex and protracted pro- cess (Lewis, 1997). In macaque monkeys, the number of excitatory synapses in the PFC increases rapidly during the third trimester of gestation and first 3 postnatal months, remains at a plateau for the following year, and then declines substantially during adolescence until sta- ble adult levels are achieved (Bourgeois et al., 1994). In contrast, the number of inhibitory synapses increases pre- natally and then appears to remain relatively constant throughout postnatal development (Bourgeois et al., 1994). Despite this apparent stability in the total number of PFC inhibitory synapses, developmental changes in the function of these synapses may occur by means of shifts in Grant sponsor: U.S. Public Health Service; Grant number: MH45156; Grant number: MH43784. *Correspondence to: David A. Lewis, Department of Psychiatry, Univer- sity of Pittsburgh, 3811 O’Hara Street, W1651 BST, Pittsburgh, PA 15213. E-mail: lewisda@msx.upmc.edu Received 12 March 2003; Revised 15 May 2003; Accepted 19 May 2003 DOI 10.1002/cne.10833 Published online the week of September 1, 2003 in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 465:385– 400 (2003) © 2003 WILEY-LISS, INC.