1. The postnatal maturation of intracortical inhibitory circuitry and the development of responses to applied gamma-aminobutyric acid (GABA) and baclofen were studied in pyramidal and nonpyramidal neurons from layers II and III of the rat primary somatosensory and primary visual cortex, in vitro. 2. Depolarizing spontaneous inhibitory postsynaptic potentials (IPSPs) could be recorded in approximately 70% of the young (postnatal day 4-10; P4-10), juvenile (P11-16), and adult cells (P28-41), respectively, when they were loaded with nitrate. At all ages these spontaneous events could be blocked by application of the GABA(A) receptor antagonist bicuculline methiodide (BMI), indicating that they were mediated by activation of GABA(A) receptors. 3. In 122 of the 130 adult cells tested, standardized electrical stimulation of the white matter or layer VI evoked a brief excitatory postsynaptic potential (EPSP), followed by both a fast (f-) and a long-latency (l-)IPSP. Similar stimuli evoked a biphasic IPSP in only 51 of the 98 juvenile and in only 1 of the 56 young neurons studied. The mean peak conductance of the f-IPSP and the 1-IPSP increased significantly from 50.2 and 7.5 nS, respectively, in juvenile cells to 84.2 and 18.0 nS, respectively, in adult neurons. 4. Application of the N-methyl-D-aspartate (NMDA) receptor antagonist D-amino-phosphonovaleric acid (D-APV) to juvenile cells induced a significant negative shift in the reversal potential of both the f-IPSP and l-IPSP. This effect was accompanied by a reduction in the peak conductance during these events by 31 and 48%, respectively, indicating that a prominent long-lasting NMDA receptor-mediated EPSP occurs concurrent with the early and late IPSP in immature neurons. In adult neurons, D-APV had no significant effect on the reversal potential of the f- and l-IPSP, although the peak conductance decreased by 20 and 5%, respectively, suggesting that there was a smaller concurrent activation of NMDA receptors in this age group. 5. The functional maturation of GABA(A) and GABA(B) receptors was studied using focal applications of GABA to the soma and the apical dendrite. Somatic GABA applications to adult neurons held at depolarized membrane potentials evoked a triphasic response, consisting of 1) a GABA(A)-mediated hyperpolarizing fast component (GABA(hf); reversal potential, -76 mV), 2) a GABA(A)-mediated depolarizing phase (GABA(d); -54 mV), and 3) a hyperpolarizing late response (GABA(hl); -80 mV). The GABA(d) response could be demonstrated at all ages in almost every neuron. The percentage of cells with a somatic GABA(hf) and GABA(hl) response increased with age from 17% each in young animals to 96 and 74%, respectively, in adult rats. 6. GABA(d) response could be obtained from every cell studied in all three age groups when GABA was applied to the apical dendrite. A dendrite GABA(hf) response could be demonstrated in 14% of the young neurons, and in 50 and 35% of the juvenile and adult cells, respectively. The percentage of cells with a dendritic GABA(hl) increased from 7% in young animals to 38 and 44% in juvenile and adult rats, respectively. 7. Baclofen evoked a long-lasting, small-amplitude hyperpolarization of the membrane potential in 60% of the juvenile (reversal potential, -94 mV) and in 88% of the adult neurons (-90 mV), when applied near the apical dendrite. A somatic baclofen response could be demonstrated in 75% of the juvenile (-86 mV) and 75% of the adult cells (-90 mV). In slices from young animals, only 23% of the cells showed a somatic baclofen response, and a dendritic response was never observed. 8. Age-dependent alterations in the efficiency of Cl- -extruding mechanisms were investigated by loading cells with Cl- and using changes in the amplitude of the positive-going f-IPSP as an index of changes in intracellular Cl- concentration ([Cl-]i). Whereas Cl- was extruded with a time constant of 6.8 s in adult neurons, 5 of 7 Cl- -loaded juvenile cells responded to orthodromic stimulation with prolonged depolarizations and prominent burst discharges that persisted for the entire impalement and were presumably related to increases in [Cl-]i. These findings suggest that outward Cl- transport is relatively ineffective in immature neurons. 9. Our data indicate that GABAergic inhibition in rat neocortex matures gradually during the first postnatal month. Whereas GABA(A) receptors are already present as early as P4, GABA(B) receptors become functionally active in the middle of the second postnatal week, at a time when inhibitory innervation begins and orthodromically evoked IPSPs can first be demonstrated. The relative inefficacy of GABAergic inhibition in juvenile cortex promotes the expression of NMDA receptor-mediated activity at this age, which in turn may play a role in developmental plasticity and increased seizure susceptibility of immature cortex.
