Robust tonic GABA currents can inhibit cell firing in mouse newborn neocortical pyramidal cells.

Within the hippocampus and neocortex, GABA is considered to be excitatory in early development due to a relatively depolarized Cl(-) reversal potential (E(Cl)). Although the depolarizing nature of synaptic GABAergic events has been well established, it is unknown whether cortical tonic currents mediated by extrasynaptically located GABA(A) receptors (GABA(A) Rs) are also excitatory. Here we examined the development of tonic currents in the neocortex and their effect on neuronal excitability. Mean tonic current, recorded from layer 5 (L5) pyramidal cells of the mouse somatosensory cortex, is robust in newborns [postnatal day (P)2-4] then decreases dramatically by the second postnatal week (P7-10 and P30-40). Pharmacological studies, in combination with Western blot analysis, show that neonatal tonic currents are partially mediated by the GABA(A) R α5 subunit, and probably the δ subunit. In newborns, the charge due to tonic current accounts for nearly 100% of the total GABA charge, a contribution that decreases to < 50% in mature tissue. Current clamp recordings show that tonic current contributes to large fluctuations in the membrane potential that may disrupt its stability. Bath application of 5 µM GABA, to induce tonic currents, markedly decreased cell firing frequency in most recorded cells while increasing it in others. Gramicidin perforated patch recordings show heterogeneity in E(Cl) recorded from P2-5 L5 pyramidal cells. Together, these findings demonstrate that tonic currents activated by low GABA concentrations can dominate GABAergic transmission in newborn neocortical pyramidal cells and that tonic currents can exert heterogeneous effects on neuronal excitability.

GABAB receptors in maintenance of neocortical circuit function.

Activation of metabotropic GABAB receptors (GABABRs) enhances tonic GABA current and substantially increases the frequency of spontaneous seizures. Despite the and pro-epileptic consequences of GABABR activation, mice lacking functional GABAB receptors (GABAB1R KO mice) exhibit clonic and rare absence seizures. To examine these mutant mice further, we recorded excitatory and inhibitory synaptic inputs and tonic mutant GABA currents from Layer 2 neocortical pyramidal neurons of GABAB1R WT and KO mice (P30-40). Tonic current was increased while the frequency of synaptic inputs was unchanged in KO mice relative to WT littermates. The neocortical laminar distribution of interneuron subtypes derived from the medial ganglionic eminence (MGE) was also not statistically different in KO mice relative to WT while the number of calretinin-positive, caudal GE-derived cells in Layer 1 was reduced. Transplantation of MGE progenitors obtained from KO mice lacking functional GABAB1R did not increase tonic inhibition in the host brain above that of media-injected controls. Taken together, these results suggest a complex role for GABAB receptors in mediating neocortical circuit function.

Neocortical integration of transplanted GABA progenitor cells from wild type and GABA(B) receptor knockout mouse donors.

Most cortical interneurons originate in a region of the embryonic subpallium called the medial ganglionic eminence (MGE). When MGE cells are transplanted into cerebral cortex, these progenitors migrate extensively and differentiate into functional inhibitory neurons. Although MGE progenitors have therapeutic potential following transplantation, it is unknown precisely how these cells distribute within neocortical lamina of the recipient brain. Here we transplanted mouse embryonic day 12.5 MGE progenitors into postnatal neocortex and evaluated laminar distribution of interneuron subtypes using double- and triple-label immunohistochemistry. Studies were performed using wild type (WT) or donor mice lacking a metabotropic GABA(B) receptor subunit (GABA(B1)R KO). MGE-derived neurons from WT and GABA(B1)R KO mice preferentially and densely distributed in neocortical layers 2/3, 5 and 6. As expected, MGE-derived neurons differentiated into parvalbumin+ and somatostatin+ interneurons within these neocortical lamina. Our findings provide insights into the anatomical integration of MGE-derived interneurons following transplantation.