Probes for PARVALBUMIN

N-methyl-D-aspartate receptor (NMDAR) hypofunction during brain development is likely to contribute to the manifestation of schizophrenia (SCZ) in young adulthood. The cellular targets of NMDAR hypofunction appear to be at least in part corticolimbic fast-spiking (FS) interneurons. However, functional alterations in parvalbumin (PV)-positive FS interneurons following NMDAR hypofunction are poorly understood. Paired patch-clamp recordings from murine cortical PV interneurons and pyramidal neurons revealed that genetic deletion of NMDAR subunit Grin1 in prospective PV interneurons before the second postnatal week impaired evoked- and synchronized-GABA release. Whereas intrinsic excitability and spiking characteristics were also disturbed by Grin1 deletion, neither restoring their excitability by K+ channel blockade nor increasing extracellular Ca2+ rescued the GABA release. GABA release was also insensitive to the Cav2.1 channel antagonist ω-agatoxin IVA. Heterozygous deletion of Cacna1a gene (encoding Cav2.1) in PV interneurons produced a similar GABA release phenotype as the Grin1 mutants. Treatment with the Cav2.1/2.2 channel agonist GV-58 augmented somatic Ca2+ currents and GABA release in Cacna1a-haploinsufficient PV interneurons, but failed to enhance GABA release in the Grin1-deleted PV interneurons. Taken together, our results suggest that Grin1 deletion in prospective PV interneurons impairs proper maturation of membrane excitability and Cav2.1-recruited evoked GABA release. This may increase synaptic excitatory/inhibitory ratio in principal neurons, contributing to the emergence of SCZ-like phenotypes.

The proper development and function of telencephalic GABAergic interneurons is critical for maintaining the excitation and inhibition (E/I) balance in cortical circuits. Glutamate contributes to cortical interneuron (CIN) development via N-methyl-D-aspartate receptors (NMDARs). NMDAR activation requires the binding of a co-agonist, either glycine or D-serine. D-serine (co-agonist at many mature forebrain synapses) is racemized by the neuronal enzyme serine racemase (SR) from L-serine. We utilized constitutive SR knockout (SR-/-) mice to investigate the effect of D-serine availability on the development of CINs and inhibitory synapses in the prelimbic cortex (PrL). We found that most immature Lhx6 + CINs expressed SR and the obligatory NMDAR subunit NR1. At embryonic day 15, SR-/- mice had an accumulation of GABA and increased mitotic proliferation in the ganglionic eminence and fewer Gad1 + (glutamic acid decarboxylase 67 kDa; GAD67) cells in the E18 neocortex. Lhx6 + cells develop into parvalbumin (PV+) and somatostatin (Sst+) CINs. In the PrL of postnatal day (PND) 16 SR-/- mice, there was a significant decrease in GAD67+ and PV+, but not SST + CIN density, which was associated with reduced inhibitory postsynaptic potentials in layer 2/3 pyramidal neurons. These results demonstrate that D-serine availability is essential for prenatal CIN development and postnatal cortical circuit maturation.

Prefrontal cortex (PFC) is a site of information convergence important for behaviors relevant to psychiatric disorders. Despite the importance of inhibitory GABAergic parvalbumin-expressing (PV+) interneurons to PFC circuit function and decades of interest in N-methyl-D-aspartate receptors (NMDARs) in these neurons, examples of defined circuit functions that depend on PV+ interneuron NMDARs have been elusive. Indeed, it remains controversial whether all PV+ interneurons contain functional NMDARs in adult PFC, which has major consequences for hypotheses of the pathogenesis of psychiatric disorders. Using a combination of fluorescent in situ hybridization, pathway-specific optogenetics, cell-type-specific gene ablation, and electrophysiological recordings from PV+ interneurons, here we resolve this controversy. We found that nearly 100% of PV+ interneurons in adult medial PFC (mPFC) express transcripts encoding GluN1 and GluN2B, and they have functional NMDARs. By optogenetically stimulating corticocortical and thalamocortical inputs to mPFC, we show that synaptic NMDAR contribution to PV+ interneuron EPSCs is pathway-specific, which likely explains earlier reports of PV+ interneurons without synaptic NMDAR currents. Lastly, we report a major contribution of NMDARs in PV+ interneurons to thalamus-mediated feedforward inhibition in adult mPFC circuits, suggesting molecular and circuit-based mechanisms for cognitive impairment under conditions of reduced NMDAR function. These findings represent an important conceptual advance that has major implications for hypotheses of the pathogenesis of psychiatric disorders.

Cortical parvalbumin (Pvalb)-expressing neurons provide robust inhibition to neighboring pyramidal neurons, crucial for the proper functioning of cortical networks. This class of inhibitory neurons undergoes extensive synaptic formation and maturation during the first weeks after birth and continue to dynamically maintain their synaptic output throughout adulthood. While several transcription factors, such as Nkx2-1, Lhx6, and Sox6, are known to be necessary for the differentiation of progenitors into Pvalb+ neurons, which transcriptional programs underlie the postnatal maturation and maintenance of Pvalb+ neurons' innervation and synaptic function remains largely unknown. Because Sox6 is continuously expressed in Pvalb+ neurons until adulthood, we utilized conditional knockout strategies to investigate its putative role in the postnatal maturation and synaptic function of cortical Pvalb+ neurons in mice of both sexes. We found that early postnatal loss of Sox6 in Pvalb+ neurons leads to failure of synaptic bouton growth, whereas later removal in mature Pvalb+ neurons in the adult causes shrinkage of already established synaptic boutons. Paired recordings between Pvalb+ neurons and pyramidal neurons revealed reduced release probability and increased failure rate of Pvalb+ neurons' synaptic output. Furthermore, Pvalb+ neurons lacking Sox6 display reduced expression of full-length tropomyosin-receptor kinase B (TrkB), a key modulator of GABAergic transmission. Once re-expressed in neurons lacking Sox6, TrkB was sufficient to rescue the morphological synaptic phenotype. Finally, we showed that Sox6 mRNA levels were increased by motor training. Our data thus suggest a constitutive role for Sox6 in the maintenance of synaptic output from Pvalb+ neurons into adulthood.Significance statement:Cortical parvalbumin-expressing (Pvalb+) inhibitory neurons provide robust inhibition to neighboring pyramidal neurons, crucial for the proper functioning of cortical networks. These inhibitory neurons undergo extensive synaptic formation and maturation during the first weeks after birth and continue to dynamically maintain their synaptic output throughout adulthood. However, it remains largely unknown which transcriptional programs underlie the postnatal maturation and maintenance of Pvalb+ neurons. Here we show that the transcription factor Sox6 cell-autonomously regulates the synaptic maintenance and output of Pvalb+ neurons until adulthood, leaving unaffected other maturational features of this neuronal population.