Dopamine D2 receptors (D2Rs) play a major role in the function of the prefrontal cortex (PFC), and may contribute to prefrontal dysfunction in conditions such as schizophrenia. Here we report that in mouse PFC, D2Rs are selectively expressed by a subtype of layer V pyramidal neurons that have thick apical tufts, prominent h-current, and subcortical projections. Within this subpopulation, the D2R agonist quinpirole elicits a novel afterdepolarization that generates voltage fluctuations and spiking for hundreds of milliseconds. Surprisingly, this afterdepolarization is masked in quiescent brain slices, but is readily unmasked by physiologic levels of synaptic input which activate NMDA receptors, possibly explaining why this phenomenon has not been reported previously. Notably, we could still elicit this afterdepolarization for some time after the cessation of synaptic stimulation. Besides NMDA receptors, the quinpirole-induced afterdepolarization also depended on L-type Ca2+ channels and was blocked by selective L-type antagonist nimodipine. To confirm that D2Rs can elicit this afterdepolarization by enhancing Ca2+ (and Ca2+-dependent) currents, we measured whole-cell Ca2+ potentials that occur after blocking Na+ and K+ channels, and found quinpirole enhanced these potentials, while the selective D2R antagonist (−)sulpiride had the opposite effect. Thus, D2Rs can elicit a Ca2+-channel dependent afterdepolarization that powerfully modulates activity in specific prefrontal neurons. Through this mechanism, D2Rs might enhance outputs to subcortical structures, contribute to reward related persistent firing, or increase the level of noise in prefrontal circuits.

SUMMARY Layer 5 pyramidal neurons comprise at least two subtypes: thick-tufted, subcortically-projecting Type A neurons, with prominent h-current, and thin-tufted, callosally-projecting Type B neurons, which lack prominent h-current. Using optogenetic stimulation, we find that these subtypes receive distinct forms of input that could subserve divergent functions. Repeatedly stimulating callosal inputs evokes progressively smaller excitatory responses in Type B but not Type A neurons. Callosal inputs also elicit more spikes in Type A neurons. Surprisingly, these effects arise via distinct mechanisms. Differences in the dynamics of excitatory responses reflect differences in presynaptic input, whereas differences in spiking depend on postsynaptic mechanisms. We also find that fast-spiking parvalbumin interneurons, but not somatostatin interneurons, preferentially inhibit Type A neurons, which leads to greater feedforward inhibition in this subtype. These differences may enable Type A neurons to detect salient inputs that are focused in space and time, while Type B neurons integrate across these dimensions.

Previously, we identified progressive alterations in spontaneous EPSCs and IPSCs in the striatum of the R6/2 mouse model of Huntington's disease (HD). Medium-sized spiny neurons from these mice displayed a lower frequency of EPSCs, and a population of cells exhibited an increased frequency of IPSCs beginning at ϳ40 d, a time point when the overt behavioral phenotype begins. The cortex provides the major excitatory drive to the striatum and is affected during disease progression. We examined spontaneous EPSCs and IPSCs of somatosensory cortical pyramidal neurons in layers II/III in slices from three different mouse models of HD: the R6/2, the YAC128, and the CAG140 knock-in. Results revealed that spontaneous EPSCs occurred at a higher frequency, and evoked EPSCs were larger in behaviorally phenotypic mice whereas spontaneous IPSCs were initially increased in frequency in all models and subsequently decreased in R6/2 mice after they displayed the typical R6/2 overt behavioral phenotype. Changes in miniature IPSCs and evoked IPSC paired-pulse ratios suggested altered probability of GABA release. Also, in R6/2 mice, blockade of GABA A receptors induced complex discharges in slices and seizures in vivo at all ages. In conclusion, altered excitatory and inhibitory inputs to pyramidal neurons in the cortex in HD appear to be a prevailing deficit throughout the development of the disease. Furthermore, the differences between synaptic phenotypes in cortex and striatum are important for the development of future therapeutic approaches, which may need to be targeted early in the development of the phenotype.

The current dopamine (DA) hypothesis of schizophrenia postulates striatal hyperdopaminergia and cortical hypodopaminergia. Although partial agonists at DA D2 receptors (D2Rs), like aripiprazole, were developed to simultaneously target both phenomena, they do not effectively improve cortical dysfunction. In this study, we investigate the potential for newly developed β-arrestin2 (βarr2)-biased D2R partial agonists to simultaneously target hyperand hypodopaminergia. Using neuron-specific βarr2-KO mice, we show that the antipsychotic-like effects of a βarr2-biased D2R ligand are driven through both striatal antagonism and cortical agonism of D2R-βarr2 signaling. Furthermore, βarr2-biased D2R agonism enhances firing of cortical fast-spiking interneurons. This enhanced cortical agonism of the biased ligand can be attributed to a lack of G-protein signaling and elevated expression of βarr2 and G proteincoupled receptor (GPCR) kinase 2 in the cortex versus the striatum. Therefore, we propose that βarr2-biased D2R ligands that exert region-selective actions could provide a path to develop more effective antipsychotic therapies.arrestin | antipsychotics | biased signaling | dopamine D2R | fast-spiking interneurons G protein-coupled receptors (GPCRs) represent the largest family of receptors in the human genome and are one of the most common targets of pharmaceutical drugs (1, 2). Upon ligand binding, GPCRs activate downstream G protein-dependent signaling pathways followed by phosphorylation of the receptor by G protein-coupled receptor kinases (GRKs) (3). Phosphorylation enhances association of the GPCR with β-arrestins (βarrs), and this combined process mediates desensitization of G-protein signaling (4) and internalization of GPCRs (5-7). Two isoforms of βarrs, βarr1 and βarr2, are widely coexpressed in most tissues in mammals and are 80% identical, but they can have either overlapping or distinct functions (8, 9). It is now firmly established that GPCRs activate downstream signaling pathways through not only canonical G-protein pathways but also, the ability of βarrs to scaffold distinct intracellular signaling complexes (10-12). Elucidation of these distinct G-protein and βarr signaling pathways has provided support for the concept of functional selectivity or biased signaling, wherein each signaling pathway has the ability to mediate distinct physiological responses (13). There are now several physiologically relevant examples of selective engagement of signaling pathways or selective GPCR ligands that target these different signaling pathways (13-15). Therefore, leveraging the concept of GPCR functional selectivity holds promise for the development of more selective therapeutic approaches. Dopamine (DA) is a catecholamine neurotransmitter that has been implicated in movement, reward, and cognition (16-19) as well as CNS disorders, such as schizophrenia, attention deficit hyperactivity disorder, Parkinson's disease, and obsessive-compulsive disorder (20-23). DA mediates its effects via GPCRs belonging to two major ...

SUMMARY The precise connectivity of inputs and outputs is critical for cerebral cortex function; however, the cellular mechanisms that establish these connections are poorly understood. Here, we show that the secreted molecule Sonic Hedgehog (Shh) is involved in synapse formation of a specific cortical circuit. Shh is expressed in layer V corticofugal projection neurons and the Shh receptor, Brother of CDO (Boc), is expressed in local and callosal projection neurons of layer II/III that synapse onto the subcortical projection neurons. Layer V neurons of mice lacking functional Shh exhibit decreased synapses. Conversely, the loss of functional Boc leads to a reduction in the strength of synaptic connections onto layer Vb, but not layer II/III, pyramidal neurons. These results demonstrate that Shh is expressed in postsynaptic target cells while Boc is expressed in a complementary population of presynaptic input neurons, and they function to guide the formation of cortical microcircuitry.

The D3 dopamine receptor, a member of the G i -coupled D2 family of dopamine receptors, is expressed throughout limbic circuits affected in neuropsychiatric disorders, including prefrontal cortex (PFC). These receptors are important for prefrontal executive function because pharmacological and genetic manipulations that affect prefrontal D3 receptors alter anxiety, social interaction, and reversal learning. However, the mechanisms by which D3 receptors regulate prefrontal circuits and whether D3 receptors regulate specific prefrontal subnetworks remains unknown. Here, we combine dopamine receptor reporter lines, anatomical tracing techniques, and electrophysiology to show that D3 receptor expression defines a novel subclass of layer 5 glutamatergic pyramidal cell in mouse PFC (either sex). D3-receptor-expressing pyramidal neurons are electrophysiologically and anatomically separable from neighboring neurons expressing D1 or D2 receptors based on their dendritic morphology and subthreshold and suprathreshold intrinsic excitability. D3-receptorexpressing neurons send axonal projections to intratelencephalic (IT) targets, including contralateral cortex, nucleus accumbens, and basolateral amygdala. Within these neurons, D3 receptor activation was found to regulate low-voltage-activated Ca V 3.2 calcium channels localized to the axon initial segment, which suppressed action potential (AP) excitability, particularly when APs occurred at high frequency. Therefore, these data indicate that D3 receptors regulate the excitability of a unique, IT prefrontal cell population, thereby defining novel circuitry and cellular actions for D3 receptors in PFC.

Zuranolone (SAGE-217) is a novel, synthetic, clinical stage neuroactive steroid GABA A receptor positive allosteric modulator designed with the pharmacokinetic properties to support oral daily dosing. In vitro , zuranolone enhanced GABA A receptor current at nine unique human recombinant receptor subtypes, including representative receptors for both synaptic (γ subunit-containing) and extrasynaptic (δ subunit-containing) configurations. At a representative synaptic subunit configuration, α 1 β 2 γ 2 , zuranolone potentiated GABA currents synergistically with the benzodiazepine diazepam, consistent with the non-competitive activity and distinct binding sites of the two classes of compounds at synaptic receptors. In a brain slice preparation, zuranolone produced a sustained increase in GABA currents consistent with metabotropic trafficking of GABA A receptors to the cell surface. In vivo , zuranolone exhibited potent activity, indicating its ability to modulate GABA A receptors in the central nervous system after oral dosing by protecting against chemo-convulsant seizures in a mouse model and enhancing electroencephalogram β-frequency power in rats. Together, these data establish zuranolone as a potent and efficacious neuroactive steroid GABA A receptor positive allosteric modulator with drug-like properties and CNS exposure in preclinical models. Recent clinical data support the therapeutic promise of neuroactive steroid GABA A receptor positive modulators for treating mood disorders; brexanolone is the first therapeutic approved specifically for the treatment of postpartum depression. Zuranolone is currently under clinical investigation for the treatment of major depressive episodes in major depressive disorder, postpartum depression, and bipolar depression.

Dysfunction of inhibitory circuits in the rostral anterior cingulate cortex underlies the affective (aversive), but not the sensory-discriminative features (hypersensitivity) of the pain experience. To restore inhibitory controls, we transplanted inhibitory interneuron progenitor cells into the rostral anterior cingulate cortex in a chemotherapy-induced neuropathic pain model. The transplants integrated, exerted a GABA-A mediated inhibition of host pyramidal cells and blocked gabapentin preference (i.e. relieved ongoing pain) in a conditioned place preference paradigm. Surprisingly, pain aversiveness persisted when the transplants populated both the rostral and posterior anterior cingulate cortex. We conclude that selective and long lasting inhibition of the rostral anterior cingulate cortex, in the mouse, has a profound pain relieving effect against nerve injury-induced neuropathic pain. However, the interplay between the rostral and posterior anterior cingulate cortices must be considered when examining circuits that influence ongoing pain and pain aversiveness.