Laminarly orthogonal excitation of fast-spiking and low-threshold-spiking interneurons in mouse motor cortex.

In motor cortex, long-range output to subcortical motor circuits depends on excitatory and inhibitory inputs converging on projection neurons in layers 5A/B. How interneurons interconnect with these projection neurons, and whether these microcircuits are interneuron and/or projection specific, is unclear. We found that fast-spiking interneurons received strong intralaminar (horizontal) excitation from pyramidal neurons in layers 5A/B including corticostriatal and corticospinal neurons, implicating them in mediating disynaptic recurrent, feedforward, and feedback inhibition within and across the two projection classes. Low-threshold-spiking (LTS) interneurons were instead strongly excited by descending interlaminar (vertical) input from layer 2/3 pyramidal neurons, implicating them in mediating disynaptic feedforward inhibition to both projection classes. Furthermore, in a novel pattern, lower layer 2/3 preferentially excited interneurons in one layer (5A/LTS) and excitatory neurons in another (5B/corticospinal). Thus, these inhibitory microcircuits in mouse motor cortex follow an orderly arrangement that is laminarly orthogonalized by interneuron-specific, projection-nonspecific connectivity.

NMDA receptor phosphorylation at a site affected in schizophrenia controls synaptic and behavioral plasticity.

Phosphorylation of the NR1 subunit of NMDA receptors (NMDARs) at serine (S) 897 is markedly reduced in schizophrenia patients. However, the role of NR1 S897 phosphorylation in normal synaptic function and adaptive behaviors are unknown. To address these questions, we generated mice in which the NR1 S897 is replaced with alanine (A). This knock-in mutation causes severe impairment in NMDAR synaptic incorporation and NMDAR-mediated synaptic transmission. Furthermore, the phosphomutant animals have reduced AMPA receptor (AMPAR)-mediated synaptic transmission, decreased AMPAR GluR1 subunit in the synapse, and impaired long-term potentiation. Finally, the mutant mice exhibit behavioral deficits in social interaction and sensorimotor gating. Our results suggest that an impairment in NR1 phosphorylation leads to glutamatergic hypofunction that can contribute to behavioral deficits associated with psychiatric disorders.

In vivo imaging of C. elegans ASH neurons: cellular response and adaptation to chemical repellents.

ASH sensory neurons are required in Caenorhabditis elegans for a wide range of avoidance behaviors in response to chemical repellents, high osmotic solutions and nose touch. The ASH neurons are therefore hypothesized to be polymodal nociceptive neurons. To understand the nature of polymodal sensory response and adaptation at the cellular level, we expressed the calcium indicator protein cameleon in ASH and analyzed intracellular Ca(2+) responses following stimulation with chemical repellents, osmotic shock and nose touch. We found that a variety of noxious stimuli evoked strong responses in ASH including quinine, denatonium, detergents, heavy metals, both hyper- and hypo-osmotic shock and nose touch. We observed that repeated chemical stimulation led to a reversible reduction in the magnitude of the sensory response, indicating that adaptation occurs within the ASH sensory neuron. A key component of ASH adaptation is GPC-1, a G-protein gamma-subunit expressed specifically in chemosensory neurons. We hypothesize that G-protein gamma-subunit heterogeneity provides a mechanism for repellent-specific adaptation, which could facilitate discrimination of a variety of repellents by these polymodal sensory neurons.

Specific polyunsaturated fatty acids drive TRPV-dependent sensory signaling in vivo.

A variety of lipid and lipid-derived molecules can modulate TRP cation channel activity, but the identity of the lipids that affect TRP channel function in vivo is unknown. Here, we use genetic and behavioral analysis in the nematode C. elegans to implicate a subset of 20-carbon polyunsaturated fatty acids (PUFAs) in TRPV channel-dependent olfactory and nociceptive behaviors. Olfactory and nociceptive TRPV signaling are sustained by overlapping but nonidentical sets of 20-carbon PUFAs including eicosapentaenoic acid (EPA) and arachidonic acid (AA). PUFAs act upstream of TRPV family channels in sensory transduction. Short-term dietary supplementation with PUFAs can rescue PUFA biosynthetic mutants, and exogenous PUFAs elicit rapid TRPV-dependent calcium transients in sensory neurons, bypassing the normal requirement for PUFA synthesis. These results suggest that a subset of PUFAs with omega-3 and omega-6 acyl groups act as endogenous modulators of TRPV signal transduction.

G protein-coupled receptor kinase function is essential for chemosensation in C. elegans.

G protein-coupled receptors (GPCRs) mediate diverse signaling processes, including olfaction. G protein-coupled receptor kinases (GRKs) are important regulators of G protein signal transduction that specifically phosphorylate activated GPCRs to terminate signaling. Despite previously described roles for GRKs in GPCR signal downregulation, animals lacking C. elegans G protein-coupled receptor kinase-2 (Ce-grk-2) function are not hypersensitive to odorants. Instead, decreased Ce-grk-2 function in adult sensory neurons profoundly disrupts chemosensation, based on both behavioral analysis and Ca(2+) imaging. Although mammalian arrestin proteins cooperate with GRKs in receptor desensitization, loss of C. elegans arrestin-1 (arr-1) does not disrupt chemosensation. Either overexpression of the C. elegans Galpha subunit odr-3 or loss of eat-16, which encodes a regulator of G protein signaling (RGS) protein, restores chemosensation in Ce-grk-2 mutants. These results demonstrate that loss of GRK function can lead to reduced GPCR signal transduction and suggest an important role for RGS proteins in the regulation of chemosensation.