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Direct gene delivery to the neurons of interest, without affecting other neuron populations in the cerebral cortex, represent a challenge owing to the heterogeneity and cellular complexity of the brain. Genetic modulation of corticospinal motor neurons (CSMN) is required for developing effective and long-term treatment strategies for motor neuron diseases, in which voluntary movement is impaired. Adeno-associated viruses (AAV) have been widely used for neuronal transduction studies owing to long-term and stable gene expression as well as low immunoreactivity in humans. Here we report that AAV2-2 transduces CSMN with high efficiency upon direct cortex injection and that transduction efficiencies are similar during presymptomatic and symptomatic stages in hSOD1(G93A) transgenic amyotrophic lateral sclerosis (ALS) mice. Our findings reveal that choice of promoter improves selectivity as AAV2-2 chicken β-actin promoter injection results in about 70% CSMN transduction, the highest percentage reported to date. CSMN transduction in both wild-type and transgenic ALS mice allows detailed analysis of single axon fibers within the corticospinal tract in both cervical and lumbar spinal cord and reveals circuitry defects, which mainly occur between CSMN and spinal motor neurons in hSOD1(G93A) transgenic ALS mice. Our findings set the stage for CSMN gene therapy in ALS and related motor neuron diseases.

The dynamic range of visual coding is extended by having separate ganglion cell types that respond to light increments and decrements. Although the primordial color vision system in mammals contains a well-characterized ganglion cell that responds to blue light increments (a blue On center cell), less is known about ganglion cells that respond to blue light decrements (blue Off center cells). We identified a regular mosaic of blue Off center ganglion cells in the ground squirrel. Contrary to the standard scheme, blue Off responses came from a blue On bipolar and inverting amacrine cell.

Rod photoreceptors were recently shown to contact 'Off' cone bipolar cells, providing an alternative pathway for rod signal flow in the mammalian retina. By recording from pairs of rods and Off cone bipolar cells in the ground squirrel (Spermophilus tridecemlineatus), we measured the synaptic responses of mammalian rods unfiltered by the slow kinetics of the rod bipolar cell response. We show that vesicle fusion and turnover in mammalian rods is fast, and that this new pathway can mediate rapid signaling.

The mammalian retina is fundamentally dichromatic, with trichromacy only recently emerging in some primates. In dichromats, an array of short wavelength-sensitive (S, blue) and middle wavelength-sensitive (M, green) cones is sampled by approximately ten bipolar cell types, and the sampling pattern determines how retinal ganglion cells and ultimately higher visual centers encode color and luminance. By recording from cone-bipolar cell pairs in the retina of the ground squirrel, we show that the bipolar cell types sample cone signals in three ways: one type receives input exclusively from S-cones, two types receive mixed S/M-cone input and the remaining types receive an almost pure M-cone signal. Bipolar cells that carry S- or M-cone signals can have a role in color discrimination and may contact color-opponent ganglion cells. Bipolar cells that sum signals from S- and M-cones may signal to ganglion cells that encode luminance.

The distinct absorbance spectra of the cone photopigments form the basis of color vision, but ultrastructural and physiological evidence shows that mammalian cones are electrically coupled. Coupling between cones of the same spectral type should average voltage noise in adjacent photoreceptors and improve the ability to resolve low-contrast spatial patterns. However, indiscriminate coupling between spectral types could compromise color vision by smearing chromatic information across channels. Here we show, by measuring the junctional conductance between green-green and blue-green cone pairs in slices from the dichromatic ground-squirrel retina, that green-green cone pairs are routinely coupled with an average conductance of 220 pS, whereas coupling is undetectable in blue-green cone pairs. Together with a lack of tracer coupling and the selective localization of connexin proteins, our results show that signals in blue and green cones are processed separately in the photoreceptor layer.

Light produces a graded hyperpolarization in retinal photoreceptors that decreases their release of synaptic neurotransmitter. Cone photoreceptors use glutamate as a neurotransmitter with which to communicate with two types of bipolar cell. Activation of metabotropic glutamate receptors in 'On' bipolar cells initiates a second-messenger cascade that can amplify small synaptic inputs from cones. In contrast, it is not known how the ionotropic glutamate receptors that are activated in 'Off' bipolar cells are optimized for transmitting small, graded signals. Here we show, by recording from a cone and a synaptically connected 'Off' bipolar cell in slices of retina from the ground squirrel, that transmission is mediated by glutamate receptors of the kainate-preferring subtype. In the dark, a cone releases sufficient neurotransmitter to desensitize most postsynaptic kainate receptors. The small postsynaptic current that persists (<5% of maximum) is quickly modulated by changes in presynaptic voltage. Since recovery from desensitization is slow (the decay time constant is roughly 500 milliseconds), little recovery can occur during the brief (roughly 100-millisecond) hyperpolarization that is produced in cones by a flash of light. By limiting the postsynaptic current, receptor desensitization prevents saturation of the 'Off' bipolar cell's voltage response and allows the synapse to operate over the cone's entire physiological voltage range.