Motor skill learning is characterized by improved performance and reduced motor variability. The neural mechanisms that couple skill level and variability, however, are not known. The zebra finch, a songbird, presents a unique opportunity to address this question because production of learned song and induction of vocal variability are instantiated in distinct circuits that converge on a motor cortex analogue controlling vocal output. To probe the interplay between learning and variability, we made intracellular recordings from neurons in this area, characterizing how their inputs from the functionally distinct pathways change throughout song development. We found that inputs that drive stereotyped song-patterns are strengthened and pruned, while inputs that induce variability remain unchanged. A simple network model showed that strengthening and pruning of action-specific connections reduces the sensitivity of motor control circuits to variable input and neural ‘noise’. This identifies a simple and general mechanism for learning-related regulation of motor variability.DOI: http://dx.doi.org/10.7554/eLife.03697.001

Deficits in fast-spiking inhibitory interneurons (FSINs) within the dorsolateral prefrontal cortex (dlPFC) are hypothesized to underlie cognitive impairment associated with schizophrenia. Though representing a minority of interneurons, this key cell type coordinates broad neural network gamma-frequency oscillations, associated with cognition and cognitive flexibility. Here we report expression of GluN2D mRNA selectively in parvalbumin positive cells of human postmortem dlPFC tissue, but not pyramidal neurons, with little to no GluN2C expression in either cell type. In acute murine mPFC slices the GluN2C/D selective positive allosteric modulator (PAM), CIQ(+), increased the intrinsic excitability as well as enhanced NMDAR-mediated EPSCs onto FSINs. This increase in intrinsic excitability with GluN2C/D PAM was also observed in the Dlx 5/6+/-FSIN developmental deficit model with reported FSIN hypoexcitability. Together these data speak to selective modulation of FSINs by a GluN2D PAM, providing a potential mechanism to counter the FSIN-deficit seen in schizophrenia.

Abnormal hippocampal activity has been linked to impaired cognitive performance in Alzheimer's disease and schizophrenia, leading to a hypothesis that normalization of this activity may be therapeutically beneficial. Our work suggests that one approach for hippocampal normalization may be through activation of the M4 muscarinic acetylcholine receptor. We used a brain penetrant M4 muscarinic acetylcholine receptor selective activator, PT-3763, to show dose-dependent attenuation of field potentials in Schaffer collateral (CA3-CA1) and recurrent associational connections (CA3-CA3) ex vivo in hippocampal slices. In vivo, systemic administration of PT-3763 led to attenuation of glutamate release in CA3 as measured by amperometry and to a dose-dependent decrease in population CA1 pyramidal activity as measured by fiber photometry. This decrease in population activity was also evident with a localized administration of the compound to the recorded site. Finally, PT-3763 reversed scopolamine-induced deficit in Morris water maze. Our results suggest that M4 muscarinic acetylcholine receptor activation may be a suitable therapeutic treatment in diseases associated with hyperactive hippocampal activity.

In this study, we reveal the existence of a novel use-dependent phenomenon in potassium channels, which we refer to as cumulative activation (CA). CA consists of an increase in current amplitude in response to repetitive depolarizing step pulses to the same potential. CA persists for up to 20 s and is similar to a phenomenon called "voltage-dependent facilitation" observed in some calcium channels. The KVS-1 K ϩ channel, which exhibits CA, is a rapidly activating and inactivating voltage-dependent potassium channel expressed in chemosensory and other neurons of Caenorhabditis elegans. It is unusual in being most closely related to the Shab (Kv2) family of potassium channels, which typically behave like delayed rectifier K ϩ channels in other species. The magnitude of CA depends on the frequency, voltage, and duration of the depolarizing step pulse. CA also radically changes the activation and inactivation kinetics of the channel, suggesting that the channel may undergo a physical modification in a use-dependent manner; thus, a model that closely simulates the behavior of the channel postulates the existence of two populations of channels, unmodified and modified. Use-dependent changes in the behavior of potassium channels, such as CA observed in KVS-1, could be involved in functional mechanisms of cellular plasticity such as synaptic depression that represent the cellular basis of learning and memory.

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