Chemogenetic Suppression of the Subthalamic Nucleus Induces Attentional Deficits and Impulsive Action in a Five-Choice Serial Reaction Time Task in Mice

The subthalamic nucleus (STN), a key component of the basal ganglia circuitry, receives inputs from broad cerebral cortical areas and relays cortical activity to subcortical structures. Recent human and animal studies have suggested that executive function, which is assumed to consist of a set of different cognitive processes for controlling behavior, depends on precise information processing between the cerebral cortex and subcortical structures, leading to the idea that the STN contains neurons that transmit the information required for cognitive processing through their activity, and is involved in such cognitive control directly and dynamically. On the other hand, the STN activity also affects intracellular signal transduction and gene expression profiles influencing plasticity in other basal ganglia components. The STN may also indirectly contribute to information processing for cognitive control in other brain areas by regulating slower signaling mechanisms. However, the precise correspondence and causal relationship between the STN activity and cognitive processes are not fully understood. To address how the STN activity is involved in cognitive processes for controlling behavior, we applied Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-based chemogenetic manipulation of neural activity to behavioral analysis using a touchscreen operant platform. We subjected mice selectively expressing DREADD receptors in the STN neurons to a five-choice serial reaction time task, which has been developed to quantitatively measure executive function. Chemogenetic suppression of the STN activity reversibly impaired attention, especially required under highly demanding conditions, and increased impulsivity but not compulsivity. These findings, taken together with the results of previous lesion studies, suggest that the STN activity, directly and indirectly, participates in cognitive processing for controlling behavior, and dynamically regulates specific types of subprocesses in cognitive control probably through fast synaptic transmission.

Generation of a MOR-CreER knock-in mouse line to study cells and neural circuits involved in mu opioid receptor signaling

Mu opioid receptor (MOR) is involved in various brain functions, such as pain modulation, reward processing, and addictive behaviors, and mediates the main pharmacologic effects of morphine and other opioid compounds. To gain genetic access to MOR-expressing cells, and to study physiological and pathological roles of MOR signaling, we generated a MOR-CreER knock-in mouse line, in which the stop codon of the Oprm1 gene was replaced by a DNA fragment encoding a T2A peptide and tamoxifen (Tm)-inducible Cre recombinase. We show that the MOR-CreER allele undergoes Tm-dependent recombination in a discrete subtype of neurons that express MOR in the adult nervous system, including the olfactory bulb, cerebral cortex, striosome compartments in the striatum, hippocampus, amygdala, thalamus, hypothalamus, interpeduncular nucleus, superior and inferior colliculi, periaqueductal gray, parabrachial nuclei, cochlear nucleus, raphe nuclei, pontine and medullary reticular formation, ambiguus nucleus, solitary nucleus, spinal cord, and dorsal root ganglia. The MOR-CreER mouse line combined with a Cre-dependent adeno-associated virus vector enables robust gene manipulation in the MOR-enriched striosomes. Furthermore, Tm treatment during prenatal development effectively induces Cre-mediated recombination. Thus, the MOR-CreER mouse is a powerful tool to study MOR-expressing cells with conditional gene manipulation in developing and mature neural tissues.

In vivo calcium imaging from dentate granule cells with wide-field fluorescence microscopy

A combination of genetically-encoded calcium indicators and micro-optics has enabled monitoring of large-scale dynamics of neuronal activity from behaving animals. In these studies, wide-field microscopy is often used to visualize neural activity. However, this method lacks optical sectioning capability, and therefore its axial resolution is generally poor. At present, it is unclear whether wide-field microscopy can visualize activity of densely packed small neurons at cellular resolution. To examine the applicability of wide-field microscopy for small-sized neurons, we recorded calcium activity of dentate granule cells having a small soma diameter of approximately 10 micrometers. Using a combination of high numerical aperture (0.8) objective lens and independent component analysis-based image segmentation technique, activity of putative single granule cell activity was separated from wide-field calcium imaging data. The result encourages wider application of wide-field microscopy in in vivo neurophysiology.

Distinct dopaminergic control of the direct and indirect pathways in reward-based and avoidance learning behaviors

The nucleus accumbens (NAc) plays a pivotal role in reward and aversive learning and learning flexibility. Outputs of the NAc are transmitted through two parallel routes termed the direct and indirect pathways and controlled by the dopamine (DA) neurotransmitter. To explore how reward-based and avoidance learning is controlled in the NAc of the mouse, we developed the reversible neurotransmission-blocking (RNB) technique, in which transmission of each pathway could be selectively and reversibly blocked by the pathway-specific expression of transmission-blocking tetanus toxin and the asymmetric RNB technique, in which one side of the NAc was blocked by the RNB technique and the other intact side was pharmacologically manipulated by a transmitter agonist or antagonist. Our studies demonstrated that the activation of D1 receptors in the direct pathway and the inactivation of D2 receptors in the indirect pathway are key determinants that distinctly control reward-based and avoidance learning, respectively. The D2 receptor inactivation is also critical for flexibility of reward learning. Furthermore, reward and aversive learning is regulated by a set of common downstream receptors and signaling cascades, all of which are involved in the induction of long-term potentiation at cortico-accumbens synapses of the two pathways. In this article, we review our studies that specify the regulatory mechanisms of each pathway in learning behavior and propose a mechanistic model to explain how dynamic DA modulation promotes selection of actions that achieve reward-seeking outcomes and avoid aversive ones. The biological significance of the network organization consisting of two parallel transmission pathways is also discussed from the point of effective and prompt selection of neural outcomes in the neural network.

Spatio-temporal control of neural activity in vivo using fluorescence microendoscopy

Controlling neural activity with high spatio-temporal resolution is desired for studying how neural circuit dynamics control animal behavior. Conventional methods for manipulating neural activity, such as electrical microstimulation or pharmacological blockade, have poor spatial and/or temporal resolution. Algal protein channelrhodopsin-2 (ChR2) enables millisecond-precision control of neural activity. However, a photostimulation method for high spatial resolution mapping in vivo is yet to be established. Here, we report a novel optical/electrical probe, consisting of optical fiber bundles and metal electrodes. Optical fiber bundles were used as a brain-insertable endoscope for image transfer and stimulating light delivery. Light-induced activity from ChR2-expressing neurons was detected with electrodes bundled to the endoscope, enabling verification of light-evoked action potentials. Photostimulation through optical fiber bundles of transgenic mice expressing ChR2 in layer 5 cortical neurons resulted in single-whisker movement, indicating spatially restricted activation of neurons in vivo. The probe system described here and a combination of various photoactive molecules will facilitate studies on the causal link between specific neural activity patterns and behavior.

Membrane-proximal region of glutamate receptor delta2 subunit is critical for long-term depression and interaction with protein interacting with C kinase 1 in a cerebellar Purkinje neuron

The glutamate receptor delta2 subunit (GluRdelta2) is selectively expressed in cerebellar Purkinje neurons (PNs) and is involved in the long-term depression (LTD). However, little is known about the mechanism of its action. Acute expression of the wild-type GluRdelta2 in the GluRdelta2-deficient PN rescued the induction of LTD, suggesting the direct role of GluRdelta2 in LTD. To identify the critical region of GluRdelta2 necessary for LTD, we constructed and expressed various mutant GluRdelta2 proteins in the GluRdelta2-deficient PNs. The mutant GluRdelta2 possessing the membrane-proximal 21 aa residues in the C-terminal cytoplasmic region rescued the induction of LTD, whereas the mutant with membrane-proximal 13 aa failed. In addition, overexpression of 865 approximately 871 aa of GluRdelta2 (corresponding to membrane-proximal 14-20 aa) fused to EGFP (enhanced green fluorescent protein) suppressed LTD in a wild-type PN. These results suggest that 865 approximately 871 aa of GluRdelta2 play an essential role in LTD. We next identified protein interacting with C kinase 1 (PICK1) as a molecule interacting with the membrane-proximal C-terminal region of GluRdelta2 by yeast two-hybrid screening. PICK1 plays an essential role in LTD. It colocalized with GluRdelta2 at spines of PNs, and immunoprecipitation assays showed that GluRdelta2 bound to PICK1 mainly through 865-871 aa. These results indicate that 865-871 aa of GluRdelta2 are essential for both LTD and interaction with PICK1, and suggest that interaction between GluRdelta2 and PICK1 might be critical for the induction of LTD.

MR imaging of Wallerian degeneration in the brainstem: temporal relationships

Degeneration of the myelin sheath and axon distal to the most proximal site of axonal interruption secondary to axonal disease has been called wallerian degeneration. On MR imaging, wallerian degeneration of the pyramidal tract can be observed as an abnormal signal intensity, showing prolonged T1 and T2 relaxation times that correspond to the corticospinal tract, with or without shrinkage of the ipsilateral cerebral peduncle and pons. Review of MR studies in 150 cases of supratentorial cerebrovascular accidents showed abnormal signal alterations in the ipsilateral brainstem in 33 of the cases. Abnormal intensity in the ipsilateral brainstem was seen as early as 5 weeks after the supratentorial ictus and was fully evident after 10 weeks in all 33 cases. Signal alterations were strongest at about 3-6 months when compared with alterations seen at 10 weeks or even 10 months after the ictus. Shrinkage of the ipsilateral brainstem appeared as early as 8 months and was demonstrated in all cases 13 months after the ictus. MR seems to be the most effective technique for early detection of wallerian degeneration and may provide insight into its pathophysiological and chemical changes.

Hydroxyapatite-reactive salivary protein revealed by iso-electrofocusing electrophoresis

Salivary protein involvement in the formation of acquired enamel pellicle, so far, has been discussed in terms of hydroxyapatite (HA)-reactive salivary proteins only from the parotid gland. This study was undertaken to seek this type of protein in the human whole (mixed) saliva and to investigate its normal and pathological variations. Several kinds of hydroxyapatite, either biogenous or synthesized by solid phase reaction, were used as a powder (250 mesh). HA was incubated with concentrated whole saliva at 25 degrees for 30 min. After centrifugation and filtration salivary proteins were analysed on a Multiphor isoelectrofocusing gel electrophoresis. The control salivary proteins were separated into three major groups; acidic (A1-A8), neutral neutral (N1-N4), and basic (B1-B3) isoelectric point (pI). In the HA incubated sample, one of the major neutral bands (NI) preferentially disappeared at about pI 7.5. This NI band was missing or scarce in the parotid saliva and had an amino acid composition rich in glycine, lysine, serine, glutamic acid, aspartic acid, and histidine. This protein was considered to be one of the major HA-reactive proteins in human whole saliva.