Hiroyuki Katagiri

Facilitation of long-term potentiation and memory in mice lacking nociceptin receptors

The peptide nociceptin (also named orphanin FQ) acts in the brain to produce various pharmacological effects, including hyperalgesia and hypolocomotion,. The nociceptin receptor uses guanine-nucleotide-binding proteins to mediate the inhibition of adenylyl cyclase, the activation of potassium channels and inhibition of calcium channels. It has been shown using knockout mice that the nociceptin receptor is not required for regulation of nociceptive responses or locomotion activity, but modulates the auditory function. Here we show that mice lacking the nociceptin receptor possess greater learning ability and have better memory than control mice. Histological analysis revealed the expression of both the nociceptin precursor and the nociceptin receptor in the hippocampus, thought to take part in aspects of learning and memory. Moreover, the receptor-deficient mice showed larger long-term potentiation in the hippocampal CA1 region than control mice, without apparent changes in presynaptic or postsynaptic electrophysiological properties. These results show that the loss of the nociceptin receptor results in a gain-of-function mutation in both the memory process and the long-term potentiation mechanism in CA1, perhaps as a result of altered intracellular signal transduction systems in neurons.

Separable features of visual cortical plasticity revealed by N-methyl-d-aspartate receptor 2A signaling

How individual receptive field properties are formed in the maturing sensory neocortex remains largely unknown. The shortening of N-methyl-d-aspartate (NMDA) receptor currents by 2A subunit (NR2A) insertion has been proposed to delimit the critical period for experience-dependent refinement of circuits in visual cortex. In mice engineered to maintain prolonged NMDA responses by targeted deletion of NR2A, the sensitivity to monocular deprivation was surprisingly weakened but restricted to the typical critical period and delayed normally by dark rearing from birth. Orientation preference instead failed to mature, occluding further effects of dark rearing. Interestingly, a full ocular dominance plasticity (but not orientation bias) was selectively restored by enhanced inhibition, reflecting an imbalanced excitation in the absence of NR2A. Many of the downstream pathways involved in NMDA signaling are coupled to the receptor through a variety of protein–protein interactions and adaptor molecules. To further investigate a mechanistic dissociation of receptive field properties in the developing visual system, mice carrying a targeted disruption of the NR2A-associated 95-kDa postsynaptic density (PSD95) scaffolding protein were analyzed. Although the development and plasticity of ocular dominance was unaffected, orientation preference again failed to mature in these mice. Taken together, our results demonstrate that the cellular basis generating individual sensory response properties is separable in the developing neocortex.

Requirement of appropriate glutamate concentrations in the synaptic cleft for hippocampal LTP induction

Although glutamate transporters maintain low extracellular levels of the excitatory neurotransmitter glutamate in the nervous system, little is known about their roles in synaptic plasticity. Here, using knockout mice lacking GLT‐1, that is the most abundant glial subtype of glutamate transporters, we showed that long‐term potentiation (LTP) induced by tetanic stimulation in mutant mice was impaired in the hippocampal CA1 region. When tetanic stimulation was applied in the presence of low concentrations of an N‐methyl‐d‐aspartate (NMDA) receptor antagonist, the impairment was overcome. Consistent with these results, the increased glutamate in the synaptic cleft of mutant mice preferentially activated NMDA receptors. Furthermore, analyses of mutant mice revealed that the magnitude of NMDA receptor‐dependent transient synaptic potentiation during low‐frequency stimulation depended on the concentration of glutamate in the synaptic cleft. These findings suggest that GLT‐1 plays critical roles in LTP induction, as well as in short‐term potentiation, through regulation of extracellular levels of glutamate, which enables appropriate NMDA receptor activation.

Optimization of somatic inhibition at critical period onset in mouse visual cortex.

Local GABAergic circuits trigger visual cortical plasticity in early postnatal life. How these diverse connections contribute to critical period onset was investigated by nonstationary fluctuation analysis following laser photo-uncaging of GABA onto discrete sites upon individual pyramidal cells in slices of mouse visual cortex. The GABA(A) receptor number decreased on the soma-proximal dendrite (SPD), but not at the axon initial segment, with age and sensory deprivation. Benzodiazepine sensitivity was also higher on the immature SPD. Too many or too few SPD receptors in immature or dark-reared mice, respectively, were adjusted to critical period levels by benzodiazepine treatment in vivo, which engages ocular dominance plasticity in these animal models. Combining GAD65 deletion with dark rearing from birth confirmed that an intermediate number of SPD receptors enable plasticity. Site-specific optimization of perisomatic GABA response may thus trigger experience-dependent development in visual cortex.

Experience-dependent slow-wave sleep development.

Sleep enhances plasticity in neocortex, and thereby improves sensory learning. Here we show that sleep itself undergoes changes as a consequence of waking experience during a late critical period in cats and mice. Dark-rearing produced a robust and reversible decrement of slow-wave electrical activity during sleep that was restricted to visual cortex and impaired by gene-targeted reduction of NMDA receptor function.

Calcium- and calmodulin-dependent phosphorylation of AMPA type glutamate receptor subunits by endogenous protein kinases in the post-synaptic density.

We have detected immunoreactivities of AMPA receptor subunits GluR1-4 in post-synaptic density (PSD) fraction and tested whether they can be phosphorylated by endogenous kinases. Incubation of PSD with Ca2+ and calmodulin increased phosphorylation of GluR1 and GluR2/3. The phosphorylation of GluR1 was largely blocked by a Ca2+/calmodulin-dependent protein kinase type II inhibitor. Thus Ca2+/calmodulin-dependent phosphorylation of glutamate receptor may be a mechanism underlying enhanced post-synaptic receptor responsiveness in LTP.

The reduction of growth hormone enhances the efficacy of basal synaptic transmission and LTP in the rat hippocampal CA1 region

Here we investigated the distribution of GluR1 subunit of AMPA type glutamate receptor (AMPAR) by immunoelectron microscopy. In the developing phase of RISE, the density of GluR1-immunoreactive signal (IRS) increased 1.7 times that of unstimulated specimen, in contrast to unchanged IRS density in the late phase of conventional LTP. When we classified the location of the GluR1-IRS to 4 classes (intradendritic [= in the dendritic cytoplasm], epidendritic [= on the dendritic surface], spinal and others), the epidendritic and spinal populations increased in both RISE and LTP specimens. It is known that AMPAR lacking GluR2 permeates Ca2+. Western blotting analyses revealed increased expression of GluR1, but not of GluR2 in RISE specimen. The above results suggest that the population of increased AMPAR during the RISE development is GluR2-lacking.This notion coincides well with our previous result that Ca2+-permeable AMPAR (suppressed by JoroSpider Toxin) appeared transiently during the RISE development. So we correlated the location of the spinal GluR1 population to the size of postsynaptic density (PSD), assuming that small synapses are newly formed ones. Relative frequency distribution analysis revealed that the GluR1-IRS was expressed preferentially in the small-PSD (i.e. ≤300 nm) synapses. The present results support a hypothesis that the increased expression of GluR1 in the developing phase of RISE, through forming Ca2+-permeable AMPAR, leads to the synaptogenesis.