Mineto Yokoi

Impairment of hippocampal mossy fiber LTD in mice lacking mGluR2

Subtype 2 of the metabotropic glutamate receptor (mGluR2) is expressed in the presynaptic elements of hippocampal mossy fiber—CA3 synapses. Knockout mice deficient in mGluR2 showed no histological changes and no alterations in basal synaptic transmission, paired-pulse facilitation, or tetanus-induced long-term potentiation (LTP) at the mossy fiber—CA3 synapses. Long-term depression (LTD) induced by low-frequency stimulation, however, was almost fully abolished. The mutant mice performed normally in water maze learning tasks. Thus, the presynaptic mGluR2 is essential for inducing LTD at the mossy fiber—CA3 synapses, but this hippocampal LTD does not seem to be required for spatial learning.

Ablation of cerebellar Golgi cells disrupts synaptic integration involving GABA inhibition and NMDA receptor activation in motor coordination.

The role of inhibitory Golgi cells in cerebellar function was investigated by selectively ablating Golgi cells expressing human interleukin-2 receptor alpha subunit in transgenic mice, using the immunotoxin-mediated cell targeting technique. Golgi cell disruption caused severe acute motor disorders. These mice showed gradual recovery but retained a continuing inability to perform compound movements. Optical and electrical recordings combined with immunocytological analysis indicated that elimination of Golgi cells not only reduces GABA-mediated inhibition but also attenuates functional NMDA receptors in granule cells. These results demonstrate that synaptic integration involving both GABA inhibition and NMDA receptor activation is essential for compound motor coordination. Furthermore, this integration can adapt after Golgi cell elimination so as not to evoke overexcitation by the reduction of NMDA receptors.

Increased social interaction in mice deficient of the striatal medium spiny neuron-specific phosphodiesterase 10A2.

Cyclic nucleotide phosphodiesterase 10A (PDE10A) is a member of phosphodiesterase families that degrade cAMP and/or cGMP in distinct intracellular sites. PDE10A has a dual activity on hydrolysis of both cAMP and cGMP, and is prominently expressed in the striatum and the testis. Previous studies suggested that PDE10A is involved in regulation of locomotor activity and potentially related to psychosis, but concrete physiological roles of PDE10A remains elusive yet. In this study, we genetically inactivated PDE10A2, a prominent isoform of PDE10A in the brain, in mice, and demonstrate that PDE10A2 deficiency results in increased social interaction without any major influence on different other behaviors, along with increased levels of striatal cAMP. We also demonstrate that PDE10A2 is selectively distributed in medium spiny neurons, but not interneurons, of the striatal complex. Thus, our results establish a physiological role for PDE10A2 in regulating cAMP pathway and social interaction, and suggest that cAMP signaling cascade in striatal medium spiny neurons might be involved in regulating social interaction behavior in mice.

Experience-dependent plasticity without long-term depression by type 2 metabotropic glutamate receptors in developing visual cortex

Synaptic depression is thought to underlie the loss of cortical responsiveness to an eye deprived of vision. Here, we establish a fundamental role for type 2 metabotropic glutamate receptors (mGluR2) in long-term depression (LTD) of synaptic transmission within primary visual cortex. Direct mGluR2 activation by (2S,2′R,3′R-2-(2′,3′-dicarboxycyclopropyl)glycine (DCG-IV) persistently depressed layer 2/3 field potentials in slices of mouse binocular zone when stimulated concomitantly. Chemical LTD was independent of N-methyl-d-aspartate (NMDA) receptors but occluded conventional LTD by low-frequency stimulation, indicating shared downstream events. Antagonists or targeted disruption of mGluR2 conversely prevented LTD induction by electrical low-frequency stimulation to layer 4. In contrast, Schaeffer collateral synapses did not exhibit chemical LTD, revealing hippocampal area CA1, naturally devoid of mGluR2, to be an inappropriate model for neocortical plasticity. Moreover, monocular deprivation remained effective in mice lacking mGluR2, and receptor expression levels were unchanged during the critical period in wild-type mice, indicating that experience-dependent plasticity is independent of LTD induction in visual cortex. Short-term depression that was unaffected by mGluR2 deletion may better reflect circuit refinement in vivo.

Striatal Medium Spiny Neurons Terminate in a Distinct Region in the Lateral Hypothalamic Area and Do Not Directly Innervate Orexin/Hypocretin- or Melanin-Concentrating Hormone-Containing Neurons

Neuronal circuits including medium spiny neurons (MSNs) in the nucleus accumbens (NAc) and melanin-concentrating hormone (MCH)-containing neurons in the lateral hypothalamic area (LHA) are hypothesized to play an important role in hedonic feeding. A reciprocal connection between NAc MSNs and MCH-containing neurons is proposed to form a neuronal circuit that is involved in hedonic feeding. Although NAc MSNs have been shown to receive projection from MCH-containing neurons, it is not known whether MCH-containing neurons in the LHA also receive direct inputs from NAc MSNs. Here, we developed a genetic approach that allows us to visualize almost all striatal MSNs including NAc MSNs. We demonstrate that striatal MSNs terminate in a distinct region within the anterior LHA, and that the terminal area of striatal MSNs in this region contains glutamatergic neurons and is distinctly separate from orexin/hypocretin- or MCH-containing neurons. These observations suggest that NAc MSNs do not directly innervate MCH-containing neurons, but may indirectly signal MCH-containing neurons via glutamatergic neurons in the anterior LHA.

Conditional genetic labeling of mitral cells of the mouse accessory olfactory bulb to visualize the organization of their apical dendritic tufts

Chemical information captured through the vomeronasal sensory neurons is transmitted to mitral cells in the accessory olfactory bulb (AOB). Morphological characterization of AOB mitral cells is crucial to reveal the mechanisms underlying pheromone and other chemical information processing. Here, we developed a conditional genetic approach to visualize single AOB mitral cells in mice and analyzed the distribution of their apical dendritic tufts and cell bodies. We found that there is a subpopulation of superficial AOB mitral cells with small cell bodies and simple dendritic arborization. We also showed that segregation of the anterior and posterior AOB appears incomplete at the level of the mitral cell positions. Furthermore, we demonstrated that significant population of the anterior AOB mitral cells has multiple apical dendritic tufts organized in a quite small range along the dorsal-ventral axis. These findings have important implications in how various chemical signals may be processed in the AOB.

Regulation of synaptic currents by mGluR2 at reciprocal synapses in the mouse accessory olfactory bulb

The throughput of information from the accessory olfactory bulb (AOB) to downstream structures is controlled by reciprocal dendrodendritic inhibition of mitral cells by granule cells. Given the high expression levels of mGluR2, a metabotropic glutamate receptor, in the AOB and the fact that the activation of mGluR2 permits the formation of a specific olfactory memory, we reasoned that mGluR2 might play an important role in regulating dendrodendritic inhibition. To test this hypothesis, we examined the effects of pharmacological and genetic manipulations of mGluR2 on synaptic responses measured from mitral or granule cells in slice preparations from 23‐ to 36‐day‐old Balb/c mice. To evoke dendrodendritic inhibition, a depolarizing voltage step from –70 to 0 mV or a threshold current stimulus adjusted to elicit action potential(s) was applied to a mitral cell using either a nystatin‐perforated or conventional whole‐cell configuration. We found that an agonist for group II metabotropic glutamate receptors (mGluR2/mGluR3), DCG‐IV [(2S,1′R,2′R,3′R)‐2‐(2,3‐dicarboxycyclopropyl)glycine], suppressed, whereas the mGluR2/mGluR3 antagonist LY341495 [(αS)‐α‐amino‐α‐[(1S,2S)‐2‐carboxycyclopropyl]‐9H‐xanthine‐9‐propanoic acid] enhanced dendrodendritic inhibition. Genetic ablation of mGluR2 markedly impaired the effects of DCG‐IV and LY341495 on dendrodendritic inhibition. DCG‐IV reduced both the frequency and the amplitude of spontaneous miniature excitatory postsynaptic currents recorded from granule cells. Additionally, DCG‐IV inhibited high‐voltage‐activated calcium currents in both mitral and granule cells. These results suggest that mGluR2 reduces dendrodendritic inhibition by inhibiting synaptic transmission between mitral cells and granule cells in the AOB.

Inducible expression of retrograde transynaptic genetic tracer in mice

A key step towards understanding the development and function of the central nervous system is by characterizing the connections between neurons. Tetanus toxin C fragment (TTC) is transynaptically and retrogradely transported without the toxins pathogenic effect, and therefore, recently it has been used as a genetic tracer combined with beta-galactosidase or green fluorescent protein. Here, we introduce a new fusion construct, APTTC, consisting of the truncated human placental alkaline phosphatase with TTC, and generating the transgenic mouse line, (tetracycline operator) tetO-APTTC, for inducible expression of APTTC regulated by tetO. We demonstrate that APTTC is transported retrogradely and transynaptically, and allows us to robustly visualize the inputs of the expressing neurons when transgenetically expressed in mice, exemplified in the striatal neuronal circuit. Therefore, tetO-APTTC transgenic mouse line can be widely used for visualization of neuronal connectivity when combined with mice carrying tetracycline-controlled transactivator (tTA) in any specific neurons.

205 An essential role for presynaptic metabotropic glutamate receptors in the induction of mossy fiber ltd

-203.. MULTIPLE CLIMBING FIBER INNERVATION OF CEREBELLAR PLJRKINJE CELLS IN METABOTROPIC GLUTAMATE RECEPTOR-l (mGluR1) MUTANT MICE. MASANOBU KANO’~2, 2 1 01, Japan. “Dept. of Physiol.. Jichi Med. Sch., Minamikawachi-machi. Tochigi 329-04. Janan. 3Dept. of Anat., Hokkaido Uni. Sch. of Med.. Sapporo 606. Japan. 4Med. Inst. for Biorepulation. Kvushu Uni.. Fukuoka 812-82. Japan. 3Howard Hushes Medical Institute, Cent. for Lcarnine and Memorv. MIT, Cambridpe. MA 02139. USA.