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Neurotransmission in the hippocampus is modulated variously through presynaptic metabotropic glutamate receptors (mGluRs). To establish the precise localization of presynaptic mGluRs in the rat hippocampus, we used subtype-specific antibodies for eight mGluRs (mGluR1-mGluR8) for immunohistochemistry combined with lesioning of the three major hippocampal pathways: the perforant path, mossy fiber, and Schaffer collateral. Immunoreactivity for group II (mGluR2) and group III (mGluR4a, mGluR7a, mGluR7b, and mGluR8) mGluRs was predominantly localized to presynaptic elements, whereas that for group I mGluRs (mGluR1 and mGluR5) was localized to postsynaptic elements. The medial perforant path was strongly immunoreactive for mGluR2 and mGluR7a throughout the hippocampus, and the lateral perforant path was prominently immunoreactive for mGluR8 in the dentate gyrus and CA3 area. The mossy fiber was labeled for mGluR2, mGluR7a, and mGluR7b, whereas the Schaffer collateral was labeled only for mGluR7a. Electron microscopy further revealed the spatial segregation of group II and group III mGluRs within presynaptic elements. Immunolabeling for the group III receptors was predominantly observed in presynaptic active zones of asymmetrical and symmetrical synapses, whereas that for the group II receptor (mGluR2) was found in preterminal rather than terminal portions of axons. Target cell-specific segregation of receptors, first reported for mGluR7a , was also apparent for the other group III mGluRs, suggesting that transmitter release is differentially regulated by 2-amino-4-phosphonobutyrate-sensitive mGluRs in individual synapses on single axons according to the identity of postsynaptic neurons. Key words: metabotropic glutamate receptor; hippocampus; perforant path; mossy fiber; Schaffer collateral; axon terminal; preterminal; immunohistochemistry; lesionMetabotropic glutamate receptors (mGluRs) have various modulatory f unctions on neuronal excitability, transmitter release, and synaptic plasticity in the C NS (Pin and Duvoisin, 1995). These f unctions have been studied most extensively in the hippocampus because of its roles in learning and memory and of its architecture, which is compartmentalized well with the three major excitatory pathways: the perforant path, mossy fiber, and Schaffer collateral. The mGluRs consist of at least eight subtypes that are classified into three groups (Nakanishi and Masu, 1994;Pin and Duvoisin, 1995). Group I mGluRs (mGluR1/mGluR5) are selectively activated by 3,5-dihydroxyphenylglycine (DHPG) (Schoepp et al., 1994) and coupled to inositol phospholipid hydrolysis. On the other hand, group II mGluRs (mGluR2/ mGluR3) and group III mGluRs (mGluR4/mGluR6/mGluR7/ mGluR8), which are linked to inhibition of the cAM P cascade in receptor-transfected cell lines, are selectively activated by 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) (Hayashi et al., 1993) and 2-amino-4-phosphonobutyrate (L-AP4), respectively.Excitability of hippocampal neurons is modulated directly through group I mGluRs (Davies et...

Taking advantage of the restricted expression of metabotropic glutamate receptor subtype 6 (mGluR6) in retinal ON bipolar cells, we generated knockout mice lacking mGluR6 expression. The homozygous mutant mice showed a loss of ON responses but unchanged OFF responses to light. The mutant mice displayed no obvious changes in retinal cell organization nor in the projection of optic fibers to the brain. Furthermore, the mGluR6-deficient mice showed visual behavioral responses to light stimulation as examined by shuttle box avoidance behavior experiments using light exposure as a conditioned stimulus. The results demonstrate that mGluR6 is essential in synaptic transmission to the ON bipolar cell and that the OFF response provides an important means for transmitting visual information.

The chemogenetic technology Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) affords remotely reversible control of cellular signaling, neuronal activity and behavior. Although the combination of muscarinic-based DREADDs with clozapine-Noxide (CNO) has been widely used, sluggish kinetics, metabolic liabilities, and potential offtarget effects of CNO represent areas for improvement. Here we provide a new high affinity and selective agonist deschloroclozapine (DCZ) for muscarinic-based DREADDs. Positron emission tomography revealed that DCZ selectively bound to and occupied DREADDs in both mice and monkeys. Systemic delivery of low doses of DCZ (1 or 3 μg/kg) enhanced neuronal activity via hM3Dq within minutes in mice and monkeys. Intramuscular injections of DCZ (100 μg/kg) reversibly induced spatial working memory deficits in monkeys expressing hM4Di in the prefrontal cortex. DCZ represents the most potent, selective, metabolically stable and fast-acting DREADD agonist reported with utility in both mice and non-human primates for a variety of applications.

Parkinson's disease is characterized by degeneration of nigral dopaminergic neurons, leading to a wide variety of psychomotor dysfunctions. Accumulated evidence suggests that abnormally synchronized oscillations in the basal ganglia contribute to the expression of Parkinsonian motor symptoms. However, the mechanism that generates abnormal oscillations in a dopamine-depleted state remains poorly understood. We addressed this question by examining basal ganglia neuronal activity in two 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated Parkinsonian monkeys. We found that systemic administration of l-3,4-dihydroxyphenylalanine (l-DOPA; dopamine precursor) decreased abnormal neuronal oscillations (8-15 Hz) in the internal segment of the globus pallidus (GPi) and the subthalamic nucleus (STN) during the ON state when Parkinsonian signs were alleviated and during l-DOPA-induced dyskinesia. GPi oscillations and parkinsonian signs were suppressed by silencing of the STN with infusion of muscimol (GABA(A) receptor agonist). Intrapallidal microinjection of a mixture of 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; N-methyl-d-aspartate receptor antagonist) and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX; AMPA/kainate receptor antagonist) also decreased the oscillations in the GPi and the external segment of the globus pallidus (GPe). Neuronal oscillations in the STN were suppressed after intrasubthalamic microinjection of CPP/NBQX to block glutamatergic afferents of the STN. The STN oscillations were further reduced by muscimol inactivation of the GPe to block GABAergic inputs from the GPe. These results suggest that, in the dopamine-depleted state, glutamatergic inputs to the STN and reciprocal GPe-STN interconnections are both important for the generation and amplification of the oscillatory activity of STN neurons, which is subsequently transmitted to the GPi, thus contributing to the symptomatic expression of Parkinson's disease.