Group II metabotropic glutamate receptors reduce excitatory but not inhibitory neurotransmission in rat barrel cortex in vivo

GPCR interactions involving metabotropic glutamate receptors and their relevance to the pathophysiology and treatment of CNS disorders

Cellular responses to metabotropic glutamate (mGlu) receptor activation are shaped by mechanisms of receptor-receptor interaction. mGlu receptor subtypes form homodimers, intra- or inter-group heterodimers, and heteromeric complexes with other G protein-coupled receptors (GPCRs). In addition, mGlu receptors may functionally interact with other receptors through the βγ subunits released from G proteins in response to receptor activation or other mechanisms. Here, we discuss the interactions between (i) mGlu₁ and GABA_B receptors in cerebellar Purkinje cells; (ii) mGlu₂ and 5-HT_2Aserotonergic receptors in the prefrontal cortex; (iii) mGlu₅ and A_2A receptors or mGlu₅ and D₁ dopamine receptors in medium spiny projection neurons of the indirect and direct pathways of the basal ganglia motor circuit; (iv) mGlu₅ and A_2A receptors in relation to the pathophysiology of Alzheimer's disease; and (v) mGlu₇ and A₁ adenosine or α- or β₁ adrenergic receptors. In addition, we describe in detail a novel form of non-heterodimeric interaction between mGlu₃ and mGlu₅ receptors, which appears to be critically involved in mechanisms of activity-dependent synaptic plasticity in the prefrontal cortex and hippocampus. Finally, we highlight the potential implication of these interactions in the pathophysiology and treatment of cerebellar disorders, schizophrenia, Alzheimer's disease, Parkinson's disease, l-DOPA-induced dyskinesias, stress-related disorders, and cognitive dysfunctions.

The Glutamatergic System in Primary Somatosensory Neurons and Its Involvement in Sensory Input-Dependent Plasticity

Glutamate is the most common neurotransmitter in both the central and the peripheral nervous system. Glutamate is present in all types of neurons in sensory ganglia, and is released not only from their peripheral and central axon terminals but also from their cell bodies. Consistently, these neurons express ionotropic and metabotropic receptors, as well as other molecules involved in the synthesis, transport and release of the neurotransmitter. Primary sensory neurons are the first neurons in the sensory channels, which receive information from the periphery, and are thus key players in the sensory transduction and in the transmission of this information to higher centers in the pathway. These neurons are tightly enclosed by satellite glial cells, which also express several ionotropic and metabotropic glutamate receptors, and display increases in intracellular calcium accompanying the release of glutamate. One of the main interests in our group has been the study of the implication of the peripheral nervous system in sensory-dependent plasticity. Recently, we have provided novel evidence in favor of morphological changes in first- and second-order neurons of the trigeminal system after sustained alterations of the sensory input. Moreover, these anatomical changes are paralleled by several molecular changes, among which those related to glutamatergic neurotransmission are particularly relevant. In this review, we will describe the state of the art of the glutamatergic system in sensory ganglia and its involvement in input-dependent plasticity, a fundamental ground for advancing our knowledge of the neural mechanisms of learning and adaptation, reaction to injury, and chronic pain.

Developmental decline in modulation of glutamatergic synapses in layer IV of the barrel cortex by group II metabotropic glutamate receptors

Previously, we demonstrated that group II metabotropic glutamate receptors (mGluRs) reduce glutamate release from thalamocortical synapses during early postnatal development (P7-11). To further examine the role of group II mGluRs in the modulation of somatosensory circuitry, we determined whether group II mGluRs continue to modulate thalamocortical synapses until adulthood and whether these receptors also modulate intra-cortical synapses in the barrel cortex. To address these issues, we examined the effect of the group II mGluR agonists on thalamocortical excitatory postsynaptic currents (EPSCs) and intra-barrel EPSCs in slices from animals of different ages (P7-53). We found that the depression of thalamocortical EPSCs by group II mGluRs rapidly declined after the second postnatal week. In contrast, adenosine continued to depress thalamocortical EPSCs via a presynaptic mechanism in young adult mice (P30-50). Activation of group II mGluRs also reduced intra-barrel EPSCs through a postsynaptic mechanism in young mice (P7-11). Similar to the thalamocortical synapses, the group II mGluR modulation of intra-barrel excitatory synapses declined with development. In young adult animals (P30-50), group II mGluR stimulation had little effect on intra-barrel EPSCs but did hyperpolarize the neurons. Together our results demonstrate that group II mGluRs modulate barrel cortex circuitry by presynaptic and postsynaptic mechanisms depending on the source of the synapse and that this modulation declines with development.

A modulatory effect of the feedback from higher visual areas to V1 in the mouse

Using a mouse brain slice preparation, we studied the modulatory effects of a feedback projection from higher visual cortical areas, mostly or exclusively area LM (or V2), on two inputs to layer 4 cells in the first visual area (V1). The two inputs to these cells were geniculocortical and an unspecified intracortical input, possibly involving layer 6 cells. We found that activation of metabotropic glutamate receptors (mGluRs) from stimulation of the feedback projection reduced the evoked excitatory postsynaptic currents of both of these inputs to layer 4 but that this modulation acts in an input-specific way. Reducing the strength of the geniculocortical input in adults involved both presynaptic and postsynaptic group I mGluRs (although in younger animals presynaptic group II mGluRs were also involved), whereas modulation of the intracortical input acted entirely via postsynaptic group II mGluRs. These results demonstrate that one of the effects of this feedback pathway is to control the gain of geniculocortical transmission.

Modulatory effects of metabotropic glutamate receptors on local cortical circuits

Glutamatergic pathways in various thalamic and cortical circuits have been classified into two types: Class 1 and Class 2, where it has been suggested that Class 1 carries the main information for processing, and Class 2 is mainly modulatory. We now extend this to the local circuitry of visual cortex of the mouse by demonstrating the modulatory actions on the Class 1 pathway from layer 4 to layers 2/3 of a Class 2 input from adjacent locations in layers 2/3. We found that this Class 2 input produces a long-lasting hyperpolarization and suppresses the initial responses of input from layer 4 and that this involves the postsynaptic activation of Group II metabotropic glutamate receptors. This modulation also shifts the paired pulse ratio of the layer 4 input from depression to facilitation.

Methodology for rapid measures of glutamate release in rat brain slices using ceramic-based microelectrode arrays: basic characterization and drug pharmacology

Excessive excitability or hyperexcitability of glutamate-containing neurons in the brain has been proposed as a possible explanation for anxiety, stress-induced disorders, epilepsy, and some neurodegenerative diseases. However, direct measurement of glutamate on a rapid time scale has proven to be difficult. Here we adapted enzyme-based microelectrode arrays (MEA) capable of detecting glutamate in vivo, to assess the effectiveness of hyperexcitability modulators on glutamate release in brain slices of the rat neocortex. Using glutamate oxidase coated ceramic MEAs coupled with constant voltage amperometry, we measured resting glutamate levels and synaptic overflow of glutamate after K(+) stimulation in brain slices. MEAs reproducibly detected glutamate on a second-by-second time scale in the brain slice preparation after depolarization with high K(+) to evoke glutamate release. This stimulus-evoked glutamate release was robust, reproducible, and calcium dependent. The K(+)-evoked glutamate release was modulated by ligands to the α(2)δ subunit of voltage sensitive calcium channels (PD-0332334 and PD-0200390). Meanwhile, agonists to Group II metabotropic glutamate (mGlu) receptors (LY379268 and LY354740), which are known to alter hyperexcitability of glutamate neurons, attenuated K(+)-evoked glutamate release but did not alter resting glutamate levels. This new MEA technology provides a means of directly measuring the chemical messengers involved in glutamate neurotransmission and thereby helping to reveal the role multiple glutamatergic system components have on glutamate signaling.

Presynaptic inhibitory terminals are functionally abnormal in a rat model of posttraumatic epilepsy

Partially isolated "undercut" neocortex with intact pial circulation is a well-established model of posttraumatic epileptogenesis. Results of previous experiments showed a decreased frequency of miniature inhibitory postsynaptic currents (mIPSCs) in layer V pyramidal (Pyr) neurons of undercuts. We further examined possible functional abnormalities in GABAergic inhibition in rat epileptogenic neocortical slices in vitro by recording whole cell monosynaptic IPSCs in layer V Pyr cells and fast-spiking (FS) GABAergic interneurons using a paired pulse paradigm. Compared with controls, IPSCs in Pyr neurons of injured slices showed increased threshold and decreased peak amplitude at threshold, decreased input/output slopes, increased failure rates, and a shift from paired pulse depression toward paired pulse facilitation (increased paired pulse ratio or PPR). Increasing [Ca(2+)](o) from 2 to 4 mM partially reversed these abnormalities in Pyr cells of the epileptogenic tissue. IPSCs onto FS cells also had an increased PPR and failures. Blockade of GABA(B) receptors did not affect the paired results. These findings suggest that there are functional alterations in GABAergic presynaptic terminals onto both Pyr and FS cells in this model of posttraumatic epileptogenesis.