The segregation and expression of glutamate receptor subunits in cultured hippocampal neurons

Distribution of AMPA-selective glutamate receptor subunits in the human hippocampus and cerebellum

The distribution of AMPA-selective subunits, GluR1-4, was determined in the human hippocampus and cerebellum by in situ hybridization and immunocytochemistry. In the hippocampus, in situ hybridization revealed that GluR1 and GluR2 mRNAs were similarly distributed and highly expressed in the dentate gyrus, with lower levels in the CA regions. GluR3 and GluR4 mRNAs were expressed at very low levels. Immunocytochemical studies showed that GluR1- and GluR2/3-immunoreactivity were highest in the dentate molecular and granular layers. In the CA regions, GluR1 and GluR2/3 staining was observed in pyramidal cell bodies and surrounding neuropil and was more intense in CA4/3/2 compared with CA1. GluR4-immunoreactivity was low throughout the hippocampus. In the cerebellum, GluR1 and GluR4 transcripts were expressed in the granular and Purkinje cell/Bergmann glia layers. GluR2 mRNA was highly expressed in the granular layer and individual Purkinje cells, while GluR3 mRNA was not detectable in the cerebellum. GluR1- and GluR4-immunoreactivity were localized to Purkinje cells and putative Bergmann glia, as well as their processes extending into the molecular layer. GluR2/3 staining was intense in Purkinje cells, with moderate staining in the granular layer. Thus, GluR1-4 subunits are differentially distributed in the hippocampus and cerebellum. In addition, the distribution of subunit mRNA and protein correlate well with each other and with the glutamatergic neuroanatomy of the hippocampus and cerebellum.

Subcellular localization and molecular pharmacology of distinct populations of [3H]-AMPA binding sites in rat hippocampus

1. The subcellular distributions of [3H]-alpha-amino-3-hydroxy-5- methylisoxazolepropionate ([3H]-AMPA) and [3H]-kainate binding sites in rat hippocampus were investigated by cell fractionation techniques. 2. Two major populations of [3H]-AMPA sites were detected with the majority of binding located intracellularly in the microsomal (P3) fraction. Most of the remaining sites were in the synaptosomal membrane fraction but some were also present in the nuclear fraction. In contrast, essentially all of the [3H]-kainate binding sites were in the synaptosomal membrane fraction. 3. Saturation binding analysis yielded KD and Bmax values for [3H]-AMPA of 147 nM and 2.6 pmol mg-1 protein respectively for the synaptosomal membrane-associated sites and 129 nM and 5.3 pmol mg-1 protein respectively for the microsomal sites. 4. Both main populations of [3H]-AMPA sites displayed the same rank order of inhibition by competitive ligands, the apparent Mr values of GluR1 subunits were equivalent, suggesting the same degree of post-translational modification and the hydrodynamic properties of the receptor complexes were identical. 5. These data are consistent with the hypothesis that the movement of AMPA receptors between cellular compartments in the postsynaptic neurone could constitute one mechanism underlying long-term potentiation in the hippocampus.

Selective clustering of glutamate and gamma-aminobutyric acid receptors opposite terminals releasing the corresponding neurotransmitters

Several immunocytochemical and physiological studies have demonstrated a concentration of neurotransmitter receptors at postsynaptic sites on neurons, but an overall picture of receptor distribution has not emerged. In particular, it has not been clear whether receptor clusters are selectively localized opposite terminals that release the corresponding neurotransmitter. By using antibodies against the excitatory glutamate receptor subunit GluR1 and the inhibitory type A gamma-aminobutyric acid (GABA) receptor beta 2/3 subunits, we show that these different receptor types cluster at distinct postsynaptic sites on cultured rat hippocampal neurons. The GABAA receptor beta 2/3 subunits clustered on cell bodies and dendritic shafts opposite GABAergic terminals, whereas GluR1 clustered mainly on dendritic spines and was associated with glutamatergic synapses. Chronic blockade of evoked transmitter release did not block receptor clustering at postsynaptic sites. These results suggest that complex mechanisms involving nerve terminal-specific signals are required to allow different postsynaptic receptor types to cluster opposite only appropriate presynaptic terminals.

Cellular and synaptic localization of NMDA and non-NMDA receptor subunits in neocortex: organizational features related to cortical circuitry, function and disease

Excitatory amino acid (EAA) receptors are an important component of neocortical circuitry as a result of their role as the principal mediators of excitatory synaptic activity, as well as their involvement in use-dependent modifications of synaptic efficacy, excitoxicity and cell death. The diversity in the effects generated by EAA-receptor activation can be attributed to multiple receptor subtypes, each of which is composed of multimeric assemblies of functionally distinct receptor subunits. The use of subunit-specific antibodies and molecular probes now makes it feasible to localize individual receptor subunits anatomically with a high level of cellular and synaptic resolution. Initial studies of the distribution of immunocytochemically localized EAA-receptor subunits suggest that particular subunit combinations exhibit a differential cellular, laminar and regional distribution in the neocortex. While such patterns might indicate that the functional heterogeneity of EAA-receptor-linked circuits, and the cell types in which they operate, are based partly on differential subunit parcellation, a definitive integration of these anatomical details into current schemes of cortical circuitry and organization awaits many further studies. Ideally, such studies should link a high level of molecular precision regarding subunit localization with synaptic details of identified connections and neurochemical features of neocortical cells.

Histological and ultrastructural localization of the kainate receptor subunits, KA2 and GluR6/7, in the rat nervous system using selective antipeptide antibodies

Kainate receptors are found throughout many regions of the brain and presumably contribute to responses of neurons to glutamate and other excitatory amino acids. Two affinity-purified polyclonal antibodies that recognize the kainate binding subunits, KA2 and GluR6, were made using C-terminus peptides. A previous study demonstrated that each antibody is specific for its subunit, although antibody to GluR6 recognizes GluR7 to some extent (hence the designation GluR6/7). Vibratome sections immunostained with either antibody showed light to moderate staining in many structures in the brain as well as in cervical spinal cord, dorsal root and vestibular ganglia, and pineal and pituitary glands. Moderate levels were seen in the olfactory bulb, cerebral cortex, caudate/putamen, and hypothalamus, whereas much of the thalamus was stained lightly. In the hippocampus, CA3 pyramidal cells were stained more densely than CA1 pyramidal cells--the difference more evident with antibody to GluR6/7. In addition, neuropilar staining was densest in the stratum lucidum of the CA3 region. In the brainstem, staining was moderate to moderately dense in a number of sensory, motor, and reticular nuclei. The moderately dense staining in the reticulothalamic nucleus and pontine nuclei with antibody to GluR6/7 may represent its recognition of GluR7. In the cerebellum, staining was moderate in granular and molecular layers with antibody to KA2 and in the molecular layer with antibody to GluR6/7, whereas it was moderately dense to dense in the granular layer with the GluR6/7 antibody. Outside of the brain, densest staining was seen with antibody to KA2 in the intermediate lobe of the pituitary gland. Ultrastructural localization of immunostaining was examined in the hippocampus, cerebral cortex, and cerebellar cortex. Typically, major staining was in postsynaptic densities apposed by unstained presynaptic terminals with round or mainly round vesicles and in associated dendrites. The light microscope pattern of staining was fairly similar to that of previous [3H]kainate binding and in situ hybridization studies. In addition, comparison with previous studies on distribution of other types of glutamate receptors indicates that KA2 and GluR6/7 are found with various other subunits in many of the same cell populations throughout the nervous system.

Distribution of AMPA selective glutamate receptors in the thalamus of adult rats and during postnatal development. A light and ultrastructural immunocytochemical study

The regional, cellular and subcellular distribution of AMPA receptors was demonstrated immunocytochemically within the thalamus of adult and young (from 1 to 20 days postnatal, P1-P20) rats. The antipeptide antibodies used recognize individual subunit proteins of the AMPA-preferring glutamate receptor, i.e., GluR1, GluR2-3 and GluR4. Our results demonstrate that these AMPA receptor subunits are generally not highly expressed in the thalamus, as compared to other brain areas and that they are enriched differentially within different thalamic nuclei. GluR1 is mostly found in intralaminar and midline nuclei throughout life, whereas GluR2-3 is moderately expressed in the thalamus, with no major developmental changes. GluR4 is the predominant subunit expressed in the reticular nucleus in adult rats, but not in young animals, where until P9 it is instead present in the ventrobasal complex. Samples of paraventricular and lateral geniculate nuclei stained with GluR1 and of reticular nucleus as well as ventrobasal complex stained with GluR4 were used for the ultrastructural study. In all the samples, labelling was in the somatic and dendritic cytoplasm, with dense patches of reaction product apposing post-synaptic densities of terminals with round clear vesicles and asymmetric specializations. Glial staining was observed only with the GluR1 antiserum and there was no evidence of labelled synaptic terminals. The differential distribution of GluR subunits in the thalamus suggests that certain subunits may participate more than others in mediating post-synaptic responses in distinct neuronal populations and also that other GluR types may be involved in the thalamic networks.

Light and electron microscopic immunocytochemical localization of AMPA-selective glutamate receptors in the rat spinal cord

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors are probably the most widespread excitatory neurotransmitter receptors of the central nervous system, and they play a role in most normal and pathological neural activities. However, previous detailed studies of AMPA subunit distribution have been limited mainly to the brain. Thus, a comprehensive study of AMPA receptor subunit distribution was carried out on sections of rat spinal cord and dorsal root ganglia, which were immunolabeled with antibodies made against peptides corresponding to C-terminal portions of the AMPA receptor subunits: GluR1, GluR2/3, and GluR4. In the spinal cord, labeling was most prominent in the superficial dorsal horn, motoneurons, and nuclei containing preganglionic autonomic neurons. Immunostaining also was observed in neurons in other regions including those known to contain Renshaw cells and Ia inhibitory cells. Although overall immunostaining was lighter with antibody to GluR1 than with GluR2/3 and 4, there were neurons that preferentially stained with antibody to GluR1. These "GluR1 intense" neurons were usually fusiform and most concentrated in lamina X. In dorsal root ganglia, immunostaining of ganglion cell bodies was moderate to dense with antibody to GluR2/3 and light to moderate with antibody to GluR4. Possible neuroglia in the spinal cord (mainly GluR2/3 and 4) and satellite cells in dorsal root ganglia (GluR4) were immunostained. Electron microscopic studies of the superficial dorsal horn and lateral motor column showed staining that was restricted mainly to postsynaptic densities and associated dendritic and cell body cytoplasm. In dorsal horn, colocalization of dense-cored vesicles with clear, round synaptic vesicles was observed in unstained presynaptic terminals apposed to stained postsynaptic densities. Subsynaptic dense bodies (Taxi-bodies) were associated with some stained postsynaptic densities in both the superficial dorsal horn and lateral motor column. Based on several morphological features including vesicle structure and presence of Taxi-bodies, it is likely that at least some of the postsynaptic staining seen in this study is apposed to glutamatergic input from primary sensory afferent terminals.