Progression of change in NMDA, non-NMDA, and metabotropic glutamate receptor function at the developing corticothalamic synapse

Homer as both a scaffold and transduction molecule

Increasing evidence shows that scaffold proteins not only control membrane assembly of receptors and channels, but also modulate intracellular signaling by assembled receptors. The Homer family of proteins act as scaffolds to bind clusters of proteins and glutamate receptors at postsynaptic sites. We review results of cloning and gene expression of this protein family, and summarize roles in glutamate receptor function and intracellular signaling in neurons. Homer proteins trigger the localization of metabotropic glutamate receptor subtype 5 (mGlu5 receptor) to the postsynaptic plasma membrane. They can also alter the kinetics and peak amplitude of the intracellular Ca2+ responses of mGlu1 and mGlu5 receptors. Homer proteins can either prevent or promote spontaneous activation of these receptors, depending on the type of Homer protein isoform expressed.

Astrocytes, spontaneity, and the developing thalamus

Recent studies in the ventrobasal (VB) thalamus have shown that astrocytes display spontaneous intracellular calcium [Ca(2+)](i) oscillations early postnatally. [Ca(2+)](i) oscillations are correlated in groups of up to five astrocytes, and propagate between cells. NMDA receptor-mediated, long lasting inward currents in thalamocortical (TC) neurons of the VB complex are correlated to [Ca(2+)](i) increases in neighbouring astrocytes, and stimulation of astrocytic [Ca(2+)](i) increases also lead to inward currents in neurons. These findings suggest that astrocytes are spontaneously active and can induce neuronal activity, a reversal of the previously held view of neuron-glia interactions in the central nervous system. This activity occurs at an important period in the development of the thalamus and therefore suggests a potential functional role in a variety of processes. Along with data on the neurotransmitter receptor repertoire of thalamic astrocytes these findings enlarge the body of knowledge on astrocytes in the thalamus, and further contribute to the emerging field of astrocyte-neuron and neuron-astrocyte interactions in the central nervous system.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Auditory thalamocortical synaptic transmission in vitro

To facilitate an understanding of auditory thalamocortical mechanisms, we have developed a mouse brain-slice preparation with a functional connection between the ventral division of the medial geniculate (MGv) and the primary auditory cortex (ACx). Here we present the basic characteristics of the slice in terms of physiology (intracellular and extracellular recordings, including current source density analysis), pharmacology (including glutamate receptor involvement), and anatomy (gross anatomy, Nissl, parvalbumin immunocytochemistry, and tract tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). Thalamocortical transmission in this preparation (the "primary" slice) involves both alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid/kainate and N-methyl-D-aspartate-type glutamate receptors that appear to mediate monosynaptic inputs to layers 3-4 of ACx. MGv stimulation also initiates disynaptic inhibitory postsynaptic potentials and longer-duration intracortical, polysynaptic activity. Important differences between responses elicited by MGv versus conventional columnar ("on-beam") stimulation emphasize the necessity of thalamic activation to infer thalamocortical mechanisms. We also introduce a second slice preparation, the "shell" slice, obtained from the brain region immediately ventral to the primary slice, that may contain a nonprimary thalamocortical pathway to temporal cortex. In the shell slice, stimulation of the thalamus or the region immediately ventral to it appears to produce fast activation of synapses in cortical layer 1 followed by robust intracortical polysynaptic activity. The layer 1 responses may result from orthodromic activation of nonprimary thalamocortical pathways; however, a plausible alternative could involve antidromic activation of corticotectal neurons and their layer 1 collaterals. The primary and shell slices will provide useful tools to investigate mechanisms of information processing in the ACx.

GABA(B) and NMDA receptors contribute to spindle-like oscillations in rat thalamus in vitro

Thalamic slice preparations, in which intrathalamic connectivity between the reticular nucleus and relay nuclei is maintained, are capable of sustaining rhythmic burst firing activity in rodents and ferret. These in vitro oscillations occur spontaneously in the ferret and have frequencies (6-10 Hz) within the range of sleep spindles observed in vivo. In the rat, mainly lower frequency (2-4 Hz) oscillations, evoked under conditions of low bath [Mg(2+)] and/or GABA(A) receptor blockade, have been described. Here we show that faster rhythms in the range of 4-9 Hz can be evoked in rat thalamic slices by electrical stimulation of the internal capsule and also occur spontaneously. When bath [Mg(2+)] was 2 mM, these spindle-like oscillations were most common in a brief developmental time window, peaking at postnatal day 12 (P12). The oscillations were almost completely blocked by the GABA(A) receptor antagonist picrotoxin, and, in some cases, the frequency of oscillations was increased by the GABA(B) receptor antagonist CGP-35348. The selective blockade of N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors by the antagonists 2-amino-5-phosphonovaleric acid or 1,2,3,4-Tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), respectively, significantly shortened oscillations but did not completely block them. A combination of the two drugs was necessary to abolish oscillatory activity. The barbituate pentobarbital, which enhances GABA(A)R responses, initially slowed and synchronized oscillations before completely blocking them. When bath [Mg(2+)] was reduced from 2 to 0.65 mM, evoked oscillations became more robust and were often accompanied by spontaneously arising oscillations. Under these conditions, GABA(A) receptor blockade no longer inhibited oscillations, but instead converted them into the slow, synchronous rhythms that have been observed in other studies. The effects of GABA(B) or NMDA receptor blockade were more pronounced in 0.65 mM than in 2 mM external [Mg(2+)]. Thus spindle-like oscillations occur in rat thalamic slices in vitro, and we find that, in addition to the previously demonstrated contributions of GABA(A) and AMPA receptors to these oscillations, NMDA and GABA(B) receptors are also involved. The strong influence of external [Mg(2+)] on GABAergic pharmacology and a contribution of NMDA receptors during oscillations suggest a link between the excitability of NMDA receptors and the activation of GABA(B)R-mediated inhibitory postsynaptic currents.

Spontaneous astrocytic Ca2+ oscillations in situ drive NMDAR-mediated neuronal excitation

Astrocytes respond to chemical, electrical and mechanical stimuli with transient increases in intracellular calcium concentration ([Ca2+]i). We now show that astrocytes in situ display intrinsic [Ca2+]i oscillations that are not driven by neuronal activity. These spontaneous astrocytic oscillations can propagate as waves to neighboring astrocytes and trigger slowly decaying NMDA receptor-mediated inward currents in neurons located along the wave path. These findings show that astrocytes in situ can act as a primary source for generating neuronal activity in the mammalian central nervous system.

Differences in quantal amplitude reflect GluR4- subunit number at corticothalamic synapses on two populations of thalamic neurons

Low-frequency thalamocortical oscillations that underlie drowsiness and slow-wave sleep depend on rhythmic inhibition of relay cells by neurons in the reticular nucleus (RTN) under the influence of corticothalamic fibers that branch to innervate RTN neurons and relay neurons. To generate oscillations, input to RTN predictably should be stronger so disynaptic inhibition of relay cells overcomes direct corticothalamic excitation. Amplitudes of excitatory postsynaptic conductances (EPSCs) evoked in RTN neurons by minimal stimulation of corticothalamic fibers were 2.4 times larger than in relay neurons, and quantal size of RTN EPSCs was 2.6 times greater. GluR4-receptor subunits labeled at corticothalamic synapses on RTN neurons outnumbered those on relay cells by 3.7 times, providing a basis for differences in synaptic strength.

Synaptic activation of the group I metabotropic glutamate receptor mGlu1 on the thalamocortical neurons of the rat dorsal lateral geniculate nucleus in vitro

Intracellular recordings were made from thalamocortical neurons in slices of rat dorsal lateral geniculate nucleus in vitro, where ionotropic glutamate receptors and ionotropic and metabotropic GABA receptors had been blocked. The activation of specific metabotropic glutamate receptors by exogenous agonists and by the electrical stimulation of the corticothalamic pathway was then assessed using selective antagonists. The specific group I agonist (S)-3, 5-dihydoxyphenylglycine and the non-selective agonist (1S, 3R)-1-aminocyclo-pentane-1,3-dicarboxylic acid both caused a concentration-dependent depolarization of membrane potential. These effects were associated with an increase in the apparent input resistance, and a more robust expression of both the depolarizing sag of the voltage response and the low-threshold Ca(2+) potential and an increase in thalamocortical neuron excitability. However, group I agonists selective for the mGlu5 receptor and agonists selective for group II and III receptors did not have these effects. Consequently, these data suggested that these actions were mediated specifically by the group I mGlu1 receptor. The activation of cortical fibres, with trains of 50 stimuli at 50Hz, resulted in a two-component depolarizing response. The first part of this synaptic response and the agonist-induced depolarization of membrane potential were depressed by the novel group I receptor antagonists LY367366 and LY367385, which are active at mGlu1 receptors. However, they were not blocked by 6-methyl-2-(phenylethyl)-pyridine, a highly selective mGlu5 receptor antagonist.Thus, the membrane potential depolarization of thalamocortical neurons caused either by exogenous agonists or by the stimulation of cortical fibres resulted from the specific activation of mGlu1 but not mGlu5 receptors. This result is consistent with the location of this receptor type on the distal dendrites of thalamocortical neurons in the dorsal lateral geniculate nucleus of the thalamus.

Corticothalamic resonance, states of vigilance and mentation

During various states of vigilance, brain oscillations are grouped together through reciprocal connections between the neocortex and thalamus. The coherent activity in corticothalamic networks, under the control of brainstem and forebrain modulatory systems, requires investigations in intact-brain animals. During behavioral states associated with brain disconnection from the external world, the large-scale synchronization of low-frequency oscillations is accompanied by the inhibition of synaptic transmission through thalamocortical neurons. Despite the coherent oscillatory activity, on the functional side there is dissociation between the thalamus and neocortex during slow-wave sleep. While dorsal thalamic neurons undergo inhibitory processes due to the prolonged spike-bursts of thalamic reticular neurons, the cortex displays, periodically, a rich spontaneous activity and preserves the capacity to process internally generated signals that dominate the state of sleep. In vivo experiments using simultaneous intracellular recordings from thalamic and cortical neurons show that short-term plasticity processes occur after prolonged and rhythmic spike-bursts fired by thalamic and cortical neurons during slow-wave sleep oscillations. This may serve to support resonant phenomena and reorganize corticothalamic circuitry, determine which synaptic modifications, formed during the waking state, are to be consolidated and generate a peculiar kind of dreaming mentation. In contrast to the long-range coherent oscillations that occur at low frequencies during slow-wave sleep, the sustained fast oscillations that characterize alert states are synchronized over restricted territories and are associated with discrete and differentiated patterns of conscious events.

Developmental remodeling of the retinogeniculate synapse

Anatomical rearrangement of retinogeniculate connections contributes to the refinement of synaptic circuits in the developing visual system, but the underlying changes in synaptic function are unclear. Here, we study such changes in mouse brain slices. Each geniculate cell receives a surprisingly large number of retinal inputs (>20) well after eye-specific zones are formed. All but one to three of these inputs are eliminated over a 3-week period spanning eye opening. Remaining inputs are strengthened approximately 50-fold, in part through an increase in quantal size, but primarily through an increase in the number of release sites. Changes in release probability do not contribute significantly. Thus, a redistribution of release sites from many inputs to few inputs at this late developmental stage contributes to the precise receptive fields of thalamic relay neurons.

Expression of functional metabotropic glutamate receptors in primary cultured rat osteoblasts. Cross-talk with N-methyl-D-aspartate receptors

Osteoblasts and osteoclasts express functional N-methyl-d-aspartate (NMDA) receptors, which participate in regulation of bone matrix. In rat femoral osteoblasts held in whole cell clamp there is a robust NMDA current but little if any response to l-glutamate. We have investigated expression of metabotropic glutamate receptors (mGluRs) in these cells. By reverse transcription polymerase chain reaction, we have detected expression of mGluR1b (but not mGluR1a, 2, 3, 4, 5, or 6). Blockade of mGluRs with (+/-)-alpha-methyl-carboxyphenyl-glycine resulted in an enlarged l-glutamate-induced current that resembled the response to NMDA. Conversely, prior stimulation of mGluRs with trans-(+/-)-1-amino-1, 3-cyclopentanedicarboxylic acid (1S,3R-ACPD; mGluR agonist) reduced the NMDA-induced current by 77%. Monitoring of [Ca(2+)](i) showed that NMDA induced a sustained elevation of [Ca(2+)](i), which was dependent upon [Ca(2+)](o). Treatment with 1S,3R-ACPD generated an initial transient that was independent of [Ca(2+)](o), followed by a sustained, [Ca(2+)](o)-dependent phase, a response consistent with phospholipase C-mediated mobilization of stored Ca(2+). Investigations of the interaction between the two receptors confirmed inhibitory modulation of the NMDA receptor-induced rise in [Ca(2+)](i) by mGluRs. Parathyroid hormone, which also activates phospholipase C in osteoblasts, had a similar inhibitory effect on the NMDA receptor-induced [Ca(2+)](i) response. Elevation of [Ca(2+)](i) mediated by mGluR activation was reduced by subsequent stimulation of NMDA receptors. This is the first description of mGluRs in bone and shows that complex glutamatergic signaling can occur in this tissue.

Cortical and subcortical contributions to activity-dependent plasticity in primate somatosensory cortex

After manipulations of the periphery that reduce or enhance input to the somatosensory cortex, affected parts of the body representation will contract or expand, often over many millimeters. Various mechanisms, including divergence of preexisting connections, expression of latent synapses, and sprouting of new synapses, have been proposed to explain such phenomena, which probably underlie altered sensory experiences associated with limb amputation and peripheral nerve injury in humans. Putative cortical mechanisms have received the greatest emphasis but there is increasing evidence for substantial reorganization in subcortical structures, including the brainstem and thalamus, that may be of sufficient extent to account for or play a large part in representational plasticity in somatosensory cortex. Recent studies show that divergence of ascending connections is considerable and sufficient to ensure that small alterations in map topography at brainstem and thalamic levels will be amplified in the projection to the cortex. In the long term, slow, deafferentation-dependent transneuronal atrophy at brainstem, thalamic, and even cortical levels are operational in promoting reorganizational changes, and the extent to which surviving connections can maintain a map is a key to understanding differences between central and peripheral deafferentation.

Action potential backpropagation and somato-dendritic distribution of ion channels in thalamocortical neurons

Thalamocortical (TC) neurons of the dorsal thalamus integrate sensory inputs in an attentionally relevant manner during wakefulness and exhibit complex network-driven and intrinsic oscillatory activity during sleep. Despite these complex intrinsic and network functions, little is known about the dendritic distribution of ion channels in TC neurons or the role such channel distributions may play in synaptic integration. Here we demonstrate with simultaneous somatic and dendritic recordings from TC neurons in brain slices that action potentials evoked by sensory or cortical excitatory postsynaptic potentials are initiated near the soma and backpropagate into the dendrites of TC neurons. Cell-attached recordings demonstrated that TC neuron dendrites contain a nonuniform distribution of sodium but a roughly uniform density of potassium channels across the somatodendritic area examined that corresponds to approximately half the average path length of TC neuron dendrites. Dendritic action potential backpropagation was found to be active, but compromised by dendritic branching, such that action potentials may fail to invade relatively distal dendrites. We have also observed that calcium channels are nonuniformly distributed in the dendrites of TC neurons. Low-threshold calcium channels were found to be concentrated at proximal dendritic locations, sites known to receive excitatory synaptic connections from primary afferents, suggesting that they play a key role in the amplification of sensory inputs to TC neurons.

Properties of miniature glutamatergic EPSCs in neurons of the locomotor regions of the developing zebrafish

As a first step in understanding the development of synaptic activation in the locomotor network of the zebrafish, we examined the properties of spontaneous, glutamatergic miniature excitatory postsynaptic currents (mEPSCs). Whole cell patch-clamp recordings were obtained from visually identified hindbrain reticulospinal neurons and spinal motoneurons of curarized zebrafish 1-5 days postfertilization (larvae hatch after the 2nd day of embryogenesis). In the presence of tetrodotoxin (TTX) and blockers of inhibitory receptors (strychnine and picrotoxin), we detected fast glutamatergic mEPSCs that were blocked by the AMPA/kainate receptor-selective antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). At positive voltages or in the absence of Mg(2+), a second, slower component of the mEPSCs was revealed that the N-methyl-D-aspartate (NMDA) receptor-selective antagonist DL-2-amino-5-phosphonovalerate (AP-5) abolished. In the presence of both CNQX and AP-5, all mEPSCs were eliminated. The NMDA component of reticulospinal mEPSCs had a large single-channel conductance estimated to be 48 pS. Larval AMPA/kainate and NMDA components of the mEPSCs decayed with biexponential time courses that changed little during development. At all stages examined, approximately one-half of synapses had only NMDA responses (lacking AMPA/kainate receptors), whereas the remainder of the synapses were composed of a mixture of AMPA/kainate and NMDA receptors. There was an overall increase in the frequency and amplitude of mEPSCs with an NMDA component in reticulospinal (but not motoneurons) during development. These results indicate that glutamate is a prominent excitatory transmitter in the locomotor regions of the developing zebrafish and that it activates either NMDA receptors alone at functionally silent synapses or together with AMPA/kainate receptors.

Presynaptic long-term potentiation in corticothalamic synapses

The thalamus and neocortex are two highly organized and complex brain structures that work in concert with each other. The largest synaptic input to the thalamus arrives from the neocortex via corticothalamic fibers. Using brain slices, we describe long-term potentiation (LTP) in corticothalamic fibers contacting the ventrobasal thalamus. Corticothalamic LTP is input-specific, NMDA receptor-independent, and reversible. The induction of corticothalamic LTP is entirely presynaptic and Ca(2+)-dependent. The expression of corticothalamic LTP is associated with a decrease in paired-pulse facilitation (PPF) and blocked by an inhibitor of the cAMP-dependent protein kinase A (PKA). Consistent with an involvement of cAMP and PKA, activation of adenylyl cyclase induced a synaptic enhancement that was associated with a decrease in PPF and occluded LTP. Corticothalamic LTP may serve to enhance the efficacy of cortico-cortical communication via the thalamus and/or to mediate experience-dependent long-term modifications of thalamocortical receptive fields.

Ca(2+)-permeable AMPA receptors and spontaneous presynaptic transmitter release at developing excitatory spinal synapses

At many mature vertebrate glutamatergic synapses, excitatory transmission strength and plasticity are regulated by AMPA and NMDA receptor (AMPA-R and NMDA-R) activation and by patterns of presynaptic transmitter release. Both receptors potentially direct neuronal differentiation by mediating postsynaptic Ca(2+) influx during early development. However, the development of synaptic receptor expression and colocalization has been examined developmentally in only a few systems, and changes in release properties at neuronal synapses have not been characterized extensively. We recorded miniature EPSCs (mEPSCs) from spinal interneurons in Xenopus embryos and larvae. In mature 5-8 d larvae, approximately 70% of mEPSCs in Mg(2+)-free saline are composed of both a fast AMPA-R-mediated component and a slower NMDA-R-mediated decay, indicating receptor colocalization at most synapses. By contrast, in 39-40 hr embryos approximately 65% of mEPSCs are exclusively fast, suggesting that these synapses initially express predominantly AMPA-R. In a physiological Mg(2+) concentration (1 mM), mEPSCs throughout development are mainly AMPA-R-mediated at negative potentials. Embryonic synaptic AMPA-R are highly Ca(2+)-permeable, mEPSC amplitude is over twofold larger than at mature synapses, and mEPSCs frequently occur in bursts consistent with asynchronous multiquantal release. AMPA-R function in this motor pathway thus appears to be independent of previous NMDA-R activation, unlike other regions of the developing nervous system, ensuring a greater reliability for embryonic excitatory transmission. Early spontaneous excitatory activity is specialized to promote AMPA-R-mediated synaptic Ca(2+) influx, which likely has significant roles in neuronal development.

Coupling of mGluR/Homer and PSD-95 complexes by the Shank family of postsynaptic density proteins

Shank is a recently described family of postsynaptic proteins that function as part of the NMDA receptor-associated PSD-95 complex (Naisbitt et al., 1999 [this issue of Neuron]). Here, we report that Shank proteins also bind to Homer. Homer proteins form multivalent complexes that bind proline-rich motifs in group 1 metabotropic glutamate receptors and inositol trisphosphate receptors, thereby coupling these receptors in a signaling complex. A single Homer-binding site is identified in Shank, and Shank and Homer coimmunoprecipitate from brain and colocalize at postsynaptic densities. Moreover, Shank clusters mGluR5 in heterologous cells in the presence of Homer and mediates the coclustering of Homer with PSD-95/GKAP. Thus, Shank may cross-link Homer and PSD-95 complexes in the PSD and play a role in the signaling mechanisms of both mGluRs and NMDA receptors.

Synchronized paroxysmal activity in the developing thalamocortical network mediated by corticothalamic projections and "silent" synapses

In mouse thalamocortical slices in vitro, the potassium channel blocker 4-AP and GABAA receptor antagonist bicuculline together induced spontaneous prolonged depolarizations in layer VI neurons from postnatal day 2 (P2), in ventroposterior nucleus neurons (VP) from P7, and in reticular nucleus neurons (RTN) from P8. Dual whole-cell recordings revealed that prolonged bursts were synchronized in layer VI, VP, and RTN. Bursts were present in cortex isolated from thalamus, but not in thalamus isolated from cortex, indicating that bursts originated in cortex and propagated to thalamus. Prolonged bursts were synchronized in layer VI when vertical cuts extended from pia mater through layers IV or V, but were no longer synchronized when cuts extended through layer VI and white matter. In voltage-clamp recordings before P10, burst conductance of all three neuronal populations was dominated by the NMDA receptor-mediated conductance, and therefore synapses were "silent". In cortex and RTN, after P10, bursts were associated with strong AMPA/kainate receptor-mediated conductances, and synapses had become "functional"; silent synapses persisted in a large proportion of VP cells after P10. Before P9, the NMDA receptor antagonist APV or the non-NMDA receptor antagonist CNQX blocked the prolonged bursts. After P9, CNQX continued to block the prolonged bursts, but APV merely shortened their duration. Thus, NMDA receptor-based silent synapses are essential for paroxysmal corticothalamic activity during early postnatal development, and connections between layer VI neurons are sufficient for horizontal cortical synchronization.

Nucleus- and cell-specific expression of NMDA and non-NMDA receptor subunits in monkey thalamus

Subcortical and corticothalamic inputs excite thalamic neurons via a diversity of glutamate receptor subtypes. Differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA) receptor subunits (GluR1-4; GluR5-7; NR1, NR2A-D) on a nucleus- and cell type-specific basis was examined by quantitative in situ hybridization histochemistry and by immunocytochemical staining for receptor subunits and colocalized gamma-aminobutyric acid (GABA) or calcium binding proteins. Levels of NMDA subunit expression, except NR2C, are higher than for the most highly expressed AMPA (GluR1,3,4) and kainate (GluR6) receptor subunits. Expression of NR2C, GluR2, GluR5, and GluR7 is extremely low. Major differences distinguish the reticular nucleus and the dorsal thalamus and, within the dorsal thalamus, the intralaminar and other nuclei. In the reticular nucleus, GluR4 is by far the most prominent, and NMDA receptors are at comparatively low levels. In the dorsal thalamus, NMDA receptors predominate. Anterior intralaminar nuclei are more enriched in GluR4 and GluR6 subunits than other nuclei, whereas posterior intralaminar nuclei are enriched in GluR1 and differ among themselves in relative NMDA receptor subunit expression. GABAergic intrinsic neurons of the dorsal thalamus express much higher levels of GluR1 and GluR6 receptor subunits than do parvalbumin- or calbindin-immunoreactive relay cells and low or absent NMDA receptors. Relay cells are dominated by NMDA receptors, along with GluR3 and GluR6 subunits not expressed by GABA cells. High levels of NR2B are found in astrocytes. Differences in NMDA and non-NMDA receptor profiles will affect functional properties of the thalamic GABAergic and relay cells.