Enrichment of glutamate immunoreactivity in lemniscal terminals in the ventropostero lateral thalamic nucleus of the rat: an immunogold and WGA-HRP study

Connections from the rat dorsal column nuclei (DCN) to the periaqueductal gray matter (PAG)

Electrical stimulation of the dorsal columns (DCs; spinal cord stimulation; SCS) has been proposed to treat chronic neuropathic pain. SCS may activate a dual mechanism that would affect both the spinal cord and supraspinal levels. Stimulation of DCs or DC nuclei (DCN) in animals where neuropathic pain has been induced causes activation of brainstem centers including the periaqueductal gray (PAG), which is involved in the endogenous pain suppression system. Biotinylated dextran-amine (BDA) was iontophoretically injected into the DCN to analyze the ascending projection directed to the PAG. Separate injections into the gracile nucleus (GrN) and the cuneate nucleus (CunN) showed BDA-positive fibers terminating in different regions of the contralateral PAG. GrN-PAG afferents terminated in the caudal and middle portions of PAG-l, whereas CunN-PAG fibers terminated in the middle and rostral portions of PAG-l. Based on the DCN somatotopic map, the GrN sends information to the PAG from the contralateral hindlimb and the tail and the CunN from the contralateral forelimb, shoulder, neck and ear. This somatotopic organization is consistent with earlier electrophysiological and PAG stimulation studies. These fibers could form part of the DCs-brainstem-spinal cord loop, which may be involved in the inhibitory effects of SCS on neuropathic pain.

Dynamics of sensory thalamocortical synaptic networks during information processing states

The thalamocortical network consists of the pathways that interconnect the thalamus and neocortex, including thalamic sensory afferents, corticothalamic and thalamocortical pathways. These pathways are essential to acquire, analyze, store and retrieve sensory information. However, sensory information processing mostly occurs during behavioral arousal, when activity in thalamus and neocortex consists of an electrographic sign of low amplitude fast activity, known as activation, which is caused by several neuromodulator systems that project to the thalamocortical network. Logically, in order to understand how the thalamocortical network processes sensory information it is essential to study its response properties during states of activation. This paper reviews the temporal and spatial response properties of synaptic pathways in the whisker thalamocortical network of rodents during activated states as compared to quiescent (non-activated) states. The evidence shows that these pathways are differentially regulated via the effects of neuromodulators as behavioral contingencies demand. Thus, during activated states, the temporal and spatial response properties of pathways in the thalamocortical network are transformed to allow the processing of sensory information.

Glutamate, but not aspartate, is enriched in trigeminothalamic tract terminals and associated with their synaptic vesicles in the rat nucleus submedius

To examine the possible roles of glutamate and aspartate as neurotransmitters in the nucleus submedius (Sm) of rats, the distributions of these amino acids were examined by electron microscopic immunogold labeling. High levels of glutamate were detected in trigeminothalamic tract terminals anterogradely labeled with horseradish peroxidase conjugates. These terminals also displayed a positive correlation between the densities of synaptic vesicles and gold particles signaling glutamate. In contrast, aspartate levels in such terminals were low and displayed no correlation with the density of synaptic vesicles. Terminals of presumed cortical origin contained the highest estimated levels of glutamate, but the positive correlation between glutamate signal and synaptic vesicle density did not reach statistical significance, presumably due to technical factors. The latter terminals also contained relatively high levels of aspartate, though without any correlation to synaptic vesicle density. The present findings provide strong support for glutamate, but not aspartate, as a trigeminothalamic tract neurotransmitter responsible for the fast synaptic transmission of nociceptive signals to neurons in the rat nucleus submedius. Aspartate presumably serves metabolic roles in these terminals. With respect to terminals of presumed cortical origin, our data are not at odds with the notion that also these terminals use glutamate as their neurotransmitter. Our findings do not support a neurotransmitter role for aspartate in the latter terminals, although such a role cannot be entirely refuted.

Role of thalamic phospholipase C[beta]4 mediated by metabotropic glutamate receptor type 1 in inflammatory pain

Phospholipase C (PLC) beta4, one of the four isoforms of PLCbetas, is the sole isoform expressed in the mouse ventral posterolateral thalamic nucleus (VPL), a key station in pain processing. The mouse thalamus also has been shown to express a high level of metabotropic glutamate receptor type 1 (mGluR1), which stimulates PLCbetas through activation of Galphaq/11 protein. It is therefore expected that the thalamic mGluR1-PLCbeta4 cascade may play a functional role in nociceptive transmission. To test this hypothesis, we first studied behavioral responses to various nociceptive stimuli in PLCbeta4 knock-out mice. We performed the formalin test and found no difference in the pain behavior in the first phase of the formalin test, which is attributed to acute nociception, between PLCbeta4 knock-out and wild-type mice. Consistent with this result, acute pain responses in the hot plate and tail flick tests were also unaffected in the PLCbeta4 knock-out mice. However, the nociceptive behavior in the second phase of the formalin test, resulting from the tissue inflammation, was attenuated in PLCbeta4 knock-out mice. In the dorsal horn of the spinal cord where PLCbeta1 and PLCbeta4 mRNAs are expressed, no difference was found between the wild-type and knock-out mice in the number of Fos-like immunoreactive neurons, which represent neuronal activity in the second phase in the formalin test. Thus, it is unlikely that spinal PLCbeta4 is involved in the formalin-induced inflammatory pain. Next, we found that pretreatment with PLC inhibitors, mGluR1 antagonists, or both, by either intracerebroventricular or intrathalamic injection, attenuated the formalin-induced pain behavior in the second phase in wild-type mice. Furthermore, activation of mGluR1 at the VPL enhanced pain behavior in the second phase in the wild-type mice. In contrast, PLCbeta4 knock-out mice did not show such enhancement, indicating that mGluR1 is connected to PLCbeta4 in the VPL. Finally, in parallel with the behavioral results, we showed in an electrophysiological study that the time course of firing discharges in VPL corresponds well to that of pain behavior in the formalin test in both wild-type and PLCbeta4 knock-out mice. These findings indicate that the thalamic mGluR1-PLCbeta4 cascade is indispensable for the formalin-induced inflammatory pain by regulating the response of VPL neurons.

The lemniscal-cuneate recurrent excitation is suppressed by strychnine and enhanced by GABAA antagonists in the anaesthetized cat

In the somatosensory system, cuneolemniscal (CL) cells fire high frequency doublets of spikes facilitating the transmission of sensory information to diencephalic target cells. We studied how lemniscal feedback affects ascending transmission of cutaneous neurons of the middle cuneate nucleus. Electrical stimulation of the contralateral medial lemniscus and of the skin at sites evoking responses with minimal threshold induced recurrent activation of CL cells at a latency of 1-3.5 ms. The lemniscal feedback activation was suppressed by increasing the stimulating intensity at the same sites, suggesting recurrent-mediated lateral inhibition. The glycine antagonist strychnine blocked the recurrent excitatory responses while GABAA antagonists uncovered those obscured by stronger stimulation. CL cells sharing a common receptive field (RF) potentiate one another by recurrent activation and disinhibition, the disinhibition being produced by serial interactions between glycinergic and GABAergic interneurons. Conversely, CL cells with different RFs inhibit each other through recurrent GABA-mediated inhibition. The lemniscal feedback would thus enhance the surround antagonism of a centre response by increasing the spatial resolution and the transmission of weak signals.

Extracellular glutamate, aspartate and arginine increase in the ventral posterolateral thalamic nucleus during nociceptive stimulation

Although there is evidence that the thalamus plays a remarkable role in pain processing few in vivo studies on the thalamic neurochemical correlates of pain have been done. In the present experiments a combination of capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF) and microdialysis in freely moving rats was used to measure extracellular arginine, glutamate and aspartate in the thalamus during the formalin test. Microdialysis probes were implanted in the left ventral posterolateral (VPL) nucleus of the thalamus in rats. Samples were collected every 30 s, derivatized with fluorescein isothyocyanate and injected into a CZE-LIF instrument. After nine baseline samples, a subcutaneous formalin (5%, 50 microl) injection in the right hind paw caused an increase of arginine, glutamate and aspartate that lasted for about 3 min. These increases were calcium and nerve impulse dependent. These results indicate that the release of arginine, glutamate and aspartate may mediate rapid pain neural transmission in the VPL nucleus of the thalamus.

The role of CNS NMDA receptors and nitric oxide in visceral hyperalgesia

The studies summarized here document the role of NMDA receptors and nitric oxide in the lumbosacral spinal cord and rostral ventromedial medulla in the maintenance of visceral hyperalgesia. Experiments were conducted in rats in which drugs were administered into either the lumbosacral intrathecal space or directly into the rostral ventromedial medulla. The visceral stimulus was noxious colorectal distension, administered before and 3 h after intracolonic instillation of either saline or 25% zymosan. The visceromotor response to colonic distension was quantified and found to be significantly enhanced in rats in which the colon had previously been treated with zymosan. Enhanced responses to distension were attenuated dose-dependently by intrathecal administration of the NMDA receptor channel blocker MK-801 and by inhibition of the neuronal isoform of nitric oxide synthase (nNOS). In corresponding studies wherein drugs were administered directly into the rostral ventromedial medulla, NMDA receptor antagonism and NOS inhibition dose-dependently attenuated exaggerated responses to colonic distension. Taken together, these data suggest that zymosan-produced visceral hyperalgesia is influenced both at the level of the spinal cord and rostral ventromedial medulla, and that descending facilitatory influences from the rostral ventromedial medulla are important to the maintenance of visceral hyperalgesia.

Modulation of nociceptive transmission by NMDA/glycine site receptor in the ventroposterolateral nucleus of the thalamus

NMDA-type glutamate receptors are involved in the generation and maintenance of altered pain states. In the present study, we examined the effect of an NMDA-glycine site antagonist, GV196771A [E-4, 6-dichloro-3-(2-oxo-1-phenyl-pyrrolidin-3-ylidenemethyl)-1H- indole-2- carboxylic acid sodium salt], on responses to noxious stimuli both in normal rats and during peripheral mononeuropathy induced by chronic constriction injury (CCI) of the sciatic nerve. In one series of experiments, activity of nociceptive neurons in the ventroposterolateral (VPL) nucleus of the thalamus was recorded in response to pressure stimuli to the contralateral hindpaw. Intravenous injection (iv) of the glycine antagonist had no effect on these cells in normal rats. When tested in rats with CCI induced 2-3 weeks previously, however, GV196771A (0.125, 0.5 and 2.0mg/kg) blocked responses to noxious stimulation in a dose-dependent and reversible manner. Morphine (0.5mg/kg, iv) and the NMDA channel blocker MK801 (0.1mg/kg, iv) suppressed noxious stimulus-evoked activity of VPL neurons in both normal and CCI-treated rats. MK801 also decreased the responses of non-nociceptive neurons to brush stimulation in both sets of animals, in contrast to the glycine antagonist which did not alter the responses of these cells. Similar results were obtained from a series of behavior experiments in which the latency for paw withdrawal from heat stimulation was measured in normal and CCI-treated rats. GV196771A (3 and 10mg/kg) injected orally, reduced the hyperalgesic response in the treated rats but did not change the withdrawal latency in normal rats. Taken together, these findings suggest that block of the NMDA receptor decreases nociceptive transmission in the thalamus and can modulate hyperalgesic states. GV196771A and glycine antagonists in general may represent innovative and safe agents for the treatment of neuropathic pain.

Immunoelectron microscopic study of glutamate inputs from the retrosplenial granular cortex to identified thalamocortical projection neurons in the anterior thalamus of the rat

We have carried out an ultrastructural study to determine the characteristics and distribution of glutamate-containing constituents of the anterodorsal (AD) and anteroventral (AV) thalamic nuclei in adult rats. We used a polyclonal antibody to glutamate and a postembedding immunogold detection method in animals in which the neurons of AD/AV projecting to the cortex had been retrogradely labelled and the terminals of corticothalamic afferents anterogradely labelled by injection of cholera toxin-horseradish peroxidase (HRP) into the retrosplenial granular cortex. The heaviest immunogold labelling was over axon terminals 0.42 to 2.2 microm in diameter containing round synaptic vesicles and establishing Gray type 1 (asymmetric) synaptic contact (type 1 terminals) on HRP-labelled or non-labelled dendrites. Mean gold particle densities over such terminals were 3-4 times higher than the densities over the dendrites to which they were presynaptic and 5-6 times higher than over terminals establishing Gray type 2 (symmetric) synaptic contacts (type 2 terminals). Gold particle densities over neuronal cell bodies and dendrites and over a subpopulation of myelinated axons were intermediate between the densities over type 1 and type 2 terminals. In adjacent serial sections immunoreacted for gamma aminobutyric acid, type 2 terminals were heavily immunolabelled whereas type 1 terminals and other profiles with moderate gold particle densities after glutamate immunoreaction displayed very low labelling. A subpopulation of small type 1 axon terminals (up to 1 microm diameter) contained HRP reaction product identifying them as cortical in origin; they contacted small dendritic profiles (most <1 microm diameter) many of which also contained HRP reaction product. We conclude that terminals of the corticothalamic projection from retrosplenial granular cortex to AD/AV are glutamatergic and innervate predominantly distal dendrites of thalamocortical projection neurons.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Glutamate-like immunoreactivity in ascending spinofugal afferents to the rat periaqueductal grey

The midbrain periaqueductal gray is a key structure for the mediation of an integrated defence behaviour. Although a prominent role for glutamate in PAG mechanisms is supported by both behavioural and morphological studies, whether PAG afferents conveying somatosensory information constitute a source of glutamatergic input to the PAG remains unknown. Here, we have compared the projection pattern of orthogradely-labelled spinoannular fibres with the distribution of glutamate-like immunoreactivity in the PAG at the light microscopic level. Transaxonal labelling was observed throughout the whole rostrocaudal axis of the PAG except for the dorsolateral regions. Cell-processes and terminal-reminiscent puncta were strongly immunoreactive in all PAG regions, including the dorsolateral areas. To ascertain whether glutamate-immunoreactive puncta observed at light microscopy indeed constituted axon terminals of the spinoannular system, glutamate-like immunoreactivity was assessed in orthogradely-labelled synaptic terminals using a post-embedding immunogold procedure for electron microscopy. Quantitative analysis of gold particle densities revealed over twice as strong an immunoreactivity in anatomically-identified spinoannular axon terminals as in dendrites postsynaptic to them, perikarya and inhibitory Gray II synapses, as well as an over 5-fold heavier immunolabelling than in glial profiles. These findings reveal that glutamate is accumulated in synaptic terminals of the spinoannular system, supporting a neurotransmitter role for this acidic amino acid in spinofugal afferents to the PAG.

Immunoreactivity for the GABA transporter-1 and GABA transporter-3 is restricted to astrocytes in the rat thalamus. A light and electron-microscopic immunolocalization

GABA plasma membrane transporters mediate GABA uptake into presynaptic terminals and surrounding glial processes and thus play a key role in shaping the time course and spatial extent of GABA's action. In the present study we have investigated the cellular and subcellular localization of two GABA transporters (1 and 3) in the rat thalamus using affinity-purified polyclonal antibodies. GABA transporter-1 and -3 immunoreactivity, detected with immunoperoxidase and immunofluorescence methods, is present throughout the thalamus in small punctate structures scattered in the neuropil among unlabelled neuronal perikarya. Labelling for GABA transporter-3 is always more intense than that for GABA transporter-1. Astrocytic processes, identified by their immunoreactivity for glial fibrillary acidic protein, express both GABA transporters. Ultrastructural investigations confirm that GABA transporter-1 and -3 labelling is restricted to astrocytes. Labelled astrocytes are adjacent to terminals making either symmetric or asymmetric synaptic contacts, and are close to neuronal profiles that do not form synaptic contacts in the plane of the section. In double-labelled thin sections some GABA transporter-1- or -3-positive astrocytic processes, detected with immunoperoxidase labelling, surround GABA-positive terminals, detected with antibodies to GABA and immunogold labelling. These findings demonstrate that in rat thalamus the GABA uptake system mediated by GABA transporter-1 and -3 is localized exclusively to astrocytes near the synapses and in the neuropil, and absent from GABAergic terminals. Astrocytes play therefore an important role in mediating GABA transmission in the thalamus, compared to cortical regions.

A simplified procedure for the physical development of the sulphide silver method to reveal synaptic zinc in combination with immunocytochemistry at light and electron microscopy

The pool of zinc present in excitatory synaptic terminals in normal and pathological conditions (for instance the status epilepticus induced by kainic acid) can be stained by a silver sulphide method followed by physical development of the insoluble zinc-sulphide complexes. In this study we applied a previously described simple and rapid developing procedure that reveals synaptic zinc, to the study of normal and pathological hippocampi and combined it with pre and postembedding immunocytochemical methods to detect different antigens. Normal and kainic acid-treated rats were perfused with fixative solutions containing sodium sulphide and 50 microm-thick vibratome sections of the hippocampi were incubated in a commercial developing solution (IntenSE M, Amersham). The developed vibratome sections were then (1) mounted for light microscopy or osmicated and epon-embedded for electron microscopy; or (2) processed for the preembedding immunoenzymatic detection of various antigens (GABA, parvalbumin, calbindin) with light and electron microscopy. Thin sections from epon-embedded samples were also processed for the postembedding immunogold localization of glutamate. This very simple and rapid procedure gives rise to zinc-specific staining, comparable to that obtained with classical developing methods and good preservation of both antigenicity and ultrastructure. It is therefore possible to detect, in the same thick or thin section, zinc reaction product and different antigens.

Subcellular distribution of AMPA and NMDA receptor subunit immunoreactivity in ventral posterior and reticular nuclei of rat and cat thalamus

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) selective glutamate receptors mediate excitatory neurotransmission in the somatosensory thalamus, but morphological localization of the receptors at identified thalamic synapses has been lacking. The authors used electron microscopic immunocytochemistry to localize AMPA selective GluR 2/3 subunits (GluR2/3) and NMDA receptor subunit 1 (NMDAR1) in rat and cat ventral posterior lateral nucleus (VPL) and in the associated sector of the reticular nucleus (RTN). Light microscopy showed that GluR2/3 and NMDAR1 immunolabeled neurons are homogeneously distributed in both nuclei. The relationship between glutamate receptor labeled profiles and glutamate or gamma-aminobutyric acid (GABA) labeled synapses was revealed by combining preembedding and postembedding immunostaining at the electron microscopic level. GluR2/3 and NMDAR1 immunoreactivity was located in somata and in proximal and distal dendrites of VPL relay cells and of RTN cells. Immunoreactivity was concentrated in postsynaptic densities of glutamatergic synapses and absent from postsynaptic densities of GABAergic synapses. In the cat, GluR2/3 and NMDAR1 immunoreactivity was also localized in GABAergic interneurons, including their presynaptic dendrites (PSD). Of the GluR2/3 and NMDAR1 labeled thalamic synapses observed, 10-29% were lemniscal (RL) type synapses in VPL; 60-70% were corticothalamic (RS) type synapses in the VPL and RTN. In the cat, 7-19% were identified as PSD profiles, and more NMDAR1 labeled PSD were found in the VPL than in the RTN. The main findings were as follows: 1) AMPA selective GluR2/3 and NMDAR1 share similar distribution patterns in the rat and cat somatosensory thalamus, 2) both glutamate receptors are likely to be colocalized at postsynaptic densities of both RL and RS synapses, and 3) localization of the glutamate receptor proteins in GABAergic dendrites in the cat thalamus indicates that glutamatergic transmission to GABAergic neurons is also mediated by both NMDA and non-NMDA receptors.

Thalamic-projecting preprocholecystokinin messenger RNA-expressing neurons in the dorsal column nuclei of the rat

This study aimed at investigating the expression of preprocholecystokinin messenger RNA among thalamic-projecting neurons in the dorsal column nuclei of the rat. Thalamic-projecting neurons were identified by injection of cholera toxin subunit b into the ventroposterolateral nucleus. Following immunohistochemical detection of retrogradely transported tracer substance, the expression of preprocholecystokinin messenger RNA in the projection neurons of the dorsal column nuclei was detected by in situ hybridization, using autoradiographic visualization of a 35S-labeled RNA probe complementary to preprocholecystokinin messenger RNA. Many preprocholecystokinin-expressing neurons were seen in the dorsal column nuclei. A large proportion of these neurons were also labeled with cholera toxin. The double-labeled neurons, as well as neurons single-labeled with preprocholecystokinin messenger RNA or cholera toxin, were preferentially found within the middle region of the dorsal column nuclei, located just caudal to the obex. These findings demonstrate that neurons in the dorsal column nuclei express preprocholecystokinin messenger RNA, and show that these neurons provide a peptidergic projection from the dorsal column nuclei to the ventroposterolateral nucleus of the thalamus. These observations suggest that cholecystokinin may be involved in the transmission of somatosensory (tactile) information from the dorsal column nuclei to the thalamus.

Heterogeneity of AMPA receptors in the dorsal column nuclei of the rat

We have combined immunocytochemistry with retrograde tracing to demonstrate that projecting neurons in the gracile and cuneate nuclei express predominantly the GluR3 subunit of the AMPA receptor while interneurons in these nuclei express predominantly the GluR1 subunit. Interneurons expressing the GluR2 subunit are also present. It is speculated that the two classes of interneurons may release different inhibitory transmitters.

Postnatal development of GABA-immunoreactive terminals in the reticular and ventrobasal nuclei of the rat thalamus: a light and electron microscopic study

The postnatal development of inhibitory GABAergic circuits in the thalamic reticular and ventrobasal nuclei was studied in rats ranging from the day of birth to the end of the third postnatal week by means of a postembedding immunogold staining procedure to visualize GABA. In the reticular nucleus, GABA labeling was present from birth in cell bodies, dendrites, growth cones and a few synaptic terminals, whereas in the ventrobasal nucleus it was exclusively in axonal processes identifiable as growth cones, vesicle-rich profiles and synaptic terminals. In both nuclei, GABA-labeled synaptic terminals were, however, very scarce and immature in neonatal animals and they became numerous and morphologically mature only after the end of the second postnatal week. These findings suggest that inhibitory synaptic responses in the somatosensory thalamus are not yet fully mature throughout the first two postnatal weeks and support the hypothesis that GABA may initially play trophic roles. The relatively late maturation of the thalamic GABAergic system may have important functional consequences, as the reticulothalamic circuits are responsible for the generation of spindle wave oscillations whose cellular mechanisms are also involved in the generation of spike-and-wave (absence) seizures in humans and in animal models.

GABAergic neurons in mammalian thalamus: a marker of thalamic complexity?

The present study evaluated the occurrence, distribution, and number of GABAergic neurons in the thalamus of different mammalian species (bat, mouse, rat, guinea pig, rabbit, cat, monkey, humans), by means of light microscopical immunoenzymatic localization of GABA or of its biosynthetic enzyme glutamic acid decarboxylase and by ultrastructural immunogold detection of GABA. Our data demonstrated that: 1) GABAergic local circuit neurons were detected in the thalamic visual domain in all the species analyzed, whereas in other thalamic nuclei their presence and number varied among species; 2) the number of GABAergic local circuit neurons progressively increased in the dorsal thalamus of species with more complex behavior; 3) the presence of local circuit neurons conferred a similar intrinsic organization to the dorsal thalamic nuclei, characterized by complex synaptic arrangements; 4) in the reticular thalamic nucleus, whose neurons were GABA-immunoreactive in all the examined species, the cellular density decreased from the bat to humans. These findings strongly suggest that thalamic GABAergic local circuit neurons are not directly related to the ability to perform specific sensorimotor tasks, but they are likely to reflect an increasing complexity of the local information processing that occurs at thalamic level.

Immunohistochemical studies of localization and co-localization of glutamate, aspartate and GABA in the anterior thalamic nuclei, retrosplenial granular cortex, thalamic reticular nucleus and mammillary nuclei of the rat

Localization and possible co-localization of glutamate, aspartate and GABA immunoreactivities was examined in the anterior thalamic nuclei, retrosplenial granular cortex, thalamic reticular nucleus and mammillary nuclei of the rat by double antigen immunohistochemistry using diaminobenzidine and benzidine dihydrochloride in one series and double immunofluorescence labelling with rhodamine and fluorescein in a second series of animals. In three of these regions, retrosplenial granular cortex, anterior thalamic nuclei, and mammillary nuclei, glutamate immunoreactivity was co-localized with aspartate immunoreactivity in a majority of the projection neurons (pyramidal neurons, predominantly in layers V and VI in retrosplenial granular cortex; rounded polygonal multipolar neurons throughout the rostrocaudal extent of the anterior thalamic and mammillary nuclei). None of the cells showing glutamate and/or aspartate immunoreactivity in these regions also displayed GABA immunoreactivity, which was present in non-pyramidal cells in the retrosplenial granular cortex (chiefly in layers I-III) and in small numbers of cells within the anterior thalamic nuclei. In the thalamic reticular nucleus, in contrast, most neurons were immunoreactive for GABA and in the majority of these neurons glutamate (and/or aspartate) immunoreactivity was co-localized with GABA.

Role of N-methyl-D-aspartate and metabotropic glutamate receptors in corticothalamic excitatory postsynaptic potentials in vivo

The ventrobasal thalamus is the principal somatosensory thalamic relay nucleus, and it receives two major sources of excitatory input: firstly an input from ascending sensory afferents, and secondly a descending projection from the primary somatosensory cortex. There is considerable anatomical evidence to suggest that both of these projections utilise the excitatory amino acid L-glutamate as their neurotransmitter. Previous work from this laboratory has shown that the sensory input to the rat ventrobasal thalamus in vivo is mediated by ionotropic excitatory amino acid receptors of both the N-methyl-D-aspartate and non-N-methyl-D-aspartate type. These findings are consistent with data from other studies in various thalamic relay nuclei. In contrast, there are considerably less data available concerning the synaptic pharmacology of the corticothalamic projection although there have been both speculation and studies concerning the functional significance of this pathway. There is some evidence to suggest an involvement of N-methyl-D-aspartate receptors and metabotropic glutamate receptors. The aim of this study was to determine which excitatory amino acid receptors might mediate cortically-elicited excitatory postsynaptic potential in the ventrobasal thalamus in vivo. Intracellular recordings were made, and neurotransmitter antagonists were applied on to rat ventrobasal thalamus neurons by microiontophoresis. Cortically-elicited excitatory postsynaptic potentials were reduced by the N-methyl-D-aspartate antagonist 3-[(+/-)-2-carboxy-piperazin-4-yl]-propyl-1-phosphonate, or the Group I metabotropic antagonist (S)-4-carboxyphenylglycine. These data indicate that both N-methyl-D-aspartate receptors and Group I (possibly metabotropic glutamate receptors type I) metabotropic receptors are involved in the mediation of corticothalamic transmission. Such a transmitter mechanism would allow a modulatory system that could selectively enhance other excitatory inputs. Some of these data have been reported in abstract form.

Functions of ionotropic and metabotropic glutamate receptors in sensory transmission in the mammalian thalamus

The thalamic relay nuclei play a pivotal role in gating and processing sensory information en route to the cerebral cortex. The major ascending sensory afferents and the descending cortico-fugal afferents to the thalamus almost certainly use the excitatory amino acid L-glutamate as their transmitter. This paper reviews the nature of this transmission in terms of the receptor types which may be used (NMDA, AMPA, kainate and metabotropic glutamate receptors), their electrophysiological and pharmacological properties, and their differential location in the thalamus on neurones, terminals and glial elements. Whilst AMPA receptors, probably of more than one variety, are likely to mediate fast transmission in the thalamus, the contributions of NMDA receptors and metabotropic glutamate receptors to sensory responses under different stimulus conditions may be more varied. This is discussed in the context of the possible functional significance of the interplay of L-glutamate-gated currents with intrinsic membrane currents of thalamic neurones. The interaction of L-glutamate transmission with other modulators (acetylcholine, noradrenaline, serotonin, glycine, D-serine, nitric oxide, arginine, redox agents) is considered.

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.