Evidence for glutamate as the neurotransmitter of corticothalamic and corticorubral pathways

Functionally Distinct Circuits Are Linked by Heterocellular Electrical Synapses in the Thalamic Reticular Nucleus

The thalamic reticular nucleus (TRN) inhibits sensory thalamocortical relay neurons and is a key regulator of sensory attention as well as sleep and wake states. Recent developments have identified two distinct genetic subtypes of TRN neurons, calbindin-expressing (CB) and somatostatin-expressing (SOM) neurons. These subtypes differ in localization within the TRN, electrophysiological properties, and importantly, targeting of thalamocortical relay channels. CB neurons send inhibition to and receive excitation from first-order thalamic relay nuclei, while SOM neurons send inhibition to and receive excitation from higher-order thalamic areas. These differences create distinct channels of information flow. It is unknown whether TRN neurons form electrical synapses between SOM and CB neurons and consequently bridge first-order and higher-order thalamic channels. Here, we use GFP reporter mice to label and record from CB-expressing and SOM-expressing TRN neurons. We confirm that GFP expression properly differentiates TRN subtypes based on electrophysiological differences, and we identified electrical synapses between pairs of neurons with and without common GFP expression for both CB and SOM types. That is, electrical synapses link both within and across subtypes of neurons in the TRN, forming either homocellular or heterocellular synapses. Therefore, we conclude that electrical synapses within the TRN provide a substrate for functionally linking thalamocortical first-order and higher-order channels within the TRN.

On the Diverse Functions of Electrical Synapses

Electrical synapses are the neurophysiological product of gap junctional pores between neurons that allow bidirectional flow of current between neurons. They are expressed throughout the mammalian nervous system, including cortex, hippocampus, thalamus, retina, cerebellum, and inferior olive. Classically, the function of electrical synapses has been associated with synchrony, logically following that continuous conductance provided by gap junctions facilitates the reduction of voltage differences between coupled neurons. Indeed, electrical synapses promote synchrony at many anatomical and frequency ranges across the brain. However, a growing body of literature shows there is greater complexity to the computational function of electrical synapses. The paired membranes that embed electrical synapses act as low-pass filters, and as such, electrical synapses can preferentially transfer spike after hyperpolarizations, effectively providing spike-dependent inhibition. Other functions include driving asynchronous firing, improving signal to noise ratio, aiding in discrimination of dissimilar inputs, or dampening signals by shunting current. The diverse ways by which electrical synapses contribute to neuronal integration merits furthers study. Here we review how functions of electrical synapses vary across circuits and brain regions and depend critically on the context of the neurons and brain circuits involved. Computational modeling of electrical synapses embedded in multi-cellular models and experiments utilizing optical control and measurement of cellular activity will be essential in determining the specific roles performed by electrical synapses in varying contexts.

Corticospinal vs Rubrospinal Revisited: An Evolutionary Perspective for Sensorimotor Integration

The knowledge about how different subsystems participate and interplay in sensorimotor control is fundamental to understand motor deficits associated with CNS injury and movement recovery. The role of corticospinal (CS) and rubrospinal (RS) projections in motor control has been extensively studied and compared, and it is clear that both systems are important for skilled movement. However, during phylogeny, the emerging cerebral cortex took a higher hierarchical role controlling rubro-cerebellar circuits. Here, we present anatomical, neurophysiological, and behavioral evidence suggesting that both systems modulate complex segmental neuronal networks in a parallel way, which is important for sensorimotor integration at spinal cord level. We also highlight that, although specializations exist, both systems could be complementary and potentially subserve motor recovery associated with CNS damage.

GABAergic and glutamatergic effects on nigrostriatal and mesolimbic dopamine release in the rat

In this review, a series of experiments is presented, in which γ-amino butyric acid (GABA)ergic and glutamatergic effects on dopamine function in the rat nigrostriatal and mesolimbic system was systematically assessed after pharmacological challenge with GABAA receptor (R) and and N-methyl d-aspartate (NMDA)R agonists and antagonists. In these studies, [123I]iodobenzamide binding to the D2/3R was mesured in nucleus accumbens (NAC), caudateputamen (CP), substantia nigra/ventral tegmental area (SN/VTA), frontal (FC), motor (MC) and parietal cortex (PC) as well as anterior (aHIPP) and posterior hippocampus (pHIPP) with small animal SPECT in baseline and after injection of either the GABAAR agonist muscimol (1 mg/kg), the GABAAR antagonist bicuculline (1 mg/kg), the NMDAR agonist d-cycloserine (20 mg/kg) or the NMDAR antagonist amantadine (40 mg/kg). Muscimol reduced D2/3R binding in NAC, CP, SN/VTA, THAL and pHIPP, while, after amantadine, decreases were confined to NAC, CP and THAL. In contrast, d-cycloserine elevated D2/3R binding in NAC, SN/VTA, THAL, frontal cortex, motor cortex, PC, aHIPP and pHIPP, while, after bicuculline, increases were confined to CP and THAL. Taken together, similar actions on regional dopamine levels were exterted by the GABAAR agonist and the NMDAR antagonist on the one side and by the GABAAR antagonist and the NMDAR agonist on the other, with agonistic action, however, affecting more brain regions. Thereby, network analysis suggests different roles of GABAARs and NMDARs in the mediation of nigrostriatal, nigrothalamocortical and mesolimbocortical dopamine function.

Physiological mechanisms of thalamic ventral intermediate nucleus stimulation for tremor suppression

Ventral intermediate thalamic deep brain stimulation is a standard therapy for the treatment of medically refractory essential tremor and tremor-dominant Parkinson's disease. Despite the therapeutic benefits, the mechanisms of action are varied and complex, and the pathophysiology and genesis of tremor remain unsubstantiated. This intraoperative study investigated the effects of high frequency microstimulation on both neuronal firing and tremor suppression simultaneously. In each of nine essential tremor and two Parkinson's disease patients who underwent stereotactic neurosurgery, two closely spaced (600 µm) microelectrodes were advanced into the ventral intermediate nucleus. One microelectrode recorded action potential firing while the adjacent electrode delivered stimulation trains at 100 Hz and 200 Hz (2-5 s, 100 µA, 150 µs). A triaxial accelerometer was used to measure postural tremor of the contralateral hand. At 200 Hz, stimulation led to 68 ± 8% (P < 0.001) inhibition of neuronal firing and a 53 ± 5% (P < 0.001) reduction in tremor, while 100 Hz reduced firing by 26 ± 12% (not significant) with a 17 ± 6% (P < 0.05) tremor reduction. The degree of cell inhibition and tremor suppression were significantly correlated (P < 0.001). We also found that the most ventroposterior stimulation sites, closest to the border of the ventral caudal nucleus, had the best effect on tremor. Finally, prior to the inhibition of neuronal firing, microstimulation caused a transient driving of neuronal activity at stimulus onset (61% of sites), which gave rise to a tremor phase reset (73% of these sites). This was likely due to activation of the excitatory glutamatergic cortical and cerebellar afferents to the ventral intermediate nucleus. Temporal characteristics of the driving responses (duration, number of spikes, and onset latency) significantly differed between 100 Hz and 200 Hz stimulation trains. The subsequent inhibition of neuronal activity was likely due to synaptic fatigue. Thalamic neuronal inhibition seems necessary for tremor reduction and may function in effect as a thalamic filter to uncouple thalamo-cortical from cortico-spinal reflex loops. Additionally, our findings shed light on the gating properties of the ventral intermediate nucleus within the cerebello-thalamo-cortical tremor network, provide insight for the optimization of deep brain stimulation technologies, and may inform controlled clinical studies for assessing optimal target locations for the treatment of tremor.

Interactive responses of a thalamic neuron to formalin induced lasting pain in behaving mice

Thalamocortical (TC) neurons are known to relay incoming sensory information to the cortex via firing in tonic or burst mode. However, it is still unclear how respective firing modes of a single thalamic relay neuron contribute to pain perception under consciousness. Some studies report that bursting could increase pain in hyperalgesic conditions while others suggest the contrary. However, since previous studies were done under either neuropathic pain conditions or often under anesthesia, the mechanism of thalamic pain modulation under awake conditions is not well understood. We therefore characterized the thalamic firing patterns of behaving mice in response to nociceptive pain induced by inflammation. Our results demonstrated that nociceptive pain responses were positively correlated with tonic firing and negatively correlated with burst firing of individual TC neurons. Furthermore, burst properties such as intra-burst-interval (IntraBI) also turned out to be reliably correlated with the changes of nociceptive pain responses. In addition, brain stimulation experiments revealed that only bursts with specific bursting patterns could significantly abolish behavioral nociceptive responses. The results indicate that specific patterns of bursting activity in thalamocortical relay neurons play a critical role in controlling long-lasting inflammatory pain in awake and behaving mice.

Prefrontal cortex and drug abuse vulnerability: translation to prevention and treatment interventions

Vulnerability to drug abuse is related to both reward seeking and impulsivity, two constructs thought to have a biological basis in the prefrontal cortex (PFC). This review addresses similarities and differences in neuroanatomy, neurochemistry and behavior associated with PFC function in rodents and humans. Emphasis is placed on monoamine and amino acid neurotransmitter systems located in anatomically distinct subregions: medial prefrontal cortex (mPFC); lateral prefrontal cortex (lPFC); anterior cingulate cortex (ACC); and orbitofrontal cortex (OFC). While there are complex interconnections and overlapping functions among these regions, each is thought to be involved in various functions related to health-related risk behaviors and drug abuse vulnerability. Among the various functions implicated, evidence suggests that mPFC is involved in reward processing, attention and drug reinstatement; lPFC is involved in decision-making, behavioral inhibition and attentional gating; ACC is involved in attention, emotional processing and self-monitoring; and OFC is involved in behavioral inhibition, signaling of expected outcomes and reward/punishment sensitivity. Individual differences (e.g., age and sex) influence functioning of these regions, which, in turn, impacts drug abuse vulnerability. Implications for the development of drug abuse prevention and treatment strategies aimed at engaging PFC inhibitory processes that may reduce risk-related behaviors are discussed, including the design of effective public service announcements, cognitive exercises, physical activity, direct current stimulation, feedback control training and pharmacotherapies. A major challenge in drug abuse prevention and treatment rests with improving intervention strategies aimed at strengthening PFC inhibitory systems among at-risk individuals.

VPM and PoM nuclei of the rat somatosensory thalamus: intrinsic neuronal properties and corticothalamic feedback

Sensory information originating in individual whisker follicles ascends through focused projections to the brainstem, then to the ventral posteromedial nucleus (VPM) of the thalamus, and finally into barrels of the primary somatosensory cortex (S1). By contrast, the posteromedial complex (PoM) of the thalamus receives more diffuse sensory projections from the brainstem and projects to the interbarrel septa of S1. Both VPM and PoM receive abundant corticothalamic projections from S1. Using a thalamocortical slice preparation, we characterized differences in intrinsic neuronal properties and in responses to corticothalamic feedback in neurons of VPM and PoM. Due to the plane of the slice, the majority of our observed responses came from activation of layer VI because most or all of the layer V axons terminating in PoM are cut. We found that VPM neurons exhibit higher firing rates than PoM neurons when stimulated with injected current. Stimulation of corticothalamic fibers evoked monosynaptic excitation, disynaptic inhibition, or a combination of the two in both nuclei. A few differences in the feedback responses emerged: purely excitatory postsynaptic potentials (EPSPs) in VPM were smaller and facilitated more than those in PoM, and only the EPSPs in VPM had a strong NMDA component. For both nuclei, some of the feedback responses were purely disynaptic inhibitory postsynaptic potentials (IPSPs) from the thalamic reticular nucleus (TRN). This was due to EPSP failures within VPM and PoM combined with greater reliability of S1-originating synapses onto TRN. These findings suggest that despite the exclusively excitatory nature of corticothalamic fibers, activation of cortex can trigger excitation or inhibition in thalamic relay neurons.

Deep brain stimulation

Deep brain stimulation (DBS) has provided remarkable benefits for people with a variety of neurologic conditions. Stimulation of the ventral intermediate nucleus of the thalamus can dramatically relieve tremor associated with essential tremor or Parkinson disease (PD). Similarly, stimulation of the subthalamic nucleus or the internal segment of the globus pallidus can substantially reduce bradykinesia, rigidity, tremor, and gait difficulties in people with PD. Multiple groups are attempting to extend this mode of treatment to other conditions. Yet, the precise mechanism of action of DBS remains uncertain. Such studies have importance that extends beyond clinical therapeutics. Investigations of the mechanisms of action of DBS have the potential to clarify fundamental issues such as the functional anatomy of selected brain circuits and the relationship between activity in those circuits and behavior. Although we review relevant clinical issues, we emphasize the importance of current and future investigations on these topics.

The thalamic reticular nucleus: structure, function and concept

On the basis of theoretical, anatomical, psychological and physiological considerations, Francis Crick (1984) proposed that, during selective attention, the thalamic reticular nucleus (TRN) controls the internal attentional searchlight that simultaneously highlights all the neural circuits called on by the object of attention. In other words, he submitted that during either perception, or the preparation and execution of any cognitive and/or motor task, the TRN sets all the corresponding thalamocortical (TC) circuits in motion. Over the last two decades, behavioural, electrophysiological, anatomical and neurochemical findings have been accumulating, supporting the complex nature of the TRN and raising questions about the validity of this speculative hypothesis. Indeed, our knowledge of the actual functioning of the TRN is still sprinkled with unresolved questions. Therefore, the time has come to join forces and discuss some recent cellular and network findings concerning this diencephalic GABAergic structure, which plays important roles during various states of consciousness. On the whole, the present critical survey emphasizes the TRN's complexity, and provides arguments combining anatomy, physiology and cognitive psychology.

Distribution and properties of GABA(B) antagonist [3H]CGP 62349 binding in the rhesus monkey thalamus and basal ganglia and the influence of lesions in the reticular thalamic nucleus

GABA(B) receptors are believed to be associated with the efferents of the nucleus reticularis thalami, which is implicated in the regulation of activity in the thalamocortical-corticothalamic circuit and plays a role in absence seizures. Yet, the distribution of GABA(B) receptors in the thalamus has only been studied in the rat, and there is no comparable information in primates. The potent GABA(B) receptor antagonist [3H]CGP 62349 was used to study the distribution and binding properties of the receptor in control monkeys and those with small ibotenic acid lesions in the anterodorsal segment of the nucleus reticularis thalami. Eight-micrometer-thick cryostat sections of the fresh frozen brains were incubated in the presence of varying concentrations of the ligand. Autoradiographs were analysed using a quantitative image analysis technique, and binding parameters were calculated for select thalamic nuclei as well as basal ganglia structures present in the same sections. The overall number of GABA(B) binding sites in the monkey thalamus and basal ganglia was several-fold higher than previously reported values for the rat. In the thalamus, the receptors were distributed rather uniformly and the binding densities and affinities were high (Bmax range of 245.5-437.9 fmol/ mg of tissue, Kd range of 0.136-0.604 nM). In the basal ganglia, the number of binding sites and the affinities were lower (Bmax range of 51.1-244.2 fmol/mg of tissue; K(d) range of 0.416-1.394 nM), and the differences between nuclei were more pronounced, with striatum and substantia nigra pars compacta displaying the highest binding densities. Seven days post-lesion, a 20-30% decrease in Bmax values (P < 0.05) was found in the nuclei receiving input from the lesioned nucleus reticularis thalami sector (the mediodorsal nucleus and densicellular and magnocellular parts of the ventral anterior nucleus) without changes in affinity. No significant changes were detected in any other structures. The results of the lesioning experiments suggest that a portion of thalamic GABA(B) receptors is in a presynaptic location on the nucleus reticularis thalami efferents. The overall distribution pattern in the thalamus also suggests a partial association of GABA(B) receptors with corticothalamic terminals presynaptically.

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.

Adenosinergic modulation of ethanol-induced motor incoordination in the rat motor cortex

1. On going work in our laboratory has shown that adenosine modulates ethanol-induced motor incoordination (EIMI) when given systemically as well as directly into the cerebral ventricles, cerebellum and corpus striatum of the rat and/or mouse. 2. The objective of this study was to determine what effect adenosine agonists and antagonists would have within the rat motor cortex on EIMI. 3. The participation of the motor cortex in EIMI was suggested when microinfusion of the anti-ethanol compound, Ro15-4513, an inverse agonist of the benzodiazepine binding site, directly into the motor cortex significantly attenuated EIMI. Further, the adenosine agonists N6-cyclohexyladenosine (CHA) and 2-p-(2-carboxyethyl)-phenethylamino-5'-N-carboxaminoadenosine++ + hydrochloride (CGS-21680) significantly accentuated EIMI in a dose-related manner. The adenosine A1 receptor-selective agonist, CHA, appeared most potent in this modulatory effect when compared to the A2-selective agonist, CGS-21680. 4. The extent of diffusion of the adenosine drugs within the cortical tissue after their microinfusion was also checked by measuring the dispersion of microinfused [3H]CHA. The [3H]CHA dispersion study indirectly confirmed that the results of the present investigation were based on the effect of adenosine drugs within the motor cortex only. 5. Accentuation by the A1- and A2-selective adenosine agonists was significantly attenuated by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2 receptor-selective antagonist 8-(3-chlorostyryl)caffeine (CSC) further suggesting modulation mainly by the A1-subtype. 6. Pretreatment of the motor cortex with pertussis toxin (PT) significantly reduced the capacity of both A1- and A2-selective adenosine agonists to accentuate EIMI suggesting the involvement of a PT-sensitive Gi/Go protein. 7. These data support earlier work which showed that adenosine modulates EIMI within the central nervous system (CNS), most likely via the A1 receptor, and moreover, extend that work by including the motor cortex as a brain area participating in the adenosinergic modulation of ethanol-induced motor impairment.

Pallidotomy for hemiballismus: efficacy and characteristics of neuronal activity

A patient with unremitting, medically intractable hemiballismus underwent a pallidotomy that abolished his involuntary movements. Firing rates of cells in the internal segment of the globus pallidus (GPi) recorded during this procedure were significantly lower than those observed during pallidotomy for Parkinson's disease, either "on" or "off" medication. Firing patterns in hemiballismus were characterized by low-frequency modulation of the firing rate. These results are consistent with the hyperkinetic model, which suggests that hemiballismus results from decreased inhibition of the pallidal relay nucleus of the thalamus by the GPi. The efficacy of surgery in the case of hemiballismus demonstrates that pallidotomy can be an effective treatment for this condition and suggests that patterned neuronal activity in the GPi is important in the mechanism of hyperkinetic disorders.

Mode of termination of pallidal afferents to the thalamus: a light and electron microscopic study with anterograde tracers and immunocytochemistry in Macaca mulatta

The mode of termination of individual pallidothalamic fibers in the densicellular subdivision of the ventral anterior thalamic nucleus (VAdc) of Macaca mulatta was analyzed with light and electron microscopy after injections of anterograde tracers in the medial globus pallidus. Three tracers were utilized: tritiated leucine, biotinylated dextran amine, and wheat germ agglutinin conjugated to horseradish peroxidase in combination with postembedding immunocytochemsitry for gamma-aminobutyric acid (GABA). Pallidothalamic fibers, upon entering the VAdc, gave off several collaterals that formed plexuses of varicose terminal branches within different cell clusters. The varicosities were aligned along somata and proximal dendrites of projection neurons providing dense input to each individual cell. At the electron microscopic level, labeled boutons displayed a predominantly flat and elongated shape. They contained a moderate number of pleomorphic synaptic vesicles and very large amounts of mitochondria, displayed symmetric synaptic contacts, and were immunoreactive for GABA. In the total sample of 128 autoradiographically labeled terminals, 64% were in synaptic contact with somata and primary dendrites of projection neurons, 14% formed synapses on proximal dendrites of undefined order, and only 7% established synaptic contacts on distal dendrites. Fifteen percent of the labeled boutons established synapses on distal dendrites of GABAergic local circuit neurons (LCN). Pallidal boutons were also found in complex synaptic arrangements: triads with three GABAergic synapses, and serial synapses with LCN dendrites that in turn established synaptic contacts on projection neuron somata or dendrites. These anatomical results suggest a dual effect of pallidal afferents to projection neurons: direct inhibition and disinhibition mediated by LCN. The findings indicate that the fine structure of pallidothalamic terminals in the monkey is similar to that described earlier in the cat. There are, however, interspecies differences in the distribution of pallidal input on postsynaptic targets and its participation in complex synaptic arrangements.

Effect of NMDA on the activity of cortical glutaminergic structures in delayed visual differentiation in monkeys

The effect of intracortical perfusion with the glutamate agonist NMDA on visual recognition and short-term memory, as well as an on the responses of visual cortex neurons, were studied in rhesus macaques. A microdialysis technique was used in combination with multichannel microelectrode recording of single cortical cells in the immediate vicinity of the dialysis tube in a behavioral experiment in which the monkey had to solve a task involving delayed visual differentiation of stimuli of different colors. NMDA altered the characteristics of recognition in monkeys. The duration of information storage in short-term memory was increased significantly (2-4-fold), and there was a significant reduction in the motor response time for all delay periods. These changes were accompanied by a significant rearrangement of neuron activity in the visual cortex at all stages of the behavioral task. At different stages of the task, 70-85% of the neurons showed 2-5-fold increases in activity, while 6-20% showed reductions in activity. These results demonstrate an involvement of visual cortex glutaminergic structures in the process of visual recognition and short-term memory, as well as a nootropic effect obtained by intracortical administration of NMDA.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Glutamate and aspartate immunoreactivity in the reciprocal projections between the anterior thalamic nuclei and the retrosplenial granular cortex in the rat

We have used retrograde and anterograde labelling with wheat germ agglutinin-horseradish peroxidase and immunohistochemistry with antibodies against glutamate and aspartate to examine the reciprocal connections between the anterior thalamic nuclei and the retrosplenial granular cortex in the rat, and to characterize those projection neurones that contain glutamate and/or aspartate. Injections into superficial layers of the retrosplenial granular cortex resulted in retrogradely labelled cell bodies in the anterodorsal, anteroventral, and to a lesser extent the anteromedial subnuclei. Approximately 70% of these cell bodies were also immunolabelled for glutamate or aspartate. Injections confined to deep layers (V-VI) resulted in the presence, in anterior thalamic neuropil, of anterogradely labelled fibre and terminal-like structures, many of which appeared to be immunolabelled for glutamate or aspartate. Injections into the anterior thalamic nuclei resulted in retrogradely labelled pyramidal cells in layers V-VI of the retrosplenial granular cortex. Most (90-95%) of these cells were immunolabelled for glutamate or aspartate. Thus, approximately 70% of thalamocortical and 90-95% of corticothalamic projection neurones in these circuits may use glutamate and/or aspartate as neurotransmitters.

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.

Bilateral intranigral NMDA blockade increases status duration and neuronal injury from systemic kainic acid

Limbic seizures may be under the regulation of the substantia nigra, pars reticulata (SNpr). Using microinjection of the NMDA antagonist AP7, we investigated the role of SNpr in modulating seizures induced by kainic acid. Seizure severity was analyzed electrographically and neural injury assessed by measurement of heat shock protein (HSP) expression and acid fuchsin (AF) staining of vulnerable hippocampal cells. Intranigral injection of AP-7 increased the duration of electrographic seizure discharges and the number of HSP-positive and acid fuchsin stained cells in all hippocampal sectors, suggesting that blockade of the NMDA receptors in SNpr enhanced neural injury.

Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of 3HD-aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Cells retrogradely labeled by WGA-HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with 3H-D-aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with 3H-D-aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with 3H-D-aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using 3H-GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of 3H-GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests 3H-GABA has limited usefulness as a transmitter-specific retrograde tracer.

Distribution of neurofibrillary tangle formation and [3H]-D-aspartate receptor binding in the thalamus in the normal elderly brain, in Alzheimer's disease and in Parkinson's disease

The overactivity of glutamatergic neurons may underlie some neurodegenerative disorders, including Alzheimer's disease (AD). We explored the relationship between glutamatergic transmission and neurofibrillary tangle formation by measuring [3H]-D-aspartate binding activity and the proportion of neurons containing tangles within individual thalamic nuclei in five AD cases. Five elderly normal and five Parkinson's disease (PD) cases were used as controls. A highly significant correlation between [3H]-D-aspartate binding and tangle counts in Alzheimer's disease suggests that those thalamic nuclei which normally receive a relatively dense glutamatergic afferent input are predisposed to tangle formation. There were no significant differences in individual thalamic nuclear [3H]-D-aspartate binding between controls and the AD and PD groups.

Excitatory amino acid transmitters and their receptors in neural circuits of the cerebral neocortex

In 1954, L-glutamate (Glu) and L-aspartate (Asp) were first suggested as being excitatory synaptic transmitters in the cerebral cortex. Since then, evidence has mounted steadily in favor of the view that Glu and Asp are major excitatory transmitters in the neocortex. Many of the experimental studies which reported how Glu/Asp came to satisfy the criteria for transmitters in the neocortex are reviewed here, according to the methods employed. Since the question of which particular synaptic sites in cortical neural circuits Glu/Asp operate as excitatory transmitters has not previously been reviewed, particular attention is given to efferent, afferent and intrinsic neural circuits of the visual and somatosensory cortices, where circuitry is relatively clearly delineated. Recent studies using chemical assays of released amino acids, high-affinity uptake mechanisms of Glu/Asp from nerve terminals, the direct micro-iontophoretic administration of Glu/Asp antagonists, and immunocytochemical techniques have demonstrated that almost all corticofugal efferent projections employ Glu/Asp as excitatory synaptic transmitters. Evidence indicating that thalamocortical afferent projections, including geniculocortical projections and some intrinsic connections are glutamatergic, is also reviewed. Thus, the results highlighted here indicate that the main framework of neocortical circuitry is operated by Glu/Asp. Pharmacological studies indicate that synaptic receptors for Glu/Asp can be classified into a few subtypes, including N-methyl-D-aspartate (NMDA) and quisqualate/kainate (non-NMDA) types. Some evidence indicating the sites of operation of NMDA and non-NMDA receptors in neocortical circuitry is reviewed, and the distinct, functional significance of these two types of Glu/Asp receptors in information processing in the neocortex is proposed.

Inhibitory control by substantia nigra of generalized epilepsy in the cat

Previous investigations have shown that the basal ganglia may exert a regulating influence on cortical epilepsy. Stimulation of the caudate nucleus enhances cortical penicillin (PCN) spikes. Stimulation of globus pallidus internus reduces cortical spike frequency. Since the substantia nigra pars reticulata (SNpr) seems to have an inhibitory action on the ventro-anterior (VA) and ventro-lateral (VL) thalamic nuclei and thalamic neurones send an excitatory influence to the cortex, we undertook an investigation to study nigral influence on cortical epilepsy induced by PCN. Experiments were conducted on encéphale isolé cats in which steady interictal activity was induced by means of parenteral PCN administration (feline generalized PCN epilepsy). Variations occurring in cortical PCN spikes following activation of either pars compacta (SNpc) or SNpr were analyzed. Electrical stimulation of SNpc reduced spike frequency and amplitude in 19% of the total number of stimulations; SNpr stimulation significantly inhibited cortical spikes, especially in the precruciate gyrus, in 80% of cases. The experimental findings constitute an electrophysiological feature of the control exerted by SNpr on the thalamo-cortical re-exciting loop. A putative preferential role of SNpr in the regulation of abnormal phenomena involving the neocortex is emphasized.

Afferents to the rat red nucleus studied by means of D-[3H]aspartate, [3H]choline and non-selective tracers

Following injection of horseradish peroxidase-labeled wheat germ agglutinin or of rhodamine-labeled microspheres as non-selective tracers into the rat red nucleus, the origins of the corticorubral and cerebellorubral pathways, as well as a considerable number of other brain structures including dorsal raphé nucleus, zona incerta and several hypothalamic nuclei showed retrogradely labeled perikarya. Labeling patterns obtained with horseradish peroxidase-labeled wheat germ agglutinin compared well with those observed following application of rhodamine-labeled microspheres which produced injection sites restricted to the small nucleus. In these latter cases, counterstaining with phosphine allowed a better definition of anatomical structures. After D-[3H]aspartate application, retrogradely labeled perikarya were observed in cerebral cortex (layer V), zona incerta, dorsal raphé nucleus and in several other structures also labeled by non-selective tracers. Following application of [3H]choline and using an improved autoradiographic method, perikaryal labeling was massive within nucleus interpositus, while it was absent in dorsal raphé nucleus, cerebral cortex and zona incerta. Retrograde tracing experiments with D-[3H]aspartate and [3H]choline revealed that these transmitter related compounds are selective markers for two subsets of afferents to the red nucleus. The transmitter specificity of the selective labeling with [3H]choline in the cerebellorubral pathway is supported only in part by the results obtained with other methods. The selective labeling with D-[3H]aspartate in the corticorubral pathway, on the other hand, is consistent with its transmitter specificity.

Evidence for reactive synaptogenesis in the ventrolateral thalamus and red nucleus of the rat: changes in high affinity glutamate uptake and numbers of corticofugal fiber terminals

High affinity glutamate uptake into corticofugal fiber terminals was measured in the ventrolateral thalamus and red nucleus at varying time intervals after lesions were made by kainic acid in the contralateral interpositus nucleus of the cerebellum in rats. Under similar conditions the density of cortical fiber terminals was estimated using the Fink-Heimer impregnation technique. 1. Glutamate uptake steadily increased in the ventrolateral thalamus up to 60 days after lesions in the contralateral cerebellum. 2. Similar changes were noted in the red nucleus. 3. The changes were dependent on the integrity of corticofugal fibers to the thalamus and red nucleus. 4. No changes in uptake of gamma-aminobutyric acid were noted. 5. Saturation curves for glutamate uptake suggested a change in the maximal number of transport sites. 6. Fink-Heimer degeneration studies showed an increase in cortical terminals in the ipsilateral ventrolateral thalamus and in both rostral and caudal regions of the red nucleus following lesions in the contralateral interpositus nucleus. The data are consistent with an increase in the number of cortical fiber terminals in reaction to loss of cerebellar input to the ventrolateral thalamus and red nucleus. This study correlates anatomical and biochemical evidence for collateral sprouting in a model based on electrophysiologic data in the red nucleus and extends the model to include the thalamus.

The action of the corticofugal pathway on sensory thalamic nuclei: a hypothesis

The N-methyl-D-aspartate receptor has recently attracted great interest due to its nonlinear current-voltage behavior. In order to evoke a large depolarizing postsynaptic current, the synaptic-induced conductance change must be paired with a postsynaptic depolarization. This temporally tuned AND gate could underlie a number of different operations throughout the nervous system. We propose that the synapses made by the optical nerve onto projection cells in the mammalian dorsal lateral geniculate nucleus are of the N-methyl-D-aspartate type. [In this Commentary, we have pooled data regarding sensory thalamic nuclei from a number of different mammalian species. Unless otherwise mentioned, we have referred to the dorsal division of the cat lateral geniculate nucleus.] About half of all synapses in these cells--located almost exclusively in the peripheral two-thirds of the dendritic tree--are associated with axons originating in layer VI of visual cortex. It then follows that the massive corticogeniculate pathway controls the gain of the retinogeniculate pathway via its action on the N-methyl-D-aspartate receptors. Thus, near-simultaneous activation of the retinal and the cortical input will transiently enhance the geniculate cell response. Generalizing to other thalamic sensory nuclei, afferent information will be routed through the thalamus and on to the cortex as long as cortical activity is congruent with sensory input to the thalamus. Experimental evidence argues for such a mechanism to control the gain of the somatosensory input to the ventrobasal thalamic nucleus.

Intrinsic circuitry involving the local axon collaterals of corticothalamic projection cells in mouse SmI cortex

The objective of this study was to identify the components involved in a local synaptic circuit in the mouse cerebral cortex. The local axon collaterals of corticothalamic (CT) projection cells in the posteromedial barrel subfield area of primary somatosensory cortex were labeled by the retrograde transport of horseradish peroxidase injected into the ipsilateral thalamus. Thalamocortical (TC) axon terminals in the same region of cortex were labeled by lesion induced degeneration. CT axon terminals synapsed preferentially with dendritic shafts, whereas TC axon terminals synapsed mainly with dendritic spines. Some dendrites received both CT and TC synapses. Dendrites were interpreted to belong to nonspiny multipolar cells. These results indicate that a reciprocal synaptic relationship exists in the cortex between nonspiny multipolar cells and CT projection cells. Both CT projection cells and nonspiny multipolar neurons have been shown previously to receive TC synapses (White and Hersh: J. Neurocytol. 11:137-157, '82; White, Benshalom, and Hersch: J. Comp. Neurol. 229:311-320, '84). These findings imply that a triadic relationship involving afferent input and populations of CT projection and intrinsic neurons is a basic feature of the synaptic organization of the cerebral cortex.

2-Amino-7-phosphonoheptanoic acid inhibits insulin-induced convulsions and striatal aspartate accumulation in rats with frontal cortical ablation

Pretreatment of rats with the excitatory amino acid antagonist 2-amino-7-phosphonoheptanoic acid (2-APH; 0.5 mmol/kg, i.p.) protected against insulin-induced clonic seizures. Complete protection was observed in 38% of the rats and partial protection in an additional 50%. Lesioning of the corticostriatal pathway by frontal cortical ablation caused decreases in the striatal levels of aspartate (-28%) and glutamate (-18%), an increase in striatal glutamine level (45%), and decreased high-affinity uptake of D-[3H]aspartate (-27%) in the lesioned dorsal neostriatum. Insulin-induced hypoglycemia caused a predicted sharp increase in aspartate level (165%) and decreased glutamate (-20%) and glutamine (-38%) levels in the intact striatum. Pretreatment of rats with 2-APH significantly reversed the insulin-induced changes in striatal aspartate, glutamate, and glutamine levels, especially in the intact hemisphere. In normoglycemic control rats, the "metabolic," i.e., concentration in the lesioned hemisphere, aspartate pool constituted 72% and the "synaptic," i.e., the concentration difference between the intact and lesioned hemispheres, 28% of the total striatal aspartate pool. 2-APH had no effect on the level of "metabolic" aspartate in the striata of normoglycemic rats but caused an almost complete suppression of "synaptic" aspartate. Following insulin-induced hypoglycemia, the "metabolic" aspartate pool doubled, whereas the "synaptic" aspartate pool increased 3.5-fold in the absence of 2-APH. The insulin-induced rise in "synaptic" aspartate level was almost completely blocked by 2-APH (a 5% rise instead of a 3.5-fold rise).(ABSTRACT TRUNCATED AT 250 WORDS)

Hemidecortication selectively alters release of glutamate in perfusates collected from cerebral cortex of unrestrained rats

The effect of hemidecortication on the in vivo release of amino acids was examined in different areas of the cerebral cortex of the freely-moving rat. After one side of the cortex was lesioned by aspiration, four guide tubes for push-pull perfusion were implanted chronically on the contralateral side so as to rest above the frontal, parietal, temporal and occipital areas of the cortex. After 10-14 days elapsed, each of these regions was perfused with an artificial cerebrospinal fluid (CSF) at a rate of 25.0 microliter/min. Two types of assays were undertaken to determine the release of either newly synthesized amino acids from [14C]glucose precursor or the actual endogenous content in samples of perfusate. The separation of the [14C]amino acids was performed by thin layer chromatography, whereas endogenous amino acids were separated by HPLC with electrochemical detection and quantitated in the range of 1.0-10.0 picomoles. When compared to the control group, samples collected in the hemidecorticate rat showed no significant differences in the new synthesis of glutamate, aspartate, glutamine, glycine, and GABA from the precursor. On the other hand, the analysis of the endogenous amino acid neurotransmitters revealed that the levels of glutamic acid and glutamine declined in samples obtained from the parietal and frontal cortex, respectively. These results implicate further the potential role of glutamic acid as a neurotransmitter of interhemispheric connections in the cerebral cortex.

Localization of glutamate in trigeminothalamic projection neurons: a combined retrograde transport-immunohistochemical study

Trigeminothalamic projection neurons are important components of the pathways for conscious perception of pain, temperature, and tactile sensation from the orofacial region. The neurotransmitters utilized by trigeminal neurons projecting to the thalamus are unknown. By use of a monoclonal antibody specific for fixative-modified glutamate and a polyclonal antiserum against glutaminase, we recently identified neurons in the trigeminal sensory complex that contain glutamate-like immunoreactivity (Glu-LI) and glutaminase-like immunoreactivity. In the present study, we utilized combined retrograde transport-immunohistochemical techniques to localize putative glutamatergic trigeminothalamic neurons. Following injection of the retrograde tracer, wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP), into the ventroposterior medial thalamus (VPM), the number of neuronal profiles that were double-labeled with WGA:HRP and Glu-LI was greatest in principal sensory nucleus (Pr5), followed by subnuclei interpolaris (Sp5I) and caudalis (Sp5C). The average percentages of projection neurons double-labeled with Glu-LI were approximately 60-70% in Pr5 and Sp5I and 40% in Sp5C. The majority of double-labeled profiles in Sp5C were located in the magnocellular layer, as opposed to the marginal and substantia gelatinosa layers. A large injection site that spread into the intralaminar thalamic nuclei and nucleus submedius--areas implicated in the processing of nociceptive information--resulted in an increase in the ratio of single-labeled to double-labeled projection profiles in Sp5C. These results suggest that glutamate may be the neurotransmitter for a majority of trigeminothalamic projection neurons located in Sp5I and Pr5. However, on the basis of anatomical association, glutamate does not appear to be the major transmitter for neurons in Sp5C that forward nociceptive information to the thalamus.

Glutamic acid as a putative transmitter of the interhemispheric corticocortical connections in the rat

The effect of hemidecortication on the endogenous levels of amino acids in medial, sulcal, and dorsal frontal cortex as well as in parietal, temporal, and occipital cortex of the rat was investigated. Under aseptic conditions, the right cerebral cortex was aspirated by suction. Then, 21 days later, the content of glutamic acid, aspartic acid, gamma-aminobutyric acid, glycine, serine, threonine, and alanine was analyzed in six areas of the intact contralateral cortex using GLC. The results demonstrated a specific decrease in the endogenous levels of glutamic acid in both parietal and temporal cortex after hemidecortication of the contralateral side. This finding suggests that glutamic acid may serve as a neurotransmitter for some of the interhemispheric corticoparietal and corticotemporal fibers. In a follow-up experiment, the effect of a frontal lesion on the endogenous levels of the same amino acids in the striatum was also examined. In this case, the glutamic acid content exhibited a decrease of 31% relative to the control value. This observation confirms the earlier finding of a glutamate-containing pathway from the frontal cortex to the striatum.

A new method for producing a specific and high titre antibody against glutamate using colloidal gold as a carrier

Antiserum against the glutamate was developed using colloidal gold as a carrier for the antigen. Glutamate was bound directly to colloidal gold without using cross-linking reagents such as glutaraldehyde, formaldehyde, or carbodiimide. Repeated injection of the colloidal gold-glutamate complex into the rabbit yielded a specific, high titre antiserum. Specificity was verified by radioimmunoassay and histochemically with absorption controls. The antiserum immunohistochemically detected numerous glutamate-containing neurons in the rat brain.

Lack of change in high affinity glutamate uptake in nucleus tractus solitarius following removal of the nodose ganglion

High affinity glutamate uptake in the intermediate portion of the nucleus tractus solitarius (NTS) was not altered by prior removal of one nodose ganglion or by prior midline transection at the level of the NTS. In contrast, glutamate uptake in a tissue punch of the septal nucleus was significantly reduced by prior transection of the fornix. Studies also compared septal glutamate uptake in crude homogenate, supernatant resulting from centrifugation at 3000 X g, and the pellet resulting from centrifugation at 17,000 X g. The results show quantitative differences between measures of glutamate uptake in these different fractions, but qualitatively the results were similar. (These results, do, however, suggest that glutamate uptake might be most appropriately measured in the pellet resulting from centrifugation at 17,000 X g.) The observation that removal of the nodose ganglion did not effect glutamate uptake in the NTS is therefore not due to the method by which glutamate uptake was assessed. These studies fail to replicate the previous observation that glutamate uptake in the NTS decreases following removal of the nodose ganglion, and thus question the hypothesis that glutamate is the neurotransmitter of vagal afferent fibers.

Lesions of connections of the medial prefrontal cortex in rats: differential effects on self-stimulation and spontaneous motor activity

The effects of lesions of the mediodorsal nucleus of the thalamus (MD) and the nucleus caudate putamen (CP) on self-stimulation (SS) of the medial prefrontal cortex (MPC) were investigated. After bilateral electrolytic lesions of the MD or the anteromedial segment of the CP, SS rate and spontaneous motor activity (SMA) were measured. Both MD and CP lesions induced a significant decrease in SS. After 8 days post-lesion, SS rate recovered to pre-lesion levels. SMA did not change significantly after MD lesion. However, SMA showed a significant decrease the 1st and the 5th days after lesioning the CP. The results suggest that the MD and possibly the CP are associated with SS, although it appears that they do not have an essential but rather a modulatory role. The recovery of SS occurs within a few days, presumably due to the compensatory effect of other pathways and structures. The results are also discussed in relation to the effect found after electrolytic lesion of ventrotegmental area and locus coeruleus.

Alterations in L-glutamate binding in Alzheimer's and Huntington's diseases

Brain sections from patients who had died with senile dementia of the Alzheimer's type (SDAT), Huntington's disease (HD), or no neurologic disease were studied by autoradiography to measure sodium-independent L-[3H]glutamate binding. In brain sections from SDAT patients, glutamate binding was normal in the caudate, putamen, and claustrum but was lower than normal in the cortex. The decreased cortical binding represented a reduction in numbers of binding sites, not a change in binding affinity, and appeared to be the result of a specific decrease in numbers of the low-affinity quisqualate binding site. No significant changes in cortical binding of other ligands were observed. In brains from Huntington's disease patients, glutamate binding was lower in the caudate and putamen than in the same regions of brains from control and SDAT patients but was normal in the cortex. It is possible that development of positron-emitting probes for glutamate receptors may permit diagnosis of SDAT in vivo by means of positron emission tomographic scanning.

Differential labelling of afferents to thalamic centromedian-parafascicular nuclei with [3H]choline and D-[3H]aspartate: further evidence for transmitter specific retrograde labelling

Microinjections of [3H]choline into the rat centromedian-parafascicular nuclei resulted in retrograde labelling of medium sized neurons in brainstem areas considered to comprise the origin of cholinergic innervation; the ipsilateral pedunculopontine tegmental, laterodorsal tegmental and parabrachial nuclei, and, bilaterally, in bulbar reticular formation. In contrast, D-[3H]aspartate labelled the origin of the corticothalamic input, which is believed to use glutamate and/or aspartate as transmitter. In addition, D-[3H]aspartate labelled smaller cells in mesencephalic and pontine periventricular gray, and parabrachial nuclei. It was concluded that retrograde labelling with [3H]choline and D-[3H]aspartate are selective phenomena with specificity for cholinergic and acidic amino acid connections, although occasional labelling of substantia nigra neurons with [3H]choline indicated that unspecific labelling may occur.

Topographic changes in high-affinity glutamate uptake in the cat red nucleus, substantia nigra, thalamus, and caudate nucleus after lesions of sensorimotor cortical areas

The distribution of presumed glutamatergic projections from sensorimotor cortical areas to the red nucleus (RN), the substantia nigra (SN), the ventrolateral thalamic complex, and the caudate nucleus (CN) was investigated in the cat. For this purpose, the changes in the sodium-dependent high-affinity glutamate uptake (HAGU) rate were measured in homogenates of tissue microdissected from various parts of these subcortical structures after chronic surgical ablation of sensorimotor cortical areas. After 8 to 10 days survival, significant reductions in HAGU activity were noted in the structures studied on the side ipsilateral to the operated cortex. Within each structure, various quantitative or qualitative changes were observed. Higher decreases in HAGU activity were found in the caudal part of the RN, the ventrolateral thalamic nucleus, and the dorsolateral part of the CN than in the other parts of these structures. The lateral part of the SN showed a large decrease in HAGU rate and its medial part a small but significant increase. Referring to the anatomic data concerning the organization of cortical projections from sensorimotor areas to the structures studied, it was shown that our results support the view that glutamate could act as a neurotransmitter along various corticosubcortical pathways.

Putative amino acid neurotransmitters and the nucleus dorsomedialis thalamus-prefrontal cortex pathway in the rat

Endogenous levels of putative amino acid neurotransmitters (glycine, glutamic acid, aspartic acid, and GABA) in medial and sulcal prefrontal cortex of the rat were analyzed using gas liquid chromatography. No changes were found in the levels of these amino acids in medial and sulcal prefrontal cortex after lesion of the nucleus dorsomedialis of the thalamus suggesting, therefore, that the NDMT-prefrontal cortex pathway is not mediated by these amino acids.