On the excitatory post-synaptic potential evoked by stimulation of the optic tract in the rat lateral geniculate nucleus

Target Interneuron Preference in Thalamocortical Pathways Determines the Temporal Structure of Cortical Responses

Sensory processing relies on fast detection of changes in environment, as well as integration of contextual cues over time. The mechanisms by which local circuits of the cerebral cortex simultaneously perform these opposite processes remain obscure. Thalamic “specific” nuclei relay sensory information, whereas “nonspecific” nuclei convey information on the environmental and behavioral contexts. We expressed channelrhodopsin in the ventrobasal specific (sensory) or the rhomboid nonspecific (contextual) thalamic nuclei. By selectively activating each thalamic pathway, we found that nonspecific inputs powerfully activate adapting (slow-responding) interneurons but weakly connect fast-spiking interneurons, whereas specific inputs exhibit opposite interneuron preference. Specific inputs thereby induce rapid feedforward inhibition that limits response duration, whereas, in the same cortical area, nonspecific inputs elicit delayed feedforward inhibition that enables lasting recurrent excitation. Using a mean field model, we confirm that cortical response dynamics depends on the type of interneuron targeted by thalamocortical inputs and show that efficient recruitment of adapting interneurons prolongs the cortical response and allows the summation of sensory and contextual inputs. Hence, target choice between slow- and fast-responding inhibitory neurons endows cortical networks with a simple computational solution to perform both sensory detection and integration.

On Parallel Streams through the Mouse Dorsal Lateral Geniculate Nucleus

The mouse visual system is an emerging model for the study of cortical and thalamic circuit function. To maximize the usefulness of this model system, it is important to analyze the similarities and differences between the organization of all levels of the murid visual system with other, better studied systems (e.g., non-human primates and the domestic cat). While the understanding of mouse retina and cortex has expanded rapidly, less is known about mouse dorsal lateral geniculate nucleus (dLGN). Here, we study whether parallel processing streams exist in mouse dLGN. We use a battery of stimuli that have been previously shown to successfully distinguish parallel streams in other species: electrical stimulation of the optic chiasm, contrast-reversing stationary gratings at varying spatial phase, drifting sinusoidal gratings, dense noise for receptive field reconstruction, and frozen contrast-modulating noise. As in the optic nerves of domestic cats and non-human primates, we find evidence for multiple conduction velocity groups after optic chiasm stimulation. As in so-called “visual mammals”, we find a subpopulation of mouse dLGN cells showing non-linear spatial summation. However, differences in stimulus selectivity and sensitivity do not provide sufficient basis for identification of clearly distinct classes of relay cells. Nevertheless, consistent with presumptively homologous status of dLGNs of all mammals, there are substantial similarities between response properties of mouse dLGN neurons and those of cats and primates.

Intrinsic electrical properties of mammalian neurons and CNS function: a historical perspective

This brief review summarizes work done in mammalian neuroscience concerning the intrinsic electrophysiological properties of four neuronal types; Cerebellar Purkinje cells, inferior olivary cells, thalamic cells, and some cortical interneurons. It is a personal perspective addressing an interesting time in neuroscience when the reflex view of brain function, as the paradigm to understand global neuroscience, began to be modified toward one in which sensory input modulates rather than dictates brain function. The perspective of the paper is not a comprehensive description of the intrinsic electrical properties of all nerve cells but rather addresses a set of cell types that provide indicative examples of mechanisms that modulate brain function.

Orexin-dependent activation of layer VIb enhances cortical network activity and integration of non-specific thalamocortical inputs

Neocortical layer VI is critically involved in thalamocortical activity changes during the sleep/wake cycle. It receives dense projections from thalamic nuclei sensitive to the wake-promoting neuropeptides orexins, and its deepest part, layer VIb, is the only cortical lamina reactive to orexins. This convergence of wake-promoting inputs prompted us to investigate how layer VIb can modulate cortical arousal, using patch-clamp recordings and optogenetics in rat brain slices. We found that the majority of layer VIb neurons were excited by nicotinic agonists and orexin through the activation of nicotinic receptors containing α4-α5-β2 subunits and OX2 receptor, respectively. Specific effects of orexin on layer VIb neurons were potentiated by low nicotine concentrations and we used this paradigm to explore their intracortical projections. Co-application of nicotine and orexin increased the frequency of excitatory post-synaptic currents in the ipsilateral cortex, with maximal effect in infragranular layers and minimal effect in layer IV, as well as in the contralateral cortex. The ability of layer VIb to relay thalamocortical inputs was tested using photostimulation of channelrhodopsin-expressing fibers from the orexin-sensitive rhomboid nucleus in the parietal cortex. Photostimulation induced robust excitatory currents in layer VIa neurons that were not pre-synaptically modulated by orexin, but exhibited a delayed, orexin-dependent, component. Activation of layer VIb by orexin enhanced the reliability and spike-timing precision of layer VIa responses to rhomboid inputs. These results indicate that layer VIb acts as an orexin-gated excitatory feedforward loop that potentiates thalamocortical arousal.

Retinogeniculate EPSPs recorded intracellularly in the ferret lateral geniculate nucleus in vitro: Role of NMDA receptors

We used an in vitro preparation of the ferret lateral geniculate nucleus (LGN) to examine the role of the NMDA class of excitatory amino acid (EAA) receptors in retinogeniculate transmission. Intracellular recordings revealed that blockade of NMDA receptors both shortened the time course and reduced the amplitude of fast and slow components of excitatory postsynaptic potentials (EPSPs) evoked by optic tract stimulation. The amplitude and width of the EPSPs mediated by NMDA receptors increased as membrane potential was depolarized towards spike threshold. Individual LGN cells were influenced to varying extents by blockade of NMDA receptors; NMDA and non-NMDA receptor blockade together attenuated severely the entire retinogeniculate EPSP. The dependence of all components of retinogeniculate EPSPs (and action potentials) on NMDA receptor activation supports the hypothesis that the NMDA receptor participates in fast (<10 ms) synaptic events underlying conventional retinogeniculate transmission. The voltage dependence of the NMDA receptor-gated conductance suggests strongly that the transmission of retinal information through the LGN is subject to modulation by extraretinal inputs that affect the membrane potential of LGN neurons.

Cortical and collicular inputs to cells in the rat paralaminar thalamic nuclei adjacent to the medial geniculate body

The paralaminar nuclei, including the medial division of the medial geniculate nucleus, surround the auditory thalamus medially and ventrally. This multimodal area receives convergent inputs from auditory, visual, and somatosensory structures and sends divergent outputs to cortical layer 1, amygdala, basal ganglia, and elsewhere. Studies implicate this region in the modulation of cortical 40-Hz oscillations, cortical information binding, and the conditioned fear response. We recently showed that the basic anatomy and intrinsic physiology of paralaminar cells are unlike that of neurons elsewhere in sensory thalamus. Here we evaluate the synaptic inputs to paralaminar cells from the inferior and superior colliculi and the cortex. Combined physiological and anatomical evidence indicates that paralaminar cells receive both excitatory and inhibitory inputs from both colliculi and excitatory cortical inputs. Excitatory inputs from all three sources typically generate small summating EPSPs composed of AMPA and NMDA components and terminate primarily on smaller dendrites and occasionally on dendritic spines. The cortical input shows strong paired-pulse facilitation (PPF), whereas both collicular inputs show weak PPF or paired-pulse depression (PPD). EPSPs of cells with no low-threshold calcium conductance do not evoke a burst response when the cell is hyperpolarized. Longer-latency EPSPs were seen and our evidence indicates that these arise from axon collateral inputs of other synaptically activated paralaminar cells. The inhibitory collicular inputs are GABAergic, activate GABA(A) receptors, and terminate on dendrites. Their activation can greatly alter EPSP-generated spike number and timing.

Changes in firing pattern of lateral geniculate neurons caused by membrane potential dependent modulation of retinal input through NMDA receptors

An optimal visual stimulus flashed on the receptive field of a retinal ganglion cell typically evokes a strong transient response followed by weaker sustained firing. Thalamocortical (TC) neurons in the dorsal lateral geniculate nucleus, which receive their sensory input from retina, respond similarly except that the gain, in particular of the sustained component, changes with level of arousal. Several lines of evidence suggest that retinal input to TC neurons through NMDA receptors plays a key role in generation of the sustained response, but the mechanisms for the state-dependent variation in this component are unclear. We used a slice preparation to study responses of TC neurons evoked by trains of electrical pulses to the retinal afferents at frequencies in the range of visual responses in vivo. Despite synaptic depression, the pharmacologically isolated NMDA component gave a pronounced build-up of depolarization through temporal summation of the NMDA receptor mediated EPSPs. This depolarization could provide sustained firing, the frequency of which depended on the holding potential. We suggest that the variation of sustained response in vivo is caused mainly by the state-dependent modulation of the membrane potential of TC neurons which shifts the NMDA receptor mediated depolarization closer to or further away from the firing threshold. The pharmacologically isolated AMPA receptor EPSPs were rather ineffective in spike generation. However, together with the depolarization evoked by the NMDA component, the AMPA component contributed significantly to spike generation, and was necessary for the precise timing of the generated spikes.

Hyperpolarisation rectification in cat lateral geniculate neurons modulated by intact corticothalamic projections

The intrinsic properties of thalamic neurons are influenced by synaptic activities in ascending pathways and corticofugal projections, as well as by the actions of neurotransmitters released by generalised modulatory systems. We focused on the effects of corticothalamic projections on the hyperpolarisation-activated cation current Ih. Intracellular recordings of thalamocortical neurons in the dorsal lateral geniculate (dLG) nucleus were performed in cats under ketamine-xylazine anaesthesia. At variance with the conventional way of recording intracellularly from thalamic neurons after partial or total ablation of the grey and white matter overlying the dLG, we preserved intact corticothalamic neuronal loops. Stimulating electrodes inserted into the optic tract and light-emitting-diodes as photic stimulation were used to identify the dLG neurons. The expression of the depolarising sag due to Ih depended on the state of cortical networks. Thalamic dLG Ih, induced by hyperpolarising current steps, was detected during the periods of cortical disfacilitation that occur during the cortical slow (< 1 Hz) oscillation, whereas Ih was absent during the active (depolarised) periods. The possibility that the excitatory corticothalamic projections could preclude the generation of the Ih was tested by applying a concentrated K+ solution (3 M) to the primary visual cortex. The same dLG neurons that did not display Ih before application of K+ were able to produce hyperpolarisation-activated depolarising sags during K+-induced cortical depression. Our data suggest that the thalamic clock-like delta oscillation, which results from an interplay between Ih and the low-threshold calcium current (IT), as described in preparations without cerebral cortex, is prevented in dLG neurons when corticothalamic loops are intact.

Glutamate transport, glutamine synthetase and phosphate-activated glutaminase in rat CNS white matter. A quantitative study

Glutamatergic signal transduction occurs in CNS white matter, but quantitative data on glutamate uptake and metabolism are lacking. We report that the level of the astrocytic glutamate transporter GLT in rat fimbria and corpus callosum was approximately 35% of that in parietal cortex; uptake of [3H]glutamate was 24 and 43%, respectively, of the cortical value. In fimbria and corpus callosum levels of synaptic proteins, synapsin I and synaptophysin were 15-20% of those in cortex; the activities of glutamine synthetase and phosphate-activated glutaminase, enzymes involved in metabolism of transmitter glutamate, were 11-25% of cortical values, and activities of aspartate and alanine aminotransferases were 50-70% of cortical values. The glutamate level in fimbria and corpus callosum was 5-6 nmol/mg tissue, half the cortical value. These data suggest a certain capacity for glutamatergic neurotransmission. In optic and trigeminal nerves, [3H]glutamate uptake was < 10% of the cortical uptake. Formation of [14C]glutamate from [U-14C]glucose in fimbria and corpus callosum of awake rats was 30% of cortical values, in optic nerve it was 13%, illustrating extensive glutamate metabolism in white matter in vivo. Glutamate transporters in brain white matter may be important both physiologically and during energy failure when reversal of glutamate uptake may contribute to excitotoxicity.

The relay of high-frequency sensory signals in the Whisker-to-barreloid pathway

The present study investigated the operational features of whisker-evoked EPSPs in barreloid cells and the ability of the whisker-to-barreloid pathway to relay high rates of whisker deflection in lightly anesthetized rats. Results show that lemniscal EPSPs are single-fiber events with fast rise times (<500 microsec) that strongly depress at short inter-EPSP intervals. They occur at short latencies (3.84 +/- 0.96 msec) with little jitters (<300 microsec) after electrical stimulation of the whisker follicle. Waveform analysis indicates that one to three lemniscal axons converge on individual barreloid cells to produce EPSPs of similar rise times but different amplitudes. When challenged by high rates of whisker deflection, cells in the whisker-to-barreloid pathway demonstrate a remarkable frequency-following ability. Primary vibrissa afferents could follow in a phase-locked manner trains of sinusoidal deflections at up to 1 kHz. Although trigeminothalamic cells could still faithfully follow deflection rates of 200-300 Hz, the actual frequency-following ability of individual cells depends on the amplitude, velocity, and direction of displacements. The discharges of trigeminothalamic cells induce corresponding phase-locked EPSPs in barreloid cells, which trigger burst discharges at stimulus onset. During the following cycles of the stimulus train, few action potentials ensue because of the strong synaptic depression at lemniscal synapses. It is concluded that the whisker-to-barreloid pathway can relay vibratory inputs with a high degree of temporal precision, but that the relay of this information to the cerebral cortex requires the action of modulators, and possibly phase-locked discharges among an ensemble of relay cells.

The relay of high-frequency sensory signals in the Whisker-to-barreloid pathway

The present study investigated the operational features of whisker-evoked EPSPs in barreloid cells and the ability of the whisker-to-barreloid pathway to relay high rates of whisker deflection in lightly anesthetized rats. Results show that lemniscal EPSPs are single-fiber events with fast rise times (<500 microsec) that strongly depress at short inter-EPSP intervals. They occur at short latencies (3.84 +/- 0.96 msec) with little jitters (<300 microsec) after electrical stimulation of the whisker follicle. Waveform analysis indicates that one to three lemniscal axons converge on individual barreloid cells to produce EPSPs of similar rise times but different amplitudes. When challenged by high rates of whisker deflection, cells in the whisker-to-barreloid pathway demonstrate a remarkable frequency-following ability. Primary vibrissa afferents could follow in a phase-locked manner trains of sinusoidal deflections at up to 1 kHz. Although trigeminothalamic cells could still faithfully follow deflection rates of 200-300 Hz, the actual frequency-following ability of individual cells depends on the amplitude, velocity, and direction of displacements. The discharges of trigeminothalamic cells induce corresponding phase-locked EPSPs in barreloid cells, which trigger burst discharges at stimulus onset. During the following cycles of the stimulus train, few action potentials ensue because of the strong synaptic depression at lemniscal synapses. It is concluded that the whisker-to-barreloid pathway can relay vibratory inputs with a high degree of temporal precision, but that the relay of this information to the cerebral cortex requires the action of modulators, and possibly phase-locked discharges among an ensemble of relay cells.

Reductions in N-acetylaspartylglutamate and the 67 kDa form of glutamic acid decarboxylase immunoreactivities in the visual system of albino and pigmented rats after optic nerve transections

This study compares the immunohistochemical distributions of N-acetylaspartylglutamate (NAAG) and the large isoform of the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase (GAD(67)) in the visual system of albino and pigmented rats. Most retinal ganglion cells and their axons were strongly immunoreactive for NAAG, whereas GAD(67) immunoreactivity was very sparse in these cells and projections. In retinorecipient zones, NAAG and GAD(67) immunoreactivities occurred in distinct populations of neurons and in dense networks of strongly immunoreactive fibers and synapses. Dual-labeling immunohistochemistry indicated that principal neurons were stained for NAAG, whereas local interneurons were stained for GAD(67). In contrast to the distribution observed in retinorecipient zones, most or all neurons were doubly stained for NAAG and GAD(67) in the thalamic reticular nucleus. Ten days after unilateral optic nerve transection, NAAG-immunoreactive fibers and synapses were substantially reduced in all contralateral retinal terminal zones. The posttransection pattern of NAAG-immunoreactive synaptic loss demarcated the contralateral and ipsilateral divisions of the retinal projections. In addition, an apparent transynaptic reduction in GAD(67) immunoreactivity was observed in some deafferented areas, such as the lateral geniculate. These findings suggest a complicated picture in which NAAG and GABA are segregated in distinct neuronal populations in primary visual targets, yet they are colocalized in neurons of the thalamic reticular nucleus. This is consistent with NAAG acting as a neurotransmitter release modulator that is coreleased with a variety of classical transmitters in specific neural pathways.

Glutamate receptor functions in sensory relay in the thalamus

It is known that glutamate is a major excitatory transmitter of sensory and cortical afferents to the thalamus. These actions are mediated via several distinct receptors with postsynaptic excitatory effects predominantly mediated by ionotropic receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate varieties (NMDA). However, there are also other kinds of glutamate receptor present in the thalamus, notably the metabotropic and kainate types, and these may have more complex or subtle roles in sensory transmission. This paper describes recent electrophysiological experiments done in vitro and in vivo which aim to determine how the metabotropic and kainate receptor types can influence transmission through the sensory thalamic relay. A particular focus will be how such mechanisms might operate under physiological conditions.

Effects of paired-pulse and repetitive stimulation on neurons in the rat medial geniculate body

Many behaviorally relevant sounds, including language, are composed of brief, rapid, repetitive acoustic features. Recent studies suggest that abnormalities in producing and understanding spoken language are correlated with abnormal neural responsiveness to such auditory stimuli at higher auditory levels [Tallal et al., Science 271 (1996) 81-84; Wright et al., Nature 387 (1997) 176-178; Nagarajan et al., Proc. Natl. Acad. Sci. USA 96 (1999) 6483-6488] and with abnormal anatomical features in the auditory thalamus [Galaburda et al., Proc. Natl. Acad. Sci. USA 91 (1994) 8010-8013]. To begin to understand potential mechanisms for normal and abnormal transfer of sensory information to the cortex, we recorded the intracellular responses of medial geniculate body thalamocortical neurons in a rat brain slice preparation. Inferior colliculus or corticothalamic axons were excited by pairs or trains of electrical stimuli. Neurons receiving only excitatory collicular input had tufted dendritic morphology and displayed strong paired-pulse depression of their large, short-latency excitatory postsynaptic potentials. In contrast, geniculate neurons receiving excitatory and inhibitory collicular inputs could have stellate or tufted morphology and displayed much weaker depression or even paired-pulse facilitation of their smaller, longer-latency excitatory postsynaptic potentials. Depression was not blocked by ionotropic glutamate, GABA(A) or GABA(B) receptor antagonists. Facilitation was unaffected by GABA(A) receptor antagonists but was diminished by N-methyl-D-aspartate (NMDA) receptor blockade. Similar stimulation of the corticothalamic input always elicited paired-pulse facilitation. The NMDA-independent facilitation of the second cortical excitatory postsynaptic potential lasted longer and was more pronounced than that seen for the excitatory collicular inputs. Paired-pulse stimulation of isolated collicular inhibitory postsynaptic potentials generated little change in the second GABA(A) potential amplitude measured from the resting potential, but the GABA(B) amplitude was sensitive to the interstimulus interval. Train stimuli applied to collicular or cortical inputs generated intra-train responses that were often predicted by their paired-pulse behavior. Long-lasting responses following train stimulation of the collicular inputs were uncommon. In contrast, corticothalamic inputs often generated long-lasting depolarizing responses that were dependent on activation of a metabotropic glutamate receptor. Our results demonstrate that during repetitive afferent firing there are input-specific mechanisms controlling synaptic strength and membrane potential over short and long time scales. Furthermore, they suggest that there may be two classes of excitatory collicular input to medial geniculate neurons and a single class of small-terminal corticothalamic inputs, each of which has distinct features.

Sensory Excitatory Postsynaptic Potentials Mediated by NMDA and non-NMDA Receptors in the Thalamus in vivo

Excitatory amino acid neurotransmitters, such as l-glutamate, act at several receptors in the brain, which are sometimes referred to as N-methyl-d-aspartate (NMDA) and non-NMDA receptors. Extensive in vitro work indicates that both NMDA receptors and non-NMDA receptors contribute to excitatory postsynaptic potentials (epsps). The contribution of NMDA receptors to epsps in vivo under physiological conditions is, however, almost unknown. The receptors that mediate the epsps evoked in thalamic relay cells by natural stimulation of sensory afferents have been investigated in anaesthetized rats, and we report the first pharmacological characterization of an excitatory amino acid receptor-mediated epsp in vivo involving both non-NMDA receptors and, in particular, NMDA receptors.

Distribution of metabotropic glutamate receptors in the superior colliculus of the adult rat, ferret and cat

The distribution of different metabotropic glutamate receptors (mGluRs 1a, 1b, 1c, 2/3, 4 and 5) has been compared in the superior colliculus of the rat, cat and ferret using immunohistochemical techniques and light microscopy. We found that although there are differences in labelling patterns between the species, there are also substantial similarities. In general, there was only light staining for the various mGluR1 splice variants, whereas labelling for the other Group I receptor, mGluR5, was heavier and with a pattern which suggested that at least some label arose from retinal afferents to the superficial superior colliculus. A further consistent feature in all species was labelling of astrocytes in the optic nerve/optic tract, superficial superior colliculus and brain at the collicular level with the antibody directed towards the Group II receptors, mGluR2 and mGluR3. Staining for the Group III receptor, mGluR4, was dense in the superficial superior colliculus in all species, with characteristics suggesting nerve fibre staining. mGluR4 staining was seen in the cat optic nerve/optic tract. One source of mGluR4 staining in the superior colliculus may thus be retinal axons, although other sources cannot be entirely excluded. These results demonstrate that distributions of mGluRs in these species have significant similarities but also some differences, suggesting that within the superior colliculus there may be some preservation of functional roles for some of the different receptor types. This is particularly so for the Group II and Group III receptors, which appear to have specific and distinct roles in the modulation of visual responses.

AMPA receptor properties at the synapse between retinal afferents and thalamocortical cells in the dorsal lateral geniculate nucleus of the rat

The thalamocortical (TC) cells in dorsal lateral geniculate nucleus transfer signals from retinal afferents to the primary visual cortex. The excitatory retinal input to the TC cells is mediated by ionotropic receptors of the N-methyl-D-aspartate (NMDA) and non-NMDA type. In the present study the excitatory postsynaptic current (EPSC) mediated by non-NMDA receptors in this synapse was characterised by means of voltage-clamp recordings from TC neurons in rat thalamic slices. The specific alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist GYKI-53655 fully blocked the non-NMDA mediated EPSC, evoked by optic tract stimulation. The EPSC peak amplitudes were linearly related to the command voltage, suggesting that the receptor complex includes the GluR2 subunit. The EPSC amplitude and decay time increased during application of the desensitisation blocker, cyclothiazide, showing that the EPSC was partly controlled by fast desensitisation.

Nitric oxide, microglial activities and neuronal cell death in the lateral geniculate nucleus of glaucomatous rats

The present study was initiated to investigate neuronal degeneration, microglial reactivity and possible roles of NO in the lateral geniculate nucleus (LGN) of glaucomatous rats. An experimental one-eye glaucoma model was created by cauterization of the limbal-derived veins. Neuronal cell viability was studied by immunostaining with antibody against neuronal nuclei. Changes of expressions of nitric oxide synthase I (NOS I), NOS II, ED 1, OX6 and OX42 in the LGN were studied by immunohistochemistry. NADPH-d histochemistry was also employed. In the experimental glaucomatous rats, the number of NeuN labelled neurons was significantly decreased in both the ipsi- and contra-lateral sides of the ventral LGN (vLGN) but not the dorsal LGN (dLGN) at 1 month post-operation and beyond. Expressions of NOS I and NADPH-d were notably increased from 1 week post-operation in the ipsilateral vLGN. In the contralateral side of the vLGN, however, this change was only observed from 1 month post-operation. No NOS II immunoreaction was observed in LGN of both the normal control and glaucomatous rats. Increased microglial reactivity as indicated by OX-42 immunoreactivity was first observed in both sides of the LGN at 1 week post-operation, and this was most significant especially at 1 and 2 months post-operation. The present results suggest that NO and microglial cells may play some important roles in the pathologic processes of neuronal degeneration in the LGN of glaucomatous rats.

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

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

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

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

Actions of 8-bromo-cyclic-GMP on neurones in the rat thalamus in vivo and in vitro

The diffusible intercellular messenger nitric oxide may have a modulatory role in the thalamus and this action may be mediated via activation of soluble guanylate cyclase. In order to investigate this possibility, we applied the cyclic-GMP analogue 8-Bromo-cyclic-GMP (8-Br-cGMP) onto neurones in the ventrobasal and lateral geniculate nuclei of the thalamus in anaesthetised rats, and compared its effects with those of a nitric oxide donor. 8-Br-cGMP enhanced the responses of neurones to iontophoretically applied NMDA and AMPA. Furthermore, somatosensory and visual responses of ventrobasal and lateral geniculate neurones were enhanced to 274+/-76% and 217+/-69% of control values, respectively. These effects were similar to those seen with nitric oxide donors in this study and previous work from this laboratory. When applied to thalamic neurones in an in vitro slice preparation, 8-Br-cGMP caused a membrane depolarisation associated with a decrease in input resistance. These findings indicate that activation of guanylate cyclase can cause a membrane depolarisation of thalamic neurones in vitro, and that this effect is sufficient to enhance action responses to ionotropic glutamate receptor stimulation via either exogenous agonists or sensory stimulation.

Activity-evoked extracellular pH shifts in slices of rat dorsal lateral geniculate nucleus

Activity-dependent extracellular pH shifts were studied in slices of the rat dorsal lateral geniculate nucleus (dLGN) using double-barreled pH-sensitive microelectrodes. In 26 mM HCO3--buffered media, afferent activation (10 Hz, 5 s) elicited an early alkaline shift of 0.04+/-0.02 pH units associated with a later, slow acid shift of 0.05+/-0.03 pH units. Extracellular pH shifts in the ventral lateral geniculate nucleus were rare, and limited to acidifications of approximately 0.02 pH units. The alkaline shift in the dLGN increased in the presence of benzolamide (1-2 microM), an extracellular carbonic anhydrase inhibitor. The mean alkaline shift in benzolamide was 0.10+/-0.05 pH units. In 26 mM HEPES-buffered saline, the alkaline response averaged 0.09+/-0.03 pH units. The alkaline shifts persisted in 100 microM picrotoxin (PiTX) but were blocked by 25 microM CNQX/50 microM APV. If stimulation intensity was raised in the presence of CNQX/APV, a second alkalinization arose, presumably due to direct activation of dLGN neurons. The direct responses were amplified by benzolamide, and blocked by either 0 Ca2+/EGTA, Cd2+ or TTX. In 0 Ca2+, addition of 500 microM-5 mM Ba2+ restored the alkalosis. Alkaline shifts evoked with extracellular Ba2+ were larger and faster than those elicited by equimolar Ca2+. In summary, synchronous activation in the dLGN results in an extracellular H+ sink, via a Ca2+-dependent mechanism, similar to activity-dependent alkaline shifts in hippocampus.

Dynamic clamp study of Ih modulation of burst firing and delta oscillations in thalamocortical neurons in vitro

The dynamic clamp technique was used in thalamocortical neurons of the rat and cat dorsal lateral geniculate nucleus in vitro to investigate the effects of the hyperpolarization-activated cation current, Ih, and of its neuromodulation on burst firing and delta oscillations. Specific block of endogenous Ih using 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)pyridinium chloride (ZD7288) (300 microM) abolished the depolarizing "sag" response to negative current steps, markedly increased the latency and shortened the duration of the low-threshold Ca2+ potentials, and decreased the number of action potentials in the burst evoked by the low-threshold Ca2+ potential. Subsequent introduction of artificial Ih using the dynamic clamp re-instated the "sag" and all the original properties of the low-threshold Ca2+ potential. In the absence of ZD7288, introduction of artificial outward Ih with the intention of abolishing endogenous Ih removed the depolarizing "sag" and produced similar effects on the low-threshold Ca2+ potentials as those observed during the pharmacological block of Ih. Application of ZD7288 to thalamocortical neurons displaying delta oscillations led to a reduction in the voltage range of their existence or to a complete cessation of this behaviour. A subsequent introduction of artificial Ih re-enabled the generation of delta oscillations. In the presence of ZD7288, physiologically relevant positive shifts in the voltage-dependence of artificial Ih increased the amplitude and duration of the low-threshold Ca2+ potential and increased the likelihood of delta oscillations while negative shifts had opposite effects. These results highlight the important difference between the dependence of burst firing and oscillations on membrane potential and their dependence on the properties of Ih, and demonstrate that the modulation by Ih of low-threshold Ca2+ potentials and burst firing in thalamocortical neurons, as well as the ability of these neurons to generate delta oscillations, is more elaborate than previously described.

Very slow oscillatory activities in lateral geniculate neurons of freely moving and anesthetized rats

In urethane anesthetized rats many lateral geniculate neurons display a strong very slow oscillatory behavior in the range of 0.025-0.01 Hz. One of the aims of the present study was to determine whether very slow oscillatory activity in this range can also be obtained in barbiturate anesthetized and in awake animals, respectively. Although very slow oscillations were found in geniculate neurons both during awakeness and during anesthesia, significant differences in peak frequencies of oscillations under the three experimental conditions (barbiturate, urethane, awake) were demonstrated. In addition, we have tested the influence of glutamate antagonists and GABA agonists as well as antagonists on the very slow oscillatory activity in urethane anesthetized rats. Very slow oscillatory activity which could be blocked by the continuous illumination of the eyes was re-induced by iontophoresis of NMDA and non-NMDA glutamate antagonists. GABA(A) as well as GABA(B) agonists also caused a significant re-induction of very slow oscillatory activity under light conditions. In the dark, muscimol, a GABA(A) agonist, significantly enhanced the very slow oscillatory activity, i.e. muscimol either induced it or reduced the frequency of very slow oscillations. For the whole sample, GABA antagonists did not have a significant influence on the very slow oscillatory activity. Autocorrelation analysis based on the spike interval histograms and determination of the spectrum of autocorrelograms revealed the significance of periodicity.

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

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

Characterization of sensory and corticothalamic excitatory inputs to rat thalamocortical neurones in vitro

1. Using an in vitro slice preparation of the rat dorsal lateral geniculate nucleus (dLGN), the properties of retinogeniculate and corticothalamic inputs to thalamocortical (TC) neurones were examined in the absence of GABAergic inhibition. 2. The retinogeniculate EPSP evoked at low frequency (>= 0.1 Hz) consisted of one or two fast-rising (0.8 +/- 0.1 ms), large-amplitude (10.3 +/- 1.6 mV) unitary events, while the corticothalamic EPSP had a graded relationship with stimulus intensity, owing to its slower-rising (2.9 +/- 0.4 ms), smaller-amplitude (1.3 +/- 0.3 mV) estimated unitary components. 3. The retinogeniculate EPSP exhibited a paired-pulse depression of 60.3 +/- 5.6 % at 10 Hz, while the corticothalamic EPSP exhibited a paired-pulse facilitation of > 150 %. This frequency-dependent depression of the retinogeniculate EPSP was maximal after the second stimulus, while the frequency-dependent facilitation of the corticothalamic EPSP was maximal after the fourth or fifth stimulus, at interstimulus frequencies of 1-10 Hz. 4. There was a short-term enhancement of the >= 0.1 Hz corticothalamic EPSP (64.6 +/- 9.2 %), but not the retinogeniculate EPSP, following trains of stimuli at 50 Hz. 5. The >= 0.1 Hz corticothalamic EPSP was markedly depressed by the non-NMDA antagonist 1-(4-amino-phenyl)-4-methyl-7,8-methylene-dioxy-5H-2, 3-benzodiazepine (GYKI 52466), but only modestly by the NMDA antagonist 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid ((RS)-CPP), and completely blocked by the co-application of GYKI 52466, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), (RS)-CPP and (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801). Likewise, the corticothalamic responses to trains of stimuli (1-500 Hz) were greatly reduced by this combination of ionotropic glutamate receptor antagonists. 6. In the presence of GYKI 52466, CNQX, (RS)-CPP and MK-801, residual corticothalamic responses and slow EPSPs, with a time to peak of 2-10 s, could be generated following trains of five to fifty stimuli. Neither of these responses were occluded by 1S,3R-1-aminocyclopentane-1, 3-dicarboxylic acid (1S,3R-ACPD), suggesting they are not mediated via group I and II metabotropic glutamate receptors.

The 'window' component of the low threshold Ca2+ current produces input signal amplification and bistability in cat and rat thalamocortical neurones

1. The mechanism underlying a novel form of input signal amplification and bistability was investigated by intracellular recording in rat and cat thalamocortical (TC) neurones maintained in slices and by computer simulation with a biophysical model of these neurones. 2. In a narrow membrane potential range centred around -60 mV, TC neurones challenged with small (10-50 pA), short (50-200 ms) current steps produced a stereotyped, large amplitude hyperpolarization (> 20 mV) terminated by the burst firing of action potentials, leading to amplification of the duration and amplitude of the input signal, that is hereafter referred to as input signal amplification. 3. In the same voltage range centred around -60 mV, single evoked EPSPs and IPSPs also produced input signal amplification, indicating that this behaviour can be triggered by physiologically relevant stimuli. In addition, a novel, intrinsic, low frequency oscillation, characterized by a peculiar voltage dependence of its frequency and by the presence of plateau potentials on the falling phase of low threshold Ca2+ potentials, was recorded. 4. Blockade of pure Na+ and K+ currents by tetrodotoxin (1 microM) and Ba2+ (0.1-2.0 mM), respectively, did not affect input signal amplification, neither did the presence of excitatory or inhibitory amino acid receptor antagonists in the perfusion medium. 5. A decrease in [Ca2+]o (from 2 to 1 mM) and an increase in [Mg2+]o (from 2 to 10 mM), or the addition of Ni2+ (2-3 mM), abolished input signal amplification, while an increase in [Ca2+]o (from 2 to 8 mM) generated this behaviour in neurones where it was absent in control conditions. These results indicate the involvement of the low threshold Ca2+ current (IT) in input signal amplification, since the other Ca2+ currents of TC neurones are activated at potentials more positive than -40 mV. 6. Blockade of the slow inward mixed cationic current (Ih) by 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium++ + chloride (ZD 7288)(100-300 microM) did not affect the expression of the large amplitude hyperpolarization, but abolished the subsequent repolarization to the original membrane potential. In this condition, therefore, input signal amplification was replaced by bistable membrane behaviour, where two stable membrane potentials separated by 15-30 mV could be switched between by small current steps. 7. Computer simulation with a model of a TC neurone, which contained only IT, Ih, K+ leak current (ILeak) and those currents responsible for action potentials, accurately reproduced the qualitative and quantitative properties of input signal amplification, bistability and low frequency oscillation, and indicated that these phenomena will occur at some value of the injected DC if, and only if, the 'window' component of IT (IT,Window) and the leak conductance (gLeak) satisfy the relation (dIT,Window/dV)max > gLeak. 8. The physiological implications of these findings for the electroresponsiveness of TC neurones are discussed, and, as IT is widely expressed in the central nervous system, we suggest that 'window' IT will markedly affect the integrative properties of many neurones.

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.

Effects of angiotensin II and IV on geniculate activity in nontransgenic and transgenic rats

Microiontophoretic ejection of angiotensin II and angiotensin IV in the vicinity of geniculate neurons was used to study the effects of these peptides on the discharge rate and the discharge pattern of extracellularly recorded activity. The main aim of the experiments was to study the effects of angiotensins in different strains of rats anesthetized with urethane (normotensive Wistar, normotensive Sprague-Dawley and hypertensive, transgenic (TGR(mREN2)27) rats). Both angiotensins mostly increased the spontaneous activity of angiotensin-sensitive geniculate neurons in all strains. Angiotensin II reduced the number of bursts in most neurons, whereas angiotensin IV significantly enhanced it. Inhibitory effects of angiotensins on spontaneous as well as on light-evoked activity could be effectively blocked by GABA(A) or GABA(B) receptor antagonists. Therefore, it can be supposed that angiotensin-containing afferent fibers innervate both projection and local circuit neurons of the dorsal lateral geniculate nucleus. In addition, angiotensin II suppressed excitation induced by glutamate receptor agonists in most neurons tested. Angiotensin-induced effects could be blocked by specific receptor antagonists. There were no significant differences in the effects of angiotensins in the various strains of rats, except for the latencies of the neuronal responses to the iontophoretic ejection of angiotensins.

Thalamic relay of spontaneous retinal activity prior to vision

Before vision, retinal ganglion cells produce spontaneous waves of action potentials. A crucial question is whether this spontaneous activity is transmitted to lateral geniculate nucleus (LGN) neurons. Using a novel in vitro preparation, we report that LGN neurons receive periodic barrages of postsynaptic currents from the retina that drive them to fire bursts of action potentials. Groups of LGN neurons are highly correlated in their firing. Experiments in wild-type and NMDAR1 knockout mice show that NMDA receptor activation is not necessary for firing. The transmission of the highly correlated retinal activity to the LGN supports the hypothesis that retinal waves drive retinogeniculate synaptic remodeling. Because LGN neurons are driven to fire action potentials, this spontaneous activity could also act more centrally to influence synaptic modification within the developing visual cortex.

Unilateral eyeball enucleation differentially alters AMPA-, NMDA- and kainate glutamate receptor binding in the newborn rat brain

The aim of the present work was to evaluate the neurochemical effects of early unilateral visual deprivation as a model of impaired visual maturation. For this purpose, binding to the different ionotropic glutamate receptor subtypes was quantified in vision-related and vision-unrelated brain structures of control and unilaterally deprived newborn rats. At post-natal (PN) day 10, male Sprague-Dawley rats underwent either unilateral eyeball enucleation (enucleation group, n = 12) or sham operation (control group, n = 12). In each group, brains were obtained either at post-natal day 20 (n = 6) or post-natal day 30 (n = 6) and processed for quantitative in vitro autoradiography selective for NMDA, kainate, and AMPA glutamate-binding sites, as well as for the presynaptic adenosine A1 receptor as a control of the deafferentation efficacy. In control animals, quantitative autoradiography revealed an increase in NMDA (e.g. +45% in superior colliculus) and kainate receptor binding (e.g. +55% in visual cortex, layer IV) from post-natal day 20 to post-natal day 30, associated with stable levels of AMPA receptor binding, in the vision-related structures. In the deafferented visual structures, monocular enucleation induced a marked decrease in A1 site density (e.g. -38 to -52%, in the superficial layer of superior colliculi, at PN day 20 and PN day 30, respectively) in parallel with a mild increase in both NMDA (e.g. +8 to 9%, in superior colliculi and visual cortex, layer IV at PN day 30, respectively) and AMPA (e.g. +16%, in layer IV of the visual cortex at PN day 30). Superimposed on marked bilateral decreases at PN day 30 in the enucleated rats, kainate receptor binding also revealed a slight but significant decrease (-5%) in the deafferented superior colliculus as compared to the non-deafferented side. The present findings (different time-courses of, and differential effects of deafferentation on, the NMDA, kainate and AMPA glutamate receptor subtypes throughout the visual brain structures) further support the involvement of these receptors in distinctive roles during maturation of the visual system.

Trends in the anatomical organization and functional significance of the mammalian thalamus

The last decade has witnessed major changes in the experimental approach to the study of the thalamus and to the analysis of the anatomical and functional interrelations between thalamic nuclei and cortical areas. The present review focuses on the novel anatomical approaches to thalamo-cortical connections and thalamic functions in the historical framework of the classical studies on the thalamus. In the light of the most recent data it is here discussed that: a) the thalamus can subserve different functions according to functional changes in the cortical and subcortical afferent systems; b) the multifarious thalamic cellular entities play a crucial role in the different functional states.

N-acetylaspartylglutamate immunoreactivity in human retina

The acidic dipeptide N-acetylaspartylglutamate (NAAG), which satisfies many of the criteria for a neurotransmitter, was identified immunohistochemically within two human retinae. We observed NAAG immunoreactivity in retinal ganglion cells, their dendrites in the inner plexiform layer, and their axons in the optic nerve fiber layer. The vast majority of ganglion cells were stained, including displaced ganglion cells, ganglion cells of different sizes, and those whose dendrites arborized in the inner and outer sublaminae of the inner plexiform layer, that is, presumed On- and Off- cells. The sizes of labeled and unlabeled cells in the ganglion cell layer, as measured in counterstained material, suggest that the unlabeled cells consist primarily or only of displaced amacrine cells. We also saw immunoreactivity in small cells along the inner margin of the inner nuclear layer, presumably amacrine cells, and in small cells with little cytoplasm in the inner plexiform and ganglion cell layers, presumably displaced amacrine cells. These results are consistent with a role for NAAG in the transmission of visual information from the retina to the rest of the brain. Further, they are similar to those reported previously in rat, cat and monkey, thus demonstrating the relevance of previous studies to humans.

Studies on the identity of the rat optic nerve transmitter

The possible role of glutamate, aspartate, sulfur-containing excitatory amino acids and gamma-glutamyl peptides as major transmitters in the rat optic nerve was evaluated. Four days following optic nerve lesion the K(+)-evoked Ca(2+)-dependent glutamate release was reduced to 31 +/- 16% (+/- S.D., n = 9) comparing release from slices of the denervated (contralateral to the lesion) and non-denervated (ipsilateral) superior colliculus, indicative of a major transmitter function for glutamate. However, significant decreases in glutamate release could not be detected seven days following the lesion (n = 5). Other studies have shown that optic nerve denervation induce formation of synapses of non-retinal origin and cause other cellular changes which may reduce the effect of deafferentation on glutamate release after 7 days. No significant change was observed in aspartate release following the lesion. The concentrations of cysteine sulfinate, cysteate, homocysteine sulfinate, homocysteate and O-sulfo-serine in the optic layers of the superior colliculus were below 1 nmol/g tissue (n = 6). Theoretical considerations indicate that this level is too low for a function of any of these as a major optic nerve transmitter. All postsynaptic components in the rat superior colliculus response, evoked by electrical optic nerve stimulation, were reduced by kynurenate (1-10 mM), a broad spectrum glutamate-receptor antagonist. The study gives further support for the view that glutamate is a major transmitter in the rat optic nerve.

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.

Serotonergic innervation of the lateral geniculate nucleus of the rat during postnatal development: a light and electron microscopic immunocytochemical analysis

The serotonergic innervation of the developing lateral geniculate nucleus of the rat was studied with immunocytochemical techniques at the light and electron microscope levels. A relatively small number of thick serotonergic fibers were observed at the time of birth, distributed more densely in the ventral portion of the nucleus and in the intergeniculate leaflet than in the dorsal lateral geniculate nucleus. By the end of the first postnatal week, this distribution pattern was more clearly established, but the number of immunoreactive fibers was increased. Thereafter, and until the adult pattern was established at the end of the third postnatal week, serotonergic fibers increased further in number and changed morphologically (e.g., they became finer and more ramified with closely spaced varicosities), but their pattern of distribution remained unchanged. Electron microscopical analysis of the dorsal lateral geniculate nucleus revealed that the vast majority of serotonin varicosities formed asymmetrical synapses with dendritic shafts; axosomatic synapses were a feature of the nucleus only at the time of birth. The proportion of serotonin varicosities forming synapses increased gradually from birth to reach a peak at the end of the second postnatal week, then declined markedly in the following week before increasing again at a later stage. It may be speculated that synapses formed during the first two weeks of life may be related to the involvement of serotonin in the morphogenesis of the lateral geniculate nucleus, whereas those formed later in development may be involved in the mediation of neurotransmitter effects.

Gamma-hydroxybutyrate promotes oscillatory activity of rat and cat thalamocortical neurons by a tonic GABAB, receptor-mediated hyperpolarization

The actions of gamma-hydroxybutyrate, a drug known to lead to an increase in nocturnal slow wave sleep and induce epileptic states following systemic application, on the membrane properties of thalamocortical neurons from brain slices of the rat and cat dorsal lateral geniculate nucleus were studied using sharp electrode intracellular recordings. Gamma-hydroxybutyrate applied by addition to the perfusion medium led to a concentration-dependent and reversible hyperpolarization of the membrane potential accompanied by a decrease in apparent input resistance (0.1 mM: 2.3 +/- 0.3 mV, 9.5 +/- 1.0%; 10 mM: 11.3 +/- 1.3 mV, 37.5 +/- 10.8%, respectively). In six of seven neurons the iontophoretic or bath (0.1-0.2 mM) application of low concentrations of gamma-hydroxybutyrate led to a hyperpolarization accompanied by the appearance of low-frequency (< 4 Hz) membrane potential oscillations crowned by bursts of action potentials, when the membrane potential of these neurons was initially set depolarized to the range where ongoing oscillatory activity occurred. The gamma-hydroxybutyrate-elicited hyperpolarization was reversibly antagonized by the co-application of the GABAB receptor antagonist CGP 35348 (0.4-1 mM), but was not affected by the putative gamma-hydroxybutyrate receptor antagonist NCS 382 (0.1-5 mM) or tetrodotoxin (1 microM), suggesting that gamma-hydroxybutyrate tonically activates postsynaptic GABAB receptors. The gamma-hydroxybutyrate-induced promotion of oscillatory activity and action potential burst firing of thalamocortical neurons may be one mechanism by which gamma-hydroxybutyrate leads to an increase in the deep stages of sleep and the generation of electroencephalogram and behavioural patterns typical of absence epilepsy.

Alterations in cortical visual evoked response following ethanol feeding in adult rats

A study was performed on the visual-evoked response (VER) in adult rats that were given an ethanol containing liquid diet for 2 months and examined directly after the exposure period or subjected to a gradual decrease in ethanol over 3 days and total abstinence for 1 week. Control rats showed a first negative peak (N1) directly following the first positive peak (P1). In ethanol-exposed rats examined without withdrawal, the VER showed an increase in onset latency and a marked distorsion of the N1 region. The existing N1 potential was very sensitive to high-frequency stimulation. The alterations were partly normalized 1 week after withdrawal. There was no increase in latency to onset of the response or to P1. There remained an increase of latency and a reduced relative amplitude upon high-frequency stimulation of the N1 peak in ethanol-exposed rats compared with controls. The mechanisms underlying the changes in the cortical potentials are not clear, but they may be related to the cholinergic, glutamatergic/NMDA and/or noradrenergic cortical systems. The lack of persistent changes in onset and P1 latency may be related to the circumstance that the retinogeniculate impulses are transmitted over glutamatergic kainate receptors, which are relatively resistant to ethanol.

Visual and somatosensory evoked potentials are mediated by excitatory amino acid receptors in the thalamus

In pentobarbital-anaesthetized rats early somatosensory evoked potentials (SEPs) were recorded from the sensory cortex in response to electrical stimulation of the contralateral forepaw and visual evoked potentials (VEPs) from the primary visual cortex in response to single light flashes. Microapplication of the specific non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the ventro-basal thalamus (VB) resulted in a pronounced decrease in amplitude and an increase in latency of SEPs, whereas injection of DNQX into the dorsal lateral geniculate nucleus (DGL) induced a pronounced decrease in amplitude and an increase in latency of VEPs. These changes were: (1) dose-dependent (DNQX 0.01-1.0 nmol), (2) receptor-specific, and (3) site-specific. In contrast, the specific NMDA antagonist 2-amino-7-phosphonoheptanoate (AP7; 0.5-5 nmol) did not affect SEPs after microapplication into the BV and less potently reduced the amplitude and increased the latency of VEPs after microapplication into the DGL. The present findings are consistent with the assumption that an excitatory amino acid serves as transmitter at synapses in the rat thalamus mediating the nervous impulses responsible for the generation of SEPs and of VEPs. In addition the results suggest that this transmitter preferentially interacts with non-NMDA receptors.

Sensory input and burst firing output of rat and cat thalamocortical cells: the role of NMDA and non-NMDA receptors

1. Intracellular and patch-clamp recordings were obtained from thalamocortical (TC) cells in the rat and cat dorsal lateral geniculate nucleus (dLGN) in vitro to study the role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the synaptic potential and burst firing evoked by electrical stimulation of the optic tract. 2. At membrane potentials more positive than -65 mV, the sensory synaptic potential consisted of a fast EPSP that was followed by a smaller, slower component. At membrane potentials more negative than -65 mV, this slower component became more prominent owing to the presence of a low-threshold (LT) Ca2+ potential, which in turn evoked a high-frequency (> 150 Hz) burst of action potentials. The lower, but not the upper limit of the range of membrane potential over which burst firing occurred was dependent on the amplitude of the fast EPSP. 3. The non-NMDA receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5-10 microM) and 1-(4-amino-phenyl)-4-methyl-7,8-methylene-dioxy-5H-2,3- benzodiazepine (GYKI 52466, 100 microM) greatly depressed the fast EPSP, abolished the burst firing generated by the LT Ca2+ potential, and left a relatively small, slow EPSP, which was sensitive to the NMDA antagonist DL-2-amino-5-phosphonovaleric acid (DL-AP5, 50-100 microM). 4. In the absence of CNQX or GYKI 52466, DL-AP5 depressed the slow but not the fast EPSP. DL-AP5 also increased the latency of the first action potential evoked by the LT Ca2+ potential or even abolished the LT Ca2+ potential and associated burst firing. The latter effect was only present when this type of firing occurred within a small membrane potential range. 5. DL-AP5 had no effect on the properties of the LT Ca2+ current IT, indicating that its effect on the burst firing was not mediated by a direct action on IT. 6. The response of TC cells to high-frequency (100 Hz) stimulation consisted of an initial burst firing response, followed by a sustained depolarization that could reach firing threshold. This sustained depolarization was markedly depressed by DL-AP5 but not by CNQX. 7. These results demonstrate that with low-frequency stimulation of the sensory afferents, the generation of TC cell output in the rat and cat dLGN is mainly controlled by non-NMDA receptors, while the contribution of NMDA receptors is limited to the burst firing generated by the LT Ca2+ potential, and depends on the membrane potential range over which this type of firing occurs.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo cerebral incorporation of radiolabeled fatty acids after acute unilateral orbital enucleation in adult hooded Long-Evans rats

We examined effects of acute unilateral enucleation on incorporation from blood of intravenously injected unsaturated [1-14C]arachidonic acid ([14C]AA) and [1-14C]docosahexaenoic acid ([14C]DHA), and of saturated [9,10-3H]palmitic acid ([3H]PA), into visual and nonvisual brain areas of awake adult Long-Evans hooded rats. Regional cerebral metabolic rate for glucose (rCMRglc) values also were assessed with 2-deoxy-D-[1-14C]glucose ([14C]DG). One day after unilateral enucleation, an awake rat was placed in a brightly lit visual stimulation box with black and white striped walls, and a radiolabeled fatty acid was infused for 5 min or [14C]DG was injected as a bolus. [14C]DG also was injected in a group of rats kept in the dark for 4 h. Fifteen minutes after starting an infusion of a radiolabeled fatty acid, or 45 min after injecting [14C]DG, the rat was killed and the brain was prepared for quantitative autoradiography. Incorporation coefficients k* of fatty acids, or rCMRglc values, were calculated in homologous brain regions contralateral and ipsilateral to enucleation. As compared with ipsilateral regions, rCMRglc was reduced significantly (by as much as -39%) in contralateral visual areas, including the superior colliculus, lateral geniculate body, and layers I, IV, and V of the primary (striate) and secondary (association, extrastriate) visual cortices. Enucleation did not affect incorporation of [3H]PA into contralateral visual regions, but reduced incorporation of [14C]AA and of [14C]DHA by -18.5 to -2.1%. Percent reductions were correlated with percent reductions in rCMRglc in most but not all regions. No effects were noted at any of nine non-visual structures that were examined. These results indicate that enucleation acutely reduces neuronal activity in contralateral visual areas of the awake rat and that the reductions are coupled to reduced incorporation of unsaturated fatty acids into sn-2 regions of phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine. Reduced fatty acid incorporation likely reflects reduced activity of phospholipases A2 and/or phospholipase C.

Segregation of direction selective neurons and synaptic organization of inhibitory intranuclear connections in the medial terminal nucleus of the rat: an electrophysiological and immunoelectron microscopical study

A combined electrophysiological and morphological investigation of the medial terminal nucleus (MTN) in the rat was undertaken, aimed at a better understanding of the relationship between structure and function in this nucleus. The locations of upward and downward direction selective units in the MTN were documented with extracellular electrophysiological recording. By means of tracer experiments, with Phaseolus vulgaris-leucoagglutinin, biocytin, and cholera toxin subunit B-horseradish peroxidase, the internal connections of the MTN, its retinal afferents, and the projection neurons to the inferior olive were visualized. Terminals originating from the retina and from internal connections were characterized at the ultrastructural level. Their termination pattern on cells in the MTN, including identified inferior olive projection neurons, were determined. Additionally, postembedding GABA immunocytochemistry was performed to identify GABAergic elements. From reconstructions of the positions of electrophysiologically recorded units in the MTN, a local segregation between upward and downward direction selective units was revealed. Upward direction selective units were found in the dorsal part and ventromedially, whereas downward direction selective units were found ventral and laterally in the MTN. The MTN receives optic fibers via two separate routes which, based on their trajectory, presumably terminate in different parts of the MTN: the inferior fascicle of the accessory optic tract in the dorsal part, and the posterior fiber bundle of the superior fascicle in the ventral part of the MTN. A correspondence has been found between the segregation of direction selective units and the areas in the MTN where the retinal fibers from the two pathways distribute. It is, therefore, proposed that the inferior fasciculus conveys upward direction selectivity and the posterior fiber bundle downward direction selectivity, and that the two fiber bundles terminate segregated in the MTN. After anterograde tracing from the eye, retinal terminals were found evenly distributed throughout the MTN. They are characterized as GABA negative R-type terminals. After retrograde tracing from the inferior olive, identified MTN-inferior olive projection neurons were found in the dorsal MTN and medially in the ventral MTN. Their location in the MTN suggests that MTN-inferior olive projection neurons are upward direction selective. MTN-inferior olive projection neurons are large non-GABAergic cells, with a variable form. A majority of both F- and R-type terminals were found to make synaptic contacts on the dendrites of MTN cells. MTN-inferior olive projection neurons did not differ from other neurons in this respect.

Intrinsic and synaptically generated delta (1-4 Hz) rhythms in dorsal lateral geniculate neurons and their modulation by light-induced fast (30-70 Hz) events

Thalamocortical neurons of cat dorsal lateral geniculate nucleus were recorded under urethane anesthesia. Neurons were identified by antidromic invasion from the internal capsule and by orthodromic stimulation from the optic chiasm or light stimuli. An intrinsic oscillation within the frequency of sleep delta waves (1-4 Hz) was induced by hyperpolarizing current pulses triggering a rhythmic sequence of low-threshold spikes alternating with after hyperpolarizing potentials. The increased propensity to oscillation after blockage of inputs arising in the retina indicates that afferent synaptic drives interfere with the intrinsic oscillation of lateral geniculate cells. The relatively rare occurrence of this type of oscillation in impaled neurons, as compared with extracellular recordings in the same nucleus or to intracellular recordings in other dorsal thalamic nuclei, suggests that the interplay between the two intrinsic currents generating delta oscillation is particularly critical in lateral geniculate cells. Another type of delta oscillation was characterized by excitatory postsynaptic potentials which gave rise to action potentials or to low-threshold spikes at more depolarized or hyperpolarized levels, respectively. It is suggested that this rhythm reflects synaptic coupling by intranuclear recurrent axonal collaterals. Light stimulation induced fast (30-70 Hz) excitatory events that were blocked after lidocaine injections into the eye. In all tested cells, changes in the ambient luminosity of the experimental room blocked the intrinsic as well as the synaptic oscillation within the delta frequency. In some cells, this suppressing effect was associated with depolarization and increased firing rate. These results demonstrate different types of sleep delta oscillations in visual thalamic neurons and show that they are modulated not only by brainstem regulatory systems, but also by specific drives along the visual channel.

Membrane and synaptic properties of developing lateral geniculate nucleus neurons during retinogeniculate axon segregation

During the first postnatal month in the ferret (Mustela putorius furo), the projections from the retina to the lateral geniculate nucleus (LGN) become segregated into eye-specific layers and ON and OFF sublayers, a process that is thought to depend in part on neuronal activity. Remarkably, virtually nothing is known about the physiological features of LGN neurons during this period. We have recorded intracellularly from 46 A-layer neurons in slices of the ferret LGN between the ages of postnatal days 7 and 33. The passive membrane properties and current-voltage relationships of the developing neurons were similar in many, though not all, respects to those of adult LGN neurons. Action potentials in younger animals were smaller in amplitude and longer in duration than in older animals, but cells at all ages were capable of producing spike trains whose latency and spike number varied with stimulus intensity. In addition, cells at all ages responded with low-threshold potentials upon release from hyperpolarization. Slightly more than half of the LGN neurons responded to optic tract stimulation with excitatory postsynaptic potentials (EPSPs), inhibitory postsynaptic potentials (IPSPs), or EPSP-IPSP pairs, beginning with the youngest ages. Thus, as early as the second postnatal week, and much before the onset of pattern vision, LGN neurons have many of the membrane and synaptic properties of adult thalamic neurons. These data are consistent with LGN cells playing a significant role in activity-dependent reshaping of the retinogeniculate pathway.

NMDA as well as non-NMDA receptor antagonists can prevent the phase-shifting effects of light on the circadian system of the golden hamster

The present experiments were designed to evaluate whether the intraventricular administration of excitatory amino acid (EAA) receptor antagonists would prevent light-induced phase shifts of the circadian rhythm of wheel-running activity in the hamster. Administration of the non-N-methyl-D-aspartate (non-NMDA) antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) blocked light-induced phase advances and delays. Similarly, administration of the competitive NMDA receptor antagonist, 3(2-carboxypiperazin-4-yl)-propyl-l-phosphonic acid (CPP), prevented light-induced phase advances and delays. Neither drug by itself caused any consistent effect on the phase of the rhythm. These data provide further evidence that EAA receptors mediate the effects of light on the circadian system, and suggest that both NMDA and non-NMDA receptor types may be involved.