The ventral and dorsal lateral geniculate nucleus of the rat: intracellular recordings in vitro

Traumatic Optic Neuropathy Is Associated with Visual Impairment, Neurodegeneration, and Endoplasmic Reticulum Stress in Adolescent Mice

Traumatic brain injury (TBI) results in a number of impairments, often including visual symptoms. In some cases, visual impairments after head trauma are mediated by traumatic injury to the optic nerve, termed traumatic optic neuropathy (TON), which has few effective options for treatment. Using a murine closed-head weight-drop model of head trauma, we previously reported in adult mice that there is relatively selective injury to the optic tract and thalamic/brainstem projections of the visual system. In the current study, we performed blunt head trauma on adolescent C57BL/6 mice and investigated visual impairment in the primary visual system, now including the retina and using behavioral and histologic methods at new time points. After injury, mice displayed evidence of decreased optomotor responses illustrated by decreased optokinetic nystagmus. There did not appear to be a significant change in circadian locomotor behavior patterns, although there was an overall decrease in locomotor behavior in mice with head injury. There was evidence of axonal degeneration of optic nerve fibers with associated retinal ganglion cell death. There was also evidence of astrogliosis and microgliosis in major central targets of optic nerve projections. Further, there was elevated expression of endoplasmic reticulum (ER) stress markers in retinas of injured mice. Visual impairment, histologic markers of gliosis and neurodegeneration, and elevated ER stress marker expression persisted for at least 30 days after injury. The current results extend our previous findings in adult mice into adolescent mice, provide direct evidence of retinal ganglion cell injury after head trauma and suggest that axonal degeneration is associated with elevated ER stress in this model of TON.

Fast-Slow Bursters in the Unfolding of a High Codimension Singularity and the Ultra-slow Transitions of Classes

Bursting is a phenomenon found in a variety of physical and biological systems. For example, in neuroscience, bursting is believed to play a key role in the way information is transferred in the nervous system. In this work, we propose a model that, appropriately tuned, can display several types of bursting behaviors. The model contains two subsystems acting at different time scales. For the fast subsystem we use the planar unfolding of a high codimension singularity. In its bifurcation diagram, we locate paths that underlie the right sequence of bifurcations necessary for bursting. The slow subsystem steers the fast one back and forth along these paths leading to bursting behavior. The model is able to produce almost all the classes of bursting predicted for systems with a planar fast subsystem. Transitions between classes can be obtained through an ultra-slow modulation of the model's parameters. A detailed exploration of the parameter space allows predicting possible transitions. This provides a single framework to understand the coexistence of diverse bursting patterns in physical and biological systems or in models.

T-type calcium channels cause bursts of spikes in motor but not sensory thalamic neurons during mimicry of natural patterns of synaptic input

Although neurons within intact nervous systems can be classified as ‘sensory’ or ‘motor,’ it is not known whether there is any general distinction between sensory and motor neurons at the cellular or molecular levels. Here, we extend and test a theory according to which activation of certain subtypes of voltage-gated ion channel (VGC) generate patterns of spikes in neurons of motor systems, whereas VGC are proposed to counteract patterns in sensory neurons. We previously reported experimental evidence for the theory from visual thalamus, where we found that T-type calcium channels (TtCCs) did not cause bursts of spikes but instead served the function of ‘predictive homeostasis’ to maximize the causal and informational link between retinogeniculate excitation and spike output. Here, we have recorded neurons in brain slices from eight sensory and motor regions of rat thalamus while mimicking key features of natural excitatory and inhibitory post-synaptic potentials. As predicted by theory, TtCC did cause bursts of spikes in motor thalamus. TtCC-mediated responses in motor thalamus were activated at more hyperpolarized potentials and caused larger depolarizations with more spikes than in visual and auditory thalamus. Somatosensory thalamus is known to be more closely connected to motor regions relative to auditory and visual thalamus, and likewise the strength of its TtCC responses was intermediate between these regions and motor thalamus. We also observed lower input resistance, as well as limited evidence of stronger hyperpolarization-induced (‘H-type’) depolarization, in nuclei closer to motor output. These findings support our theory of a specific difference between sensory and motor neurons at the cellular level.

Resolving the detailed structure of cortical and thalamic neurons in the adult rat brain with refined biotinylated dextran amine labeling

Biotinylated dextran amine (BDA) has been used frequently for both anterograde and retrograde pathway tracing in the central nervous system. Typically, BDA labels axons and cell somas in sufficient detail to identify their topographical location accurately. However, BDA labeling often has proved to be inadequate to resolve the fine structural details of axon arbors or the dendrites of neurons at a distance from the site of BDA injection. To overcome this limitation, we varied several experimental parameters associated with the BDA labeling of neurons in the adult rat brain in order to improve the sensitivity of the method. Specifically, we compared the effect on labeling sensitivity of: (a) using 3,000 or 10,000 MW BDA; (b) injecting different volumes of BDA; (c) co-injecting BDA with NMDA; and (d) employing various post-injection survival times. Following the extracellular injection of BDA into the visual cortex, labeled cells and axons were observed in both cortical and thalamic areas of all animals studied. However, the detailed morphology of axon arbors and distal dendrites was evident only under optimal conditions for BDA labeling that take into account the: molecular weight of the BDA used, concentration and volume of BDA injected, post-injection survival time, and toning of the resolved BDA with gold and silver. In these instances, anterogradely labeled axons and retrogradely labeled dendrites were resolved in fine detail, approximating that which can be achieved with intracellularly injected compounds such as biocytin or fluorescent dyes.

Hyperpolarization-activated cation channels: from genes to function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in heart and nervous system. The current produced by HCN channels has been known as I(h) (or I(f) or I(q)). I(h) has also been designated as pacemaker current, because it plays a key role in controlling rhythmic activity of cardiac pacemaker cells and spontaneously firing neurons. Extensive studies over the last decade have provided convincing evidence that I(h) is also involved in a number of basic physiological processes that are not directly associated with rhythmicity. Examples for these non-pacemaking functions of I(h) are the determination of the resting membrane potential, dendritic integration, synaptic transmission, and learning. In this review we summarize recent insights into the structure, function, and cellular regulation of HCN channels. We also discuss in detail the different aspects of HCN channel physiology in the heart and nervous system. To this end, evidence on the role of individual HCN channel types arising from the analysis of HCN knockout mouse models is discussed. Finally, we provide an overview of the impact of HCN channels on the pathogenesis of several diseases and discuss recent attempts to establish HCN channels as drug targets.

Preserving information in neural transmission

Along most neural pathways, the spike trains transmitted from one neuron to the next are altered. In the process, neurons can either achieve a more efficient stimulus representation, or extract some biologically important stimulus parameter, or succeed at both. We recorded the inputs from single retinal ganglion cells and the outputs from connected lateral geniculate neurons in the macaque to examine how visual signals are relayed from retina to cortex. We found that geniculate neurons re-encoded multiple temporal stimulus features to yield output spikes that carried more information about stimuli than was available in each input spike. The coding transformation of some relay neurons occurred with no decrement in information rate, despite output spike rates that averaged half the input spike rates. This preservation of transmitted information was achieved by the short-term summation of inputs that geniculate neurons require to spike. A reduced model of the retinal and geniculate visual responses, based on two stimulus features and their associated nonlinearities, could account for >85% of the total information available in the spike trains and the preserved information transmission. These results apply to neurons operating on a single time-varying input, suggesting that synaptic temporal integration can alter the temporal receptive field properties to create a more efficient representation of visual signals in the thalamus than the retina.

Explicit calcium bursting stochastic resonance

In the present work the influence of internal noise resulting from small cell volume on bursting calcium oscillations is studied. With the internal noise switched on, the center of the main peak in the PSD (power spectrum density) was modified by internal noise. With increasing of the cell volume, the calculated signal-to-noise ratio (SNR) undergoes a maximum, which is referred in the present work as explicit bursting stochastic resonance. In addition, another quantity, the correlation time is used to measure the coherence of bursting oscillations. We demonstrate that the correlation time of the oscillations also exhibits a maximum at a certain cell volume.

Distinct roles of metabotropic glutamate receptor activation on inhibitory signaling in the ventral lateral geniculate nucleus

The ventral lateral geniculate nucleus (vLGN) has been implicated in numerous functions including circadian rhythms, brightness discrimination, pupillary light reflex, and other visuomotor functions. The contribution of inhibitory mechanisms in the regulation of vLGN neuron excitability remains unexplored. We examined the actions of metabotropic glutamate receptor (mGluR) activation on the intrinsic excitability and inhibitory synaptic transmission in different lamina of vLGN. Activation of mGluRs exerts distinct pre- and postsynaptic actions in vLGN neurons. In the lateral magnocellular subdivision of vLGN (vLGNl), the general mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) enhanced the frequency of GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSC) that persisted in the presence of sodium channel blocker tetrodotoxin (TTX) in a subpopulation of neurons (TTX insensitive). This increase is attributed to the increased output of dendritic GABA release from vLGN interneurons. In contrast, in the medial subdivision of vLGN (vLGNm), the mGluR agonist-mediated increase in sIPSC frequency was completely blocked by TTX. The selective Group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) increased sIPSC frequency, whereas the selective Group II mGluR agonist (2R, 4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) significantly decreased sIPSC frequency in vLGNl neurons. Optic tract stimulation also produced an mGluR-dependent increase in sIPSC frequency in vLGNl neurons. In contrast, we were unable to synaptically evoke alterations in sIPSC activity in vLGNm neurons. In addition to these presynaptic actions, DHPG depolarized both vLGNl and vLGNm neurons. In vLGN interneurons, mGluR activation produced opposing actions: APDC hyperpolarized the membrane potential, whereas DHPG produced a membrane depolarization. The present findings demonstrate diverse actions of mGluRs on vLGN neurons localized within different vLGN lamina. Considering these different lamina are coupled with distinct functional roles, thus these diverse actions may be involved in distinctive forms of visual and visuomotor information processing.

Contrasting the functional properties of GABAergic axon terminals with single and multiple synapses in the thalamus

Diverse sources of GABAergic inhibition are a major feature of cortical networks, but distinct inhibitory input systems have not been systematically characterized in the thalamus. Here, we contrasted the properties of two independent GABAergic pathways in the posterior thalamic nucleus of rat, one input from the reticular thalamic nucleus (nRT), and one "extrareticular" input from the anterior pretectal nucleus (APT). The vast majority of nRT-thalamic terminals formed single synapses per postsynaptic target and innervated thin distal dendrites of relay cells. In contrast, single APT-thalamic terminals formed synaptic contacts exclusively via multiple, closely spaced synapses on thick relay cell dendrites. Quantal analysis demonstrated that the two inputs displayed comparable quantal amplitudes, release probabilities, and multiple release sites. The morphological and physiological data together indicated multiple, single-site contacts for nRT and multisite contacts for APT axons. The contrasting synaptic arrangements of the two pathways were paralleled by different short-term plasticities. The multisite APT-thalamic pathway showed larger charge transfer during 50-100 Hz stimulation compared with the nRT pathway and a greater persistent inhibition accruing during stimulation trains. Our results demonstrate that the two inhibitory systems are morpho-functionally distinct and suggest and that multisite GABAergic terminals are tailored for maintained synaptic inhibition even at high presynaptic firing rates. These data explain the efficacy of extrareticular inhibition in timing relay cell activity in sensory and motor thalamic nuclei. Finally, based on the classic nomenclature and the difference between reticular and extrareticular terminals, we define a novel, multisite GABAergic terminal type (F3) in the thalamus.

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.

Structural and functional composition of the developing retinogeniculate pathway in the mouse

The advent of transgenic mice has made the developing retinogeniculate pathway a model system for targeting potential mechanisms that underlie the refinement of sensory connections. However, a detailed characterization of the form and function of this pathway is lacking. Here we use a variety of anatomical and electrophysiological techniques to delineate the structural and functional changes occurring in the lateral geniculate nucleus (LGN) of dorsal thalamus of the C57/BL6 mouse. During the first two postnatal weeks there is an age-related recession in the amount of terminal space occupied by retinal axons arising from the two eyes. During the first postnatal week, crossed and uncrossed axons show substantial overlap throughout most of the LGN. Between the first and second week retinal arbors show significant pruning, so that by the time of natural eye opening (P12–14) segregation is complete and retinal projections are organized into distinct eye-specific domains. During this time of rapid anatomical rearrangement, LGN cells could be readily distinguished using immunocytochemical markers that stain for NMDA receptors, GABA receptors, L-type Ca2+channels, and the neurofilament protein SMI-32. Moreover, the membrane properties and synaptic responses of developing LGN cells are remarkably stable and resemble those of mature neurons. However, there are some notable developmental changes in synaptic connectivity. At early ages, LGN cells are binocularly responsive and receive input from as many as 11 different retinal ganglion cells. Optic tract stimulation also evokes plateau-like depolarizations that are mediated by the activation of L-type Ca2+channels. As retinal inputs from the two eyes segregate into nonoverlapping territories, there is a loss of binocular responsiveness, a decrease in retinal convergence, and a reduction in the incidence of plateau potentials. These data serve as a working framework for the assessment of phenotypes of genetically altered strains as well as provide some insight as to the molecular mechanisms underlying the refinement of retinogeniculate connections.

Paradoxical potentiation of neuronal T-type Ca2+ current by ATP at resting membrane potential

Despite the marked influence on neuronal physiology of the low-voltage activated T-type Ca(2+) currents, little is known about the intracellular pathways and neurotransmitters involved in their regulations. Here, we report that in thalamocortical neurons a phosphorylation mechanism induces an increase both in the current amplitude (1.5 +/- 0.27-fold in the ventrobasal nucleus) and its inactivation kinetics. Dialysis of the neuron with an ATP-free solution suppresses the T-current potentiation, whereas it becomes irreversible in the presence of ATPgammaS. Phosphorylation occurs when the channels are inactivated and is slowly removed when they recover from inactivation and remain in closed states (time constants of the induction and removal of the potentiation: 579 +/- 143 msec and 4.9 +/- 1.1 sec, respectively, at 25 degrees C). The resulting apparent voltage sensitivity of this regulation follows the voltage dependence of the current steady-state inactivation. Thus, the current is paradoxically inhibited when the preceding hyperpolarization is lengthened, and maximal currents are generated after transient hyperpolarizations with a duration (0.7-1.5 sec) that is defined by the balance between the kinetics of the dephosphorylation and deinactivation. In addition, the phosphorylation will facilitate the generation of T current at resting membrane potential. This potentiation, which is specific to sensory thalamocortical neurons, would markedly influence the electroresponsiveness of these neurons and represent the first evidence of a regulation of native Cav3.1 channels.

Membrane bistability in thalamic reticular neurons during spindle oscillations

The thalamic reticular (RE) nucleus is a major source of inhibition in the thalamus. It plays a crucial role in regulating the excitability of thalamocortical networks and in generating some sleep rhythms. Current-clamp intracellular recordings of RE neurons in cats under barbiturate anesthesia revealed the presence of membrane bistability in approximately 20% of neurons. Bistability consisted of two alternate membrane potentials, separated by approximately 17-20 mV. While non-bistable (common) RE neurons fired rhythmic spike-bursts during spindles, bistable RE neurons fired tonically, with burst modulation, throughout spindle sequences. Bistability was strongly voltage dependent and only expressed under resting conditions (i.e. no current injection). The transition from the silent to the active state was a regenerative event that could be activated by brief depolarization, whereas brief hyperpolarizations could switch the membrane potential from the active to the silent state. These effects outlasted the current pulses. Corticothalamic stimulation could also switch the membrane potential from silent to active states. Addition of QX-314 in the recording micropipette either abolished or disrupted membrane bistability, suggesting I(Na(p)) to be responsible for its generation. Thalamocortical cells presented various patterns of spindling that reflected the membrane bistability in RE neurons. Finally, experimental data and computer simulations predicted a role for RE neurons' membrane bistability in inducing various patterns of spindling in target thalamocortical cells. We conclude that membrane bistability of RE neurons is an intrinsic property, likely generated by I(Na(p)) and modulated by cortical influences, as well as a factor that determines different patterns of spindle rhythms in thalamocortical neurons.

Nature of inhibitory postsynaptic activity in developing relay cells of the lateral geniculate nucleus

Using intracellular recordings in an isolated (in vitro) brain stem preparation, we examined the inhibitory postsynaptic responses of developing neurons in the dorsal lateral geniculate nucleus (LGN) of the rat. As early as postnatal day (P) 1-2, 31% of all excitatory postsynaptic (EPSP) activity evoked by electrical stimulation of the optic tract was followed by inhibitory postsynaptic potentials (IPSPs). By P5, 98% of all retinally evoked EPSPs were followed by IPSP activity. During the first postnatal week, IPSPs were mediated largely by GABA(A) receptors. Additional GABA(B)-mediated IPSPs emerged at P3-4 but were not prevalent until after the first postnatal week. Experiments involving the separate stimulation of each optic nerve indicated that developing LGN cells were binocularly innervated. At P11-14, it was common to evoke EPSP/IPSP pairs by stimulating either the contralateral or ipsilateral optic nerve. During the third postnatal week, binocular excitatory responses were encountered far less frequently. However, a number of cells still maintained a binocular inhibitory response. These results provide insight about the ontogeny and nature of postsynaptic inhibitory activity in the LGN during the period of retinogeniculate axon segregation.

Distinct firing properties of higher order thalamic relay neurons

It has been proposed that the thalamus is composed of at least two types of nuclei. First-order relay nuclei transmit signals from the periphery to the cortex while higher order nuclei may route information from one cortical area to another. Although much is known about the functional properties of relay neurons in first-order nuclei, little is known about relay neurons belonging to higher-order nuclei. We investigated the electrophysiological properties of relay cells in a higher-order thalamic nucleus using in vitro intracellular recordings from thalamic slices of the rat's lateral posterior nucleus (LPN). We found neurons of the LPN possess many of the same membrane properties as first-order relay neurons. These included low-threshold calcium spikes (IT) and burst firing, a mixed cation conductance (IH) that prevented membrane hyperpolarization, and a transient K+ conductance that delayed spike firing (IA). The repetitive firing characteristics of LPN neurons were more distinct. One group of cells, located in the more caudal regions of the LPN responded to depolarizing current pulses with a train of action potentials or in a regular spiking (RS) mode. This form of firing showed a steep but highly linear increase in firing frequency with increasing levels of membrane depolarization. Another group of cells, located in the more rostral regions of the LPN, responded to depolarizing current pulses with clusters of high-frequency bursts or in a clustered spiking (CS) mode. The overall firing frequency rose nonlinearly with membrane depolarization, but the frequency of a given burst remained relatively constant. The caudal LPN receives input from the superior colliculus, whereas the rostral LPN receives input from layers V and VI of the visual cortex. Thus the RS and CS cells may be driven by subcortical and cortical inputs respectively, and the distinct temporal properties of their response modes may be a necessary component of the LPN circuitry.

Novel neuronal and astrocytic mechanisms in thalamocortical loop dynamics

In this review, we summarize three sets of findings that have recently been observed in thalamic astrocytes and neurons, and discuss their significance for thalamocortical loop dynamics. (i) A physiologically relevant 'window' component of the low-voltage-activated, T-type Ca(2+) current (I(Twindow)) plays an essential part in the slow (less than 1 Hz) sleep oscillation in adult thalamocortical (TC) neurons, indicating that the expression of this fundamental sleep rhythm in these neurons is not a simple reflection of cortical network activity. It is also likely that I(Twindow) underlies one of the cellular mechanisms enabling TC neurons to produce burst firing in response to novel sensory stimuli. (ii) Both electrophysiological and dye-injection experiments support the existence of gap junction-mediated coupling among young and adult TC neurons. This finding indicates that electrical coupling-mediated synchronization might be implicated in the high and low frequency oscillatory activities expressed by this type of thalamic neuron. (iii) Spontaneous intracellular Ca(2+) ([Ca(2+)](i)) waves propagating among thalamic astrocytes are able to elicit large and long-lasting N-methyl-D-aspartate-mediated currents in TC neurons. The peculiar developmental profile within the first two postnatal weeks of these astrocytic [Ca(2+)](i) transients and the selective activation of these glutamate receptors point to a role for this astrocyte-to-neuron signalling mechanism in the topographic wiring of the thalamocortical loop. As some of these novel cellular and intracellular properties are not restricted to thalamic astrocytes and neurons, their significance may well apply to (patho)physiological functions of glial and neuronal elements in other brain areas.

The Study of Passive Membrane Properties and Morphology Reveals Neuronal Differences Along the Sagittal Axis of the Ventral Periaqueductal Grey Matter

The membrane properties of the neurons located in the ventral part of the periaqueductal grey (PAG) of the guinea-pig were studied using an in vitro slice preparation. Cells had low values of resting membrane potential (-53.3 +/- 1.3 mV, mean +/- standard error), high input resistance (195. +/- 16.2 M ohm) and moderate values of membrane time constant (12.6 +/- 0.7 ms). The last two parameters changed as recordings were made along the sagittal axis, higher values corresponding to the more rostral cells. Three main neuronal types-fusiform, triangular and stellate-were found in the ventral PAG using intracellular injection of Lucifer yellow. A study of the cell number and cell density was carried out in coronal and sagittal sections of the ventral PAG. This analysis showed a clear gradient of size in this region arising from the gradual disappearance of large (17 to 40 microm) neurons in the caudorostral direction. The neuronal density also increased in this direction. Therefore, some electrotonic and morphological parameters differ along the sagittal axis. These findings suggest a larger neuronal heterogeneity of the caudal part of the PAG, and might contribute to a functional segregation of this region.

Synaptic Currents in Thalamo-cortical Neurons of the Rat Lateral Geniculate Nucleus

Thalamo-cortical neurons were identified in slices of the rat dorsal lateral geniculate nucleus and whole-cell currents were recorded using the patch-clamp technique. Postsynaptic currents occurring spontaneously, or elicited by extracellular stimulation in the vicinity of the recorded neuron, were analysed. Spontaneous postsynaptic currents were observed in every recorded neuron. At a holding potential of - 60 mV, and with a high internal Cl-, the currents were inward and had amplitudes ranging from < 10 to 425 pA. All the spontaneous currents were blocked by 10 microM bicuculline, indicating that they were due to the activation of postsynaptic gamma-aminobutyric acid (GABAA) receptors. The 10-90% rise time of these spontaneous GABAergic currents was 0.86 +/- 0.19 ms. Their time course of decay could be fitted to an exponential function with one time constant of 18.19 +/- 3.02 ms (mean +/- SD), or two time constants of 4.47 +/- 0.77 and 33.27 +/- 3.74 ms. This activity was frequently organized in bursts. Stimulus-evoked postsynaptic currents were recorded and shown to be due to the activation of glutamatergic receptors. Under similar experimental conditions a bicuculline-sensitive component was also recorded. These stimulus-evoked GABAergic currents had a 10 - 90% rise time of 1.93 +/- 0.54 ms. Their time course of decay could also be fitted to an exponential function with one time constant of 24.42 ms or two time constants of 10.26 +/- 2.46 and 49.30 +/- 10.98 ms. The difference in the time course between spontaneous and evoked GABAergic currents suggests that these responses may arise from synapses having different locations.

Molecular diversity of pacemaker ion channels

Ionic currents activated by hyperpolarization and regulated by cyclic nucleotides were first discovered more than 20 years ago. Recently the molecular identity of the underlying channels has been unveiled. The structural features of the protein sequences are discussed and related to the mechanisms of activation, selectivity for cyclic nucleotides, and ion permeation. Coverage includes a comparison of the biophysical properties of recombinant and native channels and their significance for the physiological functions of these channels.

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

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

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.

Spindle-like thalamocortical synchronization in a rat brain slice preparation

We obtained rat brain slices (550-650 microm) that contained part of the frontoparietal cortex along with a portion of the thalamic ventrobasal complex (VB) and of the reticular nucleus (RTN). Maintained reciprocal thalamocortical connectivity was demonstrated by VB stimulation, which elicited orthodromic and antidromic responses in the cortex, along with re-entry of thalamocortical firing originating in VB neurons excited by cortical output activity. In addition, orthodromic responses were recorded in VB and RTN following stimuli delivered in the cortex. Spontaneous and stimulus-induced coherent rhythmic oscillations (duration = 0.4-3.5 s; frequency = 9-16 Hz) occurred in cortex, VB, and RTN during application of medium containing low concentrations of the K(+) channel blocker 4-aminopyridine (0.5-1 microM). This activity, which resembled electroencephalograph (EEG) spindles recorded in vivo, disappeared in both cortex and thalamus during application of the excitatory amino acid receptor antagonist kynurenic acid in VB (n = 6). By contrast, cortical application of kynurenic acid (n = 4) abolished spindle-like oscillations at this site, but not those recorded in VB, where their frequency was higher than under control conditions. Our findings demonstrate the preservation of reciprocally interconnected cortical and thalamic neuron networks that generate thalamocortical spindle-like oscillations in an in vitro rat brain slice. As shown in intact animals, these oscillations originate in the thalamus where they are presumably caused by interactions between RTN and VB neurons. We propose that this preparation may help to analyze thalamocortical synchronization and to understand the physiopathogenesis of absence attacks.

Pacemaker oscillations in heart and brain: a key role for hyperpolarization-activated cation channels

Rhythmic activity of single cells or multicellular networks is a common feature of all organisms. The oscillatory activity is characterized by time intervals of several seconds up to many hours. Cellular rhythms govern the beating of the heart, the swimming behavior of sperm, cycles of sleep and wakefulness, breathing, and the release of hormones. Many neurons in the brain and cardiac cells are characterized by endogenous rhythmic activity, which relies on a complex interplay between several distinct ion channels. In particular, one type of ion channel plays a prominent role in the control of rhythmic electrical activity since it determines the frequency of the oscillations. The activity of the channels is thus setting the "pace" of the oscillations; therefore, these channels are often referred to as "pacemaker" channels. Despite their obvious important physiological function, it was not until recently that genes encoding pacemaker channels have been identified. Because both hyperpolarization and cyclic nucleotides are key elements that control their activity, pacemaker channels have now been designated hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels. The molecular identification of the channels and the upcoming studies on their properties in heterologous systems will certainly enhance our understanding of "pacemaking" in physiological systems. This review gives a brief insight into the physiological importance of these channels and sums up what we have learned since the first cloning of genes succeeded (for recent reviews, see also Clapham 1998; Luthi and McCormick 1998a; Biel et al. 1999; Ludwig, Zong, Hofmann, et al. 1999; Santoro and Tibbs 1999).

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.

Group III metabotropic glutamate receptors control corticothalamic synaptic transmission in the rat thalamus in vitro

1. Corticothalamic (CT) EPSPs evoked at <= 0.1 Hz were recorded from thalamocortical neurones in the rat dorsal lateral geniculate nucleus in vitro, with both GABAA and GABAB receptors blocked. 2. The group III metabotropic glutamate (mGlu) receptor agonists L-2-amino-4-phosphono-butyric acid (L-AP4) and O-phospho-L-serine (L-SOP) both caused a concentration-dependent depression of the CT EPSP. The maximum depression and EC50 values for these effects were 64.4 +/- 3.8 % and 88.0 +/- 24.7 microM for L-AP4, and 42.0 +/- 2.5 % and 958 +/- 492 microM for L-SOP, respectively (means +/- s.e.m.). Neither agonist had any effect on membrane potential or input resistance. 3. The depression of the CT EPSP caused by L-AP4 was reversed using the group III antagonist (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4, 1 mM), and the group II/III antagonist LY341495 (3 microM), but not using the group II antagonist (2S)-alpha-ethylglutamic acid (300 microM). The potencies of L-AP4, L-SOP and LY341495 indicate that this action of L-AP4 is mediated via mGlu7 and mGlu8 and not mGlu4 receptors. 4. Neither MAP4 nor LY341495 had any effect on the CT EPSPs evoked by 10 Hz trains of five stimuli, indicating the lack of endogenous activation of group III mGlu receptors in the thalamus during short bursts of cortical input. However, the magnitude of the depression caused by L-AP4 indicates that any physiological activation of group III mGlu receptors would have a profound effect on the CT input to the thalamus, and hence cortical control of thalamic function.

Anatomic, intrinsic, and synaptic properties of dorsal and ventral division neurons in rat medial geniculate body

Anatomic, intrinsic, and synaptic properties of dorsal and ventral division neurons in rat medial geniculate body. Presently little is known about what basic synaptic and cellular mechanisms are employed by thalamocortical neurons in the two main divisions of the auditory thalamus to elicit their distinct responses to sound. Using intracellular recording and labeling methods, we characterized anatomic features, membrane properties, and synaptic inputs of thalamocortical neurons in the dorsal (MGD) and ventral (MGV) divisions in brain slices of rat medial geniculate body. Quantitative analysis of dendritic morphology demonstrated that tufted neurons in both divisions had shorter dendrites, smaller dendritic tree areas, more profuse branching, and a greater dendritic polarization compared with stellate neurons, which were only found in MGD. Tufted neuron dendritic polarization was not as strong or consistent as earlier Golgi studies suggested. MGV and MGD cells had similar intrinsic properties except for an increased prevalence of a depolarizing sag potential in MGV neurons. The sag was the only intrinsic property correlated with cell morphology, seen only in tufted neurons in either division. Many MGV and MGD neurons received excitatory and inhibitory inferior colliculus (IC) inputs (designated IN/EX or EX/IN depending on excitation/inhibition sequence). However, a significant number only received excitatory inputs (EX/O) and a few only inhibitory (IN/O). Both MGV and MGD cells displayed similar proportions of response combinations, but suprathreshold EX/O responses only were observed in tufted neurons. Excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) had multiple distinguishable amplitude levels implying convergence. Excitatory inputs activated alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors the relative contributions of which were variable. For IN/EX cells with suprathreshold inputs, first-spike timing was independent of membrane potential unlike that of EX/O cells. Stimulation of corticothalamic (CT) and thalamic reticular nucleus (TRN) axons evoked a GABAA IPSP, EPSP, GABAB IPSP sequence in most neurons with both morphologies in both divisions. TRN IPSPs and CT EPSPs were graded in amplitude, again suggesting convergence. CT inputs activated AMPA and NMDA receptors. The NMDA component of both IC and CT inputs had an unusual voltage dependence with a detectable DL-2-amino-5-phosphonovaleric acid-sensitive component even below -70 mV. First-spike latencies of CT evoked action potentials were sensitive to membrane potential regardless of whether the TRN IPSP was present. Overall, our in vitro data indicate that reported regional differences in the in vivo responses of MGV and MGD cells to auditory stimuli are not well correlated with major differences in intrinsic membrane features or synaptic responses between cell types.

Current clamp and modeling studies of low-threshold calcium spikes in cells of the cat's lateral geniculate nucleus

Current clamp and modeling studies of low-threshold calcium spikes in cells of the cat's lateral geniculate nucleus. All thalamic relay cells display a voltage-dependent low-threshold Ca2+ spike that plays an important role in relay of information to cortex. We investigated activation properties of this spike in relay cells of the cat's lateral geniculate nucleus using the combined approach of current-clamp intracellular recording from thalamic slices and simulations with a reduced model based on voltage-clamp data. Our experimental data from 42 relay cells showed that the actual Ca2+ spike activates in a nearly all-or-none manner and in this regard is similar to the conventional Na+/K+ action potential except that its voltage dependency is more hyperpolarized and its kinetics are slower. When the cell's membrane potential was hyperpolarized sufficiently to deinactivate much of the low-threshold Ca2+ current (IT) underlying the Ca2+ spike, depolarizing current injections typically produced a purely ohmic response when subthreshold and a full-blown Ca2+ spike of nearly invariant amplitude when suprathreshold. The transition between the ohmic response and activated Ca2+ spikes was abrupt and reflected a difference in depolarizing inputs of <1 mV. However, activation of a full-blown Ca2+ spike was preceded by a slower period of depolarization that was graded with the amplitude of current injection, and the full-blown Ca2+ spike activated when this slower depolarization reached a sufficient membrane potential, a quasithreshold. As a result, the latency of the evoked Ca2+ spike became less with stronger activating inputs because a stronger input produced a stronger depolarization that reached the critical membrane potential earlier. Although Ca2+ spikes were activated in a nearly all-or-none manner from a given holding potential, their actual amplitudes were related to these holding potentials, which, in turn, determined the level of IT deinactivation. Our simulations could reproduce all of the main experimental observations. They further suggest that the voltage-dependent K+ conductance underlying IA, which is known to delay firing in many cells, does not seem to contribute to the variable latency seen in activation of Ca2+ spikes. Instead the simulations indicate that the activation of IT starts initially with a slow and graded depolarization until enough of the underling transient (or T) Ca2+ channels are recruited to produce a fast, "autocatalytic" depolarization seen as the Ca2+ spike. This can produce variable latency dependent on the strength of the initial activation of T channels. The nearly all-or-none nature of Ca2+ spike activation suggests that when a burst of action potentials normally is evoked as a result of a Ca2+ spike and transmitted to cortex, this signal is largely invariant with the amplitude of the input activating the relay cell.

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.

Cellular mechanisms underlying two muscarinic receptor-mediated depolarizing responses in relay cells of the rat lateral geniculate nucleus

We used the whole-cell recording technique in an in vitro preparation to examine the electrophysiological actions of the muscarinic receptors on relay cells in the rat lateral geniculate nucleus. Drop application of the muscarinic agonist acetyl-beta-methylcholine resulted in a slow depolarization that persisted for several minutes. The response was insensitive to the nicotinic antagonist hexamethonium, but was blocked by atropine, a muscarinic antagonist. The response was also insensitive to blockade of synaptic transmission by tetrodotoxin, indicating a direct muscarinic effect. The muscarinic depolarization consisted of two components that were somewhat separated in time. The early portion of the muscarinic response was mediated by a large inward current with little change in input resistance, while the later portion was mediated by a small inward current associated with a large increase in input resistance. Pharmacological agents were used to distinguish the two components. Drop application of McN-A-343, an ml receptor agonist, could only mimic the later component of the muscarinic response. This was supported by the result that the later component was blocked by low concentrations of pirenzepine. These data suggest that the ml receptor only mediates the late component of the muscarinic response, while the early component is mainly mediated by the m3 receptor. The idea that both ml and m3 receptors were involved in the muscarinic depolarization was further supported by voltage-clamp analysis. This revealed that activation of the ml receptor was associated with a decrease in an inward potassium current, IKleak, while activation of the m3 receptor was likely associated with both a decrease in IKleak and an increase in the hyperpolarization-activated cation current Ih. In summary, our data suggest that muscarinic responses in geniculate relay cells result from the activation of two receptors, which modulate IKleak and Ih. Given the fact that the ascending aminergic systems also depolarize geniculate relay cells via two receptors acting on IKleak and Ih, we concluded that ascending activating systems use common mechanisms to enact the depolarizing form of arousal in relay neurons.

Electrophysiological and neurochemical study of the rat geniculo-cortical pathway. Evidence for glutamatergic neurotransmission

The projection from the dorsal lateral geniculate nucleus to the primary visual cortex of the rat was studied electrophysiologically. Electrical stimulation of the dorsal lateral geniculate nucleus and the optic tract produced three types of responses on neurons of area 17: excitation followed by inhibition, excitation and inhibition. These results extend and confirm, in adult rats, previous studies done in rat geniculate-visual cortex cocultures preparations in vitro. The role of glutamate in the neurotransmission of the rat geniculo-cortical pathway was also investigated. In a first set of experiments, the effects of kynurenate, an antagonist of glutamate receptors, on visual cortex neurons with a monosynaptic excitatory response to dorsal lateral geniculate nucleus stimulation were studied. Microiontophoresis of kynurenate in area 17 neurons selectively suppressed the excitatory response to dorsal lateral geniculate nucleus and optic tract stimulation. In a second set of experiments, the effects of electrical stimulation of the dorsal lateral geniculate nucleus and the optic tract on the release of amino acids in the rat visual cortex in vivo were studied. Using the push-pull method, we perfused a discrete region of the visual cortex with artificial cerebrospinal fluid (CSF), and the amino acid content of the perfusates was analysed by high performance liquid chromatography (HPLC). Stimulation of either the dorsal lateral geniculate nucleus or the optic tract significantly increased glutamate release in area 17. The rest of the amino acids studied did not show significant changes. The results provide evidence for the participation of glutamate in the neurotransmission of the geniculo-cortical pathway in the rat.

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.

Physiological properties of neurons in the optic layer of the rat's superior colliculus

We made intracellular recordings from 74 neurons in the optic layer of the rat superior colliculus (SC). Resting membrane potentials were -62.3 +/- 6.2 (SD) mV, and input resistances were 37.9 +/- 10.1 MOmega. Optic layer neurons had large sodium spikes (74.2 +/- 12.3 mV) with an overshoot of 12 mV and a half-amplitude duration of 0.75 +/- 0.2 ms. Each sodium spike was followed by two afterhyperpolarizations (AHPs), one of short duration and one of longer duration, which were mediated by tetraethylammonium (TEA)-sensitive (IC) or apamin-sensitive (IAHP) calcium-activated potassium currents, respectively. Sodium spikes were also followed by an afterdepolarization (ADP), which was only revealed when the AHPs were blocked by TEA or apamin. In response to hyperpolarizing current pulses, optic layer neurons showed an inward rectification mediated by H channels. At the break of the current pulse, there was a rebound low-threshold spike (LTS) with a short duration of <25 ms. The LTS usually induced two sodium spikes (doublet). Most optic layer neurons (84%) behaved as intrinsically bursting cells. They responded to suprathreshold depolarization with an initial burst (or doublet) followed by a train of regular single spikes. The remaining 16% of cells acted as chattering cells with high-frequency gamma (20-80 Hz) rhythmic burst firing within a narrow range of depolarized potentials. The interburst frequency was voltage dependent and also time dependent, i.e., showed frequency adaptation. Unmasking the ADP with either TEA or apamin converted all of the tested intrinsically bursting cells into chattering cells, indicating that the ADP played a crucial role in the generation of rhythmic burst firing. Optic layer neurons receive direct retinal excitation mediated by both N-methyl--aspartate (NMDA) and non-NMDA receptors. Optic tract (OT) stimulation also led to gamma-aminobutyric acid-A (GABAA) receptor-mediated inhibition, the main effect of which was to curtail the excitatory response to retinal inputs by shunting the excitatory postsynaptic current. Intracellular staining with biocytin showed that the optic layer neurons that we recorded from were mostly either wide-field vertical neurons or other cells with predominately superficially projecting dendrites. These cells were similar to calbindin immunoreactive cells seen in the optic layer. The characteristics of these optic layer neurons, such as prominent AHPs, strong shunting effect of inhibition, and short-lasting LTS, suggest that they respond transiently to retinal inputs. This is consistent with a function for these cells as the first relay station in the extrageniculate visual pathway.

Sodium current in rat and cat thalamocortical neurons: role of a non-inactivating component in tonic and burst firing

The properties of the Na+ current present in thalamocortical neurons of the dorsal lateral geniculate nucleus were investigated in dissociated neonate rat and cat neurons and in neurons from slices of neonate and adult rats using patch and sharp electrode recordings. The steady-state activation and inactivation of the transient Na+ current (INa) was well fitted with a Boltzmann curve (voltage of half-maximal activation and inactivation, V1/2, -29.84 mV and -70.04 mV, respectively). Steady-state activation and inactivation curves showed a small region of overlap, indicating the occurrence of a INa window current. INa decay could be fitted with a single exponential function, consistent with the presence of only one channel type. Voltage ramp and step protocols showed the presence of a noninactivating component of the Na+ current (INaP) that activated at potentials more negative (V1/2 = -56.93 mV) than those of INa. The maximal amplitude of INaP was approximately 2.5% of INa, thus significantly greater than the calculated contribution (0.2%) of the INa window component. Comparison of results from dissociated neurons and neurons in slices suggested a dendritic as well as a somatic localization of INaP. Inclusion of papain in the patch electrode removed the fast inactivation of INa and induced a current with voltage-dependence (V1/2 = -56.92) and activation parameters similar to those of INaP. Current-clamp recordings with sharp electrodes showed that INaP contributed to depolarizations evoked from potentials of approximately -60 mV and unexpectedly to the amplitude and latency of low-threshold Ca2+ potentials, suggesting that this noninactivating component of the Na+ channel population plays an important role in the integrative properties of thalamocortical neurons during both tonic and burst-firing patterns.

Sodium current in rat and cat thalamocortical neurons: role of a non-inactivating component in tonic and burst firing

The properties of the Na+ current present in thalamocortical neurons of the dorsal lateral geniculate nucleus were investigated in dissociated neonate rat and cat neurons and in neurons from slices of neonate and adult rats using patch and sharp electrode recordings. The steady-state activation and inactivation of the transient Na+ current (INa) was well fitted with a Boltzmann curve (voltage of half-maximal activation and inactivation, V1/2, -29.84 mV and -70.04 mV, respectively). Steady-state activation and inactivation curves showed a small region of overlap, indicating the occurrence of a INa window current. INa decay could be fitted with a single exponential function, consistent with the presence of only one channel type. Voltage ramp and step protocols showed the presence of a noninactivating component of the Na+ current (INaP) that activated at potentials more negative (V1/2 = -56.93 mV) than those of INa. The maximal amplitude of INaP was approximately 2.5% of INa, thus significantly greater than the calculated contribution (0.2%) of the INa window component. Comparison of results from dissociated neurons and neurons in slices suggested a dendritic as well as a somatic localization of INaP. Inclusion of papain in the patch electrode removed the fast inactivation of INa and induced a current with voltage-dependence (V1/2 = -56.92) and activation parameters similar to those of INaP. Current-clamp recordings with sharp electrodes showed that INaP contributed to depolarizations evoked from potentials of approximately -60 mV and unexpectedly to the amplitude and latency of low-threshold Ca2+ potentials, suggesting that this noninactivating component of the Na+ channel population plays an important role in the integrative properties of thalamocortical neurons during both tonic and burst-firing patterns.

Electrophysiological properties of dorsal lateral geniculate neurons in brain slices from ME7 scrapie-infected mice

Electrophysiological recordings using conventional intracellular techniques were obtained from dorsal lateral geniculate nucleus (dLGN) neurons in brain slices from ME7 scrapie-infected mice at specific time points throughout the incubation period of the disease. Comparisons were made with age-matched control mice. A number of dLGN neurons from control and scrapie-infected mice were injected with biocytin in order to examine their cellular morphology. Mice were infected with ME7 scrapie by an intraocular route and the mean (+/- SEM) incubation period of the disease was 276 +/- 3.5 days. Our results indicate that there were no differences in the electrophysiological or morphological parameters of neurons recorded in ME7 scrapie-infected and age-matched control mice at any stage of the disease up to 240 days postinoculation. After this time, however, no detectable electrical activity was recorded in the dLGN. This study demonstrates that in the ME7 scrapie-infected dLGN, relay neurons with normal physiological and morphological properties are present even at an advanced stage of the disease at a time when the dLGN is known to be subject to marked pathological changes and a profound neuronal loss.

Morphology and membrane properties of neurones in the cat ventrobasal thalamus in vitro

1. The morphological (n = 66) and electrophysiological (n = 41) properties of eighty-six thalamocortical (TC) neurones and those of one interneurone in the cat ventrobasal (VB) thalamus were examined using an in vitro slice preparation. The resting membrane potential for thirty-seven TC neurones was -61.9 +/- 0.7 mV, with thirteen neurones exhibiting delta oscillation with and without DC injection. 2. The voltage-current relationships of TC neurones were highly non-linear, with a mean peak input resistance of 254.4 M omega and a mean steady-state input resistance of 80.6 M omega between -60 and -75 mV. At potentials more positive than -60 mV, outward rectification led to a mean steady-state input resistance of 13.3 M omega. At potentials more negative than -75 mV, there was inward rectification, consisting of a fast component leading to a mean peak input resistance of 14.5 M omega, and a slow time-dependent component leading to a mean steady-state input resistance of 10.6 M omega. 3. Above -60 mV, three types of firing were exhibited by TC neurones. The first was an accelerating pattern associated with little spike broadening and a late component in the spike after-hyperpolarization. The second was an accommodating or intermittent pattern associated with spike broadening, while the third was a burst-suppressed pattern of firing also associated with spike broadening, but with broader spikes of a smaller amplitude. All TC neurones evoked high frequency (310-520 Hz) burst firing mediated by a low threshold Ca2+ potential. 4. Morphologically TC neurones were divided into two groups: Type I (n = 31 neurones) which had larger soma, dendritic arbors that occupied more space, thicker primary dendrites and daughter dendrites that followed a more direct course than Type II (n = 35). The only electrophysiological differences were that Type I neurones (n = 16) had smaller peak input and outward rectification resistance and spike after-hyperpolarization, but greater peak inward rectification resistance, and exhibited delta oscillation less often than Type II (n = 13). 5. The morphologically identified interneurone exhibited no outward rectification, only moderate inward rectification, and no high frequency firing associated with the offset of negative current steps below -55 mV. This interneurone had a regular accommodating firing pattern, but the spike after-hyperpolarization had a late component, unlike the accommodating firing in TC neurones. 6. Therefore, the differentiation of TC neuronal types in the cat VB thalamus based on their morphology was reflected by differences in peak input resistance, outward rectification and spike after-hyperpolarization, which could be accounted for by their difference in soma size. More importantly, the firing pattern of the majority of TC neurones in the cat VB thalamus were different from those of TC neurones in other sensory thalamic nuclei. 7. Thalamocortical neurones in the cat VB thalamus were also clearly distinguishable from the interneurone based on the presence of their prominent outward rectification, peak inward rectification and robust low threshold Ca2+ potentials.

Tonic activation of presynaptic GABA(B) receptors on thalamic sensory afferents

The presence and role of presynaptic GABA(B) receptors in the control of excitatory amino acid-medicated transmission were investigated (using sharp electrode recordings) in the rat dorsal lateral geniculate nucleus and ventrobasal thalamus in vitro by comparing the effects of the selective GABA(B) receptor agonist, (+ or -)-baclofen, and of two antagonists, CGP 35348 and 2-hydroxy-saclofen, on the excitatory postsynaptic potentials evoked in thalamocortical neurons by stimulation of the sensory afferents. Application of CGP 35348 alone blocked the GABA(B) receptor-mediated inhibitory postsynaptic potential evoked in the dorsal lateral geniculate nucleus by stimulation of the optic tract (n = 5), but had no effect on the resting membrane potential and input resistance of thalamocortical cells (n = 6). In contrast, 2-hydroxy-saclofen caused a hyperpolarization (6.9 + or - 0.5 mV, n = 10) and a decrease in the apparent input resistance (26.3 + or - 2.6%, n = 10). This effect of 2-hydroxy-saclofen was antagonized by CGP 35348. When bicuculline was present in the perfusion medium and following intracellular injection of QX 314, GABA(A) and GABA(B) receptors in the recorded neurons were blocked. Under this condition, application of baclofen decreased the amplitude of the medial lemniscus- and optic tract-evoked excitatory postsynaptic potentials in the two thalamic nuclei investigated. This effect was fully antagonized by CGP 35348 and only partially by 2-hydroxy-saclofen. CGP 35348 alone increased (19.3 + or - 4.3%, n = 5) and 2-hydroxy-saclofen alone decreased (29.9 + or - 8.6%, n = 5) the amplitude of the excitatory postsynaptic potential. This effect of 2-hydroxy-saclofen was not blocked by CGP 35348. These results indicate that presynaptic GABA(B) receptors are present on the terminals of the sensory afferents in the rat dorsal lateral geniculate nucleus and in the ventrobasal thalamus. These receptors are tonically activated by endogenous GABA, at least in vitro, and provide a negative control mechanism by which the excitatory amino acid-mediated transmission within these nuclei can be regulated. In contrast, the endogenous GABA level is not sufficient for a tonic activation of postsynaptic GABA(B) receptors. Furthermore, these results indicate that 2-hydroxy-saclofen acts as a partial agonist on postsynaptic CGP 35348-sensitive GABA(B) receptors, and that, in addition to its antagonist action on presynaptic CGP 35348-sensitive GABA(B) receptors, it also has an effect on either presynaptic, CGP 35348-insensitive GABA(B) receptors and/or another presynaptic receptor type.

Control of slow oscillations in the thalamocortical neuron: a computer model

We investigated computer models of a single thalamocortical neuron to assess the interaction of intrinsic voltage-sensitive channels and cortical synaptic input in producing the range of oscillation frequencies observed in these cells in vivo. A morphologically detailed model with Hodgkin-Huxley-like ion channels demonstrated that intrinsic properties would be sufficient to readily produce 3 to 6 Hz oscillations. Hyperpolarization of the model cell reduced its oscillation frequency monotonically whether through current injection or modulation of a potassium conductance, simulating the response to a neuromodulatory input. We performed detailed analysis of highly reduced models to determine the mechanism of this frequency control. The interburst interval was controlled by two different mechanisms depending on whether or not the pacemaker current, IH, was present. In the absence of IH, depolarization during the interburst interval occurred at the same rate with different current injections. The voltage difference from the nadir to threshold for the low-threshold calcium current, IT, determined the interburst interval. In contrast, with IH present, the rate of depolarization depended on injected current. With the full model, simulated repetitive cortical synaptic input entrained oscillations up to approximately double the natural frequency. Cortical input readily produced phase resetting as well. Our findings suggest that neither ascending brainstem control altering underlying hyperpolarization, nor descending drive by repetitive cortical inputs, would alone be sufficient to produce the range of oscillation frequencies seen in thalamocortical neurons. Instead, intrinsic neuronal mechanisms would dominate for generating the delta range (0.5-4 Hz) oscillations seen during slow wave sleep, whereas synaptic interactions with cortex and the thalamic reticular nucleus would be required for faster oscillations in the frequency range of spindling (7-14 Hz).

Electrophysiological and morphological properties of interneurones in the rat dorsal lateral geniculate nucleus in vitro

1. Intracellular recordings were made from putative interneurones (n = 24) and thalamocortical (TC) projection neurones (n = 45) in slice preparations of the rat dorsal lateral geniculate nucleus (dLGN) in order to compare the electrophysiological properties of these neuronal types. 2. Intracellular injection of biocytin to electrophysiologically identified neurones (n = 34) revealed the morphology of putative interneurones (n = 4) to be similar to class B and that of TC neurones (n = 30) to be similar to class A Golgi-impregnated neurones. 3. Interneurones had resting membrane potentials (-52 mV) relatively positive to those of TC neurones (-63 mV), shorter time constants (36.8 and 58.2 ms, respectively), but similar steady-state input resistances (164 and 180 M omega, respectively). Steady-state voltage-current relationships were nearly linear in interneurones, but highly non-linear in TC neurones. 4. The structure of action potential firing evoked at the break of hyperpolarizing voltage transients was dependent upon neuronal type. Interneurones fired a single action potential or a burst of action potentials with a maximum frequency of < 130 Hz, whilst TC neurones fired a high frequency burst with a minimum frequency of > 250 Hz. In addition, well-defined burst firing of action potentials in response to depolarizing voltage excursions, from membrane potentials negative to -65 mV, could be evoked in TC neurones, but not in interneurones. 5. The directly evoked action potentials of interneurones were characterized by an initial slow pre-potential preceding the fast upstroke of the action potential. The amplitude and width of interneurones' action potentials were smaller than those of TC neurones and the amplitude and duration of the single action potential after-hyperpolarization were greater in interneurones. Both interneurones and TC neurones fired action potentials repetitively in response to suprathreshold voltage excursions, with interneurones demonstrating a greater degree of spike-frequency adaptation. Following a train of action potentials, interneurones and TC neurones generated a slow after-hyperpolarizing potential: in interneurones but not TC neurones this potential was followed by a slow depolarizing potential. 6. An intrinsic, subthreshold membrane potential oscillatory activity with a mean frequency of approximately 8 Hz was observed in interneurones. 7. Electrical stimulation of the optic tract evoked in interneurones apparently pure EPSPs, pure IPSPs or a mixture of EPSPs and IPSPs. EPSPs were found to be biphasic and mediated by the activation of non-N-methyl-D-aspartate (NMDA) and NMDA excitatory amino acid receptors. IPSPs and the response to the iontophoretic application of GABA were found to reverse between -65 and -70 mV. The application of GABAB receptor agonists failed to affect the membrane properties of six of seven interneurones tested. In addition spontaneous EPSPs and IPSPs were recorded in interneurones. 8. These results demonstrate that the electrophysiological properties of putative interneurones are distinct from those of TC neurones of the rat dLGN. The implications of these findings for the control of visual responsiveness of TC neurones are discussed.

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.

Topological and phenomenological classification of bursting oscillations

We describe a classification scheme for bursting oscillations which encompasses many of those found in the literature on bursting in excitable media. This is an extension of the scheme of Rinzel (in Mathematical Topics in Population Biology, Springer, Berlin, 1987), put in the context of a sequence of horizontal cuts through a two-parameter bifurcation diagram. We use this to describe the phenomenological character of different types of bursting, addressing the issue of how well the bursting can be characterized given the limited amount of information often available in experimental settings.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: in vivo electrophysiology

In vivo intracellular recordings were obtained from identified thalamocortical neurons in the ventroanterior-ventrolateral complex in urethane-anesthetized rats. This thalamic nucleus has few interneurons. Neurons that responded to cerebellar stimulation were injected intracellularly with horseradish peroxidase or biocytin and examined with light and electron microscopy (see companion paper). Intrinsic membrane properties and voltage-dependent rhythmic activity of cerebellar-responsive ventroanterior-ventrolateral neurons were similar to those described previously for thalamic neurons. Thus, in addition to conventional "fast" Na(+)-dependent spikes, rat ventroanterior-ventrolateral neurons had "slow" Ca(2+)-mediated low-threshold spikes and membrane conductances that supported rhythmic oscillations. Two modes of spontaneous activity were observed: (i) a tonic firing pattern that consisted of irregularly occurring fast spikes that predominated when the membrane potential was more positive than about -60 mV, and (ii) a rhythmic firing pattern, observed when the membrane potential was more negative than about -65 mV, composed of periodic (4-8 Hz) membrane hyperpolarizations and ramp depolarizations that often produced a low-threshold spike and a burst of fast spikes. In some neurons, spontaneous fast prepotentials were also observed, often with a relatively constant rate (up to 70 Hz). Cerebellar stimulation elicited excitatory postsynaptic potentials that in some cases appeared to be all-or-none and were similar in form to fast prepotentials. Stimulation of ipsilateral motor cortex elicited a short-latency antidromic response followed by a monosynaptic excitatory postsynaptic potential, which had a slower rise time than excitatory postsynaptic potentials evoked from cerebellum, suggesting that cortical inputs were electrotonically distal to cerebellar inputs. In the presence of moderate membrane hyperpolarization, the cortically evoked excitatory postsynaptic potential was followed by a long-lasting hyperpolarization (100-400 ms duration), a rebound depolarization and one or two cycles resembling spontaneous rhythmic activity. Membrane conductance was increased during the initial component of the long hyperpolarization, much of which was probably due to an inhibitory postsynaptic potential. In contrast, membrane conductance was unchanged or slightly decreased during the latter three-quarters of the long hyperpolarization. The amplitude of this component of the long hyperpolarization usually decreased when the membrane was hyperpolarized with intracellular current injection. Thus, both disfacilitation and an inhibitory postsynaptic potential may have contributed to the latter portion of the cortically-evoked long hyperpolarization. The cortically-evoked inhibitory postsynaptic potentials likely originated predominantly from feedforward activation of GABAergic neurons in the thalamic reticular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: light and electron microscopy

The morphology and synaptic organization of neurons in the ventroanterior-ventrolateral nucleus of rats was examined using in vivo intracellular staining techniques. Neurons were characterized electrophysiologically based on intrinsic membrane properties and synaptic responses to stimulation of motor cortex and cerebellar nuclei, as described in the companion paper. Cerebellar-responsive neurons were stained intracellularly with either horseradish peroxidase or biocytin. All stained ventroanterior-ventrolateral nucleus neurons were identified as thalamocortical neurons on anatomical (and often electrophysiological) grounds, consistent with previous findings that rat ventroanterior-ventrolateral nucleus is interneuron-sparse. Ventroanterior-ventrolateral nucleus neurons had three to eight thick primary dendrites. Proximal dendrites often exhibited a tufted branching pattern, from which many thinner, higher order dendrites arose. Dendrites branched to form a funnel-like infiltration of the neuropil that resulted in a spherical, roughly homogeneous dendritic field. The axon originated from the cell body or a proximal dendrite and coursed laterally and dorsally to innervate motor cortex. One to five axon collaterals were emitted in the rostral dorsolateral sector of the thalamic reticular nucleus; collaterals were not observed in the ventroanterior-ventrolateral nucleus or other nuclei in dorsal thalamus. The synaptic organization of the ventroanterior-ventrolateral nucleus was examined with electron microscopy, including two intracellularly labeled ventroanterior-ventrolateral nucleus neurons that were shown electrophysiologically to receive monosynaptic inputs from the cerebellum. The neuropil of rat ventroanterior-ventrolateral nucleus lacked the complexity and diversity found in corresponding thalamic nuclei of felines and primates, due to the paucity of interneurons. Vesicle-containing dendrites, dendrodendritic synapses and glomeruli were not observed. Three broad classes of presynaptic terminals were identified. (1) Small round boutons: small boutons containing densely-packed, small round vesicles that formed asymmetric synapses predominantly with the distal dendrites of thalamocortical neurons. These were the most prevalent type of bouton in the ventroanterior-ventrolateral nucleus (78% of presynaptic elements) and likely arose from the cerebral cortex. (2) Large round boutons: large terminals with loosely packed small round vesicles that made multiple asymmetric synapses with proximal and intermediate dendrites. Large round boutons comprised 8% of the neuropil, and likely arose from the cerebellar nuclei. (3) Medium size boutons with pleomorphic vesicles: medium-sized profiles containing pleomorphic vesicles that formed symmetric synapses with proximal, intermediate and distal dendrites and, less frequently, with cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

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)